A selective surface is one which is good at absorbing energy but which does not radiate much energy back to the atmosphere.
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A well designed system using any good quality collector (i.e. evacuated tube, selective surface flat plate or non-selective surface flat plate) can provide a significant contribution to a householder's hot water requirement.
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Types of solar collectors
There are three main types of solar collector for Active
Solar Heating Systems. These are:
Evacuated tubes
Flat plate collectors
Unglazed plastic collectors
Flat plate collectors
A flat-plate collector consists of an absorber, a transparent cover, a frame, and insulation. Usually an iron-poor solar safety glass is used as a transparent cover, as it transmits a great amount of the short-wave light spectrum
.
Only very little of the heat emitted by the absorber escapes the cover (greenhouse effect).
In addition, the transparent cover prevents wind and breezes from carrying the collected heat away (convection). Together with the frame, the cover protects the absorber from adverse weather conditions. Typical frame materials include aluminum and galvanized steel; sometimes fibre glass-reinforced plastic is used.
The insulation on the back of the absorber and on the side-walls lessens the heat loss through conduction. Insulation is usually of polyurethane foam or mineral wool.
Evacuated-tube collectors
In this type of vacuum collector, the absorber strip is located in an evacuated and pressure proof glass tube. The heat transfer fluid flows through the absorber directly in a U-tube or in counter-current in a tube-in-tube system. Several single tubes, serially interconnected, or tubes connected to each other via manifold, make up the solar collector. A heat pipe collector incorporates a special fluid which begins to vaporize even at low temperatures. The steam rises in the individual heat pipes and warms up the carrier fluid in the main pipe by means of a heat exchanger. The condensed liquid then flows back into the base of the heat pipe.
The pipes must be angled at a specific degree above horizontal so that the process of vaporizing and condensing functions. There are two types of collector connection to the solar circulation system. Either the heat exchanger extends directly into the manifold ("wet connection") or it is connected to the manifold by a heat-conducting material ("dry connection"). A "dry connection" allows to exchange individual tubes without emptying the entire system of its fluid. Evacuated tubes offer the advantage that they work efficiently with high absorber temperatures and with low radiation.
Solar Flat Plate v Evacuated Tubes
• Evacuated tube collectors, though much more expensive
than flat plate collectors, achieve both higher temperatures and higher
efficiencies:
• They perform well in both direct and diffuse solar radiation.
This characteristic, combined with the fact that the vacuum minimizes
heat losses to the outdoors, makes these collectors particularly useful
in areas with cold, cloudy winters.
• Because of the circular shape of the evacuated tube, sunlight
is perpendicular to the absorber for most of the day. For comparison,
in a flat-plate collector that is in a fixed position, the sun is only
perpendicular to the collector at noon.
Evacuated tubes and flat plate collectors are used
for Active solar Heating systems for houses and other types of buildings.
The less expensive unglazed plastic type of collector is used exclusively
for outdoor swimming pools where lower water temperatures are required.
Conclusion: - A well designed system using any good quality collector (i.e. evacuated tube, selective surface flat plate or non-selective surface flat plate) can provide a significant contribution to a householder's hot water requirement.