where E – is the exergy of heat Q supplying by collector per day, (MJ); x – is a number of years in service; / = 0.12 – is
the constant whose value is obtained in Xiao et al. (1998) for ideal heating device; b = 31/0.28 = 110.7 – is a coefficient (where 31 – is a number of days in July and 0.28 – is the part of year’s heat supplied by collector in July).