Thermally Charging the House & the Future of Solar

With our recent acquisition of an AC unit to help us through the unfortunately more common heat waves, it became evident that we would need to upgrade our charge controller. The old one could take in no more than 80A from the panels, and the AC unit consumes 70A which leaves us with very little to run everything else in the house. So I bought a nice charge controller, rated for 100A but which can do up to 150A.

It is much bigger than the previous one so I made a mess of my wiring panel. I’m not going to worry about it for now. I don’t know when this will happen, but soon enough, we will build a root cellar and move the solar setup into it. Then I’ll have a good opportunity to rewire properly.

I’m still figuring out how the new controller behaves and what it delivers in reality. We can run the AC as long as the panels produce 70A or more, doing it outside of this means the batteries will get drained very quick.

Now, early June, it seems as though we have 6 hours of 70+A to work with. 6 hours during which we’ll run the AC no matter the temperature outside to thermally charge the house as much as possible. This is exactly what we do in the Winter with stove heat. We don’t just want the desired temperature in the room, we want it embedded in every wall, every mattress, every piece of furniture and as deep as possible.

I got the setup ready in the nick of time for a heat wave. During these 6 hours, the house is really nice, a real refuge from the heat. This AC unit at 8000BTU doesn’t make it nice and cool, but it does keep it comfortable, much more so than previous Summers. In this way, the AC is already amazing.

Towards the end of the day, with the Sun still pouring heat but the panels not making 70+A, the house remains comfy from the accumulated cold, but starts to warm up. When the Sun goes bellow the horizon, the house is just warm and we switch to window fans to bring in fresh outside air.

The new charge controller is MPPT, all the previous ones I owned were PWM. I tried to understand how MPPT works but I still don’t quite get it. The graphs for PWM are show a pretty obvious “algorithm”: open the flood gates when the voltage is low, close them when it’s right. Apparently MPPT charge controllers are more efficient. But they do cost a lot more and I can’t say I ever had an issue with the PWMs. It makes sense to milk your panels as much as you can when you have a limited surface to install them on (roof, boat, RV). In our case we have more than enough panels that PWMs were a non-issue. Except if you want a nice charge controller that does better at higher amperages.

While the PWM graph reflects the Sun’s path over the panels through a day:

The fancy pants MPPT aligns itself to the load instead of turning input from the panels on and off:

And I mean, it really aligns itself to the load.

It’s almost a shame, I don’t know how much I can pull from the panels because you don’t see the peak unless you reach it with your load. That’s what a fancy charge controller does I guess. It came with a whole bunch of monitoring gizmos, but they are unnecessary as my monitoring is decoupled from the controller. Really, all I want from it is the ability to go beyond 80A, and it certainly does that.

So here we are today with 18 ~110W panels and the biggest charge controller we can get. This is pretty much the maximum for a run of the mill off grid system. But these charge controllers can be run in parallel, and I would like to explore wind power. My crystal ball tells me that in the next 5 years, we will have more panels and wind capabilities. In some ways it’s simple engineering curiosity. In other ways, we’ve made cloudy days a non-issue, it would be nice to make Winter one too. And maybe we could push for an electric vehicle, or a bigger AC, or both. We recently got a little wind spinner toy for the garden, I look at it go through the day, and I’m getting curious about wind. We’ll see, for now we’re just enjoying a nice step which should carry us for a while, and the luxury of air conditioning.

Gratuitous panel pic, I enjoy their sight

Cheating the Laws of the Universe – a LOT of Amps

With our recent solar upgrade, we are able to produce much more than our charge controller can take. In fact, on days of perfect sun (full exposure, perfect angle) we need to turn off half of the array or the charge controller, which can only take up to 80A shuts off. This sounds silly, but on cloudy days, having all these panels is invaluable. So right now we’re a bit in a manual mode of turning panels on and off based on the weather.

3 years ago we had a brutal heat wave which sent us away from our house. I vowed then to have some sort of A/C capabilities before the next one. One of the nicest thing we’ve done to ourselves was getting a “regular” fridge. While at the time, it pushed our solar install to the limits, today it’s really not a big deal to keep going, even though it’s by far our biggest consumer of electrons. So we bought an 8000BTU A/C unit, I was expecting it to be maybe “a couple of fridges” worth of power consumption, but let me tell you… This thing is 7 fridges put together! Ouch!

We’ve been testing it now to make sure we won’t have any bad surprises when the heat comes.

We’ll never have the battery bank to store and supply 70A through the night. Also true, when the Sun shines, the solar panels can most definitely keep 70A coming and more. We’ve gotten good with stove heat, at managing not heat itself, but how to buffer it in the house, to buy us time through a cold night. It looks like we’ll have to do exactly that, but with cold. Run the A/C all day while the Sun gives us more energy than we can do anything with, to buffer the house as much as possible against the heat. Nights usually provide respite from heat in the Summer, in the Winter this is also true but it’s the opposite of a respite when you are fighting the cold :).

So our panels are more than enough, our storage is essentially null for the purpose of A/C, and our charge controller is too close to its limit of 80A to funnel all the panel energy to the A/C while doing the few other things we need electricity for.

I think this tells us we need to upgrade the charge controller. This way we can have a more pleasant Summer, and more specifically mitigation of heat waves.

All for free, money wise and carbon wise. Amidst these mundane concerns of solar system design to tackle such a juggernaut device as an A/C, it is easy to forget how beautiful and elegant it is that the hotter the Sun is, the more we can turn it into Cold. How often are problems their own solutions?