3.14

Solar Cooling and Refrigeration Systems

GG Maidment and A Paurine, London South Bank University, London, UK
© 2012 Elsevier Ltd. All rights reserved.

3.14.1
3.14.2
3.14.3
3.14.4
3.14.4.1
3.14.4.2
3.14.4.3
3.14.4.4
3.14.4.5
3.14.4.6
3.14.4.7
3.14.5
3.14.5.1
5.14.5.2
5.14.5.3
3.14.6
3.14.7
5.14.8
References

Introduction
Solar-Powered Cooling
Need for Solar-Powered Cooling
Solar-Powered Cooling Technologies

Desiccant Cooling System
Solid Desiccant
Liquid Desiccant
Absorption Systems
Adsorption Systems
Ejector Systems
Photovoltaic–Compression Systems
Relative Comparison of Solar Cooling Technologies
Solar Coefficient of Performance
Capital Cost Comparison
Life-Cycle Cost Comparison
Application of Solar Cooling System
Integration with Solar Hot Water and Solar Tthermal Systems for Cost-Effectiveness
Conclusions

481

481

481

482

482

483

483

484


486

489

489

490

490

491

491

492

492

493

493


3.14.1 Introduction
Thermally driven cooling offers a more sustainable and low-energy solution for refrigeration and air-conditioning applications. For
most cooling applications, there is a coincidence between peak solar gain and peak cooling demand. By using solar thermal energy
to drive a cooling cycle, it is possible to produce cooling virtually coincident with the demand for cold, and thus solar-powered
cooling is a potential technology for domestic, commercial, and industrial buildings. The coincidence of cooling with demand is
shown in Figure 1.

This chapter presents an overview of the state-of-the-art of solar cooling. It describes the general theory, the technologies, and
their relative performance and applications. Several competing technologies for solar energy collection for sorption and vapor
compression refrigeration are compared in terms of efficiency, life-cycle cost (LCC), and primary energy basis.

3.14.2 Solar-Powered Cooling
Solar cooling is a technology for converting heat collected from the sun into useful cooling into refrigeration and air-conditioning
applications. Solar thermal energy is collected and used by a thermally driven cooling process, which in turn is normally used to
generate chilled water or conditioned air for use in the building. A typical solar cooling scheme essentially includes three
components. These include the solar collector for harnessing solar energy by converting it into heat or mechanical work, a
refrigeration or air-conditioning plant for producing cooling, and a heat sink for heat rejection. A diagram of the main components
of a solar cooling scheme is shown in Figure 2.

3.14.3 Need for Solar-Powered Cooling
Many buildings require cooling to offset heat gains. In most temperate countries, solar heat gains represent a large proportion of the
overall load to the building. For the United Kingdom, according to Jones [2], for the typical hypothetical office block, the solar gain
contributes between 25% and 40% of the total cooling load. Low-energy and more sustainable cooling systems have been proposed
as an alternative to traditional energy-intensive methods. Interest in solar cooling systems was first shown during the energy crisis of
1970s. These systems used solar thermal energy to energize absorption cycles or light to provide electrical power from photovoltaic
(PV) panels for vapor compression refrigeration cycles. As these systems utilize solar energy, they require minimal grid-derived
electricity, unlike conventional vapor compression equipment. The main advantage of such systems is that they provide virtually
‘free cooling’ that is coincident with the occurrence of solar gains. Also, solar energy is freely available in moderate to hot climates
where more than 50% of the world’s population reside [3].

Comprehensive Renewable Energy, Volume 3

doi:10.1016/B978-0-08-087872-0.00314-0

481



482

Applications

G
[W]

P
[W]
3

1

Congruence
1 Global radiation
2 Cooling load
2

3 Heating load
Surplus of solar in summertime

Jan

Dec

Figure 1 Relationship between incidence of solar radiation and cooling [1].

Solar
collector


Heat or
work

Refrigeration or
cooling plant

Cooling

Building or
process

Heat sink
Figure 2 Scheme of a typical solar cooling.

