Review of solar refrigeration and cooling systems

Review of solar refrigeration and cooling systems

Energy and Buildings 67 (2013) 286–297 Contents lists available at ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locate/enbu...

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Energy and Buildings 67 (2013) 286–297

Contents lists available at ScienceDirect

Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

Review

Review of solar refrigeration and cooling systems Ioan Sarbu ∗ , Calin Sebarchievici Department of Building Services Engineering, “Politehnica” University of Timisoara, Piata Bisericii 4A, 300233 Timisoara, Romania

a r t i c l e

i n f o

Article history: Received 22 May 2013 Received in revised form 30 July 2013 Accepted 14 August 2013 Keywords: Renewable energy Solar refrigeration technology PV system Thermo-mechanical cooling Desiccant solar system Absorption cooling Adsorption cooling

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 and environmental issues. This paper provides a detailed review of different solar refrigeration and cooling methods. There are presented theoretical basis and practical applications for cooling systems within various working fluids assisted by solar energy and their recent advances. Thermally powered refrigeration technologies are classified into two categories: sorption technology (open systems or closed systems) and thermo-mechanical technology (ejector system). Solid and liquid desiccant cycles represent the open system. The liquid desiccant system has a higher thermal coefficient of performance (COP) than the solid desiccant system. Absorption and adsorption technologies represent the closed system. The adsorption cooling typically needs lower heat source temperatures than the absorption cooling. Based on COP, the absorption systems are preferred to the adsorption systems, the higher temperature issues can be easily handled 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. The study also refers to solar hybrid cooling systems with heterogeneous composite pairs, to a comparison of various solar cooling systems, and to some use suggestions of these systems. © 2013 Elsevier B.V. All rights reserved.

Contents 1.

2. 3. 4. 5. 6.

7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Renewable energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Solar energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar refrigeration technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar photovoltaic cooling systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar thermo-electrical cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar thermo-mechanical cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar thermal cooling techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Open sorption systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1. Liquid desiccant system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2. Solid desiccant system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3. Desiccant solar cooling system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Closed sorption systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. Absorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Solar absorption cooling systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Adsorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4. Solar adsorption cooling systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of various solar refrigeration technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +40 256403991; fax: +40 256403987. E-mail address: [email protected] (I. Sarbu). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.08.022

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1. Introduction Energy security is the ability of a nation to deliver the energy resources needed to ensure its welfare and implies secure supply and stable prices. Energy is vital for progress and development of a nation’s economy. The economic growth and technological advancement of every country depends on it [1] and the amount of available energy reflects that country’s quality of life. Economy, population and per capita energy consumption have caused the increase in demand for energy during the last few decades. Fossil fuels continue to supply much of the energy used worldwide, and oil remains the primary energy sources. Therefore, fossil fuels are the major contributor to global warming. Along with the global warming impacts and climate changes, the demands for airconditioning and refrigeration have increased. Encouraged by the successful worldwide effort to protect the ozone layer, scientists and engineers have been committed to minimize and reverse the harming environmental effects of global warming. Global warming occurs when carbon dioxide, released mostly from the burning of fossil fuels (oil, natural gas, and coal) and other gases, such as methane, nitrous oxide, ozone, chlorofluorocarbons (CFCs), hydro-chlorofluorocarbons (HCFCs) and water vapour, accumulate in the lower atmosphere. As results of the rapid growth in world population and the economy total world energy consumption is projected to increase by 71% from 2003 to 2030 [2]. The awareness of global warming has been intensified in recent times and has reinvigorated the search for energy sources that are independent of fossil fuels and contribute less to global warming. The Vienna Convention for the Protection of the Ozone Layer (1985), the Kyoto Protocol on Global Warming (1998) and the five amendments of the Montreal Protocol (1987) all discussed the reduction of CFCs to protect the ozonosphere, but the situation continues to decline. The European Commission (EC) Regulation 2037/2000, implemented on 1 October 2000, works to control and schedule all the ozone depleting materials; all HCFCs will be prohibited by 2015 [3,4]. The European strategy to decrease the energy dependence rests on two objectives: the diversification of the various sources of supply and policies to control consumption. The key to diversification is renewable energy sources (RES), because they have significant potential to contribute to a sustainable development [5]. 1.1. Renewable energy The term “renewable energy” refers to energy that is produced from natural resource having the characteristics of inexhaustibility over time and natural renewability. Renewable energy sources include wind, solar, geothermal, biomass and hydro energies [6]. An efficient utilization of renewable resources has a significant potential in both stimulating the economy and reducing pollution. Thus, many governments started to implement various policies that support renewable generation. One of the key components of any renewable energy policy is setting of renewable energy targets [7]. There have been numerous efforts undertaken by developed countries to implement different renewable energy technologies. The use of wind energy has increased over the last few years [8]. For example, the Netherlands, Germany, India and Malaysia are using wind turbines for producing electricity [9]. In north-western Iran, mineral materials are used for the production of geothermal energy and in Iceland, seventy percent (70%) or their factories utilize geothermal energy for industrial purposes [10]. Although Romania has a high potential of renewable energy sources, in 2010 the RES share in final energy consumption was 23.4%. Anyway, Romania ranked the second place in the European Union concerning the share of energy from renewable sources gross final consumption between 2006 and 2010 [11]

