Green power for African farming: sun, wind and biomass
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CTA. 1991. Green power for African farming: sun, wind and biomass. Spore 31. CTA, Wageningen, The Netherlands.
Permanent link to cite or share this item: http://hdl.handle.net/10568/45429
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Africa's farmers will need increasing supplies of energy if agriculture is to meet the demands of the 21st century. But energy is already a scarce commodity. Although there will always be some reliance on petroleum products, this disparity between...
Africa's farmers will need increasing supplies of energy if agriculture is to meet the demands of the 21st century. But energy is already a scarce commodity. Although there will always be some reliance on petroleum products, this disparity between demand and supply can be, and to a certain extent already is being, diminished by the use of alternative energy sources. Africa currently depends on a mixture of 'modern' (electricity, oil) and 'traditional' (wood, animal traction, and human muscle power) energy. However, demographic growth and the changing rural scene in Africa create particular needs which cannot be met by present resources. Rural areas require energy at low cost because money is scarce. They also require energy from a locally available source because villages are not always connected to the main power grid. And those sources must be renewable since there are ecological constraints on the mass exploitation of trees. Both African and 'northern' research centres, and also industry in 'the north', are now coming up with technical solutions to meet the challenge of agriculture's ever increasing demand for energy. In the past two decades Africa has become a big experimental area for a vast range of ideas, some of which have turned out to be less than appropriate and were swiftly dropped. However others, after considerable trial and error, look very promising. To avoid the common mistake of failing to distinguish between energy for agriculture and energy for the rural population, specialists now refer to 'domestic' end 'agricultural' energy. The needs of domestic energy include those for social and family life: drawing or pumping water for drinking, washing cooking and sometimes heating; lighting; radio and TV; and personal transport. Agricultural needs are for pumping drinking water for stock, farm machinery, spraying and post-harvest equipment (mills, dehuskers, cleaners, peelers and driers), and transport of goods. Rural and agricultural - a common confusion However, the two have some common requirements. The extraction of water, for whatever purpose, can be achieved in the same way, while the technical aspects of transport are no different throughout the world in either town or country, and the consumption of petroleum products takes place everywhere. Both traditional and 'modern' power can fuel the desired modernization of African farming. Modernization inevitably means mechanization with either electric or combustion engines, and even if the use of these is not yet widespread in the African countryside, now is the time to think of appropriate and economical solutions for the future. In most places the majority of agricultural machines will continue using diesel for the foreseeable future except in those sugar producing areas which can make alcohol as an alternative fuel. Smaller fixed engines, on the other hand, can use alternative energy sources - wind, sun and biomass. The first two although inexhaustible and free, nonetheless require investment in wind and solar generators and are also subject to wind strength, cloud and nightfall. Man can, however, more easily control the use of biomass. In the past, the wind and river flow have been the main means of driving machines. Africa has few places where the wind blows with sufficient strength and regularity, except on the coast, and here wind power is put to good use. Islands have particular advantages and the Cape Verde archipelago provides one of the best examples where hundreds of windpumps have created many small oases in an otherwise virtually barren landscape. The basic materials used to build the windpumps were generally imported by the local farmers and, despite their extreme age, they are still working, maintained by workshops in the small townships. Larger, modern windpower projects have also been launched on a national scale. Wind energy for coastal areas The Senegalese coastline is also fairly well equipped with windpumps, and many seaside villages are able to make use of the fresh water which lies deep beneath the saline water table. The equipment for these is often based on the Savonius rotor, produced with the help of the University of Dakar. It consists of two petrol drums cut lengthways and mounted on a vertical shaft. This system is rather less complicated than windpumps with modern aerodynamically designed rotor blades, but it has the twin advantages of turning on a vertical axis, which simplifies its working parts, and of being of self-limiting rotation even in high winds. In Mauritania the recently launched 'Alizes' project will install 101 multipaddle windpumps capable of pumping 15m3 per day, and built on site with the help of IT Dello, a French NGO. According to one of those concerned in the work however, the machines currently in use, although labeled 'appropriate', were unreliable and expensive to maintain. It is crucial that any project should be big enough both to train its own maintenance personnel and eventually be taken over by the local people. These factors are the key to the success of the introduction of any new technology. The agricultural windpump in these three countries, as in most of Africa, pumps the water directly without having to go through the extra stage of producing electricity. As long as the water table is not too deep and there is wind, this is the most efficient way to irrigate small areas in the dry season. During the winter there is less wind, but rainfall is more plentiful. Solar-powered wells Solar power is ubiquitous, free and inexhaustible, but has been utilized for pumping water for only 20 years. Using the sun for drying cereals, coffee and cocoa beans, fish and meat is far older. Solar drying equipment uses a 'greenhouse effect' to raise the temperature within a confined space, and thus dry products faster, more thoroughly and more hygienically, which