Solar Energy for Developing Countries
Review paper prepared for the
United Nations Environment Programme
By
Dr. Mahmoud Abdel Halim Saleh
Professor, National Research Center
Cairo – Egypt
October – 1978
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I – Introduction
Over two billion people representing more than half the world population, live in the developing countries. Most of the developing countries, beside the extremely low standard of living of their population, have several features in common such as:
i- The majority of the population is employed in agriculture.ii- A large percentage of the people live in rural areas in scattered small villages, with population not more than five hundred persons each.
iii- Practically, neither commercial energy sources nor suitable transport facilities are available in these small villages, expect in the form of human or animal labour. The women in these villages spend most of their time collecting wood, twigs, animal dung and vegetable waste to use as cooking fuel. They walk miles everyday after wells searching for cooking fuel and water to drink.
iv- Due to the lack of commercial energy sources, the agricultural methods in these areas are very primitive hence; the result is lower crop yields and gradual depletion of the natural resources.
v- The people in these areas are not willing to accept easily sophisticated technological developments, due to the low standard of education. Even slightly educated people prefer to migrate from these villages to the urban areas, seeking for industrial and other types of work which are limited. This increases the neglect of the rural areas, and results in further migration to crowded cities, widening the gap between rural and urban standards of living.
vi- Extensive central power generation facilities and distribution systems are unusually non-existent and almost unsuitable for rural areas. The derived end-use energy costs from conventional fuels can be substantially greater in these environments. On the other hand, energy systems using renewable sources, specially the solar source, have a definite economic potential.
vii- Most of the developing countries lie within the “solar – belt” i.e., in the latitude range 35° N - 35 ° S which is characterized by the higher intensity of solar radiation.
When these situations are recognized, it becomes increasingly apparent that in the developing countries, particularly those short of fossil fuels, the utilization of the non-depleting solar energy, offers appropriate solution for meeting a part of their future energy needs especially in rural areas. There is, therefore, ample scope to improve the economic conditions of the people living in rural areas of the developing countries, by the use of the non-polluting solar energy. In addition, the solar energy may be utilized in the urban sectors of the developing countries using sophisticated technologies developed in industrialized countries, after adaptation to the local conditions.
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Several successful trials have been made-in a very limited number of developing countries – for commercial exploitation and local manufacture of solar energy equipment, based on well-known and accepted low temperature technology. These are flat plate collectors, solar water heaters, grain driers, cookers, water stills and space heaters. Also, some solar projects, based on medium and high temperature technology, have been installed in a few developing countries by industrialized countries. These plants are still in the prototype stage. The exact commercial opportunity, with special reference to local manufacturing potential in developing countries, is not yet assessed. Some of the installed projects are based on using concentrating parabolic collectors with and without tracking, others are based on the use of photovoltaic cells and a third group is based on using solar pumps. The developing countries must concentrate on evaluating, adapting and absorbing these imported technologies. In addition, there exist a number of institutions in both developing and industrialized countries that are engaged in research and development of solar equipment, with particular emphasis on the needs of developing countries. These institutions have already developed sound technological concepts, and transformed them into manufacturing prototypes.
II – Energy Needs for Rural Development
Many developing countries have considered, in their plans, the construction of large power plants (thermal, hydro-electric…etc) as essential components of their socio-economic development. These large power systems, though undoubtfully essential, have failed to provide the minimum basic energy needs of the rural areas where the majority of population of these countries live. An illustrative example of this situation is the non-uniform pattern of the energy development in India over the past thirty years, which has been described as “urban islands of energy-affluence amidst vast oceans of rural energy-deprivations.” The amount of energy consumed is rural sectors of India did not exceed 12% of the total energy generated in the whole country. Even this very small percentage was distributed in a highly inequitable manner among the villages. Therefore, the energy systems based on centralized energy production are not adequate for the micro-economic requirements of the rural areas of developing countries. The use of small-scale decentralized, renewable energy source and in particular the solar source, for rural areas seems to be the appropriate solution of the energy problems in many developing countries, especially the poorer ones. The attractiveness of the solar sources arises from the increasing prices of oil, increasing costs of most conventional centralized power stations and also, from ravages of deforestation in many regions. In some rural areas of developing countries, where firewood is used as an energy source for cooking and heating, the over exploitation of forests threatens their renewability.
The increase of the per capita energy consumption in rural areas is a necessary condition for accelerating the socio-economic development. Obviously, there are other social, economic and political factors governing this development. Studies showed that the per capita GNP increases with the per capita energy consumption in an approximately linear fashion. However, this quasi-linear relation has a great slope at its lowest part, i.e. the standard of living rises very rapidly when energy consumption increases from its lowest values. Therefore, the provision of even very small amounts of energy may have a considerable impact on the standard of living in rural areas. The agricultural productivity, rural-based industries, health, education
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opportunities…etc could be much improved when even low power devices are available. The harnessing of solar energy making use of the recent developments in solar technologies could fill a part of the energy gap especially in rural areas.
III. Solar Technologies for Developing Countries
Most significant solar technologies have been developed in the industrialized countries. Some of the technological achievements in such nations could not be easily or wisely transferred to the developing societies. This review is mainly concerned with the solar technologies appropriate for the developing societies, either transferred from industrialized nations or locally developed in these societies. The appropriateness of solar energy technologies is judged by their respect to some criteria and contingencies. The appropriate technologies should fulfill a real need of the society such as water, food, health, clothing, shelter, employment…etc. They must be under the overall control of the utilizer, or at least within the control of the community or society, in which they are used. This does not mean that less developed areas should not consider the use of more sophisticated solar technologies, but that these technologies must provide as much as possible independence of operation. The developing countries must allocate some resources which will permit these technologies to operate should the supply of spare parts or technical expertise cut-off. Moreover, the solar equipment must be environmentally appropriate and compatible with the overall ecosystem and must make use of suitable materials which are able to resist degradation from local climatic and operating conditions.
In this chapter, solar energy technologies which could meet the immediate and the foreseen needs of the developing societies are briefly reviewed. Some of these technologies have been already utilized in the developing countries and others are being field tested in these countries. The review is confined to those technologies utilizing the direct solar source rather than its indirect forms such as wind energy, hydro-energy….etc.