3.14.4 Solar-Powered Cooling Technologies
There have been numerous projects worldwide relating to the systems used for converting solar thermal energy into useful cooling.
These systems have included the use of flat-plates, evacuated-tube, PV, and concentrating solar collectors in combination with
desiccant cooling, adsorption chillers, absorption chillers, vapor compression systems, and ejector refrigeration system.
The relative efficiency for each of these solar cooling systems is determined based on the efficiency of the collector device (ηcoll)
and the coefficient of performance (COP) of the cooling cycle. The overall system efficiency, also referred to as solar coefficient of
performance (SCOP), is indicated in eqn [2].
Qu
 COP
GT

½1Š

SCOP ¼ ηcoll  COP

½2Š


SCOP ¼
or

where ηcoll is collector efficiency, Qu is the useful heat gained by the collector, and GT is the solar insolation.
The SCOP is used because it gives a simple but combined index of system efficiency as well as capital cost. Therefore, it should be
noted that the collector and heat rejection component size and cost for technologies (described below) are significantly affected by
SCOP.

3.14.4.1

Desiccant Cooling System

In a desiccant cooling system, air can be passed over common solid desiccants such as zeolite or silica gel for dehumidification and
to sensibly cool the air well below ambient temperature conditions in some form of evaporative cooling process. Also, liquid
dessicants such as lithium or calcium chloride have been used for air dehumidification processes. In either case, the desiccant
requires regeneration and this can be achieved using solar thermal energy to dry it out, in a cost-effective, low energy, and
continuously repeating cycle.
A number of desiccant-based solar cooling demonstration projects have been cited in the literature reviews conducted by Lu et al.
[16], Pesaran and Wipke [15], Ahmed et al. [4], and Gommed and Grossman [18]. The main advantage of desiccant-based systems
when combined with solar cooling is that regeneration takes place at relatively low temperatures, a factor suited for use with solar
energy.


Solar Cooling and Refrigeration Systems

3.14.4.2

483


Solid Desiccant

A range of solar air-conditioning systems utilizing solid desiccants in open-cycle configuration have been reported. The original
concept was applied to the Pennington cycle. A system that produces dehumidification and cooling as modeled by Halliday et al.
[12] is shown in Figure 3.
It should be noted that in Figure 3 a rotating desiccant wheel containing silica gel particles is deployed to dehumidify and
provide supplementary sensible cooling of incoming outside air. The desiccant wheel operates such that as a heat exchanger sensibly
increases the temperature of the process air while decreasing its latent heat. The thermal effectiveness (εT,DW) of the desiccant wheel
is given by the following expression, where notation relates to the numbering in Figure 3:
�
�
_ process air Cp ðT8 ′ − T7 Þ þ ðg8 − g7 Þhf g
m
ε T ; DW ¼
½3Š
_ regeneration air ðh4 − h3 Þ
m
where T8′ is the temperature of process air at vapor pressure similar to that of the outside air entering the desiccant wheel and
equivalent to dry bulb temperature of air leaving the desiccant wheel and g and hfg are moisture content and vaporization latent heat
of water, respectively.
The effectiveness of the thermal wheel can also be expressed in the terms of the vaporization latent heat rate of the adsorbed
water and the regeneration input heat rate into the system. The regeneration effectiveness (εR,DW) of the desiccant wheel is given by
the following expression:
εR ; DW ¼

_ process air ðg8 −g7 Þhf g
m
_ regeneration air ðh4 − h3 Þ
m


½4Š

By using a perfectly designed and optimized desiccant wheel with high regeneration effectiveness, it is possible to achieve high COP
with desiccant-based cooling systems.

3.14.4.3

Liquid Desiccant

Liquid-based desiccant systems are described in the literature [5] and these are reported to also clean the air and improve indoor air
quality. There are some liquid desiccant cooling systems that use water–lithium chloride (LiCl–H2O) and water–calcium chloride
(CaCl2–H2O) solutions for sorption purposes. In comparison with solid desiccant cooling systems, the liquid desiccant cooling
systems have higher rate of air dehumidification at the same range of driving temperatures and in that they have high energy storage
capacity when used in the concentrated solutions. Figure 4 shows a distinctive liquid desiccant solar cooling system.
The effectiveness of the liquid desiccant solar cooling can be defined based on the process and regeneration air inlets and outlets.
Assuming there is minimal or no heat loss from the system described in Figure 4, the cooling effectiveness (εC,LD) of the liquid
desiccant system is given by the following expression:
εC ; LD ¼

