Solar refrigeration system will be used more and more with the decrease of conventional energy sources and the increase of environmental pollution in future. Solar refrigeration can be used in freezers, refrigerators, building air conditioning systems, food preservation, ice-making, cooler etc...
Solar energy is a very large, inexhaustible source of energy. 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. Also, solar energy is much cleaner than most of the conventional energy sources. Thus, the use of solar energy may be an option to meet the ever increasing energy demand. The expected shortage of conventional energy sources such as fossil fuels and the associated detrimental effect on the environment are the driving forces for using solar energy. Refrigeration and air conditioning systems are among the most suitable fields for the application of solar energy.
Refrigeration is a process in which work is done on a system to move heat from lower temperature to higher temperature to get cooling effect. Refrigeration is done to maintain the temperature of certain space at a temperature lower than the surrounding. The mechanical device extracts heat from the refrigerated space maintained at a lower temperature and rejects it to the surrounding that is at relatively higher temperature to achieve the cooling effect. Refrigeration is used to provide favourable condition for storing of food products and preservation of medicine. It is also used to provide comfort through the process of air conditioning in hot and humid places. Solar refrigeration system is operated using electricity directly produced from solar radiation using photovoltaic cell or using radiant heat from the sun collected by the different types of solar collectors. It is expected that this type of refrigeration system will be used more and more with the decrease of conventional energy sources and the increase of environmental pollution in future. Solar refrigeration can be used in freezers, refrigerators, building air conditioning systems, food preservation, ice-making, cooler etc.
Types of Solar Refrigeration System
Solar refrigeration systems can be classified in three different categories. They are:
i. Photovoltaic operated refrigeration system
ii. Solar mechanical refrigeration
iii. Absorption refrigeration
Photovoltaic Operated Refrigeration System
In this system, solar radiation is directly converted to direct current electricity using semiconducting materials. The operation of a PV-powered solar refrigeration cycle is simple. Solar photovoltaic panels produce DC electrical power that can be used to operate a DC motor, which is coupled to the compressor of a vapour compression refrigeration system. The process that makes the refrigeration possible is the conversion of sunlight into DC electrical power, achieved by the PV panel. The DC electrical power drives the compressor to circulate refrigerant through a vapour compression refrigeration loop that extracts heat from an insulated enclosure. This enclosure includes the thermal reservoir and a phase change material. Figure 1 shows the schematic diagram of a photovoltaic operated refrigeration system.
Figure 1: Schematic diagram of a photovoltaic operated refrigeration system
Solar Mechanical Refrigeration
In this type of refrigeration system, required compressor power to drive the compressor in refrigeration cycle is provided by a solar Rankine cycle. Sunlight strikes the solar panel which drives a Rankine cycle and produces work in the turbine. This work is then utilized to run the compressor of the vapour compression refrigeration system. The schematic diagram of a solar mechanical refrigeration system has been shown in figure 2.
Figure 2: Schematic diagram of a solar mechanical vapour compression refrigeration system
Solar Absorption Refrigeration System
In this system, low grade energy as heat from solar panel is used as input for chilling purpose.
Figure 3: Schematic diagram of a solar absorption refrigeration system
Figure 3 shows the schematic diagram of a solar absorption refrigeration system. This system is different from a conventional vapour compression refrigeration system. Basic components of such refrigeration system are absorber, generator, solar panel, condenser, expansion valve, evaporator, DC battery and fan. The compressor in the vapour compression system is replaced by a generator, absorber and pump. Refrigerant (NH3) in the evaporator absorbs the heat from the refrigerated space and gets evaporated. It is then passed to absorber where it is dissolved with absorbent (H2O) and pumped to generator. Electrical energy from solar panel is utilized for heating in the generator and the refrigerant enters into condenser. The refrigerant is converted to liquid in the condenser and the pressure of the liquid refrigerant is dropped to the evaporator pressure with the help of an expansion device (ED). The main advantage of absorption system is compression of liquid instead of vapour which results in less mechanical work requirement as input. But the system is much expansive compared to compression refrigeration system. Other than ammonia-water combination, few more refrigerant - absorber pairs have been tried which are listed in table 1.
