Flat plate collectors.ppt (Size: 2.78 MB / Downloads: 48)
Solar collectors are heat exchangers that use solar radiation to heat a working fluid, usually liquid or air. They can be classified in three groups:
- Evacuated-tube collectors
- Focusing collectors.
In flat-plate collectors there is no optical concentration of sunlight and they are generally stationary . In addition to this their outlet temperature capability is below 100 °C
However to reach higher temparatures evacuated-tube collectors and focusing collectors are used. In evacuated-tube collectors they use vacuun to reduce heat lost and to protect the absorber coating from deteration.By this way they can reach temperatures up to 140 °C and they can collect both direct and diffuse solar radiation
And focusing collectors, they are not stable and they follow the sun to get direct radiation; theycan not utilize diffuse radiation. And they are also capable of producing high temperatures .
A flat plate collector is basically a black surface that is placed at a convenient path of the sun. And a typical flat plate collector is a metal box with a glass or plastic cover (called glazing) on top and a dark-colored absorber plate on the bottom. The sides and bottom of the collector are usually insulated to minimize heat loss.
Components of a typical flat plate collector:
It is usually made of copper, steel or plastic. The surface is covered with a flat black material of high absorptance.If copper or steel is used it is possible to apply a selective coating that maximizes the absorption of solar energy and minimizes the radiation emitted by plate.
The flow passages conduct the working fluid through the collector. If the working fluid is a liquid , the flow passage is usually a tube that is attached to or is a part of absorber plate. If the working fluid is air , the flow passage should be below the absorber plate to minimize heat losses.
Absorber plate & Flow passages
Copper, which has high conductivity and is corrosion- resistant, is the material for absorber plates, but because copper is expensive, steel is also widely used. For a copper plate 0.05 cm thick with 1.25-cm tubes spaced 15 cm apart in good thermal contact with the copper, the fin efficiency is better than 97 percent.
The surface of the absorber plate determines how much of the incident solar radiation is absorbed and how much is emitted at a given temperature. Flat black paint which is widely used as a coating has an absorbance of about 95 percent for incident shortwave solar radiation. It is durable and easy to apply .