PCM based Solar Refrigeration Systems

Source : http://inhabitat.com/solar-powered-camel-clinics-carry-medicine-across-the-desert/

Today, we are living a lifestyle, which demands energy consumption and this demand will continue to rise. Conventional methods of power production like burning of fossil fuels are extensively used to meet the ever increasing energy demands throughout the world. However, the fossil fuels are scarce. Renewable Energy sources like Solar Energy can be harnessed to meet our energy demand. There are number of ways to utilize solar energy in a cleaner and sustainable approach and to do that we need to develop the right technology which is economically sound and certainly advantageous than our conventional methods of energy generation.

Use of Solar Energy for heating or electricity production is intuitive but driving a refrigeration process from solar energy may not be that intuitive. Phase Change Material (PCM) has revolutionized the Vapor Compression Cycle based Solar Refrigerator. PCMs are special thermal energy storage materials which changes phase from solid to liquid or vice versa at a particular phase change temperature. They have very high latent heat capacity and can store large amounts of thermal energy when available. In the Vapor Compression Cycle, when the irradiation from the sun is greater, a compressor can be run to obtain cooling such that PCM freezes. The frozen PCM has the ability to maintain the lower temperature during the time when compressor cannot be operated due to unavailability of the solar energy. The thermal storage system can reduce the refrigeration system size by elimination of bulky storage alternatives like the electrical batteries or fuel tank.

Commercially, Solar Energy is used for Ice making, Air Conditioning and for other temperature control application. Few good examples are, the Fisherman in the village of Maruta, which is located on Mexican pacific coast, 18^oN of equator, to able to store fish on ice which is produced without the use of Electricity and they used Solar Powered Vapor Absorption Cycle based refrigerator [1]. The world’s first automatic commercial Photo Voltaic Ice- making machine was designed by SunWize and it was installed in the year 1999 in order to serve the inland fishing community of Chorreras in Chihuahua, Mexico [2]. This system which was priced at USD$ 38,000, had a Coefficient of Performance of 0.65 and produced an average of 75 kg of ice/day. 97 % of total power required in the refrigeration process of this ice machine was obtained from solar collector and the rest was supplemented using the conventional backup propane generator.

Another commercially available, Solar based Ice making machine was made by a company named Energy Concepts. They named their product as ISAAC Solar Icemaker. This system makes use of parabolic trough solar collector with no electrical or fuel input, no expensive material of construction, and this system simply operates in two modes. In one mode, liquid ammonia refrigerant is obtained by providing heat with Solar Energy during the day time and in the night ice is formed by reabsorption of ammonia.[3] ISAAC can produce 5 kg/m² of ice per sunny day and this system can be deployed for off grid use in the remote areas.

Figure 1 World's first PV ice-maker developed by SunWize in the heart of the Chihuahuan desert for the fishermen of Chorreras. Source: Photovoltaics for Rural Development in Latin America: A Quarter Century of Lessons Learned

Austin Solar AC is another company which provides heating and cooling services using the Vapor Absorption Cycle. [4] The desorption process for Refrigerant-Water in Vapor Absorption Cycle needs high temperature in the range of 120^oC-130^oC and these can provided by the use of large solar collectors.[5] As a result, Vapor Absorption Cycle based  solar refrigerators are bulky as compared to Vapor Compression Cycle based Solar Refrigerator.

In a Vapor Compression Cycle, a mechanical power is needed to drive the compressor which increases the pressure and temperature of the refrigerant. The mechanical energy input for running a compressor is where “Solar Energy” can play a key role and the point to ponder is “Whether solar energy can produce the amount of power to drive the compressor throughout the day?” It turns out that, we will need more than just the solar energy to obtain refrigeration and another challenge is maintaining the lower temperatures for the desired period. A photo voltaic (PV) can convert solar energy and produce a DC current to run a DC motor for the compressor. The operation characteristic of the PV governs how efficiently we can run a compressor. The figure below shows the Current/ Power vs voltage for PV for different operating conditions.

