3.15

Solar-Assisted Heat Pumps

DA Chwieduk, Warsaw University of Technology, Warsaw, Poland
© 2012 Elsevier Ltd. All rights reserved.

3.15.1
3.15.2
3.15.2.1
3.15.2.2
3.15.2.3
3.15.2.4
3.15.3
3.15.3.1
3.15.3.2
3.15.3.3
3.15.3.4
3.15.3.5
3.15.3.6
3.15.4
3.15.4.1
3.15.4.2
3.15.4.3
3.15.4.4
References

Introduction to the Concept of Solar-Assisted Heat Pumps
Heat Pump Fundamentals
Principles of Heat Pump Operation
Thermodynamic Cycles

Classification of Heat Pumps
Renewable Heat Sources
Solar-Assisted Heat Pump System
Classification, Configurations, and Functions
Direct Solar-Assisted Heat Pump Systems
Series Solar-Assisted Heat Pump Systems
Parallel Solar-Assisted Heat Pump
Dual-Source Solar-Assisted Heat Pump
Other Configurations
Solar-Assisted Heat Pump System with Seasonal Storage
Fundamental Options of Seasonal Energy Storage
Classification and Evaluation of Seasonal Ground Storage
Heat and Mass Transfer in the Ground Store, General Consideration
Applications

495

495

495

497

501

502

506

506


507

508

511

516

517

519

519

521

524

525

527


3.15.1 Introduction to the Concept of Solar-Assisted Heat Pumps
There are some limitations on the use of solar radiation for heating purposes, mainly because of its stochastic and intermittent
character. There are changes in solar radiation availability in the long term, that is, over a year, and also in the short term, that is, over
a day. When solar energy is used for space heating, the time and peak values of available solar radiation are quite opposite to the
time and peak values of space heating demand. This is especially true in high-latitude countries, where in winter the solar radiation
level is low and the duration of solar irradiation is short, and consequently there is a cold climate and long heating season with high

heating demands.
In many high-latitude regions, heating of buildings is a major component of the total energy used in the building sector. Usually
‘traditional’ solar active heating systems alone cannot provide all the heating needs. There are different options to solve this problem.
One of them is to couple a solar heating system with a heat pump in one combined heating system, as can take place in a small- or
medium-scale application, that is, in single-family houses and multifamily or public buildings, respectively. This type of a heating
system is called a solar-assisted heat pump (SAHP) system. Another option is to use the seasonal solar energy storage in the form of
sensible heat of a large storage volume. Usually the temperature of the heat stored is too low to be used directly for heating. The
low-temperature heat stored can be converted into higher temperature heat by applying a heat pump. In this way also a heat pump is
incorporated into the heating system. Such heating systems can be used for medium- or large-scale applications, that is, in multifamily
or public buildings, or blocks of buildings (i.e., in communes and small city districts), respectively. In the case of medium-scale
applications, the heating system is termed a solar-assisted heat pump system with seasonal storage (SAHPSS) system, and a common
example is a solar-assisted heat pump system with ground storage (SAHPGS). In the case of large-scale applications, such a system is
called a central solar heating plant with seasonal storage (CSHPSS). There are a variety of underground thermal energy storage (UTES)
system configurations and modes of operation. Solar collectors and a heat pump are the major system components.
This chapter describes the concept, classification, and operation of SAHP systems, including systems with seasonal storage. For
better understanding of the idea of SAHP system operation and application, the fundamentals of heat pumps are initially presented.
Heat pumps applied in SAHP systems are vapor compression heat pumps, and this type of heat pumps are described and analyzed.

3.15.2 Heat Pump Fundamentals
3.15.2.1

Principles of Heat Pump Operation

Heat pumps, refrigerators, and heat engines are heat machines. Their operation is based on thermodynamic processes that are
governed by the first and second laws of thermodynamics [1, 2]. Heat pumps and refrigerators operate in reversed cycles compared
with heat engines. The processes and energy flows are in the opposite direction to those in the power cycles of heat engines, as
presented schematically in Figure 1.

Comprehensive Renewable Energy, Volume 3


doi:10.1016/B978-0-08-087872-0.00321-8

495


496

Applications

Power cycle

Reverse cycle

Heat source
at T2

Heat sink
at T2

Q2

Engine

Q2
Refrigerator
or
Heat pump

Wout


Win

Q1

Q1
Heat sink
at T1

Heat source
at T1

Figure 1 Concept of power cycle for a heat engine and reverse cycle for a refrigerator or a heat pump.

A heat engine’s function is to generate work using a heat source. In a heat engine, the high-temperature T2 heat source is used
giving the heat input Q2 to get the energy output in the form of work W. However, there is an amount of heat Q1 at lower
temperature T1 that must be removed to the heat sink to fulfill the first law of thermodynamics. The first law of thermodynamics is
the conservation of energy law, and can be written (see Figure 1) as follows:
Q2 ¼ jWj þ jQ1 j

½1Š

Unlike a heat engine, a heat pump’s function is to lift a certain quantity of heat Q1 from a heat source at a lower temperature level T1
to a heat sink at higher temperature T2. However, to fulfill the first law of thermodynamics, the upgrading of heat must be done by
the work W that is supplied to the machine. Following this and Figure 1, it can be seen that the amount of heat Q2 to the heat sink is
the sum of heat Q1 extracted from the heat source and the amount of work W required in the process, and the first law of
thermodynamics, described by eqn [1], is accomplished.
As it has been already mentioned, a heat pump is used to supply heat Q2 at high temperature T2. Conversely, a refrigerator is
used to extract heat Q1 at low temperature T1. It means that a refrigerator’s function is to cool down a given heat source extracting
a certain quantity of heat Q1 from this source (at lower temperature T1). In a refrigerator, as in a heat pump, to fulfill the first law
of thermodynamics, to extract the heat from the heat source the work W must be supplied to drive the cycle and heat must

be removed at a higher temperature. In practice, the functions of both heat pump and refrigerator can be combined in one
machine, when both heating and cooling are required simultaneously. For example, such situations can be found at a sport center
where some chillers are used to cool an ice skating area and at the same time they also provide heat for hot water for swimming
pools, operating like a heat pump.
The efficiency of a heat engine, a refrigerator, and a heat pump is defined using the first law of thermodynamics. In the case of a
heat pump and refrigerator, efficiency is measured by the coefficient of performance (COP), which is the ratio of the energy that is
used for heating (at the heat sink) or for cooling (at the heat source) to the work that has to be supplied to drive the cycle. The
efficiency of a heat engine, a refrigerator, and a heat pump can be expressed by eqns [2a], [2b], and [2c], respectively, in the
following way:
η¼
COPr ¼

W
Q2 − jQ1 j
jQ1 j
¼
¼1−
Q2
Q2
Q2

½2aŠ



Q2 −jWj  Q2 
jQ1 j
jQ1 j
¼
¼

¼
−1
jWj
Q2 − jQ1 j
jWj
W

½2bŠ

Q2
Q2
¼
jWj Q2 − jQ1 j

½2cŠ

COPhp ¼

It can be seen from eqn [2a] that the efficiency of the heat engine is always lower than 1, and the COP of a refrigerator (eqn [2b]) is
one less than that of a heat pump (eqn [2c]).
Summarizing, a heat engine is a machine generating work from the heat that is provided to the process and rejecting some
amount of the heat at a lower temperature. Conversely, a heat pump is a machine that lifts a certain quantity of heat from a lower
temperature level to a higher temperature level using the work provided to the machine. A refrigerator is a machine that extracts a
certain quantity of heat from a lower temperature level and transfers it to a higher temperature level also using work provided to the
machine.


Solar-Assisted Heat Pumps

497


According to the second law of thermodynamics, heat cannot flow from a lower to a higher temperature without the expenditure
of energy. This law for the reversible cycle can be expressed by the following equation:
X Qi
¼0
Ti

½3aŠ

Taking into account eqn [3a] and referring to the nomenclature used before, the following can be written:
Q1 Q2
−
¼0
T1 T2

½3bŠ

Referring to eqn [1], eqn [3b] can be rewritten in the following way:
Q1 Q 1 þ W
−
¼0
T2
T1

½4Š

Q1 Q 1 W
−
−
¼0

T1 T2 T2

½5Š

After rearranging the terms, it is as follows:

Because T2 > T1, the difference between the first and the second term of eqn [5] is larger than zero, and the work input must be
sufficiently big to make the sum to be equal to 0. Thus, there is a minimum work that is required for the machine, that is, the heat
pump, to operate reversibly. For the vapor compression heat pumps considered, the work supplied is in the form of mechanical
energy provided by electrical energy to drive the heat pump compressor. There are other types of heat pumps, where the energy
required to drive the system is supplied in the form of heat, and these are sorption heat pumps (not analyzed in this chapter).

3.15.2.2

Thermodynamic Cycles

In principle, to achieve the reversible cycle in a heat pump, a condensable fluid must realize a reversed Carnot cycle. This allows heat
to be input and output at a constant temperature by means of boiling and condensation. This meets the requirement that all heat
transfer to and from the system must be reversible. The Carnot cycle [1–4] for a heat pump in a temperature–entropy diagram is
shown in Figure 2 on the left and the main components of a heat pump based on the Carnot cycle are shown on the right.
The following processes presented and numbered in Figure 2 take place:
1–2 Isentropic compression. Two-phase liquid–gas of the working fluid from the evaporator flows into the compressor and is
compressed to the required level of pressure and temperature.
2–3 Constant pressure and temperature heat rejection – condensation. Vapor of the working fluid at high pressure and temperature
flows from the compressor to the condenser. At constant pressure and temperature, the working fluid condenses giving up
the heat (latent heat – condensation heat) to the sink, for example, space heating medium that provides the heat required by
the space heating demand.
3–4 Isentropic expansion. Liquid of the working fluid from the condenser flows into the ideal expansion device (at high pressure
and temperature) and is expanded to the required level of pressure and temperature to close the reversible cycle.
4–1 Constant pressure heat absorption (evaporation). The two-phase liquid–gas working fluid at low pressure and temperature

flows from the expansion valve to the evaporator. At constant pressure and temperature, the working fluid evaporates, because
of the existing temperature gradient between working fluid and the low-temperature heat source. Then the process repeats.

T

Q2
3

T2

3

Q2

4

2

2
Win

T1

Condenser

Expansion
valve

Compressor


1
Q1
Δs

4

Evaporator

s
Q1

Figure 2 The Carnot cycle for a heat pump and schematic concept of a vapor compression heat pump.

1

Win


498

Applications

The ideal reversed Carnot cycle is realized between heat source and heat sink that have constant temperature, that is, T1 = const and
T2 = const. The transition process between these two states (lines) is adiabatic, and can be written in the following way:
dQ
¼ dS ¼ 0
T

½6Š


Equation [6] is true for only reversible adiabatic processes and then S = const. Analyzing Figure 2 it can be seen that two processes
are adiabatic: 1–2 compression and 3–4 expansion. Because of the equity of entropy differences, S4 – S1 = S3 – S2, the efficiency of the
Carnot cycle for a heat engine (eqn [2]) is as follows:
η¼

T1 ðS4 −S1 Þ
T1
jW j
jQ1 j
¼1−
¼1−
¼1−
Q2
Q2
T2 ðS3 −S2 Þ
T2

½7aŠ

The same can be written for the efficiency (i.e., COP) of a refrigerator and a heat pump operating in reversible adiabatic processes
1–2 and 3–4 (Figure 2), and according to eqns [2b] and [2c] respectively, it is as follows:
COPr ¼

T1 ΔS
T1
jQ1 j
¼
¼
Q2 −jQ1 j ðT2 −T1 ÞΔS ðT2 −T1 Þ


½7bŠ

T2 ΔS
T2
jQ2 j
¼
¼
ðT2 −T1 ÞΔS ðT2 −T1 Þ
jQ2 j −Q1

½7cŠ

COPhp ¼

The work W that is necessary to drive the machine is equal to the area limited by lines 1–2, 2–3, 3–4, and 4–1, which
represent all ongoing processes, as shown in Figure 2. The Carnot cycle has the maximum possible COP between any two
temperature levels T2 and T1 and is used as an ideal cycle for comparison with practical ones. From eqns [7b] and [7c], it is
clear that the COP is the biggest when the temperature difference between T2 and T1 is the smallest (the work W – the
matched area – is the smallest). It means that the operation of a heat pump and refrigerator is most efficient when the
temperature of a heat source is as close as possible to the temperature of a heat sink. This is very important for the practical
selection of heat sources and heat sinks.
The main components of a heat pump specified below are responsible for processes presented and numbered in Figure 2:
1–2 Compressor.

2–3 Condenser.

