Why is this apparent design flaw present in solar ESS systems? - Page 1
Forum section: electronics / projects

author: 5U4GB
author_rank: Super Contributor
timestamp: August 21, 2023, 10:57:07 am
content: This one is a bit hard to put into a coherent subject line/question, the full problem is this: Say you have a solar ESS (energy storage system, so PV panels, inverters, batteries) for which you put the lower-powered stuff that you care about onto the backed-up circuit and the higher-powered stuff that's not important and that'd drain the batteries very quickly on the non-backed-up circuit. Say the first lot is 0.5kW, the second is 9.5kW. You connect it all up and find that 10kW is being drawn from the batteries, not 0.5kW as expected, and pretty soon they're completely discharged.The explanation is in the internal wiring of the control system, I'm using Enphase here as an example but others do this too (assume residential so only L1 is present):Since the sole consumption CT is on the common bus, the controller can only see the overall draw of 0.5 + 9.5kW, so it pushes 10kW onto the bus to drive the flow at the CT to zero, powering a load that you specifically set up to not be powered from battery.This design is so obviously broken that I assume there's some valid technical reason why adding a second consumption CT for the backup load and only drawing that much from the battery isn't going to work, particularly since the issue is common across several vendors. Anyone know why it's done this way?(And because I know someone's going to ask this: Because the backed-up load was carefully set up to not draw large amounts of power, avoiding bang-bang charge-discharge cycling the battery).

author: moffy
author_rank: Super Contributor
timestamp: August 21, 2023, 11:47:04 am
content: What you need is the house wiring split into 2 groups/circuits for backup purposes:1. 0.5kW2. 9.5kWThen when external power fails a contactor opens and disconnects the 9.5kW load, leaving only the 0.5kW load. Organising wiring in this way in addition to the normal circuits such as power points, lights etc. would be very unusual. The best way might be to have say one power point circuit dedicated for backup and plug everything you want backed up into that circuit.

author: 5U4GB
author_rank: Super Contributor
timestamp: August 21, 2023, 12:51:40 pm
content: That's exactly how it's set up, due to the multi-step process required to insert images the wrong image got inserted, the post now has the correct one. So with this layout, with the grid connected, the battery is supplying 10kW instead of the planned 0.5kW.

author: moffy
author_rank: Super Contributor
timestamp: August 21, 2023, 01:19:33 pm
content: So effectively you only want the batteries to charge when the grid is connected, no discharge, the solar panels can use the grid for excess power and you only want the batteries to supply power when the grid is unavailable for the 0.5kW load? Then you are using the batteries as a UPS for the 0.5kW load, which is a different configuration from what you have, except you also want the batteries to charge from the solar if no grid is available. I am trying to state in words the necessary logic to try and understand the problem better.

author: 5U4GB
author_rank: Super Contributor
timestamp: August 21, 2023, 01:22:14 pm
content: No, I want the batteries to drive the 0.5kW backup load, not the 9.5kW non-backup load, however the only ESS configuration available, at least of the couple I've looked at, drive both the backup and non-backup load from the batteries.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 21, 2023, 02:14:10 pm
content: Just to understand: when the grid shuts off, the 9.5 kW load is disconnected. Your concern is when the grid is present and both loads are active, the battery is being used for more than the 0.5 kW allocated to the backup circuit?

author: 5U4GB
author_rank: Super Contributor
timestamp: August 21, 2023, 02:23:52 pm
content: Yup. 99% of the time the grid is active and the 9.5kW non-backed-up load is present alongside the 0.5kW backed-up load, so the battery draw is 10kW from both loads rather than the 0.5kW of the backed-up load.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 21, 2023, 02:50:07 pm
content: So what you want is:1) if the solar power is > 10 kW, power all loads, recharge the battery with the excess, and export if the battery is full2) else if the solar power is > 9.5 kW, power both loads with solar, use the battery for unmet demand up to the 0.5 kW of the backup load.3) else if solar power < 9.5 kW, use 0.5 kW from the battery, import the remaining shortfall from the grid?Except that you want those thresholds to be based on the actual instantaneous current draw of the two loads?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 21, 2023, 03:09:19 pm
content: I'd say the reason this is uncommon is not technical but design for how they are usually used. These systems are designed and installed in order to minimize total import when export credits are low or performing load shifting for time of use rates. These are where the financial incentives push for battery storage and as you say it operates in grid tie mode 99% of the time. Backup power is a secondary goal.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 21, 2023, 03:35:42 pm
content: If the 9.5 kW loads are disconnected when the power company's supply fails, how come the battery is being drained of 0.5 kW plus 9.5 kW in that situation?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 21, 2023, 05:48:49 pm
content: The issue the OP has is not with the grid failure, it's with grid present. Most or all solar storage systems give battery priority over grid for the entire load. I'm assuming the issue is that either the solar or storage system is not big enough to hit zero import for a typical day so sometime in the evening the batteries hit zero. Then if there is a power failure at night there isn't any battery for the backup load.Does the storage system have a reserve percentage setting? Like don't discharge below 20% when on grid? That seems like it would solve the problem I described but I'm not sure I understand the specific reason the OP doesn't like this behavior.

