太陽能熱水

Technology Outline

2) Solar Collector Perforrmance

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)

SolarCollectorPerforrmance。以上文字說明這張圖片。

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.