As far as I know, there are currently two main approaches to utilising solar radiation for maximum energy conversion to electricity. These are either direct conversion to electricity, using photovoltaic arrays, or the use of simple mirrors to concentrate heat onto a pipe carrying a working fluid to a heat exchanger of some kind.

Edit: There are earlier posts regarding mirrors and votes to close on those grounds but I don't invoke mirrors of any kind in this OP.

My question is: Is it possible, (or is it already current practice in some solar arrays?) to combine these two methods, for increased efficiency in the capture of solar radiation?

This would involve using a photovoltaic solar cell array in a parabolic shaped configuration, with it's reflected radiation then being utilised to increase the temperature of a working fluid, (such as water, or whatever fluid is suitable for the temperature range involved), held within a "pipe" above the solar array.

If this is not feasible, is this because the overall efficiency of the proposed system in this OP is already comparable (or even inferior to) existing systems?

Solar Array

I appreciate that the fluid channel will block some of the solar radiation reaching the PV array and decrease it's efficiency because of both the shadow of the channel and the array of struts needed to hold the fluid channel in place.

This wiki article, from the comment below by sanchises PV Cells and Liquid cooling, is a variation on the above, and may/may not be more efficient than the proposal in this post.

  • $\begingroup$ PV cells are generally most efficient when sunlight hits it perpendicular. I believe this is used with 'flat' panels though, for combined PV and (residential) heating purposes. EDIT: yep. See this Wikipedia article. $\endgroup$
    – Sanchises
    Apr 12, 2015 at 13:38
  • $\begingroup$ An interesting application along your lines of thought is the Solar Pumped Laser (see iopscience.iop.org/1555-6611/23/6/065801/article), although this has interest aside from efficiency increase. $\endgroup$ Apr 12, 2015 at 14:09
  • $\begingroup$ This is not a duplicate of the "why do solar panels have mirrors". This question is asking about hybrid photovoltaic-thermal energy conversion which has some very interesting thermodynamic properties. Vote to keep open. $\endgroup$
    – boyfarrell
    Apr 12, 2015 at 14:48
  • $\begingroup$ @boyfarrell - you are right, there is a difference; I have retracted my close vote. $\endgroup$
    – Floris
    Apr 12, 2015 at 21:39

1 Answer 1


Wikipedia has a good article on hybrid photovoltaic-thermal systems. As you proposed they consist of a solar cell with a thermal collector at the rear.

Solar energy conversion is a fascinating topic from a thermodynamic perspective and has been summarised beautifully by the work of De Vos,

A hybrid photovoltaic-thermal device can be modelled as an endoreversible engine: an irreversible engine where all the irreversibilities are restricted to the coupling of the engine to the external world. By coupling we mean exchanges of particles (the photovoltaic component) and heat (the thermal component).

Endoreversible engine for hybrid conversion

Where $U=Q + \mu N$ represents the flow of energy which has a low-quality component $Q$ and high-quality component $\mu N$. The low-quality component is termed so because it is accompanied by an entropy flow $S=Q/T$ where as the high-quality component is not accompanied by an entropy flow. Thus the total energy flow is accompanied by an entropy flow of $(U -\mu N)/T$.

The photovoltaic component will operate on the flow of particles $N$ and the thermal component will operate on the heat flow $Q$.

For the solar case, $T_1=5762K$, $T_2=300K$, $\mu_1=0$, $\mu_2=0$, thus we solve for $T_3$ and $\mu_3$ to give the hybrid (pv/pt) power conversion efficiency,

Hybrid conversion efficiency without solar concentration.

So quite a significant boost is possible with hybrid conversion. This calculation assume that the sunlight is not concentrated.

Now if we concentrated sunlight to maximum amount 42600$\times$.

Hybrid conversion efficiency with 46200 times solar concentration.

As concentration is increased the hybrid efficiency is boosted only slightly by the photovoltaic process. This is because of dramatic reduction in irreversibility as the concentration of sunlight is increased due to matching of the absorption and emission solid angles and also the much high temperatures that can be reached by the collector allow the Carnot energy to run very efficiently.

  • $\begingroup$ thanks very much for that answer. I just have to ask, (not that I doubt you), if we concentrated sunlight to maximum amount 42600× , how physically speaking is this number achieved? If it's a case of a separate OP, that's no problem...regards $\endgroup$
    – user74893
    Apr 12, 2015 at 17:33
  • $\begingroup$ Easy to explain but open a new question because it will involve another diagram and I don't want to clutter this answer. $\endgroup$
    – boyfarrell
    Apr 12, 2015 at 17:40

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