Overview of solar refrigeration technology

The refrigeration system which can directly utilize solar energy to drive the system and produce the useful refrigeration is classified as a heat-powered refrigeration system. Absorption refrigeration and adsorption refrigeration systems are already extensively well known. However, there is another system which is called an ejector refrigeration system.

​Absorption refrigeration, adsorption refrigeration and ejector refrigeration systems

In case of the absorption refrigeration system, the refrigerant vapor at low-pressure leaving from the evaporator will be absorbed into the absorber. Inside the absorber, there is the chemical reaction which creates the solution of refrigerant and absorbent. After that, this solution will be pressurized in order to supply to the generator where the high pressure and temperature refrigerant vapor will be separated from the solution and continuously flow to the condenser. Most of the energy used in this system is heat energy. For adsorption refrigeration system, in contrast with the absorption system, the low-pressure refrigerant vapor will be adsorbed by the porous solid adsorbent. For example, water and silica gel, if the silica gel is cooled by using cooling water, it can efficiently absorb the low pressure refrigerant vapor (water) from the evaporator. In another hand, if the silica gel that already contains the refrigerant vapor is heated up by hot water, the refrigerant vapor will be separated from silica gel with higher temperature and pressure. This hot refrigerant vapor will flow to the condenser to achieve the condensation process. Ejector refrigeration system is another one system that can use heat energy to drive the system. However, it is less complicated than other systems because it needs only one refrigerant substance within the system. Moreover, there is no chemical reaction within the system. The ejector and boiler are used as compressor for ejector refrigeration system.

The application of solar cooling system

According to the present market, there are only two systems, absorption and adsorption refrigeration systems, which are available commercially. In case of ejector refrigeration system, there is no information about commercial availability. Considering to absorption refrigeration system, it can approximately produce the chilled water at 5°C with the cooling capacity in the range of 50 to 8000 kW. Steam generated by steam boiler or hot water produced by solar water heater panel can be used to drive the system. The temperature of hot water should be in the range of 80 to 180°C depending on the capacity of the absorption refrigeration system. In case of the adsorption refrigeration system, several companies have also introduced to the present refrigerating machine market. The cooling capacity of the adsorption refrigeration system is less than 1000kW but it can be driven by hot water with rather low temperature of 60°C. The temperature of chilled water generated by adsorption system is almost equal to the chilled water produced by absorption system. The advantage of the adsorption refrigeration system is that there is no internal corrosion problem because it uses a solid as the adsorbent. Unlike the absorption system, it normally use Lithium bromide to absorb a refrigerant. In case of ejector refrigeration system, it is still in the process of efficiency development. There are some Thai and aboard research groups has built a prototype in order to test and to apply to use in real application. According to published researches, it is confirmed that there is the possibility to use the ejector refrigeration system in air conditioning application.

The applications in Thailand

For Thailand, the development and demonstration projects of air conditioning system powered by solar energy has been already launched by Naresuan University, Phitsanulok. The absorption refrigeration system with a capacity of 10 TR is used in this project. Heat energy which is used to drive the system is obtained from hot water generated by evacuated tube solar collector with the area of 72 m2. The results showed that it can replace the electricity used for air conditioning system by 81%c. Another good example can be found at Nomjitt building. In order to reduce the electricity used for air conditioning system, the company has installed the absorption refrigeration system which is powered by solar energy. For adsorption refrigeration and ejector systems, it hasn’t been applied to use in Thailand. These are some examples of solar-powered refrigeration system which is already applied to use in Thailand. However, the most important drawback of solar-powered cooling systems is that the major expense of this system is the cost of solar collector. Moreover, when a solar refrigeration system is applied in Thailand where the weather is different from western countries, the sufficient knowledge and understanding of the system is very important to stable the system operation and to obtain the maximum efficiency.