Is it worth using a battery to take advantage of cheap overnight electricity?

[andrewducker]

I was chatting about gas boilers being phased out (kinda, maybe), and wondering about the costs involved in switching away from burning fossil fuels to heat my house to using electricity instead.

Electrical radiators are 100% efficient (you literally turn all of the electricity into heat). So you don't have to worry about central heating systems - you can just put a heater wherever you want one.

The problem is that gas is under 3p/kWh and electricity is 13p/kWh.

Storage heaters are one way around this cost - they work by heating ceramic bricks (or similar) at night (when electricity is cheaper) and releasing the heat when you need it. An alternative would be a battery to transfer the electricity itself from nighttime to daytime. You'd need a *really* big one for heating though.

Checking here there's a graph halfway down how much energy homes in different countries use each year for heating. Britain comes out at just over 12,000 kWh. (Which is about £360 with gas, or £1,440 with electricity.) Assuming you use central heating for half the year, that's about 67kWh/day. Probably in some kind of normal curve, but let's ignore that for the moment.

There's an electrical tariff in the UK called "Economy 7" which gives you cheaper electricity overnight. Just under 9p/kWh. If you had that, then you could charge a battery up overnight, and heat the house up during the day. Again, for simplicity's sake, let's assume you sleep under a huge pile of blankets, and only need the heating on in the morning and evenings.

So you want to shift 66kWh each day. You can get a battery pack installed for a bit over £500/kWh. So you're going to need to spend £33k up front to get a few installed.

And you then save 4p/kWh. Or £480/year. So your payback time is...about 66 years. Which would be an even bigger problem when you take into account that most batteries are only warrantied for 6,000 cycles. Or under 20 years if we cycle once a day.

I'd say that really, really wasn't worth it. And we'd need battery costs to come down by a a factor of ten to make it worthwhile.

(Have I made a mistake in my maths? Or missed a point somewhere? If so, leave a comment!)

Comments

[naath]

You want a heatpump, because they use much less elecricity, but sadly have very high instalation costs when retrofitted. I am hoping for subsidies...

[errolwi]

This was done in NZ, linked with minimum insulation levels. The price charged by certified installers increased by about as much as the subsidy, but at least insulation improved!
Sadly when you install a heat pump in a maritime temperate climate (e.g. Auckland), people quickly realise that they can be used to reduce humidity during summer. Not only did power overall usage not decrease as much as expected, the usage pattern changed in ways that the electricity supply system finds more difficult to accommodate (lots of hydro, so seasonal changes matter).

[channelpenguin]

In the UK, I (pretty much always) had economy 7 with storage heaters and a very large hot water tank with immersion heater. That's the system I guess it's designed to attach to. Washing machine can also be programmed to run at night.

Insulation is the best way to get efficient.

Also, underfloor heating!

[andrewducker]

How does underfloor heating make things more efficient?

(The conversation started with people hating being stuck with storage heaters, because they mean having to predict how much heat you're going to want the next day. Which is never going to be efficient)

[toothycat]

Larger surface area for the heating means you can get away with lower temperatures, which is what makes the system more efficient.

[andrewducker]

Aaaaah, thank you!

[movingfinger]

I have underfloor heating in my lower level, installed over concrete slab (with the usual layers, engineered wood on top) and I find that it does have a slow cool-down and overall beneficial warming effect to the rest of the house (which is on forced-air heating/cooling with a new variable-speed heat pump). When the power goes out, which is sadly often, that area takes a long time to cool even with the terrible windows (which I cannot use heavy blinds, etc., on because of the young and energetic cat), because the slab holds heat.

The heat pump I chose functions to -10F (I'm in the US, sorry), it was designed for the region I live in and that's about as cold as we generally expect to get. Other models (especially older models) have a higher bottom of their range. I believe wall-mounted models have a restricted range, but there may be some designed especially for Northern Europe now, and I encourage anyone looking into a new system to get cooling if they can. I have the impression that ground-source is more common in Europe than the US.

My impression of battery systems is that they are best used in conjunction with whole-house solar or a generator. Line-delivered power is too expensive to make it cost effective to store it yourself.

[toothycat]

We have air conditioning. It can heat as well as cool. The coefficient of performance for our systems is just under 4; i.e., for 1kw energy in, a little under 4kw heat is pumped into/out of the house. So on paper the cost is not that much more than gas. However:

* there's only a point to the exercise if the electricity is renewable. This is currently more than 13p/kwh to guarantee - our tariff is more like 15p/kwh. Electricity from non-renewables produces around twice as much greenhouse gas as getting the same amount of energy by burning gas.

* domestic air conditioning currently available in the UK, at a price range which is affordable (at least for us), has an operating temperature range which does not go much below 0C for heating. So an alternative way of heating the house is required at precisely the times of the year heating is most needed.