_ Process Air ðgB −gC Þhfg
m
_ Regeneration Air ðhE −hD Þ
m

½5Š

where g and hfg are moisture content and vaporization latent heat of water, respectively.
Alternatively, the εC,LD can be defined in terms of the temperature of the salt solution in the system:

C

P ð1 À 2 Þ ðT2 −T1 Þ
εC ; LD ¼ 
CP ð3 À 2 Þ ðT3 −T2 Þ

½6Š



where C
P ð1 À 2 Þ ≈ CP ð3 À 2 Þ is the average specific heat capacities of the salt solutions in the two heat exchangers and T is the
temperature of the salt solution.
Evap.
cooler
Exhaust
air

Outside
air

6

7

Desiccant
wheel

4

+


+

Regen.
coil
8

Solar
coil

3

2

1

Return
air

10

Supply
air

Heating
coil
9

Thermal
wheel


+
Solar
coil

+

−

Cooling
coil

Figure 3 Solar desiccant cooling system. Developed from Halliday SP, Beggs CB, and Sleigh PA (2007) The use of solar desiccant cooling in the UK: a
feasibility study. Applied Thermal Engineering 22: 1327–1338 [12].


484

Applications

Supplementary
cooling battery

−

Air passing
through a soak
media

1
Incoming

outside
PROCESS
air

A

B

C Treated supply air

(CaCl2 or LiCl)
dilute salt solution
Supplementary
heater battery

Supplementary
cooling
exchanger

Liquid desiccant
heat exchanger
2
Hot & humid
exhaust air
E

+

Solar collector
(CaCl2 or LiCl)

concentrated salt
solution

Incoming outside
REGENERATION air

D

3

Liquid desiccant
regenerating
heat exchanger

Figure 4 Liquid desiccant solar cooling system.

Therefore, eqn [5] simplifies to the following expression below:
εC ; LD ¼ ðT2 −T1 ÞðT3 −T2 Þ− 1

½7Š

The cooling effectiveness in this case is similar to relative efficiency, and therefore, the SCOP for liquid solar cooling system can be
established as follows:
SCOP ¼ ηcoll εC ; LD

½8Š

where ηcoll is solar collector efficiency.
Liquid desiccants are not popular in the supply airstream due to possible health risks such as Legionnaires’ disease and the risk of
desiccant droplet carryover causing corrosion in metal ducts. A number of liquid desiccant cooling systems have been developed to

eliminate the use of the humidifier and desiccant in the supply airstream. One arrangement utilizes two heat exchangers, where the
cooled airstream of the first indirect evaporative liquid-desiccant air-cooling heat exchanger is used in a second heat exchanger to
cool a clean supply airstream without direct contact with either the cooling water or the liquid desiccants. This is shown in Figure 5.
Although the use of the second heat exchanger eliminates the possibility health risks and ductwork metal corrosion due to
droplet carryover, it does impinge on the overall effectiveness of the cooling system. Since the liquid desiccant flowing from heat
exchanger 2 (HTX 2) via pip. 5 in Figure 5 is minimal, it does not significantly influence the performance of the regenerating
heat exchanger. Therefore, the overall SCOP can be defined in terms of incoming primary outside process air (IPOPA) onto the heat
exchanger 1 (HTX1) and incoming outside regeneration air (IORA) onto the liquid desiccant regenerating heat exchanger using the
following expression:
SCOP ¼ ηcoll

_ IPOPA ðgA − gB Þhfg
εHTX2 m
_ IORA ðhH −hG Þ
m

½9Š

Alternatively, this can be defined in terms of the incoming secondary outside process air (ISOPA) onto HTX 2. Since the air will
not be in direct contact with the liquid desiccant, it will be sensibly cooled only, and therefore, this can be established using the
following expression:
SCOP ¼ ηcoll

_ ISOPA ðhE − hF Þ
m
_ IORA ðhH − hG Þ
m

½10Š


where εHTX2 is effectiveness of the heat exchanger.
While there are relatively few suppliers/installations of solar-based desiccant systems, they have been used extensively in certain
niche applications where the ability to independently control air humidity at low levels provides additional benefits.

