The easiest way of using collected solar radiation is for low temperature heating purposes. Most of the low-temperature solar heating systems depend on the use of glazing, because it has the ability to transmit visible light and to block infrared radiation. High-temperature solar collectors employ mirrors and lenses. Solar thermal engines are an extension of active solar heating and help to produce high temperatures to drive steam turbines to produce electric power. Solar ponds and even ocean thermal energy conversion devices that operate on the solar-induced temperature difference between the top and the bottom of the world's oceans may cover many hectares.
Another way of benefiting from solar radiation is by passive solar heating devices which have different meanings. For instance, in the narrow sense, it means the absorption of solar energy directly into a building to reduce the energy required for heating the habitable space. Passive solar heating systems are integral parts of the building and mostly use air to circulate the collected energy without pumps or fans. In the broad sense, passive solar heating means low-energy building designs, which are effective in reducing the heat demand to the point where small passive solar gains make a significant contribution in winter.
It is well known that black surfaces absorb solar radiation more than any other color and, therefore, when a surface is blackened it will absorb most of the incident solar radiation. Continuous flow of solar radiation onto such a surface will increase its temperature. This will continue until the heat gain from the solar radiation is in equilibrium with the heat loss from the collector. Of course, among the heat losses, there are two types, namely, naturally unavoidable losses and losses due to human uses. The heat can be transmitted to where it is needed through pipes soldered to the metal plate which is heated due to exposure to solar radiation. The heat balance of a collector will have three components in relation as follows (ASHRAE 1981; Dunn 1986):
Absorbed heat - Lost heat = Removed heat by coolant
It is possible to define the coefficient of efficiency for the collector as
Efficiency coefficient = (absorbed heat - lost heat)/incident solar radiation.
In practice, the collectors must be designed in such a manner that the efficiency becomes high. In order to achieve such a goal there are two methods, either the reduction of heat losses or the increase of the incident solar radiation and, hence, the heat absorbed per unit area. For low-temperature collectors heat loss reduction methodology is suitable. It is possible to reduce heat loss by using transparent cover plates, by using specially treated absorber surfaces, and by evacuating the space between the cover plate and the absorber surface. In contrast, for high-temperature solar collectors the efficiency must be increased by increasing the incident radiation through the concentrators. Of course, for this purpose only direct radiation is considered.
There are three heat transfers that should be considered in any solar energy design for efficiency. For any solar radiation collector to work efficiently it is necessary to reduce the heat losses or to minimize them. As a material is heated by solar radiation, it seeks to reach equilibrium with its surroundings by conduction, convection, and radiation processes.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.