The negative environmental impacts of burning fossil fuels have forced the energy research community to seriously consider renewable sources, such as naturally available solar energy. Solar refrigeration technologies harness the energy of the sun and use it to run a cooling system. This type of solar application is an attractive option for the preservation of food and the refrigeration of vaccines and medicines in areas with a high intensity of solar radiation and no electricity supply. Refrigeration systems those use environment-friendly refrigerants provide a sustainability advantage when compared to other refrigerant selections. However, the energy use associated with refrigeration system operation and the environmental impacts associated with its generation and distribution often outweighs the choice of refrigerant. To minimise environmental impacts associated with refrigeration system operation, it is wise to evaluate the prospects of a clean source of energy such as solar energy.
Solar energy
Direct use of solar energy is attractive because of its universal availability, low environmental impact, and low or no ongoing fuel cost. Research has demonstrated that solar energy is an ideal source for low temperature heating applications such as space and domestic hot water heating. Solar heating applications are intuitive since, when solar energy is absorbed on a surface, the surface temperature rises, providing a heating potential. The use of solar energy to provide refrigeration is rather less intuitive. The power from the sun intercepted by the earth is approximately 1.8 ×1011 MW, which is much larger than the present consumption rate on the earth of all commercial energy sources. Thus, in principle, solar energy could supply all the present and future energy needs of the world on the continuing basis. Also, solar energy is a clean source of energy, free and available in adequate quantities in almost all parts of the world.
Types of refrigeration systems
The refrigeration effect can be obtained using different principles and accordingly different refrigeration systems have been evolved. Each refrigeration system has its own merits and demerits, and also, specific use. The most common refrigeration systems that can be used for multipurpose applications are vapour compression refrigeration system and sorption refrigeration system. Moreover, solar energy can be used here in different forms.
Vapour compression refrigeration cycle
The basic operating principle of vapour compression refrigeration cycle that forms the foundation for nearly all conventional refrigeration systems is discussed here. A schematic diagram of the vapour compression refrigeration system is shown in Figure 1 and the corresponding pressure-enthalpy (p-h) diagram for the refrigerant is shown in Figure 2.
In the vapour compression cycle, cooling or refrigeration effect is obtained in the evaporator as low temperature refrigerant entering the evaporator as a mixture of liquid and vapour at state 4 is vapourised by thermal input from the load. The remaining equipments in the system reclaim the refrigerant and restore it to a condition in which it can be used again to provide cooling or refrigeration effect. The vapour leaving the evaporator at state 1 in a saturated (1a) or slightly superheated (1b) condition enters a compressor that raises the pressure and, consequently, the temperature of the refrigerant. The high pressure hot refrigerant at state 2 enters the condenser (a heat exchanger) that uses ambient air or water to cool the refrigerant to its saturation temperature prior to fully condensing to a liquid at state 3. The high pressure liquid is then throttled to a lower pressure, which causes some of the refrigerant to vaporise as its temperature is reduced. The low temperature liquid that remains is available to produce useful refrigeration. The performance of the system is expressed by a term called COP (Coefficient of Performance) which is defined as the ratio of refrigeration effect and the power input into the compressor.
Photovoltaic cell operated refrigeration cycle
Photovoltaic (PV) cell directly converts solar radiation to Direct Current (DC) electricity using semiconducting materials. Solar photovoltaic panels produce DC power that can be used to operate a DC motor that is coupled to the compressor of a vapour compression refrigeration system. The major considerations in designing a PV-refrigeration cycle involve appropriately matching the electrical characteristics of the motor driving the compressor with the available current and voltage being produced by the PV array. Unfortunately, PV modules will operate over a wide range of conditions that are rarely as favourable as the rating condition. In addition, the power produced by a PV array is as variable as the solar resource from which it is derived. The performance of a PV module, expressed in terms of its current voltage and power-voltage characteristics, principally depends on the solar radiation and module temperature. Figure 3 shows current and power vs. voltage plots for a 1.32 m2 single crystalline PV module at the reference condition and four operating conditions.
