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Energy and Buildings 67 (2013) 286–297

Contents lists available at ScienceDirect

Energy and Buildings

j ourna l ho me pa g e: www.elsev ier .com/ locate /enbui ld

eview

eview of solar refrigeration and cooling systems

oan Sarbu ∗, Calin Sebarchieviciepartment of Building Services Engineering, “Politehnica” University of Timisoara, Piata Bisericii 4A, 300233 Timisoara, Romania

r t i c l e i n f o

rticle history:eceived 22 May 2013eceived in revised form 30 July 2013ccepted 14 August 2013

eywords:enewable energyolar refrigeration technologyV system

a b s t r a c t

Providing cooling by utilizing renewable energy such as solar energy is a key solution to the energy andenvironmental issues. This paper provides a detailed review of different solar refrigeration and coolingmethods. There are presented theoretical basis and practical applications for cooling systems within var-ious working fluids assisted by solar energy and their recent advances. Thermally powered refrigerationtechnologies are classified into two categories: sorption technology (open systems or closed systems) andthermo-mechanical technology (ejector system). Solid and liquid desiccant cycles represent the open sys-tem. The liquid desiccant system has a higher thermal coefficient of performance (COP) than the soliddesiccant system. Absorption and adsorption technologies represent the closed system. The adsorption

hermo-mechanical coolingesiccant solar systembsorption coolingdsorption cooling

cooling typically needs lower heat source temperatures than the absorption cooling. Based on COP, theabsorption systems are preferred to the adsorption systems, the higher temperature issues can be easilyhandled with solar adsorption systems. The ejector system represents the thermo-mechanical cooling,and has a higher thermal COP but require a higher heat source temperature than other systems. Thestudy also refers to solar hybrid cooling systems with heterogeneous composite pairs, to a comparisonof various solar cooling systems, and to some use suggestions of these systems.

© 2013 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2871.1. Renewable energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2871.2. Solar energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

2. Solar refrigeration technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2873. Solar photovoltaic cooling systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2884. Solar thermo-electrical cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2895. Solar thermo-mechanical cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2896. Solar thermal cooling techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

6.1. Open sorption systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2906.1.1. Liquid desiccant system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2906.1.2. Solid desiccant system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2916.1.3. Desiccant solar cooling system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

6.2. Closed sorption systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2916.2.1. Absorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2916.2.2. Solar absorption cooling systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2926.2.3. Adsorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2946.2.4. Solar adsorption cooling systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

7. Comparison of various solar refrigeration technologies . . . . . . . . . . . . . . . . . . .8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +40 256403991; fax: +40 256403987.E-mail address: ioan.sarbu@ct.upt.ro (I. Sarbu).

378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.enbuild.2013.08.022

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I. Sarbu, C. Sebarchievici / Ener

. Introduction

Energy security is the ability of a nation to deliver the energyesources needed to ensure its welfare and implies secure supplynd stable prices. Energy is vital for progress and developmentf a nation’s economy. The economic growth and technologicaldvancement of every country depends on it [1] and the amountf available energy reflects that country’s quality of life. Econ-my, population and per capita energy consumption have causedhe increase in demand for energy during the last few decades.ossil fuels continue to supply much of the energy used world-ide, and oil remains the primary energy sources. Therefore, fossil

uels are the major contributor to global warming. Along with thelobal warming impacts and climate changes, the demands for air-onditioning and refrigeration have increased.

Encouraged by the successful worldwide effort to protect thezone layer, scientists and engineers have been committed to min-mize and reverse the harming environmental effects of global

arming. Global warming occurs when carbon dioxide, releasedostly from the burning of fossil fuels (oil, natural gas, and coal)

nd other gases, such as methane, nitrous oxide, ozone, chloroflu-rocarbons (CFCs), hydro-chlorofluorocarbons (HCFCs) and waterapour, accumulate in the lower atmosphere. As results of the rapidrowth in world population and the economy total world energyonsumption is projected to increase by 71% from 2003 to 20302]. The awareness of global warming has been intensified in recentimes and has reinvigorated the search for energy sources that arendependent of fossil fuels and contribute less to global warming.

The Vienna Convention for the Protection of the Ozone Layer1985), the Kyoto Protocol on Global Warming (1998) and the fivemendments of the Montreal Protocol (1987) all discussed theeduction of CFCs to protect the ozonosphere, but the situationontinues to decline. The European Commission (EC) Regulation037/2000, implemented on 1 October 2000, works to control andchedule all the ozone depleting materials; all HCFCs will be pro-ibited by 2015 [3,4].

The European strategy to decrease the energy dependence restsn two objectives: the diversification of the various sources of sup-ly and policies to control consumption. The key to diversification

s renewable energy sources (RES), because they have significantotential to contribute to a sustainable development [5].

.1. Renewable energy

The term “renewable energy” refers to energy that is producedrom natural resource having the characteristics of inexhaustibil-ty over time and natural renewability. Renewable energy sourcesnclude wind, solar, geothermal, biomass and hydro energies [6].n efficient utilization of renewable resources has a significantotential in both stimulating the economy and reducing pollution.hus, many governments started to implement various policies thatupport renewable generation. One of the key components of anyenewable energy policy is setting of renewable energy targets [7].

There have been numerous efforts undertaken by developedountries to implement different renewable energy technologies.he use of wind energy has increased over the last few years [8].or example, the Netherlands, Germany, India and Malaysia aresing wind turbines for producing electricity [9]. In north-western

ran, mineral materials are used for the production of geothermalnergy and in Iceland, seventy percent (70%) or their factories uti-ize geothermal energy for industrial purposes [10].

Although Romania has a high potential of renewable energy

ources, in 2010 the RES share in final energy consumption was3.4%. Anyway, Romania ranked the second place in the Europeannion concerning the share of energy from renewable sources grossnal consumption between 2006 and 2010 [11]

Buildings 67 (2013) 286–297 287

Among the energy sources alternative to fossil fuels, renewableenergy sources such as solar and wind are the more available.

1.2. Solar energy

In recent years, scientists have increasingly paid more atten-tion to solar energy. There is a sudden demand in the utilization ofsolar energy for various applications such as water heating, build-ing heating/cooling, cooking, power generation and refrigeration[12].

Solar energy is the result of electromagnetic radiation releasedfrom the Sun by the thermonuclear reactions occurring inside itscore. All of the energy resources on earth originate from the sun(directly or indirectly), except for nuclear, tidal and geothermalenergy. The sun actually transmits a vast amount of solar energyto the surface of the earth [13]. The term “solar constant” signi-fies the radiation influx of solar energy. The mean value of solarconstant is equal to 1368 W/m2 [14].

