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 

How passive cooling technology can help to meet Food Inflation

An average Indian is spending much more on buying fruits, vegetables and milk products than cereals and pulses. The latest National Sample Survey Office (NSSO) data shows that spending of an Indian has more than doubled on buying fruit, vegetables and milk products in the last five years whereas expenditure on cereals and pulses has increased by only between 33 to 75% (Chauhan, 2013). Today’s food inflation is less about food grain, more about fruit and vegetables, eggs, fish, meat and dairy products. This article focuses mainly on these commodities presenting different challenges and inefficiencies in the present value chain, highlighting the role of phase change materials in improving the situation.

The largest consumer segment to focus is the urban areas. This segment is crucial in virtue of its high population growth rate that has increased many-folds from 79 million in 1961 to 377.1 million in 2011. This has led to putting an increasing pressure on the agriculture sector to produce more. However, more than the productivity increment, the development of effective food supply chain is of utmost important to satisfy the hunger of the growing population. The agriculture value chain is of central substance to all the farmers, processors, logistics partners, wholesalers and retailers. The supply-side constraints are influencing the food prices largely both at local, national and global level. Market imperfections, like lack of proper infrastructure in rural areas, shortage of storage and transportation facilities further add to food inflation (Ministry of Agriculture, 2012). All of these points converge to the point, that there is urgent need to deal with the inefficiencies in the whole agriculture value chain.

Need to optimize consumption of fuel

An increase in fuel prices has both obvious and more subtle impacts on inflation. One type of impact is straightforward - fuel prices (along with lighting) have a share of about a tenth in the overall basket of goods used by the government to calculate consumer inflation. So any increases in petrol or diesel or LPG prices will automatically result in the inflation rate inching higher (Celestine, 2013). In 2011, the RBI, in a study, assessed that a 10% increase in domestic fuel prices could raise overall wholesale prices by about one percentage point in the short run. In the longer-run though, that 10% impact, would cause inflation to rise by about two percentage points. 

Need to counter the effects of long transportation time

The farmers incur labor costs for loading and off-loading of agriculture produce and weighing costs also, which increases their total cost of selling their agricultural produce, reducing their income. It is estimated that the total cost of transportation of the agriculture produce from farms to the wholesale markets accounts for nearly 10% of the total value of agriculture produce in many cases. The total time taken by the farmer to transport the produce sometimes takes 3-4 hours which is a big pain. Many times, farmers don’t even negotiate the price of their agriculture produce with the broker before going to wholesale market. As a result, they are bound to sell at the given price at that particular time in order to save the costs of transporting the agriculture produce back to their farms.

The situation gets worse when these commodities are transported from one state to other through the network of middlemen and wholesalers again causing high loss in terms of both quality and quantity of the horticulture commodities. The degradation of the horticulture commodities caused due to longer transportation time attracts adulteration in the horticulture commodities. Various types of harmful chemicals and injections are used to improve the visibility of the commodity so that it seems to be fresh to local consumers. As most of the wholesale and retail markets don’t have any effective mechanism to check the quality and artificial freshness of the horticulture commodities, it has severe harmful consequences on health and environment.

 Passive cooling Technology

With the above backdrop to the problem areas resulting in high cost of fruits & vegetables, one solution that can positively impact the cost of storage and transportation of temperature sensitive commodities is phase change Materials (PCM) technology. PCMs are passive cooling materials which have the potential to reduce 80% and more of the operating cost incurred due to diesel consumption. Phase Change Materials fall under the sub category of energy exchanging smart materials. Energy exchanging smart materials is defined as those materials that are able to store latent and sensible energy in the form of light, heat, electricity or hydrogen and exhibit reversibility. A PCM has the ability to store and release large amounts of heat/energy while maintaining a constant temperature.

PCMs are engineered to change their phase (solid to liquid or vice versa) at a specific temperature and one should look at the following three factors to qualify a PCM;

1)      High thermal storage capacity in the form of latent heat (200KJ/Kg or above)

2)      Constant temperature maintenance during the release of stored energy.

3)      Guaranteed repeatability in performance for over 3000 times.

The benefits of the above features are;

·         Precise temperature control allowing not more than +/- 1°C  of error.

·         Longer duration of retention period – upto 18 hours due to high latent heat.

·         Reduction in the overall weight of the freezer due to high energy storage to weight ratio of the PCMs.

         

                                   Value addition -  Three functions in one product

 PCM’s have tremendous potential to fulfill the growing need of energy for cooling and heating applications across various industries.  The PCM based transportation application market offers a large potential in developing countries as these account for 60% of the global exports. India though accounts for just 1.5% of the total export, the demand is growing at a CAGR of 20.61% and 7.21% for fruits & vegetables respectively (Trade, 2013). The total production of fruits & vegetable accounts for 14% of the world’s production (Trade, 2013). Passive cooling technologies can drastically reduce the price to the consumer while reducing the cost of wastage & transportation at the upstream of the supply chain.

In temperature control transportation 80 – 90% of operating cost can be reduce by reducing the dependability on diesel. PCM based trucks once charged for 8-10 hours using electricity can provide refrigeration for about 12 hours during the journey. This puts lesser pressure on the farmers or sellers to sell the commodity on the same day there by enabling them negotiate. PCM helps in using the energy when it is cheap and store it for use during operation. The concept can be used even for cold stores similarly to save on the diesel cost during power outages by keeping energy stored in PCM for backup. In the above figure 1 shows a comparison of the savings in operating cost between a conventional reefer truck and a truck using phase change materials. This is achieved at a little or no difference in the capital cost which is paid pack within a year.

The phase change material market is estimated to grow from $460 million in 2013 to $1,150 million by 2018, growing with a CAGR of 20.1% during the same period (Markets, 2013) . Factors such as government policies, unpredictable climatic changes and lack of infrastructures will continue to contribute to rising inflation. It is the need of the hour to adopt and implement innovative technologies such as phase change materials to prevent further losses.