3.14.4.4

Absorption Systems

The absorption cycle consists of four basic components operating at two pressure conditions and uses an absorbent–refrigerant
solution such as water lithium bromide (LiBr-H2O) as the working fluid. These components include the evaporator, absorber,
generator, and condenser as shown in Figure 6.


Solar Cooling and Refrigeration Systems

Supplementary
cooling battery

−

Incoming primary
outside PROCESS
air

485

Primary warm
exhaust air

Incoming

secondary outside
PROCESS air

A
E

D
Cooling
water

1
(CaCl2 or LiCl)
concentrated salt
solution

HTX 1
B

F

C
HTX 2

2

Supplementary
heater battery

Treated
supply cool

air

3
H

Hot & humid
exhaust air

+

G
Incoming outside
REGENERATION air

4

Liquid desiccant
regenerating
heat exchanger

Figure 5 Solar air cooling system with indirect evaporative liquid desiccant.

Weak solution
absorbent

High pressure refrigerant
Condenser

Strong solution
absorbent


Generator
QG

QC
High-pressure refrigerant

Ex.valve

Low-pressure refrigerant
Low-pressure refrigerant
Evaporator

Absorber
QA

QE

Figure 6 The basic absorption cycle.

The system works such that high-pressure liquid refrigerant flows from the condenser through an expansion device, which
reduces the pressure, to the evaporator condition. The refrigerant evaporates in the evaporator, cooling the secondary air or water
(cooled medium) and the resulting low-pressure vapor passes to the absorber, where it is absorbed into a strong solution absorbent.
It is necessary to continually reconcentrate the solution to maintain the low evaporation temperature required. The ‘weak’
solution with high percentage of refrigerant is recirculated to the generator where most of the refrigerant is boiled off and the
resulting ‘strong’ solution is passed back to the absorber via an expansion device. The heat input into absorption chillers can be
supplied via an array of solar thermal collectors.
There are many different variants of absorption chillers including half-, single-, double-, and triple-effect systems. There are also
open- and closed-cycle absorption units that utilize liquid/vapor, solid/vapor, and sorbent/refrigerant combinations. The variants of
solar absorption system are detailed in the paper by Syed et al. [6]. The main practical difference between these systems is the driving

temperature required for regeneration of the solution and their relative COPs as well as capital cost and complexity. The most
common cycle is the single-effect LiBr–H2O system that requires heat at around 80 °C will typically achieve a COP of around 0.7 for
a chilled water application.
The COP of a typical absorption chiller can be expressed by assuming an ideal heat engine operating in a Carnot cycle. Therefore,
relation between work and heat for an ideal heat engine is given by the second law of thermodynamics:


486

Applications

Carnot efficiency :

W
Th −Ts
¼
Th
QG

½11Š

where W (kW) is the work, QG (kW) is the heat input rate, Th (K) is the temperature of heat source, and Ts (K) is the temperature of
heat sink.
The relation between work required and refrigeration load for an ideal mechanical refrigeration machine operating as a reverse
Carnot cycle is as follows:
W
Tl −Ts
Carnot efficiency :
¼
½12Š

Tl
Qs
where Qs (kW) is the refrigeration load, Tl (K) is the temperature of refrigeration load, and Ts (K) is the temperature of heat sink
(assumed to be the same as for heat engine).
The COP of the two ideal cycles is given by the following expression:
COP ¼