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Among the energy sources alternative to fossil fuels, renewable energy sources such as solar and wind are the more available. 1.2. Solar energy In recent years, scientists have increasingly paid more attention to solar energy. There is a sudden demand in the utilization of solar energy for various applications such as water heating, building heating/cooling, cooking, power generation and refrigeration [12]. Solar energy is the result of electromagnetic radiation released from the Sun by the thermonuclear reactions occurring inside its core. All of the energy resources on earth originate from the sun (directly or indirectly), except for nuclear, tidal and geothermal energy. The sun actually transmits a vast amount of solar energy to the surface of the earth [13]. The term “solar constant” signifies the radiation influx of solar energy. The mean value of solar constant is equal to 1368 W/m2 [14]. In Romania the annual solar energy flow ranges between 1000–1300 kWh/m2 /year in more than half of the country. This climate allows the operation of solar collectors from March until October, with conversion efficiency between 40% and 90% [15]. Thus, an important solar potential exist. Most countries are now accepting that solar energy has enormous potential because of its cleanliness, low price and natural availability. For example, it is being used commercially in solar power plants. Sweden has been operating a solar power plant since 2001. Romania’s experience in solar energy represents a competitive advantage for the future development of this area, the country being a pioneer in this field. Between 1970 and 1980 were installed around 800,000 m2 of solar collectors that placed the country third worldwide in the total surface of photovoltaic cells. Between 1984 and 1985 was achieved the peak of solar installations, but after 1990 unfavourable macroeconomic developments led to the abandonment of the production and investments in the solar energy field. Today about 10% of the former installed collector area is still in operation [16]. In recent years, many countries have been facing difficulties with the issue of refrigeration systems. Specifically, the demand of air conditioning for both commercial and residential buildings during the summer is ever-increasing [13]. There is a lack of electricity and storage in developing countries to accommodate high energy consumptive systems such as refrigeration and cooling. The solar cooling techniques can reduce the environmental impact and the energy consumption issues raised by conventional refrigeration and air-conditioning systems. Therefore, in this paper are presented theoretical basis and practical applications for cooling technologies within various working fluids assisted by solar energy and their recent advances. Also, a comparison of various solar cooling systems is performed and some suggestions about the use of these systems are given. 2. Solar refrigeration technology Solar refrigeration offers a wide variety of cooling techniques powered by solar collector-based thermally driven cycles and photovoltaic (PV)-based electrical cooling systems. Fig. 1 shows a schematic diagram of a solar thermal cooling system. The solar collection and storage system consists of a solar collector (SC) connected through pipes to the heat storage. Solar collectors transform solar radiation into heat and transfer that heat to the heat transfer fluid in the collector. The fluid is then stored in a thermal storage tank (ST) to be subsequently utilized for various applications. The thermal AC (air-conditioning) unit is run by the hot refrigerant coming from the storage tank, and the refrigerant circulates

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Fig. 1. Schematic of a solar thermal cooling system. Fig. 2. Global PV-based solar electricity production for four decades.

through the entire system. Since solar energy is time-dependent, the successful utilization of all these cooling systems is to a very large degree dependent on the thermal storage tank employed. The various stages of thermal storage integrated solar cooling systems are shown in Table 1 [17]. In comparison with conventional electrically driven compression systems, substantial primary energy savings can be expected from solar cooling, thus aiding in conserving energy and preserving the environment. Solar refrigeration technology engages a system where solar power is used for cooling purposes. Cooling can be achieved through four basic methods: solar PV cooling, solar thermoelectrical cooling, solar thermo-mechanical cooling, and solar thermal cooling. The first is a PV-based solar energy system, where solar energy is converted into electrical energy and used for refrigeration much like conventional methods [18]. The second one produce cool by thermoelectric processes [19,20]. The third one converts the thermal energy to mechanical energy, which is utilized to produce the refrigeration effect. The fourth method utilizes a solar thermal refrigeration system, where a solar collector directly heats the refrigerant through collector tubes instead of using solar electric power [13]. The performance of refrigeration systems is determined based on energy indicators of these systems. The COP (coefficient of performance) can be calculated as follows: COP =

Eu Ec

(1)

where Eu is the cooling usable energy; Ec is the consumed energy by system. Also, energy efficiency ratio (EES), in British thermal unit per Watt-hours (Btu/(Wh)) is defined by equation: EER = 3.413COP

(2)

where 3.413 is the transformation factor from Watt to Btu/h. Detailed discussion of each solar refrigeration technology follows. 3. Solar photovoltaic cooling systems A PV cell is basically a solid-state semiconductor device that converts light energy into electrical energy. To accommodate the huge demand for electricity, PV-based electricity generation has been rapidly increasing around the world alongside conventional power plants over the past two decades. Fig. 2 shows a comparative representation of the development of solar PV systems in different countries [21]. While the output of a PV cell is typically direct current (DC) electricity, most domestic and industrial electrical appliances use alternating current (AC). Therefore, a complete PV cooling system typically 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 (from the sun) into DC electrical energy. • The battery is used for storing DC voltages at a charging mode when sunlight is available and supplying DC electrical energy in a discharging mode in the absence of daylight. A battery charge regulator can be used to protect the battery form overcharging. • The inverter is an electrical circuit that converts the DC electrical power into AC and then delivers the electrical energy to the AC loads. • The vapour compression AC unit is actually a conventional cooling or refrigeration system that is run by the power received from the inverter.

Table 1 Stages and options in solar cooling techniques. Source

Conversion

Sun

Solar PV (electrical)

Solar thermal 1. Flat plate collector 2. Evacuated tube collector 3. Concentrated collector

Thermal storage (hot energy)

Production of cool energy

Thermal storage (cool energy)

1. Vapor compression Thermoelectric

1. Sensible 2. Latent 3. Thermo-chemical

1. Ejector 2. Desiccant Absorption (a) Single-effect (b) Half-effect (c) Double-effect (d) Triple-effect 4. Adsorption

Applications 1. Air conditioning (a) Office (b) Building (c) Hotel (d) Laboratory Process industries (a) Dairy (b) Pharmaceutical (c) Chemical Food preservation (a) Vegetables (b) Fruits (c) Meat and fish

