Lakhasly

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.

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. the absorber can heat upto 350K, and this emittance spectrum is higher than 2 micron. Since [alpha = epsilon] , the absorptance should be as low as possible above 2 micron to limit radiation losses.

Collector Performance:

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

Power ouput {Qout} = \tau E A{c} - Q_{ref} - Q_{conv} - Q_{rad}

is reduced by losses due to reflection, convection and radiation.

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.

Losses: Q_{RC}=a1A_{c}\left ( T_{c}-T_{a}}\right )+a2A_{c}\left ( T_{c}-T_{a}\right )^{2} = a A_{c}\left ( T_{c}-T_{a} \right )

a1 and a2 are loss coefficients. Evacuated tube collector loss coefficients arer much lower than Flat plate collectors. The loss coeffiecients are experimentally determined. For a Flat Plate Collector, optical efficiency is 0.7-0.8, a1(W/m^2K) = 2.5-3.8, a2(W/m^2K^2) = 0.007-0.23.

Efficiency: \eta {c}= {Q{out}}/{E A_{c}} = \eta {o} - {Q{RC}}/{E A_{c}}}

\eta {c}= \eta {o} - {a1(T{c}-T{a})+a2(T_{c}-T_{a})/{2}}{E}}]

Collector Flow Rate:

Q_{out}=\hat{m}C_{p}(T_{out}-T_{in})

\hat{m}= Q_{out}/C_{p}(T_{out}-T_{in})

The collector flow rate should be high so that efficiency does not go down from high collector temperatures. On the other hand, flow rate should not be too high, in order to maintain the energy transfer demands within acceptable range.

{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.