III.1. Solar Collectors
The solar collector is a central element in nearly all dispersed solar energy applications. It is usually the most costly component of the solar energy system and hence its capital cost is a determining factor in evaluating the system economics.
For most applications in developing countries, either flat plate collectors or low level concentrating stationary collectors are adequate. The relatively simple flat plate collector consists of a blackened absorber plate covered by one or more glass or plastic covers. Since glass is transparent to the incident solar radiation, but opaque to the re-radiated energy, the solar energy is trapped. Most flat plate collectors use liquids, such as water, being the best transfer medium which flows in tubes bonded to the absorber plate. Although the water is the most common cooling fluid, air can also be used for heating or drying applications.
Depending on the temperature requirements, a variety of techniques for improving collector performance are available, These include the use of selective absorber coatings,
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multiple glazing, partial vaccum in the space between the absorber and cover plates, honey-comb structures to reduce convective and re-radiation losses and solar concentration.
Low solar concentration levels(less than 10 x) can be achieved with compound parabolic concentrators which require only a few collectors tile adjustments during the year.
For applications requiring temperatures higher than those achievable with flat plate collector or compound parabolic concentrators, tracking concentrators can be used. The most common type is the linear parabolic trough concentrators, in which the cross section of the reflector perpendicular to the collector’s major axis is a constant paraboloid, and the energy is focused on a line rather than a point. Only single axis tracking is needed, and concentration ratios are typically in the 20x – 40x range. The instantaneous efficiency exhibits less sensitivity to outlet temperature, than is typical for flat plates and generally is 40% - 60%. Also other types are available including the parabolic dish, Fresnel lens, and slat type concentrators. The parabolic trough collector is closest to commercial production.
The initial capital cost of various collector systems ranges could be roughly estimated as follows:-Conventional flat plate collectors: 86 – 140 $ / m²Heat trap collectors 130-170 $ / m²A very rough estimate for the costs of the evacuated tube collectors is 215 -430 $ / m² since not exact commercial price is established. There is no commercial experience in low level concentrating collectors. These costs expected to be reduced as a result of increased experience in collector manufacture and production level.
III.2. Photovoltaic Solar Collectors
The simplest and most acceptable means for converting sunlight into electrical energy is by using photovoltaic cells. The solar to electrical energy conversion takes place without involving any rotating parts, which leads to longer lifetime and decreased maintenance costs. Another significant advantage of the photovoltaic cells is that they have nearly constant efficiency independent of the electric loading. On the other hand the capital cost of the photovoltaic cells is still comparatively high and their efficiency is still low. The capital cost of silicon cells which are used now in all available terrestrial solar panels is 15-30 $/ peak watt*. This cost must be reduced by two orders of magnitude at least for the silicon cells to be competitive with the conventional electrical energy converters.
A significant amount of research work is currently going on the industrialized nations, and very few developing countries, to reduce the capital cost by decreasing the costs of the manufacturing processes, improving the efficiency, and by using solar concentration. The achieved efficiencies for silicon cells are 16% for single crystal, 10% for poly-crystalline and 5% for amorphous. An increase in the efficiencies by 2-5% is aimed to be reached in the near future. The expected improvements in the efficiency of commercially available single crystal silicon cells are very small. However a 30-50-% cost reduction is anticipated by improving the manufacturing processes of such cells.
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The most promising alternate materials for photovoltaic cells are until recently, the Cadmium Sulphide and Gallium Arsenide. The Cadmium Sulphide cells have advantage of low cost due to the cheaper manufacturing processes and cheaper material. On the other hand, the material instabilities and the low efficiency (the highest reported is about 8% with 5-6% being typical for production units) of Cadmium Sulphide cells, represent their major disadvantages. Efficiency less than 10% could not be acceptable for terrestrial applications. The good optical electronic characteristics of Gallium Arsenide cells leading to higher conversion efficiencies (18%) make them adequate for terrestrial applications when compared with silicon cells. In silicon the absorption of photons occurs in depth of 100-200 microns, while in gallium Arsenide the same absorption occurs in depth of 1-2 microns. Therefore, much less material is used in Gallium Arsenide cells when compared with silicon cells, thus compensating for the higher material costs. The very good high temperature characteristics of Gallium Arsenide cells make attractive for use with concentrators.
The use of solar concentrators reduces the cost of photovoltaic panels, since the expensive photovoltaic panel area can be replaced by inexpensive reflector area of the compound parabolic reflectors or Fersnel Lenses. However, out of the most recent published reports photovoltaic cells have not been used with concentrators in terrestrial applications. This is due to the fact that the solar panels are usually installed in remote areas, and the use of concentrators complicates the system and makes it less reliable, requiring frequent inspection and maintenance.
III.3. Solar Distillation
The solar water distillation is one of the very attractive applications of solar energy to meet the needs of arid and semi-arid zones in developing countries using simple technologies. The first known modern solar water still was of a large basin type installed more than a century ago in Northern Chile. This solar still supplied fresh water to a mining community for many years.
The solar still usually consists of a basin with black bottom to absorb sunlight. The basin contains the saline water and is covered with transparent glass or plastic sheets which form an air tight enclosure. The glass or plastic covers, slope towards a collection trough. During sunshine, solar energy passes through the transparent cover, and is absorbed by the saline water and the basin liner. The absorbed energy evaporates some of the saline water in the basin which increases the humidity close to the water surface, thus producing convection currents within the still enclosure. The warmer humid air rises up to the cooler glass or plastic covers, where part of the water vapour condenses on their surface and slides down to the collection trough. The condensed fresh water is then taken from the ends; Saline water should be added at a rate of at least twice the rate of production of fresh water, to prevent preciptation.
All efforts in the field of solar water desalination are concentrated on small scale plants which could supply drinking water to small communities.
The initial cost of the solar still is the determining factor of the cost of the output fresh water. Research and development in this area is directed towards reducing the initial costs of the stills, by selecting various geometrical configurations of the stills. Also, the efficiency of solar
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stills is improved by injecting dyes into the saline water. This injection of dyes leads to more effective absorption of solar radiation by the saline water in the stills.
Several solar stills of different sizes have been installed in developing countries Greece, India, Mexico, Somalia Haiti, Pakistan, and Tunisia ….etc.
