CEN Keymark Scheme for Solar Thermal Products

Collector performance parameters

• q = A*(n0*G - a1*dT - a2*dT²) [W]

• G : Solar irradiance on collector plane [W/m²]
• dT: Temperature difference between collector mean fluid temperature and ambient air temperature [K] (Kelvin)

• n0
: Optical efficiency
• Combined efficiency of the transparent cover and the absorber [-]
• a1
: 1st order heat loss coefficient
• Heat loss cofficient at collector fluid temperature equal to ambient temperature [W/K]
• a2
: 2nd order heat loss coefficient
• Temperature dependent term of heat loss coefficient [W/K²]
• The heat loss coefficient of a collector depends on the temperature. High values of a2
indicates large temperature influence
• The higher the value of a2
the more "hookednosed" the power/efficiency curve)

• A: Collector area corresponding to the performance parameters
• Two sets of performance parameters are determined according to European Standard EN12975-2 - one set corresponding to the absorber area and one set corresponding to the aperture area. It is important to use corresponding area / parameters. Normally the aperture area is used, which is: The area through which the solar radiation enters the solar collector

Collector efficiency curves

• Compare your collector with typical collectors: Download Excel-file
• Compare your collector with typical collectors: Run interactive file (requires Microsoft® Internet Explorer 4.01 Service Pack 1 (SP1) or later and the Microsoft Office XP Web Components)