Qs
Tl ðTh −Ts Þ
¼
T h ðTl −Ts Þ
QG

½13Š

Absorption-type cooling is the most common type of solar cooling technology used in practice. According to a survey of pilot plants
by International Energy Agency [7], small capacity machines in the range of 1–10 kW were mainly solar-energized as they can be
operated with low-grade heat of temperatures below 100 °C. Most of the machines in the survey were air-cooled, and during good
climatic conditions, a theoretical COP in the range of 0.7–0.75 was attained. In a more recent survey [8], it was reported that
absorption technology dominated the other technologies applied in solar cooling industry, that is, it is incorporated in 67% of the
installations. Moreover, it was reported that there has been growth in small-scale systems (< 20 kW), which were not really on the
market a few years ago.
A number of commercial solar-powered absorption products are on the market now, such as that shown in Figure 7 marketed by Solar
Polar. This is reported to provide better integration of the components, and also offers scope to provide combined cooling and heating.
The main issues reported with the application of LiBr absorption chillers were that of mismatch between cooling load and chiller
capacity for commonly used residential applications in Europe [8]. There are also spatial constraints reported in accommodating a
water-cooled chiller in a typical three- to four-bed house. This problem has now been reportedly overcome with a smaller
commercially available air-cooled (rotary) LiBr-H2O chiller of 4.5 kW nominal cooling capacity to replace a split DX unit.
Some pictures of commercially available packaged units are shown in Figure 8. These are designed to be installed outside the
building and piped to high-wall fan coil unit in an adjacent room. There is no external condenser circuit and the hot water cylinder

tee-offs are piped directly to the chiller, hence saving cost and space.

3.14.4.5

Adsorption Systems

The adsorption cycle is similar to absorption cycle, but it uses solid sorbent rather than a liquid. The system uses an adsorption
medium such as zeolite and activated carbon together with a refrigerant to achieve a cooling effect. The natural mineral zeolite has
the property to attract water vapor and to incorporate it in its internal crystal lattice while releasing heat at the same time. The
operation of adsorption heat pumps and refrigerators is therefore based on the ability of porous solids (the adsorbent) to adsorb
vapor (the adsorbate or refrigerant) when at low temperature and to desorb it when heated. The adsorption system is a

Cooler

unit


Solar
collector
tubes

Insulated
ducts

Refrigerator

Figure 7 Solar Polar’s commercial product.


Solar Cooling and Refrigeration Systems


487

Figure 8 Rotartica commercial product.

four-temperature discontinuous cycle that consists of one or several adsorbers connected to heating sources, condenser, and
evaporator. Figure 9 below shows a single adsorbent bed intermittent adsorption cycle.
The adsorption cycle presented in Figure 9 consists of four stage processes that are detailed in Figure 10.
The processes demonstrated in Figure 10 are as follows:
• Process A to B. Heating and pressurization, where the adsorbent is heated while the adsorber is closed and therefore raising the
pressure from the evaporating to the condensing pressure.
• Process B to C. Desorption (generation) and condensation, where the adsorbent temperature continues to increase and hence
inducing desorption of vapor which is then passed into condenser for condensation by rejecting heat to the environment. The
heat necessary to regenerate the adsorbent is a low-grade heat source such as solar energy or waste heat.
• Process C to D. Cooling and depressurization, where the adsorbent releases heat while the adsorber is closed and therefore
decreasing the pressure from the condensing to evaporating the pressure.
• Process D to A. Adsorption and evaporation, where the adsorbent temperature continues to decrease and hence inducing
adsorption of vapor being vaporized in the evaporator. The heat of evaporation is drawn from the space by means cooling
medium, which is usually air or water in the case of air conditioning.

QC
Condenser

Adsorbent
bed

+

Intermediate
refrigerant

receiver

Supplementary
heater battery

Solar
collector

Flow & return
cooling media

Evaporator

−

Air handling unit
cooling battery

Figure 9 Schematic diagram of an intermittent adsorption cycle.


488

Applications

Decreasing isosters
Ln(P)

Liquid–vapor
equilibrium

1

Pc
Condenser

B

Sensible heating
desorption
C

Valve (c)
Isosteric
sensible
heating

Throttling
valve

Pe

Evaporator

2

A

Valve (e)

Isosteric

sensible
cooling
D

Sensible cooling
adsorption
Tevap

Tcond

Tmax

−1/T

Figure 10 Clapeyron diagram for an adsorption system [9].