In Romania the annual solar energy flow ranges between1000–1300 kWh/m2/year in more than half of the country. Thisclimate allows the operation of solar collectors from March untilOctober, with conversion efficiency between 40% and 90% [15].Thus, an important solar potential exist.

Most countries are now accepting that solar energy has enor-mous potential because of its cleanliness, low price and naturalavailability. For example, it is being used commercially in solarpower plants. Sweden has been operating a solar power plant since2001. Romania’s experience in solar energy represents a competi-tive advantage for the future development of this area, the countrybeing a pioneer in this field. Between 1970 and 1980 were installedaround 800,000 m2 of solar collectors that placed the country thirdworldwide in the total surface of photovoltaic cells. Between 1984and 1985 was achieved the peak of solar installations, but after1990 unfavourable macroeconomic developments led to the aban-donment of the production and investments in the solar energyfield. Today about 10% of the former installed collector area is stillin operation [16].

In recent years, many countries have been facing difficultieswith the issue of refrigeration systems. Specifically, the demandof air conditioning for both commercial and residential buildingsduring the summer is ever-increasing [13]. There is a lack of elec-tricity and storage in developing countries to accommodate highenergy consumptive systems such as refrigeration and cooling.

The solar cooling techniques can reduce the environmentalimpact and the energy consumption issues raised by conventionalrefrigeration and air-conditioning systems. Therefore, in this paperare presented theoretical basis and practical applications for cool-ing technologies within various working fluids assisted by solarenergy and their recent advances. Also, a comparison of varioussolar cooling systems is performed and some suggestions aboutthe use of these systems are given.

2. Solar refrigeration technology

Solar refrigeration offers a wide variety of cooling techniquespowered by solar collector-based thermally driven cycles and pho-tovoltaic (PV)-based electrical cooling systems. Fig. 1 shows aschematic diagram of a solar thermal cooling system. The solarcollection and storage system consists of a solar collector (SC) con-nected through pipes to the heat storage. Solar collectors transformsolar radiation into heat and transfer that heat to the heat transfer

fluid in the collector. The fluid is then stored in a thermal stor-age tank (ST) to be subsequently utilized for various applications.The thermal AC (air-conditioning) unit is run by the hot refriger-ant coming from the storage tank, and the refrigerant circulates

288 I. Sarbu, C. Sebarchievici / Energy and Buildings 67 (2013) 286–297

ttlvatsi

ptetsepclahe

of

C

wb

W

E

TS

Fig. 1. Schematic of a solar thermal cooling system.

hrough the entire system. Since solar energy is time-dependent,he successful utilization of all these cooling systems is to a veryarge degree dependent on the thermal storage tank employed. Thearious stages of thermal storage integrated solar cooling systemsre shown in Table 1 [17]. In comparison with conventional elec-rically driven compression systems, substantial primary energyavings can be expected from solar cooling, thus aiding in conserv-ng energy and preserving the environment.

Solar refrigeration technology engages a system where solarower is used for cooling purposes. Cooling can be achievedhrough four basic methods: solar PV cooling, solar thermo-lectrical cooling, solar thermo-mechanical cooling, and solarhermal cooling. The first is a PV-based solar energy system, whereolar energy is converted into electrical energy and used for refrig-ration much like conventional methods [18]. The second oneroduce cool by thermoelectric processes [19,20]. The third oneonverts the thermal energy to mechanical energy, which is uti-ized to produce the refrigeration effect. The fourth method utilizes

solar thermal refrigeration system, where a solar collector directlyeats the refrigerant through collector tubes instead of using solarlectric power [13].

The performance of refrigeration systems is determined basedn energy indicators of these systems. The COP (coefficient of per-ormance) can be calculated as follows:

OP = Eu

Ec(1)

here Eu is the cooling usable energy; Ec is the consumed energyy system.

Also, energy efficiency ratio (EES), in British thermal unit peratt-hours (Btu/(Wh)) is defined by equation:

ER = 3.413COP (2)

able 1tages and options in solar cooling techniques.

Source Conversion Thermal storage (hotenergy)

Producenergy

Sun Solar PV (electrical) 1. VapoThermo

Solar thermal1. Flat plate collector2. Evacuated tubecollector3. Concentratedcollector

1. Sensible2. Latent3. Thermo-chemical

1. Eject2. DesicAbsorp(a) Sing(b) Half(c) Dou(d) Trip4. Adso

Fig. 2. Global PV-based solar electricity production for four decades.

where 3.413 is the transformation factor from Watt to Btu/h.Detailed discussion of each solar refrigeration technology fol-

lows.

3. Solar photovoltaic cooling systems

A PV cell is basically a solid-state semiconductor device thatconverts light energy into electrical energy. To accommodate thehuge demand for electricity, PV-based electricity generation hasbeen rapidly increasing around the world alongside conventionalpower plants over the past two decades. Fig. 2 shows a comparativerepresentation of the development of solar PV systems in differentcountries [21].

While the output of a PV cell is typically direct current (DC)electricity, most domestic and industrial electrical appliances usealternating current (AC). Therefore, a complete PV cooling systemtypically consists of four basic components: photovoltaic modules,a battery, an inverter circuit and a vapour compression AC unit [22].

• The PV cells produce electricity by converting light energy (fromthe sun) into DC electrical energy.

• The battery is used for storing DC voltages at a charging modewhen sunlight is available and supplying DC electrical energy ina discharging mode in the absence of daylight. A battery chargeregulator can be used to protect the battery form overcharging.

• The inverter is an electrical circuit that converts the DC electricalpower into AC and then delivers the electrical energy to the AC

loads.

• The vapour compression AC unit is actually a conventional coolingor refrigeration system that is run by the power received fromthe inverter.

tion of cool Thermal storage (coolenergy)

Applications

r compressionelectric

1. Air conditioning(a) Office(b) Building(c) Hotel(d) LaboratoryProcess industries(a) Dairy(b) Pharmaceutical(c) ChemicalFood preservation(a) Vegetables(b) Fruits(c) Meat and fish

orcant

tionle-effect-effectble-effectle-effectrption

1. Sensible2. Latent3. Thermo-chemical

I. Sarbu, C. Sebarchievici / Energy and Buildings 67 (2013) 286–297 289

hgcn

4

ippebdbwturtbm

deTattTd

Fig. 3. Schematic of a stand-alone PV system.