1. Sensible 2. Latent 3. Thermo-chemical

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

The PV system can perform as a standalone system (Fig. 3), a hybrid system (working with an oil/hydro/gas power plant) or as a grid or utility intertie systems. Though the efficiency of PV modules can be increased by using inverters, their COP and efficiency are still not desirable. 4. Solar thermo-electrical cooling In solar electric cooling, power produced by the solar PV devices is supplied either to the Peltier cooling systems. It is possible to produce cool by thermoelectric processes, using the principle of producing electricity from solar energy through thermoelectric effect and the principle of producing cool by Peltier effect. It have been produced such thermoelectric refrigerators, with the principle diagram in Fig. 4. Thermoelectric generator consists of a small number of thermocouples that produce a low thermoelectric power but which can easily produce a high electric current. It has the advantage that can operate with a low level heat source and is therefore useful to convert solar energy into electricity. The thermoelectric refrigerator is also composed of a small number of thermocouples through which run the current produced by the generator. The combination of the two parts is compatible with use as thermoelectric materials of the semiconductors based on Bi2 Te3 [23]. Vella et al. [19] shown that a thermoelectric generator, which draws its heat from solar energy, is a particularly suitable source of electrical power for the operation of a thermoelectric refrigerator. They developed the theory of the combined thermoelectric generator and refrigerator and determined the ratio of the numbers of thermocouples needed for the two devices. A 4-couple thermoelectric generator has been used to power a single-couple refrigerator. Temperatures below 0 ◦ C have been achieved for a temperature difference 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, in which the electron gas serves as the working fluid. In recent years, concerns of environmental pollution due to the use of CFCs in conventional domestic refrigeration systems have encouraged increasing activities in research and development of domestic refrigerators using Peltier modules. Moreover, recent progress in thermoelectric and related fields have led to significant reductions in fabrication costs of Peltier modules and heat exchangers together with moderate improvements in the module performance. Although the COP of a Peltier module is lower than that of conventional compressor unit, efforts have been made to develop domestic thermoelectric cooling systems to exploit the advantages associated with this solid-state energy conversion technology [18]. Other applications of this technology are air conditioning and medical instruments. 5. Solar thermo-mechanical cooling In the thermo-mechanical solar cooling system, the thermal energy is converted to the mechanical energy. Then the mechanical energy is utilized to produce the refrigeration effect. The steam ejector system represents the thermo-mechanical cooling technology. Fig. 5 illustrates the steam ejector system integrated with a parabolic solar collector SC. The steam produced by the solar collector is passing through the steam jet ejector E. During this process, the evaporator pressure is reduced, and water is vaporized in the evaporator V by absorbing the heat from the cold water. When cooling is not needed, the steam turbines can be used to produce electricity. Most of the steam ejector system require steam at pressures in the range of 0.1–1.0 MPa, and temperatures in the range of 120–180 ◦ C [24]. However, Loehrke [25] proposed and demonstrated that the steam ejector system could be operated using low-temperature solar heat by reducing the operating pressure under atmospheric pressure. Khattab and Barakat [26] later proved this concept by developing a detailed mathematical model of the solar steam ejector cycles operating at low pressure and low temperature for the air-conditioning application. The working fluid used in a solar ejector cooling system lead to different performance depending on operating conditions. In order to compare the performance of different used working fluids, in Table 2 are presented the following values: tg – the generating temperature; tc – the condensation temperature achieved in condenser C (37 ◦ C for cooling with cooling tower, 30 ◦ C for cooling with cold water); pg – the pressure in the generator G (maximum pressure in the system); pe – the pressure in the evaporator V (minimum pressure in the system); COP – the theoretical coefficient of performance; ej – the ejector efficiency; COPr = ej ·COP – the real coefficient of performance; QSC – the heat needed to be supplied by solar collector in generator to achieve a cooling power

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

tg (◦ C) 85

H2 O 130 85 R-11 130 R-21

85

Propane

85

Butane

85

NH3

85 130

tc (◦ C)

pg (kPa)

pe (kPa)

COP

37 30 37 30 37 30 37 30 37 30 37 30 37 30 37 37

392.2 392.2 475.5 475.5 460.8 460.8 784.0 784.0 754.9 754.9 2745 2745 882.3 882.3 2157 2157

0.88 0.88 0.88 0.88 50.0 50.1 50.1 50.1 90.2 90.2 539 539 147 147 520 520

0.913 0.184 1.471 0.217 2.076 0.226 2.887 0.223 0.936 0.196 1.947 0.226 1.708 0.226 3.121 0.216 0.790 0.172 1.162 0.209 0.496 0.078 1.038 0.198 0.423 0.091 0.666 0.170 Not possible solution 0.348 0.016

