Solar thermal collectors and applications

(Ti — Ta) Gt n = Frto: — c1 — c2 (71) n = Fr no where 0out = T0Ut/T0, = Tjn/To, Ns is the entropy generation number and M is the mass flow number given by: S T N _ SgenT ° Ns Q* and M = mcpT0 Q* (67) and if we denote c0 = FRra and x = (Ti — Ta)/Gt then: n = c0 — c1x — c2Gtx2 (72) And for concentrating collectors the efficiency can be written as: If the inlet temperature is fixed вт = 1, then the entropy generation rate is a function of only M and Uout. These parameters are interdependent because the collector outlet temperature depends on the mass flow rate. c!(Tj — Ta). CGb c2(Tj — Ta)2. CGb (73) and if we denote k0 = FRno, k1 = c1/C, k2 = c2/C and y = (Ti — Ta)/Gb : then: n = k0 — k1y — k2Gby2 (74) . Performance of solar collectors

ASHRAE Standard 93:1986 [108] for testing the thermal performance of collectors is undoubtedly the one most often used to evaluate the performance of flat-plate and concen­trating solar collectors. The thermal performance of the solar collector is determined partly by obtaining values of instantaneous efficiency for different combinations of incident radiation, ambient temperature, and inlet fluid temperature. This requires experimental measurement of the rate of incident solar radiation falling onto the solar collector as well as the rate of energy addition to the transfer fluid as it passes through the collector, all under steady state or quasi-steady-state conditions. In addition, tests must be performed to determine the transient thermal response characteristics of the collector. The variation of steady-state thermal efficiency with incident angles between the direct beam and the normal to collector aperture area at various sun and collector positions is also required [108].

ASHRAE Standard 93:1986 [108] gives information on testing solar energy collectors using single-phase fluids and no significant internal storage. The data can be used to predict performance in any location and under any weather conditions where load, weather, and insolation are known.

Solar thermal collectors and applications

Collector thermal efficiency

In reality the heat loss coefficient UL in Eqs (2) and (42) is not constant but is a function of collector inlet and ambient temperatures. Therefore: TOC o "1-5" h …

Global climate change

The term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapour and clouds) in keeping the surface of the earth warm. Recently however, …

Limitations of simulations

Simulations are powerful tools for process design offering a number of advantages as outlined in the previous sections. However, there are limits to their use. For example, it is easy …

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