On-grid domestic battery storage

zeupater · 18 February 2019 at 12:03AM

1961Nick wrote: »

6000 cycles @ 85% DOD is accurate.

The batteries are rated down to 90% DOD but the maximum inverter setting is 85%.

Losses don't appear to be anywhere near 15% but I don't have enough data yet to be sure of a figure. The 'vampire' standing loss is 10w.

Even on your miserly figures, the chance of breaking even is now a reality......unless the inverter packs in after 10 years!:sad:

Hi

I think you misunderstand ... the standard for battery cycle performance is degredation to 70% of the capacity available when new, so the manufacturer is saying that after 6000 cycles the remaining maximum capacity of a usable 8.2kWh system would be 5.7kWh, so the average degredation of capacity over the 6000cycles would be half of 30%(100%-70%), that's the 85% used ...

For information, the model also applied 85% maxium capacity DOD, protecting the top 10% & bottom 5% to extend useful battery life.

The losses used are direct from the manufacturer's specifications which give max efficiencies of - charging 94.5% & discharge 94%, giving a maximum figure of 88.8%, but note that is discribed as maximum whereas many other manufacturers specify an average of around 92.5% ... that's 85.5% round trip efficiency .... to this you can add the standby power consumption at ~22kWh/year (specified 5W) and natural self discharge (standing losses) that all rechargable batteries 'suffer' of up to 5% for the first 24hrs plus ~1.5%/month(Av) on a battery that averages (say) 50% charge and you have another 2.52% of daily loss, so around 83% round trip efficiency, so the 85% used could be considered 'generous' ...

Anyway, the 'miserly' figures are the result of crunching the battery system's specifications through a model involving millions of calculations which resolves to individual timeslices approximating to single kettle boil cycles matched to actual PV performance data in the same timeslices for a year ... the model is likely far more accurate than any published paper I've read concerning the subject considering the standard methodology is to apply solar data based on PVGIS averages! ....

Considering the increase in self consumption of you PV generation, have you considered running your calculations on zero standing charge tariffs and completely dumping the idea of using off peak energy (E7 etc) as it may actually work out cheaper?? ... you can compare using the energy purchase information detailed in the previous post by simply applying the appropriate tariffs ...

HTH
Z

Martyn1981 · 18 February 2019 at 8:26AM

1961Nick wrote: »

But you still need to factor energy price inflation into the calculation if you believe energy prices will rise?

Hiya Nick, really interesting what you are doing, and hopefully it will save money, but regardless we need early adopters.

Regarding the bit I quoted, am I right in assuming you mean energy price inflation above just normal inflation? That's a very fair point, and one I used to keep in my back pocket as a small bonus. However, looking at longer term predictions now by the NAO, and the shockingly fast decline in RE generation costs, it looks like the expected energy price rises (baseline - before inflation) are reducing, and will be less significant. So much progress this decade on RE costs, it's almost impossible to believe.

But, here's a bonus income, have you read about the schemes you can join where 'a company' pays you for the right to export some of the battery leccy, or for the leccy export, or both, as they then trade the combined total (from 1,000's of systems) on the market at peak price times?

Most schemes today in the UK, seem to be linked to who supplies the batt, but in Aus, anyone can join such schemes.

Potentially such schemes could be a good source of additional income, but will depend on peak prices, and the number of people who join.

1961Nick · 18 February 2019 at 9:38AM

zeupater wrote: »

Hi

I think you misunderstand ... the standard for battery cycle performance is degredation to 70% of the capacity available when new, so the manufacturer is saying that after 6000 cycles the remaining maximum capacity of a usable 8.2kWh system would be 5.7kWh, so the average degredation of capacity over the 6000cycles would be half of 30%(100%-70%), that's the 85% used ...

For information, the model also applied 85% maxium capacity DOD, protecting the top 10% & bottom 5% to extend useful battery life.