The PV system can perform as a standalone system (Fig. 3), aybrid system (working with an oil/hydro/gas power plant) or as arid or utility intertie systems. Though the efficiency of PV modulesan be increased by using inverters, their COP and efficiency are stillot desirable.

. Solar thermo-electrical cooling

In solar electric cooling, power produced by the solar PV devicess supplied either to the Peltier cooling systems. It is possible toroduce cool by thermoelectric processes, using the principle ofroducing electricity from solar energy through thermoelectricffect and the principle of producing cool by Peltier effect. It haveeen produced such thermoelectric refrigerators, with the principleiagram in Fig. 4. Thermoelectric generator consists of a small num-er of thermocouples that produce a low thermoelectric power buthich can easily produce a high electric current. It has the advan-

age that can operate with a low level heat source and is thereforeseful to convert solar energy into electricity. The thermoelectricefrigerator is also composed of a small number of thermocoupleshrough which run the current produced by the generator. The com-ination of the two parts is compatible with use as thermoelectricaterials of the semiconductors based on Bi2Te3 [23].Vella et al. [19] shown that a thermoelectric generator, which

raws its heat from solar energy, is a particularly suitable source oflectrical power for the operation of a thermoelectric refrigerator.hey developed the theory of the combined thermoelectric gener-tor and refrigerator and determined the ratio of the numbers ofhermocouples needed for the two devices. A 4-couple thermoelec-

ric generator has been used to power a single-couple refrigerator.emperatures below 0 ◦C have been achieved for a temperatureifference across the generator of about 40 K.

Fig. 4. Schematic of solar thermo-electrical cooling system.

Fig. 5. Schematic of steam jet solar cooling system.

The thermoelectric refrigerator is a unique cooling system, inwhich the electron gas serves as the working fluid. In recentyears, concerns of environmental pollution due to the use of CFCsin conventional domestic refrigeration systems have encouragedincreasing activities in research and development of domesticrefrigerators using Peltier modules. Moreover, recent progress inthermoelectric and related fields have led to significant reduc-tions in fabrication costs of Peltier modules and heat exchangerstogether with moderate improvements in the module performance.Although the COP of a Peltier module is lower than that of conven-tional compressor unit, efforts have been made to develop domesticthermoelectric cooling systems to exploit the advantages associ-ated with this solid-state energy conversion technology [18]. Otherapplications of this technology are air conditioning and medicalinstruments.

5. Solar thermo-mechanical cooling

In the thermo-mechanical solar cooling system, the thermalenergy is converted to the mechanical energy. Then the mechanicalenergy is utilized to produce the refrigeration effect.

The steam ejector system represents the thermo-mechanicalcooling technology. Fig. 5 illustrates the steam ejector system inte-grated with a parabolic solar collector SC. The steam produced bythe solar collector is passing through the steam jet ejector E. Dur-ing this process, the evaporator pressure is reduced, and water isvaporized in the evaporator V by absorbing the heat from the coldwater.

When cooling is not needed, the steam turbines can be usedto produce electricity. Most of the steam ejector system requiresteam at pressures in the range of 0.1–1.0 MPa, and temperaturesin the range of 120–180 ◦C [24]. However, Loehrke [25] proposedand demonstrated that the steam ejector system could be operatedusing low-temperature solar heat by reducing the operating pres-sure under atmospheric pressure. Khattab and Barakat [26] laterproved this concept by developing a detailed mathematical modelof the solar steam ejector cycles operating at low pressure and lowtemperature for the air-conditioning application.

The working fluid used in a solar ejector cooling system lead todifferent performance depending on operating conditions. In orderto compare the performance of different used working fluids, inTable 2 are presented the following values: tg – the generatingtemperature; tc – the condensation temperature achieved in con-denser C (37 ◦C for cooling with cooling tower, 30 ◦C for coolingwith cold water); pg – the pressure in the generator G (maximumpressure in the system); pe – the pressure in the evaporator V (min-

imum pressure in the system); COP – the theoretical coefficientof performance; �ej – the ejector efficiency; COPr = �ej·COP – thereal coefficient of performance; QSC – the heat needed to be sup-plied by solar collector in generator to achieve a cooling power

290 I. Sarbu, C. Sebarchievici / Energy and Buildings 67 (2013) 286–297

Table 2Performances of different working fluids used in solar steam ejector systems.

Working fluid tg (◦C) tc (◦C) pg (kPa) pe (kPa) COP �ej COPr QSC (W) ASC (m2)

H2O85

37 392.2 0.88 0.913 0.184 0.168 69,130 17330 392.2 0.88 1.471 0.217 0.319 36,396 91

13037 475.5 0.88 2.076 0.226 0.469 24,717 6230 475.5 0.88 2.887 0.223 0.645 17,979 45

R-1185

37 460.8 50.0 0.936 0.196 0.183 63,428 15930 460.8 50.1 1.947 0.226 0.440 26,356 66

13037 784.0 50.1 1.708 0.226 0.386 26,911 6730 784.0 50.1 3.121 0.216 0.675 17,175 43

R-21 8537 754.9 90.2 0.790 0.172 0.135 85,630 21530 754.9 90.2 1.162 0.209 0.242 47,866 120

Propane 8537 2745 539 0.496 0.078 0.039 298,969 75030 2745 539 1.038 0.198 0.209 56,585 14237 882.3 147 0.423 0.091 0.038 302,475 758

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Butane 85 30 882.3 147

NH3 85 37 2157 520

130 37 2157 520

f 1.16 × 104 W; ASC – the solar collector area, assuming a solarux of 0.8 kW/m2 and capture efficiency of 0.5, for achieve a cool-

ng capacity of 1.16 × 104 W. Considering one flat-plate collector,he possible temperature for which can easily provide solar heat isg = 85 ◦C, and for a parabolic-cylinder concentrating collector cane adopted tg = 130 ◦C.

Analyzing the COPr values from this table results as the compet-tive working fluids: water and Freon, among which best is R-11.

ater and R-11 have comparable COPr, but operating pressures inhe system are very different. Thus, for the use of flat plate collec-ors (tg = 85 ◦C), steam ejector cooling system works completely inepression (pe and pg is less than atmospheric pressure). So if water

s used as refrigerant leakage problems are to be solved to avoid airntering the system.

Various experimental studies [27–29] have examinated theffect of the operation conditions such as the generator temper-ture, evaporator temperature and condenser temperature, theeometrical conditions, the system conditions such as refrigerantnd collector selections on the performance of the system. Otheresearchers [30,31] have presented numerical methods of simulat-ng the ejector and studied the performance of system.

