Solar preheater for an electric water heater.

[Ross]

So i have a point of use 2.5 gallon (yeah a little thing) electric water heater. I'd like to build a simple roof top black hose coil under glass water heater to preheat or heat the water for 8 out of 12 months (which is variable here but should be doable) I'd like it to do 100% for 3 or 4 moths running 120F water in and the rest at anything above 80 without risking freezing. (?) So bypass and drain plumbing is a must and understood, I can hook up a snifter valve to blow it out too. So what size pipe and how much glass to house it is the problem. Is there a site that can size that for me? Other ideas?

[SolarGary]

Hi Ross,
The pipe coil under glass may get hotter than the pipe coil can take -- especially if its a good sun day, and the pipe coil has been drained. The HDPE black pipe that most people use for the pipe coil heaters is only rated to about 130F -- it will take more than that in practice, but stagnated glazed collectors can go well over 250F.

PEX is good for somewhat higher temps, but it needs to be protected from direct sun exposure.

The PEX collector I had for a couple of years had a steep tilt to reduce summer stagnation temps, but it still got above 230F on a few occasions:
The $1000 Solar Water Heating System
The logged temps:
$1000 Solar Water Heating System -- Performance


You might take a look at this free download book:
The Integral Passive Solar Water Heater Book
Its got some simple roof solar water heaters (e.g. the Japanese pillow heaters) -- there might be something there that would do it.

Just for reference, a 300 ft coil of the HPDE pipe holds about 12 gallons of water and a 300 ft coil of PEX holds about 9 gallons (they are both nominal 1 inch, but the HDPE has a larger ID).

Gary

[Jim-mi]

How about a "batch" type heater.
A black tank "under glass"
Inside a box painted black of course.
I have read about up to 30 gallon tanks used.
Copper pipe for any thing in the sun.
Plumbed for "easy" draining.


have fun

[Ross]

That might be better Jim I'll check out Gary's link though. thanks!!

[SolarGary]

Originally Posted by Ross

That might be better Jim I'll check out Gary's link though. thanks!!

The link is pretty much what Jim suggested -- the book is basically a very complete rundown on batch solar water heaters of all types.

There are some more of them here: Solar Water Heating Projects and Plans

Hard to beat for simplicity if you can find a tank. I've heard from some people who managed to get new tanks for plumbing or hardware places that had tanks in which the outer casing was damaged and it was worthless to them -- since you take the outer casing off anyway, this works fine for a batch heater.
A metal beer keg might be another way to go since you don't need much volume.
Or, the book has some batch heater configurations that don't use metal tanks at all (like the Japan Pillow heaters).

Gary

[GoldenCityMuse]

I was always thinking about a car radiator in a black box with glass on it.

[YuccaFlatsRanch]

The fact that I am not using one of these in south Texas is criminal and I am guilty as charged.

[wendle]

Don't forget to check ebay and craislist. I managed to find some large heavy made solar water heating panels for a swimming pool. I use a drain back system so the water doesn't go directly into the tap water. My heat exchanger is pex pipe coiled inside a water trough surrounded by apx 6 inches of wool.