Figure 2 Single PV cell current, voltage and power plot highlighting the Maximum Power Point curve
Source: Digikey Electronics, http://www.digikey.com/en/articles/techzone/2013/jul/addressing-the-challenges-of-power-management-in-wireless-sensor-networks-wsns

From the Fig 2, it can be inferred that, there exists a value of voltage for which PV cell power output is maximum for the different intensity of irradiation. A DC motor power characterises should be matched closely with that of maximum power point curve in-order to perform an efficient job to run a compressor with available intensity of irradiation from the sun.

The novel technology of using the Direct PV along with PCM in solar refrigeration is patented by innovators at NASA’s Johnson Space Centre. These refrigeration system finds application in the rural areas where grid electricity is unavailable and solar energy is abundant.[6] A company named SunDanzer, has commercialized the PCM based Solar Refigerator. They caters to refrigeration needs of the household with its chest style freezers, refrigeration needs of the medicine field with its proprietary PCMs for storing heat sensitive vaccine in solar PV driven freezers and refrigeration needs of the military by reducing its fuel consumption in battlefield by use of solar driven potable water cooling, storage and air conditioning systems.[7]

Another, similar technology was developed in an international project partnered by Greenpeace technology, GTZ, UNICEF, UNEP, WHO, industrial partners and Danish Technological Institute. They developed a product named SolarChill- a Solar PV refrigerator which runs without the electrical battery. The main objective of the SolarChill Project is to help deliver vaccines and refrigeration to the rural poor. Successful trials of this refrigerator was carried out at Copenhagen, Indonesia, and Cuba and it was found that vaccine can be kept between 0-8 ^oC after the PCM is frozen, for outside ambient temperature of 20 ^oC. This technology uses ice as a phase change material which can provide 62 % more energy than conventional 50 Ah-12 V batteries. Newer versions of SolarChill are aimed for optimization with regard to control strategy for different climatic condition, reduction in cost and module area. [8]

Figure 3. First SolarChill Prototype
Source : SolarChill - a solar PV refrigerator without battery

PCM can be used to create a low cost solar driven refrigerator. The usage of PCM as a thermal storage application is tested for commercial refrigerators by Centre of Excellence –Renewable and Sustainable Energy studies, Jaipur. [9] They used a 165 L Videocon refrigerator which had a R134 Refrigerant, solar panels, Solar Sine wave UPS along with the battery. Three Sets of Experiment was performed on the system. In case one, the refrigerator was operated using solar energy with no load inside the refrigerator. In second case, refrigerator is operated by loading 2kg PCM.  In third case, backup obtained due to energy stored in PCM was tested by shutting off the compressor. The experiment was performed for 6 hours in all three cases. While in the unloaded operation, the temperature of freezer section reached -5 ^oC and goes up to -6.8 ^oC in 6 hours while in the vegetable section temperature reaches up to 10 ^oC. In loading condition, a minimum temperature of only -2^oC was attained in the freezer while vegetable section managed to attain 10 ^oC which is agreeable as energy is consumed in freezing of Ice gel packs. In the backup test the PCM was able to maintain the temperature around 5 ^oC during the 6 hours of operation. The cost of procuring the system in India is much lower as compared to what available in the developed markets and with efficient design, optimizing heat losses and removing dependence on battery, a tailored solution can be made that can cater to the needs of refrigeration in remote areas for India as well as other developing markets.



About the Author

Kunal Bhagat works as a Associate- Application Engineering at Pluss Advanced Technologies Pvt. Ltd. He holds a Dual Degree (Masters+Bachelors) in Mechanical Engineering with Specialization in Thermal and Fluids from IIT Bombay. Prior to joining Pluss, he has worked in the Designing and Testing of PCM based heat exchangers at Thermal Energy Service Solution Pvt. Ltd. He also has a experience in CFD analysis of PCM based storage systems for Concentrated Solar Power application. He has completed IARC’ Centre for United Nation course, “Rio + 22 Sustainable Energy for All” which highlights sustainable energy entrepreneurship, increase use of renewable energy and tackling issues of energy crisis