“The area of a solar cell required to generate one watt under typical noon-time solar conditions (0.1 watt cm²) is referred to as “peak watt” of capacity, and the cost of solar cells is usually given in terms of “US- Dollars per peak watt”.
The solar distillation plants installed in Greece, in Nisiros and Fiskardho, are considered of the most important plants of the world. These stills are manufactured of Aluminum structure with sheet glass walls a covers. The black absorbing surfaces are made of thin sheets of butyl rubber sealed with silicone rubber. The main drawbacks of such constructions are that some of the materials used in them are not often available in developing countries. They need high quality technicians for their manufacture and their cost is relatively high. On the other hand this type of solar stills seems to be more reliable.
In India, a solar water distillation plant of capacity 0.9 m³/day is installed in Bhavnagar. A larger plant of a capacity of 5 m³/day is under construction. The total cost of this installation is approximately 28000$ at about 14$ per m² of the effecti9ve evaporating surface. Simple and reliable technology is used in the construction of this plant, which enables to transfer it to other developing countries.
In Somalia, UNIDO and UNICEF are financing the implementation of solar water desalination project. The net are of the evaporating surface of the stills is about 2000 m². The anticipated fresh water output of the plant, including collected rain fall is 5-6 m³ / day. The plant is composed of 15 blocks, each consist of a six symmetrical and intercommunicating designs of about 13m long and 1.5 m wide. The plant design is a modified version of the design, prepared by Indian Central Sales and Marine Chemicals Research Institute, for its use in India. In the modified design, the piping design is improved, the passages between the stills are decreased, and the inclination of the glass cover is changed from15 º to 20 º, the aluminum substituted the wood in supporting the upper side of the sheet glass, and the locations of the water tanks are changed.
In Mexico, a solar water distillation plant, involving three operating units with rated output of 1-1.5 m³ / day, is under construction. Each unit is composed of individual stills laid on the ground, and connected together with PVC piping. The still consists of fiber glass tray 0.93 x 1.063 m² with a blackened bottom and a glass cover sloping at 30º to the horizontal. The cost of the produced fresh water is 3-4 $/m³ in Mexico.
The utilization of the solar energy for water distillation in developing countries is not limited to the above mentioned projects. More plants are already installed, and under construction in different developing countries.
III.4. Solar Cooking
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In developing countries rich in sunlight, and desperately poor of fossil fuels, fire wood is basically used for cooking. The inefficiency of the stoves using fire wood, the continuous increase of population and the depletion of the fire wood supply, raise the urgent need for developing countries, to find some alternative systems for cooking, even to save the remaining trees. The solar cookers may be appropriate substitutes, in spite of the unsuccessful attempts to introduce them for use. The main difficulties in utilization of the simple solar cookers are the high initial cost, the unavailability to use them indoors, in the evening or in cloudy weathers and the variance in cooking techniques required between solar cookers and traditional practices. The cooking methods are culture bound and adaptations generally evolve slowly. It needs perhaps long time to convince people who cook indoors, in a crouched position, over slow burning fires, to change these habits.
There are two basic types of the solar cookers for outdoor use. The solar hot box, which is in the form of a box with double glazing, is used outdoors in the sun and oriented manually. Higher temperatures inside the cooking chamber are obtained by addition of reflectors. The parabolic reflector cooker concentrates the sun rays on a focal area in which the cooking pot is placed. For indoor cooking the detached solar collector and cooking chamber unit may be used. In these units a working fluid, either water or oil is heated in a separate collector, whether a flat plate collector in case of water or concentrating collectors in case of oil. The heated fluid is transferred to a separate, insulated cooking chamber which can be located indoors where the cooking is done. A new cooking system is proposed, in which chemical heat storage is used. Inexpensive and commonly available chemicals, such as Calcium Chloride, Magnesium Chloride or Ammonia could be used in small systems that require little maintenance. A portable heat package of 20 kg weight is expected to store enough thermal energy to cook a meal (2 kWh) after being exposed to concentrated sunlight for one hour. The heat could be released at any desired time. In some communities each family may have its own complete cooking system, including a solar concentrator and a number of heat packages. In other conditions a communal solar concentrator may be more appropriate and economical with collectively “charged” packages, being carried to the concentrators by individuals. The central concentrator when not charging the heat packages may be used for a variety of other purposes. This proposed system could be used indoors at any time, whether the sun is shining or not.
All the above mentioned systems have not been intensively field tested, and no comprehensive study of solar cooking technology has been made till now. A lot of work is needed to test, modify and adapt these technologies, to the conditions of the developing countries.
III.5. Solar Drying
The use of solar energy to dry and preserve the surplus of the agricultural products is one of the oldest applications of solar energy. Solar drying in its traditional form is still used in many developing countries. The traditional technique is to spread the products to be dried on the ground or platforms, often with no pre-treatment and to expose it to solar radiation and wind. The products are turned regularly until sufficiently dried so that they can be stored for later
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consumption. This technique though cheap, often leaves products of inferior quality due to the absence of control on the rate of drying which is a determining factor for the quality of the dried products. Furthermore, in such a technique the drying process is intermittent being affected by cloudiness and rain. The products are affected by dust and atmospheric pollution, are not safe from intrusion by people and animals, and are subject to infestation by insects. Consequently, artificial solar drying becomes increasingly attractive especially with the increasing price of oil, which is used as a fuel for most of artificial driers, operating in the advanced segments of the developing countries. There will be no social problems in utilizing solar driers since traditional sun-drying is a wide-spread technology practiced in many of the developing countries both is rural areas and urban sectors.
In drying applications, the driers are classified into small scale and large scale driers. The small scale systems are used by individual farmers who produce only a modest amount of agricultural products for their family use, or for sale in the local market in the immediate vicinity. The small scale driers are usually passive systems using only solar or wind energy for their operation. They include, in addition to sun or natural driers, direct solar driers. In direct solar driers, the product is placed in an enclosure with a transparent cover or side panels. The heat generated by absorption of solar radiation on the product, and on the internal surfaces of the drying chamber, evaporates the moisture by circulating currents of the heated air. Large scale systems use convective driers with external power source. Where conventional electric power supplies are available, they are utilizes to supply fans and blowers, in order to increase the efficiency, and to improve the operating performance. In convective solar driers the air is heated in a separate solar collector, and then ducted to the drying chamber to dehydrate the product. There are also mixed-mode solar driers, in which combined action of the directly incident solar radiation on the product, and air preheated in a solar air heater, furnish the heat required to complete drying, which depends mainly on the rate of evaporation of the moisture in the product. Increasing the drying rate, reduces the cost of the drying process, but produces inferior qualities of the dried product, also excessive temperature should be avoided. Furthermore, low drying rates must be used for products having the tendency to form crust. Other solar drying systems, such as the use of greenhouses to dehydrate product and the use of heat extracted from underside of roofs, a system which was utilized since a very long time, may be appropriate for application in developing countries. Also, solar timber kilns are considered as special category of solar driers.
