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post #48 of Old 11-28-2017
colemj
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Re: Solar heating element for water heater

Quote:
Originally Posted by misfits View Post
You know that would be a pretty cool idea.
IF your PV system completed charging your house bank & there were still a couple hours of good sunlight left, this would work providing your PV system had the nugget to drive a 300 watt element.

The 300 watt DC element is capable of raising the temperature in a 6 gal tank 20 degrees in an hour.
One watt equals 3.413 BTU's in my world. A 300 watt element would equal 1023 Btu's per hour.
One BTU raises the temperature of a pound of water one degree. Water weighs 8.34 pounds per gallon.
6 gallons X 8.34 X 20 degree rise= 1000 Btu's

So if you were some place that the ambient water temperature of your heater was 70 degrees, hour later it's at 90. Not a bad deal in my book.

It would also be pretty easy to make a small open loop thermal solar panel & just let it work by convection. You'd have to valve it off so at night it didn't operate the opposite way. All the above sounds like a good science experiment to me
This is the same analysis I did above, only in imperial units all the way, where I only brought it down to them at the end.

If anyone is interested in different imperial units, raising that water 20F would take 110 mile-pounds

However, your assumption of 100% solar efficiency is off. Assuming late daylight hours and non-optimal incident angle, one can really only expect ~40% efficiency from the solar. So a 300W array would only put out 120W.

Mark

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