Circular reasoning (Rolfe's solar energy system)

[Gadget Geek]

I've been looking at Home Assistant set up and I see you can install it on a laptop via a Linux Live USB. I have both of these so I think I'll give it a go. Any advice gratefully received.

 

[Rolfe]

I have watched the video, and he actually confirmed my suspicion that it's quite a complicated thing to do, and that it can go wrong if the weather doesn't pan out as expected. I do have the system he says would be worth a try, an 8 kW array with a 5 kW export limit, and it does allow the excess to go into the battery if there's still room there when it would otherwise start clipping. But even so he reckons the game probably isn't worth the candle.

I might give it a try manually once or twice when the forecast looks reliable and see just how tricky it is.

 

[johnb80]

I've been looking at Home Assistant set up and I see you can install it on a laptop via a Linux Live USB.

It will run on several different platforms including Raspberry Pi. Linux install is dead easy, YouTube is your friend, almost everything is on there.

I have both of these so I think I'll give it a go. Any advice gratefully received.

Use YouTube all the way through from setup and all of the goodies you want to install. It’s not the most intuitive program but you can get by.

 

[Rolfe]

We had a reliable-looking forecast for full sun all day so I thought I would do a bit of experimenting. Turned out to be very successful.

First, I cancelled the usual instruction for the battery to charge from midnight on off-peak electricity, so that it sat empty all night while the house ran from the grid. After 5.30 am this was peak-rate power, but I thought I would just swallow that first time to see what would happen. In the event the house used 190 watt-hours between 5.30 and sunrise, which is something like 3.5p extra compared to using the same amount of off-peak power, so not breaking the bank just this once.

I could see that the solar would start to power the house from about 7 am, so in order to prevent the battery from charging and so gobbling the initial (unclipped) export, I set the battery to export from 7 am. So far so good, and when I woke up all was progressing nicely as anticipated. House running from the solar with all the excess being exported, and the battery down about 2%.

I was expecting clipping to start about 10 am and thought I would have to take the battery off its export setting then in order to let it charge from the unclipped excess, but to my surprise (and gratification) I didn't have to do anything. As soon as the grid export limit was reached the excess started to go into the battery without me doing anything. This continued throughout the day, with a perfect arc of generation peaking at just over 7 kW at about 12.30 (my array is slightly east-angled).

During this time the battery charged up to over 60%, which is more than enough for the house needs for the rest of the day, and indeed will leave some to export just before 11.30 pm.

Clipping stopped about 3 pm, at which point the battery, which was still set to export, tried to go into export mode to keep the maximum allowed export going, which I didn't particularly want it to do. I fiddled around a bit and eventually just switched the battery to idle mode and left it to its own devices. Its own devices turned out to be to keep charging from the solar, but only at 210 W. No idea why, but it got itself over 70% during this time. Maybe I should just have left it on export, but set it to export only down to whatever percentage was in it at the time. Might try that tomorrow because it should be easy to automate.

The solar stopped powering the house while I was typing this, about 5.30, so I took the battery off idle to let it power the house for the rest of the day, and carry on just as I normally would, exporting anything that's left around 10 pm.

This graph is a bit messy because I was fiddling with the settings to see what it would do, but you get the basic idea. I would say I got 6-7 kWh into the battery that would otherwise have been wasted to clipping.

Tomorrow is also showing continuous sunshine, so I'll give it another go, this time letting the battery charge 5.25-5.30 am to cover the period from 5.30 am till sunrise (in high summer this won't be necessary as sunrise is around 5.30 am anyway) and setting the battery export limit higher than I expect to recover during the day so that it won't start exporting as soon as clipping stops.

My feeling is that if I set the battery export to finish at 5.30, that will let the battery take over for the rest of the evening, and all I will actually have to do is check the situation around 10 pm to decide how much I want to export from the battery and when to start it.

