There are basically two types of solar collector: non concentrating stationary and concentrating. A non concentrating collector has the same area for intercepting and for absorbing solar radiation where as a sun tracking concentrating solar collector usually has concave reflecting surfaces to intercept and focuses the sun's beam radiation to a smaller receiving area thereby increasing the radiation flux.
Stationary Collectors:
These collectors are permanently fixed in position and do not track the sun. Three types of collectors fall in this category:
1. Flat plate collectors (FPC)
2. Stationary compound parabolic collectors (CPC)
3. Evacuated tube collectors (ETC)
1. Flat-plate Collectors (FPC):
A typical flat-plate solar collector is shown in Fig. (1). When solar radiation passes through a transparent cover and impinges ·on the blackened absorber surface of high absorptivity, a large portion of this energy is absorbed by the plate and then transferred to the transport medium in the fluid tubes to be carried away for storage or use. The underside of the absorber plate and the side of casing. are well insulted to reduce conduction losses. The liquid tubes can be welded to the absorbing plate. The liquid tubes are connected at both ends by large diameter header tubes.
The transparent cover is used to reduce convection losses from the absorber plate through the restraint of the stagnant air layer between the absorber plate and the glass.It also reduces radiation losses from the collector as the glass is transparent to the short wave radiation received by the sun but it is nearly opaque to long-wave thermal radiation emitted by the absorber plate (greenhouse effect).
Flat plate collectors (FPC) are by far the most used type of collector. Flat-plate collectors are usually employed for low temperature application up to 80°C. Flat plate collectors are permanently fixed i n position and require no tracking of the sun. The collectors should be oriented directly towards the equator, facing south in the northern hemisphere and north
in the southern. Flat-plate collectors have been built in a wide variety of designs and from many different materials. They have been used to heat fluids such as water, water plus antifreeze additive, or air. The collector should also have a long effective life, despite the adverse effects of the sun's ultraviolet radiation, corrosion and clogging because of acidity, alkalinity or hardness of the heat transfer fluid, freezing of water, or deposition of dust or moisture on the glazing.
2. Compound Parabolic Collectors (CPC):
Compound parabolic collectors (CPC) have the capability of reflecting to the absorber all of the incident radiation within wide limits. The necessity of moving the concentrator to accommodate the changing solar orientation can be reduced by using a through with to sections of a parabola facing each other as shown in Fig. (2).
Compound parabolic concentrators, can accept incoming radiation over a relatively wide range of angles. By using multiple internal reflections,
any radiation that is entering the aperture, within the collector acceptance angle, finds its way to the absorber surface located at the bottom of the collector. The absorber can be cylindrical as shown in Fig. (2) the lower portion of the reflector (BC and CD) is circular while the
upper portions (AB and DE) -are parabolic. As the upper part of a CPC contribute little to the radiation reaching the absorber, they are usually truncated thus forming a shorter
version of the CPC, which is also cheaper. CPC's are usually covered with glass to avoid dust and other materials from entering the collector and thus reducing the reflectivity of its walls.
These collectors are more useful as linear or through-type concentrators. The orientation of a CPC collector is related to its acceptance angle, [Sc in Fig. (2)]. Also depending on the collector acceptance angle, the collector can be stationary or tracking. A CPC concentrator can be oriented with its long axis along either the north-south or the east-west direction and its aperture is tilted directly towards the equator at an angle equal to the local latitude. When oriented along the north-south direction the collector must track the sun by turning its axis so as to face the sun continuously. As the acceptance angle of the concentrator along its long axis is wide, seasonal tilt adjustment is not necessary. It can also be stationary but radiation will only be received during the hours when the sun is within the collector acceptance angle. When the concentrator is oriented with its long axis along the east-west direction, with a little seasonal adjustment in tilt angle the collector is able to catch the sun's rays effectively through its wide acceptance angle along its long axis.
Two basis types of CPC collectors have been designed; the symmetric and the asymmetric. These usually employ two main types of absorbers; fin type with pipe and tubular absorbers.
3. Evacuated Tube Collectors (ETC):
Evacuated heat pipe solar collectors (tubes) consist of a heat pipe inside a vacuum-sealed tube, as shown in Fig. (3). Evacuated tube collectors have demonstrated that the combination of a selective surface and an effective convection suppressor can result in good performance at high temperatures. The vacuum envelope reduces convection and conduction losses, so the collectors can operate at higher temperatures ( 150°C). Both direct and diffuse radiation can be collected.
Evacuated tube collectors use liquid-vapor phase change materials to transfer heat at high efficiency. These collectors feature a heat pipe (a highly efficient thermal conductor) placed inside a vacuum-sealed tube. The pipe, which is a sealed copper pipe, is then attached to a black copper fin that fills the tube (absorber plate). Protruding from the top of the tube is a metal tip attached to the sealed pipe (condenser). The heat pipe contains a small amount of fluid (e.g. methanol) that undergoes an evaporating-condensing cycle. In this cycle, solar heat evaporates the liquid, and the vapor travels to the heat sink region where it condenses and releases its latent heat. The condensed fluid return back to the solar collector and the process is repeated. When these tubes are mounted, the metal tips up, into a heat exchanger (manifold) as shown in Fig. (3). Water, or glycol, flows through the manifold and picks up the heat from tubes. Because no evaporation or condensation above the phase-change temperature is possible, the heat pipe offers inherent protection from freezing and overheating. This self-limiting temperature control is a unique feature of the evacuated heat pipe collector.
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