3–4 Theoretically it could be a turbine; however, a Carnot cycle is impractical for power generation, therefore it is expansion valve

(and in practice a throttle).
4–1 Evaporator.

To determine the COP of an ideal Carnot cycle heat pump, eqn [7c] can be used. For example, if outside air at 0 °C (273 K) is a heat
source and the air inside the house at 20 °C (293 K) is a heat sink, then the COP of the heat pump considered is equal to 14.65. In
practice, it is not possible and a real machine would have a COP much smaller, of about 3–4, as is described in the following
paragraphs. In reality, a Carnot cycle cannot easily be used for a heat pump (or refrigeration) cycle and the reversed Rankine,
Perkins/Elmer, or Linde cycles are used, all being vapor compression cycles [3, 4]. Figure 3 presents the basic vapor compression
cycle for heat pumping in a temperature–entropy diagram.
The following processes presented and numbered in Figure 3 take place:
1′–2′ Isentropic compression. Saturated dry vapor (of the working fluid) at low pressure flows from the evaporator into the
compressor and is compressed to the required level of pressure and temperature in the superheating vapor region.

T
2�
x=0
T2

T1

3

4

x=1
Q2

4�

2

1


1�

Q1
s
Figure 3 The vapor compression cycle for a heat pump presented in a temperature–entropy diagram.


Solar-Assisted Heat Pumps

499

2′–2 Isobaric heat rejection. Superheated vapor of the working fluid at high pressure and temperature flows from the compressor to

the condenser. The heat rejection takes place due to temperature gradient and represents desuperheating of the vapor.

2–3 Isobaric and isothermal heat rejection – condensation. Desuperheated vapor (of the working fluid) at high pressure flows

through the condenser. At constant pressure and temperature, vapor condenses giving up the heat to the sink (e.g., space
heating medium). Saturated liquid leaves the condenser. There is no pressure drop in the condenser and connecting piping,
3–4′ Isenthalpic expansion. Saturated liquid (of the working fluid) from the condenser flows into the throttle (at high pressure and
temperature) and is expanded to the required low pressure and temperature. Expansion losses mean that the cycle is not
reversible (not isentropic expansion process).
4′–1′ Isobaric and isothermal evaporation. Two-phase mixture of the working fluid at low pressure and temperature from the
throttle flows through the evaporator. At constant pressure and temperature, the working fluid evaporates, taking heat from
the low-temperature heat source. There is no pressure drop in the evaporator and connecting piping.
Comparing the evaporation process of this cycle and the Carnot cycle, it is evident that now the end of the evaporation process is
exactly on the saturation line (point 1′). It must be underlined that in reality stopping evaporation at just the right dryness fraction,
as it is in the Carnot cycle, is very difficult. In addition, real compressors might be damaged because of compressing two-phase
mixtures (saturated liquid and vapor) to a saturated vapor state.
In practice, to ensure that the evaporation is really completed and there is only one-phase fluid (saturated vapor) at the

compressor inlet, a small amount of superheat (typically a few degrees) is transferred to the vapor just after leaving the evaporator.
This is shown in Figure 4 in the T–S diagram where state 1″ (beginning of ‘suction’) is slightly superheated.
Using a throttle in the place of an ideal expansion device means that the expansion process is not reversible. There is saturated
liquid leaving the condenser which flows into the throttle. However, the reduction in pressure in the throttle causes some of the
liquid to boil and a two-phase mixture is formed. As a result, the temperature of the working fluid drops. The whole process is
isenthalpic, because the enthalpy of the stream of fluid is the same before and after the throttle. However, two phase mixture leaving
the throttle contains liquid with lower enthalpy than the fluid before throttle and vapor with higher enthalpy than the fluid before
throttle. This means that the working fluid enters the evaporator at point 4′ not 4 and the entropy of expansion process in the
throttle increases and the process is irreversible.
Entropy difference at the heat source is not equal to that of the sink, the former being bigger than the latter. At such cycle, the
work input W to the device must be bigger than that for the Carnot cycle (the rectangular area matched in Figure 2). As a
consequence, the vapor compression COP is reduced in comparison with a Carnot cycle working between the same temperature
limits, and can be written as follows:
COPhp ¼

T2 ΔS2 − 3
T2
ΔS2 − 3
jQ2 j
¼
¼
¼ COPhpCar ηhpCar < COPhpCar
ðT2 −T1 ÞΔS4 ′ − 1
ðT2 −T1 Þ ΔS4 ′ − 1
jQ2 j −Q1

½8Š

As mentioned above, to ensure complete evaporation, the quantity of heat absorbed from the heat source is increased (evaporation
ends at point 1″ not at 1 for the Carnot cycle, or 1′ for the Linde cycle), as shown in Figure 4. This effect of slightly overheating the

dry vapor before it enters the compressor results in an increase in the refrigeration effect, which is positive for a refrigerator, but not
for a heat pump.
The vapor compression cycle is very often presented as the ln(p)–h diagram. An example of such diagram for the vapor
compression cycle is shown in Figure 5.
In Figure 5, most of the same state points are presented in the ln(p)–h diagram as before (see Figure 4). However, there is an extra
point 2‴, which represents the state of the desuperheated vapor after the compression, which in reality is not an isentropic process.
The points in Figure 5 represent the following processes:

T
2�

T2

3

Q2

2

1�

Carnot
T1

4 4�

1

1�


Q1
s
Figure 4 T–S diagram of the ideal vapor compression with the beginning of ‘suction’ slightly superheated.


500

Applications

h2 �3�

np

Q2

3

4 4�

2

1 1�

2�

2�

1�

Q1

h1�4�

h1� 2 �

h

Figure 5 The vapor compression cycle presented in a ln(p)–h diagram.

1″–2‴ Nonisentropic compression in the superheating vapor region, 1″–2″ isentropic compression, dotted line.

2‴–2 Isobaric heat rejection representing the desuperheating of the vapor.

2–3 Isobaric and isothermal heat rejection – condensation.

3–4′ Isenthalpic expansion.

4′–1′ Isobaric and isothermal evaporation.

Analyzing the ln(p)–h diagram, it can be seen that

q1 = h1″ – h4′ = h1″4′ – isobaric evaporation (w = 0);
q2 = h3 – h2‴ = –h2‴3 – isobaric condensation (w = 0);
win = h1″ – h2‴ = –h1″2‴ – adiabatic compression process (q = 0).
It is very convenient to use the ln(p)–h diagram to determine the COP of a heat pump, because the COPs are simple ratios of length
(enthalpies) in the ln(p)–h diagram, as presented in Figure 5. A COP of a refrigerator and a heat pump (referring to eqns [2b] and
[2c], respectively) can be expressed as a function of enthalpy. The enthalpies of different refrigerants as functions of pressure and
temperature can be found in the literature (e.g., in tables and charts [5]). Thus, the COP of a refrigerator and a heat pump, using the
diagram in Figure 5, can be expressed as follows:
h1 ″ 4 ″
h1 ″ 2 ‴


½9aŠ

h2 ‴ 3
h1 ″ 2 ‴

½9bŠ

COPr ¼ −
COPhp ¼

The COP of a heat pump described by eqn [9b] is higher than the real one and there are many reasons for this. First, in practice,
liquid leaving a condenser is often subcooled to ensure that only the liquid phase of the working fluid enters the throttle. Therefore,
apart from the so-called ‘superheat horn region’ (see Figures 3 and 4), there is a ‘subcooled region’ outside the saturated line (point
3 is moved into the liquid region, to the left). This makes the input work W to drive the machine bigger than for a Carnot cycle, as
indicated by the increased area enclosed by the cycle in a T–S diagram, and as a consequence the COP is smaller. It must be also
underlined that in real heat exchangers there is a temperature difference between evaporating working fluid and a heat source
(e.g., about 10 °C) and between condensing working fluid and a sink (e.g., 5 °C). It makes the temperature difference in a vapor
compression heat pump between a heat source and a heat sink bigger than that in an ideal Carnot cycle heat pump, and makes the
COP smaller. Another reason for the reduction in COP is the nonideal compression process. Compressors operate with a certain
isentropic efficiency and in addition electric motors driving a compressor operate with an efficiency less than one. Therefore, in
practice, COP drops to 3–4.
It should be mentioned that nowadays some compressor heat pumps can offer apart from the heating an additional
function by being able to cool buildings. There are two main different methods for cooling with a heat pump. In the first
method, a heat pump can operate in a reversible cycle, so in summer it operates like a refrigerator (the fundamentals have
been described in the beginning of this chapter). In the second method, the so-called direct ‘natural cooling’ takes place. It
means that a heat pump is switched off, except for the control unit and the circulation pumps. The ‘natural cooling’ is applied
in ground and underground water heat pumps when the brine of underground heat exchangers or the groundwater system
absorbs the heat from the heating circuit (e.g., floor heating system) in a building and transfers it to the heat source medium
in the ground. This method can also be considered as thermal regeneration of the ground heat source (cooled during heating

season and heated during summer).


Solar-Assisted Heat Pumps

501

One of the most important issues of heat pumps is requirements for refrigerants. Refrigerants should have low pressures
(all subcritical) at heat exchange temperatures. They must be nontoxic, not flammable, and nonpolluting, that is, environmentally
safe. Considering their influence on the environment, the following indexes are usually used:
• ODP – ozone depletion potential;
• GWP – global warming potential;
• TEWI – total equivalent warming impact (for the whole system).
It is also required that the refrigerant’s density is high for low-volume flow rate and thus a smaller compressor, piping diameter, and
heat exchangers can be used. In the past, the most popular refrigerant for domestic and light commercial heat pumps was R22.
Nowadays, it has been replaced mainly by R 134a (a hydrofluorocarbon (HFC)). However, there are a number of new (and old)
refrigerants such as other HFCs, propane and butane mixtures, ammonia, or carbon dioxide. Another important issue is the
selection of a heat source suitable for heating demands and the type of heat pump as described below.

3.15.2.3

Classification of Heat Pumps

As has been described in previous paragraphs, the general principle of a heat pump operation is to extract heat from a low-temperature
heat source and to transfer it to a heat sink at a higher temperature. The useful energy output must be significantly greater than
additional energy required to drive a heat pump to achieve a real reduction in primary energy use. Heat pumps can use renewable
energy or waste heat as a heat source. Energy extracted from these sources is converted into useful heat in the low-temperature range.
This low-temperature heat can be applied with high efficiency, for example, for space heating and domestic hot water (DHW) [6].
A number of different classifications of heat pumps can be made; the main one is according to the form of energy that is used to
drive them. In this case, the following types are considered:

• mechanically driven, that is, compressor heat pumps;
• thermally driven, that is, sorption heat pumps.
In this chapter, the mechanically driven compressor heat pumps are considered. Among compressor heat pumps, we can list
• electrical heat pumps in which the compressor is driven by electricity (e.g., by an electric motor);
• heat pumps in which the compressor is driven by an internal combustion engine; these heat pumps operate with natural gas,
diesel, or biofuel (rapeseed oil).
The heat pumps considered can be classified according to the type of the end user as follows:
• domestic heat pumps for small- and large-scale application used for space heating and DHW;
• domestic heat pumps for small- and large-scale application used for space heating, cooling, and DHW; these heat pumps can
work depending on the season of the year (winter and summer) in heating or cooling mode, which means that a heat pump
function can be reversed;
• light commercial heat pumps for different heating purposes (applied in offices, schools, hotels, hospitals, public buildings);
• heat pumps with dehumidification function (for swimming pools; for drying of vegetables, fruits, plants, etc.);
• large commercial and industrial heat pumps (for town districts and towns, industrial applications). These mainly use waste heat
sources. The sources of waste heat can be sewage, exhaust gases, or technological waste heat in air, water, and vapor.
Classification can also be made according to the heat pump’s construction. There are two fundamental options:
• compact or unitary heat pump – all the components are in one compact unit, there is a heat exchanger between a heat pump
evaporator and a heat source, and between a heat pump condenser and a heat sink. Usually the heat source is outside the building
heated; however, when the waste heat is used it could be also inside a building;
• split heat pumps – the components are split (divided) usually into two units: one of them is located in a separate room or outside
a building. Usually an evaporator or a condenser can be located directly at the heat source or at the heat sink, respectively.
It means that evaporation and/or condensation take part directly at a heat source or at a heat sink. When the evaporator is located
directly at a heat source, such systems are also called direct expansion.
The other way of classification can be done in accordance with the role of a heat pump in a heating system, and it is as follows:
• monovalent heat pump – it operates throughout the year in monovalent mode providing all heating requirements by itself;
• bivalent or hybrid heat pump – to provide all heating requirements it has to operate in conjunction with another heating device
or system.
Heat pumps can also be classified according to the type of a heat source medium, which can be generally air, water,
and brine (antifreeze mixture). They can use waste or renewable energy heat sources. Renewable energy sources are



502

Applications

mainly used for domestic and light commercial (including swimming pools) applications. The renewable energy sources are
as follows:
•
•
•
•
•

ambient air;
ground (soil);
geothermal water;
surface water;
solar radiation.

