Solar Energy in Refrigeration
& Air Conditioning
Solar energy is the result of electromagnetic radiation released from the sun by the thermonuclear reactions occurring inside its core. Thus, the use of solar energy to produce refrigeration and air conditioning can be a viable option to replace conventional cooling systems…
- Bijan Kumar Mandal,
Madhu Sruthi Emani
The conventional air cooling systems around the world are dominated by Vapour Compression Refrigeration (VCR) machines, which have high electricity consumption and contribute to high peak loads during hot seasons. Solar assisted refrigeration and air conditioning systems represent sustainable and environment friendly alternatives to traditional air cooling systems. Interest in solar cooling systems first began in 1970s when the energy crisis occurred throughout the world. Within a few years, many development projects were started. However, in the recent years, the need for solar cooling systems has gained increased momentum due to greater awareness of the necessity to reduce emission of greenhouse and ozone depleting gases which are released from the conventional refrigeration systems. Solar energy is the result of electromagnetic radiation released from the sun by the thermonuclear reactions occurring inside its core. Thus, the use of solar energy to produce refrigeration and air conditioning can be a viable option to replace conventional cooling systems. Solar refrigeration offers a wide variety of cooling techniques. The classification of solar cooling systems has been shown in figure 1.
Figure 1: Classification of Solar Cooling Systems
The solar electrical powered refrigeration systems can be categorised into photovoltaic and thermoelectric cooling. The photovoltaic (PV) based solar energy system converts solar energy into electrical energy and then utilises it for producing the refrigeration, similar to conventional methods. The solar powered thermo electric cooling devices work on the principle of ‘Petlier Effect’. A practical thermoelectric cooler consists of two or more elements of semiconductor material that are connected electrically in series and thermally in parallel. These thermoelectric elements and their electrical interconnects typically are mounted between two ceramic substrates. The substrates serve to hold the overall structure together mechanically and to insulate the individual elements electrically from one another and from external mounting surfaces. Thermoelectric devices contain no chlorofluorocarbons, so it is environment friendly and it is fully reversible cycles, precise temperature control, and work efficiently in sensitive application. The main disadvantage of thermo-electric is low COP but it does have high potential in specific application, such as cooling electronic devices, where thermo-electric is preferred due to small size and consume very less electricity.
Figure 2 shows a representation of the solar PV cooling system. 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. 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. The PV cells produce electricity by converting light energy 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 from 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. The PV system can perform as a standalone system, 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 within the desirable range. Due to the advantages of solar thermal systems over solar photovoltaic systems, recently more research has been carried out in the field of solar thermal cooling systems.
Figure 2: Solar Photovoltaic Cooling System
During the past decade, the efficiency of the solar photovoltaic collectors increased only slightly (10–15%), contrary to that of the solar thermal collectors. Also, the electrically driven systems are characterised by the limited useful power that can be achieved by solar means and by their fairly high initial cost. Thus, more attention has been paid to the solar thermal-driven refrigeration technologies in the recent years. The thermo-mechanical and sorption systems are the popular subcategories under solar thermally driven systems.
Solar thermo-mechanical cooling systems have received a renewed attention in recent years due to their advantages such as ability to produce low refrigeration temperatures by using appropriate working fluids, ability to produce electricity when cooling is not needed by coupling the prime mover with an electric generator, maintaining high performance at off-design conditions and utilization of a wide range of temperatures from solar collectors. Also, the new environment-friendly refrigerants proposed for thermo-mechanical cooling cycles make thermo-mechanical cooling technologies attractive to investigate. In a thermo-mechanical cooling system, the heat gained from a solar collector is converted into mechanical work, which is used to compress the working fluid in a vapour compression refrigeration (VCR) cycle directly i.e. ejector cooling cycle or indirectly i.e. coupled with an Organic Rankine Cycle. Hybrid solar thermo-mechanical cooling with conventional cooling systems also offers a great potential for reduction in energy demand for buildings.
Solar Ejector Cooling Systems
Solar ejector cooling cycles are a suitable option to harvest the solar energy with low-temperature collectors and fulfill the cooling requirements of the building. Ejector cooling systems are basically the same as conventional vapour compression based cooling systems. The only difference is substituting the mechanical compressor with an ejector, which is considered as a thermally driven compressor. The solar ejector cooling system has three circulating loops as shown in figure 3(a). The solar loop consists of a pump, solar collector, generator heat exchanger and heat storage tank. The solar loop provides heat to the generator. This heat is absorbed by the working fluid in the power loop, producing high temperature, high pressure vapour. The high pressure vapour flows through the ejector where it accelerates as it passes through the nozzle. Flow pressure decreases as its velocity increases. At the nozzle exit, the primary fluid pressure becomes lower than entertained flow pressure. At this point, the entertained flow is sucked into the ejector and is mixed with the primary flow. The fluid mixture emerges from the mixing chamber, and as it enters the diffuser its velocity decreases and pressure increases. The pressure of the emerging flow is slightly above the condenser pressure. In the refrigeration loop, the fluid inside the condenser becomes liquid by rejecting heat to the environment. A portion of the liquid is pumped to the generator to complete the power loop. The rest of the liquid is expanded through a throttling valve and enters the evaporator as a mixture of vapour and liquid. By absorbing heat from the cooled space in the evaporator, all the fluid transforms to vapour, and enters the ejector, which completes the refrigeration cycle. According to literature available, the COP of ejector cooling systems is low compared to other heat driven cooling systems. However, simple construction and low maintenance requirements make them appealing for building cooling. Design variables including the ejector geometrical parameters, operation conditions and working fluid affect the performance of a solar ejector cooling system.