of 1.16 × 104 W; ASC – the solar collector area, assuming a solar flux of 0.8 kW/m2 and capture efficiency of 0.5, for achieve a cooling capacity of 1.16 × 104 W. Considering one flat-plate collector, the possible temperature for which can easily provide solar heat is tg = 85 ◦ C, and for a parabolic-cylinder concentrating collector can be adopted tg = 130 ◦ C. Analyzing the COPr values from this table results as the competitive working fluids: water and Freon, among which best is R-11. Water and R-11 have comparable COPr , but operating pressures in the system are very different. Thus, for the use of flat plate collectors (tg = 85 ◦ C), steam ejector cooling system works completely in depression (pe and pg is less than atmospheric pressure). So if water is used as refrigerant leakage problems are to be solved to avoid air entering the system. Various experimental studies [27–29] have examinated the effect of the operation conditions such as the generator temperature, evaporator temperature and condenser temperature, the geometrical conditions, the system conditions such as refrigerant and collector selections on the performance of the system. Other researchers [30,31] have presented numerical methods of simulating the ejector and studied the performance of system. Nehad [27,28] compared the theoretical performance of the ejector system working with R-717, R-11, R-12, R-113 and R-114. Then he chose R-113 as a refrigerant for the experiment since it has a higher COPr , a reasonable operating pressure, and is non-toxic. Eames et al. [29] reported that the measured COPr of the singlestage ejector system using H2 O as its working fluid ranged from 0.178 to 0.586 at a generating temperature tg of 120–140 ◦ C, an evaporation temperature te of 5–10 ◦ C, and a condensation temperature tc of 26.5–36.3 ◦ C. Vidal et al. [30] analyzed the solar ejector system using R-141b as its refrigerant by using the TRNSYS and EES simulation software. The system was designed to deliver 10.5 kW of cooling with 80 m2 of flat-plate collector tilted 22◦ from the horizontal and a 4 m3 hot-water storage tank. They reported the maximum COPr of 0.22 at tg = 80 ◦ C, te = 8 ◦ C, and tc = 32 ◦ C. They also concluded that an efficient ejector system could only work in a region with decent solar radiation and where a sufficiently low condenser temperature could be kept. Grazzini and Rocchetti [32] theoretically investigated the performance of the two-stage ejector system. They reported that the COPr of the two-stage ejector system ranged from 0.13–0.53 at tg = 110–120 ◦ C, te = 7–12 ◦ C, and tc = 30–40 ◦ C. Along with other researches results [33,34] show that the low ejection efficiency leads to values of COPr for solar ejector cooling systems smaller than in the case of solar absorption cooling systems. The performance of the ejector system depends on the mass flow rate ratio through the motive nozzle and the suction nozzle.

ej

COPr

QSC (W)

ASC (m2 )

0.168 0.319 0.469 0.645 0.183 0.440 0.386 0.675 0.135 0.242 0.039 0.209 0.038 0.113

69,130 36,396 24,717 17,979 63,428 26,356 26,911 17,175 85,630 47,866 298,969 56,585 302,475 102,284

173 91 62 45 159 66 67 43 215 120 750 142 758 256

0.005

2,130,150

5338

The ejector systems are mostly used in air conditioning applications, but they can be used in chemical and metallurgical industry for air cooling in areas with higher heat dissipation. 6. Solar thermal cooling techniques Solar thermal cooling (Fig. 1) is becoming more popular because a thermal solar collector directly converts light into heat. For example, Otanicar et al. [22] described a thermal system that is capable of absorbing more than 95% of incident solar radiation, depending on the medium. Sorption technology is utilized in thermal cooling techniques. The cooling effect is obtained from the chemical or physical changes between the sorbent and the refrigerant. Sorption technology can be classified either as open sorption systems or closed sorption systems [2]. 6.1. Open sorption systems Open system refers to solid or liquid desiccant systems that are used for either dehumidification or humidification. Basically, desiccant systems transfer moisture from one airstream to another by using two processes. In the sorption process the desiccant system transfer moisture from the air into a desiccant material by using the difference in the water vapour pressure of the humid air and the desiccant. If the desiccant material is dry and cold, then its surface vapour pressure is lower than that of the moist air, and moisture in the air is attracted and absorbed to the desiccant material. In desorption (regeneration) process, the captured moisture is released to the airstream by increasing the desiccant temperature. After regeneration, the desiccant material is cooled down by the cold airstream. Then it is ready to absorb the moisture again. When these processes are cycled, the desiccant system can transfer the moisture continuously by changing the desiccant surface vapour pressures, as illustrated in Fig. 6. To drive this cycle, thermal energy is needed during the desorption process. The difference between solid and liquid desiccants is their reaction to moisture. 6.1.1. Liquid desiccant system Materials 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 absorbs the moisture from the process air. Afterward, the liquid falls to a sump, is pumped, and is sprayed back into the air. While absorbing moisture, the desiccant becomes warmer and the partial vapour pressure is increased. The concentration of desiccant decreases and the water content increases. A small amount of liquid desiccant is

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Several studies performed on the description and operation of desiccant cooling systems by different researchers [36,37]. Systems that use rotary desiccant wheel to dehumidify the air are the most popular desiccant cooling systems and studied by different researchers [38,39]. They showed that desiccant cooling systems are viable alternative to vapour compression systems.

Fig. 6. Process of moisture transfer by desiccant.

taken continuously from the sump to the regenerator to remove the water that is picked up. The desiccant is also sprayed into the air. The desiccant is heated before it contacts the air so that the partial pressure of the desiccant is higher than that of the air. Therefore, the moisture is transported to the regeneration air (process 2–3 in Fig. 6). The regeneration air leaves the regenerator in a hot and humid condition. As the liquid desiccant solution returns to the sump of the conditioner, it is drier more concentrated, and still at high vapour pressure and temperature. Before being sprayed into the air, the liquid desiccant is cooled to the required temperature by a cooling tower or chiller (process 3–1 in Fig. 6). The favourable feature of the liquid desiccant system is the fact that the liquid desiccants can be regenerated at temperatures below 80 ◦ C so that low temperature heat sources can be utilized. In efforts to reduce a building’s energy consumption, designers have successfully integrated liquid desiccant equipment with standard absorption chillers [35]. In a more general approach, the absorption chiller is modified so that rejected heat from its absorber can be used to help regenerate liquid desiccants. 6.1.2. Solid desiccant system The solid desiccant system is constructed by placing a thin layer of desiccant material, such as silica gel, on a support structure [2]. The desiccant wheel rotates slowly between the process and the regeneration airstreams. It is divided into two sections for the regeneration air and the process air. Process air flows through the first part of the wheel, and the moisture is removed due to the lower partial vapour pressure in the desiccant material. To regenerate the desiccant, the wheel passes the hot reactivation air, and the process can start again. For solid desiccant materials, the increase of drybulb temperature of the process air is a result of the adsorption heat. This consists of the vaporization latent heat of the adsorbed moisture and the heat of wetting. The heat of wetting is approximately 20% of the vaporization heat [35]. Both liquid and solid desiccants may be used in equipment designed for drying air and gases at atmospheric or elevated pressures (schools, theatres, restaurants, hospitals). Regardless of pressure levels, basic principles remain the same, and only the desiccant towers or chambers require special design consideration. Desiccant capacity and actual dew-point performance depend on the specific equipment used, characteristics of the various desiccants, initial temperature and moisture content of the gas to be dried, reactivation methods, etc. Factory-assembled units are available up to a capacity of about 38 m3 /s.