Although a considerable amount of driers are utilized in different developing countries, a lot of technical problems need to be solved, before widespread use in rural areas. One of these problems is how to find out efficient and cheap methods for collection of solar energy, transfer of heat to the working fluid and absorption of heat by the product to be dried.
Hereafter, some selected solar driers utilized in different developing countries, are briefly discussed:
1. In Colombia, an experimental prototype of a vertical natural drying system for cassava particles have been built and tested. The system is composed of two wire-mesh panels supported by two wooden uprights. The drier has two openings on the top and bottom for easy loading and unloading. A wooden roof covers the drier to protect the product from rains and to allow
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continuous drying, day and night. The drier was tested for different distances between the wire-mesh panels i.e. with different loadings of cassava. Good quality dried cassava is obtained from this type of driers.
2. A direct- solar drier of the cabinet type has been used in Syrian Arab Republic for small scale drying of agricultural products. The drier is simply a rectangular container insulated at its sides and base, which have blackened interior surfaces. The container is covered with a double layered transparent cover. The product to be dried is placed on perforated trays. Ventilation takes place by natural convection, as the heated air inside the container passes out of holes drilled in the outlet parts, which are located in the upper parts of the container. Fresh air is drawn through holes drilled in the base.
3. Another types of direct driers known as see-saw drier, was developed in the Ivory Cost and the design was modified by a joint effort of the government of Ghana and FAO. It was designed to dry coffee and coca-beans. The drier consists of a rectangular tray formed in wood. The product to be dried is placed on Bamboo matting base. All the interior surfaces of the drier are coated with black paint. The drier is covered by a transparent PVC layer. The drying frame is mounted on a north-south trestle, having a height equal to one fourth of the frame length. The see-saw motion is thus in the east-west vertical plane, such that the drying frame can face the sun all-over the day. Air circulation takes place, also by natural convection.
4. A prototype of a glass-roof dryer was built in Brazil. It is similar to the green-house having a special ridge cap, made of folded zinc sheet. This cap allows for the outlet of the warm air, while the fresh air is to enter through side shutters. The main additional feature in this prototype is that gas heaters are situated under a part of the drying plate form, to supply heat during raining periods or during night for continuous drying process. Air flow inside the drier is regulated by six wooden shutters, which can be opened or closed independently.
5. A large drier was installed in Barbados to dry corn. This dryer handles 770 kg of freshly shelled corn per day in two stages. The moisture content is supposed to be reduced from 30% to 18% in the first stage, in which a mobile solar air heated drying cart is used, in which air blown through a diffuser duct, using centrifugal fan. Then corn is fed into the blower and transferred to storage – bin drier, where the moisture content of the corn is reduced gradually to the equilibrium moisture content of 13%.
6. A solar timber – seasoning kiln was installed in India, to dry timber at higher rates than those obtained with traditional air – drying method. The kiln consists of a wooden fame structure, which is oriented lengthwise in an east-west axis. The whole structure of the drier, except the north wall, is covered with a double layered transparent polyethylene film. The north wall is sheathed with plywood. The roof of the kiln faces the south and is titled at an angle 27º. A false ceiling is installed inside the kiln above the wood stacks, and also a false north wall are installed covering the total length of the kiln, and extends from the floor up to the false ceiling. A fan driven by a one H.P reversible electric motor, and mounted at the center of the false wall, is used for forced air circulation. The particular design of the kiln allows the circulation of large quantities of warmed air (up to 60º) in either a single – pass or a recalculating mode. This leads to rapid drying of timber, without undue dehydration cracks and wraps.
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Several other types of timber kilns, were constructed and tested in different developing countries such as Puerto- Rico, Philippines, Uganda, Ghana, Madagascar, Tanzania…etc
III.6. Small Scale Solar Power Plants
The conversion of solar energy into mechanical or electrical energy seems to be one of the most important applications of solar energy, both in industrialized and developing countries. The interest and needs of developing countries are limited, at least up to the end of this century, to the small- scale power plants of output powers not exceeding 100 kW. The needs of developing countries are seen particularly in providing electricity to small villages pumping water for irrigation, running small working machines and supplying electrical energy to radio-stations, or telecommunication equipment in remote areas.
The solar-electric energy conversion is accomplished directly with photovoltaic cells. The solar-mechanical energy conversion is performed by using heat engines which can operate at relatively low temperature (95 - 315º C) including organic Rankine – cycle (ORC) and Stirling engines. Although both engines are in development, the ORC is closest to be commercially available. The output mechanical power may be utilized directly to drive irrigation pumps, or may be converted into electrical energy using a conventional electric generators.
The ORC engine consists of a boiler, an expander (turbine, piston or a positive displacement rotary machine), a condenser and a feed pump. The boiler is simply a heat exchanger which can transfer heat from solar collectors (flat plate, parabolic, etc……) into the working fluid. The expander produces the shaft power when supplied with pressurized vapour of the working fluid. The condenser consists mostly of a water – cooled heat exchanger. The feed pump may be reciprocating or centrifugal. Various working fluids may be used. For temperatures below 200º C, R- 11, R -113 and R-114 refrigerants are good working fluids. The R -11 has low pressure and consequently low performance at the temperatures that can be achieved by flat plate collectors. The main drawback of R – 113 is that it needs a regulator, but R -114 is ideally suited to the expected temperature for power generation.
