太陽能熱水

Technology Outline

1) Solar Collectors

The solar collector is by far the most widely used solar energy conversion device, and there are millions in use around the world. Solar collectors can be classified into two major types based on design, i.e. flat-plate collectors and evacuated-tube collectors, with the latter further divided into glass-glass type and glass-metal type.

(a) Flat-plate solar collectors

A flat-plate solar collector contains a metal absorber plate (made of copper or aluminum) enclosed in an insulated rectangular box with a glass or plastic cover. The absorber is usually painted black to maximize heat absorption. The tubes for the heat transfer medium (i.e. water), which are usually made of copper, are connected conductively to the absorber. When the solar radiation hits the absorber, the major part of it is absorbed and a small part reflected. The absorbed heat is conducted to the tubes or channels for the heat transfer medium.

(b) Evacuated-tube solar collectors

i. Glass-glass type

The collector contains parallel rows of transparent tubes. Each tube is made up of an outer glass tube and an inner glass tube. The inner tube is coated with an absorber coating that absorbs solar energy well but minimizes radiant heat loss. A thermal conducting plate with a U-tube is inserted into the inner glass tube. The water to be heated flows in the U-tube. Air is removed from the space between the outer glass tube and the inner glass tube to form a vacuum so as to reduce conductive heat loss.

ii. Glass-metal type

Glass-metal tubes are further divided into direct flow-through type and heat-pipe type.

For direct flow-through evacuated-tube collectors, the absorber in the form of metallic fins or metallic cylinder is installed inside the glass tube. Air is removed from the glass tube to create a vacuum. Water flows in U-pipe which is attached to the absorber inside the glass tube.

For heat-pipe evacuated-tube collectors, a heat pipe is attached to the absorber inside the vacuum glass tube. The heat pipe is filled with a working fluid with low boiling point (such as alcohol). At the upper end of the heat pipe is a condenser bulb where heat exchange takes place. The tubes are mounted, with the condenser bulbs up, into a manifold (or storage tank in the case of packaged solar water heater). Heat energy collected by the absorber fins vaporizes the working fluid, which rises into the condenser bulb in the form of vapour. Water from the recirculation loop flows through manifold and picks up the heat from the condenser bulbs. The condensate of the working fluid then returns to the collector heating zone by gravity.