Combining solar panels and passive water heating?

We had planned to put our solar panels (electric) on the roof of our house, but was told that an old house like ours it wouldn't be a good idea. So we decided to put them on a roof in front of the water tanks - to provide shade for the tanks and create a workshop underneath. But a friend told us that they get awfully hot on the back, so putting them in from of cold-water tanks might not be a good idea.

So this is where we thought: Why not use this heat to warm up the hot water? Run some tubing on the back and have the water from the hot water tank run through it?

Do you see any problems with this idea?

I think it can provide some useful water heating. However, it won't allow for temperatures nearly so high as conventional water heaters. So, one would have to store a larger volume of this heated water to put the energy to use... and this means larger tanks. You might circulate the water in the tanks through the tubing set behind the panels (this is probably what you were thinking) using a small pump. I recommend a small pump with a magnetic drive impeller. A model sold under the name "Pondmaster" is available that might interest you. There are also DC magnetic drive pumps available for a low price that are manufactured in China, and designed specifically for solar water heating purposes. You could operate one of these small pumps on a thermostat to heat up the water in the tanks to 100F or so. It seems very reasonable to me, and a very good idea.

Oh we don't want to use more electricity - I was thinking we could do it passively?

I was worried that the water wouldn't cool the solar panels sufficiently (compared to air), and they wouldn't perform as efficiently?

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Dawn Hoff wrote:Oh we don't want to use more electricity - I was thinking we could do it passively?

I was worried that the water wouldn't cool the solar panels sufficiently (compared to air), and they wouldn't perform as efficiently?

There are a lot of advantages to using a small pump. It would draw no more than 20 watts and operate only a few hours each day and only while the panels are generating, so there would be no battery losses (assuming an off grid system). Best of all is a pump makes it so much easier to design the system as the alternative would require a thermosiphon. Sure, a thermosiphon can be done, but you would be more restricted on how you place the panels relative to the water storage tanks.

I don't anticipate a problem with excessive panel temperature. However, it's impossible to know without specifics on the design.

Well we haven't decided a design yet, so I would love pointers as to what we definitely shouldn't do. I'll give a pump some thought, but I have an engineering crush on the passive design

Dawn Hoff wrote:Well we haven't decided a design yet, so I would love pointers as to what we definitely shouldn't do. I'll give a pump some thought, but I have an engineering crush on the passive design

I know what you mean, thermosiphon is elegant. Here is a youtube video featuring an interesting solar thermosiphon project. http://www.youtube.com/watch?v=mF5mT8v4n8M