Another way of heat pump classification is according to the type of heat sink medium, Water and air are the two main heating
mediums depending on the type of heating system. Sometimes, heat pumps are named (classified) according to the type of heat
source and heat sink medium in the following way:
•
•
•
•
•
•
•
•


air–air;
air–water;
water–air;
water–water;
ground–water, or brine–water;
ground–air, or brine–air;
solar–water (SAHP-w);
solar–air (SAHP-a).

However, it can be mentioned that the last four types are also called in general ground source heat pumps and solar (solar-assisted)
heat pumps (the last two) respectively. Selection of a heat source suitable for a heat sink and for a given heating demand is very
important and influences considerably the heat pump operation and in turn its performance (COP).

3.15.2.4

Renewable Heat Sources

Generally, when heat pumps are considered for effective use, the following characteristic features of a heat source are taken into
account [6]:
•
•
•
•
•
•
•
•

good availability;

coherency between the source and the user;
high thermal capacity;
constant in time and of relatively high temperature;
natural energy equilibrium of the source (environment) and its physical characteristics are not affected by heat extraction;
high purity (to avoid corrosion);
no pollution, damage to environment, and other ecological issues;
low cost of heat extraction.

Renewable energy heat sources are described below and their characteristic features that have been just mentioned are analyzed briefly.
The simplified idea of utilizing different renewable energy sources for a heat pump at a single-family house is presented in Figure 6.
Considering availability of heat sources ambient air is the best. Unfortunately, it is not coherent with space heating demand. When
space heating demand is the highest, the air temperature is the lowest. The temperature of ambient air is not constant in time and it can
fluctuate very rapidly. Ambient air (heat source) heat pumps operate usually with a COP of about 3. When the temperature of ambient
air is just above 0 °C, then the problem with ice formation on the evaporator surface can occur. This surface ice effect causes worse heat
transfer conditions and together with the low ambient air temperature they result in low thermal performance of a heat pump, that is,
low COP. Nowadays, these possible problems are overcome by applying regular automatic defrosting (the high-temperature working
fluid from the compressor, instead of flowing into the condenser, is recirculated into the evaporator). Modern air–water heat pumps
can operate down to an outside temperature of − 15 °C. However, then their COP is much lower than 3 and they no longer can meet
the heating demand completely. Air heat pumps operate in bivalent (dual) mode using an auxiliary heater in times of low outside
temperature. Usually, this bivalent mode is monoenergetic mode, that is, electric heater is used. The heating water is preheated by
the heat pump, to the selected flow temperature, and then an electric heating cartridge is used to provide auxiliary peak heat. It should
be mentioned that an air–water heat exchanger must circulate a large volume of air (e.g., 3000–4000 m3 h−1). As a consequence, they
can generate a lot of noise. Air heat pumps can be constructed and installed as compact heat pumps and in this case air supply duct is
used to supply outside air to the heat pump evaporator inside the building and exhaust air duct is used to take off the air used. In this
case, a problem with noise generation is very likely. Therefore, the split construction is used very often, and the intake and outtake of air
and evaporator are located outside the building, and other heat pump elements inside the building.
There are also heat pumps utilizing exhausted air as a heat source. They are applied mainly in buildings with very low energy
demand, for example, in passive buildings, where they are coupled with domestic ventilation system with heat recuperation. The



Solar-Assisted Heat Pumps

503

Solar radiation

Ambient air

Heat pump

Ground

Well

Underground water

Figure 6 Idea of utilizing different renewable energy sources for a heat pump at a single-family house.

part of heat of air extracted from a building from the ventilation system that cannot be recovered in a direct way (in a recuperative
heat exchanger) is used as a heat source for an integral exhaust air–water heat pump. This type of a heat pump can also be used in
other so-called ‘low-energy buildings’, but it cannot operate only in monovalent mode and an electrical supplementary heater is
usually used to provide the additional auxiliary heating energy required to meet the total heat demand.
The main advantages of air heat pumps, apart from very good availability of a heat source, are the following:
• heat extraction from outside air does not disturb the natural energy equilibrium of a heat source (environment) and its physical
characteristics;
• high purity;
• there is no pollution because of extraction and exhaustion of air cooled, and no damage to the environment;
• there are very low costs of heat extraction, the lowest among all types of renewable energy source heat pumps, and the method of
heat extraction is the simplest one.
However, the main disadvantage, that is, no coherency between a heat source and a heat sink (space heating demand), causes the

operation of a heat pump with low COP, and as a consequence a higher amount of electricity is used to drive the air heat pump than
that required, for example, for the ground source one.
The ground constitutes a suitable heat source for a heat pump considering small-scale low-temperature heating systems [7]. The
seasonal temperature fluctuations are much smaller than those of the ambient air even at small depths. Ground at small depth is
under the influence of solar radiation, rain, melt snow (water), and other environmental factors. At a depth of 2 m, underground
temperatures range from 2 to 13 °C during the heating season (in most European countries). With the increase of ground depth
down to 10 m the temperature becomes nearly constant throughout the whole year and is approximately equal to the annual mean
outside (ambient) air temperature [8]. With the further increase of depth, the ground temperature increases but relatively very
slowly. The influence of geothermal energy is weak even at a depth of 50 or 100 m [9]. The energy flowing from deeper layers
upward represents only 0.063–0.1 W m−2.
Energy stored in a natural way in the ground medium is extracted by means of large-area horizontal plastic pipework buried
underground or longer length plastic tubes set into drilled vertical ducts or bore holes [7, 10]. Heat exchangers are installed in an
area next to the building. In horizontal heat exchanger systems, the plastic pipes (e.g., polyurethane (PE)) are buried underground at
a depth of between 1.2 and 1.5 m. Individual pipe runs (loops) are usually limited to a length of 100 m. If the length of pipe runs is
too big, then there is pressure drop in piping and as a consequence the required pump capacity would be too great. All loops have to
be of the same length, because the pressure drop must be the same to achieve identical flow conditions in every pipe run. As a
consequence all heat exchanger loops can extract heat evenly from the ground medium. Usually, heat extracted from the ground is
transferred via water and antifreeze mixture (brine) to an evaporator of a heat pump. Because of the liquid used, these heat pumps


504

Applications

are called brine/water heat pump (brine in the primary and water in the secondary (heating) circuit). In the ground outside the
building or just directly inside the building, there is a header duct with a brine distribution that consists of two brine distributors,
flow and return, where the pipe ends come together. Return and flow headers are installed slightly higher than piping (venting). It is
important that each loop is able to be shut off separately. A circulation pump circulates the brine through the pipes that extract the
heat stored underground. The heat extraction from the ground varies from 10 W m−2 (in the case of underground areas of dry sandy
soil) to 35 W m−2 (in the case of ground with groundwater ways).

In central European countries, the freezing zone in the soil is 1 m and in some regions 1.5 m deep. More to the north
(high-latitude countries) the freezing depth is bigger. This makes it preferable to use vertical ground heat exchangers rather than
horizontal ones [6]. Heat is extracted from the ground by vertical ground heat exchangers that are mainly in the form of U tubes
(so-called ground ducts or probes), double U tubes (so-called duplex probes), or concentric tubes (popular in the past, not at
present). These tubes are coupled with a heat pump evaporator similar to the horizontal heat exchanger system. Water and
antifreeze mixture (brine) circulate in pipes and the pipe ends come together in a header duct with a brine distribution (with
flow and return distributors) located outside or inside the building. If a header duct is located outside the building, it is
recommended to insulate the underground collector pipes (flow and return). The location of heat exchanger tubes can vary, and
they can be set into the ground in rectangular, hexagonal, or cylindrical configuration. The distance between vertical tubes depends
on the thermal and hydrological characteristics of the ground. Usually, this distance can be at least about 4 m for single U tubes and
5 m for double U tubes. Possible heat flux extraction for vertical ground probes depends on the type of the ground [7] and it can be
at a level of 20 W m−1 of a tube length for dry sediment with relatively low thermal conductivity (lower than 1.5 W m−1 K−1) or even
70 W m−1 (of a tube length) for solid rock with high thermal conductivity (higher than 3 W m−1 K−1). In the case of ground formed
by gravel and sand with waterways, the specific heat extraction for double U tube can be about 55–65 W m−1, and for moist clay and
loam about 40 W m−1. Under standard hydrological conditions, an average possible heat flux extraction (so-called probe capacity)
of 50 W m−1 probe length can be expected (according to Reference [11]).
The most important advantage of a ground heat pump, especially with vertical heat exchangers, is the fact that heating of a
building can be accomplished in a monovalent mode of operation. Of course, at the end of the heating season, due to heat
extraction the ground medium is cooled down. However, because of the natural heat and mass processes, that is, influence of
ambient environment from the top, undisturbed ground surrounding the sides, and geothermal energy from the bottom,
the ground can come back to its initial thermal balance. This way, in an annual cycle, the natural thermal state of the ground
source cannot be disturbed. When bigger heat demand is expected, then it is very good to apply artificial charging of the soil, for
example, by solar energy [12] (as described in Section 3.15.4), to ensure return to initial undisturbed thermal conditions. Of course,
when heating requirements of a building are rather small, then it is quite sufficient to apply ground heat pumps, using the ground as
a natural heat source without artificial charging of the ground medium.
Ground source heat pumps can use the earth in direct or indirect mode. In direct mode, heat exchangers buried horizontally or
set vertically in the ground constitute the evaporator coils (evaporation process takes place just in the ground medium). When
ground heat exchangers constitute separated closed loops or pipe runs and are coupled with a heat pump evaporator located inside a
building, then a heat pump uses the ground in indirect mode. It means that heat exchange between the ground body and working
fluid in the heat pump evaporator (refrigerant) is accomplished through an additional medium. In this case, a heat carrier fluid

(brine), as an additional medium, circulates in the ground heat exchangers.
The possibility of utilizing ground as a heat source varies from place to place and depends mainly on local geology and size of
system. To design a system it is good to make a general review of a place proposed for location and positioning of the ground system
and to estimate heat demand and its distribution in time. Having made this preliminary study on ground source potential and
heating needs, the selection of the type and configuration of the ground heat pump can be made. In most ground heat pump
systems for small-scale applications, only very rough analysis is needed. This analysis becomes more complicated when the size of
the system, that is, heat demand, is bigger. It is very useful to know the geological and hydrogeological characteristics of the ground.
As a result, the thermal behavior of a ground system can be predicted more easily. Geological and hydrological conditions are not so
important in the case of small-scale ground-coupled heat pump systems. When a high level of underground water (waterways)
exists, then it is a great advantage for a ground system. Heat from the surrounding undisturbed ground body at a higher temperature
is transferred more quickly to the ducts and probes of a ground system. In the case of small systems, very often there is not much area
available to install the system; therefore an analysis of disposition and arrangements of the system components, especially tubes of
ground heat exchangers, is needed.
As mentioned before, vertical ground heat exchanger systems are more recommended than horizontal coils, especially in
high-latitude countries. The main reasons are as follows:
• Heat extraction conditions are better deeper in the ground than just approximately 1 m below the earth surface, especially when
freezing phenomena develop. Deeper in the ground the temperature is higher (than just below the earth surface), and a heat
pump uses the heat source of the higher temperature and operates in better thermal conditions. As a consequence, the COP is
higher (it can be at a level of 4–4.5), and hence a smaller amount of electricity is used to drive the heat pump.
• Problem with ice formation on the evaporator surface (of a heat pump) does not occur; there is no need for defrosting. The
heating mode can be more predictable and stable.
• The operation of a heat pump can be accomplished in a monovalent mode, as the heat pump is the only heating device providing
all heating requirements.