The losses used are direct from the manufacturer's specifications which give max efficiencies of - charging 94.5% & discharge 94%, giving a maximum figure of 88.8%, but note that is discribed as maximum whereas many other manufacturers specify an average of around 92.5% ... that's 85.5% round trip efficiency .... to this you can add the standby power consumption at ~22kWh/year (specified 5W) and natural self discharge (standing losses) that all rechargable batteries 'suffer' of up to 5% for the first 24hrs plus ~1.5%/month(Av) on a battery that averages (say) 50% charge and you have another 2.52% of daily loss, so around 83% round trip efficiency, so the 85% used could be considered 'generous' ...

Anyway, the 'miserly' figures are the result of crunching the battery system's specifications through a model involving millions of calculations which resolves to individual timeslices approximating to single kettle boil cycles matched to actual PV performance data in the same timeslices for a year ... the model is likely far more accurate than any published paper I've read concerning the subject considering the standard methodology is to apply solar data based on PVGIS averages! ....

Considering the increase in self consumption of you PV generation, have you considered running your calculations on zero standing charge tariffs and completely dumping the idea of using off peak energy (E7 etc) as it may actually work out cheaper?? ... you can compare using the energy purchase information detailed in the previous post by simply applying the appropriate tariffs ...

HTH
Z

Interesting....

The numbers I'm getting are around 5% on the charge & discharge which aren't dissimilar to the specification you are quoting. Clearly new batteries are likely to be at the top end.

I'm not particularly bothered about degradation because I'll add more batteries when the price drops - that should happen just before the US2000 Plus is superseded. The redundant capacity will also be insurance against a battery failing in the future. Additionally, reducing the DOD to 20% or even 25% will prolong the life of the batteries & leave some charge for the EPS.

I've looked at E7 & traffic light tariffs but the 'capital' cost of each kWh charged/discharged is around 8.5p. Add that to the 8p cost of imported cheap rate energy & 90% round trip efficiency, and it's not really viable.

It's a learning curve at the moment & I'm going to need 12 months of data before I have a full picture of the economics. The good thing is that the worst case scenario only leaves me a few pounds out of pocket & there may even be an upside.

Coastalwatch · 18 February 2019 at 10:22AM

Martyn1981 wrote: »

But, here's a bonus income, have you read about the schemes you can join where 'a company' pays you for the right to export some of the battery leccy, or for the leccy export, or both, as they then trade the combined total (from 1,000's of systems) on the market at peak price times?

Certainly an interesting development as most of us have excess energy in the summer we can't use and probably over and above the 50% export figure for which we receive payment. This effectively being donated to Utilities who then receive circa 15p/kWh from those living nearby. With the wholesale price of leccy being around 5p/kWh then it's something I should welcome but to then have to pay three times that figure for effectivley retreiving it seems a little steep.

Martyn1981 wrote: »

Most schemes today in the UK, seem to be linked to who supplies the batt, but in Aus, anyone can join such schemes.

I understand from a friend, so only heresay, that similar schemes exist in the USA whereby a percentage handling charge is applied for the electricity previously exported and subsequently purchased back.
If only!

NigeWick · 18 February 2019 at 10:57AM

Martyn1981 wrote: »

we need early adopters.

Yep, I'm using up my inheritance. 4kW solar system, Tesla PW2 plus another 600W of solar because of the way VAT is set. And now 64kWh Hyundai Kona Premium SE.

mmmmikey · 19 February 2019 at 9:47PM

Martyn1981 wrote: »

But, here's a bonus income, have you read about the schemes you can join where 'a company' pays you for the right to export some of the battery leccy, or for the leccy export, or both, as they then trade the combined total (from 1,000's of systems) on the market at peak price times?

I've signed up for a Grid Services contract with EDF scheme through Powervault.

The way this particular scheme works is fairly complex. They're not making money by buying electricity when it is cheap, paying you to store it for them and then selling it when electrcity is expensive.

What they're doing is paying you so they can use your battery to smooth out very short term unplanned changes in supply or demand, making money via the Frequency Response market.

Software is built in to the Powervault battery system to support this.

There's some good videos about frequency response on the National Grid website - or Google "Firm Frequency Response".