Above 200º C fluids with high thermal stability such as Benzene or Toluene must be used. The efficiency of a solar-ORC system which combines and ORC – engine and a solar collector, is the product of the collector and the engine efficiencies. Since the collector efficiency decrease with temperature, while the engine efficiency increase with temperature, an optimum operating temperature must exist to maximize the overall efficiency of the system. Typical values of the optimum temperature and efficiency for different types of collectors are given in the following table:
Type of collector Optimum Temperature º C Efficiency
Flat plate 110 5% Evacuated tube 150 10% Fresnel lens 200 10%
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With tracking concentrators the efficiency increases gradually and may reach 12% at 350º C. The above values are estimated under the assumptions of a solar intensity equals to 948 W / m², and a maximum cycle temperature 95% of the collector temperature.
The cost of solar – ORC systems decreases with the increase of size and depends on the type of collector used. For a 100 kW (peak) system the cost is $2500 – 5700 /kW with flat plate collectors and $1000 – 3800 /kW with tracking concentrators. However, even with this high capital cost the solar powered ORC – systems can find applications in rural areas of developing countries, especially for water pumping, for irrigation and drinking purposes. The ORC- system almost do not need any maintenance, a single man without any special experience can operate the system and only once per year the system is to be inspected by a qualified technician.
Another type of solar water pumping installations is being developed in India. This type of solar pumps operates without any moving parts. A mixture of petroleum liquids with boiling temperatures 35 - 40º C is evaporated in flat plate collectors, and then flows to a closed tank full of water located in a well. The pressure of the vapour forces the water to rise upwards. The vapour of the working fluid condenses during night and then the process is repeated again in the next day. The method is very simple but the amount of the pumped water is very small and many technical problems appeared while building the first prototype.
Several experimental solar power plants have been installed in different developing countries. Some selected examples of these power plants are reviewed below.
1.a. In Niger an experimental educational TV station powered by photovoltaic cells is constructed and operated since 1968.
b. The approach of the airport of Medina in Saudi Arabia was facilitated since 1973 by illuminating the seven mountain peaks leading to it with light beacons powered by photovoltaic generators.
c. A prototype of a photovoltaic solar water pumping system for rural needs has been developed and is in operation in India during the last year. Two imported silicon solar panels are used for direct conversion of solar energy to electrical energy which is fed to a d. c. motor- pump set. The first panel consists of 4 – modules of 25 peak watt each. The second panel consists of 8 modules of 10 – peak watt each. These panels have been connected for 24 volt nominal operation and were utilized to a 125 – watt pump. The set operates only for six hours during the day.
d. French research group has installed a solar pumping system, using 400 – watt photovoltaic panel, a d.c. motor and a reciprocating pump in Dakar, Senegal. It is reported that the system has an efficiency of 20 – 25%.
2.a. The most powerful solar water pumping system in the developing world is the 30 kW systems operating in Mexico since September 1975. The system was built by SOFRETES (Société Française d’ Etude Thermique et d’ Energie Solaire). The area of the solar collector used is 1500 m². The system uses a turbine as an expander with R -11 as a working fluid. The
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turbine drives a 7400 r.p.m. alternator feeding an electric – centrifugal pump. The discharge rating of the system is 150 m³ / hr at a head of 40 m.
b. More than 40 one-kilowatt solar water pumps using two – cylinder reciprocating expanders and butane or Freon as working fluids, have been installed by SOFERETES and are now being tested in twelve developing countries :- Brazil, Cameron, Cape Verde, Chad, India, Mali, Mauritania, Mexico, Niger, Senegal, United Arab Emirates and Upper Volta. It was claimed that each system operates 5-7 hrs daily and can supply 30 m³ of water per day for a total discharge head of 20 m.
3. A 10 kW power generation plant was developed at New – Mexico, U.S. by a team of experts working for the United Nations Environment Programme (UNEP) before being sent to a project executed by UNEP in Sri – Lanka. The main feature of this project is the integration of different renewable sources of energy, namely solar energy, wing energy and biogas generated from organic waste to produce electrical energy. In this project the expander is a rotating engine developed by sun power systems to use mainly industrial waste energy. However, the working fluid namely Freon could \be evaporated by hot water obtained from flat – plate solar collectors. The engine is based on Rankine cycle. It drives 60 Hz alternator at speed of 1800 r.p.m. It was reported that one of the prototypes has been tested, without significant technical problems.
4. a. A 10 kW solar power station was installed, and being now tested in Madras – India since January 1978. The power plant was built within the scope of a common project, between West Germany and India. The complete heat circulation unit was developed by the German side, while the Indians constructed and financed the collector surface, the heat storage and the necessary building. In this power plant, solar energy is collected using aluminum roll bonded; double glazed flat plate collectors with mirror boosters. The water heated by the collector is pumped at 95º C into a thermal storage tank. The hot water from the tank is pumped to an evaporator where the heat energy is transferred to the working fluid which is R–114. The superheated R-114 operates a rotating helical expander. A voltage controlled 3 – phase, 230 volt, 50Hz alternator is coupled to the expander shaft, through an adjustable gear transmission system, to maintain the alternator speed constant, despite the fluctuations which may occur in the expander speed. The plant auxiliaries such as pumps and control equipments are supplied from the main bus bar of the system via a transformer rectifier set which charges at the same time a storage battery. During the starting up of the plant the auxiliaries are fed from the storage batteries. Different electrical load patterns are stimulated by a bank of incandescent lamps. The system is designed for a full load output of 10 kW and a part load at night of 2.5 kW.
b. The most recent experimental solar power plant started operation in May 1978 in Cairo–Egypt. It is worth mentioning that the first known solar power plant in the world was built also in Cairo in 1912 by Frank Shauman. The present prototype plant was designed and manufactured by West Germany, and the testing is being carried by mutual cooperation of the Germans and the Egyptians. In that plant water is solar heated by a combination of a 400 m² flat plate collectors, and 200m² parabolic troughs, tracking the sun from east to west. A part of the absorbed heat is fed by means of a heat exchanger, into the working fluid which is Freon R-113 and the other part of heat is fed into a thermal hot water storage tank. After sun-set this stored heat energy, is used to run the plant for a period ranges from two up to five hours, at reduced output. The Freon is
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evaporated by the energy supplied from the hot water vapour and then expanded in 42000 r.p.m steam turbine producing mechanical energy. The turbine used is a redesigned exhaust–driven turbo–super charger, namely a radial turbine. A permanent magnet 3–phase, 700 Hz, 15 kVA synchronous generator is directly coupled on the shaft of the turbine. The 3–phase high frequency output of the generator is rectified and then inverted by means of a mechanical D.C./A.C. converter into 220/380 volt, 50Hz output. A load battery of capacity 8 kWh fed continuously during day time ensures that there is enough energy for starting the plant the next morning. The turbine speed is controlled by using fast hydraulic control valve before the turbine to regulate the mass flow. A computerized data acquisition system is attached to the plant. The overall estimated efficiency of the system is about 3% in spite of the fact that the turbine-generator combination has an efficiency of 96%. This very low overall efficiency is due to the poor Carnot efficiency (working temperature is 105oC and condensation temperature is36.5ºC), and the electrical energy consumed in all the auxiliaries of the plant (control unit, data acquisition…etc). The plant starts producing electrical energy around 9.0’ o’clock in the morning, and its full output power is obtained within one hour. The plant is designed for a full net output of 10 kW for 4 hours. Additional thermal energy is stored and converted into electrical energy after sunset at reduced output of 3 kW. The output energy is estimated to be 80 kWh per day and 20 MWh per year with approximately 3000 hours of sunshine per annum.
