On-grid domestic battery storage

mmmmikey

Back to batteries...

In order to give myself more flexibility ( for example, being able to have hot food and do the ironing on the same day ) I've just bought a second battery system to run alongside the PowerVault.

"mmmikey B" is an altogether simpler affair based on a low frequency inverter charger and bank of AGM lead acid batteries. The batteries will be charged overnight on E7 night rate electricty at a cost of 4.7p per kWh. Now I am on metered export following my smart meter install this costs about the same as using spare solar. If I used solar for charging I will lose 5.38p in export payments for every kWh I use - allowing for round trip efficiency losses they will probably end up about the same.

This new system will provide power for lighting and a low power ring main which the fridge, computer, TV, broadband router, etc. run off. I've only had to have a minimal amount of electrical work done in the house to achieve this. The inverter is off-grid, i.e. not grid tied - the circuits are completely separate from the other grid powered circuits.

In effect what I have is a simple UPS. The charging input for the inverter/charge is connected to a timeswitch. During the E7 night rate period the power is on and passes straight through the inverter/charger to the lights etc. When the power switches off the inverter/charger delivers battery power to the lights etc. An advantage of this is that the lights will stay on during a power cut, but the implications of this configuration is that if the battery goes flat I'll have no lights. I can manually revert to grid power lights etc. if needed though.

My thinking is that I only need a very simple system for my predictable known loads, and because the inverter is on or off there are no switching lags, etc. The PowerVault is left with higher power appliances like the kettle, toaster, microwave, cooker, etc. which it is much better at dealing with. In the event I want to push the boat out and cook the veg at the same time as roasting the chicken instead of having the veg as a starter, and I end up exceeding the PowerVault inverter capacity as a result, the more sophisticted PowerVault system can handle this and use a combination of grid and battery power. The new system doesn't need to do this - because the load is predictable and low I've just sized it accordingly, with a bit of contingency.

The choice of batteries was driven by the choice of inverter/charger - I specifically wanted a simple, robust device and my choice limited me to lead acid batteries. The economics of this look OK - my experience of using lead acid batteries in caravans is that as long as you never abuse them (even once) you can make them last a long time. Time will tell on this. I've bought 4 120Ah 12v AGM batteries which are wired in series with a balancer to keep them all happy. This give me a total nominal capacity of 5.8kWh - according to the specs 90% of this should be usable (AGM being much better than standard lead acid batteries in this respect) but I'm only expecting to use 50% in order to maximise their life and I suspect this is more realistic anyway.

It's going to be difficult to assess the economic benefits of this because they are laregly driven by heating in the shoulder months and it's difficult to factor for changing weather. I'm not overly concerned though, because the decision to buy was as much based on long term convenience. It's one thing monitoring and micro-manging energy usage when I'm in the house on my own and in the initial flush of enthusiasm - it's another thing controlling use when friends are round and I expect that the novelty of planning my day around energy monitors will wear off at some stage. Also, even if the immediate cash benefits are small, taking some of the load off the PowerVault should make it last longer so there are benefits there.

That's the theory - will let you know how I get on.

p.s. PowerVault calibration/optimisation is work in progress and I'm expecting to be able to squeeze a bit more out of it each day once this is done. Following discussion with PowerVault (they are as good as ever) I'm collecting data and then they will start tweaking settings.

Hope this is of some interest.....

orrery

joefizz wrote: »

'Which leads to a further thought - how on earth is the AC 'wave' regulated across the whole of the country?'
Ahh, thats the big question going forward, with so much of renewables being generated from DC then its becoming more and more of a problem.

Eh? Surely that should be "less of a problem"?


The voltage and the frequency were inextricably linked when all generation was by rotating machine and large industrial kit required frequency to be controlled (even clocks used to count the 50Hz to maintain time and it worked well so the CEGB had to correct for frequency fluctuations by forcing the same error in the other direction so it all averaged out).


