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Why haven’t solar heat and photovoltaic panels been combined in one product?

Stephen Fields | Posted in General Questions on

The availability of appropriately oriented roof space for both would seem to make it a no-brainer to apply a thin film photovoltaic onto the cover of a solar heat panel. Sure, some efficiency of heat gain might be lost but you’d be getting electricity generation!

Any ideas?

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  1. Daniel Morrison | | #1

    Because it makes sense?

    Actually, there's a house up in northern CT, right near my house in fact, that ran PEX tubing under a metal roof with PV film on them. The single roof provides hot water and electricity. I think Stephen Winter Associates did it.

    Thanks for reminding me of a great house to cover in



    The "Main Stream Green" home built by Cherokee investment partners here in Cary NC used a system by Dawn Solar to accomplish this. basically it's just a staple up radiant system that goes under the roof sheathing after you install the solar PV and harvests the heat off the underside of the sheathing by way of a propylene glycol solution that brings heat from the attic down to a pre-heat tank upstream of the water heater. It makes up for it's lack of efficiency by it's size. I'm unclear what the plan is when you need to re-roof somewhere down the line at which time you would need to figure out how not to punch the pipes full of holes without pulling the pipes off the underside of the roof (in this case they have covered the pipes with spray foam.)

    The advantage is that the PV panels work better if they have cooling on the back so your point is absolutely correct that this is a great idea just waiting for fuller realization.



    I take that partially back. the Dawn System is like a radiant system but it installs above the roof deck between sleepers like a radiant floor and then you can install metal roofing or solar PV over the top of it. So it's a bit more fully thought out than I made it appear in my first reply.

    It still lacks the efficiency of a dedicated solar panel and makes up for that by it's large size.


  4. GBA Editor
    Martin Holladay | | #4

    Many researchers have built combination solar collectors over the years — that is, units integrating PV with solar thermal collection. While these have some advantages — obviously, the circulating fluid lowers the temperature of the PV array, improving PV efficiency — they have several disadvantages that prevent them from achieving commercial success. These disadvantages:

    1. Like a Swiss Army knife, such solar panels represent a compromise. You usually end up with a not-very-good solar collector married to a not-very-good PV array.

    2. Once the solar thermal piping develops a leak, you have a maintenance nightmare. Solar thermal collectors usually develop problems sooner than PV modules, and once one element fails, the entire assembly has to come off the roof for repair or replacement.

  5. Riversong | | #5

    SunDrum Solar offers combination solar thermal/PV systems that have a combined efficiency of greater than 50%.

    One of their units was installed on the Needham MA zero-energy house:

  6. TKunhardt | | #6

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