On-grid domestic battery storage

zeupater

telatech wrote: »

Thanks. I thought that charging from the grid was possible with a DC coupled battery. My PV setup uses a Solaredge Inverter and according to various online documents it is possible to charge the battery using a profile. Only issue is that their interface is limited to the LG battery only. I wish other cheaper batteries were compatible with their charging interface.

Hi

The issue is that the panels feed DC to the inverter which converts to mains AC before the energy is measured by the TGM .... charging the DC batteries from AC mains on the pre-inverter side of the TGM would show up as an anomalous issue when submitting FiT readings which would ultimately lead to
investigation, almost certainly leading to the invalidation the remainder of your FiT contract & the possibility of legal action for fraudulent activities!

HTH
Z

NigeWick

ASavvyBuyer wrote: »

Eligibility for what?

The electricity tariff you are on with Ecotricity, or the Solar PV FIT payments?

It's to do with FiT.

mmmmikey

Hi - I have just ordered a Powervault system and have the option of either AC coupling "downstream" of the generation meter or DC coupling "upstream" of the meter, which gives rise to similar questions re: FIT payments as per the last few posts.



In a nutshell, my understading is that if you have a DC coupling upstream of the generation meter and charge via the grid, when you use that electricty (i.e. discharge the batteries) it will flow through the generation meter making it look like you're generating more electricity than you really are, increasing your FIT payments which is clearly not allowed.



It's possible to get around this with a 2-way generation meter (i.e. it goes up when electricity flows one way and down when it flows the other) but all this needs to be agreed with whoever makes your FIT payments.


Hope this helps clarify things, Mike

Martyn1981

mmmmikey wrote: »

Hi - I have just ordered a Powervault system and have the option of either AC coupling "downstream" of the generation meter or DC coupling "upstream" of the meter, which gives rise to similar questions re: FIT payments as per the last few posts.



In a nutshell, my understading is that if you have a DC coupling upstream of the generation meter and charge via the grid, when you use that electricty (i.e. discharge the batteries) it will flow through the generation meter making it look like you're generating more electricity than you really are, increasing your FIT payments which is clearly not allowed.



It's possible to get around this with a 2-way generation meter (i.e. it goes up when electricity flows one way and down when it flows the other) but all this needs to be agreed with whoever makes your FIT payments.


Hope this helps clarify things, Mike

Have to admit, I've no idea how this would work or not. With DC side storage, the inverter (I think) simply directs the excess DC to the batts, it doesn't actually do anything with it, so to speak.

Then it acts 'normally' changing the DC to AC, but with the feed from the batts , instead of (or in addition too) the PV.

For a grid supply, such as E7, the leccy would have to go to the inverter as AC and be changed to DC, then fed to the batts, and I don't know if 'normal' inverters, or even DC coupled inverters can do that. Anyone know?

I'm sure there are inverters that can take feeds from both DC and AC, perhaps the islanding types?

Your post has certainly raised some interesting ponderings for me.

mmmmikey

Hi in answer to the question above....


The Powervault unit includes an inverter and charge controller and this can be connected in one of two ways.


One option is to connect it to the AC supply completely independently of the solar panels. The batteries are charged either when the unit detects "spare" solar power which is being exported to the grid, or from power from the grid. If you're charging from the grid you would set the timer to do this during the E7 cheap rate period. One way or the other, in this mode the unit is essentially just working as an AC battery charger - like a car battery charger, for instance. Connected like this, the Powervault has no effect on your FIT payments.



The other option is to connect the solar panels directly to inputs on the Powervault and allow the Powervault inverter to replace the existing inverter, which becomes redundant. In this mode, grid charging works as above but charging from the panels is DC to DC - i.e. the Powervault circuitry includes a charge controller which functions like the controller on a small solar system (such as you might install in a caravan). In this setup, because the battery is upstream of the generation meter, when you discharge the battery in runs through the meter and if you've charged the battery with grid power this will be measured by the meter as you use it, which, other things being equal would increase your FIT payments. To counter this, you have a two way meter which goes down when you charge the battery through the grid.


Both options have their own pros and cons when it comes to efficiency.


For the AC connection, inefficiency is introduced because you are converting DC solar power to AC power via your solar inverter then converting it back to DC via the charging circuitry in the Powervault. However, your FIT payments are completely unaffected.


For the DC connection, solar charging is more efficient. However, the power you put into the batteries from the grid is less than the power you get out due to charging and battery efficiency. This efficiency loss has the effect of reducing your FIT payments because for grid charging the generation meter runs backwards more during charging than forwards during use.


