The thermal performance of the flat-plate collectors were
evaluated by the ASHRAE Standard 86-93. The collecting efficiency,
water temperature and heat gain were compared in the same solar
irradiation. The tests have performed from 9:00 to 16:00. The
volume flow rate of woking fluids is 140 L/h. The heat gain of fluids
can be calculated using Eq. (1). The heat gain of fluids can also be
expressed in terms of the energy absorbed by the absorber and the
energy lost from the absorber as given by Eq. (2) [25].