What Are Solar Collectors
A solar collector is an installation that captures solar energy contained from sunlight and converts it into heat. Since almost the entire spectrum of solar radiation is used to produce heat, these collectors efficiency is around 60% to 75%.
From a functional perspective, the main component of the solar absorber is the element that converts sunlight into heat energy and produces heat. After this, heat energy is taken from the collector and either stored or used directly.
To reduce inevitable heat loss, there is need for a thermal insulation of the absorbing element from the environment. Depending on the technique used there are different purposes which this can be used for:
- collectors which are using common insulating materials.
- collecting the thermal insulation is achieved by means of a vacuum but with expensive manufacturing technology.
- collectors that rely on simple techniques which are used to heat swimming pools.
Types of Solar Collectors
Flat Plate Collectors
In principle, a solar collector has a rectangular metal housing in which the other elements are mounted. Through a glass window the sunlight falls on a surface that absorbs almost the entire spectral range. The resulting heat energy is not lost as a result of the insulated collector. The heat convection to the outside is limited by one or more windows. In the vacuum collectors this aspect is almost entirely eliminated. Heat radiation due to its own temperature, is also prevented by the window glass which is opaque for higher wavelengths. This heat is retained within the manifold, leading thermal equilibrium at a temperature higher than in the situation without a window. This effect is known as a greenhouse effect. The modern solar collectors use special glass containing as low iron as possible and with increased resistance to hail and snow load.
An absorbent element, in particular in the vacuum manifolds, may have a wavelength selectivity so that, on the one hand can absorb a wide range of solar radiation and, on the other hand, can have an emission as low as possible in the near-infrared range in order to reduce the emission of heat.
The heat absorbing element gives off the heat flowing through copper or aluminium pipes attached. The heat agent transporting heat to the user or to a storage container. Some have open solar heating circuits, which means that through the collector pipes flows even water, necessary for the user. In regions with higher risk of frost, separate circuits are usually used. The primary circuit pertaining to the collector contains a frost-resistant fluid (antifreeze). Heat is transferred from the primary circuit through a water heat exchanger from the secondary circuit, the user.
Evacuated Tube Collectors
Evacuated tube collectors consist of parallel tubes behind which are reflectors, concentrating the solar radiation. Evacuated tubes consist of two concentric glass tubes in between which is space. The internal tube is surrounded by an absorbent, attached to a copper tube through which a heating agent circulates. The vacuum between the tubes minimises heat loss by convection and conduction, allowing superior performance.
Due to higher temperatures, heating system may require special elements to eliminate the danger of overheating. Such collectors are more efficient in areas with moderate temperature, justifying their use in warm areas with only the technical installations where you need higher temperatures. Another advantage is that the adsorbent surface is always perpendicular to the direction of sunlight energy absorbed during the day. The technology used in the manufacture of this type of collector is similar to that of thermal power plants with parabolic troughs.
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