Although the heating and cooling provided by a single generator is discontinuous, it can be made continuous by operating two or
more generators out of phase. Figure 11 shows a schematic diagram of a typical conventional adsorption system with two adsorbers
(generators).
The heating and cooling water media interchange between the two adsorbent beds and therefore ensuring the adsorption of
vapor is continuous in order to maintain the cooling condition requirements.
Assuming, heat supplied to the systems in Figures 9 and 11 is derived from solar energy, the SCOP can be established based on
the two ideal Carnot cycles and this is given by the following expression:
SCOP ¼ ηColl

Tevap ðTs −TCond Þ
�
�
Ts Tevap −TCond


½14Š

where Ts > Tmax (K) is the temperature of heat source, Tevap (K) is the temperature of refrigeration load, and TCond (K) is the
temperature of heat sink.
There are a few solar-powered adsorption systems operating. According to Solair [8], 11% of the systems installed utilize
adsorption cycles.

QC
Condenser

+ Supplementary
heater battery
Absorbent
bed 1

Absorbent
bed 2
Evaporator

−

Air handling unit
cooling battery

Figure 11 Schematic diagram of a continuous adsorption cycle.

Solar
collector

Flow & return

cooling media


Solar Cooling and Refrigeration Systems

3.14.4.6

489

Ejector Systems

An ejector cooling system (ECS) is a mechanical system utilizing the Rankine cycle and the gas dynamic effect of an ejector
(a thermal compression process). The basic ejector refrigeration cycle is illustrated in Figure 12. The system consists of two loops,
the power loop and the refrigeration loop. In the power loop, low-grade heat, QG, is used in a generator to evaporate high-pressure
liquid refrigerant (process 1–2). The high-pressure vapor generated, known as the primary fluid, flows through the ejector where it
accelerates through the nozzle. The reduction in pressure that occurs induces vapor from the evaporator, known as the secondary
fluid, at point 3. The two fluids mix in the mixing chamber before entering the diffuser section where the flow decelerates and
pressure recovery occurs. The mixed fluid then flows to the condenser where it is condensed rejecting heat, QC to the environment.
A portion of the liquid exiting the condenser at point 5 is then pumped to the boiler for the completion of the power cycle. The
remainder of the liquid is expanded through an expansion device and enters the evaporator of the refrigeration loop at point 6 as a
mixture of liquid and vapor. The refrigerant evaporates in the evaporator producing a refrigeration effect, QE, and the resulting vapor
is then drawn into the ejector at point 3.
The ECS is reported to compete with absorption on the grounds of simplicity, reliability, and low installation cost; however, its
COP is much lower (typically 0.3 for a single-effect system).
The efficiency of the integrated cycle or SCOP for an ECS is established using the following equation:
�
�
h5 −h4
SCOP ¼ ηcoll 
½15Š

h1 −h6
_ g (entrainment ratio), ηcoll is collector efficiency, and h is vaporization latent heat of the refrigerant.
_ e =m
where  ¼ m
The SCOP of 0.25 was reported at evaporating, condensing, and generating temperatures of 8 °C, 32 °C, and 95 °C, respectively.
Furthermore, Mostofizadeh and Bohne [13] reported a COP of 0.3–0.35 at evaporator, condenser, and generator operating
temperatures of 4 °C, 40 °C, and 90 °C.
Huang et al. [10] have reported on the development of a solar-driven ECS of 10.5 kW cooling capacity with a 65 m2 double-glazed
flat-plate solar collector. Wolpert and Riffat [17] theoretically obtained a COP of 0.62 for a PTC-driven steam ejector system of 13 kW
cooling capacity. The experimentally derived COP of their system operating in Loughborough, UK, was only 0.3.

3.14.4.7

Photovoltaic–Compression Systems

These systems combine PV cells with electrically driven vapor compression refrigeration systems. PV cells convert insolation to DC
electricity, which is then inverted into AC to produce shaft power for an electromechanical compressor. According to Best and
Pilatowski [14], these systems have strong market pull mainly due to the lower cost and higher COP of the refrigeration machine. A
typical system is shown in Figure 13.
However, research is needed to improve the efficiency and lower the cost of PV panels. Various materials for PV cells such as
cadmium sulfide (CdS), amorphous silicon (a-Si), copper indium diselenide (CuInSe2), cadmium telluride (CdTe), and poly­
crystalline silicon have been tested. It has been established that the maximum power delivered is limited by the relatively low
efficiency of the panel (< 20%). Results of the Solair project have shown promise in air-conditioning and cold storage. A 1 kW
(COP = 2.5–4) prototype air conditioner was built and connected to a 1.2 kW output array of PV cells activated by threshold
insolation of 450 W m−2. The cold storage prototype was built and tested in Spain. Results show that the operation has been
satisfactory over a range of climatic conditions and a variety of foodstuffs.

