0.06 X 1008
Qu = AcFR[S - UL(T - Ta)] = (1.2 X 4) X 0.614[801 - 6.5(323 - 288)]
Finally, the collector efficiency is n = =_^_= 0.396
3.5 PRACTiCAL CONSiDERATiONS FOR FLAT-PLATE COLLECTORS
For various reasons, the actual performance of a flat-plate collector may be different from the one obtained from the theoretical analysis presented in this section. The first reason is that the fluid flowing through the collector may not be uniform through all risers due to manufacturing errors. The section of the collector receiving a lower flow rate will have a lower FR and therefore inferior performance. Leaks in air collectors are another reason for poorer performance. Additionally, for multi-panel collectors with serpentine absorbers, which are installed one next to the other, the edge losses are limited to the first and last collectors of the array, resulting in an improved UL compared to that of a single collector.
Problems related to freeze protection of collectors are dealt with in Chapter 5. The effect of dust collected on the glass cover of the collector in an urban environment seems to have a negligible effect, and occasional rainfall is adequate to clean the surface. For those wishing to account for dust in temperate climates, it is suggested that radiation absorbed by the collector plate is reduced by 1% and for dry and for dusty climates by 2% (Duffie and Beckman, 1991). Degradation of the cover materials, however, can affect transmittance and seriously affect the long-term performance of the collector. This is more important in plastic collector covers. The same applies for the absorber plate coating. Additionally, the mechanical design of the collector may affect its performance, as for example the penetration of water or moisture into the collector, which would condense on the underside of the glass, thus significantly reducing its properties. A description of quality tests to verify the ability of the collector to withstand this and other effects is given in Chapter 4.
Concerning the manufacture of the collectors, it is important to have a collector casing that will withstand handling and installation and be able to enclose the collector elements and keep them clean from water and dust penetration for the life of the collector. In high latitudes, the collectors should be installed at an inclination to allow the snow to slide off their surface.
Installation of collectors is related to three elements: the transportation and handling of the collector, the installation of brackets, and manifolding. The first is related to the overall weight and size of the collector. For small (~2m2)
collectors this can be done by hand; for bigger collectors, the help of machinery is required. The bracketing should be adequate to withstand wind loading, whereas manifolding can be the most time-consuming operation, although nowadays, special bronze fittings are available that make the work easier. Attention is drawn here to the use of dissimilar materials, which can lead to electrolytic corrosion.
Attention to these factors can guarantee many years of trouble-free operation of the collectors, which is very important to both satisfy the customers and promote the use of solar energy.
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