Figure 3: Representation of (a) Solar ejector cooling system and (b) Solar Rankine cooling system
Solar Rankine Cooling Systems
These systems were investigated widely in the 1970s and 1980s. Research on these systems was almost on hold over the past two decades. But due to recent advancements in Organic Rankine Cycle (ORC) equipment and introduction of new environmentally friendly working fluids in recent years, the research on this topic is receiving more attention. The main idea behind the Solar Rankine Cooling System is utilization of solar heat to produce mechanical work and drive a conventional VCR Cycle. The schematic representation of Solar Rankine cooling system has been shown in figure 3(b).
There are two common arrangements of solar Rankine cooling system designs presented by different researchers. One configuration is using separate power and cooling cycles where the expander of the ORC and the compressor of the VCRC are coupled mechanically. In the latter design, same working fluid is used in both the loops that eliminate leakage and mixing problems. Integrated design is also simpler. Rankine cooling systems can utilize higher generation temperatures to produce cooling and, whenever the cooling demand is low, to produce electricity. In addition, hybrid solar thermo-mechanical cooling with conventional cooling systems offers a great potential for energy demand reduction for buildings.
Sorption refrigeration uses physical or chemical attraction between a pair of substances to produce the refrigeration effect. A sorption system has the unique capability of transforming thermal energy directly into cooling power. The sorption technology can be classified into open sorption system and closed sorption system.
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 increase of heat in a solution can reverse the process. The absorption process has been represented in figure 4. The first evolution of an absorption system began in the 1700s. It was observed that in the presence of H2SO4 (sulphuric acid), ice can be made by evaporating pure H2O (water) within an evacuated container. In 1810, it was found that ice could be produced from water in a couple of vessels connected together in the presence of sulphuric acid. As the H2SO4 absorbed water vapour, ice formed on the surface of water. However, difficulties emerged with leakage and the corrosion of air into the void vessel. In 1950, a new system was introduced with a water/lithium–bromide pairing as working fluids for commercial purposes. The primary advantage of an absorption system is that it has a larger COP (coefficient of performance) than other thermally operated technologies.
Figure 4: Solar Absorption System
Faraday first introduced vapour adsorption technology in 1848, using a solid adsorbent. Adsorption cycles were first used in refrigeration and heat pumps in the early 1990s.The disadvantages of liquid–vapour systems were overcome by using solid–vapour cycles; this technology was first marketed in the 1920s. Adsorption refrigeration technology has been used for many specific applications, such as purification, separation and thermal refrigeration technologies. The important point to be noted is that the absorption is a volumetric phenomenon, whereas adsorption is a surface phenomenon. The working of the solar adsorption cooling system has been shown in figure 5. Adsorption is a process in which molecules of a fluid are attached to a surface. The surface is composed of a solid material. The molecules do not perform any chemical reaction; they merely discard energy when attached to the surface. The phase change is exothermic and the process is fully reversible. On an exposed solid surface with a gas, the gas molecules are forcibly thrusted upon the surface of that solid. Therefore, some molecules adhere to the surface and get adsorbed, while some of them rebound back. At the onset of an adsorption process, the rate of adsorption is greater because the full surface is uncovered. The adsorption rate gradually decreases as the surface becomes more inundated with the adsorbate. Meanwhile, the rate of desorption increases in parallel with the decrease of adsorption, as desorption occurs from the exposed solid surface. However, equilibrium is achieved when the adsorption rate and desorption rate are equal. This is called adsorption dynamic-equilibrium, as the number of striking molecules on the surface and rebounding molecules from the surface are equal.
Adsorption technology can accommodate high temperature heat sources without corrosion. Adsorption technology is better equipped to handle vibration issues in a cooling system than absorption technology. Because of the liquid absorbent present in an absorption system, vibrations can cause serious problems, such as flow from the absorber to condenser or from the generator to evaporator, potentially polluting the refrigerant. Adsorption is immune to this condition and can thus be used in locomotive sand fishing boats. An adsorption system is simpler to design than an absorption system. For example, to design an absorption system with a H2O/NH3 working pair, extra equipment (dephlegmate) is required, because the boiling points of water and ammonia are very close. The comparison between the solar absorption and adsorption systems has been presented in table 1.
