Solar Thermal Collector Output

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.

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.

The collector efficiency line is public solar-engineering theory (ASHRAE 93 / Hottel-Whillier-Bliss); the intercept and slope come from the collector's SRCC rating and the temperatures and sun from the site.

Estimate. AHJ and licensed professional govern.

Field names used by the API: optical_efficiency, loss_coeff, inlet_temp_f, ambient_temp_f, irradiance_btu, area_sqft, efficiency, useful_btu_per_sqft, useful_btu_hr

• ASHRAE 93 efficiency line eta = FR(ta) - FR*UL*(Ti - Ta)/G; useful heat = G x max(0, eta) x area; intercept ~0.68-0.75 glazed flat plateASHRAE 93 / Hottel-Whillier-Bliss / SRCC rating
• Operating point least efficient when hot and dim; no useful heat past stagnation (eta <= 0); unglazed pool collectors have a high intercept but steep loss slopesolar-thermal engineering practice