Solar-Assisted Heat Pumps

505

• If a part of heat exchanger tubes or one of them does not operate in an effective way or it is just out of order, this situation does not

require to stop operation of the whole system (vertical tubes of heat exchangers are connected in parallel), as the other part of the
system can overtake all heating requirements.
• They do not need a large area of ground surface.
• No excavation is needed (excavation can cause devastation of a big land (garden) area).
• They do not affect the plants growing and other gardening.
• Maintenance of the system, including makeover, is more simple and convenient for the user.
It should be added that nowadays drilling methods used to insert vertical tubes into the ground are well developed and prices are
being continuously decreased. Currently, the costs of borehole drilling including pipe inserting and working fluid infilling can lie
between 30 and 50 € m−1. Depending on the technology used for given ground conditions and advancement of equipment applied,
the drilling is usually done at a depth of 50–100 m, sometimes to 150 or even 200 m [13].
Nowadays, the majority of heat pumps that are installed are ground source heat pumps, mainly because the ground is available
everywhere (however, the available ground surface can be different), and temperature and thermal capacity are predictable and they
are relatively constant in time of a month or longer. This makes ground source heat pumps very attractive for end users because they
can operate in monovalent mode and their efficiency is relatively high. Usually COP for ground source heat pumps with vertical
heat exchangers is about 4–4.5 and for heat pumps with horizontal heat exchangers it is 3–4. To summarize, to design and select a
ground source heat pump, there is a need to estimate the expected heating load and its nature, including demand level and its
distribution in time, and expected temperature range, which can be different for space heating and DHW, and for any other use.
Apart from the specific heating requirements, a heat pump has to be selected according to its operation and thermal characteristics,
ground thermal conditions, and expected efficiency of the ground source, including type and size of ground heat exchangers and
their location and configuration.
When analyzing geothermal water as a heat source, one should consider the deep geothermal energy resources (more than
1000 m deep) (are not dealt with in this chapter) and shallow geothermal energy sources (from a few meters to a few hundred
meters). Ground water at considerable depth (aquifers) and shallow geothermal water are very interesting solutions for direct
heating or for heating via a heat pump (depending on water temperature and heat flux that are carried by them). However, there are
two main problems: high drilling and operating costs and very often poor water quality, for example, corrosive salt content.
Shallow geothermal water can be used with good efficiency if geohydrological conditions, including purity of water, are good
enough to be used for heating purposes. Most groundwater at depths of about 10 m is available at constant and relatively high
temperature. This constant temperature of between 7 and 12 °C (in central Europe) is maintained at a depth of several meters below
the earth surface. Groundwater is extracted through a supply well via a suction pump and transported to the evaporator of the
water/water heat pump. Subsequently, the cooled down water is returned via a return well. This must be done in such a way as to not

disturb water flows, and especially depletion of groundwater layers should be avoided. A special regulation should be imposed to
keep the necessary distance between a supply and return well. Usually it cannot be smaller than 5 m (this is the minimum distance
for small-scale applications). The purity of water is also very important. Therefore usually there is an intermediate circuit heat
exchanger that separates the water circuit of the supply and return wells from the evaporator circuit. This type of heat pump system is
suggested for small- to medium-scale applications. However, groundwater heat sources can also be used at a larger scale, for
example, when aquifers, groundwater reservoirs, are used as a heat source or for storage.
Surface water constitutes a heat source that cannot be used in every location and is mostly used in medium-scale applications.
The temperature of surface water is strongly dependent on ambient temperature fluctuations; therefore, its possible application for
heating purposes is limited in some climatic conditions. The average river temperature in the coldest months can be about 0 °C, and
the average lake temperature, which depends mainly on the depth of the lake, is even lower [14]. For these heat sources, detailed
analysis of the given location must be carried out. In winter, surface ice formation can occur very easily. However, with the increase
of depth, the dependence on ambient air temperature decreases. Therefore, some systems of this type have two intakes of water. One
of them is located at the surface of water and this is for the use in spring, summer, and autumn. The other intake is located deep in
the water, much below the freezing depth and this is for the use in winter.
Surface water as a heat source for a heat pump is not used very often nowadays. This heat source has too many disadvantages to
be popular. The availability is not very good, because it is limited to few locations. Coherency between the source and the user is
rather poor, because it is strictly connected with ambient air temperature. When space heating demand is high, the surface water
temperature is low, because the ambient air temperature is very low. The thermal capacity is quite big but the heat extraction can
disturb the natural state of the source, especially in winter when freezing of water can develop very quickly, and the heat cannot be
extracted. In addition, the costs of heat extraction are usually significant. Surface water (heat source) heat pumps have COP at
different levels in time, the averaged COP for the whole year’s operation can be about 3 to 3.5.
Solar energy is available during the daytime, but at different levels depending on geographical latitude, time of the day, and
season of the year. ‘Solar energy supply’ and ‘solar energy heating demand’ are quite opposite in time. When energy is needed for
space heating purposes, the time and peak values of heat demand are quite opposite to the time and peak values of available solar
radiation. However, a heat pump is a device that makes the application of solar energy for heating quite effective, as is considered in
detail in the following section.
Nowadays, the most popular heat sources for residential and commercial heat pumps are ambient air and ground. Solar energy
as a heat source is not used so often. However, recently ground source heat pumps have become increasingly popular and solar



506

Applications

collectors have been used more often not only for DHW, but also for space heating (combi systems) or even cooling (combi plus
systems). It means that heat pumps and solar collectors ‘come together’ quite often in modern heating and cooling systems to
provide all thermal energy needs. In this way, they constitute combined solar thermal and heat pump systems, termed SAHP
systems. SAHP systems are planned and installed with different levels of complexity, using different components, and are applied
for different heating and even cooling needs.

3.15.3 Solar-Assisted Heat Pump System
3.15.3.1

Classification, Configurations, and Functions

The intermittent character of solar radiation and the strong dependence of the irradiation level on time of the day and season of the
year make it necessary to store the solar energy and necessitate combining solar heating systems with other heating device to fulfill
all heating requirements. One such device is a heat pump. Combining solar thermal systems with heat pumps is especially popular
in modern low-energy buildings, because such heating systems can supply all the heating demand and no auxiliary conventional
heating is needed. A heat pump replaces the burner or other conventional device or system used for space heating. In summer when
solar irradiation is high, a solar thermal system can provide nearly all heating demand for DHW use. Combining a heat pump with
solar thermal into one heating system can contribute significantly to the reduction of fossil fuel usage and as a result in reduction in
the running costs of a heating system. The other very important advantage is that such combination allows in most cases that the
system operates in a monovalent mode, that is, not requiring any auxiliary heating device and as a result reduces fossil energy
consumption. The reduction of fossil fuel consumption is not only because of substituting the fuel (from fossil to renewable) but
also due to better operation conditions of a heat pump, which results in higher COP and in consequence in reduced electricity
demand (to drive the heat pump).
Nowadays, there are a variety of concepts of solar thermal and heat pump system combinations but there are not enough proven
results to state definitely which one is the best. Classification of SAHP systems is usually made because of the configuration of the
system; mainly it is connected with the role of solar collectors and a heat pump for heating and the mutual interaction between

them. Traditional classification of SAHP systems is according to the heat source of a heat pump and its connection to the solar
thermal part of the system, mainly solar collectors [15, 16]. Based on that, the following categories can be classified: parallel, series,
and dual-source SAHP systems. In a parallel system, a heat pump uses a heat source other than collected solar energy and solar
energy is supplied directly to the heating system or is stored in a storage tank. The solar collectors and the heat pump are generally
independent and can operate in parallel mode. In a series SAHP system, solar energy directly from solar collectors or storage is used
as a heat source for a heat pump. The heat pump is dependent on the solar collector operation. In a dual-source SAHP system, a heat
pump can use two heat sources. Collected and stored solar energy is one of the heat sources and the other renewable heat source is
usually ground or air. The heat pump is only partly dependent on solar collector operation.
There are three main functions of heating systems under consideration. They are as follows: water heating (DHW), space heating,
and space cooling. The SAHP system can provide all these three energy needs or two or only one of them. Nowadays, the
monofunction systems (only for hot water or only for space heating) are used very rarely. Usually multifunction systems are
used. They supply heat for hot water preparation (DHW) and for space heating. Some such systems can also be used for cooling.
The cooling mode is mainly achieved via a heat pump or directly (see Section 3.15.2.2).
In the last decades of the past century, solar thermal systems were used only for hot water preparation. Therefore in that time,
SAHP systems were offered and installed with ‘traditional’ solar thermal part only for water heating for DHW system and with a heat
pump for space heating [15–19]. The concept of such system represents the idea of parallel SAHP system [17], where the solar
thermal part for DHW was independent of the heat pump that was used only for space heating and it was based on a renewable heat
source other than solar heat source. In those days, because of the technology available and applied, the heat pumps were used
mainly for space heating due to the requirement to operate in relatively stable conditions (space heating demand is needed
continuously in winter and it is relatively constant during a day and night or even over a longer time span). DHW use is
characterized by high fluctuation of heating demand during a day. Without appropriate storage and automatic control (daily
automatic timetable of hot water use), there would be sudden changes (increase) in a short time in the temperature level and the
quantity of heat required for heating needs. With the increase of the temperature difference between the heat source and heat sink
(DHW system), and the increase of heat needed, the COP of a heat pump decreases. Nowadays, construction of heat pumps and
storage has been improved, automatic control is well developed, and heat pumps are used for all heating functions with high
efficiency. At present, there are many different configurations of SAHP systems. The main components and loops of these systems
can operate alternatively in series or in parallel mode; some have also extra function called active regeneration of the heat source
(mainly ground and groundwater).
Usually, a heat source is used in an indirect way to supply heat to the evaporator of a heat pump. It means that the heat source
and the evaporator are integrated through an intermediate heat exchanger loop. This loop transfers the solar energy collected or

another renewable energy extracted from the environment to the refrigerant loop of a heat pump. Sometimes, more intermediate
loops and elements are used, for example, when a working fluid circulates between solar collectors and a heat exchanger in a storage
tank, and then there is another heat exchanger in the storage connected with a heat pump evaporator. Of course, every heat
exchanger has a certain effectiveness, that influences (reduces) the final share of solar energy used. Sometimes, the source of heat for
the heat pump is used in a direct way. It means that a solar collector and a heat pump evaporator are integrated into one unit. The


Solar-Assisted Heat Pumps

507

evaporator is also the solar collector where direct evaporation of refrigerant takes place. The heat output of a heat pump condenser
can also be used in a direct way when the condenser is located directly in the space to be heated or in an indirect way through an
intermediate heat exchanger loop.
Liquid (water or antifreeze mixture) and air solar collectors can be used in SAHP systems. The heat pump can also be of different
type, for example, water–water, water–air, air–air, air–water, or brine–water. In the case of air collectors, a storage tank is not always
used, because they can provide heat directly to a heat pump evaporator. When liquid solar collectors are used, they are always
coupled with a storage tank. Storage is a very important component of the heating system and it integrates all the other components
[20]. Storage tanks can contain intermediate heat exchanger loops or loops with direct evaporation or condensation. Improvement
of the thermal performance of storage and reduction in storage volume can be achieved through utilization of not only specific heat
of the storage medium but also the latent heat gained during a phase change process (melting phenomena) of the medium. For this,
the storage tank can be filled with phase change materials (PCMs) that melt at a given temperature [21–23].
In operation, SAHPs can have high, low, or even zero interaction between the heat pump and solar collectors. A heat pump and
solar collectors can be coupled very strongly and solar energy can be even used for active regeneration of a heat source of a heat
pump, as is applied in the case of ground and groundwater heat sources. However, it shall be mentioned that the systematic
classification of SAHP systems is still an open issue. The interest in SAHP systems is growing, as evidenced by many new investments
as well as many new research tasks (e.g., International Energy Agency (IEA) Task 44 ‘Solar and Heat Pump Systems’, and IEA Solar
Heating and Cooling Program) [24].

3.15.3.2


Direct Solar-Assisted Heat Pump Systems

In a direct SAHP system, solar energy is used directly to heat the working fluid of a heat pump, that is, a refrigerant. A solar collector
and a heat pump evaporator constitute one integrated unit. Two types of solar collectors are used: unglazed solar collectors (bare
solar collector) and regular glazed solar collector with one cover. In this system, the working fluid of the heat pump, a refrigerant,
flows through an integrated solar collector-evaporator. Due to solar radiation incident on the integrated unit and the direct effect of
the ambient surroundings, the refrigerant undergoes a phase change from liquid to vapor, so the evaporation process takes place at
the location of the heat source, usually located on the roof of the building. The refrigerant is directly evaporated in an integrated
solar collector-evaporator. Such system is called a direct expansion solar-assisted heat pump system (DX SAHP). This system
represents also a split configuration.
The main advantage of a DX SAHP system is the elimination of the intermediate heat exchanger that is required for standard
SAHP systems with a closed loop of collector working fluid (antifreeze mixture), which makes the construction of the system
simpler. This also improves the thermal performance of the system only if it operates under appropriate weather conditions
[25–27]. A very important issue for DX SAHP system operation is the selection of a suitable refrigerant and it seems to be one of the
main problems for the widespread use of such systems [25]. Another very crucial and critical issue is the sizing of the collector–
evaporator panels. The thermal capacity of solar collectors (also evaporators) collecting solar energy should be matched to the heat
pumping capacity of the compressor. A collector–evaporator temperature can fluctuate rapidly. The flexible operation of the direct
SAHP system can be maintained by using compressor capacity modulation (a variable speed compressor). The refrigerant, especially
during phase change from liquid to vapor during the evaporation process, should operate in relatively stable conditions. However,
in a direct SAHP system, the temperature level can change rapidly in a short time, that is, during a few hours of a day, and over a
longer span of time. There are also huge differences in the temperature of working fluid in winter and summer; for example, in
summer a refrigerant should evaporate at a temperature of 30 °C and in winter at a temperature of −20 °C. Of course, it is possible
to find places (countries) with not so huge differences in temperature throughout the year. The idea of DX SAHP is presented in
Figure 7 on the left side and the T–S diagram representing the ongoing vapor compression cycle on the right side.

