using extra power

System

We have a concrete floor in a solar powered studio. The infloor heating piping is heated by a wood stove when we are there. I would like to use excess solar electric capacity to heat an electric coil that would be installed in the holding tank. The female thread into the tank is a 1" or 1 1/4" IP coarse thread fitting. Is dumping power this way realistic and available? WE have 800 watts of panel and 2 inverters .....a 1500watt true sine and a 3000watt modified sine . I need advice on how to control this function.

XRinger

Re: using extra power

bilbo wrote: »

We have a concrete floor in a solar powered studio. The infloor heating piping is heated by a wood stove when we are there. I would like to use excess solar electric capacity to heat an electric coil that would be installed in the holding tank. The female thread into the tank is a 1" or 1 1/4" IP coarse thread fitting. Is dumping power this way realistic and available? WE have 800 watts of panel and 2 inverters .....a 1500watt true sine and a 3000watt modified sine . I need advice on how to control this function.

It sounds like you need a 1kw heating element in your tank, and an electrician to wire it up. (With a thermostatic control).

Some wind power guys dump extra wind power watts this way all the time..

The government has done some studies on the idea of using PV to heat water.
http://www.bfrl.nist.gov/863/pvsolar/pvwater/index.htm
And they found it works pretty well. The cost of PV is the main sticking point.

Since your PV is already installed, using your storage water tank as a 'Dump Load' for un-used PV power is a dang good idea.(IMHO).

I'm currently using DC from 400w of PV to heat water. Today it's doing 412w.
Not a lot of heat.. (1,406 BTUh) but better than nothing.

I'm seriously thinking of adding two more panels for 800w total. (2,730 BTUh).
That should make a noticeable difference..

Yes, I already know that "pay-back" will occur in the year 2067.. :roll:

Be prepared to hear from people telling you that PV to heat water is nuts,
and you should spend a ton of money of a solar hot-water system..

Cariboocoot

Re: using extra power

Using PV to heat water is nuts.

But seriously, if that's all you're doing with it it is nuts.

In this case you're seeking to make use of Watts that otherwise wouldn't be "harvested". I believe there are several people on the forum who do this.

One way is with the AUX control on an Outback charge controller; have it activate a relay that connects the heating element up once "Float" is reached. You have to balance the load with the potential capture of course; can't expect to get 3000 Watts of element fired up if there's only 1000 Watts of PV!

icarus

Re: using extra power

As 'Coot suggests, if you are trying to heat this way, you would be better off using direct solar hot water heat. If, you are trying to use your excess, find a small 10-20 gallon 120vac electric heater and plumb it into the loop, driving the water heater from either the controller as 'Coot suggests, or a voltage controlled relay driver. You could also replace the 120vac heating element with a 12 vdc one. (I don't know if anyone makes a 24 or 48 vdc water heating element.

A heating element won't care if it runs on MSW or true sine wave from what I have been told.

T

XRinger

Re: using extra power

I've seen a few different AC & DC water heater dump loads on the web. (& Ebay).

Here's one site that sells DC 12,24 & 48 DCV heaters.

http://www.survivalunlimited.com/diversionloads.htm

MisterB

Re: using extra power

It sounds reasable to me. You aren't going to get nearly as much heat out of 800 watts as a wood stove but it would keep things warmer than otherwise when you aren't there.

I have been dumping some of the extra energy from my system into a small electric refregerator with a an ancient Trace Quad Function Voltage Controller connected to an equally ancient 600 watt Statpower inverter. When the system voltage gets near to float level, the controller turns on the inverter and the fridge. The fridge is really low power--less than 100 watts at full power--and in summer when I really have power and am not as likely to be home to use it, I am going to put some mineral polishing equipment--rock tumblers, slab polishers--on the circuit. This is ideal because this equipment is really noisy, is meant to run for hours unattended and I'd rather not be there when it's running and when I'm home, I will be using the power for other things.

BB.

Re: using extra power

Many of the 12/24/48 volt electric water heater elements seem to be standard 240 VAC elements simply operating at lower voltages--which is fine (and cheap).

For example, a 4.5 kW 240 VAC heating element when operated at:

• 4,500 Watts * (120v/240v)^2 = 1,125 watts
• 4,500 Watts * (48v/240v)^2 = 180 watts
• 4,500 Watts * (24v/240v)^2 = 45 watts
• 4,500 Watts * (12v/240v)^2 = 11.25 watts

You can find other size immersion heaters or run several heaters in parallel too.

-Bill

icarus

Re: using extra power

I looked into the possibility of dumping excess into my 3 way fridge, using the 120 volt heating element. I sort of gave up on the idea when I realized that at best I was likely to get ~ 1 hour a day, at some hardware cost,, all to save almost zero in propane.

I do understand how we hate to "waste" the extra power available when the CC dials down.

T