UKC

I know I'm not the only one here with an interest in this sort of subject - I'm more or less thinking aloud, and would be grateful to know what others think!

Scenario: we have 5.7kW solar PV on the roof.  Despite the car being a PHEV, we still export 75% of the energy.  We have an export meter so we get paid ~5p/kWh export; I pay ~15p/kWh to buy grid electricity.  Across a whole year I export 4400kWh, whilst importing 3000kWh.  I'm on the verge of switching the gas heating to a small ASHP and wet underfloor heating.

I'm considering buying a battery.  This would be one of the newest types, using re-purposed EV batteries; I'm loking at an 8kWh model from Powervault.  From an economic point of view it will pretty much break even over its lifetime.  Assuming it runs smoothly for the predicted ten years, the saving from the difference in price for each kWh it saves me from importing will add up to its installation price, more or less.

What I'm mulling over is whether it's a good idea in terms of overall reduction of CO2.  As it's made from recycled car batteries, I'm choosing to neglect the embedded CO2 in producing it - whilst this still won't be zero, it'll be small enough to ignore over a ten year period.

If I look just at my household CO2 it's a clear winner; it saves CO2-free energy from the PV and displaces grid electricity.  Although I pay the premium for 100% green electricity, I know full well that reducing my demand reduces overall grid emissions, probably to the tune of the UK average of around 270g/kWh.

But: using my PV to charge my battery is simply depriving the grid of that contribution.  What I am really achieving is to not supply the grid with energy when it's sunny, in return for not drawing from the grid when it's dark.  I am still using the same amount of power (actually a little more, because the cycle efficiency of this battery is in the region of 90%, so a full charge-discharge actually uses about 10% more energy than otherwise).

What I can't answer, then, is whether this is worth it or not.  If we had a magic 100% renewable grid, then clearly not.  If we ever got to the point where the daytime demand was less than the renewable supply, and we had excess renewable energy to use, then clearly it would.

I fear the answer is probably that it's not a net saving of CO2 - all I am doing is removing power from a time of day when demand is high and the nastiest types of generation such as coal are more likely to be switched on, and saving demand in the night when it's likely that I'm just displacing CCGT.  Or, at best, I'm just displacing daytime CCGT for nighttime CCGT, which doesn't really make any difference at all...

http://gridwatch.co.uk/

On average grid demand remains high well into the evening in the UK where the relative reliance on gas turbine derived power increases proportionally. As such, being able to buffer into the evening is probably a net CO2 benefit, but this can be achieved in other ways, if you can push you usage into times of low grid demand. I'm not sure what the answer is, but given that storage is going to have to be required to deleverage from combined cycle gas turbine demand, its difficult to see that that wouldn't help at the individual household level as well as at the larger infrastructural level.

http://www.dynamicdemand.co.uk/grid.htm

No.

Currently CCGT > wind > solar in terms of supply, and CCGT is the responsive element in matching supply/demand as I understand it - apart from exceptionally windy days.   So, solar displaces the responsive fossil supply either directly or via battery load shifting.  Using batteries just throws energy away in the inefficiencies, even ignoring CO2 cost embodied in the batteries.  Exporting your domestic solar doesn’t displace other domestic solar exports, it displaces CCGT, so the carbon intensity of the grid is irrelevant.

Only when day time renewable supply exceeds daytime demand with all responsive fossils turn off, but night time doesn’t, can load shifting (via batteries or scheduling usage) displace fossil outputs.  For that to happen we need a lot more solar PV.

I have AGM batteries for redundancy and I’m procrastinating on doing load shifting with them - it’ll knacker them sooner, save a bit of the bills and increase my carbon intensity.

ASHP - if you want to see our set up just message me on FB and come and stay. We've got that and Solar Thermal and it reduces our net export and pays us £1500/year for 7 years. ASHP cost £15k but we started from scratch (no pipes no radiators, so £5k was that).

This is more aimed at reducing you household CO2 than just tackling you net export.

I'd suggest a different route.

We have a 4kWp PV and a gizmo that recognises when we are exporting and diverts the energy into two prioritised outlets.  The main one for us is our hot water thermal store, via the immersion rod.

Generally we only run the boiler at the end of the day, so the store gets a good chance to mop up the spare energy and is then topped up (if it needs it) for showers in the morning.

The gizmo cost around £300 IIRC and paid for itself very quickly in reduced gas bills.

We use the secondary outlet to run a small electric radiator in our (otherswise rather damp) cellar.

Edit: and if you do go ASHP, then I think your battery would be redundant anyway.

Thanks all.  Really helpful to think it through like this.  It is now clear to me that a battery won't save any CO2 with the current UK generation mix.  All it will ever do is exchange an increase in daytime CCGT for a decrease in nighttime CCGT (or, worse, replace that nighttime CCGT with daytime coal).  And lose ~10% in inefficiencies along the way.

It might, one day, be a net CO2 saving - but that's going to need an enormous amount more solar, enough that we need to displace solar into the night, and probably won't happen within the lifetime of a battery.  In fact, it strikes me that the best thing we could do with all the tired batteries from older EVs right now is to charge them at night on CCGT and use them to displace daytime coal - almost the exact opposite of what they're being used for..! 

Phil - thanks - I will definitely come look at your system next time we're in your corner of the world.  I am already pretty set on the ASHP - it's not worth us installing ST alongside it (mostly because we don't use that much DHW, and also because I've already covered my whole roof with PV) but if I can get a sensibly priced ASHP that can also do DHW then all the better. 

Deadeye - I'm afraid the solar diverters don't save any CO2 for someone on mains gas.  They divert your power into the hot water, so you reduce your gas usage, but somewhere a CCGT station has to produce the same amount of energy that you have just deprived the grid of, so they are more or less carbon neutral.  If all the CCGTs are spinning at full power then it'll be coal that takes the slack up, so in that instance they're carbon positive.  Of course, if you're not on mains gas and using something more carbon intensive to heat your water then they're a win.

I totally get why people install them - they do save you money if you are on 'deemed' export, because you still get the payment for the assumed 50% export regardless of the fact that you're actually using more of the energy.  But I'm on metered export, so consuming another unit of PV rather than exporting it means I get paid 5p less - and that displaces a unit of gas, also about 5p.