Collector–

evaporator



1

T

Compressor

2′
2

2′
3
2

Heat

exchanger–
condenser

4′

Valve

Storage
tank

4′

1


3
Pump

Figure 7 The idea of DX SAHP and T–S diagram for the vapor compression cycle.

s


508

Applications

The following processes presented and numbered in Figure 7 take place:
1′–2′ Nonisentropic compression. Slightly superheated refrigerant vapor (working fluid) at low pressure flows from the solar
collector–evaporator into the mechanical compressor and is compressed to the required level of pressure and temperature.
2′–2 Isobaric heat rejection. Superheated vapor of the working fluid at high pressure and temperature flows from the compressor to
the heat exchanger-condenser. The heat rejection takes place due to temperature gradient and represents the desuperheating of
the vapor.
2–3 Isobaric and isothermal heat rejection – condensation. Desuperheated vapor at high pressure flows through the heat
exchanger–condenser. Vapor condenses giving up the heat to the sink, that is, water flowing to a storage tank (then from
storage to space heating circuit).
3–4′ Isenthalpic expansion. Saturated (or slightly subcooled) refrigerant liquid from the condenser flows into the thermostatic
expansion valve (throttle) and is expanded to low pressure and temperature (nonisentropic expansion process).
4′–1′ Isobaric and isothermal evaporation at the solar collector–evaporator due to solar radiation incident on the solar collector–
evaporator. Saturated liquid/vapor mixture at low pressure and temperature from the thermostatic expansion valve flows
through the evaporator. Under solar radiation the working fluid evaporates and at low pressure flows to the compressor, and
the cycle repeats.
A collector–evaporator is under the influence of the ambient surroundings, that is, solar radiation and ambient temperature. The
energy gain causes an increase in the enthalpy of the refrigerant and it can evaporate. The energy balance of a solar collector–
evaporator under steady-state conditions can be written as follows:

_ ðh1′ − h4′ Þ ¼ F′Ac ½Gs ðταÞ −UL ðTf −Ta ފ
m

½10Š

The term on the left-hand side of eqn [10] represents the energy gained by the working fluid (refrigerant) during evaporation (m is
the mass flow rate, h is the enthalpy of the working fluid according to the state points in Figure 7) and it is equal to the solar energy
gained by the solar collector–evaporator, represented by the term on the right-hand side, that is, the useful energy Qu gained from
solar collectors (F′ is the efficiency of solar collectors, Ac the surface area of solar collectors, Gs the solar irradiation, UL the averaged
loss coefficient). It can be assumed that the working fluid temperature Tf in a collector remains constant at the saturation value at
constant saturation pressure (frictional pressure drop is neglected). Equation [10] can also be used in a quasistationary model of
solar collector–evaporator operation. During a day it is possible to observe that one quasistationary state follows the other. At every
quasistationary state temperature of the ambient air Ta and the working fluid temperature Tf of evaporation in a collecter, and solar
irradiance Gs are stable, but they are different than in the following state.
With the development of new technologies there are some novel ideas to integrate the DX SAHP system with other
innovative technologies. One of them is to replace the solar thermal collector–evaporator by PV/T (photovoltaic/thermal)
collector–evaporator. Some experimental studies on photovoltaic SAHP systems have been performed recently [28]. PV/T
collector–evaporator converts some portion of solar energy into electricity and some into heat. Heat is absorbed by the refrigerant
circulating in a heat pump loop and the refrigerant evaporates directly in a PV/T collector–evaporator. The system also consists of the
standard air evaporator, which is used when there is no or not enough solar radiation available (both evaporators are connected in
parallel). There are also two condensers, air cooled and water cooled (also connected in parallel). The water-cooled condenser is
used for space heating and DHW and the air-cooled condenser is used only for space heating. There is a variable-frequency
compressor and an electronic expansion valve in the system. With changes in the frequency of the compressor, the input power
to drive the compressor also changes. To meet the changes in the operating frequency of the compressor, the expansion valve adjusts
its position automatically. The PV DX SAHP system generates heat and electricity; therefore, it is proposed to use a comprehensive
coefficient of thermal and electrical performance COPp/t, which is defined in the following way [28]:
COPp=t ¼

Qheat þ Wp =ηpower
W


½11Š

In eqn [11], W corresponds to work input of a heat pump compressor and Wp is the output power of the PV panels. The PV output
power is transformed into equivalent thermal power, taking into account the average standard electricity generation efficiency
(ηpower).
The idea of application of direct SAHP systems was presented many years ago [29], and from time to time some researchers call
for the comeback of this technology [25–28]. However, up to now, this technology has not become popular. There are some
advantages of direct SAHP systems, but perhaps disadvantages, with the main one being uncertainty of their operation in unstable
weather conditions, are so strong that they limit significantly the interest in the implementation of these systems in practice.

3.15.3.3

Series Solar-Assisted Heat Pump Systems

A series SAHP system could be just called a solar heat pump, because solar energy is the only heat source used for the heat pump. In
most series SAHP systems, there are two main modes of system operation (it could also have auxiliary heating, and then it can be
considered as a third mode of operation):


Solar-Assisted Heat Pumps

509

• solar direct heating – if the temperature of heat collected or stored (depending on the system configuration) is high enough, then
solar energy is used directly for heating;
• solar indirect heating via a heat pump – if the temperature of heat collected or stored is too low for direct heating, then a heat
pump is used to meet the heating demand, that is, solar energy converted into useful heat is used as a heat source for a heat pump.
It should be mentioned that in the middle- and high-latitude countries, mainly liquid solar collectors are used (flat plate or vacuum
tube) with the antifreeze mixture circulating in a collector loop and a storage tank, with water as a storage medium. A storage tank is

a central component of the heating system. Storage of heat improves the thermal performance of a system and increases its
reliability [20, 30]. The location of a storage tank can differ depending on the system configuration, which influences the mode of
operation. To improve the ability to store heat, PCMs can be used (the storage tank is filled with PCM) and then heat storage takes
place due to both the specific and latent heat of the water and PCM storage medium [21–23]. In low- and middle-latitude countries,
liquid, mainly water, or air solar collectors are used.
In the case of air collectors, they are usually applied for space heating of a building, mainly by passive operation, and they are
usually integrated with the south façade of a building. The outdoor (ambient) air circulates in spaces or channels within the solar
collectors’ south facade. The air collectors can also be used actively when fans are used for forced circulation of the air or when they
(collectors) assist the evaporator of an air–air or air–water heat pump. If the solar air collectors of a solar passive system are used to
supply space heating directly very often in winter even in a warm climate (like the Mediterranean), they do not operate all the time,
because of too low ambient temperature and too low solar irradiation level [31]. However, if these collectors are used to supply
outside air heated by solar radiation as a heat source for a heat pump, then they can be used all year long and there is no need to use
any other heating device. This type of series SAHP systems can operate in a hybrid mode, that is, they are based on passive
(collectors)–active (heat pump) operation at the same time. If a heat pump is used only for space heating, it can be an air–air heat
pump. However, if it is used for a few functions, mainly space heating and cooling and DHW, then the air–water heat pump can be
used, and space heating and cooling can be accomplished, for example, by fan coils.
The idea of series SAHP systems with solar collectors was introduced at the end of the 1970s [18]. An example of a traditional
series SAHP is presented in Figure 8. The numbers given in the figure are for the main components of the system. In the series
SAHP system, the heat carrier working fluid (water or antifreeze mixture) circulates in a solar collector loop and transfers heat
collected by solar collectors to a heat exchanger in the storage tank. There is another heat exchanger in the storage tank that
couples this storage tank with a DHW storage tank and/or with the heating circuit of a building or with a heat pump evaporator.
It means that the DHW system is theoretically independent of the space heating and the DHW storage tank is equipped with an
auxiliary heater. Heating of the building can be accomplished directly from the storage tank or via a heat pump. The heat carrier
fluid, which is usually water, transfers heat from the storage tank to the heat pump evaporator. Heat pump refrigerant takes heat
out of the water, which after being cooled down comes back to the storage tank. Heat from a heat pump condenser is extracted by
the working fluid of a heating circuit to provide heat to the building. Depending on the type of heating system, water or air is used
as the space heating fluid.
To describe the operation of the standard SAHP system presented in Figure 8, the energy balance of the storage in unsteady state
can be represented as


15

11
7

14

7

7

17

1

17

3

23

23
5

4

2

13


8

10
14


12


9

11

16


9


9
21

21
20

Figure 8 Standard series SAHP system. Legend: 1, solar collector; 2, heat exchanger; 3, storage tank; 4, heat pump. 5, space heating circuit; 6, ground
heat exchanger; 7, three-way control valve; 8, temperature sensor; 9, circulating pump. 10, DHW storage tank; 11, safety valve; 12, expansion tank; 13,
solar control; 14, temperature sensor in a collector loop. 15, bleed; 16, nonreturn valve; 17, temperature sensor; 18, temperature sensor in a ground
loop. 19, circulating pump in a ground loop. 20, control; 21, cold water supply; 22, main storage tank; 23, auxiliary heater.



510

Applications
dTs
ðVcρÞ
¼ Qu ðtÞ − Qloss ðtÞ − Qhd ðtÞ − Qhp ðtÞ − QDHWd ðtÞ
dt

½12Š

The term on the left-hand side of eqn [12] expresses the storage capacity of a storage medium in a tank (Vcρ (J K−1)) and the
fluctuation of storage temperature Ts in time caused by (terms on the right-hand side) useful solar energy gains Qu supplied by solar
collectors, which is reduced because of the heat losses from the storage Qloss (with high-quality thermal insulation of the tank, the
losses can be neglected) and the heat used directly for space heating purposes Qhd and for DHW heating demand QDHWd (heat
supplied to a DHW tank) and because of heat supplied to a heat pump evaporator Qhp. The storage temperature Ts in eqn [12] refers
to storage with full mixing of storage medium (water) or can be treated as the averaged value if there is stratification effect in the
storage tank.
The main modes of operation of the series SAHP system under consideration (the heat losses from the storage are neglected,
Qloss = 0) are as follows:
• Solar DHW heating: Heat stored in the storage tank is transferred to the DHW tank. When temperature TsDHW of stored heat in the
DHW tank is high enough, that is, if TsDHW > TDHWmin, then all DHW load is supplied from the DHW tank, QDHWd = QDHW, and
the auxiliary heater is turned off, so Qaux = 0. When the temperature of water in the DHW tank is too low, TsDHW < TDHWlimit, then
the auxiliary heater is on and supplies the rest (auxiliary) of heat, thus QDHW = QDHWd + Qaux. When the temperature of water in
the DHW tank is below the minimum level, TsDHW < TDHWmin, then the auxiliary heater is on and supplies all DHW demand,
QDHW = Qaux. Depending on the solar radiation level and the difference between the temperature of solar collectors and the main
storage, the solar collector loop can operate or not.
• Solar direct heating of the building, when solar energy is used directly for heating the building: When temperature Ts of stored
heat is high enough, that is, if Ts > Tsmin, then Qhd = Qheat and the heat pump is turned off, so Qhp = 0; depending on solar
irradiation and the difference between the temperature of solar collectors and storage, the solar collector loop can operate (Qu > 0)

or not (Qu = 0).
• Solar indirect heating of the building via the heat pump: When the temperature of collected or stored heat is too low for direct
heating, that is, if Ts ≤ Tsmin, then the heat pump is used and supplies heat to meet space heating demand. Taking into account eqn
[2c], the heat extracted out of the storage tank as a heat source of the heat pump is equal to


1
½13Š
Qhp ðtÞ ¼ Qheat ðtÞ 1 −
COPhp ðtÞ
and heat supplied to a building is extracted from the heat pump condenser, so Qhpcon = Qheat and Qhd = 0, and Qu > 0 or Qu = 0,
depending on whether solar collectors operate or not.
• Solar indirect heating of a building via a heat pump and auxiliary heating: When the heat pump is used to supply heat but it
cannot provide all heating requirements (e.g., there is a limit for the lowest COP value), then the auxiliary heater is used to supply
the rest of the space heating demand, and Qhpcon + Qaux = Qheat.
The COP of a heat pump is given in general form by eqn [2c]. However, the COP is also applied to determine the thermal
performance of the whole SAHP system. The COP of the whole SAHP system is defined as the ratio of the total heat received to the
total work input to the heating system. Thus, in the case of a series SAHP system used only for heating a building, the COP can be
expressed in the following way:
COP ¼