Nehad [27,28] compared the theoretical performance of thejector system working with R-717, R-11, R-12, R-113 and R-114.hen he chose R-113 as a refrigerant for the experiment since it has

higher COPr, a reasonable operating pressure, and is non-toxic.Eames et al. [29] reported that the measured COPr of the single-

tage ejector system using H2O as its working fluid ranged from.178 to 0.586 at a generating temperature tg of 120–140 ◦C, anvaporation temperature te of 5–10 ◦C, and a condensation tem-erature tc of 26.5–36.3 ◦C.

Vidal et al. [30] analyzed the solar ejector system using R-141bs its refrigerant by using the TRNSYS and EES simulation soft-are. The system was designed to deliver 10.5 kW of cooling with

0 m2 of flat-plate collector tilted 22◦ from the horizontal and a m3 hot-water storage tank. They reported the maximum COPr of.22 at tg = 80 ◦C, te = 8 ◦C, and tc = 32 ◦C. They also concluded thatn efficient ejector system could only work in a region with decentolar radiation and where a sufficiently low condenser temperatureould be kept.

Grazzini and Rocchetti [32] theoretically investigated the per-ormance of the two-stage ejector system. They reported that theOPr of the two-stage ejector system ranged from 0.13–0.53 at

g = 110–120 ◦C, te = 7–12 ◦C, and tc = 30–40 ◦C.Along with other researches results [33,34] show that the low

jection efficiency leads to values of COPr for solar ejector coolingystems smaller than in the case of solar absorption cooling sys-ems. The performance of the ejector system depends on the massow rate ratio through the motive nozzle and the suction nozzle.

0.666 0.170 0.113 102,284 256Not possible solution0.348 0.016 0.005 2,130,150 5338

The ejector systems are mostly used in air conditioning applica-tions, but they can be used in chemical and metallurgical industryfor air cooling in areas with higher heat dissipation.

6. Solar thermal cooling techniques

Solar thermal cooling (Fig. 1) is becoming more popular becausea thermal solar collector directly converts light into heat. For exam-ple, Otanicar et al. [22] described a thermal system that is capableof absorbing more than 95% of incident solar radiation, dependingon the medium. Sorption technology is utilized in thermal cool-ing techniques. The cooling effect is obtained from the chemical orphysical changes between the sorbent and the refrigerant. Sorp-tion technology can be classified either as open sorption systemsor closed sorption systems [2].

6.1. Open sorption systems

Open system refers to solid or liquid desiccant systems that areused for either dehumidification or humidification. Basically, des-iccant systems transfer moisture from one airstream to another byusing two processes. In the sorption process the desiccant systemtransfer moisture from the air into a desiccant material by usingthe difference in the water vapour pressure of the humid air andthe desiccant. If the desiccant material is dry and cold, then itssurface vapour pressure is lower than that of the moist air, andmoisture in the air is attracted and absorbed to the desiccant mate-rial. In desorption (regeneration) process, the captured moisture isreleased to the airstream by increasing the desiccant temperature.After regeneration, the desiccant material is cooled down by thecold airstream. Then it is ready to absorb the moisture again. Whenthese processes are cycled, the desiccant system can transfer themoisture continuously by changing the desiccant surface vapourpressures, as illustrated in Fig. 6. To drive this cycle, thermal energyis needed during the desorption process. The difference betweensolid and liquid desiccants is their reaction to moisture.

6.1.1. Liquid desiccant systemMaterials typically used in liquid desiccant systems are lithium

chloride (LiCl), calcium chloride (CaCl), and lithium bromide (LiBr).The system usually consists of a conditioner and a regenerator[2]. The conditioner handles the process air to be dehumidified.The liquid desiccant is sprayed into the air and directly absorbsthe moisture from the process air. Afterward, the liquid falls to a

sump, is pumped, and is sprayed back into the air. While absorb-ing moisture, the desiccant becomes warmer and the partial vapourpressure is increased. The concentration of desiccant decreases andthe water content increases. A small amount of liquid desiccant is

I. Sarbu, C. Sebarchievici / Energy and

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aken continuously from the sump to the regenerator to remove theater that is picked up. The desiccant is also sprayed into the air.

he desiccant is heated before it contacts the air so that the partialressure of the desiccant is higher than that of the air. Therefore,he moisture is transported to the regeneration air (process 2–3n Fig. 6). The regeneration air leaves the regenerator in a hot andumid condition. As the liquid desiccant solution returns to theump of the conditioner, it is drier more concentrated, and still atigh vapour pressure and temperature. Before being sprayed intohe air, the liquid desiccant is cooled to the required temperaturey a cooling tower or chiller (process 3–1 in Fig. 6). The favourableeature of the liquid desiccant system is the fact that the liquid des-ccants can be regenerated at temperatures below 80 ◦C so that lowemperature heat sources can be utilized.

In efforts to reduce a building’s energy consumption, design-rs have successfully integrated liquid desiccant equipment withtandard absorption chillers [35]. In a more general approach, thebsorption chiller is modified so that rejected heat from its absorberan be used to help regenerate liquid desiccants.

.1.2. Solid desiccant systemThe solid desiccant system is constructed by placing a thin layer

f desiccant material, such as silica gel, on a support structure2]. The desiccant wheel rotates slowly between the process andhe regeneration airstreams. It is divided into two sections for theegeneration air and the process air. Process air flows through therst part of the wheel, and the moisture is removed due to the lowerartial vapour pressure in the desiccant material. To regenerate theesiccant, the wheel passes the hot reactivation air, and the processan start again. For solid desiccant materials, the increase of dry-ulb temperature of the process air is a result of the adsorption heat.his consists of the vaporization latent heat of the adsorbed mois-ure and the heat of wetting. The heat of wetting is approximately0% of the vaporization heat [35].

Both liquid and solid desiccants may be used in equipmentesigned for drying air and gases at atmospheric or elevatedressures (schools, theatres, restaurants, hospitals). Regardless ofressure levels, basic principles remain the same, and only the des-

ccant towers or chambers require special design consideration.Desiccant capacity and actual dew-point performance depend

n the specific equipment used, characteristics of the various des-ccants, initial temperature and moisture content of the gas to beried, reactivation methods, etc. Factory-assembled units are avail-ble up to a capacity of about 38 m3/s.

