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It's because hot water heaters pull much more power heating water than in maintaining heat. Timers have been proven again and again to be ineffectual. Insulation is a much better bet.
As for coils on the roof or elsewhere, you're going to find natural convection to be not near as effective as having a small circulation pump in the systems. It's power draw will be minimal. If it's connected to a thermostat in the box, that's good. Solar collectors can get awful hot when the sun is cooking, and a lot of them suffer glazing and sealant failures. As well, remember if it gathers heat in the day it sheds it when it's rainy or cold out, too, and conductive loss from the water can be pretty significant.
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If you have a solar powered pump, it will only pump the water around when the sun is shining and heating the water. When the sun goes down and there isn't any more heat being put in the water, no more pump.
Usta be we'd sail into Black Manele bay on Lanai and wash the boat down with hot water. The water came from a tank about a half mile up the hill and the pipe was just laid on top of the ground. Nice HOT water by the time it got down to the harbor. Of course, with that system ALL you had was hot water but it would cool off at night. Once they built that hotel over at white Manele though there isn't any more hot water in the harbor.
I suppose you could take a few lengths of black garden hose and toss them up out in your yard somewhere in the sun. Attach one to the lower drain of your water heater and one to the upper end somewhere. Run water through the hose so there won't be an air lock and it should thermo-siphon the heated water back into your tank.
"I like yard sales," he said. "All true survivalists like yard sales."
Kurt Wilson
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Not to always run in and by the snot nosed know-it-all, but there's more to it in these systems, and mystery problems that can plague them or at least greatly diminish efficiency. Since everything is expensive today, and material costs are high, it's good to thoroughly engineer such projects to insure they work, and aren't a waste of money.
The problem with "thermal pumping" alone is this: that heating and cooling water is going to give off dissolved oxygen and it's going to hang up somewhere in your system and give you a vapor lock of sorts. Maybe it won't be a problem, maybe just a little problem, maybe an intermittent problem, but it will be there. If you're really careful, know the symptoms and don't mind cutting the whole thing apart to fix it, it's not a big deal. Elsewise, get a pump. Seriously, you will save yourself unbelievable woe. I know it's not as cool as running completely on solar--get a panel to run the pump if you need to, but get a pump. Any of the types for hydronic heating systems will work, and not much capacity is needed.
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Punamom,
We built our own and it works great. Drop me an email and I'll give you the details.
Aloha,
Daniel R Diamond
Daniel R Diamond
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I think that a simple bare water tank inside an insulated box will work well enough if you don't use a lot of hot water. The simple design will minimize the previously mentioned airlock problems and no circulating pump will be required. The trade-off is the low heating capacity. Get multiple small tanks and you will get more surface area to absorb the sun.
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It would be interesting to see how a 12V heating element hooked directly to a mediunm solar PV panel would work. Nor enough power to overheat it, and if it has 2 elements, leave 1 hooked up for rainy days.
Gordon J Tilley
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WoW great responses!!
I've been under the weather but I will get to work on this project soon and let you all know how it turns out.
Perhaps someone needing work will offer an inexpensive setup to help out some families out there.
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The problems is nothing is inexpensive to build at this point.
The second problem is that in many areas on the Puna side, it would not make a bit of hot water for 4 months out of the year, which I'm not sure is economical.
Still, these "sorts" of systems are proven. Just beware of designs from Phoenix or Sacramento and think you can plug it in here in Hawaii.
Be careful of hippie engineering. I would not want to introduce a bunch of plastic and hose and stuff into my potable water supply.
By the way I'm not trying to discourage anyone. Just understand the limitations.
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JWFITZ makes some good points. I have property in Eden Roc but I don't live there yet. There have been times that I have been there all day and it has been solid overcast and rain the whole time. Had I been depending on solar hot water I would have been out of luck. Also those are the days that you really want a good hot shower. So it may be that Eden Roc, at about 1800' elevation, is too cold and rainy for solar to work consistantly.
I like the idea of using wood, either burned directly or in a wood gassification system. Trees are actually solar collectors with very effective storage. However there are pitfalls there too. I think that if everyone burned wood we would be surprised how fast we would run out, and there would be a lot of air pollution. I don't think that it would have to be that way but I think that it would be that way because the majority of society is kind of lazy and ignorant.
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I still like the idea of the simple box type solar water heater. Using a discarded water heater tank it would be about the cheapest system you could put together. It would also be one of the least efficient but that won't matter if you are prepared to work within the limits of what it can provide. If you use it for preheating in front of a conventional system you may not even be affected when the sun doesn't shine. Of course you also won't have any idea whether you just wasted your money either.