III.7. Solar Refrigeration and Cooling
The solar refrigeration and cooling is another promising area of utilization of solar energy in developing countries. However, many technical problems are to be solved before the solar refrigeration and cooling systems become economically feasible.
Mechanical or electrical energy produced by a solar power plant, could be used to drive the compressor of a conventional refrigerating or cooling system. However, this approach seems to be inefficient. The use of the heat energy of the solar radiation directly in an absorption system, is the most attractive approach in the present time at least. Among many sophisticated and expensive refrigeration and cooling technologies, the most appropriate technologies for utilization in developing countries, appear to be the evaporative cooling process and the intermittent absorption refrigeration process. Other technologies are either too sophisticated or too expensive for the developing world.
Evaporative coolers in the form of jars or jugs made of earthenware were extensively used in many developing countries in Asia and Africa for hundreds of years to cool drinking and bathing water. Other types of evaporative coolers such as basket coolers woven from bamboo or cupboard coolers manufactured from wood are also suitable for small domestic applications. These types of coolers are very cheap and may be manufactured by the local skills from materials which are available in most of the developing countries.
The intermittent absorption refrigeration systems are perhaps more sophisticated and more costly, however they are adequate for both domestic and communal applications. Solar energy could be used to power such refrigerators.
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A prototype of solar communal refrigerating plant was built in Denmark and will be tested under the tropical climatic conditions in Khartoum–Sudan. The plant has been designed to produce 720 kg of ice in a clear day. It has a collector of 4 m² area. Different absorbent/refrigerant combinations are tried among which Calcium Chloride/Amonia proved to be the best. The cost of building such a plant in a developing country is estimated to be $ 16000, and for 12years working life-time, the estimated production cost of ice is $ 0.02 per kg.
III.8. Solar Water Heating
Water heating by solar energy for domestic applications has now been accepted as a practical means of energy substitution throughout the world. Solar water heaters have been proved competitive and already very popular in many countries. The technology of this application is simple and can be adapted to local conditions in the developing countries.
The main components of the domestic type solar water heater are: - Flat plate collector, storage tank, circulation pipes, and feed water supply. The major problems in solar water heaters are their cost, efficiency, reliability and service life. Many systems use mass produced Aluminum bond duct panels for the absorber plate. Pitting and corrosion are still major problems in open – thermosyphon systems. Absorber designs employing galvanized iron sheets wrapped around galvanized iron pipes, or copper tubes and sheets constructed in the same way are also used but distinct merits of one over the other are still not clearly evident. The cost of the storage tank also contributes significantly to the total cost of the solar water heater, and its volume should be chosen to ensure hot water, day and night taking into consideration cloudy conditions.
The precise potential for domestic solar water heating in developing countries, should be assessed by examining its social acceptability cost effectiveness, and other related factors. In most of the developing countries the domestic water heating will not be an important application except in some limited cold areas. Hot water systems will be more needed for large hotels, hospitals and similar places.
III.9. Solar Space Heating and Cooling
Space heating and cooling by solar energy has been proved to be not only technically feasible, but is also becoming economically competitive with the conventional systems. For the developing countries the space heating and cooling may not have a top priority among the applications of solar energy, nevertheless the use of such systems may improve the human productivity especially under severe climatic conditions, where temperature ranges from the freezing point up to more than 48º C in the different seasons.
Solar space heating and/or cooling systems may be classified into passive and active systems. Passive systems incorporate architectural features to enable the building as a whole to respond to changing internal and external climatic conditions. This may be performed by placing windows to either let in or block out the direct rays of the sun. Movable shutters or insulation can be incorporated over windows or fixed overhangs could be positioned to block out summer sun, but let in winter solar radiation when the sun is at a lower angle. Exterior walls of the building could be designed with massive supporting structural members on the inside for large thermal
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capacitance and insulation on the outside. These and other measures could be taken without little or no increase in the initial cost of the structure.
In active solar heating systems air or water is heated in a flat plate collector by the solar radiation, and then is circulated with the help of a blower or a pump through the space to be heated. For small buildings or single rooms the warm air is naturally circulated by the stack effect and the blower or fan is dispensed with.
Active space cooling systems use solar heat-through mechanical components to produce a cooling effect. The time of greatest cooling load corresponds well with the maximum solar availability, but the conversion of heat to cooling is expensive due to the mechanical equipment involved. Higher temperature is required to operate cooling equipment than required for space heating. The same solar collector may be used for both heating and cooling applications but with lower efficiency for cooling. The most appropriate cooling technology for developing countries seems to be the absorption cycle machine.
An experimental solar space heating project is undertaken by the National Physical Laboratory (NPL) of India. The system which was designed and established by NPL, for a factory, consists of an array of flat plate collectors mounted on the top of the factory. Water is circulated in the collector by means of a pump. The heated water is stored in an insulated storage tank. The hot water from the tank is circulated by means of another pump, in a fan-oil radiator, placed inside the space to be heated. The satisfactory results obtained from this project, encouraged the NPL to design another combined space heating and cooling project for the same factory.