Buildings 67 (2013) 286–297 291

Several studies performed on the description and operation ofdesiccant cooling systems by different researchers [36,37]. Sys-tems that use rotary desiccant wheel to dehumidify the air are themost popular desiccant cooling systems and studied by differentresearchers [38,39]. They showed that desiccant cooling systemsare viable alternative to vapour compression systems.

6.1.3. Desiccant solar cooling systemDesiccant solar systems reduce the moisture of the ambient air

by utilizing thermal energy from the solar collector to regeneratedesiccants. Then the dry air is cooled through indirect and/or directevaporative stages, as shown in Fig. 7.

Since Lof [40] investigated liquid desiccant solar cooling, mostof the research on liquid desiccant solar cooling began in theearly 1990s. Moreover, the latest developments are focused on liq-uid sorption applications since the liquid sorption materials haveadvantages of higher air dehumidification at the same driving tem-perature, as well as the possibility of high energy storage by meansof hygroscopic solutions.

Ameel et al. [41] compared the performance of variousabsorbents, including LiCl, CaCl, and LiBr. They concluded that LiBroutperformed the other absorbents.

Gommed and Grossman [42] developed the prototype of theliquid desiccant cooling system assisted by the flat solar collectorsusing LiCl/H2O as its working fluid. Through the parametric study,they demonstrated that conditions of the ambient air are the majorparameters considerably affecting the dehumidification process inthe liquid desiccant system. They reported that the system provided16 kW of dehumidification capacity with a thermal COP of 0.8.

Henning et al. [43] installed a solar-assisted desiccant coolingsystem with a 20 m2 flat-plate solar collector and a 2 m3 hot-waterstorage tank. They reported that a solar fraction of the coolingbetween the solar heat and auxiliary heat provided was 76%, withan overall collector efficiency of 54% and a cooling COP of 0.6 duringtypical summer conditions. In addition, they proposed a combi-nation of a solar-assisted solid desiccant cooling system with aconventional vapour compression chiller for warm and humid cli-mates, and claimed up to 50% of primary energy savings.

6.2. Closed sorption systems

In closed sorption technology, there are two basic methods:absorption refrigeration and adsorption refrigeration.

6.2.1. Absorption refrigerationAbsorption is the process in which a substance assimilates from

one state into a different state. These two states create a strongattraction to make a strong solution or mixture. The absorptionsystem is one of the oldest refrigeration technologies. The first evo-lution of an absorption system began in the 1700s. It was observedthat in the presence of H2SO4 (sulphuric acid), ice can be made byevaporating pure H2O within an evacuated container. In 1859, aFrench engineer named Ferdinand Carre designed an installationthat used a working fluid pair of ammonia/water (NH3/H2O). In1950, a new system was introduced with a water/lithium bromide(H2O/LiBr) pairing as working fluids for commercial purposes [44].

The absorption refrigeration technology consists of a generator,a pump and an absorber that are collectively capable of com-pressing the refrigerant vapour. The evaporator draws the vapourrefrigerant by absorption into the absorber. The extra thermalenergy separates the refrigerant vapour from the rich-solution. Thecondenser condenses the refrigerant by rejecting the heat and then

the cooled liquid refrigerant is expanded by the evaporator, and thecycle is completed.

The refrigerant side of the absorption system essentially worksunder the same principle as the vapour compression system.

292 I. Sarbu, C. Sebarchievici / Energy and Buildings 67 (2013) 286–297

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vapour-refrigerant are reabsorbed by a weak-solution from resor-ber Rb and the system operates similarly to the above mentionedcycle. System pressures may be allowed as close to atmospheric

Fig. 7. Schematic of de

owever, the mechanical compressor used in the vapour compres-ion cycle is replaced by the thermal compressor in the absorptionystem. The thermal compressor consists of the absorber, the gen-rator, the solution pump, and the expansion valve. The attractiveeature of the absorption system is that any types of heat source,ncluding solar heat and waste heat, can be utilized in the desorber.

Typical refrigerant/absorbent pairs used in the absorptionystem are NH3/H2O and H2O/LiBr. The thermodynamic char-cteristics of these have been described in various studies andxperiments [45,46]. Even though NH3/H2O and H2O/LiBr pairsave been used all over the world, researchers are still looking forew pairs [47].

Based on the thermodynamic cycle of operation and solu-ion regeneration, the absorption systems can be divided intohree categories: single-, half-, and multi-effect (double-effect andriple-effect) solar absorption cycles. The single-effect and half-ffect chillers require relatively lower hot-water temperatures withespect to multi-effect systems [8].

Best absorption refrigeration technology applications are heat-ctivated refrigerators, gas-fired residential air conditioners, andarge industrial refrigeration plants.

Grossman [48] provided typical performances of the single- andulti-effect absorption system, as shown in Table 3. Typical cooling

OPs of the single-effect, double-effect, and triple-effect absorp-ion systems are 0.7, 1.2, and 1.7, respectively. The operation ofhe H2O/LiBr-based absorption system is limited in the evaporat-ng temperature and the absorber temperature, due to the freezingf the water and the solidification of the LiBr-rich solution, respec-ively. The operation of the NH3/H2O-based absorption system isot limited in either the evaporating temperature or the absorptionemperature. However, ammonia is toxic and its usage is limited tohe large capacity system.

.2.2. Solar absorption cooling systemsSolar absorption systems utilize the thermal energy from a solar

ollector to separate a refrigerant from the refrigerant/absorbentixture.As shown in Table 3, the flat plate solar collector can be used for

he single-effect cycle. However, the multi-effect absorption cyclesequire high temperatures above 85 ◦C, which can be delivered by

he evacuated tube or concentrating-type collectors.

.2.2.1. Single-effect solar absorption cycle. Recent statistics showhat most absorption cooling systems are made using single-effect

t solar cooling system.

absorption cycle with a H2O/LiBr pair, where a solar flat plate col-lector or an evacuated tubular collector with hot-water is used toimplement these systems [49].

Single-effect absorption cooling system is based on the basicabsorption cycle that contains a single absorber and generator asshown in Fig. 8. In the generator G, the refrigerant is separatedfrom the absorbent by the heat provided by the solar collector. Thevapour-refrigerant are condensed in condenser C, then laminatedin expansion valve EV1 and evaporated at low pressure and tem-perature in the evaporator V. The cooled refrigerant is absorbedin the absorber Ab by weak-solution that returns from generatorafter the lamination in the expansion valve EV2. The rich-mixturecreated in absorber is pumped by pump P and returned in G. Theusual a solution heat exchanger (SHX) can be used to improve cycleefficiency [50]. A 60% higher COP can be achieved by using the SHX[51]. The absorption being exothermic, the absorber is chilled withcooling water.

