No matter that the hype cycle wants you to think, the renewable energy
transition is the biggest thing happening in tech and it's happening faster
and faster. Despite being neck deep in it personally with offgrid solar
projects, most recently solar hot water, increasingly it becomes clear I'm
watching from the sidelines.
In Australia,
everyone gets 24 kwh of free daytime electric power now.
That's without installing any solar panels of their own, the grid just has
that much excess capacity. All it takes to save $thousands per year (and
avoid emissions) is to schedule some big loads like the hot water heater
and EV to charge during the day. To save more, drop in a home battery
that charges for free and powers the home through the evening.
In Germany, a 2 kwh plug-in home battery costs $350 and the electric
company will pay you
$130 per year to plug it into your wall.
There are similar offers throughout Europe.
In Cuba something something geopolitics, oil blockade, belt and road =>
suddenly 1GW of solar farms with another gigawatt on the way.
I'll soon visit South Carolina where with no subsidies whatsoever from a
decidedly renewable-unfriendly government, it made sense for my dad's house
to get a whole home battery and double the solar array. The resulting
system will be able to power the well pump and probably also the whole
geothermal HVAC system through the kind of month-long grid down events that
happened in Hurricane Helene.
Myself, well, I've got a by modern standards small 4 kwh home battery that
powers my house offgrid, and I've recently installed a heat pump hot water
heater. That's after about a decade pondering what solution to use for
solar hot water, to replace an aging and horrible propane instant water
heater. I've in the past considered everything from evacuated tubes to
special direct drive inverters to DC resistive MPTT dump loads. The solution
turned out to be just a big enough solar array, and plugging in a 120v hot
water heater that needs only 500 watts in heat pump mode. Plus a small
amount of code to manage when it runs.
In the time I was thinking about that, economies of scale and tech
improvements just wiped all those other possibilities off the map, it's not
economical to install and maintain a separate evactuated tube heat
collector when a pile of solar panels costs so little and when electric
hot water has gotten more than 200% efficient.
I also recently completed my permanant EV charger installation, with a new
inverter and conduit and proper wiring, and increased the car's charge rate
to 2 kw. Eliminating the need to charge anywhere except at home except
on road trips.
Coordinating when these two big loads run, to maximize solar production and
ensure that the house battery is full at the end of the day was ... not
hard at all actually? The car charger amps can be dialed up and down to
match incoming solar power fairly well, and leave some room for the hot
water heater. They both operate as more or less dump loads. More or less
because neither one can be cycled on or off very fast (to avoid wear and
tear on the car's contactor and the heat pump's compressor), so it makes
sense to leave them on and skate through short cloudy sections of the day,
as long as the house battery doesn't get too low.
How low is too low for the house battery? Depends on the time of day. The
code it's currently using, which may get tweaked over winter:
-- When the battery is charged enough to run major loads that may prevent
-- charging it further.
--
-- This varies with the hour of day. Early in the day, the battery does not
-- need to be as full to be considered well charged, since there is
-- still plenty of time for it to charge up. Later in the day, with less
-- time to charge, it needs to be more full.
wellCharged :: Hour -> Percentage
wellCharged (Hour hour)
| hour < 9 = Percentage 90 -- night
| pmhour <= 0 = Percentage 50
| pmhour <= 1 = Percentage 60
| pmhour <= 2 = Percentage 70
| pmhour <= 3 = Percentage 80
| pmhour <= 4 = Percentage 90
| otherwise = Percentage 95
where
pmhour = hour - 12
More complicated is, what to do it there's solar power to run one or the
other, but not both? This is starting to get into the territory of
microgrids now, or of demand response programs, so there's a whole industry
or three out there doing industry things geared at the kind of no-brainer
solutions I mentioned earlier. From what I've gathered, all of them
involve proprietary protocols and gear.
What I've done is to read the state of the hot water heater and car, and
prioritize hot water over the car. Except, if the car is below 10% it
urgently needs to charge.
And I found a really simple way to decide when to run the low-priority
load: Just check if the house battery's current charge will be considered
wellCharged in an hour. So if it's 2 pm, the battery needs to be 80%
charged to run the lower-priority load, and if it dips below that, that
load will turn off but the high-priority load will keep running down to 70%
battery.
Unfortunately, getting any information out of my hot water heater relies on
a vendor API server that is often down on weekends, and reverse
engineered the web page of my EVSE[1] to control it, to say nothing of the
nightmare of getting the car's state of charge from The Cloud.
Anyway, I'm pleased with having easily tweakable code and how far I've
taken this offgrid, and everything I've learned doing so, but like I said,
I'm clearly observing from the sidelines over here while the most
significant thing for all of us is going on over there. You might
appreciate my code or method, but you'll eventually be plugging in a home
battery or signing up for a free daytime power tarrif from your electric
company, or having professionals install a whole home system for
climate resiliance.
So my question is, where does free software fit into all this? There are
things like Home Assistant that do productize the kind of thing I'm doing
enough to be useful more widely. But still niche. Meanwhile there are
inverters and batteries that phone home to China, and every consumer
facing install is either "use this device" or "integrate these 3
proprietary devices".
I don't think focusing on these negatives is really useful though, I'm more
trying to understand where all this is going and then maybe get out ahead
of it in some useful way with free software. Your thoughts welcome.
[1] Obviously OpenEVSE exists, but it didn't meet my needs hardware
wise. And I could set my EVSE to use an OCPP server but it was easier to do
the screen scraping than find an appropriate one, and I have
the feeling I would not appreciate learning any more about OCPP,
in the same way I really don't want to know a lot about web browsers'
tag soup mode.
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