Inverters don't care two hoots about the frequency, they'll just synchronise with the grid and then vary the voltage to supply the required power.

joefizz

orrery wrote: »

Inverters don't care two hoots about the frequency, they'll just synchronise with the grid and then vary the voltage to supply the required power.

I guess you missed the news last couple of months then. Most of the grid is tied to frequency changes, thats the measure of a lot of the trip outs and diverters. Traditionally the frequency going wrong was serious trouble/load differences so it was all built around that.
As you mention the frequency itself isnt the issue, Japan runs on two frequencies interconnected, its how the grid handles change in frequency thats the issue.


No point having inverters track grid frequency if that makes the 'problem' worse and then shuts off the inverters making the problem worse. My solaredge inverter has a narrow frequency cut off built in for specific uk use, it works in a similar way to total mains failure, if the frequency goes outside of this the inverter shuts down, regardless of whether it can supply power or not. Saying that though there is enough equipment upstream that will shut down before it gets to my inverter, and then you black out hundreds of thousands....


In isolation its not a problem, with the way the UK grid is designed (or not) then it is. Frequency response is a growth area at the minute for precisely this reason.

Reed_Richards

mmmmikey wrote: »

.... Now I am on metered export following my smart meter install this costs about the same as using spare solar.

That was quite a long post and I must admit I found it difficult to absorb all the details in one read. But the throwaway about metered export caught my attention. I have a feeling that I have read somewhere else on this forum that you don't necessarily get put on metered export with a smart meter. At the moment I think my actual export really is about 50% of what I generate but I am looking for more ways to use the "free" electricity which might put me at a disadvantage if I had to move to metered export. So how does this come about?

mmmmikey

Reed_Richards wrote: »

That was quite a long post and I must admit I found it difficult to absorb all the details in one read. But the throwaway about metered export caught my attention. I have a feeling that I have read somewhere else on this forum that you don't necessarily get put on metered export with a smart meter. At the moment I think my actual export really is about 50% of what I generate but I am looking for more ways to use the "free" electricity which might put me at a disadvantage if I had to move to metered export. So how does this come about?

Hi Reed, my understanding is as follows:


1. You're under no obligation to have a smart meter fitted although it could quite possibly be something you would benefit from as an increasing number of tariffs come with the condition that you have a smart meter installed, and if you choose not to have a smart meter you will restrict your range of tariff options.


2. My FIT agreement makes it clear that I have no right to receive deemed payments, and if I have a meter capable of measuring export then the FIT licensee is allowed to insist that I provide export readings and move to payments based on actual export. They are also allowed to install an export meter at their own cost if they choose to do so. My assumption is that FIT agreements are standard and everyone will have agreed to the same conditions.


3. EDF's position (who are my licensee) is explained here:


https://www.edfenergy.com/content/smart-meters-and-feed-tariff-changes


I'm not sure if this is the same position held by other licensees.


4. The EDF Green Team (who look after the FIT scheme for EDF) are currently progressively switching EDF FIT customers with EDF SMETS2 smart meters to metered exports.


5. In my case metered exports are advantageous and they switched me whilst I was on the 'phone.


Hope this makes sense.

Reed_Richards

But it was established here that E.ON still use deemed export even if you have a smart meter https://forums.moneysavingexpert.com/discussion/6041279/e-on-smart-meter-query

orrery

joefizz wrote: »

No point having inverters track grid frequency if that makes the 'problem' worse and then shuts off the inverters making the problem worse.

You have no choice. I'm sure that the inverters would like to be set to 50Hz and just left, but the grid will flex as it comes on and off load. That is why the inverters must track both the frequency and phase of grid. The frequency is tightly controlled, see here:


https://gridwatch.co.uk/frequency


Clearly, if the grid is way outside of these limits then the inverter must shut down as it doesn't understand what is going on.


The more inverter power we get on the grid, the more stable it will be, as additional load will only reduce voltage for an inverter. The more rotating machines taking more of the load, then the greater will be the phase and frequency change.