Solar mechanical refrigeration
Solar mechanical refrigeration uses a conventional vapour compression system driven by mechanical power that is produced with a solar-driven heat power cycle (Rankine cycle). A storage tank can be included to provide some high temperature thermal storage. The vapour flows through a turbine or piston expander to produce mechanical power as shown in Fig. 4. Efficiency is significantly lower and solar mechanical systems are competitive only at higher temperatures for which tracking solar collectors are required. Because of its economy-of-scale, this option would only be applicable for large refrigeration systems.
Sorption refrigeration technologies
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. Among the pair of substances, the substance with a lower boiling temperature is called the refrigerant and the other is called the sorbent. Figure 5 shows a schematic diagram of a closed sorption system.
The component where sorption takes place is denoted as absorber Ab, and the one where desorption takes place is denoted as generator G. The generator receives heat Qg from the solar collector SC to regenerate the sorbent that has absorbed the refrigerant in the absorber. The refrigerant vapour generated in this process condenses in the condenser C, rejecting the condensation heat Qc to the ambient. The regenerated sorbent from the generator is sent back to the absorber, where the sorbent absorbs the refrigerant vapour coming from the evaporator E, rejecting the sorption heat Qa to ambient. In the evaporator, the liquefied refrigerant coming from the condenser and expansion valve evaporates, removing the heat Qe from the cooling load. Absorption refers to a sorption process where a liquid or solid sorbent absorbs refrigerant molecules into its interior and changes physically and/or chemically in the process. Adsorption involves a solid sorbent that attracts refrigerant molecules onto its surface by physical or chemical force and does not change its form in the process.
Absorption systems
The absorption system is one of the oldest refrigeration technologies and began in 1700s. It consists of a generator, a pump and an absorber that is collectively capable of compressing the refrigerant vapour. The evaporator draws the vapour refrigerant by absorption into the absorber. The thermal energy (may be solar or heat recovered from exhaust gases) supplied to the generator separates the refrigerant vapour from the rich solution. The refrigerant is condensed by rejecting the heat in a condenser, and then the cooled liquid refrigerant is expanded in the expansion valve before entering the evaporator to complete the cycle. The refrigerant side of the absorption system essentially works under the same principle as the vapour compression system. However, the mechanical compressor used in the vapour compression cycle is replaced by a thermal compressor in the absorption system. NH3/H2O and H2O/LiBr are typical refrigerant /absorbent pairs used in absorption systems. Each working pair has its advantages and disadvantages as listed in table 1.
The absorption systems can be divided into three categories namely single, double and triple effect solar absorption cycles. Typical cooling COPs of the single-effect, double-effect, and triple-effect absorption systems are 0.7, 1.2, and 1.7, respectively. For single effect absorption system with generator temperature of around 850C, flat plate collector may serve the purpose. But for multi-effect with generator temperature 1300C and above, compound parabolic and evacuated tube collectors are needed.
Adsorption systems
Adsorption technology was first used in refrigeration and heat pumps in the early 1990s. Solar energy is the energy source of most adsorption systems operating with the basic cycle. A solar adsorption cooling system based on the basic adsorption refrigeration cycle does not require any mechanical or electrical energy. It just needs thermal energy and it operates intermittently according to the daily cycle. This refrigerator is a closed system consisting of a solar collector SC containing the adsorbent bed, a condenser C, a receiver R equipped with a 2-way valve V, and an evaporator E as shown in Fig 6.
Comparison between absorption and adsorption Systems
Solar solar-assisted absorption and adsorption systems may be more attractive in future due to pollution-free working fluids (instead of chlorofluorocarbons) being used as refrigerants. A comparison between the absorption and adsorption refrigeration systems is presented in table 2.
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. Solar thermal with single-effect absorption systems appear to be the best option, closely followed by solar thermal with single-effect adsorption systems.
Prospects of solar based refrigeration systems
Solar-powered adsorption refrigeration devices can meet 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.
Another possibility is to use adsorption systems as thermal energy storage devices. For solar sorption systems, considerable reduction in unit cost or significant improvements in its performances at present costs are still required to increase their competitiveness and commercialisation potential. Additionally, the search for new working fluids those are environment friendly and require low operating temperatures is required.
