SOLAR ENERGY

Solar energy does not refer to a single energy technology but rather covers a diverse set of renewable energy technologies that are powered by the Sun’s heat. Some solar energy technologies, such as heating with solar panels, utilize sunlight directly.

Other types of solar energy, such as hydroelectric energy and fuels from biomass (wood, crop residues, and dung), rely on the Sun’s ability to evaporate water and grow plant material, respectively. The common feature of solar energy technologies is that, unlike oil, gas, coal, and present forms of nuclear power, solar energy is inexhaustible.

Solar energy can be divided into three main groups—heating and cooling applications, electricity generation, and fuels from biomass.

The Sun has been used for heating for centuries. The Mesa Verde cliff dwellings in Colorado, which date from AD 600, were constructed with rock projections that provide shade from the high (and hot) summer Sun but allow the rays of the lower winter Sun to penetrate.

Today a design with few or no moving parts that takes advantage of the Sun is called passive solar heating. Beginning in the late 1970s, architects increasingly became familiar with passive solar techniques. In the future, more and more new buildings will be designed to capture the Sun’s winter rays and keep out the summer rays.

Active solar heating and solar hot-water heating are variations on one theme, differing principally in cost and scale. A typical active solar-heating unit consists of tubes installed in panels that are mounted on a roof. Water (or sometimes another fluid) flowing through the tubes is heated by the Sun and is then used as a source of hot water and heat for the building.

Although the number of active solar-heating installations has grown rapidly since the 1970s, the industry has encountered simple installation and maintenance problems, involving such commonplace occurrences as water leakage and air blockage in the tubes.

Solar cooling requires a higher technology installation in which a fluid is cooled by being heated to an intermediate temperature so that it can be used to drive a refrigeration cycle. To date, relatively few commercial installations have been made.

Generation of Electricity
Large-scale hydroelectric projects are still being pursued in many developing countries. The simplest form of solar-powered electricity generation is the use of an array of collectors that heat water to produce steam to turn a turbine. Several of these facilities are in existence.

Other sources of Sun-derived electricity involve high-technology options that remain unproven commercially on a large scale. Photovoltaic cells, which convert sunlight directly into electricity, are currently being used for remote locations to power orbiting space satellites, gates at unattended railroad crossings, and irrigation pumps.

Progress is needed to lower costs before widespread use of photovoltaic cells is possible. The commercial development of still other methods seems far in the future. Ocean thermal conversion (OTC) generates electricity on offshore platforms; a turbine is turned by the power generated when cold seawater moves from great depths up to a warm surface. Also still highly speculative is the notion of using space satellites to beam electricity via microwaves down to Earth.

Electricity can be generated by a variety of technologies that ultimately depend on the effects of solar radiation. Windmills and waterfalls (themselves very old sources of mechanical energy) can be used to turn turbines to generate electricity. The energies of wind and falling water are considered forms of solar energy, because the Sun’s heating power creates wind and replenishes the water in rivers and streams.

Most existing windmill installations are relatively small, containing ten or more windmills in a grid configuration that takes advantage of wind shifts. In contrast, most electricity from hydroelectric installations comes from giant dams. Many sites suitable for large dams have already been tapped, especially in the industrialized nations. However, during the 1970s small dams used years earlier for mechanical energy were retrofitted to generate electricity.


Biomass
Fuels from biomass encompass several different forms, including alcohol fuels (mentioned earlier), dung, and wood. Wood and dung are still major fuels in some developing countries, and high oil prices have caused a resurgence of interest in wood in industrialized countries. Researchers are giving increasing attention to the development of so-called energy crops (perennial grasses and trees grown on agricultural land). There is some concern, however, that heavy reliance on agriculture for energy could drive up prices of both food and land.

D Current Status
The total amount of solar energy now being used may never be accurately estimated, because some sources are not recorded. In the early 1980s, however, two main sources of solar energy, hydroelectric energy and biomass, contributed more than twice as much as nuclear energy to the world energy supply.

Nevertheless, these two sources are limited by the availability of dam sites and the availability of land to grow trees and other plant materials, so the future development of solar energy will depend on a broad range of technological advances.

The potential of solar energy, with the exception of hydroelectricity, will remain underutilized well beyond the year 2000, because solar energy is still much more expensive than energy derived from fossil fuels. The long-term outlook for solar energy depends heavily on whether the prices of fossil fuels increase and whether environmental regulations become stricter.

For example, stricter environmental controls on burning fossil fuels may increase coal and oil prices, making solar energy a less expensive energy source in comparison.