Renewable Energy Questions/Discussion > Solar (heating or electric)

Solar to electric Water heating.

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DJ:

I would like to make up a controller / converter with an Arduino to take the (excess) output from my solar panels and use it on an old resistance  water heater.

There is -some- detail I have seen of this on another forum although the poster is an expert on such things and just posts some code and expects others to work out the components and circuit involved.  I'm way too much of a beginner for that But if given the circuit, parts and code I could probably nut it out.
I understand the power can be fed direct to a normal 240V element as we use here buy way of PWM to get the panel voltage and the element resistance matched.

I have spent hours today trying to find something like this but apart from what looks like some grossly overpriced commercial components that look very simple, I can't find anything like I want. Plenty of circuits to control a water pump on an an absorption heater but nothing to convert Low voltage Panel output to drive a standard heating element.  And please dear god, no need to tell me that this is inefficient or whatever else, I have read it 50 times already.  It IS efficient when I have power I have no other use for and an old electric HWS sitting round rusting.

I plan to use this initially anyway as a pre heater for the regular HWS.  The solar may only get the temp of this heater to 40-50 o and that feeds into the regular tank making the power I have to pay for from the grid to get it to full temp, a lot less than it would be otherwise and I am still guaranteed a full hot shower. I was also thinking that there is no reason this could not also power an electric stove element. Just needs to be sufficient power as a resistance element is a resistance element no matter how you apply it.  Blow some air across said element and you have a space heater.

If anyone knows of or has info or links to a suitable circuit with all the info that a beginner can build, I would really appreciate the heads up. Right now I don't even seem to be able to come up with the right name of what I am searching for.

frackers:
The first thing to think about is the voltage you have available from your panels and what the heater requires. Here is an example:

If the heater element in the tank is 220v and you have 8 panels of 250 watts each available, then assuming they are 60 cell panels with a maximum output at about 28 volts then just wire them in series and get a relay that can switch 220v DC (MUST be DC rated) and energise it  via the existing thermostat in the heating element. So that is 2kw at about 220volts and all its cost is the panels and a relay.

This extra relay is required because when contacts are broken on a DC circuit an arc starts up. On AC, the arc dies out due to the voltage changing all the time - on DC it doesn't "quench" and you end up with heating in places you really don't want. The thermostat contacts will be rated for the full power of the element but will be AC rated but they will be fine for the small current that a relay draws.

When the tank is up to temperature, be aware that the open circuit voltage from the panels will be up to 8 x 40 (i.e. 320) volts - another reason for having a suitably rated relay. This voltage will pull down to 220 or below when the contacts close on the tank cooling.

With a smaller solar array  upconverting the voltage a bit (but not too much otherwise you'll 'use up" the current output capacity of the panels and end up with a lower voltage) to balance the load of the heater to the panels output.

With a larger array then its making sure  you don't burn out the heater.

DJ:

I was thinking of something more like a KW and maybe 150V or less driving the heater.
I am under the highly likely incorrect impression that by using an arduino somehow to create a PWM output, the 240 element can be made to use all the current available at whatever amps it is at regardless of the voltage.

My understanding was that if you have say 1 KW @ 150V from the panels and feed it into the 240 element,  The element would only supply 250 w or whatever heating power. If you modulate or tweak the output from the panel, you could get the full 1Kw even at a much lower voltage.
The commercial unit I have seen on the net looks very small and basic and I thought this was how it was done as was the other thread I saw that only had the arduino code and was missing the rest of the info in the puzzle.

As far as the arcing of the relay/ thermostat contacts, you are saying to set up so the  thermostat switches the relay which is connected to the element?
I have bought some SSR 40A relays recently which are DC to AC. Would they be OK or is something else required? Any Difference with AC and DC being they are solid state and I imagine nothing to arc?

Pete:
Hi DJ, just wondering what sort of solar system you have. Is you system a stand alone battery setup or only a grid connect?
If you have a stand alone system there may be a simple way to do what you want.
You could use your regulator to switch the heater on. It would depend on what sort of regulator you have.
When the regulator switches to float you could use that signal going to the LED that indicates that to drive a relay that switches the panels to the water heater.
You could use your arduino for that, just program it to read the input voltage from the LED, and switch a relay on to power the water heater. Of course you would leave the thermostat in the heater so that you don't boil the tank.
There are low voltage elements available, I have seen 12 volt ones, not sure if there are other voltages too.
Otherwise you could use a cheap square wave inverter to drive the element. As it is a purely resistive load it won't mind a square wave driving it.
Hopefully others will come up with more ideas.
Pete

144VDC:
Hi DJ,
I have a stand alone (off grid) solar system and use my excess power to heat my domestic water. I have a 7000 watt array and most days 1/4 to 1/2 is excess. I use a pwm shunt regulator, that I built, to control my charge voltage. It shunts the excess power to a 120 gal. hot water tank with three 240v 3500w (10500w) elements. This system keeps us in hot water all the time.
I have a 144v battery bank, so my charge voltage is 174v and float voltage is 160v. Heater elements are just big resistors; the more voltage the more current and therefore the more wattage up to the point they burn out. My point is, any heater can be used at any voltage up to a maximum where the watt density is too high and it burns out. My elements are 4655watts @160v and my pwm duty cycle drops it down from there to whatever is needed to maintain the battery voltage.

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