IV. Environment and Solar Energy Utilization
The impacts of the utilization of solar energy on the environmental conditions, such as climate modification, pollution, material balance and ecological balance, represent a determining factor in the future commercialization of the solar technologies. The neglect of environmental factors when developing new energy sources, may adversely affect the environmental conditions in such a way that the life of the people is seriously affected. As a result of that, it would appear advantages to develop solar technologies considering, first, their environmental impacts. On the other hand, the environmental factors may have serious influences on the design features, performance and operation of solar devices. Although the topic is very important, there have been very few; to the best of the author’s knowledge, published comprehensive studies or quantitative analyses on the mutual impacts between environment and solar energy systems. In the present work, only some general remarks are outlined. Detailed studies are urgently needed in this area, to determine qualitatively and quantitatively the positive and negative environmental consequences of utilization of solar technologies in developing countries, and also the effect of the environmental factors on the performance of solar systems.
IV.1. Environmental Impacts of Solar Technologies
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Solar energy systems are pollution free, since they have no exhaust gases or particulate emission as in the case of energy systems using fossil fuels, or radiation hazards, as nuclear power systems. For each million BTU of energy produced by solar, rather than fossil fueled power plants, air pollution would be reduced by three pounds of sulphur oxides, one pound of nitrogen oxides and one pound of particulates.
The solar energy systems have no residuals, which represent a very serious problem for both fossil fuel and nuclear power systems. The radioactive waste disposal is an international power which is not yet solved.
The pollution and the residual waste problems are features of fossil fueled energy systems irrespective of their size. Traditional domestic cooking or baking stoves, agriculture driers, and small internal combustion engines driving water for drinking or irrigation, conventional water and space heating systems and conventional desalination plants, are all polluting and have residual waste, exactly in the same manner as gigantic fossil fueled power plants, but may be on different scales.
The possibility of accidents during transportation of fossil or nuclear fuels and during the operation of energy systems utilizing such fuels, may cause serious water and / or air pollution effects, and may lead to loss of property and death of living organisms such as human beings, animals and plants. This applies to small scale, as well as to large scale systems. An explosion of a domestic Kerosene or Butugas stove used for cooking, baking bread or water heating may start fire in the building where it is used, and could have the same consequences as the explosion of gigantic power station, but – of course – on a smaller scale. Solar energy systems, whether small or large, operate at much lower temperature and pressures without using any combustable materials, which make them not liable to explosions or firing.
The well – acknowledged fast exhaustion of oil resources and the five – fold increase in oil prices, since 1973, virtually guarantee that the developing countries will never depend on the petroleum as an energy source. If the energy demands of the developing countries, were all provided by coal, as a substitute for oil, an absolutely climate modification problem would result. The combustion of coal produces carbon dioxide. The addition of carbon dioxide to the air raises the earth’s temperature by retarding the heat into space (a phenomenon known as greenhouse effect). The carbon dioxide remains in the atmosphere for hundreds or perhaps thousands of years, therefore its emission impact is cumulative and irreversible on any relevant time scale. The rapid increase of the carbon dioxide percentage in the atmosphere would dramatically change the local heat balance. Fortunately, solar sources add no new heat to the global environment and when in equilibrium, they make no net contribution to the atmospheric carbon dioxide.
The resources of fossil fuels, which are mostly used in the urban sectors of developing countries, are delectable and localized in certain areas of the world. Most of the energy now consumed in the rural sectors of developing countries, comes from non-commercial sources, mainly wood. This resulted in large scale deforestation and a critical dislocation of the ecological balance leading to erosion and soil run-off. On the other hand, solar energy is evenly spread over the surface of the globe, inexhaustible and free.
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Climate modification in limited areas using solar energy, as in the case of greenhouses, is one form of controlled environmental impacts of solar technologies. The greenhouses have the advantage of reducing the intense solar radiation, and water evaporation, during daytime, and warm the weather during night, thus protecting the plants from shocks which may occur due to drop of temperature to a very low rate at night, a phenomenon which occurs in most arid zones.
Solar energy utilization in the deserts of developing countries, could change the ecological system in these arid zones, and hence could help in solving the problem of population distribution and the limited cultivated areas. In Egypt, as an example the majority of the population is living in the Nile valley, the area of which does not exceed 6% of the total area of Egyptian territory. The population density in the remaining desert part is about 3 persons/ km² while in the valley this density exceeds 1000 persons/ km². The development of solar technologies to be utilized in these large desert areas would help in the population redistribution to a more uniform pattern.
Solar systems raise, also some fundamental environmental issues for which no certain conclusions could be made whether these issues are in favour or against the utilization of solar technologies in developing countries.
Naturally, a remarkable percentage of the energy in the solar radiation is reflected from the ground back to the atmosphere another remarkable percentage is converted to heat by direct absorption on the ground or as a result of biospheric, or other natural processes. The major part of the remaining amount serves to evaporate water and lastly a small part of the solar energy is converted back into chemical energy, and stored through the growth of land or marine vegetation in all these energy exchanges. The climate is very sensitive to even small fluctuations in the local energy fluxes and especially heat fluxes. An increase in the thermal influx would hold good, if part of solar energy were diverted from its natural course and did not return to it in the short term in the immediate vicinity of the point of interception. A net of off take of a small percentage of the incident solar energy, over a wide area, would have a remarkable climatic and consequently biological repercussion which could not be predicted or assessed at present.
Although the amount of energy extracted from the atmosphere by solar energy systems will be very small, compared with the total energy available, one can not predict the impact of wide- spread use of these technologies, whether there is a maximum limit of the energy to be extracted before unforeseen changes in the climate, or the earth’s energy balance begin to occur.
Among the impact of the solar energy systems on the climatic conditions, one can mention also, the altered patterns of local precipitation and the change in the local transpiration rates due to the extensive paving of land surfaces. Studies carried out on models of large scale, solar energy conversion plants showed that some major changes on the physical parameters of the region containing the plant, may take place. These parameters are the albedo (reflectivity w.r.t. a specific wave length or some wave length distribution of radiation) , surface roughness, surface porosity, surface emissivity, Bowen ratio ( ratio of sensible to latent heat flux at the surface) surface thermal conductivity, surface heat capacity, local energy balance, and re-emission ratio(ratio of energy absorbed and released in the presence of the solar facilities to that in the
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absence of these facility). These parameters represent the boundary conditions of the local climatic system. The changes in these parameters due to installation of solar plant are still unquanitified as functions of the plants configurations. More studies are needed to find out whether small scale solar energy systems will cause remarkable changes in these physical parameters or not.
