Nowadays, the world increasingly pays attention to “renewable energy” day by day and there are many attempts to use a renewable energy instead of the conventional energy converted from fossil fuels. To obtain the energy from fossil fuels, the transformation process can be the cause of the global warming problem. In case of Thailand, the renewable energy that is widely used is “solar energy”. Commonly, we would think about to use the solar energy to generate electricity. However, the solar energy can be used to drive the refrigeration system as well. We will not talk about to use solar energy to generate electricity first and then uses that converted electricity to drive the refrigeration system. But we will talk about refrigeration system that can be directly powered by solar energy. The refrigeration system which is mentioned in this article is “absorption refrigeration system”.
Absorption refrigeration system driven by solar energy
Figure 1 shows a schematic diagram of a solar powered absorption refrigeration system. Water can be applied to use with this system as its refrigerant (1-2-3-4). The refrigerant will release heat (Qc) to cool water generated by cooling tower, and it will be condensed to be saturated liquid at the condenser (1-2). Then, liquid refrigerant will flow through expansion valve (2-3) and the phase of liquid refrigerant will be changed to mixture before entering the evaporator. The refrigeration will be produced at the evaporator. The phase of refrigerant will gradually be changed to saturated vapor and then leaving out of evaporator. The refrigerant will be working with absorbent (Ammonia or Lithium bromide) and mixed together as solution.
Strong solution will be flowed from the generator and passed through expansion vale (8-9-10). This results in the reduction of pressure and temperature while the concentration of strong solution remains constant. Then, the strong solution will enter the absorber where the vapor refrigerant from the evaporator will be absorbed by strong solution. So, the concentration of strong solution will be reduced, and it becomes a weak solution. During the absorption process, some amount of heat will be generated due to the chemical reaction. To maintain the temperature of absorber, the excess heat generated by chemical reaction will be released to cool water from cooling tower. The weak solution will then be pumped to heat exchanger (5-6-7). This causes temperature of weak solution is increased while the temperature of strong solution from generator (8-9) is decreased. The installation of heat exchanger between generator and absorber will improve the coefficient of performance (COP) of the system due to the energy used at generator will be reduced, lower than the system without heat exchanger. The utilization of solar energy of this system will take place at generator. The generator will operate with parabolic trough collector which mainly consists of two parts. The first one is parabolic trough reflector (concentrator). The second one is evacuated tube receiver which is used to absorb the heat energy from parabolic trough reflector. The working principle of parabolic trough collector is shown in figure 2. The heat transfer fluid contained inside the receiver tube is Therminol VP-1 which the maximum operating temperature is 400°C. Therminol VP-1 will absorb heat energy from sunlight and the temperature of Therminol VP-1 will be increased. Then, it will be used to generate steam at storage tank and the steam will be used as the heat energy for generator of absorption refrigeration system. Nowadays, some commercial building in Thailand has already been using the absorption refrigeration system driven by solar energy. However, the solar energy receiver used in the system is flat plate solar collector which the efficiency is lower than parabolic trough collector.
Article by: Pornphimol Winyuchakrit (Ph.D.)