6.1.3. Desiccant solar cooling system Desiccant solar systems reduce the moisture of the ambient air by utilizing thermal energy from the solar collector to regenerate desiccants. Then the dry air is cooled through indirect and/or direct evaporative stages, as shown in Fig. 7. Since Lof [40] investigated liquid desiccant solar cooling, most of the research on liquid desiccant solar cooling began in the early 1990s. Moreover, the latest developments are focused on liquid sorption applications since the liquid sorption materials have advantages of higher air dehumidification at the same driving temperature, as well as the possibility of high energy storage by means of hygroscopic solutions. Ameel et al. [41] compared the performance of various absorbents, including LiCl, CaCl, and LiBr. They concluded that LiBr outperformed the other absorbents. Gommed and Grossman [42] developed the prototype of the liquid desiccant cooling system assisted by the flat solar collectors using LiCl/H2 O as its working fluid. Through the parametric study, they demonstrated that conditions of the ambient air are the major parameters considerably affecting the dehumidification process in the liquid desiccant system. They reported that the system provided 16 kW of dehumidification capacity with a thermal COP of 0.8. Henning et al. [43] installed a solar-assisted desiccant cooling system with a 20 m2 flat-plate solar collector and a 2 m3 hot-water storage tank. They reported that a solar fraction of the cooling between the solar heat and auxiliary heat provided was 76%, with an overall collector efficiency of 54% and a cooling COP of 0.6 during typical summer conditions. In addition, they proposed a combination of a solar-assisted solid desiccant cooling system with a conventional vapour compression chiller for warm and humid climates, 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 refrigeration Absorption is the process in which a substance assimilates from one state into a different state. These two states create a strong attraction to make a strong solution or mixture. The absorption system is one of the oldest refrigeration technologies. The first evolution of an absorption system began in the 1700s. It was observed that in the presence of H2 SO4 (sulphuric acid), ice can be made by evaporating pure H2 O within an evacuated container. In 1859, a French engineer named Ferdinand Carre designed an installation that used a working fluid pair of ammonia/water (NH3 /H2 O). In 1950, a new system was introduced with a water/lithium bromide (H2 O/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 compressing the refrigerant vapour. The evaporator draws the vapour refrigerant by absorption into the absorber. The extra thermal energy separates the refrigerant vapour from the rich-solution. The condenser condenses the refrigerant by rejecting the heat and then the cooled liquid refrigerant is expanded by the evaporator, and the cycle is completed. The refrigerant side of the absorption system essentially works under the same principle as the vapour compression system.

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Fig. 7. Schematic of desiccant solar cooling system.

However, the mechanical compressor used in the vapour compression cycle is replaced by the thermal compressor in the absorption system. The thermal compressor consists of the absorber, the generator, the solution pump, and the expansion valve. The attractive feature of the absorption system is that any types of heat source, including solar heat and waste heat, can be utilized in the desorber. Typical refrigerant/absorbent pairs used in the absorption system are NH3 /H2 O and H2 O/LiBr. The thermodynamic characteristics of these have been described in various studies and experiments [45,46]. Even though NH3 /H2 O and H2 O/LiBr pairs have been used all over the world, researchers are still looking for new pairs [47]. Based on the thermodynamic cycle of operation and solution regeneration, the absorption systems can be divided into three categories: single-, half-, and multi-effect (double-effect and triple-effect) solar absorption cycles. The single-effect and halfeffect chillers require relatively lower hot-water temperatures with respect to multi-effect systems [8]. Best absorption refrigeration technology applications are heatactivated refrigerators, gas-fired residential air conditioners, and large industrial refrigeration plants. Grossman [48] provided typical performances of the single- and multi-effect absorption system, as shown in Table 3. Typical cooling COPs of the single-effect, double-effect, and triple-effect absorption systems are 0.7, 1.2, and 1.7, respectively. The operation of the H2 O/LiBr-based absorption system is limited in the evaporating temperature and the absorber temperature, due to the freezing of the water and the solidification of the LiBr-rich solution, respectively. The operation of the NH3 /H2 O-based absorption system is not limited in either the evaporating temperature or the absorption temperature. However, ammonia is toxic and its usage is limited to the large capacity system.

absorption cycle with a H2 O/LiBr pair, where a solar flat plate collector or an evacuated tubular collector with hot-water is used to implement these systems [49]. Single-effect absorption cooling system is based on the basic absorption cycle that contains a single absorber and generator as shown in Fig. 8. In the generator G, the refrigerant is separated from the absorbent by the heat provided by the solar collector. The vapour-refrigerant are condensed in condenser C, then laminated in expansion valve EV1 and evaporated at low pressure and temperature in the evaporator V. The cooled refrigerant is absorbed in the absorber Ab by weak-solution that returns from generator after the lamination in the expansion valve EV2 . The rich-mixture created in absorber is pumped by pump P and returned in G. The usual a solution heat exchanger (SHX) can be used to improve cycle efficiency [50]. A 60% higher COP can be achieved by using the SHX [51]. The absorption being exothermic, the absorber is chilled with cooling water. For low temperature heat sources results unacceptably low values of degassing zone and vapour-refrigerant release in generator is slow down and the operation of the system becomes unstable or impossible. To improve the COP and for use lower temperatures in generator, can be used solar resorption cooling system (Fig. 9) [23]. In this case, in generator, the refrigerant is also separated from the absorbent by the heat provided by the solar collector, but vapour-refrigerant are reabsorbed by a weak-solution from resorber Rb and the system operates similarly to the above mentioned cycle. System pressures may be allowed as close to atmospheric

6.2.2. Solar absorption cooling systems Solar absorption systems utilize the thermal energy from a solar collector to separate a refrigerant from the refrigerant/absorbent mixture. As shown in Table 3, the flat plate solar collector can be used for the single-effect cycle. However, the multi-effect absorption cycles require high temperatures above 85 ◦ C, which can be delivered by the evacuated tube or concentrating-type collectors. 6.2.2.1. Single-effect solar absorption cycle. Recent statistics show that most absorption cooling systems are made using single-effect

Fig. 8. Schematic of solar absorption cooling system.

