Two concentrating collectors (collector A and collector B) have the same concentration factor of CR = 7 and the optical efficiency of ηar = 0.88. The collector temperature for both collectors is 145oC and the ambient air temperature is 27oC. The heat transfer coefficient for collector A is 2.5 W/m2·oC and that for collector B is 3.5 W/m2·oC. The solar irradiation on collector A is 600 W/m2.
(a) At what solar irradiation rate does collector B have the same efficiency as collector A? (b) What is the efficiency change of collector A when the solar irradiation increases to 900 W/m2?