Solar refrigeration technologies harness the energy of the sun and use it to run a cooling system. This type of solar application is an attractive option for the preservation of food and the refrigeration of vaccines and medicines in areas with a high intensity of solar radiation and no electricity supply (or only an unreliable supply).
Solar refrigeration first received a great deal of attention a few decades ago during the 1970s oil crisis1. To date, several solar cooling technologies have been developed and some are technically mature, but they are not yet competitive on the global cooling market. The different types of solar cooling appliances that exist today can be divided in two categories: the solar electric and the solar thermal refrigerators2.
Around 1.5 billion residential refrigerators and freezers are in use globally. The number of newly produced units has been increasing over the last decades and was more than 100 million units per year in 20083.
In the future the global demand for cooling devices is predicted to increase even further, especially in developing countries and emerging economies. Off-grid solar refrigeration devices could become a suitable alternative for countries that benefit from high levels of stable solar radiation and, in particular, for areas with no access to grid electricity. However, currently, neither the solar electric nor the sorption refrigerator technologies can compete in terms of cost or efficiency with conventional domestic devices. The actual future potential of the technology will depend greatly on it being further developed and promoted in order to survive.
The substitution of stand-alone systems that use kerosene or bottled gas can lead to tangible emission reductions. It is estimated that a one kerosene refrigerator emits between 733 and 916kg of CO2 per year. If the kerosene refrigerators that are in use today (approximately 100,000) were replaced, this could save between 73 and 91 kilotonnes of CO2 each year4. Residental cold appliances that are in use globally have an average annual electricity consumption of 460 kWh each and cause worldwide CO2 emissions of more than 425 Mio tonnes5. Refrigerators using solar energy can – up to a point – replace conventional electric refrigerators that use fossil fuel based electricity and, in doing so, reduce related greenhouse gas emissions.
The preservation of food and the availability of fresh products thanks to cooling devices can help to improve food security and nutrition. Equally, the possibility of storing medicine and vaccines in remote areas can improve heathcare and help to fight diseases that are preventable by widely used vaccines. Therefore, solar refrigeration could, in principle, play a significant role in achieving the MDGs. However, the acquistion costs of such devices are, at present, too high for most households and, in particular, for the poor population in developing countries.
The environmental impacts of refrigerators depend on the material and refrigerants used for cooling, on the energy needed to run the system and, as well, on the end-of-life disposal. In all three regards, the sorption heat driven cooling system has environmental advantages when compared to conventional or solar electric refrigerators. The chemicals used in sorption refrigerators are mainly water, ammonia, salts, silica gel, lithium bromide and lithium chloride. These refrigerants are relatively harmless to the environment and do not come into contact with the air. Therefore, running sorption refrigerators – as opposed to the many electrically driven cooling systems on the market today – would not contribute to global warming or be harmful to the environment6.
Solar powered refrigerators are most likely to replace stand-alone refrigerators, which are currently fuelled by kerosene or bottled gas, in developing countries. The replacement of these fossil fuel sources could reduce harmful greenhouse gas emissions of 0.7 – 0.9 t CO2 per year for each kerosene refrigerator4. In addition, the emissions resulting from transportation along the fuel supply chain for gas or diesel would be avoided. Moreover, if electric refrigerators were replaced by solar refigerators a lot of energy could be saved, resulting in a significant reduction in electricity usage and related emissions.
Until 2005, conventional refrigerators used refrigerants with high ozone depleting effects and global warming potential (like CFC). These old substances - and also some of the currently used substances, such as HFC 134a, have a high global warming potential. Particularly in developing countries, old appliances are still in use and their improper disposal can have dangerous impacts on the environment. Released to the environment these refrigerants and foams contribute to the destruction of the protective ozone layer above the earth.
In this respect, the emissions of solar electric appliances must also be managed and the levels of emissions depend on the refrigerants used; notably, sorption refrigerators use no harmful substances in the cooling technique.
