(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  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 .
ASHRAE Standard 93:1986  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.