For low temperature heat sources results unacceptably low val-ues of degassing zone and vapour-refrigerant release in generatoris slow down and the operation of the system becomes unstable orimpossible. To improve the COP and for use lower temperatures ingenerator, can be used solar resorption cooling system (Fig. 9) [23].

In this case, in generator, the refrigerant is also separated fromthe absorbent by the heat provided by the solar collector, but

Fig. 8. Schematic of solar absorption cooling system.

I. Sarbu, C. Sebarchievici / Energy and Buildings 67 (2013) 286–297 293

Table 3Typical performance of absorption cycles.

No. Absorption system COP EER (Btu/(Wh)) Heat source temperature (◦C) Type of solar collectors matched

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1 Single-effect 0.7 2.39

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3 Triple-effect 1.7 5.80

ressure as possible, which simplifies sealing problems, pumpsuilt and reduce temperature in generator.

A single-effect absorption cooling system is simpler than otherhen the design depends on the types of working fluids. The

ystem shoes better performance with non-volatile absorbents as2O/LiBr. If volatile working pair such as NH3/H2O is used, then anxtra rectifier should be used before the condenser to provide pureefrigerant [44].

A low cost non-concentrating flat plate or evacuated tubeolar collector is sufficient to obtain the required temperature forhe generator. Though economical, its COP is lower. For obtain-ng a higher COP, multi-effect systems such as double-effect andriple-effect absorption chillers are used, which are run by steamroduced from concentrating solar collectors.

Nakahara et al. [52] developed a single-effect H2O/LiBr absorp-ion chiller of 7 kW nominal cooling capacity, assisted by a 32.2 m2

rray of flat plate solar collectors. In their system, thermal energyroduced by the solar collector was stored in a 2.5 m3 hot-watertorage tank. Their experimental results during the summer periodhowed that the cooling capacity was 6.5 kW. The measured COP ofhe absorption system was in range of 0.4–0.8 at the generator tem-erature of 70 ◦C to 100 ◦C. Li and Sumathy [53] observed a H2O/LiBrbsorption system with a partitioned hot-water storage tank. Theystem consisted of a 38 m2 flat plate collector and a 4.7 kW absorp-ion chiller. They concluded that the system exhibited 15% moreOP (approximately 0.7) than a conventional whole-tank modeystem. Another investigation on a H2O/LiBr absorption systemonsisting of 49.9 m2 of flat plate collector was performed by Syedt al. [54]. The system performs cooling within generator tempera-ures of 65–90 ◦C, maintaining a capacity of 35 kW. They calculatedhree different COPs and achieved an average collector efficiencyf approximately 50%.

In an intermittent single-stage NH3/H2O absorption system, theolution pump is eliminated and the density difference is utilizedor the NH3/water circulation. In this way, the auxiliary power isaved. Since Trombe and Forx [55] suggested using an intermit-ent single-stage NH3/H2O absorption system assisted by the solar

nergy for ice production, several researchers [56,57] explored theeasibility of such systems

To improve the unsteady nature of the solar heat from the solarollector to the absorption system, Chen and Hihara [58] proposed

Fig. 9. Schematic of solar resorption cooling system.

Flat plate Flat plate/compound parabolic concentrator Evacuated tube/concentrating collector

a new type of absorption cycle that was co-driven both by solarenergy and electricity. In their proposed system, total energy deliv-ered to the generator could be controlled by adjusting the mass flowrate through the compressor. Their numerical simulation modelresults showed the steady COP value of 0.8 for the new cycle, whichwas higher than the conventional cycle.

Chinnappa et al. [59] proposed a conventional vapour compres-sion AC system cascaded with a solar-assisted NH3/H2O absorptionsystem. They concluded that the hybrid system achieved of aCOP = 5, which is higher than that of the vapour compression cycleat 2.55, by reducing the R-22 condensation temperature to 27 ◦C.

6.2.2.2. Half-effect solar absorption cycle. The primary feature of thehalf-effect absorption cycle is the running capability at lower tem-perature compared to others. The name “half-effect” arises fromthe COP, which is almost half that of the single-effect cycle [50].

Arivazhagan et al. [60] performed an experiment with a two-stage half-effect absorption system using the working pair ofR134a/DMAC. They were able to attain an evaporation tempera-ture of −7 ◦C with the generator temperature varying from 55 ◦C to75 ◦C. They concluded that within the optimum temperature range(65–70 ◦C), a COP of approximately 0.36 could be achieved.

Sumathy et al. [61] proposed a two-stage H2O/LiBr chiller forcooling purposes in south China. They found a cooling capacity of100 kW through the integration of a solar cooling system with thesechillers. They concluded that the system had a nearly equivalentCOP as the conventional cooling system, but at a 50% reduced cost.

Izquierdo et al. [62] designed a solar double-stage absorptionplant with H2O/LiBr, which contained flat plate collectors to feedthe generator. They reported that within a condensation temper-ature of 50 ◦C, the COP was 0.38 while providing a generationtemperature of 80 ◦C. They also performed an exergetic analysisof this system and conclude that the single-effect system had 22%more exergetic efficiency than the double-stage half-effect system.

6.2.2.3. Double-effect solar absorption cycle. Double-effect absorp-tion cooling technology was launched in 1956 for developing thesystem performance within a heat source at higher temperatures[63]. Fig. 10 illustrates a double-effect absorption system with a

H2O/LiBr pair.

The cycle begins with generator G-I providing heat to genera-tor G-II. The condenser C rejects the heat and passes the workingfluid towards the evaporator V; within this step, the required

Fig. 10. Schematic of solar assisted double-effect H2O/LiBr absorption system.

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94 I. Sarbu, C. Sebarchievici / Ener

efrigeration occurs. Then, the fluids pass through the heatxchangers HX-I and HX-II from the absorber Ab to G-I by means of

pump P. Trough this process, HX-II can pass the fluids to G-II andhen G-II passes to HX-I. The complete cycle follows three differentressure levels: high, medium and low.

Two single-effect systems effectively form a double-effectbsorption cooling system. Therefore, the COP of a double-effectystem is almost twice that of the single-effect absorption system.or example, Srikhirin et al. conducted an analysis showing thathe COP of a double-effect system is 0.96, whereas the single-effectystem has a COP of only 0.6 In the past few years, the COP of double-ffect absorption systems has reached values of 1.1–1.2 by usingas-fired absorption technology [8].