QE
6


Evaporator

3

QC
5

1

Condenser

Generator
QG

Figure 12 Basic ejector cycle.

4

2

Ejector


490

Applications

QC
Condenser


Photovoltaic
solar panel
AC

Inverter

DC

Battery

DC

Evaporator

−

Air handling
unit cooling
battery

Figure 13 Typical photovoltaic vapor compression cycle.

3.14.5 Relative Comparison of Solar Cooling Technologies
The relative use of solar cooling systems will depend on their relative performance in terms of efficiency, capital cost, and LCC. Work
carried out by Syed et al. [6] investigated these and the results of this investigation are detailed below.

3.14.5.1

Solar Coefficient of Performance


In assessing the relative efficiency of a solar cooling cycle, we are concerned with the efficiency of the cooling cycle and efficiency of
the collector device itself. Figure 14 shows the range of SCOP data reported for four competing solar cooling technologies at
different application temperatures.

Solar coefficient of performance (−)

1.2
1
0.8
0.6
0.4
0.2
0
−50

−40

−30

0
−20
−10
Cooling temperatures (°C)

10

20

Standard Single-effect LiBr/water absorption


Double-effect LiBr/water absorption

PV - Vapor compression

Improved single-effect LiBr/water absorption

Figure 14 High-level SCOP map for a range of cooling temperatures.

30


491

150
125
100
750
500

Total cost
Solar collectors
Chillers
Ancillaries
FPC+SE Absorption

Conversional(nonsolar)

Heat rejection

FPC+SE Absorption


PV+Centrifugal

PV+Water cooled


0

ETC+DE Absorption


250
ETC+SE Absorption


Capital cost of cooling (£/kW)

Solar Cooling and Refrigeration Systems

Figure 15 A comparison of the capital cost of solar cooling systems.

5.14.5.2

Capital Cost Comparison

Figure 15 subdivides the three generic families and compares the capital cost composition of solar cooling equipment normalized
per kilowatt of cooling.
The following observations from Figure 15 are noteworthy:
• The collector cost ranges from 2 to 26 times the chiller cost depending on their types and operating temperatures.
• About a sixth of the investment is required for procuring flat-plate collectors for supplying hot water at a temperature of 75 °C

compared with concentrating collectors for higher temperatures. The lower capital cost system consists of flat-plate collectors and
single-effect absorption chillers sized for low hot water temperatures.
• The lowest cost option is currently 3 times the cost of a conventional (nonsolar) vapor compression system.

5.14.5.3

Life-Cycle Cost Comparison

LCC depends on the SCOP as well as capital and running cost of systems. Only those systems that show lower SCOP could have
better LCC if they have lower capital costs. These two have the highest SCOP; however, the options that have a marginally lower
SCOP were also considered in an LCC evaluation. In practice, the SCOP will depend on the availability of solar energy and whether
additional thermal energy is required. The solar fraction SOLF is sometimes used to describe the utilization of solar energy for
cooling. Therefore, SOLF is described as a ratio of thermal solar energy or electrical solar energy input to total energy input
(including ancillary energy). It should be noted that a solar fraction of unity is achievable if a system is energized entirely with solar
energy. This is given by either of the following equations:
Qsol
Qsol þ Qgas

½16Š

Wpv
Wpv þ Wpoh

½17Š

SOLFthe ¼
SOLFw ¼

where Qsol is the solar thermal energy, Qgas is the gas auxiliary energy, and Wpv is the solar electrical energy.
Therefore, the overall LCC is given by the following equation:

� �
�
�
�
� �
�
1
i
EFLH
Â
W
þ
POH
Â
W
Â
C
þ
Q
þ
M
LCC ¼
Z
þ
C
wg
g
ng
poh
eflh

Qe
1−ð1 þ i Þ− n

½18Š

where Z is the capital cost, Qg is the generator load, Cng is the natural gas tariff, Cwg is the grid electricity tariff, EFLH is the
number of equivalent full load hours (EFLH), POH is the plant on hours which is typically 1.5 EFLH, Weflh is the electrical


