Solar Thermal Collector Output
The useful heat a flat-plate solar thermal collector delivers, by the ASHRAE 93 / Hottel-Whillier-Bliss efficiency line the SRCC prints on every rating: efficiency = optical efficiency (y-intercept, ~0.70) minus the loss coefficient (slope) x (inlet - ambient) / irradiance; useful heat = irradiance x efficiency x area. Optical 0.70, loss 0.85 Btu/hr-ft^2-F, 120 F inlet, 70 F ambient, 300 Btu/hr-ft^2 sun = 0.56 efficiency -> 168 Btu/hr per sq ft, 6,700 Btu/hr over 40 sq ft. A colder, dimmer day (140 F inlet, 40 F ambient, 250 sun) drops to 0.36 and 3,600 Btu/hr -- least efficient when heat is needed most; past stagnation (efficiency <= 0) it makes nothing. A performance estimate; the SRCC-rated intercept and slope, the incidence angle, the flow rate, and the glazing govern.
Formula and source
efficiency = optical_efficiency - loss_coeff x (inlet_temp_f - ambient_temp_f) / irradiance_btu; useful_btu_per_sqft = irradiance_btu x max(0, efficiency); useful_btu_hr = useful_btu_per_sqft x area_sqft.
Flat-plate solar thermal collector efficiency (ASHRAE 93 / Hottel-Whillier-Bliss line as reported by the SRCC collector rating), by name; the SRCC-rated intercept and slope, the incidence angle, the flow rate, and the glazing condition govern the real output.
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