Electric Water Heater Runing on 12v Bateries

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Electric Water Heater Runing on 12v Bateries

Postby nokillzone » May 20th, 2013, 1:20 pm

Well i have a Water Heater on electricity ... old one i dont use
I have a Pool and a Solar Panel kit from Harbour Freight.

I seen on ebay they sell 12v 100watt-600watt Water heating element,.
so i figure has any one seen or has done what i am planing on doing ?

Converting house water heating tank on electricity to run off batteries to heat the pool.... ??

please any comments of suggestions would be appreciated !
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Re: Electric Water Heater Runing on 12v Bateries

Postby k-ww » May 20th, 2013, 2:01 pm

Not to dash any cold water on your idea, but:

1) Household water heaters use many Kilowatts of energy to heat water [Look at the nameplate of your waterheater]. Typical hot water tanks are 10 - 100 gallons.

2) How many fills of your water heater are in the pool [1000's of gallons].

3) How much energy does it take to heat a gallon of water 10 degrees.

You are better off using a solar water setup to heat your pool and or cover it with black plastic.

The pool will loose heat many times faster than the 100 - 600W heating element can heat it.

A very roughly scaled experiment would be: use a candle to heat a pot of water - how long before it will boil?
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Re: Electric Water Heater Runing on 12v Bateries

Postby asheets » May 21st, 2013, 9:16 am

I concur with k-ww. You'd be better off using the photovoltaic panel to drive a pump hooked up to a large copper pipe grid.

But, from my experiments done way-back-when, I only got a +5degree output from the solar heater on the sunniest days, from a grid nearly half the surface area of the pool itself. That's a whole lot of copper pipe ($$$) for not a whole lot of comfort. The pool was a massive cold-sink, and impossible to heat up without a lot of propane for the flash-heater. So "better off" is really a relative term.

On the other hand, I have read from some of the folks over at the otherpower.com forum who used an electric hot water heater tank as a dump load for their wind generators. But again, their wind turbines are in the kilowatt class, as opposed to your HF panel that on a good day may only put out 50 watts.
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Re: Electric Water Heater Runing on 12v Bateries

Postby leadacid » May 22nd, 2013, 1:06 pm

Yay Math Time!

Assuming a perfectly insulated pool, 0-ohm connectors, wires, etc.
Assuming that your inlet water temperature (ground/city water) is 50F (10C)
Assuming that your desired pool temp is 86F (30C)
Assuming that your pool is relatively small 12' x 24' x 48" (3.7m x 7.3m x 1.2m) and holds 4502 gallons of water (17041.9 Liters, 17041900mL)


1 Joule is one Watt-Second. Or in other words, one watt of energy expended in one second. And 4.184 joules of heat energy is needed to bring 1 gram of water (one cubic centimeter, or one mL) up one degree C.

So, we need to raise 17041900 mL of water 20 degrees. So math time!

To raise that much water just one degree would be:
17041900 x 4.184 = 71303309.6 watt-seconds.
To raise that much water 20 degrees would be 1,426,066,192 watt-seconds.

Now, of course you don't need to heat it 1 second, so let's say that we'll do it over the course of 5 days.

5 days = 432000 seconds
1426066192 watt-seconds / 432000 seconds = 3301 Watts.

Now that 3301 watts is continuous over those 5 days. And of course, that is assuming perfect ... well ... everything. Wires, heating elements, insulation, etc. No losses anywhere, which of course isn't realistic. However, 3.3KW is a somewhat realistic number, being roughly 14 amps at 240V, something that most households would be able to support.

(As a side note, that would be 396KWh consumed, and at about $0.12 / KWh, so it would cost you roughly $47 to heat your pool)

Okay, so lets take a look at your proposed power source, the Harbor Freight solar panel. The biggest one I could find was their 45-watt kit. So!

At 45W, how long would it take to heat that pool up? 31690359.82 seconds! That's only 366.78 days! But, realistically there are only about 14 hours of daylight during the summer, and to be fair, lets take only half of that to account for times when the sunlight isn't direct and the panel isn't outputting its full 45 watts, so about 7 hours of daylight a day.

End result? 1257.5 days, or roughly 3 1/2 years.

So...I don't think that your solar panel will actually heat that pool. Especially when you consider that almost everything in this system is "leaky". The pool water won't hold its heat forever due to imperfect insulation. The heater isn't 100% efficient either, nor is the electrical system involved.

Heck, just having it out in the sun will generate more heating power directly than doing the sun-> solar panel-> electricity-> heater route.
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Re: Electric Water Heater Runing on 12v Bateries

Postby AngeloVaughn » July 21st, 2015, 11:43 pm

Woow! Amazing calculations. For me, I will just stick on solar water heater if that is the case. This a cost-effective way to generate hot water for your home
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