PVT and Hot Water from same panel?

[pickpaul]

I had an idea and did some searching...

Anaf Solar

It would seem the increase in efficiency from cooling the panels would cover the electrical needs of a 12v pump to circulate from a hot water tank but would love to hear thoughts?

Thanks, Paul.

[djmarchand]

Paul:

The whole purpose of the cooling coils is to reduce the temperature of the panels so that they are more efficient. Heating domestic hot water requires that the panels heat up to about 120 deg F to provide decent hot water. That is too high for best panel efficiency. The Anaf site says the same thing.

But if you have limited space for panels it might be nice to heat hot water with your PV panel even if not at the best efficiency. But it doesn't take much room for hot water heating. A 4 sq foot panel will do the job. See heliatos.com.

David

[pickpaul]

Thanks for the info David. My interest was primarily to squeeze most electricity from the panels I have space for (Alberg30) and I've always liked cogeneration as a concept. Maybe I could use the cold water tank as a heat sink? That way I would make more electricity with cool panels and use less electricity to heat that less cold water when I'm not running the engine or a Dickerson diesel furnace I plan on installing for next winter. Just thinking aloud and interested in input.

Thanks, Paul.

[Dsanduril]

A lot of the research in PV is around cooling the panels and/or increasing the efficiency at higher temperatures. Liquid cooling has been approached many times, but has generally proven to be not cost effective.

However, in the special situation of a boat installation it might be worth the effort/cost. The normal operating cell temperature (NOCT) of most silicon cells is around 45C (at 20C ambient temperature) and the typical power derating is around 0.4%/C. So, if you can run the panels 15C lower in temperature you should see about 6% more output. The greater the temperature difference the more you gain. Is that enough to pay for the energy and capital costs?

I like the idea in theory, not so sure about in the real world for most applications. One place it really makes sense (and have seen it in actual use) is for a heated swimming pool. Water at 25-30C is great for pool heating, and the filtration/circulation pumps for pools are energy hogs. Getting both out of the same installation is nice.

[ccannan]

Yes, cooling your solar panels will not warm up your water to 120F, but it doesn't need to. Let's say your panels can raise the temperature of the cooling the cooling water 25F (from 60F to 85F) that is 25F less that your water heater has to work to raise the water temperature (from 60F to 120F). I don't know if the amount of energy expended to circulate the cooling water would be more or less than would be saved by raising your cooling water temp. I think some of the numbers could be calculated quickly. For example, if your hot water heater has a capacity of 5 gallons, You can calculate how much energy (electrical) it would take to raise that amount of water X degrees. Likewise you can calculate how much energy (electrical) a pump would consume to pump X volume of water up X distance. Don't forget to add an additional 15% to each number for inefficiencies.

[Dsanduril]

Assuming recirculation, the system would probably only require a 10W or 20W circulation pump (my whole house uses a 25W pump for heat distribution from solar water heating). Even if you are getting a 10% improvement that means that you can't break even until you get up into the ~200W array size.

Just a starting point, you'd have to calculate for the actual system, but looks barely feasible (small system on a boat) from an electrical standpoint, the warmer water, if useful, might tip the scale in favor.

[pickpaul]

Thanks for all the thoughts. That pool info will be filed away :-)