Qhd þ Qhpcon
mc Cp ½ðTsout − Tsin Þ þ ðTconout − Tconin ފ
Qheat
¼
¼
Wtotal
W þ WpumpSd þ WpumpShp þ Wheat
Wtotal


½14Š

The numerator of eqn [14] represents the quantity of heating energy at the heat sink, that is, Qheat, that is supplied by the solar
system directly Qhd (Tsout is the supply temperature at the outlet of the storage and Tsin is the return temperature at the inlet of the
storage tank) and by a heat pump from the heat pump condenser Qhpcon to the heating circuit (water or air of specific heat Cp and
mass flow rate mc; Tconout and Tconin are the temperature at the outlet and inlet of the condenser, respectively). The denominator
represents the total work input into the SAHP system, that is, not only the work input W needed to drive the compressor of a heat
pump, but also the work input WpumpSd for the circulating pumps of the solar heating system (without DHW part) during direct
heating mode, the work input WpumpShp for the circulating pumps of the solar heating system (without DHW part) during heating
via a heat pump, and the work input Wheat for the circulating pumps of water heating circuit in a building or for fans in the case of
the air heating system.
The COP of the series SAHP system under consideration can be also written taking into account the auxiliary heating; then, the
numerator of eqn [14] represents the quantity of all heating energy supplied to the building Qheattotal, that is, by the solar system
directly Qhd, by the heat pump from the heat pump condenser Qhpcon, and by the auxiliary heater Qaux. The denominator represents
the total work input including the work input Waux for auxiliary electric heater and eqn [14] takes the following form:
COP ¼

Qhd þ Qhpcon þ Qaux
Qheattotal
¼
W þ WpumpSd þ WpumpShp þ Wheat þ Waux
Wtotal

½15Š


Solar-Assisted Heat Pumps

511


The COP given by eqns [14] and [15] should be applied only for the space heating system of the SAHP system considered. This is
due to the fact that a heat pump is coupled directly with a solar thermal system only for space heating needs. The DHW heating
system constitutes in some way an independent solar thermal system with its own auxiliary heater. The DHW system operates in
parallel to the SAHP space heating system. Therefore, even if there is a common storage tank for both systems (in result there is some
interaction between the two systems), in the case under consideration it is better not to include operation of the DHW solar heating
system in the determination of the COP of the series SAHP system.
In one of the very simple forms of a series SAHP system, the solar energy collected by the solar collectors is used only as a heat
source for the heat pump and there is no other mode of operation, so there is no direct supply of solar energy (collected by solar
collectors) to the heating system [32]. In such system, there are two storage tanks: a low-temperature storage tank and a
high-temperature storage tank. The low-temperature storage tank is located between a solar collector loop and a heat pump
evaporator and the high-temperature storage tank between a heat pump condenser and a space heating circuit in a building. This
idea was also checked experimentally [33]. A heat pump evaporator can be situated directly in a low-temperature storage tank and a
heat pump condenser directly in a high-temperature storage tank. However, it is also possible to use heat exchangers in low- and
high-temperature storage tanks to be coupled with a heat pump evaporator and condenser, respectively. Of course, an auxiliary peak
heater can be also included in the system, for example, in a high-temperature storage tank.
There are different modifications of the series SAHP system. One of them is realized through introduction of long-term storage
separate from short-term storage. However, such system is no longer a typical series SAHP system. It should be mentioned that in the
past hot water preparation (DHW) used to be an independent function (from space heating) and DHW was provided by solar
energy and auxiliary heater (usually electrical) in parallel to space heating, which was accomplished by the series SAHP system.
Heating of a building and heating of DHW were then independent. Nowadays, hot water preparation is one of the standard
functions of the SAHP system. Usually, storage (with stratification) is a central component of the heating system and space heating
and DHW are supplied by a combined heating system. The automatic control system manages the heat supply to DHW and space
heating or eventually to cooling or air conditioning system.

3.15.3.4

Parallel Solar-Assisted Heat Pump

A parallel SAHP system consists of a solar thermal part and a heat pump that uses a heat source other than solar energy [17]. Solar
liquid (water or antifreeze mixture) collectors or solar air collectors can be used. In a solar heating system based on liquid solar

collectors, solar energy can be used directly for heating purposes or through a storage tank, and an auxiliary heater can also be
applied. A heat pump uses usually the ambient air or ground as an independent heat source. If solar air collectors are used, they are
applied mainly for passive heating of a building, but because they cannot usually fulfill the space heating requirements in cold days
(even in a warm climate) the active heating of the building is realized through air-to-air or air-to-water heat pump [31].
As it has been mentioned, in the past a solar heating system (solar collectors and storage) was responsible only for hot water
heating (DHW) and a heat pump for space heating. Both systems used to operate without interaction. A standard parallel SAHP
system is presented in Figure 9. The legend to this figure is the same as for Figure 8.
This system consists of conventional solar thermal part with solar liquid collectors (water or antifreeze mixture) in a closed solar
collector loop and a storage tank. (If air collectors are used, they are integrated into the building façade and the solar collector loop is
open.) There are also heat exchangers in a storage tank for DHW and for space heating. There is another heat exchanger in the storage
tank that couples this storage with a DHW storage tank, in a similar way as a series SAHP system presented in Figure 9. The DHW
15

11
7

14

7

17

1

10

3

23
23

4

2

13

5
8

10
14

12

9

11

16

9

9
21

21
20

Figure 9 Standard parallel SAHP system. See Figure 8 caption for legends.



512

Applications

system is theoretically independent of space heating; however, some interaction exists because of a common main storage tank. The
other main component of the parallel SAHP is a conventional heat pump, which can be one of the following types: air–air,
air–water, brine (water)–water, brine (water)–air. Solar energy is given priority to meet heating requirements. There is also an
auxiliary heater for space heating. The main modes of the system operation are as follows:
• Solar DHW heating: Heat stored in the main storage tank is transferred to the DHW tank (description of this mode of operation is
the same as for series SAHP system presented in Figure 8).
• Solar space heating: When temperature Ts of stored heat is high enough, that is, if Ts > Tsmin, then Qhd = Qheat and at that time a
heat pump is off, Qhp = 0; depending on solar irradiation and the difference between the temperature of solar collectors and
storage, the solar collector loop can operate (Qu > 0) or not (Qu = 0).
• Heat pump heating: If the temperature of collected or stored heat is too low to meet heating requirements, that is, if Ts ≤ Tsmin, the
heat pump operates using heat source other than solar energy and heat supplied to the building is extracted from the heat pump
condenser, so Qhpcon = Qheat and Qhd = 0; depending on solar conditions and the difference between the temperature of solar
collectors and storage, the solar collector loop can operate (Qu > 0) and heat can be stored in a storage tank or not (Qu = 0).
• Heat pump and auxiliary heating: The rule of this mode of operation is the same as for the series heat pump shown in Figure 8,
and the only difference is in the heat source for the heat pump, which is different from solar energy. Heating load is provided by a
heat pump and an auxiliary heater, and Qhpcon + Qaux = Qheat.
In the parallel SAHP system, the total available energy of the system is the sum of energy gained from two different systems: solar
thermal and heat pump systems. Thermal description of the two systems considered is the same as they operate as stand-alone
systems. However, indirectly they influence each other, because when one heat source is used, the other is not. It means that heat is
extracted from these heat sources not so quickly and during breaks in operation if they have the ability, like ground for example,
they can recover slightly or retain the heat accumulated for later use. To describe solar thermal system operation including heating of
DHW, the standard energy balance of the storage can be expressed in a similar way as eqn [12] in the following form:
ðVcρÞ

dTs

¼ Qu ðtÞ− Qloss ðtÞ− Qhd ðtÞ− QDHWd ðtÞ
dt

½16Š

The term on the left-hand side of eqn [16] expresses (as before) storage capacity and fluctuation of storage temperature Ts in time
and that on the right-hand side gives useful solar energy Qu supplied by solar collectors, heat losses from the storage Qloss, heat
provided directly to the heating circuit Qhd to meet the space heating load, and heat provided to the DHW storage tank QDHWd.
There is no heat supplied to a heat pump evaporator Qhp.
A heat pump operates in a standard way, as conventional heat pump; therefore, the COP of the heat pump can be expressed by
the standard equation [2c]. However, it is also possible to determine the COP of the whole parallel SAHP system. Then the total
work input into the system must be included, so apart from the work input W to drive the compressor of the heat pump, it is
necessary to add the work input Wheat for the circulating pump of a water heating circuit or for the fans of an air heating system in
the building, and the work input Wpump for the circulating pumps of the solar heating system. Total heat Qheat supplied to the space
heating system (DHW system is not taken into account for COP determination, because theoretically it is an independent heating
system) during the longer time of its operation is the sum of heat extracted directly Qhd from the ‘solar’ storage tank and heat Qhpcon
supplied by the heat pump condenser and heat Qaux provided by the auxiliary heater, which can be written in the following way:
X
t

Qheat ¼

X
t

Qhd þ

X
t


Qhpcon þ∑ Qaux
t

Thus, the COP of the parallel SAHP system under consideration (see Figure 9) can be expressed in a similar way as the COP for the
series SAHP system (see Figure 8) with the difference that there is a new work input Whp, which represents the work needed for
circulation of the working fluid at the heat source of a heat pump to extract heat out of this source. In the case of a heat pump using
ambient air as the heat source, it is usually equal to zero. Referring to eqn [15], the COP of the parallel SAHP system considered
above can be written as
COP ¼

Qhd þ Qhpcon þ Qaux
Qheattotal
¼
W þ WpumpSd þ Wheat þ Waux þ Whp
Wtotal

½17Š

There are a variety of heat sources and heat sinks for a heat pump and a solar system can be based on air or liquid collectors
supplying heat to air or water heating system in the building. All these varieties can be used in parallel systems; however, some of
them are more popular than others. A parallel SAHP system can provide heat to the heating system where water or air is the heat
carrier fluid. The solar collector loop can supply heat to the water storage tank. The heat stored can be supplied to water-to-water
heat exchanger located in the water heating circuit, or to water-to-air heat exchanger located in heated (conditioned) space. The air
solar collectors can also be used; they usually operate in a passive way and supply heat directly to the space. Heat pumps use
renewable heat sources other than solar energy. An ambient air (heat source) heat pump can be used to supply heat to indoor air
(a heat sink). In such system, ‘solar’ water-to-air heat exchanger and air-to-air heat pump individually provide heat for space heating
in a building. It is also possible to use a ground source heat pump, which is especially popular in high-latitude countries. In such
system, mostly ‘solar’ water-to-water heat exchanger and ground (brine)-to-water heat pump provide heat for space heating through