The adsorption cycle can be operated at lower heat source temperatures than the absorption cycle, but its COP is also lower. Based on the coefficient of performance, the absorption cooling systems are preferred to the adsorption cooling systems, and the higher temperature issues can be easily handled with solar adsorption systems. Solar thermal with single-effect absorption system appears to be best option, closely followed by solar thermal with single-effect adsorption system and solar thermal with double-effect absorption system options at the same price level. Solar-powered adsorption refrigeration devices can meet, among things, the needs for refrigeration, air-conditioning applications and ice making, with great potential for the conservation of various goods (medicines, food supplies) in remote areas. Nevertheless, the purpose of each system and the ambient conditions dictate its configuration (type of solar collector) and working pair, and performance. Compared to the absorption systems, adsorption systems can be powered over a larger range of heat source temperatures. The adsorption systems are more robust and less sensitive to physical impacts, do not present corrosion problems due to the working pairs normally used and are less complex because they contain fewer moving parts.
Figure 5: Solar Adsorption Cooling System
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. To drive this cycle, thermal energy is needed during the desorption process.
Liquid Desiccant System
Materials typically used in liquid desiccant systems are lithium chloride (LiCl), calcium chloride (CaCl) and lithium bromide (LiBr). A liquid desiccant cooling system has been shown in figure 6. The system usually consists of a conditioner and a regenerator. 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 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. 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. 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. 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.
Figure 6: Schematic of solar liquid desiccant refrigeration
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. 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 dry bulb 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. 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 38m3/s. Several studies performed on the description and operation of desiccant cooling systems by different researchers. Systems that use rotary desiccant wheel to dehumidify the air are the most popular desiccant cooling systems and studied by different researchers. The desiccant cooling systems are viable alternative to vapour compression systems.
The comparison of various solar cooling techniques has been made by different researchers. Balaras et al. (2007) provided an overview of solar air-conditioning in Europe. For this purpose, they collected information on 54 solar powered cooling projects conducted in various locations in Europe. Figure 7 describes the annual thermal performance for the evaluated projects. The annual thermal COP is defined as the ratio of the annual cold production expressed in kWh and the annual heating input also expressed in kWh. According to the available data on actual performance, the average annual thermal COP is 0.58, slightly lower than the design thermal COP (0.65). The H2O/LiBr systems show the best performance, while the adsorption systems are generally less efficient. The lowest performance is shown by the NH3/H2O diffusion system. The grey bars indicate the systems that use flat plate collectors, the dark bars indicate the systems that use evacuated tube solar collectors, and the light bar indicates a system that uses stationary concentrating collectors.
Figure 7: Annual Thermal Performance for different solar projects
They concluded that the single-effect absorption systems have a COP in the range of 0.50–0.73, adsorption systems have a lower thermal COP of 0.59, a liquid desiccant system have a COP of 0.51, and a steam jet system has 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 0C, 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 production and the annual heat input, both expressed in Btu/(Wh). The average annual EER was around 1.98 for all systems investigated. The H2O/LiBr absorption systems have the best annual performance, while the adsorption systems have low annual performance. This result reflects the fact that 70% of the systems employed absorption technology and 75% of the solar assisted absorption systems used H2O/LiBr as their working fluid.
Figure 8 compares the solar thermal cooling cycles’ COP versus the regeneration temperature as reported from a study by Al-Alili et al.(2014). One can see that the adsorption cycles are operated using lower regeneration temperatures than other two cycles, while the ejector cycles require higher driving temperature. In addition, the absorption cycle COPs are the highest, while the ejector cycle COPs are the lowest. Also, the adsorption cycle can be operated at lower heat source temperatures than the absorption cycle, but its COP is also lower. Similar to the solid desiccant material development requirement, additional enhancement of the adsorption system’s COP is needed through new material development, system loss reduction or multi-stage approaches. Ejector cycles showed comparatively higher COP than other energy conversion technologies investigated in this study, but it requires higher operating temperatures.
Figure 8: Comparison of different thermal cycles’ COP vs. regeneration temperature
Several potential researches have been carried out on solar refrigeration systems. Although, the total number of working solar cooling units is very small at present, interest in solar cooling technologies is increasing again due to a combination of environmental consciousness and increasing prices of fossil fuels. The solar thermal cooling technologies, however, have their overall efficiencies lower than that of the vapour compression refrigeration systems. In order for the solar thermal cooling cycles to penetrate the market, their performance has to improve to a level that competes with the electric vapour compression cycles. Therefore, improving efficiency of solar thermally operated cooling technologies is an essential future research topic. The closed sorption refrigeration systems present as a good alternative to replace the classical refrigeration and air-conditioning systems with more environmentally friendly systems that can be powered by solar energy.
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