When it comes to re-stabilising the grid, the inverters come up trumps. The last outage took only a few minutes to re-stabilise the grid (about 4 minutes) due to the increasing level of battery (inverter) support - the previous similar outage took over 20 minutes. In this recent case there was good level of battery support located fairly close (the Flitwick solar farm) to the 1st station that went into fault (Little Barford near St Neots).


I see no evidence that increasing levels of inverter-based generation pose a problem, rather than a benefit. Don't forget that Australia installed a massive Tesla battery specifically for that purpose.

joefizz

orrery wrote: »

That is why the inverters must track both the frequency and phase of grid.




I see no evidence that increasing levels of inverter-based generation pose a problem, rather than a benefit. Don't forget that Australia installed a massive Tesla battery specifically for that purpose.

Youve answered your own question as it were.
Inverters as set up currently in the UK (well up to the last outage) track the frequency of the grid, the UK grid hasnt been set up to use inverter based output as the frequency of the grid. We see it a lot in reports about the frequency of the grid as if its a thing or a constant but its not and it has to originate somewhere. As you mentioned it was originally from large rotating machines and the uk grid (and most others) are based around that, nobody really thought that the large rotating machines would one day be largely redundant....



Im not one for conspiracy theories and I also dont really believe in coincidences but when national grid tweeted a few hours before the last outage about 67% of uk power being wind and that being a record its hard not to have that as a line of investigation, even if its not mentioned in public (but is mentioned in various other reports and papers)



Indeed australia have installed the Tesla battery to help, but look at the reasons why it was installed in the first place.


Im not arguing with you about batteries being one of the answers its just that its only been in the last couple of years its been thought about as a result of becoming an issue. Im not up to date with national grid but I think it was only in the last 2-3 years they have invited tenders for supply of similar and if you remember the buzz around elon musk promising to sort the battery out in australia in time or for free then you know that this is a current issue. Again Im not current on ROI but last I heard they were capping renewables at 50% until the infrastructure can catch up.
Certainly here in NI when I was at the electricity generators conference last year there was a lot about localised storage and grid balancing and how it was needed 2 years ago, not just now.
Theres a lot of money to be made in this now, hence my previous posts about the STOR facilities popping up all around the place.



In the report about the latest outage they stated that renewables were not to blame but a report a couple of months earlier said that increasing renewables use would possibly cause major outages exactly like that one due to the way the network was set up. Now again, not one for coincidences, but also not demonising renewables, this was a issue that has been forseen and reported and indeed as you mention easily sorted. I said easily as in available, its not cheap.... and therein lies the problem. Maybe with the upcoming QE for infrastructure projects all this will be sorted.

joefizz

Reed_Richards wrote: »

But it was established here that E.ON still use deemed export even if you have a smart meter https://forums.moneysavingexpert.com/discussion/6041279/e-on-smart-meter-query

Just to add in that over in NI we moved from deemed export to actual export in summer 2018. We were told a couple of months before they were thinking about it and then got the request for export reading.



Being small,compact, self contained, one export payment place we get used to trial a lot of stuff thats going to get rolled out elsewhere.
People who bought iboosts werent happy ;-)

Martyn1981

orrery wrote: »

You have no choice. I'm sure that the inverters would like to be set to 50Hz and just left, but the grid will flex as it comes on and off load. That is why the inverters must track both the frequency and phase of grid. The frequency is tightly controlled, see here:

I hope this makes sense, and is correct, as I'm simply regurgitating old news (perhaps 2013ish), but don't pretend to understand it.

So, we all know that inverters now monitor grid voltage, and because the push it up a bit, they should shutdown when it hits 253V. But I recall that in Germany they also have to monitor grid frequency, and if it goes too high, they also have to shutdown to prevent oversupply.

I think I'm right in believing that frequency will go up with over supply and under demand?