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Table 3 Typical performance of absorption cycles. No.

Absorption system

COP

EER (Btu/(Wh))

Heat source temperature (◦ C)

Type of solar collectors matched

1 2 3

Single-effect Double-effect Triple-effect

0.7 1.2 1.7

2.39 4.10 5.80

85 130 220

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

pressure as possible, which simplifies sealing problems, pumps built and reduce temperature in generator. A single-effect absorption cooling system is simpler than other when the design depends on the types of working fluids. The system shoes better performance with non-volatile absorbents as H2 O/LiBr. If volatile working pair such as NH3 /H2 O is used, then an extra rectifier should be used before the condenser to provide pure refrigerant [44]. A low cost non-concentrating flat plate or evacuated tube solar collector is sufficient to obtain the required temperature for the generator. Though economical, its COP is lower. For obtaining a higher COP, multi-effect systems such as double-effect and triple-effect absorption chillers are used, which are run by steam produced from concentrating solar collectors. Nakahara et al. [52] developed a single-effect H2 O/LiBr absorption chiller of 7 kW nominal cooling capacity, assisted by a 32.2 m2 array of flat plate solar collectors. In their system, thermal energy produced by the solar collector was stored in a 2.5 m3 hot-water storage tank. Their experimental results during the summer period showed that the cooling capacity was 6.5 kW. The measured COP of the absorption system was in range of 0.4–0.8 at the generator temperature of 70 ◦ C to 100 ◦ C. Li and Sumathy [53] observed a H2 O/LiBr absorption system with a partitioned hot-water storage tank. The system consisted of a 38 m2 flat plate collector and a 4.7 kW absorption chiller. They concluded that the system exhibited 15% more COP (approximately 0.7) than a conventional whole-tank mode system. Another investigation on a H2 O/LiBr absorption system consisting of 49.9 m2 of flat plate collector was performed by Syed et al. [54]. The system performs cooling within generator temperatures of 65–90 ◦ C, maintaining a capacity of 35 kW. They calculated three different COPs and achieved an average collector efficiency of approximately 50%. In an intermittent single-stage NH3 /H2 O absorption system, the solution pump is eliminated and the density difference is utilized for the NH3 /water circulation. In this way, the auxiliary power is saved. Since Trombe and Forx [55] suggested using an intermittent single-stage NH3 /H2 O absorption system assisted by the solar energy for ice production, several researchers [56,57] explored the feasibility of such systems To improve the unsteady nature of the solar heat from the solar collector to the absorption system, Chen and Hihara [58] proposed

Fig. 9. Schematic of solar resorption cooling system.

a new type of absorption cycle that was co-driven both by solar energy and electricity. In their proposed system, total energy delivered to the generator could be controlled by adjusting the mass flow rate through the compressor. Their numerical simulation model results showed the steady COP value of 0.8 for the new cycle, which was higher than the conventional cycle. Chinnappa et al. [59] proposed a conventional vapour compression AC system cascaded with a solar-assisted NH3 /H2 O absorption system. They concluded that the hybrid system achieved of a COP = 5, which is higher than that of the vapour compression cycle at 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 the half-effect absorption cycle is the running capability at lower temperature compared to others. The name “half-effect” arises from the COP, which is almost half that of the single-effect cycle [50]. Arivazhagan et al. [60] performed an experiment with a twostage half-effect absorption system using the working pair of R134a/DMAC. They were able to attain an evaporation temperature of −7 ◦ C with the generator temperature varying from 55 ◦ C to 75 ◦ 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 H2 O/LiBr chiller for cooling purposes in south China. They found a cooling capacity of 100 kW through the integration of a solar cooling system with these chillers. They concluded that the system had a nearly equivalent COP as the conventional cooling system, but at a 50% reduced cost. Izquierdo et al. [62] designed a solar double-stage absorption plant with H2 O/LiBr, which contained flat plate collectors to feed the generator. They reported that within a condensation temperature of 50 ◦ C, the COP was 0.38 while providing a generation temperature of 80 ◦ C. They also performed an exergetic analysis of 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 absorption cooling technology was launched in 1956 for developing the system performance within a heat source at higher temperatures [63]. Fig. 10 illustrates a double-effect absorption system with a H2 O/LiBr pair. The cycle begins with generator G-I providing heat to generator G-II. The condenser C rejects the heat and passes the working fluid towards the evaporator V; within this step, the required

Fig. 10. Schematic of solar assisted double-effect H2 O/LiBr absorption system.