Gulp - sounds complicated but I hope that makes sense! Note that I researched this in relation to my Powervault purchase, I'm not sure if other battery systems have the same option for direct connection of the solar panels to the battery or not.


p.s. on a broader but related note, because I am on an E7 tarriff where the difference between the day and night rates is ca. 15p per kWh using the battery to store cheap rate electricity makes a lot of sense to me and changes the economics of buying the battery considerably.

zeupater

Hi

All understood, but if you want to connect to an existing PV setup & retain access to FiT contract payments you effectively have two options .... install on the mains side of the TGM if you want to be able to charge with cheap rate electricity -or- install on the DC side of the inverter if you're interested in improving the overall efficiency! ....

Note - whatever the relative difference in efficiency between AC & DC connectivity, if the battery is installed on the Inverter side of the TGM your FiT payments will reduce!

HTH
Z

mmmmikey

To throw something else related into the mix, Powervault are selling their solution with an optional gird services contract from EDF. The contract gives EDF the ability to charge and discharge the battery to smooth out demand on the grid (in return for an upfront payment) and similar considerations in relation to FIT receipts apply.


They are offering £1500 off the cost of the 4kWh Powervault. It's difficult to know whether this is a good deal or not because it depends how much EDF use the battery, but in my case it looks worth doing. There's an option to opt out and return a proportion of the £1500 so the risk isn't huge. My understanding is that they will use the Powervault for frequency response rather than bulk storage. That is, when usage surges the frequency drops (presumably as the generators in various power stations bog down) so the batteries connect to the grid until the frequency recovers - i.e the expectation is that they'll draw power for a few minutes here and there rather than every night wind the wind isn't blowing and the wind turbines aren't spinning. Similarly, they won't charge up the batteries in a planned way so they can draw the power back later, they'll just use the batteries to dump power if there's a sudden drop in demand and the frequency increases.


Has anyone looked at this and have any thoughts to share?

zeupater

Hi

No use letting everyone know what you can get as a discount unless all of the others variables are considered too!

To work a battery system alongside a PV setup may be fine, however there does seem to be a general lack of understanding of how the introduction of cheap rate off peak charging interacts poorly with PV which often results in there being little (or no) benefit from taking this approach for average energy use properties as well as how various battery offering specifications are able to absorb & release energy relative to supply & demand ... this even being the case amongst the industry/suppliers themselves as evidenced by many of the performance/savings claims they're making! ....

If you look back over the past few months on this thread you'll find analysis of various storage system solutions and how they would likely perform over a typically variable irradiation pattern year in a home with a typical energy demand pattern resulting in average annual consumption ...

HTH
Z

Exiled_Tyke

mmmmikey wrote: »

To throw something else related into the mix, Powervault are selling their solution with an optional gird services contract from EDF. The contract gives EDF the ability to charge and discharge the battery to smooth out demand on the grid (in return for an upfront payment) and similar considerations in relation to FIT receipts apply.


They are offering £1500 off the cost of the 4kWh Powervault. It's difficult to know whether this is a good deal or not because it depends how much EDF use the battery, but in my case it looks worth doing. There's an option to opt out and return a proportion of the £1500 so the risk isn't huge. My understanding is that they will use the Powervault for frequency response rather than bulk storage. That is, when usage surges the frequency drops (presumably as the generators in various power stations bog down) so the batteries connect to the grid until the frequency recovers - i.e the expectation is that they'll draw power for a few minutes here and there rather than every night wind the wind isn't blowing and the wind turbines aren't spinning. Similarly, they won't charge up the batteries in a planned way so they can draw the power back later, they'll just use the batteries to dump power if there's a sudden drop in demand and the frequency increases.


Has anyone looked at this and have any thoughts to share?

I had a look and concluded that the cost (taking into account the number of expected cycles of the battery) still substantially outweighed the benefits. I'm now sorry I didn't keep the calculations but they are easy to recreate.

Coastalwatch

mmmmikey wrote: »

They are offering £1500 off the cost of the 4kWh Powervault. It's difficult to know whether this is a good deal or not because it depends how much EDF use the battery, but in my case it looks worth doing.

Has anyone looked at this and have any thoughts to share?

I did some simple calcs on the EDF offering, in our case and without taking battery degradation into account the 4kWh offering would take 16.4 years and the 8kWh 17.1 years before breaking even. By then of course it's beyond the guarantee period and probably at the end of it's useful life, if it, and we get that far!