where 0, a1 and a2 are the system parameters to be determined
through experimental findings; K is the bi-axial incident angle multiplier
which can be seen as the product of the longitudinal incident
angle multiplier and the transverse incident angle multiplier [9].
0 is known as the reference collector system efficiency, which is
equivalent to the collector efficiency when ¯Ti and Ta are the same.
Eq. (5) reduces to a first order equation when a2 is set to zero. From
Eqs. (3) and (4), the heat absorption rate at each tube is