REFRIGERATION FOR DEVELOPING COUNTRIES
Introduction
Refrigeration plays an important role in developing countries, primarily for the preservation of food, medicine, and for air conditioning. Examples of these applications are: • In agriculture and dairies: Removal of field heat immediately after harvesting of crops, storage of fruit, flowers, vegetables, milk, meat, and cooling during transport. • In retail trades: sale of fresh foods, fish and cold drinks. • Buildings, computer installations: air conditioning and temperature regulation. • Domestic: food and drink storage. • Health clinics: storage of blood, vaccines and medicine.
Figure 1a: Passive / evaporative coolers
Choice of technology
Cooling can be provided in different ways. The method adopted in industrialized countries depends heavily on grid electricity, supplied continuously and reliably to every part of the country. In contrast, refrigeration is required in developing countries to stimulate agriculture and commerce, in vast areas without a reliable electricity supply. Alternative methods are therefore necessary. A number of approaches can be considered. Three kinds of cooling technology are contrasted in Figure 1 a-c, these are: • • • Passive / evaporative Sorption heat driven Mechanical compression
Figure 1b: Sorption refrigerator
The third method, mechanical compression, is usually dependent on a reliable and continuous supply of grid or diesel generated electricity.
Figure 1c: Compression refrigerator
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Refrigeration for developing countries
Practical Action
The other two methods are therefore more suitable in non-industrialized areas. They require further development on the basis of requests from users in rural locations. Several approaches, which can be considered, are: Production of ice using electricity in regional centres; transport of this ice to agricultural areas; packing of produce to be cooled with ice in insulated containers. Electricity is either grid or diesel generated. Refrigerators, which are electrically driven, use mechanical compression technology. • In some cases refrigerators can be driven directly by mechanical shaft power, for instance where water turbines can be readily installed. • Production of ice using heat driven coolers ('HDCs') on a local level at agricultural production points. Packing of produce with ice for transport. The heat sources for HDCs are varied; it can be from wood, charcoal or agro-waste burnt in open stoves, from fossil fuel in conventional burners, or it can be from thermal solar collectors. HDCs use sorption technology. • Provision of cold storage chambers using either passive, sorption, or mechanical compression technology. If passive cooling is used, temperatures less than 10°C can rarely be achieved. • Provision of cold storage at the point of use using mechanical compression coolers drawing electricity from photovoltaic cells. This is referred to as photovoltaic cooling technology. The most suitable method of cooling chosen will depend upon various factors; the application, the degree of reliability required, the supply of power, the level of skill needed to operate and maintain, training facilities, and available finance. The different technologies should be considered with these factors in mind. As with any technology, sufficient training is especially important; it must be planned as an integral part of an implementation programme and remains a constant concern during the years following installation. This will increase reliability of the system and Time in days reduce life cycle costs dramatically. Temperatures and ventilation Different applications have different requirements for temperature control and ventilation. Figure 2 shows the temperatures needed for the storage of butter, meat, fresh fish and milk. Very often storage of vegetables is complicated by the need for careful ventilation to remove unwanted gases, and to avoid humidity conditions, which would spoil the produce. Relative humidity requirements vary depending on the moisture content of the produce. A simple method of increasing humidity is to sprinkle water on the floor. In vaccine and blood storage very careful temperature control is required. •
Temperatures (°C) Figure 2: Temperatures for safe storage
Passive / evaporative In applications where temperatures between 10-25°C are needed, passive methods can be used, see Evaporative Cooling Practical Action’s Technical Brief. These include traditional methods such as the use of porous jars or wet sack coverings, where the evaporative heat of the liquid, usually water, is drawn into the atmosphere. This method is effective where the atmosphere is naturally dry. Domestic storage devices have been designed along these lines, particularly with the use of charcoal beds, drip-fed with water.
