Solar Energy

I

Introduction

Solar Energy, radiant energy produced in the Sun as a result of nuclear fusion reactions. It is transmitted to the Earth through space by electromagnetic radiation in quanta of energy called photons, which interact with the Earth’s atmosphere and surface.

The strength of solar radiation at the outer edge of the Earth’s atmosphere when the Earth is taken to be at its average distance from the Sun is called the solar constant, the mean value of which is 1.37 kW per sq m. The intensity is not constant, however; it appears to vary by about 0.2 per cent in 30 years. The intensity of energy actually available at the Earth’s surface is less than the solar constant because of absorption and scattering of radiant energy as photons interact with the atmosphere.

The strength of the solar energy available at any point on the Earth depends, in a complicated but predictable way, on the day of the year, the time of day, and the latitude of the collection point. Furthermore, the amount of solar energy that can be collected depends on the orientation of the collecting object.

II

Natural Transformation of Solar Energy

Natural collection of solar energy occurs in the Earth’s atmosphere, oceans, and plant life. Interactions between the Sun’s energy, the oceans, and the atmosphere, for example, produce the winds, which have been used for centuries to turn windmills. Modern applications of wind energy use strong, light, weather-resistant, aerodynamically designed wind turbines that, when attached to generators, produce electricity for local, specialized use or as part of a community or regional network of electric power distribution.

Approximately 30 per cent of the solar energy reaching the outer edge of the atmosphere is consumed in the hydrological cycle, which produces rainfall and the potential energy of water in mountain streams and rivers. The power produced by these flowing waters as they pass through modern turbines is called hydroelectric power (see Hydro-Power).

Through the process of photosynthesis, solar energy contributes to the growth of plant life (biomass) that can be used as fuel, including wood and the fossil fuels that are derived from geologically ancient plant life. Fuels such as alcohol or methane can also be extracted from biomass.

The oceans also represent a form of natural collection of solar energy. As a result of the absorption of solar energy in the ocean and ocean currents, temperature gradients occur in the ocean. In some locations, these vertical variations approach 20° C (36° F) over a distance of a few hundred metres. When large masses exist at different temperatures, thermodynamic principles predict that a power-generating cycle can be created to remove energy from the high-temperature mass and transfer a lesser amount of energy to a low-temperature mass. The difference in these two heat energies manifests itself as mechanical energy (for example, output from a turbine), which can be linked with a generator to produce electricity. Such systems, called ocean thermal energy conversion (OTEC) systems, require enormous heat exchangers and other hardware in the ocean to produce electricity in the MW range.

III

Direct Collection of Solar Energy

A

Passive Solar Systems

Passive solar systems capture and use solar energy without the aid of mechanical or electrical devices. Most buildings can be regarded as simple passive solar systems, taking advantage of direct gains of solar radiation through windows or skylights. As the solar radiation strikes floors, walls, and other objects within the room it is converted to heat. Good building design can therefore help to reduce the amount of supplemental heating (such as gas or electricity) required by the building during cold periods. However, care must be taken to prevent excessive gains during hot weather, which would cause overheating. This is usually governed by simple fixed shades or movable blinds that control the amount of sunlight entering the room.

Other architectural features such as conservatories can be used to take advantage of indirect solar gains. Again solar radiation passes through the glazing of such design features to heat the floor, wall, and objects within. This raises the temperature of the air within the sun-space (the area that catches the sun) and this warm air is then transferred to the main living or working areas. True “passive” systems achieve this by natural convection (warm air rising creates air currents), but circulation fans may be used to enhance the heat transfer or to provide more control. Alternatively, the warm air can be vented from the sun-space to the outside of the building and used to create a cool air draught within the main living/working areas. An indirect passive solar system can therefore be designed both to provide heat to the main building in cold weather and to help cool the building during hot periods.

Other passive applications of solar energy include solar dryers, which harness the sun’s energy to create warm convection currents to dry crops; solar-box cookers, consisting of an insulated box with a glass lid that captures solar energy to heat food; and simple solar stills, which use solar energy to cause evaporation from salt water, the condensate being collected to provide drinking water.

B

Active Solar Systems

Active solar systems, in the main, refer to solar technologies for water or air heating. These consist of a collector that absorbs solar radiation and transfers the heat energy to a carrier fluid flowing within channels attached to the absorber plate. The percentage of the solar energy incident on the collector actually transferred to the carrier fluid is a measure of the instantaneous collector efficiency.

Solar water-heating systems, which provide hot water at a temperature suitable for washing (generally no more than 80° C/176° F and typically 40 to 50° C/104 to 122° F) are of two fundamental types: flat plate collectors and evacuated tubes. Flat plate collectors generally consist of a black absorber plate (black surfaces absorb more of the incident solar radiation and are therefore more efficient collectors than lighter surfaces), housed in an insulated weatherproof case with a glazed front. The casing helps to reduce conduction and convection heat losses to the surrounding air. The absorber plates of many modern collectors have special selective coatings applied to them that further enhance the thermal performance by reducing radiation losses.

Evacuated tube collectors reduce radiative heat losses to the environment still further by housing the absorber—in this case a metal fin—within a sealed glass vacuum tube (similar to a vacuum flask). A special fluid contained within a closed channel attached to the fin transfers the heat energy collected to the hot water circuit. Evacuated tubes are generally more efficient than flat plate collectors, but because of their greater complexity are also more expensive. Flat plate technology can also be adapted to provide warm air, as opposed to hot water, for space-heating purposes. Active solar-heating systems have been used efficiently for water- and space-heating since the early 1970s. Typical residential applications employ roof-mounted, fixed collectors. In the Northern hemisphere, they are oriented in a southerly direction; in the Southern hemisphere, they are oriented to face north. The optimum angle at which to mount collectors relative to the horizontal plane depends on the latitude of the installation. Generally, for year-round applications such as providing hot water, collectors are tilted (relative to the horizontal plane) at an angle equal to the latitude angle ± 15°, and are oriented to face true south (or north) within ± 20°.

In addition to the flat plate collectors, typical hot-water and comfort heating systems include circulating pumps, temperature sensors, automatic controllers to activate the circulating pump, and a storage device. Either air or a liquid (water or a mixture of water and antifreeze) can be used as the fluid in the solar heating system, and a rock bed or a well-insulated water storage tank typically serves as an energy storage medium.