Increasingly, the term "solar technology" is being used to include any renewable energy system that directly or indirectly depends on the sun for energy. This includes waterpower, biogas, and wood fuels, for example, which are covered in separate chapters. This chapter is therefore concerned with direct use of solar energy.

Probably the most significant direct solar technology for the South is that of crop drying, which is covered in the section CROP DRYING, PRESERVATION AND STORAGE. Solar distillation for water purification has been covered in the WATER SUPPLY section.

On this page you will find materials on passive solar architecture for house heating, passive solar cooking for the tropics, solar greenhouses, water heaters, cookers, irrigation pumps, and photovoltaic cells. A good general survey of the technologies that may some day have relevance for the South can be found in Technology for Solar Energy Utilization; most of these, however, are not economically competitive at present.

Several publications on passive solar architecture, now a booming field in the United States, are included. Passive solar design involves careful choices of building orientation, layout, location of glass windows, and materials to best take advantage of natural energy flows. Because they minimize the use of costly primary fuels for space heating and cooling, passive solar buildings will eventually dominate new construction in much of the United States. In North America and the other parts of the temperate zones, heating is usually the primary design objective. This is also true in parts of the Himalayas, the Andes, and other mountainous regions, where indigenous structures often reflect certain passive solar principles. In these areas of the South there is the potential for new applications of' recent advances in the field..By contrast, the cooling of living spaces is the primary objective of passive solar design in the tropics. Elements of Solar Architecture for Tropical Regions and Design for Climate: Guidelines for the Design of Low Cost Houses for the Climates of Kenya introduce the basic design considerations for passive solar cooling.

Conventional greenhouses consume large quantities of energy to control the climatic conditions inside. Solar greenhouses, on the other hand, are heated primarily by the sun, and are often attached to houses to provide home heating as well. Low-cost designs using plastic sheeting and local materials may be of relevance in the mountainous regions of the South for supplementary home heating and food production. In the tropics greenhouses are probably mainly of interest because the vegetables grown inside require less water. Several publications reviewed here offer a look at current U.S. solar greenhouses.

Solar water heaters for domestic hot water are becoming more widely used in the industrialized nations as a response to the "energy crisis". They are well suited to temperate regions. A typical system consists of a flat plate collector and storage tank which holds water heated to about 140 degrees Fahrenheit (60 degrees Centigrade).

In tropical regions solar water heating systems can provide hot water for bathing, washing clothes and other uses. (Water heated in a normal flat plate collector does not boil, however, so this is not directly suited to water purification schemes.) Solar water heating in these circumstances is probably best used in health centers and urban homes where there is already a demand for hot water. Katmandu, Nepal is a good example of a city in a developing country with a well-established solar hot water heater industry.

For developing countries, the cost of materials for solar hot water heaters may make them rather expensive. There are more than a dozen ways to make a basic flat plate collector: there is some potential for very low cost designs, particularly if a low pressure (gravity-fed) system is being used. Such collectors can use metal other than copper, and even replace pipes altogether by using shallow tanks. The insulating material behind the collector plate can be local natural fiber such as coconut husks or rice hulls.

Solar cookers have been mentioned as a possible alternative in fuel-short and deforested regions. The 1962 publication Evaluation of Solar Cookers offers a look at many designs, including most of those still being tested today. The high cost, awkward operation, slowness, and inconvenience of outdoor cooking have prevented this technology from finding a niche. Nevertheless, these devices remain a fixture at research centers and exhibitions, where they regularly amaze visitors. To our knowledge, despite two decades of scattered attempts, there are no examples of the successful introduction of solar cookers.

The use of solar energy to drive engines for irrigation water pumps has recently received a great deal of attention. In this application, flat plate collectors provide hot water, which is used to heat liquid gas. The gas expands and drives the engine. The gas then passes through a condenser, where it is cooled by the well water, and the cycle is then repeated. This appears to be one of those solar energy applications that is technically but not economically feasible in developing countries. Though the costs seem to be dropping below $25,000 per installed kw of capacity (the level of a few years ago) they are a long way from being affordable. Locally-built water pumping windmills appear to be a far more cost-effective alternative in areas with even relatively low average wind speeds. In fact, the most thoroughly tested solar pump designs seem likely to remain technological dead ends, built in poor countries only through the intervention of rich country aid. programs. Some of the completely new concepts in solar pump designs may prove more fruitful.

Photovoltaic cells that produce electricity from sunlight will have a place in water pumping and many other high value tasks as the price of these cells comes down significantly. For lowest cost and greatest simplicity, solar pump systems can be designed without electrical storage equipment, and such systems may one day provide important amounts of irrigation water to the world’s small farmers. The photovoltaic cells themselves will continue to be a relatively high technology, imported product in most developing countries. The foreign exchange requirements for photovoltaic cell imports are likely to be a serious barrier to widespread use. In addition, while the decentralized energy production offered is well-matched to settlement patterns in the rural South, electricity is not an energy form well-matched to the energy needs of these communities, where cooking, draft power, and transport are the important energy consuming tasks along with irrigation pumping.

A whole assortment of electrical equipment would be needed to store and make use of solar electricity, and very little of this equipment could be produced or afforded in the villages. The near-term applications are lighting and remote communications equipment in government buildings and the homes of the wealthy; these applications are already booming in some places because of the high cost of alternatives. For an excellent introduction to the subject of photovoltaics and the associated equipment, see Solar Powered Electricity and Solar Photovoltaic Products: A guide for Development Workers.

The sun’s energy is "available" everywhere, yet it is also a diffuse or low-grade energy form. This means that while solar energy is an excellent, low-cost means of creating temperature differences of tens of degrees for drying and heating, it is inevitably difficult and expensive to collect and concentrate solar energy to generate electricity or perform mechanical work. For this reason, drying, heating, and cooling are the most practical solar applications for most communities in the South.

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