Solar Water Heating
Solar Collector Performance
The characteristic curve equation of a solar collector can be a first-order equation or a second-order equation.
First-order equation: η = ηo - k * ΔT / G
Second-order equation η = ηo - k1 * ΔT / G - k2 * (ΔT)^2 / G
| Where |
η ηo k k1 k2 ΔT G |
= = = = = = = |
efficiency maximum efficiency at Δ T=0 heat loss coefficient in W/m2.K linear heat loss coefficient in W/m2.K quadratic heat loss coefficient in W/m2.K2 average temperature difference between absorber and air (K) available irradiance (W/m2) |
Above: Graph of efficiency and average temperature difference at constant irradiance level of 1000 W/m2, for typical solar collectors
Evacuated-tube collectors are usually more expensive than flat-plate collectors. However, as the k-value is lower, evacuated-tube collector can achieve a high efficiency even with large temperature difference between absorber and surrounding.