The large areas of land needed for the collectors of solar power systems, will undoubtfully, have an impact on the local ecosystem.
The installation of the solar collectors over large areas for any solar system, or on the roofs of building for solar spaces or water heating, or any other purposes, would cause aesthetic pollution which would be unacceptable and provoke reactions.
IV.2. Impact of Environmental Factors on Solar Energy Systems
Environmental factors, specially the climatic conditions, have predominant effects on the operation of solar energy systems. The performance of solar devices, rather than any other conventional or nonconventional energy systems, is very sensitive to climate variations. This is due to the fact that the energy source of such systems is the solar radiation. The climatic conditions and consequently the intensity of solar radiation, are time and space dependent,. The solar radiation intensity varies in a very wide range from hour to hour, from day to night and from season to another. However, most of the developing countries lie within the solar belt, and therefore, their solar radiation patterns are not very different. The daily and seasonal fluctuations in the solar intensity, imply the utilization of storage facilities in the majority of the solar energy systems, and limit the use of solar equipment without storage facilities, to certain specified hours in the day-time. An illustrative example is the solar cooker, which could not be used at night or in a cloudy weather.
The amount of the incoming direct solar radiation reaching the ground is, also highly depleted by the atmospheric contaminates, thus degrading the performance qualities of the solar equipment. The very fine dust encountered in many arid and desert areas of developing countries, would seriously affect the operation performance of the solar devices, particulary the solar collectors. The heat trapped by solar collectors or the energy converted by photovoltaic collectors may be reduced in some dusty areas to one half of its normal value. This large collectors areas need to be cleaned periodically. Special design consideration should be undertaken when dealing with solar devices to be installed in different climatic conditions. Any device with rotating parts should be adequately designed to meet these contingencies. An example of that, is an imported photovoltaic array installed in a research institute in West Africa, was found to diminish in output capacity after a period of only one year, due to decomposition of the rubber encapsulating material, holding the solar cells under a glazing. This decomposed material which migrated over the solar cell, reduced their output. The cells were appropriately manufactured but the encapsulating frame was not adequately adapted to the local climatic condition. Another interesting example is the fractures noticed in almost all the glass covering the linear receivers of the parabolic troughs in the 10 kW solar power plants installed in Cairo. These glass tubes of the parabolic troughs were tested of course in the manufacturing country
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Germany. However, they did not withstand the severe climatic conditions of Egypt more than few months
Another environmental factor affecting the utilization of solar energy is the considerable amount of land required for solar collectors. Therefore, the solar applications would be more suitable for a rural environment, where land is easily available rather than urban areas of developing countries. However, in many developing countries farms are small in size and an average farmer may not be willing to allocate even a small portion of extra agricultural land for the collectors, when the area involved exceeds that required for alternative energy system.
V. Society and Solar Technology
The widespread applications of solar technologies in developing countries are certainly governed by social factors. It was claimed by some solar researchers, that the major impediment to solar technology utilization in developing societies, has been neither technical (the devices work) nor economic (many simple devices can be cheaply made), but the problems have mainly social and cultural roots. The solar devices must be socially acceptable and culturally adaptable to the traditions and needs of the population in the developing countries.
Once the solar technologies have proven to be technically feasible, economically justifiable, environmentally appropriate and socially acceptable, they will be widely used in the developing societies. The increased per capita energy due to the utilization of solar technologies could be readily used to increase the standard of living, effectively to improve water and food supply, health, education and employment of population.
It is evident that for a speedy introduction the solar energy in developing countries the governmental and public awareness should be increased. Many officials in the developing countries did not want to settle for second rate solar energy resources, while the industrialized countries flourished on oil and nuclear power. Most of the industries refuse to participate in solar energy hardware development and production. Skilled manpower is badly needed to operate, maintain and repair the solar devices. Occasionally people given solar devices, have not the will to use it because the rigid time requirements of the solar technology disturbed their daily routine or because the direct use of sunlight defied their cultural traditions. In oil producing developing countries the people think that they have oil and there is no need to utilize the solar energy. Others assume that encouraging solar energy use will reduce the value of their exportable oil. The use of some solar devices implies variation in some traditional techniques such as in cooking, irrigation, etc…
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Conclusions
Part of the energy needs of developing countries, both in rural areas and urban sectors, could be satisfied, in the near future by solar energy utilization.
The utilization of appropriate solar technologies will undoubted fully accelerates the socio-economic developments, and will offer an improved quality of life, especially in rural areas. The harnessing of solar energy in the rural remote areas improves the methods of agriculture, resulting in more crops and at the same time facilitates preservation of agriculture surplus for use over all the year. Solar energy may substitute for trees which are used as firewood in cooking, baking bread, heating….etc in rural areas. In oil producing developing countries the use of solar energy leads to substantial saving in the non-renewable oil sources.
The technological developments in some solar applications such as water desalination, agriculture drying and water and space heating, reached a stage at which commercial use of these applications could start. The local industries in the developing countries must be encouraged by the governments to invest more money for production of solar equipments as collectors, driers, stills…etc. Other applications such as cooking, small scale power generation and refrigeration, though they are very essential for developing societies, still need more efforts to prove their techo-economic feasibility when operating in the environmental conditions of the developing countries. Efforts should be continued to reduce the capital costs of these devices and to adapt them to the social and cultural traditions of the developing countries.
It is mandatory for developing countries to raise the public awareness for the advantages of solar energy utilization. Also, the people must be trained to operate and maintain the solar equipment.
It is necessary for the specialists in the developing countries to be aware and to follow all the technical developments in this area in the advanced countries, even those technologies which are not expected to find application in the foreseen future in developing countries. The technologies may include large-scale terrestrial solar power systems, oceans thermal power plants, and solar satellite power stations.
The advanced countries and the international organizations must play a more active role to develop appropriate solar technologies to help in transferring well established solar technologies after their adaptation to provide expertise and to train personal for
It is well acknowledged that solar energy systems are pollution free and have the least environmental impacts when compared with other commercial energy sources, nevertheless, intensive studies should be carried out to identify and evaluate the environmental issues associated with the commercialization of solar technologies in developing societies. The effects of the environmental and social factors on the operation, performance and wide spreading of solar devices, should be carefully examined.
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