Tierney [64] performed a comparative study among four dif-erent systems with a collector of 230 m2 and concluded that theouble-effect chiller with a trough collector had the highest poten-ial savings (86%) among the four systems to handle the demandor a 50 kW load.

.2.2.4. Triple-effect absorption cycle. Triple-effect absorption cool-ng can be classified as single-loop or dual-loop cycles. Single-loopriple-effect cycles are basically double-effect cycles with an addi-ional generator and condenser. The resulting system with threeenerators and three condensers operates similarly to the double-ffect system. Primary heat concentrates absorbent solution in arst-stage generator at about 200–23 ◦C. A fluid pair other than2O/LiBr must be used for the high temperature cycle. The refrig-rant vapour produced is then used to concentrate additionalbsorbent solution in a second-stage generator at about 150 ◦C.inally, the refrigerant vapour produced in the second-stage gen-rator concentrates additional absorbent solution in a third-stageenerator at about 93 ◦C. The usual solution heat exchangers can besed to improve cycle efficiency. Theoretically, these triple-effectycles can obtain COPs of about 1.7 [35].

A double-loop triple-effect cycle consists of two cascaded single-ffect cycles. One cycle operates at normal single-effect operatingemperatures and the other at higher temperatures. The smallerigh temperature topping cycle has a generator temperature ofbout 200–230 ◦C. A fluid pair other than H2O/LiBr must be usedor the high temperature cycle. Heat is rejected from the highemperature cycle at 93 ◦C and is used as the energy input forhe conventional single-effect bottoming cycle. Theoretically, thisriple-effect cycle can obtain an overall COP of about 1.8 [35].

Multi-effect cycles are costlier but energy efficient. Double-nd triple-effect chillers employ an additional generator and heatxchanger to liberate the refrigerant from the absorbent solutionith lesser heat input. The available solar intensity, cooling capac-

ty requirements, overall performance and cost, determines theelection of a particular configuration.

Li and Sumathy [12] stressed the importance of the genera-or inlet temperature, chiller, collector choice, system design andrrangement, in the design and fabrication of a solar powered air-onditioning system. Srikhirin et al. [44] have discussed a numberf absorption refrigeration systems and related research options. Inhis section, literatures pertaining to the improvement of absorp-ion cooling systems, theoretical and experimental studies on solarbsorption cooling, and finally, on subjects with a thrust on thermaltorage integrated cooling systems are reviewed.

.2.2.5. Hybrid solar absorption cooling systems. A hybrid coolingoncept arose due to integrate different pairs or systems for obtain-ng better cooling performance. Hybrid solar absorption cooling

ystem refers to the integration of three individual cooling tech-ologies: radiant cooling, desiccant cooling and absorption cooling65]. Table 4 summarizes the above mentioned absorption coolingystems [8].

Buildings 67 (2013) 286–297

Solar absorption cooling systems are used in air conditioningapplications, for food preservation and in ice production.

6.2.3. Adsorption refrigerationAdsorption technology was first used in refrigeration and heat

pumps in the early 1990s. The adsorption process differs from theabsorption process in that absorption is a volumetric phenomenon,whereas adsorption is a surface phenomenon. The primary compo-nent of an adsorption system is a solid porous surface with a largesurface area and a large adsorptive capacity. Initially, this surfaceremains unsaturated. When a vapour molecule contacts the sur-face, an interaction occurs between the surface and molecules, andthe molecules are adsorbed on to the surface [6].

In an adsorption refrigeration technique, the working pair playsa vital role for optimal performance of the system. Thus, there aresome working pairs: silica gel/water; activated-carbon/methanol;activated-carbon/ammonia; zeolite/water; activated-carbon gran-ular and fibre adsorbent, etc.

The adsorption cycle is composed of two sorption chambers,an evaporator, and a condenser [2]. Water is vaporized under lowpressure and low temperature in the evaporator. Then the watervapour enters the sorption chamber where the solid sorbent, suchas silica gel, adsorbs the water vapour. In the other sorption cham-ber, the water vapour is released by regenerating the solid sorbentby applying the heat. Then the water vapour is condensed to liquidby the cooling water supplied from a cooling tower. By alternatingthe opening of the butterfly valves and the direction of the cool-ing and heating waters, the functions of two sorption chambers arereversed. In this way, the chilling water is obtained continuously.The adsorption cycle achieves a COP of 0.3–0.7, depending upon thedriving heat temperature of 60–95 ◦C [66].

Adsorption refrigeration technology has been used for manyspecific applications, such as purification, separation and thermalrefrigeration technologies.

6.2.4. Solar adsorption cooling systemsSolar energy can easily be used in the adsorption cooling sys-

tems. The performance of the solar adsorption cooling systems wasreported by several researchers.

Tchernev [67], Pons and Guilleminot [68], and Grenier et al.[69] reported the COP values of 0.10–0.12 with the solar poweredadsorption systems using zeolite/water, and Critoph [70] reportedthe COP value of 0.05 using activated-carbon/ammonia.

Wang et al. [71] developed a prototype of solar adsorption cool-ing system with activated-carbon/water. They concluded that theprototype system with a 2 m2 solar collector was capable to making60 kg of hot-water at 90 ◦C and producing 10 kg ice per day.

Henning and Glaser [72] conceived a pilot adsorption coolingsystem, in which the solar heat produced by vacuum tube collectorswith a surface area of 170 m2 was utilized to power the systemusing silica gel/water. The reported COP varied between 0.2 and0.3.

Luo et al. [73] used a solar adsorption cooling system for lowtemperature grain storage with silica gel/water. They reported aCOP value ranging from 0.096 to 0.13.

Sumathy et al. [74] provided literature reviews of the solaradsorption cooling technologies using various adsorption pairs andtheir performances. Table 5 summarizes the performance of thesolar adsorption cooling systems using various adsorption pairs.

The dominating technology in the European market of solarrefrigerating installations is still absorption chillers. However,some newly developing trends currently observed are directed

towards reducing the use of absorption chillers [17].

One reason for this situation is the possibility of taking advan-tage of alternative systems, such as adsorption, when the hot wateris below 90 ◦C [75]. However, the results of the solar adsorption

I. Sarbu, C. Sebarchievici / Energy and Buildings 67 (2013) 286–297 295

Table 4The characteristics of working fluids found from various absorption cooling technologies.

