Collapse early and avoid the rush

22 06 2020

The title of this post is of course a well known line from John Michael Greer’s masterful blog. I’m using it today, because apart from everything else happening in the world today, we have stretched our off grid solar power system to the limit. I always knew this would happen, but now I’m reporting on how we are literally powerless, even with state of the art equipment….

Intermittency was one of the shortcomings of solar energy that was mentioned in Planet of the Humans and poopooed by the film’s critics. Ten years old, they said, we’ve moved on they said…. Well no, we haven’t moved on. Solar energy is still intermittent.

We’ve been stretching the envelope down here in wintery Tasmania for about a month. Apart from it getting colder, the biggest change is the length of the days. Yesterday was the solstice, and with hills all around us, the sun doesn’t rise until 8am; and it sets before 4:30pm. Then there are the clouds. Lots of them. Don’t get me wrong, clouds down here are beautiful….

Winter sunrises can be stunning

It’s not unusual at this time of year to get no proper sun until 9am. and if your batteries went flat at 2am, and you’re hanging out for a coffee but you can’t even pump water from your taps, it feels like collapse. I’m the first to know if the batteries go flat in the middle of the night, my CPAP machine stops working and tries to suffocate me. Just joking. An alarm comes on to wake me up, just in case.

NiFe batteries don’t ACTUALLY go flat, the inverter spits the dummy when the voltage drops to 32.6V. with everything shut down, the battery voltage starts rising again until a critical voltage is reached and the inverter restarts. But if the blackout was caused by a biggish load, like the fridge, the voltage quickly drops again, rinse and repeat. So I have to get up and turn the fridge off, and the light circuit too, because we have two lights on sensors in the bathrooms, and two remote controlled lights in the bedroom fans which automatically come back on when the power returns. I didn’t know they were designed that way when I bought them, we have to live with it. No biggie. Just more stuff to learn.

But wait there’s more….. if the sun comes out full bore next morning, it will of course start recharging the batteries. Trouble is, I discovered, as soon as the charging voltage of 64.3V is reached, which is the entire system’s maximum operating voltage, the maximum power point tracker charging the battery bank drops the Amps coming in, going into absorption mode, and reducing the power going into the batteries…! I’ve seen our 2.2kW array reduced to producing under 1kW in full sunshine conditions.

Never having lived with off grid solar before, I had no idea of just how inefficient such systems were. So even though we’ve had the odd day or two of reasonable weather, our batteries were never fully recharged. Now that the whole fury of roaring forties winter has been unleashed on Tasmania, we’re bearing the brunt. At midday, total solar input was a paltry 20W. That’s 0.001% of full capacity. The upside is, after more than 36mm of rain by lunchtime, our tanks are overflowing again…

Now I’ve had people say to me, put more panels in, or more batteries, but in truth, none of this would help because a 4.4kW array would put 40W into the batteries instead of 20W. AND would a bigger battery bank ever get fully charged? Not to mention the fact that all that extra capacity would be simply wasted on a bright sunny day.

My son’s been onto me to install a wind turbine as backup, but since getting my personal weather station, I’ve discovered that wind speeds rarely reach the necessary 22km/hr required to reach cut in speed…..

Fans of 100% renewables always make me laugh…. Because living without fossil fuels, as imperative as the need to do so is, means taking the grid off the grid. And good luck with installing huge storage capacity with the economic calamity we’re now facing.

Things could be worse. I’ve just cooked a risotto on the AGA, and to avoid using the pump, we’re collecting the tanks overflow in buckets to washup….. we’re still living way better than at least half the people on the planet. AND for the first time ever, I’ve written a blog post on my phone. It wasn’t quite the ordeal I was expecting either…..

UPDATE

After speaking to my mate Bruce in Queensland who has also bought a bank of NiFe batteries to replace his aging lead acid ones, and who’s having the exact same problem (only with much older equipment) I decided to remove two cells out of service.

This was easy as it turned out, I just loosened three nuts, swung the connector between the two lines of cells over, and retightened the nuts. Those two cells amount to 2×1.65V = 3.7V. The battery bank’s new voltage was then 64.4V – 3.7V = 60.7V. This voltage drop now forces the MPPT to ‘overcharge’ the remaining 38 cells to, theoretically, 1.7V which will increase the battery bank’s storage capacity in the poor winter weather conditions that are normal here. This is when the flexibility of NiFe batteries really shines. Don’t worry about the ‘overcharge’, you can’t hurt these batteries, they’re pretty well indestructible, that’s why I love them. And don’t even think about doing this with other types of batteries, you’ll just kill them….

Instead of going into absorption mode, the MPPT is now in bulk mode, forcing the whole output of the panels into storage. You can even see the bank voltage has already gone up from 60.7V to 61.2V in just the time it took me to reconnect the system, check everything, and decide to take a photo…… by the end of the day, with a mixture of Sun and clouds, we managed 5.4kWh, the best result in ages. Best of all, the following day of full sun resulted in 7.2kWh, an outstanding result for winter. Our batteries are now full, and the next three days that may bring snow to the hills should be survivable…. some time after the equinox, I will put the battery bank back into normal mode, let’s see what happens…..

But wait, there’s more… by mid October, I realised that the batteries had gone rather drier than usual. All that supercharging of the cells, while it definitely worked and we haven’t any more blackouts, causes more gasing, with hydrogen and oxygen bubbling out of the electrolyte, causing its volume to drop. This is normal, but to my amazement, I ended up putting 54L of distilled water into the batteries! That’s way more than double anything I’d previously added…… It’s a good thing the batteries are indestructible and not affected by this. They didn’t even seem to have lost capacity.

At over a dollar a litre though, I’m going to have to get my act together and build myself a solar still to make my own pure water…

UPDATE: on the twin’s birthday, I’ve reconnected the two cells I took out of service to reduce the water loss in the electrolyte. After all the talk from people who probably never lived with NiFe batteries saying they have a high self discharge rate, I can tell you it’s utter rubbish. After a whole four months at zero charging, the two hibernating cells were at 1.32V, which astounded even me…. The other resting cells (I did the deed in the dark) were at 1.57V. After just six hours of charging on a cloudy day, they’ve almost already equalised.