Scribbly · 21 February 2019 at 9:59PM

I've been following this thread for a while and now my head hurts trying to keep up with all the maths. I have a newbie question I'm afraid, and I promise that I have read (some) of this thread to see whether it has been answered, but it's quite a long thread by now so I apologise if it has been asked already.

I'm about to inherit enough money to put a battery into our PV system and I've phoned a few companies for quotes, but none of them can answer the question I have about "where in the system does the battery sit?". Maybe I'm not explaining it properly.
So here goes:

We have a 2.4 PV system on the roof. Our house is empty during the day, with no heating and just the fridge/freezer and fishtank to keep running. So when it is sunny we generate electricity, some of which is used for these essentials and some is exported to the grid. If it's not sunny we get all the elec from the grid. I understand that bit.

If we put in a battery, I assume it comes in the process BEFORE we export any electricity to the grid, so that the battery gets topped up first? And then any excess elec that is generated after that goes to the grid?

Later in the day we all come home from school/work. Let's pretend it's still sunny so the PVs are still generating but when we turn the Xbox on, and a radio, and boil the kettle, we need to draw elec from somewhere as the PVs won't generate enough.

So here's my question: Will the elec that we need when we get home be taken automatically from the battery, or will it come from the grid until we reach a pre-determined level of usage and then it comes from the battery? Can we decide where it comes from?

If the battery is empty (because we've had lots of cups of tea when we got home) but we are generating elec through the PVs, how does the system decide whether to top up the battery with the PV elec (which means we are drawing elec from the grid to keep the Xbox working..), or whether to use the PV elec to operate the Xbox and leave the battery empty?

The example the battery companies keep giving me is that "I would be able to charge my electric car overnight using the elec stored in the solar battery", but I think the battery would be empty by 10pm because it will have powered the oven, kettle and (you guessed it) the Xbox all evening.

Finally, has anyone had any experience of using the IKEA batteries? They are cheap enough for us, and I like the idea of them being modular, and quite small.
Sorry for the numpty questions,
Thanks all

Martyn1981 · 22 February 2019 at 8:56AM

Hiya Scribbly. Set ups and choices will vary, and you will have some choice, but for most expect to install a battery connected to your consumer unit, so after the PV system. This is called an AC side battery as it takes charge from the AC, whereas a battery linked to the inverter will be a DC battery, and charged from the PV (via the inverter).

Your house will then have monitors on the cables looking for export. When that happens the battery will ask for charge and try to take as much of that leccy as possible, depending on the rate of charge it can handle, and how full it is.

When you then ask for 'too much' leccy later on, it will first be provided by the PV system, then when import begins from the grid, the battery will attempt to provide the shortfall, again depending on the rate it can discharge, and how much leccy it has.

The grid will continue to provide the shortfall if you are asking for more power (kW's) than the PV and batt can provide. Eg, you get home, switch on the 3kW kettle and it gets 1kW from the PV, and 2kW's from the battery, but then PV gen falls to 0.5kW and the import of 0.5kW kicks in.

Or, the battery has a discharge max of 1kW, so you get 1kW each from the PV, batt and import, then 1.5kW from import as the PV gen falls.

Well that was short and simple!

Scribbly · 23 February 2019 at 4:47PM

Thank you Martyn, that was a simple explanation! I think I’ll go for it in the next couple of weeks as we have this “spare” money coming our way (which is always nice). I’ll also look at a 1kw kettle which I read about earlier in this thread. I’ll keep you posted.....

Martyn1981 · 26 February 2019 at 9:15AM

This article is a little 'adverty', but my takeaway is that more competition is entering the domestic market:-

Siemens Enters Residential Storage Market With Junelight Smart Battery

The battery consists of up to six battery units with a net capacity of 3.3 kilowatt-hours (kWh) each and can be retrofitted and flexibly adapted to changes in consumption behavior, providing up to 19.8 kWh of storage at any time. The battery can provide output for use in the home as well as heat pumps and electric vehicle charging and is already equipped with all relevant connections for future functional expansions such as new app features and functions involving intelligent consumption control.

On-grid domestic battery storage

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