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Refrigeration for developing countries
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Nocturnal cooling in areas where clear night skies are common, can be effective. In airconditioning applications, the use of shade has been developed effectively in traditional architecture, together with evaporative cooling by fountains and roof ponds. Wherever possible passive methods should be used both in agriculture and architecture, since they can be sustained locally and are economic. Only when cooling below 10°C is needed, is it justifiable to look at active cooling technology, requiring complex machinery, and technical maintenance programmes. Sorption Heat driven coolers (HDCs) The principle of sorption refrigeration is shown in Figure 3 which illustrates the simplest type of sorption cooler and which has an intermittent cycle consisting of two phases. Continuous cycles are also possible - the Electrolux uses a continuous cycle. The general term sorption covers both liquid absorption and solid adsorption variants of this technology. Sorption units have some very important advantages. They can be designed to contain no moving parts, so that skilled maintenance personnel and replacements of components are less likely to be needed. Secondly, they are simple to manufacture; local manufacture increases local knowledge of the technology, which improves operation, maintenance and faultfinding. Thirdly, they are readily adaptable to locally available fuels, including biomass and solar energy. Finally, the refrigeration circuit does not use CFCs, which damage the environment. Sorption units are referred to as HDCs (heat-driven coolers).
Refrigeration phase: Because of low temperature in the bed an low pressure, the refrigerant evaporates in the cold box, drawing in heat, freezing water and cooling the storage space. Refrigerant vapour releases heat while it recombines with the sorbent which can either be a solid eg charcoal, calcium chloride, or a liquid eg water. Refrigerant vapour flow Nigh time cooling phase Cold box Heat Active cooling store of ice made
Heat
Refrigerant vapour flow
Day time cooling phase Cold box
Heat Heat
No cooling effect during the day Cold stored as
ice Conventional gas and kerosene-driven sorption units The heat source in conventional sorption Heat phase: At high bed temperature and systems pressure, the refrigerators is usually gas or kerosene refrigerant is driven out of the sorbent and collects in the cold flame. Units powered from gas bottles are box as a liquid used on caravans or boats. A domestic unit, often used in remote locations in Figure 3: Sorption cooler. developing countries is the kerosene-driven Illustration: Neil Noble / Practical Action. Electrolux. It has been calculated that the cost of purchasing and running one of these units is around £1000 for 10 years use. The refrigeration circuits of these devices operate reliably for many years. Maintenance of the burner assembly is required and a constant supply of wicks, burners and lamp glasses are essential. Lastly, the fuel tank must be replenished with kerosene of suitable quality. These units involve the use of hydrogen as a working fluid and cannot be designed as efficient icemakers, although they have some ice-making capacity.
Novel sorption units Novel sorption units are under development for greater efficiency in ice making and cold storage. They do not involve hydrogen as a working fluid. A great deal of emphasis is being placed on design for reliable operation in remote environments where technical maintenance services are not available. Emphasis is also placed, in some cases, on design for local manufacture. Costs and performance figures are not easily available since these units are still on trial. 3
Refrigeration for developing countries
Practical Action
Solar-powered sorption units The heat source for sorption units of the kind shown in Figure 3 can be the sun. In a simple version the heating phase ends at sunset, and the refrigeration phase occurs during the night. If the sun fails to shine for a few days, the ice made on previous days acts as a store of cold, keeping the cold box at a low temperature while it gradually melts. It is expected that a unit producing 100 kg of ice per day can be produced for £4,000 (including the cost of highly efficient solar thermal panels), giving an ice cost of £0.03 per kg. A project that Practical Action is working on in collaboration with SCORE at the University of Nottingham is a thermoacoustic device that uses thermal energy from a stove or solar energy that generates acoustic waves that then produce electricity. http://peswiki.com/index.php/Directory:Acoustic_Generator:SCORE:Stove_for_Cooking,_Refrig eration_and_Electricity More information is available from Paul Riley, Score Project Director, on +44 (0) 0115 951 5600, +44 (0)7973 426 379 paul.riley@nottingham.ac.uk or Internal Communications Manager Tara de Cozar in the University’s Communications Office on +44 (0)115 8468545, tara.decozar@nottingham.ac.uk http://www.score.uk.com/ Mechanical Compression Grid electricity Where a reliable electricity supply exists, the most economic option is to install a standard compressor driven unit. Conventional refrigerators of this kind are sold commercially. As an example, a unit making about 100 kg of flaked ice, for fisheries use, each day in tropical conditions will cost £7000, not including the cost of storage containers for the ice, or delivery. The power consumption