Solar electric refrigeration uses PV modules and batteries, both of which have negative environmental impacts during their lifecycle. The production process, materials used and disposal of PV systems currently operating in Europe is estimated to have a climate change impact of between 35g and 92g CO2 (equivalent) per kilowatt-hour generated7.
With regard to the batteries, it is essential to dispose of these correctly after use in order to avoid harmful impacts on the environment. Lead acid batteries, which are normally used, can be recycled but the appropriate recycling services are not always in place, especially in the rural areas of developing countries. The method of disposal of the batteries, i.e. recycling possibilities, must be considered when installing a solar electric refrigerator.
The demand for cooling is increasing in many parts of the world, including in those regions where there is no electricity supply and conventional fuels are difficult or expensive to obtain8. The major use for refrigeration in developing countries today is for the preservation of medicines and/or food.
Solar refrigeration offers the chance to reduce post harvest losses and to preserve agricultural products such as fruit, vegetables, dairy products, flowers, meat and fish. This could, on the one hand, prevent surplus produce from spoilage and, on the other, improve food security and variety in areas with no electricity supply.
Access to refrigeration may also benefit farmers and fishermen who sell their products to export markets. Without cooling capabilities the perishable products have to be sold immediately after harvest or treated with another conservation method like drying. This reduces the chance of negotiating good prices, because the buyer is in a better bargaining position.
Vaccines, medicines and stored blood for transfusion need to be kept within a limited temperature range both during transportation and storage. In many parts of the world where there is no reliable supply of electricity, the provision of refrigeration is a major challenge. Currently, these products are most often stored in kerosene or diesel refrigerators. The performance of these refrigerators is often inadequate, the fuel supply is complicated and expensive and greenhouse gases are emitted. Solar powered cooling can offer, therefore, a great opportunity to improve healthcare in developing countries. 2
Solar photovoltaic refrigerators are already commercially available from different companies. There is far less development potential in this case, as it is mainly a combination of already existing and proven technologies (PV system plus electrical fridge). There are also some solar thermal refrigerators commercially available but, to date, the market remains small date and there is much potential for further development. Larger refrigerators for commercial and industrial use have also been developed, but these are only experimental prototypes rather than commercial products. It is estimated that over the past five years, at least 3000 photovoltaic medical refrigerators have been installed.2
Unfortunately, there is no detailed data on the expected growth of demand for solar refrigeration technologies, whereas the market for solar cooling in general (including solar air conditioning and cooling of buildings) is expected to grow rapidly9. Improving the efficiency of the different systems, the use of new sorbent materials and the possibility of joint operation of various refrigeration cycles are key areas to develop in order to scale up the dissemination of solar powered refrigerators.
Solar powered refrigerators offer the potential benefits of lower running costs, greater reliability and a longer working lifespan than kerosene refrigerators or diesel generators, which have mainly been used for cooling in remote areas of developing countries. However, it should be noted that all types of solar refrigerator systems have far higher acquisition and installation costs than comparable kerosene and bottled gas fuelled refrigerators.
The costs for solar refrigerator vary widely, depending on the technology used. Costs can range from between US$ 1,200 (for a simple cabinet) to over US$ 7,000 for a complete system. 1, 10 A kerosene unit costs only about US$650 – US$1300, but uses 0.5 - 1.4 litres of fuel per day, making the life-cycle costs comparable to that of solar refrigerators – but solar refrigerators have no additional fuel costs.
The use of solar refrigerators also eliminates the risks of fuel supply problems and avoids fuel transportation costs in remote areas. Sorption refrigerators have, in addition, lower maintenance costs compared to kerosene and bottled gas fuelled refrigerators. The intended use of the solar refrigerator influences the acceptable price range for solar cooling. The price elasticity for medical applications where only limited capacity is needed can be much higher than for food preservation. As electricity and energy costs in general are predicted to rise in the future, the cost aspect could become a significant factor in the growth of solar cooling and refrigeration devices.