Applications

LCC difference relative to conventional (nonsolar) cooling (%)

492

700
FPC+SE absorption (75 °C/70 °C)
PV+air-cooled screw
FPC+SE absorption (85 °C/80 °C)
ETC+SE absorption (115 °C/110 °C)
ETC+DE absorption
PTC+ejector
PV+centrifugal
PV+water-cooled screw

600

500

400


300

200

100
500

1500

2500

3500

4500

5500

Equivalent full load hours (hours)
Figure 16 Annual difference in life-cycle cost between solar and nonsolar cooling systems.

energy input as shaft power to mechanical compressor, Qe is the evaporator load, Wpoh is the ancillary plant power, and M
is the maintenance cost.
An LCC comparison was carried out to consider the combined effect of capital and running cost on system performance. It was found
that due to the high capital cost of currently available solar collectors, annual LCC savings with solar cooling systems compared with the
conventional system cannot yet be realized. This is indicated in Figure 16, which provides the difference in LCC of solar cooling systems
and a conventional centrifugal vapor compression system with a base of 100% (indicating LCC equivalence) against EFLH.
As the systems are run for longer EFLH, the LCC difference diminishes due to the impact of cost of saved energy. Interestingly,
this results show that the lowest capital cost option reflects the lowest LCC difference for a number of EFLH of cooling.
The result of an LCC sensitivity study has shown that solar cooling using flat-plate collectors and single-effect absorption chillers

at the lowest driving temperatures could become economical if the collector capital cost is reduced [11].

3.14.6 Application of Solar Cooling System
A recent study funded by the European Union investigated the current and future potential use of the solar cooling systems in
Europe. It created a database of installed systems and was able to draw some key conclusions.
There is a significant potential stated for solar cooling technology and particularly in the <20 kW capacity range. The total
number of systems currently in operation in Europe is not very well known but may be estimated to 300–400 installations.
Specifically, an increase in small-size solar cooling systems for residential application has been observed in southern European
countries. In general, the increase of the sales rate of small-sized chillers within the last two years is promising. Absorption chillers
are the thermally driven systems most present on the market in both small and large applications. Their combination with flat-plate
or evacuated-tube solar collectors is quite well experienced in large systems.
Small-scale sorption chillers are now commercially available, but the market is in its infancy and there are several nontechnical
barriers which can resist penetration. At the moment, the key factors are that combining solar heating and cooling needs high effort
in design stage, which is not affordable for small applications, and small-scale sorption chillers are at the moment expensive due to
low production numbers.
Critical to expansion is the integration with a solar heating system, and if so, it is possible for solar-powered cooling to deliver
large-scale benefits.

3.14.7 Integration with Solar Hot Water and Solar Tthermal Systems for Cost-Effectiveness
A key barrier to application is the relative cost associated with the technology. However, as the solar cooling system has solar
collectors, it is possible to produce heating also and therefore improving the economics of application. A typical combined system is
illustrated in Figure 17.


Solar Cooling and Refrigeration Systems

493

Hot water


Mains cold
water
+

Supplementary
heater battery

Mains cold
water

Solar
collector

Flow & return for
cooling system
Flow & return for
Swimming pool

Figure 17 A schematic diagram of a combined solar cooling, heating, and hot water system.

5.14.8 Conclusions
This chapter highlights the what, how, and why solar-powered cooling could be utilized. It provides information on the current state
of the solar cooling market and the relative performance of different technologies. Specifically, the benefits of solar-powered cooling
are as follows:
•
•
•
•
•
•


Enables the usage of the surplus solar heat during the summer
No overheating of the solar system during the summer
Simultaneity of solar heat supply and cooling demand
Minimum operating costs for cooling, heating, and for domestic hot water
Using solar heat as a renewable energy source
Independency from fossil fuels.

To fully realize these potential benefits, some significant developments in terms of cost minimization, value engineering, and
integration of systems are required.

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[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
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[11]
[12]
[13]
[14]
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