would be in the order of 4 kW continuously. There will be extra costs in the form of replacement parts, maintenance and ancillary equipment. Water turbine driven coolers Costs can be reduced if shaft power is used directly to drive the compressor, for example from a water turbine. An auxiliary electricity supply is useful to provide control and protection functions, and for instance to drive ventilation fans. It is nevertheless feasible to design wholly mechanical cold storage and ice-making systems. Diesel generating sets The cost of operating a generator in rural areas is dependent on local conditions and must be assessed in the light of local experience. Quite often the cost can be very much higher than expected because of the need for maintenance personnel and the difficulties encountered in obtaining fuel and spare parts. If the generated electricity is not available continuously then the refrigerator should be designed as an ice-maker, allowing cold to be stored in the form of ice. Experience has shown that systems involving the storage of electricity in batteries have very high costs and are unreliable. Solar photovoltaic systems Solar energy is an intermittent power source, usually available for 12 hours every day. The intensity of insolation is very variable. It can be converted by photovoltaic cells into electricity, which is then stored in batteries, so that a continuous smooth electrical supply can be provided to power a mechanical compression refrigerator. Also see Solar Photovoltaic Refrigeration of Vaccines Practical Action’s Technical Brief. The advantage of using solar power is that it is a source that can be relied upon, never to fail for more than a few days. This reliability is very important in some cases, such as vaccine storage, where loss of temperature control can spoil the vaccines completely. The battery is designed to continue to provide electricity at night and on days when no sunshine is available. In this application, the high cost of photovoltaic cells, batteries and control equipment is justified. The size of the photovoltaic array and the battery capacity must be carefully calculated to provide an economic system.
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Refrigeration for developing countries
Practical Action
Solar refrigeration units of this kind, especially designed for vaccine preservation, are commercially available. A system providing 60-80 watts of cooling is typically priced in the range of £3000-5500. Replacement parts will tend to cost £500-1000 in the course of four years of operation. Most of this cost will be in the replacement of batteries which are designed to have a four year life but can fail in a shorter period if maintained poorly. Replacement costs are considerably reduced if skilled, technical maintenance personnel are available. Combined heating/drying/cooling system Because a refrigerator releases heat it can be used to raise temperatures in agricultural processes like crop or spice drying. The cooling effect can be used to dehumidify the air passing over the crop and the heating effect can be used to warm the air. In this, very high efficiencies can be obtained (for instance up to 7 times as much useful energy produced as required to drive the device). Such efficiencies are commonly met in timber drying plants using these principles. Practical Action is developing low cost methods of utilizing this effect, with respect to drives from small hydro turbines or from steam or diesel engines. A second example is the use of heat from a refrigerator (also known as a heat pump, exactly the same machine) to help sterilize milk, while the same refrigerator cools the milk to preserve it. Choosing the system In order to decide which refrigeration system to adopt for a particular purpose, it is necessary to consider the ongoing inputs required by each system. Table 1 lists the various systems and the inputs required for each. The choice of system is based on the foreknowledge that all the necessary inputs will continue to be available in the locality of the fridge. The mistake is often made of installing a unit with a relatively low purchase cost which later ceases to function through lack of necessary inputs.
COMMENT ENERGY SOURCE PERSONNEL SPARE PARTS 10 YEAR TOTAL COST
MECHANICAL COMPRESSION Grid Electricity available for other purposes, e.g. lighting, flaking of ice. Grid electricity. Cost of connecting/ transforming can be high. Maintenance: skilled personnel. Source of parts may be distant. Supply may be uncertain. Purchase cost, electricity, personnel, replacement parts. Purchase cost, diesel, replacement parts.
Diesel
Electricity available for other purposes, e.g. lighting, flaking of ice. Expensive. Electricity available for lighting, communications, temperature control.
Diesel generator
Maintenance: skilled personnel permanently onsite. Skilled personnel permanently available.
Source of parts may be distant. Supply may be uncertain. Battery life 24 years. Control electronics can fail.
Solar photovoltaic
Irradiation of 1020MJ/day/m². Long cloudy periods problematic.
£3500-6500 for 60-80W cooling, includes replacement costs.
SORPTION (HDCs) Conventional Well known in the field. Gas/kerosene quality must be adequate. Burner parts, wick adjustment, etc. Replacement of burner parts routine. £1000-2000 for 60-100W cooling and small maintenance cost. Purchase cost
Novel (biomass
Local
Any locally available
Maintenance of
Locally
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