* this does not solve hot water.

We are weirdos who are more tolerant of cold than of heat, so this solution is not for everyone, but as Naath points out, heatpumps do exist; these can be much more efficient if focused on heating only, and the heating is low temperature (therefore a large area is required - underfloor heating is usually suggested); you're well into saving-on-gas territory at the cost of massive installation pain (and sacrificing a chunk of garden if you go for ground source, which is more efficient).

[zz]

something something flywheels!

[danieldwilliam]

I think timeshifting electricity by not burning gas is always going to be cheaper than timeshifting electricity by burning gas and storing the electricity in a battery.

Batteries in a house *might* make sense when (if) widespread renewable penetration makes retail electricity prices highly variable across a day / week.

[andrewducker]

Good point if it was dramatically cheaper at night, then it might make more sense.

But in that case the electricity companies would do it themselves.

[danieldwilliam]

I think it will almost certainly always be cheaper for a utility or generator to do the storage. They are aggregating demand and have access to cheaper capital.

Retail batteries are probably only going to be economical if you have your own primary supply or a really strong desire for uninteruptable power.

[armiphlage]

The cost of the inverter to convert from AC to DC and back is non-trivial (bias note: my company makes both home and grid-scale inverters). The larger the inverter, the cheaper per kW of installed capacity.

It may not yet be economically feasible for individual consumers to store off-peak energy for peak usage, but my local utility just installed a 16 MWh lithium-ion battery just down the road from my factory.

https://www.newmarkettoday.ca/local-news/biggest-battery-energy-storage-facility-in-gta-now-live-in-newmarket-1712059

[andrewducker]

And, of course, wholesale energy prices vary a lot more than retail ones do. So there's much more scope for savings.

[frith]

Your calculations pretty much sum up my take on cost of installation and life expectancy of materials versus what I pay now for electricity. Namely, I'd never break even.

I wonder how big a mechanical power storage facility could be built with £33,000? I imagine a big shed where an electromechanical system lifts weights to store power, releasing power on demand by lowering the weights, something along the lines of power collection in an electric car braking system. It would be modular, easy to maintain and mostly made from recyclable materials.

[andrewducker]

So, a 1000kg mass, lifted 10m gives you... 0.027kWh. So for the 66kWh we want to store, we need to either lift 2469 tons 10m, or lift 1 ton 25km. We could comprise by lifting 50 tons 50m, I guess?

(The amount of energy you get in chemical form is really surprisingly high. An AA battery is about 3Wh can lift 1,000kg 1m.)

There are people looking into doing this kind of thing on a large scale, but I'm not entirely convinced. Here's an example.

Looking at their projected output they're saying they can do 5-8 MWh of storage.

Their lighter weight is 500 tons. Which is 1.5kWh per m (at 100% efficiency). They say a new shaft would be 150m deep. Which would store 225kwH (what they're saying they're working on for a prototype for next year).

With 5000 tons you'd get 2.25 MWh. Still not up to the levels they're estimating, but a step up. Presumably they'd need old mines to get fully ramped up.

Concrete is 2,400kg per cubic metre. So assuming you were dealing with a 10m thick block of concrete (don't want it being too thick, or you have to dig your hole deeper), you'd need it to be 14m on each side. That's a chunky bit of space, but not unmanageable. Make it 20m deep and you could get away with 10m on a side. And, of course, you could have multiple chunks suspended separately in your tunnel.

I am intrigued by what they're saying about also using it as a pressure vessel. I wonder how well that works with shafts, and how high you can raise the pressure before it finds its way out somewhere. Probably quite high. But with "interesting" failure modes.

[frith]

Ouch! I concede that the weights and distances needed to store just 66kWh are overwhelming. A water tower would have to be around the size of a spacious 5½ apartment, 10 m above the turbine. It really looks like batteries and safe hydrogen storage devices would be more practical if only they cost less and lasted longer.

[armiphlage]

Here's an interesting method that does not require a really deep hole:

https://qz.com/1355672/stacking-concrete-blocks-is-a-surprisingly-efficient-way-to-store-energy/

[andrewducker]

Yeah, that's an interesting one. But still requires a lot of space and is very expensive.

[armiphlage]

What if we did this in a shipping terminal? Lots of heavy boxes of standard size specifically designed to be lifted and put back down, all the full ones at approximately 20-22 tonnes, and you already have paid for the cranes and the storage space. You have a good idea of which containers need to be readily available (and on top of piles), and which ones are waiting a month for a ship to come in. When electricity is expensive, you reposition everything so the stacks of containers are as short as possible, given the available space in the yard and shipping schedules. When electricity is cheap, stack everything to the maximum allowable limit of eight containers.

It'd be a fun project for a summer intern to write the code, determining when the cost of stored energy outweighs the wear on the equipment.