Collector construction

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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 labor or materials. The materials most frequently used for collector plates are copper, aluminum, 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. The convective heat loss in a collector is relatively insensitive to the spacing between the absorber and the cover in the range of 15-40 mm. The back insulation of a flat-plate collector is made from fiberglass or a mineral fiber mat that will not outgas at elevated temperatures. Building-grade fiberglass is not satisfactory because the binders evaporate at high temperature and then condense on the collector cover, blocking incoming solar radiation.

Figure 3.3 shows a number of absorber plate designs for solar water and air heaters that have been used with varying degrees of success. Figure 3.3a shows a bonded sheet design, in which the fluid passages are integral to the plate to ensure good thermal conduct between the metal and the fluid. Figures 3.3b and 3.3c show fluid heaters with tubes soldered, brazed, or otherwise fastened to upper or lower surfaces of sheets or strips of copper (see also the details in Figure 3.2). 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. Figure 3.3d 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, which could melt the solder.

The major difference between air- and water-based collectors is the need to design an absorber that overcomes the heat transfer penalty caused by lower heat transfer coefficients between air and the solar absorber. Air or other gases can be heated with flat-plate collectors, particularly if some type of extended surface (Figure 3.3e) is used to counteract the low heat transfer coefficients between metal and air (Kreider, 1982). Metal or fabric matrices (Figure 3.3f) (Kreider and Kreith, 1977; Kreider, 1982), thin corrugated metal sheets (Figure 3.3g),

Water systems Air systems

Water systems Air systems

Staggered Stud Partition

FIGURE 3.3 Various types of flat-plate solar collector absorber configurations for water and air.

or porous absorbers may be used, with selective surfaces applied to the latter when a high level of performance is required. The principal requirement of these designs is a large contact area between the absorbing surface and the air. The thermal capacity of air is much lower than water, hence larger volume flow rates of air are required, resulting in higher pumping power.

Reduction of heat loss from the absorber can be accomplished either by a selective surface to reduce radiative heat transfer or by suppressing convection.

Cpc Collectors With Absorber Fins
FIGURE 3.4 Photograph of a plastic collector absorber plate.

Francia (1961) 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 Figure 3.3, is the uncovered or unglazed solar collector. These are usually low-cost units that can offer cost-effective solar thermal energy in applications such as water preheating for domestic or industrial use, heating of swimming pools (Molineaux et al., 1994), space heating, and air heating for industrial or agricultural applications. Generally, these collectors are used in cases where the operating temperature of the collector is close to ambient. These collectors, usually called panel collectors, consist of a wide absorber sheet, made of plastic, containing closed-spaced fluid passages (see Figure 3.4). Materials used for plastic panel collectors include polypropylene, polyethylene, acrylic, and polycarbonate.

Flat-plate collectors (FPCs) are by far the most-used type of collector. Flat-plate collectors are usually employed for low-temperature applications, up to 80°C, although some new types of collectors employing vacuum insulation or transparent insulation (TI) can achieve slightly higher values (Benz et al., 1998). Due to the introduction of highly selective coatings, actual standard flat-plate collectors can reach stagnation temperatures of more than 200°C. With these collectors good efficiencies can be obtained up to temperatures of about 100°C.

Lately some modern manufacturing techniques such as the use of ultrasonic welding machines have been introduced by the industry that improve both the speed and the quality of welds. This is used for welding of fins on risers, 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.

3.1.2 Compound Parabolic Collectors (CPCs)

Compound parabolic collectors (CPCs) are non-imaging concentrators. They have the capability of reflecting to the absorber all of the incident radiation

Non Imaging ConcentratorsCompound Parabolic Solar Collector

Wedge absorber Tube absorber

FIGURE 3.5 Various absorber types of CPCs.

Wedge absorber Tube absorber

FIGURE 3.5 Various absorber types of CPCs.

within wide limits. Their potential as collectors of solar energy was pointed out by Winston (1974). The necessity of moving the concentrator to accommodate the changing solar orientation can be reduced by using a trough with two sections of a parabola facing each other, as shown in Figure 3.5.

Compound parabolic concentrators can accept incoming radiation over a relatively wide range of angles. By using multiple internal reflections, any radiation entering the aperture within the collector acceptance angle finds its way to the absorber surface located at the bottom of the collector. The absorber can take a variety of configurations. It can be flat, bifacial, wedge, or cylindrical, as shown in Figure 3.5. Details on the collector shape construction are presented in Section 3.6.1.

Two basic types of CPC collectors have been designed: symmetric and asymmetric. CPCs usually employ two main types of absorbers: the fin type with a pipe and tubular absorbers. The fin type can be flat, bifacial, or wedge, as shown in Figure 3.5 for the symmetric type, and can be single channel or multichannel.

Compound parabolic collectors should have a gap between the receiver and the reflector to prevent the reflector from acting as a fin conducting heat away from the absorber. Because the gap results in a loss of reflector area and a corresponding loss of performance, it should be kept small. This is more important for flat receivers.

(a)
Parabolic Solar Reflector Cell Panel

FIGURE 3.6 Panel CPC collector with cylindrical absorbers. (a) schematic diagram. (b) Photo of a CPC panel collector installation.

FIGURE 3.6 Panel CPC collector with cylindrical absorbers. (a) schematic diagram. (b) Photo of a CPC panel collector installation.

For higher-temperature applications a tracking CPC can be used. When tracking is used, this is very rough or intermittent, since the concentration ratio is usually small and radiation can be collected and concentrated by one or more reflections on the parabolic surfaces.

Compound parabolic collectors can be manufactured either as one unit with one opening and one receiver (see Figure 3.5) or as a panel (see Figure 3.6a). When constructed as a panel, the collector looks like a flat-plate collector, as shown in Figure 3.6b.

3.1.3 Evacuated Tube Collectors (ETCs)

Conventional simple flat-plate solar collectors were developed for use in sunny, warm climates. Their benefits, however, are greatly reduced when conditions become unfavorable during cold, cloudy, and windy days. Furthermore, weathering

Heat pipe condenser

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Responses

  • clara
    Why is copper used for collector plates?
    8 years ago
  • Annikki
    How to make a cylindrical absorber cpc solar heating?
    8 years ago

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