It's not set-and-forget, but it's not an onerous thing to do to harvest that extra solar on very sunny days when clipping is going to happen. Even if the weather forecast turns out to be wildly wrong it wouldn't be a disaster, because if I woke up to a cloudy sky I could simply cancel the battery export setting and let the battery charge on whatever solar there was. It's pretty unlikely there wouldn't be enough to get at least 50% into the battery for evening use, so I still wouldn't have to import from the grid.

ETA: I note that I generated 48.1 kWh today, which of course is significantly more than if I'd just let the system clip. On Friday, which also had almost uninterrupted sun, I got 41.7 kWh. I also only imported 3.5 kWh in total, which covered the house base load up till 7 am, plus the Eddi heating the water tank from 4.30 am. Usually I import about 13.7 kWh on days when I'm not charging the car or running any kitchen appliances overnight.

So let's say I got about 6.5 kWh more than I would have got otherwise. That's about £1 in export payment, and maybe a bigger saving as I think losses from charging and discharging the battery might have been less. And it will only get better as we go into summer - it's only 6th April for goodness sake. I think my array may be capable of 8 kW in mid-June, and the solar pretty much takes over from the off-peak tariff at 5.30, and it can keep powering the house (low summer load) until up to 9 pm. Depends how many of these wall-to-wall sunshine days we get, but now I'm getting the hang of it, it's not much of a chore. This week is showing plenty of opportunity to experiment, anyway.

 

[T1 Terry]

When the clipping is more than the battery can hold (I'm guessing it is a house battery) can the MG be scheduled to start charging to use up any other excess?

I see you have mid day temps that we would be happy to drop to over night from around mid Nov to around this time of yr
Now daylight saving has finished over here, that extra hr of sunlight isn't warming the mornings up much ..... and it gets late much earlier as well

T1 Terry

 

[Bam Bam]

When the clipping is more than the battery can hold (I'm guessing it is a house battery) can the MG be scheduled to start charging to use up any other excess?

I see you have mid day temps that we would be happy to drop to over night from around mid Nov to around this time of yr
Now daylight saving has finished over here, that extra hr of sunlight isn't warming the mornings up much ..... and it gets late much earlier as well

T1 Terry

The car can't take the excess because the inverter can't convert it to AC (which the car charger needs). It can, however, put the (otherwise clipped) DC power into the DC-coupled battery.

It seems that Rolfe has got a method now.

I'm wondering if we are missing out, but because we have two equally sized East-West arrays I don't think we get as much clipping.

 

[Rolfe]

But, Bam Bam, if I put the car on the granny charger that will take power from the home battery and leave room for more solar. If it ever gets to that point I might try it, if it's convenient.

It would probably only happen in June, and of course on a very sunny day.

Terry, these temperatures are unusually warm for the time of year. Also, it was 5°C when I woke up this morning. Last night driving home from choir practice the car was pinging ice warnings.

I see the weather is forecast to get horrible again on the 13th. But this week is a good opportunity to figure out how best to set the system up for sunny days.

 

[Bam Bam]

But, Bam Bam, if I put the car on the granny charger that will take power from the home battery and leave room for more solar. If it ever gets to that point I might try it, if it's convenient.

It would probably only happen in June, and of course on a very sunny day.

Terry, these temperatures are unusually warm for the time of year. Also, it was 5°C when I woke up this morning. Last night driving home from choir practice the car was pinging ice warnings.

I see the weather is forecast to get horrible again on the 13th. But this week is a good opportunity to figure out how best to set the system up for sunny days.

Hah, yes that just occurred to me and I came back to say that.

Charge either the hot water (Eddi) or the car at a lower rate.

Do you need to use the granny charger? Does the zappi not allow you to charge at, say 4 kW? Our Givenergy unit can be set at 0.5A intervals.

 

[JohnV6]

My Ohme Home Pro charges on solar from 1.44 kW but if the PV drops below that it pulls off the grid. i.e. there is no smart solar switch just a manual over ride.

 

[Rolfe]

I could probably do it with the Zappi if I fiddled with the settings, but the granny charger is easier for that job. The Eddi is another possibility but to be honest I heat that for 7p/unit just before the end of the off-peak tariff every night and it usually does me for the day,

 

[Everest]

My Ohme Home Pro charges on solar from 1.44 kW but if the PV drops below that it pulls off the grid.