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refrigeration occurs. Then, the fluids pass through the heat exchangers HX-I and HX-II from the absorber Ab to G-I by means of a pump P. Trough this process, HX-II can pass the fluids to G-II and then G-II passes to HX-I. The complete cycle follows three different pressure levels: high, medium and low. Two single-effect systems effectively form a double-effect absorption cooling system. Therefore, the COP of a double-effect system is almost twice that of the single-effect absorption system. For example, Srikhirin et al. conducted an analysis showing that the COP of a double-effect system is 0.96, whereas the single-effect system has a COP of only 0.6 In the past few years, the COP of doubleeffect absorption systems has reached values of 1.1–1.2 by using gas-fired absorption technology [8]. Tierney [64] performed a comparative study among four different systems with a collector of 230 m2 and concluded that the double-effect chiller with a trough collector had the highest potential savings (86%) among the four systems to handle the demand for a 50 kW load. 6.2.2.4. Triple-effect absorption cycle. Triple-effect absorption cooling can be classified as single-loop or dual-loop cycles. Single-loop triple-effect cycles are basically double-effect cycles with an additional generator and condenser. The resulting system with three generators and three condensers operates similarly to the doubleeffect system. Primary heat concentrates absorbent solution in a first-stage generator at about 200–23 ◦ C. A fluid pair other than H2 O/LiBr must be used for the high temperature cycle. The refrigerant vapour produced is then used to concentrate additional absorbent solution in a second-stage generator at about 150 ◦ C. Finally, the refrigerant vapour produced in the second-stage generator concentrates additional absorbent solution in a third-stage generator at about 93 ◦ C. The usual solution heat exchangers can be used to improve cycle efficiency. Theoretically, these triple-effect cycles can obtain COPs of about 1.7 [35]. A double-loop triple-effect cycle consists of two cascaded singleeffect cycles. One cycle operates at normal single-effect operating temperatures and the other at higher temperatures. The smaller high temperature topping cycle has a generator temperature of about 200–230 ◦ C. A fluid pair other than H2 O/LiBr must be used for the high temperature cycle. Heat is rejected from the high temperature cycle at 93 ◦ C and is used as the energy input for the conventional single-effect bottoming cycle. Theoretically, this triple-effect cycle can obtain an overall COP of about 1.8 [35]. Multi-effect cycles are costlier but energy efficient. Doubleand triple-effect chillers employ an additional generator and heat exchanger to liberate the refrigerant from the absorbent solution with lesser heat input. The available solar intensity, cooling capacity requirements, overall performance and cost, determines the selection of a particular configuration. Li and Sumathy [12] stressed the importance of the generator inlet temperature, chiller, collector choice, system design and arrangement, in the design and fabrication of a solar powered airconditioning system. Srikhirin et al. [44] have discussed a number of absorption refrigeration systems and related research options. In this section, literatures pertaining to the improvement of absorption cooling systems, theoretical and experimental studies on solar absorption cooling, and finally, on subjects with a thrust on thermal storage integrated cooling systems are reviewed. 6.2.2.5. Hybrid solar absorption cooling systems. A hybrid cooling concept arose due to integrate different pairs or systems for obtaining better cooling performance. Hybrid solar absorption cooling system refers to the integration of three individual cooling technologies: radiant cooling, desiccant cooling and absorption cooling [65]. Table 4 summarizes the above mentioned absorption cooling systems [8].

Solar absorption cooling systems are used in air conditioning applications, for food preservation and in ice production. 6.2.3. Adsorption refrigeration Adsorption technology was first used in refrigeration and heat pumps in the early 1990s. The adsorption process differs from the absorption process in that absorption is a volumetric phenomenon, whereas adsorption is a surface phenomenon. The primary component of an adsorption system is a solid porous surface with a large surface area and a large adsorptive capacity. Initially, this surface remains unsaturated. When a vapour molecule contacts the surface, an interaction occurs between the surface and molecules, and the molecules are adsorbed on to the surface [6]. In an adsorption refrigeration technique, the working pair plays a vital role for optimal performance of the system. Thus, there are some working pairs: silica gel/water; activated-carbon/methanol; activated-carbon/ammonia; zeolite/water; activated-carbon granular and fibre adsorbent, etc. The adsorption cycle is composed of two sorption chambers, an evaporator, and a condenser [2]. Water is vaporized under low pressure and low temperature in the evaporator. Then the water vapour enters the sorption chamber where the solid sorbent, such as silica gel, adsorbs the water vapour. In the other sorption chamber, the water vapour is released by regenerating the solid sorbent by applying the heat. Then the water vapour is condensed to liquid by the cooling water supplied from a cooling tower. By alternating the opening of the butterfly valves and the direction of the cooling and heating waters, the functions of two sorption chambers are reversed. In this way, the chilling water is obtained continuously. The adsorption cycle achieves a COP of 0.3–0.7, depending upon the driving heat temperature of 60–95 ◦ C [66]. Adsorption refrigeration technology has been used for many specific applications, such as purification, separation and thermal refrigeration technologies. 6.2.4. Solar adsorption cooling systems Solar energy can easily be used in the adsorption cooling systems. The performance of the solar adsorption cooling systems was reported 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 powered adsorption systems using zeolite/water, and Critoph [70] reported the COP value of 0.05 using activated-carbon/ammonia. Wang et al. [71] developed a prototype of solar adsorption cooling system with activated-carbon/water. They concluded that the prototype system with a 2 m2 solar collector was capable to making 60 kg of hot-water at 90 ◦ C and producing 10 kg ice per day. Henning and Glaser [72] conceived a pilot adsorption cooling system, in which the solar heat produced by vacuum tube collectors with a surface area of 170 m2 was utilized to power the system using silica gel/water. The reported COP varied between 0.2 and 0.3. Luo et al. [73] used a solar adsorption cooling system for low temperature grain storage with silica gel/water. They reported a COP value ranging from 0.096 to 0.13. Sumathy et al. [74] provided literature reviews of the solar adsorption cooling technologies using various adsorption pairs and their performances. Table 5 summarizes the performance of the solar adsorption cooling systems using various adsorption pairs. The dominating technology in the European market of solar refrigerating 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 advantage of alternative systems, such as adsorption, when the hot water is below 90 ◦ C [75]. However, the results of the solar adsorption