Solar-Assisted Heat Pumps

513

heating circuit (floor or wall heating) in a building. An auxiliary heater and automatic control system are also included in the
system.
The operating strategy for a parallel SAHP is to give priority to the solar thermal part, then to the heat pump, and eventually, as
the final alternative, to the electric heater as the peak source. However, in the past, all components used to operate separately, one
after the other. Nowadays, there are systems that through automatic control make it possible to supply heat at the same time from a
solar thermal system and from a ‘nonsolar’ heat pump, and even from an auxiliary heater, to storage or to a heating system directly.
If a storage tank is used for heating of a building and DHW, then because of sanitary reasons (Legionella bacteria) it is necessary to
have periodically (e.g., once per week) the temperature of water for the DHW system above a certain level (usually 55 °C is the
limit). To ensure this temperature level, sometimes all three heating components operate (solar collectors, heat pumps, electric
heater). The heating circuit in a building can use water or air as the heat carrier fluid. In this modified parallel SAHP system, the
following modes of operation are possible:
• Solar heating only: If temperature Ts of stored heat is high enough, that is, if Ts > Tsmin, then Qhd = Qheat + QDHW, and the heat
pump does not operate; depending on solar and ambient conditions, the solar collector loop can operate and useful heat Qu from
solar collectors can be transferred to the storage tank.
• Solar heating and heat pump heating in parallel: If the temperature of collected or stored heat is too low to meet total heat load,
Ts ≤ Tsmin, but still this temperature is high enough (above a given temperature limit), Ts > Tslimit, to supply some heat Qsol = Qhd to
the heating system, the solar system operates providing part of the heating demands. At the same time, a heat pump operates
using a heat source other than solar energy, and provides the rest of the heat required Qhpcon, therefore
Qhd + Qhpcon = Qheat + QDHW.
• Heating via the heat pump. If the temperature of collected or stored heat is too low to meet heating requirements even partly, that
is, if Ts ≤ Tslimit, the heat pump operates using a heat source other than solar energy and provides all heating requirements
Qhpcon = Qheat + QDHW; depending on solar conditions and the temperature difference between solar collectors and storage, the
useful heat from solar collectors Qu can be collected and stored.
• Heating via heat pump and auxiliary heating: If the temperature of heat collected by solar collectors or heat stored in the storage
tank is too low to meet heating requirements even partly, that is, if Ts ≤ Tslimit, the heat pump operates using a heat source other
than solar energy and supplies Qhpcon to the heating system; however, if the COP of the heat pump drops below the limit

determined by the control system, then auxiliary heater is used to provide the rest of heat Qaux to meet all heating requirements
Qhpcon + Qaux = Qheat + QDHW; depending on solar conditions and the temperature difference between solar collectors and storage,
the useful heat from solar collectors Qu can be collected and stored.
• Solar heating, heat pump heating, and auxiliary heating: This is when the peak load must be met; if it is possible, there is heat
provided by solar collectors Qsol = Qhd and the heat pump operates providing more heat Qhpcon, and because it is not enough to
meet all heating requirements Qhd + Qhpcon < Qheat + QDHW, the auxiliary heater is turned on and it supplies the rest of the heat
required Qaux, therefore, Qu + Qnotsol + Qaux = Qheat.
Total heat supplied to the heating system can be expressed by the same equation [16] as in the case of the standard parallel SAHP
system. However, now the heat supplied for DHW needs should also be taken into account because a heat pump also accomplishes
this function. Therefore, the input work (electrical energy) of the auxiliary heater for DHW should also be included as well as the
other work input WDHW associated with this function, for example, to drive circulating pumps of DHW circulation loop and of a
regular piping network. The averaged energy performance of the parallel SAHP system for space heating and DHW, that is, the
averaged COP of the whole system, can be expressed in a similar way as for the parallel SAHP system including heat provided to the
DHW and the work input associated with this function. The COP takes the following form:
COP ¼

Qhd þ Qhpcon þ Qhaux þ QauxDHW
Qheattotal
¼
W þ WpumpSd þ Wheat þ Whaux þ Whp þ WDHW þ WauxDHW
Wtotal

½18aŠ

Assuming that the solar thermal and ‘nonsolar’ heat pump supply heat to the same heating circuit in a building with the same heat
carrier fluid and referring to eqn [14], now eqn [18] of the averaged COP of the parallel SAHP system considered takes the form:
COP ¼

Qhd þ Qhpcon þ Qaux
mc Cp ½ðTsout − Tsin Þ þ ðTconout − Tconin ފ þ Qhaux þ QhdDHW þ QhpconDHW þ QauxDHW

Qheattotal
¼
¼
Wtotal
W þ WpumpSd þ Wheat þ Whaux þ Whp þ WDHW þ WauxDHW
Wtotal
½18bŠ

The formulation of Qhd depends on the type of solar thermal system that is used. The work input symbols are the same as in eqn
[14]. If other circulation pumps or fans are used, their work should be also included in eqns [18a] and [18b].
Nowadays, one of most typical variations of the parallel SAHP system is realized through integration of all the main components
in a water storage tank with stratification. Solar collectors through a working fluid (water or antifreeze mixture) circulating in a
closed loop supply useful heat Qu to a storage tank. The heat exchanger of the solar collector loop is usually located at the lower
part of the storage tank. A heat pump uses a heat source other than solar energy and supplies heat Qhpcon extracted from this
source also to the storage tank. In some systems, it is also possible to supply heat directly to the heating system in a building


514

Applications

(not via storage) [13]. Usually the heat exchanger that links the heat pump condenser and store is located in the upper part of the
storage tank, above the solar collector heat exchanger. Sometimes, a heat pump condenser can be put directly into a storage tank. If the
storage tank is also for DHW, the inlet of cold water is located at the bottom. At the top of the tank the outlet of hot water for DHW is
installed, to extract the heat QhDHW for DHW. There is another heat exchanger in the tank, below the DHW outlet, that connects the
store with the heating circuit for space heating, usually low temperature, for example, floor heating circuit. The heat Qheat needed for
heating a building is extracted through this heat exchanger. Very often an auxiliary heater, usually an electric one, as the peak source is
also integrated into the storage tank at the top. If necessary, when the temperature of the storage tank, even at the top, is too low to
meet the heating requirements, the electric heater is turned on and it supplies auxiliary heat Qaux to the storage tank.
Most of the modern parallel SAHP systems contain a storage tank, which is a main core component of the system that integrates

all the other components. In such a configuration of the system, even if the solar thermal system and the heat pump do not have
direct contact, through the common storage tank they interact with each other. There are positive effects of this interaction, because
the solar thermal part and heat pump are complementary to each other. This makes the operation of the whole heating system very
reliable. A parallel SAHP system can provide all heating loads and there is no need to install and use any other heating device, extra
burner, or boiler. This is very convenient for the user. However, due to the fact that a heat pump and solar collectors deliver heat to
the same storage medium, sometimes operation of one part of the system, usually the heat pump, limits the operation of the other
one, that is, the solar collectors. For example, in winter in high-latitude countries, very rarely is the temperature of the working fluid
of the solar collectors higher than the temperature of the heat carrier fluid extracting heat from the heat pump condenser. As a
consequence, the heat pump operates most of the time and limits the utilization of solar energy. In addition, sometimes installers
(through the automatic control system) set too high a limit for the temperature of the working fluid of the solar collectors to
circulate. If this value is too high (e.g., above 40 °C) the working liquid does not circulate and supply heat to the storage tank in
winter and on cloudy days, which limits significantly the operation of the solar thermal part of the system.
Figure 10 presents a scheme of modern parallel SAHP system and Figure 11 presents the main components of the system in the
indoor ‘boiler room’ (heat pump in the middle, combined buffer storage at the right side). This system has been operating recently.
In Figure 10 the symbols T with numbers in indexes represent main temperature sensors linked to the control. This system contains
the following main components: solar collectors – flat plate with antifreeze mixture as a working fluid; a ground source heat pump
with U-shaped vertical heat exchangers and antifreeze mixture as a working fluid; combined buffer storage with water as a storage
medium; storage tank for DHW with peak electric heater. There is a low-temperature floor heating circuit in the building. The
combined buffer storage consists of a big tank and a small one inside the big one. The solar collector loop is closed and heat is
transferred through a heat exchanger to the big storage tank. The big tank is also supplied by a ground source heat pump. The small
tank inside the big one is used as a buffer for the DHW. There is an inlet of cold water at the bottom and an outlet of warm water at
the top. The outlet is connected to the DHW storage tank, which can be also supplied directly from the heat pump and if necessary

T1
Tg

Heat
pump
Cold water


Combined
buffer
storage

T3

T2
Cold water
Mixing valve
T6

T4

DHW storage tank
Figure 10 An example of the parallel SAHP system operating since 2010.

Electric heater

T7

Floor heating

Tin

T5


Solar-Assisted Heat Pumps

515


Figure 11 Components of the parallel SAHP system shown in Figure 10.

the electric heater can be on. Heating of the building is accomplished by the heat stored in the big tank of combined buffer storage.
Referring to eqn [12] written for the averaged storage temperature Ts, the energy balance of the combined buffer storage in unsteady
state of the system considered can be written in the following way:
ðVcρÞ

dTs
¼ Qu ðtÞ þ QhpBS ðtÞ− Qloss ðtÞ −QhDHWBS ðtÞ− Qh ðtÞ
dt

½19Š

In eqn [19], there is heat QhpBS supplied by the heat pump to the combined buffer storage. It can be the total heat provided by the heat
pump, QhpBS = Qhp, or only part QhpBS = xQhp of that heat, if there is some quantity of heat QhpDHW =(1 – x)Qhp provided by the heat
pump to the DHW storage tank. In a given time, there can also be extraction of some water heated from the small tank to feed the DHW
storage tank, QhDHWBS = mC(TDHWBS – Tin). Thus the energy balance of the DHW storage tank can be written in the following way:
ðVcρÞ

À
Á
dTDHW
¼ QhDHWBS ðtÞ þ Qhp ðtÞ− QhpBS ðtÞ þ QauxE ðtÞ− Qloss ðtÞ− QhDHW ðtÞ
dt

½20Š

There is no inlet of cold water to the DHW storage but only outlet for direct use. Some cold water is provided to the three-way valve
out of the storage tank to protect the user against too high water temperature from the DHW system.

The parallel SAHP system presented in Figure 10 supplies heat for building heating and for a DHW system. The operation of the
system is based on solar collectors and a ground source heat pump that supply heat to one or both the storage tanks. The main
modes of operation of the system considered can be described in a general way as follows:
• Solar heating only: storage tanks: combined buffer and DHW storage are supplied by solar collectors; the heat pump is off and no
auxiliary energy is used.
• Solar heating and peak auxiliary heating for DHW: Storage tanks are supplied by only solar collectors; the heat pump is off, for a
peak load (or to protect against Legionella bacteria) the auxiliary electric heater is on; depending on thermal and environmental
conditions, the useful heat Qu from solar collectors can be transferred to storage tanks.
• Solar heating and heat pump heating in parallel: If the temperature of collected or stored heat is too low to meet total heat load,
for DHW and space heating, the heat pump is switched on and supplies heat to one or two storage tanks; the useful solar energy
can be collected and stored in the combined storage tank, if possible.
• Heating via the heat pump only: When the temperature difference between the outlet of solar collectors loop and the storage (at a
given point) is below the limit value, the solar collectors do not operate, and the heat pump provides all heating requirements
and supplies one or two tanks.
• Heating via the heat pump and auxiliary heating: When there is no available solar energy and the heat pump cannot provide all
the heat for DHW, auxiliary electric heater is on during peak time.
The COP of the system under consideration that is applied for space heating and DHW can be expressed in a general way as follows:
COP ¼

Qhd þ Qhpcon þ QauxDHW
Qheattotal
¼
W þ WpumpSd þ Wheat þ Whp þ WDHW þ WauxDHW
Wtotal

½21Š

The equation above is written with the assumption that total heat requirements are provided by the system considered. The use of
electric heater (WauxDHW) is included in the total work required to accomplish all heating requirements; however, electric heater is



516

Applications

used for only DHW. In the total work WDHW there is also work required to drive circulation loop and pumps in the DHW system. Of
course it is the automatic control system that is responsible for the effective operation of the system [34].
In some parallel SAHP systems, it is possible that a heat pump can supply heat directly to the heating system (usually to the
storage tank as in the system presented in Figure 10), depending on the heating demand and temperature level of working fluid. The
automatic control of the system considered can be organized in a different way and priorities could be given to different heat
sources. The solar thermal collectors and the heat pump are not connected together. They can operate in alternative ways, that is,
each of them at a different time, but they can also operate together, both supplying heat at the same time. The main idea of the
parallel operation is to use two heat sources: solar for solar collectors and the other one (not solar) for the heat pump in a parallel
way. However, as it was presented, there is interaction between operations of the main components of the system even if they are not
coupled together. Perhaps such systems could be called flexible parallel SAHP systems. The modern control systems based on
microprocessor techniques make it possible to apply different operation strategies for different applications and heat demand.

3.15.3.5

Dual-Source Solar-Assisted Heat Pump

In a dual-source SAHP system, there are two heat sources for a heat pump [19, 21, 35]. A heat pump is equipped with two
evaporators or one evaporator but supplied by two heat sources (through two heat exchangers). One heat source is solar energy and
the other source is usually air or ground or other heat source. When air solar collectors are used, then solar energy collected is
transferred directly to the heat pump evaporator. When liquid solar collectors are used, then solar energy is absorbed by solar
collectors and can be sent directly to the heat pump evaporator or can be stored in the form of sensible heat in the storage tank. Then
heat stored can be used as heat source for the heat pump evaporator if the temperature of the storage medium (water) is high
enough. If not, the heat pump can use the other heat source (usually ground or air). Solar or the other renewable heat source is used
depending on which source results in a higher COP of the heat pump. It can be said that the dual-source SAHP system is in some
way a combination of two systems: parallel and series. A standard dual-source SAHP system with liquid (flat plate) solar collectors is

presented in Figure 12.
In a traditional dual-source SAHP system, presented in Figure 12 (numbers represent the main system components and are given
in the legend of Figure 8), the following main modes of system operation can be used:
• Solar DHW heating: Heat stored in the main storage tank is transferred to the DHW tank (description of this mode of operation is
the same as for series SAHP system presented in Figure 8).
• Solar direct heating: When solar energy is used directly for heating, if temperature Ts of stored heat is high enough to supply heat
to the heating system directly, that is, if Ts > Tsmin, then Qhd = Qheat and a heat pump does not operate, Qhp = 0; depending on the
15

11
7

14

7

7

17

1

3

23
23

12



4

2

13

8

10
14

9

11

16

5

9

9
21

21
20

18

19

6

Figure 12 Standard dual-source SAHP system. See Figure 8 caption for legends.


