Solar thermal collectors and applications

Flat-plate collectors

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 insulated to reduce conduction losses. The liquid tubes can be welded to the absorbing plate, or they can be an
integral part of the 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).

FPC are usually permanently fixed in 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. The optimum tilt angle of the collector is equal to the latitude of the location with angle variations of 10-15° more or less depending on the application [20].

A FPC generally consists of the following components as shown in Fig. 2:

Glazing. One or more sheets of glass or other diathermanous (radiation-transmitting) material.

Tubes, fins, or passages. To conduct or direct the heat transfer fluid from the inlet to the outlet.

Absorber plates. Flat, corrugated, or grooved plates, to which the tubes, fins, or passages are attached. The plate may be integral with the tubes.

Headers or manifolds. To admit and discharge the fluid. Insulation. To minimise the heat loss from the back and sides of the collector.

Container or casing. To surround the aforementioned components and keep them free from dust, moisture, etc.

FPC 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. Their major purpose is to collect as much solar energy as possible at the lower possible total cost. 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, and breakage of the glazing because of thermal expansion, hail, vandalism or other causes. These causes can be minimised by the use of tempered glass.

More details are given about the glazing and absorber plate materials in Sections 2.1.1.1 and 2.1.1.2, respectively. Most of these details apply also to other types of collectors.