Collector absorbing plates
126.96.36.199. The collector plate absorbs as much of the irradiation as possible through the glazing, while loosing as little heat as possible upward to the atmosphere and downward through the back of the casing. The collector plates transfer the retained heat to the transport fluid. The absorptance of the collector surface for shortwave solar radiation depends on the nature and colour of the coating and on the incident angle. Usually black colour is used, however various colour coatings have been proposed in Refs. [25-27] mainly for aesthetic reasons.
By suitable electrolytic or chemical treatments, surfaces can be produced with high values of solar radiation absorptance (a) and low values of longwave emittance (є).
Essentially, typical selective surfaces consist of a thin upper layer, which is highly absorbent to shortwave solar radiation but relatively transparent to longwave thermal radiation, deposited on a surface that has a high reflectance and a low emittance for longwave radiation. Selective surfaces are particularly important when the collector surface temperature is much higher than the ambient air temperature. Lately, a low-cost mechanically manufactured selective solar absorber surface method has been proposed .
An energy efficient solar collector should absorb incident solar radiation, convert it to thermal energy and deliver the thermal energy to a heat transfer medium with minimum losses at each step. It is possible to use several different design principles and physical mechanisms in order to create a selective solar absorbing surface. Solar absorbers are based on two layers with different optical properties, which are referred as tandem absorbers. A semiconducting or dielectric coating with high solar absorptance and high infrared transmittance on top of a non-selective highly reflecting material such as metal constitutes one type of tandem absorber. Another alternative is to coat a nonselective highly absorbing material with a heat mirror having a high solar transmittance and high infrared reflectance .
Today, commercial solar absorbers are made by electroplating, anodization, evaporation, sputtering and by applying solar selective paints. Much of the progress during recent years has been based on the implementation of vacuum techniques for the production of fin type absorbers used in low temperature applications. The chemical and electrochemical processes used for their commercialization were readily taken over from the metal finishing industry. The requirements of solar absorbers used in high temperature applications, however, namely extremely low thermal emittance and high temperature stability, were difficult to fulfil with conventional wet processes. Therefore, large- scale sputter deposition was developed in the late 70s. The vacuum techniques are nowadays mature, characterized by low cost and have the advantage of being less environmentally polluting than the wet processes.
For fluid-heating collectors, passages must be integral with or firmly bonded to the absorber plate. A major problem is obtaining a good thermal bond between tubes and absorber plates without incurring excessive costs for labour or materials. Material most frequently used for collector plates are copper, aluminium, and stainless steel. UV-resistant plastic extrusions are used for low temperature applications. If the entire collector area is in contact with the heat transfer fluid, the thermal conductance of the material is not important.
Fig. 3 shows a number of absorber plate designs for solar water and air heaters that have been used with varying degrees of success . Fig. 3A shows a bonded sheet design, in which the fluid passages are integral with the plate to ensure good thermal conduct between the metal and the fluid. Fig. 3B and C shows fluid heaters with tubes soldered, brazed, or otherwise fastened to upper or lower surfaces of sheets or strips of copper. Copper tubes are used most often because of their superior resistance to corrosion.
Thermal cement, clips, clamps, or twisted wires have been tried in the search for low-cost bonding methods. Fig. 3D shows the use of extruded rectangular tubing to obtain a larger heat transfer area between tube and plate. Mechanical pressure, thermal cement, or brazing may be used to make the assembly. Soft solder must be avoided because of the high plate temperature encountered at stagnation conditions.
Air or other gases can be heated with FPC, particularly if some type of extended surface (Fig. 3E) is used to counteract the low heat transfer coefficients between metal and air . Metal or fabric matrices (Fig. 3F) [13,30], or thin corrugated metal sheets (Fig. 3G) may be used, with selective surfaces applied to the latter when a high level of performance is required. The principal requirement is a large contact area between the absorbing surface and the air. Various applications of solar air collectors are reported in Refs. [31-37]. A design procedure for solar air heating systems is presented in Ref.  whereas the optimisation of the flow passage geometry is presented in Ref. .
Reduction of heat loss from the absorber can be accomplished either by a selective surface to reduce radiative heat transfer or by suppressing convection. Francia  showed that a honeycomb made of transparent material, placed in the airspace between the glazing and the absorber, was beneficial.
Another category of collectors which is not shown in Fig. 3 is the uncovered or unglazed solar collector . These are usually low-cost units which can offer cost - effective solar thermal energy in applications such as water preheating for domestic or industrial use, heating of swimming pools [42,43], space heating and air heating for industrial or agricultural applications.
FPC are by far the most used type of collector. FPC are usually employed for low temperature applications up to 100 °C, although some new types of collectors employing vacuum insulation and/or TI can achieve slightly higher values . Due to the introduction of highly selective coatings actual standard FPC can reach stagnation temperatures of more than 200 °C. With these collectors good efficiencies can be obtained up to temperatures of about 100 °C.
The characteristics of a typical water FPC are shown in Table 2.
Lately some modern manufacturing techniques have been introduced by the industry like the use of ultrasonic welding machines, which improve both the speed and the quality of welds. This is used for the welding of fins on risers in order to improve heat conduction. The greatest advantage of this method is that the welding is performed at room temperature therefore deformation of the welded parts is avoided. These collectors with selective coating are called
Fig. 3. Various types of flat-plate solar collectors.
advance FPC and the characteristics of a typical type are also shown in Table 2.