Solar-Assisted Heat Pumps

517

solar radiation level and the difference between the temperature of working fluid in solar collectors and storage medium, the solar
collector loop can operate gaining useful energy (Qu > 0) or not (Qu = 0).
• Solar indirect heating via the heat pump: If the temperature of collected or stored heat is too low for direct heating, Ts ≤ Tsmin, but
if it is higher than the temperature of the other heat source of the heat pump evaporator Ts > Tnotsol, then the heat pump can use
the heat stored to meet heating demand, so Qhpcon = Qheat and Qhd = 0, and Qu > 0 or Qu = 0, depending on whether the solar
collectors operate or not; the heat Qhp extracted out of the storage tank as a heat source for the heat pump can be calculated using
eqn [13].
• Heating via the heat pump using the renewable energy other than solar energy: When the storage temperature Ts is too low for
effective use of the heat stored as a heat source of the heat pump evaporator, and/or the temperature Tnotsol of the other heat
source is higher than the solar one, that is, Ts ≤ Tnotsol, then Qhpnotsolcon = Qheat, and Qhd = 0, Qhp = 0, and Qu > 0 or Qu = 0,
depending on whether the solar collectors operate or not.
• Heat pump and auxiliary heating: The rule of this mode of operation is the same as for the series heat pump shown in Figure 8,
with the only difference being that there are two heat sources for the heat pump: solar or the other renewable heat source is used
depending on which source results in a higher COP of the heat pump; heating load is provided by the heat pump and an auxiliary
heater, so at the heat sink it is always true that Qhpcon + Qaux = Qheat.
If needed, additional modes of operation can be applied. The automatic control system is responsible for the effective operation of
the system. The energy balance of the storage tank of the dual-source SAHP system presented in Figure 12 can be described in the
unsteady state in the same way as in the case of the series SAHP system, that is, by eqn [12]. The heat pump is used only for space
heating; therefore, the COP of the system considered is presented without DHW function and can be written (referring to eqns [15]
and [17]) in the following way:
COP ¼


Qhd þ Qhpcon þ Qaux
Qheattotal
¼
W þ WpumpSd þ WpumpShp þ Wheat þ Waux þ Whp
Wtotal

½22Š

When the heat pump of the dual-source SAHP system also supplies heating energy for DHW, then eqn [22] includes more terms,
similar to eqn [18] or [21].

3.15.3.6

Other Configurations

Studies and experiments in the past showed that the parallel systems were much better than the series systems and slightly better
than dual source [36]. However, at present, it is difficult to state definitely which system is the most effective and has the highest
thermal performance; more results of the operation of different systems are needed and comparative and optimization studies are
necessary. Nowadays, different strategies of operation are possible in one system due to well-developed automatic control systems
and any system can be a mixture of different configurations and operate as a multifunctional system in a flexible way.
Nowadays, most heat pump systems are used for heating of buildings and DHW supply. These systems must include a hot water
heater (an electric heater (booster)). The electric heater provides the auxiliary energy when there is peak demand, usually in cold
winter days. Heat pumps at the small scale, for example, for single-family houses, should be designed to cover about 60–70% of the
maximum designed heat load of a building, but not more, because of high investment costs. It means that a heat pump can cover
nearly most, up to 90–95%, of the annual heat demand of the building. The rest is covered by the auxiliary peak electric heater
included in a storage tank or in the heat pump. This construction of the heat pump makes the SAHP system configuration different
from standard series or parallel systems used in the past. However, these modern heat pumps can also be coupled to a solar system
in a series or parallel way. Therefore, it is difficult to classify them as ‘other’ configuration of the SAHP systems, but the past
definitions of series, parallel, or dual-source SAHP system also do not suit them fully.

Some of the SAHP systems using the ground as a heat source for the heat pump have a function of recharging (regeneration) the
ground (duct, borehole, etc.). An example of such a system is presented in Figure 13.
As shown in Figure 13 (the legend is the same as for Figure 8), solar collectors can be connected to the return pipe from the
evaporator going back to the ground. The antifreeze mixture (based on glycol) circulating into the solar collector loop can be sent to
the ground heat exchanger loops. Some results show that the recharging process results in an increase of the ground temperature by
only a few degrees [13]. However, these few degrees can improve much the operation of the heat pump. When solar collectors and
ground heat exchangers are linked together and the same antifreeze mixture can circulate in both loops, then apart from the
recharging effect it is also possible to use another mode of operation. When the temperature of the working fluid in solar collectors
is higher than the temperature of the working fluid in ground heat exchangers, the antifreeze mixture can circulate first in ground
heat exchangers, which behave as a preliminary heat source. Next the antifreeze mixture circulates through solar collectors gaining
more heat and then it flows into the heat pump evaporator. In this way, the heat extracted from the ground is upgraded by solar
energy and the heat pump operates longer in better thermal conditions (higher temperature of the heat source – ground upgraded
by solar), in consequence with higher COP. This type of SAHP system is a very specific configuration and it represents a hybrid of
parallel and series and dual-source SAHP system.
A solar-assisted ground storage heat pump system with latent heat energy storage [23] is another example of a rather complicated
system in configuration. Its effective operation depends on automatic control systems giving possibilities of different strategies for


518

Applications

15

7

14
22

1


DHW
23

4
5
8
7

12
11

16
9

9
7
21

21
Cold water
18
6

Figure 13 A combined SAHP system: hybrid of parallel–series–dual-source SAHP system. See Figure 8 caption for legends.

operation and it is very difficult to classify this system according to traditional categories. The main components of the system are
the following: solar collectors, latent heat storage with PCM material and a serpentine heat exchanger, U tube ground heat
exchangers, heat pump, and a heating system in the form of fan coils. These components can operate on their own, or in
cooperation with some others or with all of them. A system of valves controlled automatically play a very important role

in managing the energy flow in the system. It is possible that the same fluid can flow through different loops linked together.
The main modes of the system operation are as follows:
• Direct solar heating: The building is heated directly by heat gained by solar collectors and the heat pump is switched off;
depending on solar and ambient conditions, the useful heat Qu from solar collectors can be transferred to a latent heat storage
with PCM material.
• Direct heating from the latent heat storage: The building is heated directly from the latent heat storage; the heat pump is switched
off, and solar energy cannot be stored and used.
• Solar heating via the heat pump and storage of solar energy: Solar energy is collected by solar collectors and then stored in the
form of latent heat in storage; loading and unloading of the latent heat storage tank take place all the time; the heat stored in the
tank becomes the heat source of the heat pump evaporator, the process of supplying heat from the store to the heat pump
evaporator can be considered as the low-temperature cycle; working fluid flows out of the evaporator and comes back to solar
collectors; heat from the heat pump condenser is transferred to the fan coil system in a building, the process of supplying heat
from the condenser of the heat pump to the fan coil system can be considered as the high-temperature cycle.
• Solar heating via the heat pump: Solar heat stored in a latent heat storage tank is used as a heat source of the heat pump; heat from
the store is sent to the heat pump evaporator for the low-temperature loop; heat from the heat pump condenser is transferred to
the fan coil system in a building in a high-temperature loop; solar energy is not collected or stored.
• Heating via the ground source heat pumps: Heat extracted by ground heat exchangers is used as a heat source for the heat pump;
this constitutes the low-temperature circuit; heat from the heat pump condenser is transferred to the fan coil system in a
high-temperature loop.
• Solar heating and ground heating via the heat pump in series: Working fluid circulates first in the ground heat exchangers, which
preheats the working fluid, then the working fluid circulates through the solar collectors gaining more heat, and finally it flows
through the heat pump evaporator and comes back to ground heat exchangers; this is the low-temperature circuit; the heat from
the heat pump condenser is transferred to the fan coil system in the high-temperature circuit.
• No heating; only storing heat from solar collectors in the latent heat storage tank.
• No heating; only storing heat from solar collectors in the ground, regenerating the ground source.


Solar-Assisted Heat Pumps

519


This is a very brief description of SAHP system operation, which shows how complicated the configuration of the system can be and
how complex are the strategies of operation of such a heating system, especially when there are multiple heat sources. The
complication of the system structure makes the investment costs high but at the same time its operation is much more effective.
Apart from traditional functions such as heating or cooling, SAHP systems can be used for dehumidification including drying
purposes. Some of SAHP drying systems have already been manufactured and tested [27, 37, 38]. One of the interesting examples of
such systems [38] proposes the use of air solar collectors and an air heat pump operating in a parallel way. Even if these two heating
systems operate in parallel, it is difficult to classify this system strictly as a parallel one. The main modes of operation of the system
can explain this statement and these modes are as follows:
• Solar heating and ventilation: When solar irradiance is high enough, the ambient air is heated by solar collectors and sent to the
granary to dry grains.
• Solar heating and heat pump heating and ventilation: When solar irradiance is not high enough to meet drying needs, the
ambient air is heated by solar collectors and by an air heat pump, the airflow from both sources is mixed and sent to the granary to
dry grains, the return air (from granary) flows to the heat pump evaporator for heat recovery.
• Heat pump heating and ventilation: When solar irradiance is very low or it is nighttime, the fans of solar collectors are off and the
ambient air is heated by an air heat pump and sent to the granary to dry grains, the return air flows back to the heat pump
evaporator for heat recovery.
• Heat pump dehumidification and ventilation: It is used on rainy and cloudy days. Ambient air is sent first to the heat pump
evaporator to be cooled and dehumidified and then to the heat pump condenser to be heated again.
It is possible to find an example of SAHP systems that are combined with systems that produce electricity. One such system has been
presented in Section 3.15.2.2 and it represents DX SAHP system with PV/T collector–evaporator [28]. The other example can be the
solar-assisted geothermal heat pump coupled with small wind turbine systems for heating agricultural and residential buildings [39].
By analyzing the examples presented above it is evident how many different technologies and modification of standard
categorization of SAHP system are available nowadays and how flexible are the modes of operation of these systems.

3.15.4 Solar-Assisted Heat Pump System with Seasonal Storage
3.15.4.1

Fundamental Options of Seasonal Energy Storage


Energy storage of different forms of thermal energy is a very efficient way of energy conservation. Energy storage can very much
improve the efficiency of the whole thermal process and the rate of useful energy conversion. Heat supply and demand are often at
quite different times.
Energy storage is widespread with many typical everyday applications. In addition to the different types of energy, the basic
difference in energy storage systems is the duration of the storage period. The most common heat stores are used as short-term
storage systems (e.g., hot water boilers in domestic use). The application of long-term storage is currently much less common,
mostly due to economic reasons. However, there is a large amount of surplus heat in summer and surplus cold in winter, which
could be stored for a longer period of time, for example, for a season, to be used when it is really needed.
In the literature [20] we can find some general and important information about requirements regarding energy storage. In combina­
tion with seasonal energy storage, solar energy can make a major contribution to heating of buildings. The incoherency of the solar
radiation peak season and space heating demand creates interest in applying the ground as a seasonal storage medium of solar energy.
A seasonal storage facility can be designed in many different ways. Heat can be stored in the ground (clay, sand), in unfractured
rocks, and in water [7, 40–42]. Four fundamental options for long-term solar thermal energy storage are presented in a schematic
way in Figure 14 and they are mentioned below:
•
•
•
•

water tanks (including water pits, water–gravel, and solar ponds);
rocks (boreholes in rocks, rock caverns, pits);
soil storage (ducts in earth, earth coils);
aquifers.

A water tank contains a mass of water heated by solar collectors and stored in a tank. The water tank can be located on the ground
surface (see Figure 14, top left) or partly embedded in the ground, or fully embedded in the ground (see Figure 14, top right). Water
tanks should have appropriate storage volume, usually a few thousand cubic meters. The water tank construction is usually of
reinforced concrete. It should be insulated partly or fully depending on its position, but at least over the roof area. To ensure water
tightness, the tank can be built with extra steel liners or special high-density concrete material with very low vapor permeability can
be used. Water pits are usually expensive due to the cost of excavation. They need lit construction; however, it is possible to keep

excavation cost low if they are built on sites with soft ground. A water–gravel pit is a pit with a watertight liner and is filled with
gravel–water mixture, which forms the storage medium. This store should have thermal insulation on its top and side walls. The
specific heat capacity of the mixture is lower than water; therefore, for the same amount of heat stored, the volume of the
gravel–water store should be bigger (50%) than that of the water storage tank.