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Technical Briefs and Manuals: English
Refrigeration plays an important role in developing countries, primarily for the preservation of food, medicine, and for air conditioning. Examples of these applications are:
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:
The third method, mechanical compression,is usually dependent on a reliable and continuous supply of grid or diesel generated electricity. 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:
The most suitable method of cooling chosen will depend upon various factors; the application, the degree of reliablity 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 reliablity of the system and reduce life cycle costs dramatically.
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.
In applications where temperatures between 10-25°C are needed, passive methods can be used. 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. 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.
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).
The heat source in conventional sorption refrigerators is usually gas or kerosene flame. Units powered from gas bottles are used on caravans or boats. A domestic unit, often used in remote locations in developing countries is the kerosene-driven 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 are being developed at present 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 many units are still on trial. Projected retail prices for biomass driven units are in the order of £2000 for a 100-kg per day icemaker. Taking fuel and operation costs into account, as well as capital repayment, this represents a production cost of ice of around £0.02 per kg.
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.
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.
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.
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 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.
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. 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.
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.
This article was added to our catalogue on Monday 23 October, 2006.
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