Can you do PV and water heating in same panel?

Gooserider · April 2008

I've had a thread on this going over on Hearth.com, (one of the responses sent me here) and we haven't reached any real conclusions, so I thought I'd ask folks with more expertise...

As I mentioned in one of my other posts, my only good solar location is on our roof, which is a limited resource, only about 500 useable square feet.

Everything I've read about solar PV says it isn't terribly efficient, that only 5-10% of the energy that falls on a panel gets turned into electricity - I assume the rest mostly exits out the back side of the panel as heat...

Is there some way to capture that wasted heat, and use it for energy in other ways? I.e. use the heat for DHW or house heating, albeit perhaps at a slightly lower efficiency than a heating only panel would give? I'm visualizing something like a solar water heating panel, only using a PV panel as a top cover instead of glass...

I've seen comments that the output of a PV panel goes down as the panel gets hotter, but how hot does a PV panel get? Would running hot water (perhaps 180-190*F) actually act as a "coolant" - seems like it would if the panel normally would get over 200*F

I've never seen any products that look like they do this, but if it's a workable concept, it would seem like a natural with lots of benefits - higher total efficiency by using more of the sun's energy, getting more work out of the same finite amount of roof space, etc. For those of us in snow country, it might also give improved performance by letting you run heated water under the panels to melt off snow faster and get the PV back in service sooner.

Am I onto something, or is this one of those ideas that doesn't work in practice because the temps are wrong?

Gooserider

mike95490 · April 2008

Re: Can you do PV and water heating in same panel?

It's a good thought, and many apparently try, few succeed. If you get a leak from the water into the sealed PV panel, the panel is ruined. There goes $900. You likely don't want to thermal shock the PV, by accidentally getting a slug of cold water into it. You don't want the water to leak, and winter frost adds more problems.

Solar hot water is more efficient than solar PV, but if the demand is timed wrong, you can benefit more from the PV.

newenergy · April 2008

Re: Can you do PV and water heating in same panel?

I think I saw someone do something home made on the net somewhere.

You really couldn't do domestic hot water because it doesn't get hot enough except maybe in the desert on hot days. Domestic hot water heating panels have glass (or the like) covers and insulation to collect a lot of heat. You wouldn't want that near your PV panels.

Pool heaters on the other hand don't require very high temperatures. You could rig something up, but it would be complicated. I don't know of any commercial products and it seems like the PV/pool heater combo would have a lot of problems. One thing is the PV panels would outlast the plumbing elements. Another is that it would really limit the marketability.

edit: You could rig something up just as a pre-heater so that your conventional domestic hot water heater doesn't have as much work to do. If you want the water flow to actually cool the PV you have to keep the water flowing though. A pool heater does that, but a domestic hot water heating system doesn't. By the time you rigged up something that enabled you to add heat to the water without allowing it to get very hot and still keep it circulating, I think you are well past being practical.

Gooserider · April 2008

Re: Can you do PV and water heating in same panel?

Thanks for the replies, I guess my key question would be how hot does a PV panel get when operating if it is fastened flat to a roof with minimal hardware? I have a 12/12 roof at latitude 42.6, so I wouldn't be tilting the panel away from the roof so there would be minimal air circulation under it.

What I'd sort of been thinking in terms of for the larger system would be to have a large thermal storage tank - the wood boiler folks like used 1,000 gallon LP tanks, which I would circulate to the roof and back to store heat (possibly with a heat exchanger) as a sort of thermal battery, which would then get tapped and distributed by other needs, such as DHW, house radiant heating, pool heat, and so on.

A large part of what makes this type of setup effective is charging the storage tank up to high temperatures, the effective heating capacity is a function of how hot you can get the tank, with 180-190*F being the desired target for what can be readily and safely attained with a wood gasification boiler. I'm assuming similar temps for my solar heating panels as a working estimate, since my setup would ideally use a boiler as well to drive the tank in the winter.

In many ways the type of hydronic thermal storage system I'm talking about bears a lot of resemblance to a semi-off-grid electric setup... There is a battery bank /tank that can be charged by many sources (PV / solar, generator / wood boiler, grid / fossil burner) and which has multiple loads drawing it down, with limits on both how much the storage can be charged to and discharged from... What I'm wondering is how well the operating temps of a PV panel would match with the temps of a hydronic storage setup.

This type of thermal storage setup can be VERY effective - we have a member on hearth.com who is doing heat, DHW, and a hot tub for 3600 sq.ft a wife and three teenage daughters in northern VT with 4 cords of wood, and summertime only use of a cheapo solar pool heater array, with an oil boiler backup (He thinks his consumption of SEVEN gallons last year is excessive BTW)

My mental image is that of a high temperature DHW type solar heating panel as described by Newenergy - a well insulated box with collector plumbing inside it, only instead of topping it with clear glazing, put a PV panel on it instead... I guess the question would be if this would actually collect heat at all, and if so how much? I would want my output temps to be in the same range as I would get with a pure thermal panel, with a lower flow rate (OK since that can be made up for with more area...)

Gooserider

mike95490 · April 2008

Re: Can you do PV and water heating in same panel?

There is a writeup here, about solar water heating. Long, but interesting.

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

n3qik · April 2008

Re: Can you do PV and water heating in same panel?

The only way to get it to work, would be to use a heat pump. This would get the best transfer of heat from the panels to the water tank. The cost to run the heat pump would off set any gains in the output of the panels, would be my guess.

Gooserider · April 2008

Re: Can you do PV and water heating in same panel?

Excellent article Mike, thanks for the pointer... A fair bit of what's in it would be applicable to my setup, though it doesn't directly relate to the question of combining a PV and hydronics into one panel.

Gooserider

mike90045 wrote: »

There is a writeup here, about solar water heating. Long, but interesting.

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

Can you do PV and water heating in same panel?

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