means they will keep longer. Solar driers are the result of study and adaptation by several African research institutes (for instance in Senegal, Niger, and Mali and Malawi), and are now sufficiently sophisticated and cheap to justify their purchase by agricultural cooperatives. For highly perishable goods such as fish, solar driers can both speed up the drying process, and economize on wood. Some equipment of this type has been installed on Senegalese beaches where the fish is landed. The 'modern' use of solar power is to produce electricity, and there are two means of doing this. The thermodynamic method consists of producing electricity from a low temperature motor. The motor works by the expansion of Freon, which is a liquid at ambient temperature and a gas when warmed by contact with solar-heated water. It works like the cooling system in a refrigerator, but in reverse. Several dozen experiments in the Sahel and signal failures like that of the large thermodynamic station at Dire in Mali have led to the abandonment of this complicated system which now is used only for heating domestic water in towns. The other method, the photovoltaic cell, uses silicon cells to produce electricity directly and such solar panels have been adopted more widely. As soon as the sun's rays touch these cells they generate a current by emitting electrons. Without human intervention a pump submerged in a well, river or deep borehole will start to work automatically in sunshine and will continue for about eight hours until the sun's rays are too low to strike the panels effectively The photovoltaic system appears ideal in theory and has given rise to many a dream and hundreds of projects. In fact, as with all new technology, there have had to be many experiments, modifications and improvements to equipment and materials till they have become completely reliable. It has also been necessary to prepare the market, train personnel, organize supplies of spare parts and arrange the management of the water supplies at village level. Photovoltaic solar energy may offer a realistic alternative, but the sites for installation must be carefully selected, and the people must be truly capable of maintaining and operating it. For example, in Mali a big multi-use photovoltaic station was established for watering five ha of market garden in the morning, providing the 300 village inhabitants with drinking water at midday, and allowing a hundred or so zebus to drink in the evening. It cost approximately $US15,000 five years ago. The price has dropped considerably since. The experience acquired over the years has allowed mass production and the costs of silicon cells has been reduced. This in turn permits village collectives and private businessmen to install small-scale plants as long as the irrigated produce pays off the investment. Photovoltaic solar power is now becoming more widespread and, if the price continues to drop, could be well suited to the needs of intensified agricultural production. These are centres of attraction grouped around water supply points and could stop the exodus of young people from the land. Agricultural waste is a source of energy that is already being used in agri-industry. The shells or husks of coffee beans, peanuts and coconut are often burned in factories to power industrial driers or steam generators. Saw-mill waste (up to 40-60% of the roundwood) can be compacted and burned in industrial furnaces or put through gasproducing plants. Engines which can use the low-grade gas produced by these plants are twice as expensive as diesel motors but, if the wood-waste is free, they will pay for themselves in about two years. The gas plants can also consume wet agricultural waste to make electricity. A large rice-growing area in Cameroon runs two large gas turbines on rice waste. Growing green fuel The situation is somewhat different for the individual peasant farmer. He does not have waste available throughout the year and what he does have he may wish to use in many different ways (mulching, fodder, house-building). He has to make a choice between biogas (see box) and his other needs. One solution which does not entail having to choose between these different uses for biomass is to grow green energy. It is possible to do this anywhere where water is plentiful and accessible and as long as the yield from the trees planted rises to 30 tonnes per ha per annum. Such 'forest-farmers' need no longer depend on the uncertainties of fuel availability in order to irrigate their fields and run their farms, but can rely solely on their own crops and harvest waste. In India, village plantations of green energy are irrigated and even treated with fertilizer, and produce around 100 tonnes/ha/ pa from around 25,000 trees/ha. Eight thousand farmers in the province of Gujarat have planted more than a million trees of many different species which not only produce gas but benefit the environment as well, and provide compost. This sort of integrated approach is still largely unknown in Africa for a very good reason: in areas suffering from drastic deforestation it is difficult to extol the merits of gas-producing' wood. However, Michel Matly, an energy specialist, denies there is a problem: 'The gas-producing plant may well consume large amounts of vegetation,' he says. 'But it also promotes the production of far more than it destroys. The energy from one tree gives enough to grow six more, or to grow two and irrigate fields as well.' Prospects for developing new, low-cost energy sources for Africa based on renewable resources that are locally available therefore seem reasonably good. Methodologies based on wind power and the sun's energy will remain relatively costly, and it will often be difficult and expensive to adapt them to local conditions. They may only be appropriate in areas of intensive production. On the other hand, the use of biomass, whether produced deliberately or as a byproduct, seems to offer better prospects. The success of biomass-based technologies will depend partly on man's ingenuity and partly on the extent to which African farmers can be convinced of the multiple benefits. Despite the fuelwood shortages, it has proved difficult to establish conventional fuelwood plantations in Africa: persuading people to grow trees for gas production will be no less easy. But in the long run, it may prove more rewarding.
SubjectsCROP PRODUCTION AND PROTECTION;
- CTA Spore (English)