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CHAPTER 2: LITERATURE REVIEW

2.2 Components of a FPSC

A brief description of the main components of a typical FPSC along with their functions and materials used for manufacturing will be presented and clarified in the following sections.

2.2.1Transparent cover

Most flat-plate collectors incorporate at least one transparent cover made of glass or plastic. The cover protects the absorber and reduces the energy lost from the upper

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surface of the FPSC. The collector’s cover diminishes heat losses by convection via containment of an air layer and by radiation in that it should exhibit a high transmittance for solar radiation (wavelengths 0.3 to 2.5 µm) in order to maximize the solar input to the absorber, and intercept the thermal radiation of wavelengths greater than about (3 µm), which is emitted by the hot absorber plate. FPSC’s covers essentially perform similar functions to those of glass in a greenhouse (Gillett & Moon, 1985; Kalogirou, 2009). Therefore, optical properties of the cover plate are of considerable importance in collector design (Ting, 1980).

The main features of the transparent cover are; absorptance of solar energy (αg), which is the absorbed portion of incident solar radiation; reflectance of solar energy (Rg), which is the reflected portion of incident solar radiation; and the transmittance of solar energy (τg), which is the transmitted portion of incident solar radiation. For higher FPSC’s efficiency, the values of the absorptance and reflectance should be the least possible and transmittance’s values should be the highest possible. The absorptance (g), reflectance (Rg), and transmittance (g) of solar energy for the transparent cover can be linked according to the conservation of energy law as follows (Duffie &

Beckman, 2013);

𝑔+ 𝑅𝑔 + 𝜏𝑔 = 1 (2.1)

The most widely used material for the FPSC’s cover is glass, which may be attributed to its high transmittance, around 90% of the incoming solar radiation, and high opaqueness for solar radiation emitted by the FPSC’s absorber plate. The main disadvantages of glass are that it is brittle, relatively expensive, and has a high density (Gillett & Moon, 1985; Amrutkar et al., 2012).

The effect of glass cover thickness on the performance of a FPSC was experimentally investigated by Bakari et al. (2014). Four different thicknesses of glass,

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i.e., 3, 4, 5, and 6 mm, were used as the transparent cover for the four 0.72m2 FPSCs that were constructed. Results proved that varying the thickness of the glass cover affected the collector’s efficiency, and the highest efficiency was reached using the 4mm glass thickness. Kalogirou (2009) indicated that for a spacing between the glass cover and the absorber plate in the range of 15–40 mm, the convective heat loss in the FPSC is almost independent of spacing. Consequently, a 4mm glass with 15mm spacing was selected as the transparent cover of the FPSC used in the experimental setup of this research.

2.2.2 Absorber plate and riser tubes

The main purpose of the absorber plate is to absorb the highest possible of the solar radiation transmitting through the transparent cover of the FPSC, to waste the lowest possible heat losses, and to transfer the collected energy to the flowing heat transfer fluid in the riser tubes (Amrutkar et al., 2012; Duffie & Beckman, 2013). An absorber plate may be made from any of a wide range of materials, or in some cases from more than one material. Copper, stainless steel, mild steel, aluminum and plastics are all used (Gillett & Moon, 1985; Kalogirou, 2009). The selection of the suitable material is dependent on many factors such as thermal conductivity, weight, cost, and availability (Amrutkar et al., 2012).

The nature and quality of the bond between the riser tubes and the absorber plate has a noticeable effect on the thermal performance of the FPSC. Better bond will provide improved heat transfer from the absorber plate to the riser tubes. Brazing, welding, press-fitting, or using high temperature solder can provide this bond. It is practically important to select a bonding system which can resist both high temperatures and temperature cycling (Gillett & Moon, 1985; Badran et al., 2008).

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Using an electric resistance heater to emulate the energy input to the absorber plate from solar radiation, Badran et al. (2008) experimentally studied the bond conductance between the riser tube and absorber plate of five locally-made FPSC’s samples. All the samples were enclosed with a 5-cm thick insulation to eliminate energy loss. Through evaluating the generated heat flux of the electric heater and the energy transferred to the working fluid, the bond conductance was calculated and found to be in the range of 6.31.8 W/m K. From all the samples tested, the one that was manufactured using the press-fit method showed the highest conductance value.

The FPSC used for performing the experimental test runs in the present study was built using a 2-mm copper absorber plate and 12.7-mm copper riser tubes. The absorber plate was solder bonded to the riser tubes all over the contact length.

2.2.3Thermal insulation

The conduction heat losses from the edges and back side of the FPSC can be eliminated by applying insulation materials. An optimum thickness may be determined on the basis of cost and effectiveness. The three most important factors other than cost that should be considered when choosing insulation materials are their resistance to temperature, durability in the presence of moisture, and thermal conductivity. Common insulation materials are glass-wool, mineral-wool and polyurethane foam (Gillett &

Moon, 1985).