Heating water with pv electricity

[SteveCH]

I could not find reference to this in the Search, so here goes.

Our wood-fired water heater is about gone, I just brazed it [rusted out pin hole] and must get serious about a replacement system. Wood-fired ones like ours--about ten gal.--the AquaHeater no longer made or imported or whatever. I've been looking for several yr. Nor are there any other wood ones I can find.

Would rather not run a coil in a wood stove which I'd have to heat up in summer. Our current heater has a water jacket around the firebox and does not heat up the room. Propane I don't want to buy and at 8500 ft. MSL it can be challenging to keep a high-BTU burner going, which knocks out on-demand heaters.

Anyhow, while I ponder trying to make my own wood-fired system, which means great care as for safety issues...

We have an excess of PV electricity many days. Batteries charged up by late morning and kept topped rest of day, [Outback MX60s]. I am wondering whether it would help to use some of this extra electricity to heat or partially heat water, probably sticking to 120 v. elements via our inverter as with the MX60s using 12 v. looks like a chore as was recently written about extensively over at the Outback forum.

So, I don't know how to calculate what I can expect running, say, a 1500 w. element 120v. to heat water in a 30 gal. tank, just an example. I know there are formulas, there have to be, but I don't know them and haven't run across them on the web. I realize one must consider temperature rise and volume. I am thinking a super-insulated tank such as one of the Vaughns to minimize heat loss in the stored water.

Can anyone set me straight?

[dbcollen]

I have  an AQUAFIRE 10 gallon wood fired water heater that I no longer use, I am in northern California. no leaks as far as I know

Dustin

madriverwind(at)gotsky(dot)com

[Opera House]

I have a small system and use the fridge as a dump load.  This may seem off topic, but the control system would be ideal for heating water.  I monitor battery voltage with a LM431 and this trips a relay when the battery is fully charged.  That triggers a MC14541 timer that turns on the inverter for ten minutes.  After that time it can restart again. I think a timed control to drain down rhe battery for a period is better than systems that dump to a heater. I buy defective 2000/4000W inverters real cheap and this seems a far better way to go than costly low voltage heating elements with their associated wiring problems.  Just put the thermostat in the control loop.  Remember a 1000W 240V heating element is only 250W at 120V.  Find a tank that has two heating elements.

[BigBreaker]

Heating water with PV is a travesty.  Solar thermal is the way to go.  Check out some of the posts here about Gary's $1000 hot water system or on his site, builditsolar.com

If you have a big enough surplus of PV to heat a meaningful amount of water than you have too much PV.  Sell some panels and put a small fraction of the proceeds towards a solar thermal setup.

[GaryGary]

Hi,

It takes 1 BTU to heat 1 lb of water 1F.

A 30 gallon tank is (30 gal)(8.33 lb/gal) = 250 lbs of water.

So, heating the 30 gallons from 60F up to 120F is

(250 lbs)(120F - 60F)(1 BTU/lb-F) = 15000 BTU.

There are 3412 BTU/ KWH, so (15000 BTU/3412 BTU/KWH) = 4.4 KWH

So, it would take 4.4 KWH of excess PV electricity to heat 30 gallons of water from 60F up to 120F.  Seems like quite a bit.  If you are in a good sun location, this might be around and extra 1000 watts of PV panels?  Worth about $4000?

I'll admit to being a bit prejudiced on this, but one way to look at it is that PV collectors cost about $4 per peak watt, while DIY solar thermal panels cost about 10 cents per peak watt -- a 40 to 1 advantage for solar thermal!

The 10 cents per peak watt is:

($5/sqft) /((93 watt/sf)(0.5efic)) = 10.7 cents per peak watt

The $5 per sf is what it costs in materials to make a good DIY solar thermal collector.

The 93 watts per sqft above is the same as the 1000 watt/sq meter used to rate PV panels.

This is the system I use:

http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm

Gary

[hydrosun]

There are about 3400 btu per kwh. A btu heats 1 pound water 1 degree F. Water weighs 8 pounds per gallon. So, if you have enough extra electricity to run a 1500 watt heating element in a 30 gallon tank for one hour the temperature would rise about 21 degrees. So it would take 3 1/2 hours to go from 50 to 120 degrees. So that would work fine if you have a surplus on sunny days. But what about cloudy days? I've put copper pipe on the outside of our wood stove with plaster of paris between them. After a few days without sun our 55 gallon tank that stores heat from a solar water heater and excess solar electric heat has cooled down below a comfortable shower temperature. So we'll do a short hot fire in the wood stove. We had to do that once this year in June. We've got a large mass underground house so we aren't overheated. With alot of the stove upper surface covered with  copper pipes it doesn't get as hot by the stove anymore. In Scotland I saw wood cook stoves with water coils that had insulating covers to keep the heat level down when you only need hot water. So it is possible to modify regular wood stoves to get hot water.

Chris

[DamonHD]

Or you can use a heat-pump which gets you back about the overall efficiency you would have had with direct solar thermal, ie 50% of incoming radiation as useful heat if you are lucky...

Rgds

Damon

[SteveCH]

Thanks, everyone, the calculations were exactly what I was looking for. Yes, we have excess PV many days, as Excell Energy paid out 2.50 per watt of PV power off-grid homes produced a couple yr. ago...I beefed up our array at that time to take advantage and get a big check back. Which we did.

However, as I suspected, it would only work on seriously clear days for heating water. I just needed to make certain.

Solar water heating is elegant and would be fine, homemade equipment to make the cost affordable to us right now. The very cold outdoor temps winter time necessitate drainback or closed loop and that makes another problem for us, of room in our very small cottage, room for the tanks and etc.

For now, I'm working on a plan to salvage the firebox of the current water heater and making a new heater out of that somehow. The beauty of the current heater is that it is a self-contained stove/heater/water tank about four ft. tall and eleven inches in diameter, much smaller than any system available or made from old water heater tanks, etc.

Thanks for the input and the responses.

[BigBreaker]

I agree completely in terms of btu's per square meter but a PV driven heat pump heat is very expensive compared to direct solar thermal.

[DamonHD]

Agreed, but resistance heating seems such a shocking waste of high-grade energy!

Rgds

Damon

[madlabs]

I live in NorCal, and if the heater isn't spoken for by the OP, I'd be interested. If you can, drop me a line at jpeakall AT madlabs DOT info.

Thanks!

Jonathan

[BigBreaker]

Right - you want to avoid that at all costs.  Solar thermal is cheap enough to oversize both in terms of output and storage, unlike PV and easy to backup with propane.