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

Working fluids

Single-effect

H2 O/LiBr, NH3 /H2 O

Half-effect

H2 O/LiBr

Double-effect

H2 O/LiBr

Hybrid

Combination of mentioned pairs

Results/reference - 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 ) with flat 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] - Within the optimum temperature range of 65–70 ◦ C, the COP = 0.36 and the evaporation temperature is −7 ◦ C/[60] - The pair is capable of providing the same COP as a conventional cooling system with reducing the cost by half/[61] - The system has 22% lower exergetic efficiency compared to the single-effect system/[62] - 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] - The types of systems are widely implemented for the cooling of larger places, such as offices, markets or auditorium

Table 5 Performance of solar adsorption cooling system. Working fluids

System COP

Solar collector

System conditions

Reference

Activated carbon/methanol Activated carbon/methanol Zeolite/H2 O Zeolite/H2 O Activated carbon/NH3 Activated carbon/H2 O Silica gel/H2 O Silica gel/H2 O

0.10–0.12 0.10–0.12 0.11 0.10–0.12 0.05 0.07 0.20–0.30 0.10–0.13

Flat plate (A = 6 m2 ) Flat plate (A = 6 m2 ) Flat plate (A = 20 m2 ) Flat plate (A = 1.5 m2 ) Flat plate (A = 1 m2 ) Flat plate (A = 2 m2 ) Vacuum tube (A = 170 m2 ) –

te = −3 ◦ C, tc = 25 ◦ C, tg = 110 ◦ C te = −6 ◦ C, tg = 70–78 ◦ C te = 1 ◦ C, tc = 30 ◦ C, tg = 118 ◦ C – – – – –

[68] [74] [69] [67,68] [70] [71] [72] [73]

Table 6 Overview of thermally activated cooling systems. Specification

Process type Open

Closed

System Sorbent type Working fluid (refrigerant/sorbent)

Liquid desiccant Liquid H2 O/CaCl2 , H2 O/LiCl

Solid desiccant Solid H2 O/silica gel, H2 O/LiCl, cellulose

COP

0.74

0.51

EER [Btu/(Wh)]

2.53

1.74

Operating temperature

67 ◦ C

45–95 ◦ C

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

7. Comparison of various solar refrigeration technologies Balaras et al. [76] provided an overview of solar air-conditioning in Europe. In this purpose, they collected information on 54 solar powered cooling projects conducted in various locations in Europe. They reported the thermal COP of different solar refrigeration technologies, as shown in Table 6. They concluded that the single-effect absorption systems haves a COP in the range of 0.50–0.73, adsorption systems haves a lower thermal COP of 0.59, a liquid desiccant system have a COP of 0.51, and a steam jet system have a relatively high COP of 0.85. Regarding the operating temperature of the systems, absorption systems operated at 60–165 ◦ C, adsorption systems operated at 53–82 ◦ C, a liquid desiccant system operated at 67 ◦ C, and a steam jet system operated at 118 ◦ C. For most of these systems operated below 100 ◦ C, the flat plate solar collectors could be used, while concentrating solar collectors had to be used for driving temperatures higher than 100 ◦ C. They also compared the annual EER, which is defined as the ratio of the annual cold

Absorption cycle Liquid H2 O/LiBr, NH3 /H2 O 0.50–0.73 (single-stage) <1.3 (two-stage) 1.71–2.49 (single-stage) <4.44 (two-stage) 60–110 ◦ C (single-stage) 130–165 ◦ C (two-stage)

Thermo-mechanical Adsorption cycle Solid H2 O/silica gel

Ejector – Steam

0.59

0.85

2.01

2.90

53–82 ◦ C

118 ◦ C

production and the annual heat input, both expressed in Btu/(Wh). The average annual EER was 1.98 for all systems investigated. The H2 O/LiBr absorption systems haves the best annual performance, while the adsorption systems haves low annual performance. This result reflects the fact that 70% of the systems employed an absorption technology, and 75% of the solar assisted absorption systems used H2 O/LiBr as their working fluid. Grossman [48] claimed that the liquid sorption systems can use lower temperature heat sources (such as flat plate solar collectors) rather than the closed absorption cycle so that they have a potential of reducing the cost of the solar part of the system.

8. Conclusions Renewable energy sources have been of considerable interest because of their promising advantages. As the world population is projected to increase and the supply of the fuel is projected to decrease, the increased supply of the renewable energy for the post-fossil fuel period is inevitable. Since the cooling demand has been increased associated with the recent climate change, cooling

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technologies based on solar energy are promising technologies for the future. Along with photovoltaic systems, thermally activated cooling systems are being used all over the world for domestic and industrial cooling purposes. Solar thermal cooling systems are more suitable than conventional refrigeration systems because pollution-free working fluids (instead of chlorofluorocarbons) are used as refrigerants. Solar cooling systems can be used, either as stand-alone systems or with conventional air-conditioning systems, to improve the indoor air quality of all building types (residential buildings, offices, schools, hotels, hospitals, and laboratories). In this paper, an extensive review of the technologies related to the better utilization of solar energy for the production of cool energy is presented. The liquid desiccant system has a higher thermal COP than the solid desiccant system. The adsorption cycle needs a lower heat source temperature than the absorption cycle. The ejector system has a higher COP, but needs a higher heat source temperature than other systems. Based on the coefficient of performance, the liquid desiccant system is preferred to the solid desiccant system and the absorption cooling systems are preferred to the adsorption cooling systems, the higher temperature issues can be easily handled with solar adsorption systems. Moreover, solar hybrid cooling systems can provide higher capacity and better thermal COPs by eliminating some of the problems encountered with individual working pairs. The next few years will be the most decisive for the success of solar cooling systems that depend on the encouragement and promotional schemes offered by the policymakers, and the efforts undertaken by the manufacturers to improve the cost efficiency as well in developing better technologies. Also, a search for new working fluids that are environmentally friendly and require low operating temperatures is advised. Finally, research on the integration and control of various energy conversion systems for multiple uses (cooling, heating, water heating, and power generation) may produce synergic efficiency enhancement.

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