But Rolfe has batteries to cover any temporary shortage if there are patchy clouds around.

 

[Everest]

I note that I generated 48.1 kWh today,

Impressive

 

[JohnV6]

The charger is after the smart meter & on a separate link to the house consumer unit so my batteries won't discharge.
However today i got the car down to 11% to do a calibration charge. The Ohme and Octopus IGO decided it needs to charge the car partly during the day. So right now instead of export the PV array out put is going into the car.

 

[Rolfe]

The charger is after the smart meter & on a separate link to the house consumer unit so my batteries won't discharge.
However today i got the car down to 11% to do a calibration charge. The Ohme and Octopus IGO decided it needs to charge the car partly during the day. So right now instead of export the PV array out put is going into the car.

Will you be able to do the charge uninterrupted, though?

 

[Rolfe]

For some unknown reason the panels are generating slightly less today. It seems to be just as sunny, but they peaked at just under 6.8 kW whereas yesterday they made it to 7.1 kW. Maybe there's a slight haze? Nevertheless the battery made it to 34% about half way through the generation period, from a starting point of 9% this time. Should have plenty in there again, and some to export at the end of the day.

My attempt to prevent the system drawing from the grid between 5.30 am (end of the off-peak rate) and sunrise didn't work, although I'm not sure why. I put the home battery on charge for five minutes at the end of the off-peak price, thinking to get enough to run the house base load for an hour and a half. What actually happened was that although the battery increased from 3% to 6% it didn't run the house, which proceeded to draw 250 watt-hours from the grid over the next half-hour. No idea what it was doing with that, at that time in the morning.

However, after that the battery did take over the house load for the final hour before sunrise, while at the same time increasing its SoC from 6% to 12%, then finally dropping to 9% after the solar had taken over the house load. I've seen it do paradoxical things like this before, strange are the ways of electricity, but I don't know if it's a one-off or if it will always tend to do unpredictable things at that low SoC.

Might be better, if I can think of a way to do it, to leave 10%-15% in the battery overnight rather than exporting the lot, and having that in hand in the morning. Of course this will get less of an issue going into summer when the panels start taking over the load at 5.30 am anyway.

 

[Rolfe]

It seems to me to be pretty safe to set the system up to do this when the forecast is reasonably sunny. If it turns out that there's enough cloud so that there isn't enough clipping being avoided to put enough charge in the battery, I can always just let the battery take more of the solar for a bit to get it high enough. Not set-and-forget, but it's not going to happen very often. I don't see it as at all likely that a day forecast to be very sunny would turn out SO bad that there wasn't enough generation to do the job. (OK, maybe if a haar happened, but haar isn't common here and it usually clears to sun by lunchtime anyway.)

 

[JohnV6]

Will you be able to do the charge uninterrupted, though?

Yes IGO & Ohme have programmed it as a continuous charge , all be it at different rates. By that I mean all at 7p per unit

 

[decrep]

For some unknown reason the panels are generating slightly less today.

Was it hotter? PV panels are supposed to be less efficient in higher temps.

 

[Rolfe]

Yes IGO & Ohme have programmed it as a continuous charge , all be it at diferent rates

Excellent!

Was it hotter? PV panels are supposed to be less efficient in higher temps.

Yes, it was definitely warmer. I went out cycling in a t-shirt. Could have been that. The panels looked slightly dusty as it hasn't rained for a bit, and there were a few spots of bird-poo, so there's that too. We'll see what it looks like tomorrow.

 

[Everest]

What actually happened was that although the battery increased from 3% to 6% it didn't run the house, which proceeded to draw 250 watt-hours from the grid over the next half-hour. No idea what it was doing with that, at that time in the morning.

I would guess that is down to your inverter's settings. It is common for an inverter to have a minimum SOC% specified below which the battery power will only be used for powering the inverter and not for powering the house. IME those default values are typically in the 10% to 20% range.

Hence, you may need to have more SOC in the battery before it can be used to power the house.