The performance of a refrigeration system is judged by a parameter called coefficient of performance (COP). The COP of a solar absorption refrigeration system can be expressed as
As pump work is very small and the above expression can be approximated without much error as
General energy equation for flat plate collector is given by,
Q = Amount of solar radiation received by collector =
I = Intensity of solar radiation
and AC = Collector surface area
Useful heat gain by the collector can be written as
where, S = Incident solar flux absorbed by the absorber plate
FR = Collector heat removal factor
Ul = Overall heat loss co-efficient
Tp = Fluid temperature at inlet of collector
Solar Absorption Refrigeration System Performance
Different researchers investigated on absorption refrigeration system varying different parameters and recorded the performance of the system under such varying conditions. Karno and Ajib simulated a vapour absorption refrigeration system using acetone-zinc bromide solutions and reported that initially the COP of the system increased rapidly then the increment was found to be slightly flatter in nature with the increase in generator temperature for fixed evaporator temperature. The results are shown for condenser and absorber temperature both fixed at 28°C and refrigerant heat exchanger effectiveness at 75%. However, COP of the system increases with the increase in evaporator temperature when other parameters are kept constant. An approximate correlation for COP of the system in terms of generator temperature (TG) and evaporator temperature (TE) has also been developed which is as follows:
The variations of COP with generator temperature and evaporator temperature have been shown in figure 4. Calculated values from the simulation program have been plotted together with the values calculated from the above said correlation.
Figure 4: Variation of COP with generator temperature and evaporator temperature
Effects of evaporator temperature at different condenser or absorber temperature on COP have also been shown in figure 5. The generator temperature is kept constant at 57°C. It shows that the COP of the system decreases with the increase in condenser or absorber temperature for any evaporator temperature.
Figure 5: Effect of evaporator temperature on COP for different condenser/absorber temperature (18-32°C)
Advantages of Solar Refrigeration
Solar energy is the main source of energy that is utilized to run solar refrigerator. So, significant amount of electrical power is saved and it also causes less pollution that would have been added due to the use of power produced by the conventional power plants. The solar energy is available in every part of the world and unlike fossil fuels and nuclear power, it is a clean source of energy. Additional power from the solar collector can also be used for the other domestic purposes. The solar refrigerators can be very useful where there is no continuous supply of electricity or difficult to get conventional fuel. More importantly it is renewable in nature. Conventional refrigeration systems emit significant amount of gas which pollute the environment. This solar refrigeration system is also needed to lower the environmental impact caused due to conventional refrigeration systems. The maintenance cost of such system is considerably low compared to that of the conventional system. Those facts encourage to use solar refrigeration system whenever possible.
Disadvantages of Solar Refrigeration
Solar refrigeration systems also have some disadvantages. As solar radiation is not available throughout the day, power production is not uniform. Again it depends on the intensity of the beam radiation. Even in the hottest regions on earth, the average solar radiation flux rarely exceeds 1 kWh/m2 and the maximum radiation flux over a day is about 6 kWh/m2. These are low values from the point of view of technological utilization. So, those refrigeration systems can be used in those places where those problems are not present. To produce sufficient energy from solar system, it needs bigger collector. So, there is a need of bigger space for the collector which is another major problem for using solar refrigeration system. Initial investment to develop such set up is also large.
The variation in availability of solar radiation occurs daily because of the day-night cycle and also seasonally because of the earth’s orbital motion around the sun. In addition, variation occurs at a specific location because of local weather conditions. Consequently, the energy collected when the sun is shining must be stored for use during periods when it is not available. The need for storage significantly adds to the cost of the system. Thus, the real challenge in utilizing solar energy as an energy alternative is to address these challenges. One has to strive for the development of cheaper methods of collection and storage so that the large initial investments required at present in most applications are reduced.
Ali Karno and Salman Ajib, 2008, Thermodynamic Analysis of An Absorption Refrigeration Machine With New Working Fluid for Solar Applications. Heat Mass Transfer 45, 71 – 81.
at Indian Institute
of Engineering Science
Bijan Kumar Mandal
Indian Institute of