Absorption coolingsystems

Working fluids Results/reference

Single-effect H2O/LiBr, NH3/H2O

- A rectifier is needed to purify the refrigerant if the pair is volatile/[50]- Approximately 60% more COP can be achieved by using a solution heat exchanger/[51]- A system capacity of 70 kW can be achieved by using a vacuum tubular collector (108 m2) withflat plate collectors 9124 m2)/[8]- COP can be increased by 15% using a partitioned hot-water tank with a flat plate collector(38 m2) and chillers (4.7 kW)/[53]

Half-effect H2O/LiBr- Within the optimum temperature range of 65–70 ◦C, the COP = 0.36 and the evaporationtemperature is −7 ◦C/[60]- The pair is capable of providing the same COP as a conventional cooling system with reducingthe cost by half/[61]- The system has 22% lower exergetic efficiency compared to the single-effect system/[62]

Double-effect H2O/LiBr- The system has almost double (0.96) the COP compared to the single-effect system/[44]- The double-effect chillers with trough collectors show the maximum potential savings (86%)/[64]

Hybrid Combination of mentionedpairs

- The types of systems are widely implemented for the cooling of larger places, such as offices,markets or auditorium

Table 5Performance of solar adsorption cooling system.

Working fluids System COP Solar collector System conditions Reference

Activated carbon/methanol 0.10–0.12 Flat plate (A = 6 m2) te = −3 ◦C, tc = 25 ◦C, tg = 110 ◦C [68]Activated carbon/methanol 0.10–0.12 Flat plate (A = 6 m2) te = −6 ◦C, tg = 70–78 ◦C [74]Zeolite/H2O 0.11 Flat plate (A = 20 m2) te = 1 ◦C, tc = 30 ◦C, tg = 118 ◦C [69]Zeolite/H2O 0.10–0.12 Flat plate (A = 1.5 m2) – [67,68]Activated carbon/NH3 0.05 Flat plate (A = 1 m2) – [70]Activated carbon/H2O 0.07 Flat plate (A = 2 m2) – [71]Silica gel/H2O 0.20–0.30 Vacuum tube (A = 170 m2) – [72]Silica gel/H2O 0.10–0.13 – – [73]

Table 6Overview of thermally activated cooling systems.

Specification Process type

Open Closed Thermo-mechanical

System Liquid desiccant Solid desiccant Absorption cycle Adsorption cycle EjectorSorbent type Liquid Solid Liquid Solid –Working fluid (refrigerant/sorbent) H2O/CaCl2, H2O/LiCl H2O/silica gel, H2O/LiCl,

celluloseH2O/LiBr, NH3/H2O H2O/silica gel Steam

COP 0.74 0.510.50–0.73 (single-stage)

0.59 0.85<1.3 (two-stage)

EER [Btu/(Wh)] 2.53 1.741.71–2.49 (single-stage)

2.01 2.90

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Operating temperature 67 ◦C 45–95 ◦C

ooling systems show that its performance is yet lower than thatf the absorption system (Table 6) and needs improvement.

. Comparison of various solar refrigeration technologies

Balaras et al. [76] provided an overview of solar air-conditioningn Europe. In this purpose, they collected information on 54 solarowered cooling projects conducted in various locations in Europe.hey reported the thermal COP of different solar refrigeration tech-ologies, as shown in Table 6. They concluded that the single-effectbsorption systems haves a COP in the range of 0.50–0.73, adsorp-ion systems haves a lower thermal COP of 0.59, a liquid desiccantystem have a COP of 0.51, and a steam jet system have a rela-ively high COP of 0.85. Regarding the operating temperature ofhe systems, absorption systems operated at 60–165 ◦C, adsorptionystems operated at 53–82 ◦C, a liquid desiccant system operatedt 67 ◦C, and a steam jet system operated at 118 ◦C. For most of

hese systems operated below 100 ◦C, the flat plate solar collectorsould be used, while concentrating solar collectors had to be usedor driving temperatures higher than 100 ◦C. They also comparedhe annual EER, which is defined as the ratio of the annual cold

<4.44 (two-stage)60–110 ◦C (single-stage)

53–82 ◦C 118 ◦C130–165 ◦C (two-stage)

production and the annual heat input, both expressed in Btu/(Wh).The average annual EER was 1.98 for all systems investigated. TheH2O/LiBr absorption systems haves the best annual performance,while the adsorption systems haves low annual performance. Thisresult reflects the fact that 70% of the systems employed an absorp-tion technology, and 75% of the solar assisted absorption systemsused H2O/LiBr as their working fluid.

Grossman [48] claimed that the liquid sorption systems can uselower temperature heat sources (such as flat plate solar collectors)rather than the closed absorption cycle so that they have a potentialof reducing the cost of the solar part of the system.

8. Conclusions

Renewable energy sources have been of considerable interestbecause of their promising advantages. As the world population

is projected to increase and the supply of the fuel is projected todecrease, the increased supply of the renewable energy for thepost-fossil fuel period is inevitable. Since the cooling demand hasbeen increased associated with the recent climate change, cooling

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96 I. Sarbu, C. Sebarchievici / Ener

echnologies based on solar energy are promising technologies forhe future.

Along with photovoltaic systems, thermally activated cool-ng systems are being used all over the world for domestic andndustrial cooling purposes. Solar thermal cooling systems are

ore suitable than conventional refrigeration systems becauseollution-free working fluids (instead of chlorofluorocarbons) aresed as refrigerants. Solar cooling systems can be used, either astand-alone systems or with conventional air-conditioning sys-ems, to improve the indoor air quality of all building typesresidential buildings, offices, schools, hotels, hospitals, and lab-ratories).

In this paper, an extensive review of the technologies relatedo the better utilization of solar energy for the production of coolnergy is presented. The liquid desiccant system has a higher ther-al COP than the solid desiccant system. The adsorption cycle

eeds a lower heat source temperature than the absorption cycle.he ejector system has a higher COP, but needs a higher heat sourceemperature than other systems. Based on the coefficient of per-ormance, the liquid desiccant system is preferred to the solidesiccant system and the absorption cooling systems are preferredo the adsorption cooling systems, the higher temperature issuesan be easily handled with solar adsorption systems. Moreover,olar hybrid cooling systems can provide higher capacity and bet-er thermal COPs by eliminating some of the problems encounteredith individual working pairs.

The next few years will be the most decisive for the successf solar cooling systems that depend on the encouragement andromotional schemes offered by the policymakers, and the effortsndertaken by the manufacturers to improve the cost efficiencys well in developing better technologies. Also, a search for neworking fluids that are environmentally friendly and require low

perating temperatures is advised. Finally, research on the integra-ion and control of various energy conversion systems for multipleses (cooling, heating, water heating, and power generation) mayroduce synergic efficiency enhancement.

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