Power controller for electric heat

I need some brainstorming here. First the background and then the question.
House is 20 years old. Designed as a solar house. In the winter the interior air is heated by the sun coming thru the south facing windows. A variable speed Grainger blower sucks the air down an airshaft from the vaulted ceilings thru an electric 12Kw 240Vac heater. The warmed air gets distributed thru ducts in the 12" think insulated concrete slab. The ducts leave the slab and go to floor registers in each room.

That's the way the house is heated and energy stored in the concrete slab. It works well and I'd change nothing. The electric heat is controlled by an Arduino based thermostat that also controls the blower speed and the AC.

What I'd like to do is to control the power going into the 12Kw resistance heater. So that during the day when the sun shines. The power going into the heater is based on how much power is being produced by the PV array/inverters and will add additional heat to the slab during the day. The PV system is grid tied. So less power would go back to the grid. Instead the slab would save the PV energy as heat to warm the house at night.

When the sun is shining on a winter day the house heat does not come on and the heat stored in the concrete slab will get us thru winter nights if the temp is no colder than about 30F. Below that the backup 12Kw heater will turn on as needed.

What I am looking for is the box to do the power control of the heater. Does anyone make something like this? Software can send whatever control signal is required by this unknown power control box.

Any ideas?

SolarRich said:

What I am looking for is the box to do the power control of the heater. Does anyone make something like this? Software can send whatever control signal is required by this unknown power control box.

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There may be ‘smarter’ things but a $15 temperature control relay on Amazon could be used to supply a simple 1/0 signal beyond the thermostats, and then some code to evaluate sun efficacy in another arduino which determines when to add grid-supplied heat energy above the thermostatic call.

But other than that I’m an idiot and have zilch to offer

If your thermostat is Arduino based, is this something you programmed?
Seems pretty straightforward to add it to an an existing Arduino already doing work on the heater.

Yes, I made the thermostat. But I need a 'box' that can take an input from the thermostat to control the voltage to the heater. Say 25%, 50% or 100%. I know that I can make a triac controllers to do just this. I'd rather buy it.

Oh, you're looking to limit the 12kw heater to the instantaneous power available. Now I better understand the question.
But, sadly I'm unaware of any off the shelf options. I'm also not that well versed in anything that could handle a 12kw heater.

My level of solution would me multiple smaller elements. Something like 500w, 1kw, 2kw, 4kw, 8kw elements. Where with the correct combination of elements engaged, you can make 500 watt steps up to and well past the current 12kw heater. However, that sounds much more basic than your skill set and likely requires modifications to your current heater.

400bird said:

you're looking to limit the 12kw heater to the instantaneous power available

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That was how I took it fwiw

400bird said:

sounds much more basic than your skill set

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I was wondering about that.
Well beyond my skillset even back 20+ years ago when I could hack around a bit.

If one has the coding skills…
I’m wondering if hacking a big enough mid-tier AIO that can ‘load share’ couldn’t be used to vary/limit current output on the fly based on a different scheme than intended; it has the switching gear already, so just telling it to act differently seems possible. Yet that seems complicated to my small mind.

My suggestion would be to explore using a water ‘battery’ to store excess energy from an overpanelled array- people do this sort of thing all the time already. Even at my neanderthalic level I can see doing that with simple clunky relays and boards available on Amazon to achieve this rudimentary logic to pull it off.
Being able to code an arduino, however, brings this down to a matter of switching pump relays and water heater elements. If incorporated with importing and parsing live solar monitoring data to weight the simple if/then/else logic required this would seem the most beneficial energy storage scheme. Plus, the hot water stored could be a more controlled - thereby ‘efficient’- delivery of the stored BTUs. Only when needed. Concrete will just accept then reradiate heat regardless of thermostatic control other than the cycling of the blower.
With a water battery one could use a circulation pump to coils at the blower housing and thermostatically limit or combine with the running of the electric coils as desired.

I’m no engineer but a highly insulated hot water battery scheme makes more sense to me than switching air heating coil amperage.

For zero export, feed line voltage and grid current into the micro. Example, any time the voltage is positive, just average up the current. Depending on how current transformer is phased. Then drive heaters till you get back to or near zero. This can work off your grid tie AC or directly from array DC. Multiple heaters can be used. I designed a system which had 6 heating elements on or off. One of them would be PWM from capacitor bank for fine control. Cotrol was simple up down counter using analog PWM function.

Does your heat strip have 2 separate coils ?
Most do in increments of 5/6/10 kw giving you the possibility to go 0-50-100% with some rewiring.

No 1 coil at 12kw.
Perhaps the best way to do this is to replace the coil with one that has the 5/6/10kw as you suggest. I recall this coil was $500. Will have to look around and see what I can find.

Triac based AC power controller will work to control power. However, at 12kW (or even 2 or 3kW) it can create horrible harmonic electrical noise that can cause interference with other household loads ranging from annoying (light flickers) to serious (motor/compressor cutouts). For clean interference-free power you will need a sinusoidal power controller (e.g. industrial lighting) but that has downside of much higher cost.

solar8484 said:

Triac based AC power controller will work to control power. However, at 12kW (or even 2 or 3kW) it can create horrible harmonic electrical noise that can cause interference with other household loads ranging from annoying (light flickers) to serious (motor/compressor cutouts). For clean interference-free power you will need a sinusoidal power controller (e.g. industrial lighting) but that has downside of much higher cost.

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Exactly. I recall using triac based light dimmer and it made lots of RFI. I need to find out about these sine wave types of lighting controllers.

I don't know why I didn't think of this earlier, but what about converting to DC?
Doing anything like this at 12kw boggles my mind, but I'm picturing a 200-400 volt AC-DC converter either with adjustable voltage output (to adjust wattage) or PMW to drop energy usage.
But 12kw is a lot of juice, I don't know how feasible or costly this would be.

It just seems so inefficient to be using solar panels to generate heat. Hot water panels do a much better job at heating. I know that my hot water panels just blowing air through them produce some very hot temperatures. Why not just pick up some cheap hot water panels and blow air through them to heat your concrete?

You wanted some brainstorming so that is my two cents.

We have a hot water system for domestic hot water.
The PV system has been here nearly 20 years and has worked well. Repairs as needed.
A quick search on the internet and I've found 5kw heaters. If I could find 4kw heaters I would use 3 of them and then switch them on/off as needed. So I'd have 4kw, 8kw and 12kw.
Could a heating coil be used on DC? I don't see why not. But yeah, the cost of the equipment may be too high but worth a look. A large bridge rectifier and PWM controller good for 50A?
Lots of good ideas!