author: 5U4GB
author_rank: Super Contributor
timestamp: August 22, 2023, 10:20:02 am
content: Quote from: ejeffrey on August 21, 2023, 02:50:07 pmMore or less. Which requires an extra CT on the backed-up load to determine the draw from that.The reason for this is to deal with areas with not-guaranteed power. Say there's a 10kW battery available and the power goes out at 9pm. With a 0.5kW load there's plenty of charge still present to run through the night until the next day when at least some sunshine will refill things. With a 10kW load the battery will be drained well before the power goes out, leaving no power available and defeating the point of having a battery.Another issue, mentioned in my first post, is that the battery is being discharged at a high rate (10kW rather than 0.5kW) once the sun sets, and then recharged again at a high rate when the sun comes back the next day, which is not exactly conducive to long battery life.There are all sorts of ugly workarounds possible, e.g. rewiring the non-backed-up load to sit outside the consumption CT, setting a high state of charge limit like 90% so the ESS becomes an extremely expensive UPS, and other equally bad solutions, but I'm wondering if there's some technical reason for designing the system like this or just lack of thought about the problem?

author: BlobbyPineapple
author_rank: Newbie
timestamp: October 16, 2023, 12:48:12 am
content: I'm 2 months late to this party, but I can clear this up very easily.OP clearly is going off assumptions/bad data and does not own the type of ESS pictured.Of course it doesn't work this way, there's no fundamental flaw and installations all over the world working just fine to prove it.All you need to do is use the Enphase app to tell the system what you want to do. If you don't want that 10kW load taking battery energy when the grid is present, just configured the system that way in the app:You can set the system to various modes - "savings" or "self consumption" change battery discharge as a priority over the grid, to save on importing from the grid.You can also set it to "backup" mode where the battery won't supply any power unless the grid is down.More:https://support.enphase.com/s/article/What-are-the-various-Enphase-Encharge-Battery-smart-profiles

author: 5U4GB
author_rank: Super Contributor
timestamp: October 16, 2023, 09:23:28 am
content: Quote from: BlobbyPineapple on October 16, 2023, 12:48:12 amThat's actually a diagram of my exact setup but thanks for letting me know it isn't, I guess I'm really running a hydropower station without knowing about it.Also, while you can do what you're describing using the Enphase app, it's not what I was asking for.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: October 16, 2023, 01:34:42 pm
content: what if you connected your non-backup load via a time switch, that was set to disconnect this load during the hours of darkness? or a number of time switches, with a manual bypass for each in case you (on occasion) wanted to use a non-backup item during the hours of darkness. so, for example, your water heating would be turned off at night, but you could press a button to bypass this if you were going to use a whole load of hot water one evening and didn't want to risk having a cold shower in the morning.would this get close to what you want to achieve?cheers,rob :-)

author: 5U4GB
author_rank: Super Contributor
timestamp: October 16, 2023, 01:46:28 pm
content: That's what I'm slowly working towards at the moment, a modbus-controllable diverter (all the existing power monitoring equipment talks via modbus) but it's a lot of plumbing, both physical and software, to get it all set up, thus the slow progress. The fact that diverters are --ing expensive isn't helping, which is why I'm also looking at cobbling something together with Home Assistant automation driving relays, but I'm not sure how electrical-code-compliant the latter will be.The fact that there's an entire cottage industry of solar diverter vendors also indicates I'm not the first person to run into this issue, which is why I was surprised that it still is an issue at this point.

author: BlobbyPineapple
author_rank: Newbie
timestamp: October 16, 2023, 08:09:19 pm
content: QuoteOK, well colour me very surprised then, because if you have the system pictured, then this whole thread can be boiled down to: Use the app to set it to do what you want. Source - I have the same system and have installed ...rough count.... more than 40 of them.Either after all those words your desired behaviour is still not being clearly comunicated, or you missed a trick and just need to go in the app (with the right permissions) and set the mode you want.If it's the case that you do want to do something very unusual, consider that all the major vendor systems are set up similarly, so if in fact your needs whatever it may be are unmet by them, then by definition you have a very unusual set of constraints that are not commercially viable - or else they would exist as more than a "cottage industry". There's nothing wrong with having unusual wants, but calling in to question the design of all the major players in the market is a bit much. You just have a very unusual use case.

author: 5U4GB
author_rank: Super Contributor
timestamp: October 18, 2023, 09:28:53 pm
content: I think there's a miscommunication here as you can't use the app to do this because of the way the system is set up. @ejeffrey has summarised the issue quite well in his posts if my version didn't make sense.

author: ch_scr
author_rank: Frequent Contributor
timestamp: October 19, 2023, 11:18:07 am
content: Quote from: 5U4GB on August 22, 2023, 10:20:02 amReally, that isthesolution to the problem. In a perfect world the setup would be installed like that from the beginning, and then the whole problem would not have existed? The "non-backup" tap before the CT exists solely for the functionality you see as a problem, and the connection behind the CT providesjust the functionality you are looking for, no app settings needed at all.

author: 5U4GB
author_rank: Super Contributor
timestamp: November 05, 2023, 12:45:44 pm
content: Quote from: ch_scr on October 19, 2023, 11:18:07 amThere's some reason why that doesn't actually work, I asked both the solar installers and an electrician about this at the time and unfortunately it's sufficiently counterintuitive that I can't remember the exact details, but both of them responded to say that it won't have the intended effect. I'll check the details next time I talk to them, unless someone here jumps in first with the answer.