The absorber or receiver (flat plate or parabolic) must have high absorptance in the wavelength range below 2 micron, as the main solar spectrum is in this range.Collector Power Output:

We combine the following terms and substitute,

Q_{conv}+Q_{rad}=Q_{RC}

Q_{ref}= \tau \rho E A_{c}

[Q_{out}= \tau E A_{c} \left ( 1-rho \right ) - Q_{RC}

For absorber, \alpha = 1- \rho

So, Q_{out}= \tau \alpha EA_{c}-Q_{RC} = \eta _{o}EA_{c}-Q_{RC}

where the product of transmittance and absorptance is replaced as the Optical Efficiency of the absorber or collector.Power ouput  _{Qout} = \tau E A_{c} - Q_{ref} - Q_{conv} - Q_{rad}

is reduced by losses due to reflection, convection and radiation.{m}'=\eta _{o}E-a1(T_{c}-T_{a})-a2(T_{c}-T_{a})^{2}/ C_{p}(T_{out}-T_{in})

This is the mass flow rate rewritten as a area flux, measured in kg/m^2 hr.Collector Performance:

The collector converts solar irradiance, E, to absorber surface ( Ac ) heat.