Off-peak energy storage for cheaper heating

[Originaltwist]

Off-peak energy storage for cheaper heating

Off-peak energy plus heat pump equals unbelievably cheap heat. But how to store it?

High temperature heat pumps might make all the difference (wider Dt) so here’s a shot of my model which shows that a 1,000 litre buffer tank along with, say 300l, DHW and a bit of slab heating might work.

60kW.hrs a day for £300 a year – worth a look!

Conclusions are a bit disturbing and contra intuitive as inefficiencies are overcome by economy.

Buffer tanks should be huge

Blending down is necessary

Zoning is easy and a good thing

HP running all day is not a good idea

A much bigger heat pump is needed

A mini-split addition is even more desirable

 

I’ve sketched out how such a system might look on

Off-peak energy storage for cheaper heating

https://wordpress.com/post/originaltwist.com/7618

So, team, is this worth a look or a step too far?  N.B. I'm aware this contradicts all the wise words seen so far on this topic and I apologise for that but I genuinly can't see what will no doubt emerge as the big flaw.

[sharpener]

Yes, thermal storage is defo much cheaper than batteries and I agree in principle, but as you demonstrate @Originaltwist you have to do it on a heroic scale for it to be truly worthwhile.

 

We only have room for a 260 litre thermal store, the economics are analysed here but in brief you have to assume 20% inflation in electricity tariffs to get a 10 yr payback. I am planning on doing it as much as an interesting experiment as for its economic value.

 

The other issue is that MCS installers are not used to thinking like this and in a sellers' market mostly want to throw in standard systems at maximum profit rip-off.

[SteamyTea]

You may be able to get away with less storage, and a smaller ASHP by playing more with the tariff prices.

So rather than try and generate and store a full days worth of energy, look at the CoP and price differentials and see if you can get similar savings by say heating 500lt to 55⁰C during the cheap period, say 5 hours, the run for another 5 hours at a higher CoP, but lower temperature direct into your heating system, while also drawing from your higher temperature thermal store.

 

The main thing is that most of the time you may only need a few kWh as the weather is warmer and the house will naturally heat up during daylight hours.

It would be a shame to always have a large store topped up and then only use a few percent of the capacity.

[Alan Ambrose]

I guess the tactic of intelligently heating the slab with UFH, used by a few people here on BH, is the simplest version of this?

[SteamyTea]

14 minutes ago, Alan Ambrose said:

I guess the tactic of intelligently heating the slab with UFH, used by a few people here on BH, is the simplest version of this?

The problem with that is the high losses that a slab has, it is how they are designed to work after all.

So to get around that some weather prediction is needed ideally.

 

[Alan Ambrose]

Even with a highly insulated slab?

[joe90]

37 minutes ago, SteamyTea said:

The problem with that is the high losses that a slab has,

But most slabs have oodles of insulation  (more than your storage heaters 🤷‍♂️) yes all insulation only slows heat loss but I believe many here have proved it works. 

[MikeSharp01]

34 minutes ago, Alan Ambrose said:

Even with a highly insulated slab?

If it were insulated on the top as well then it would be a Thermal store of sorts then maybe it would be useful but as they are, By design, open on top to heat the house they are all but useless. Although you do have a load of mass at 20deg to play with if that helps.

[sharpener]

4 hours ago, SteamyTea said:

You may be able to get away with less storage, and a smaller ASHP by playing more with the tariff prices.

 

Octopus Cosy is well suited to this, you get 3 hrs overnight and another 3 hrs 1300-1600 which is ideal for a recharge before the evening peak.

 

 

It will need some more modelling before I know if this is a better bet than Intelligent Octopus Go.

 

[sharpener]

4 hours ago, MikeSharp01 said:

If it were insulated on the top as well then it would be a Thermal store of sorts then maybe it would be useful but as they are, By design, open on top to heat the house they are all but useless. Although you do have a load of mass at 20deg to play with if that helps.

 

Heat lost from the top of the slab is not wasted as it is within the thermal envelope, and to take @Tim S's recent example 37 tons of concrete beats any plausible amount of water even though it has only 1/4 the specific heat. Several ppl on here report good results, as mentioned below:

 

5 hours ago, Alan Ambrose said:

I guess the tactic of intelligently heating the slab with UFH, used by a few people here on BH, is the simplest version of this?

 

Edited April 14 by sharpener

[JohnMo]

21 hours ago, Originaltwist said:

Off-peak energy storage for cheaper heating

Off-peak energy plus heat pump equals unbelievably cheap heat. But how to store it?

High temperature heat pumps might make all the difference (wider Dt) so here’s a shot of my model which shows that a 1,000 litre buffer tank along with, say 300l, DHW and a bit of slab heating might work.

60kW.hrs a day for £300 a year – worth a look!

Conclusions are a bit disturbing and contra intuitive as inefficiencies are overcome by economy.

Buffer tanks should be huge

Blending down is necessary

Zoning is easy and a good thing

HP running all day is not a good idea

A much bigger heat pump is needed

A mini-split addition is even more desirable

 

I’ve sketched out how such a system might look on

Off-peak energy storage for cheaper heating

https://wordpress.com/post/originaltwist.com/7618

So, team, is this worth a look or a step too far?  N.B. I'm aware this contradicts all the wise words seen so far on this topic and I apologise for that but I genuinly can't see what will no doubt emerge as the big flaw.

You do need to start with this approach as the heat pump is really needed to be 3x oversized for the house (so maybe a typical MCS install). And have a smart meter that actually communicates (unlike mine). I run E7 with 6kW heat pump and 60T of floor. My heat demand is 3kW and on the coldest days I need to pump heat for 12hrs. So in my case the cost for the same amount of energy (electric 14p kWh) is £560 instead of £300.

[Originaltwist]

On 14/04/2024 at 12:41, sharpener said:

Yes, thermal storage is defo much cheaper than batteries and I agree in principle, but as you demonstrate @Originaltwist you have to do it on a heroic scale for it to be truly worthwhile.

 

We only have room for a 260 litre thermal store, the economics are analysed here but in brief you have to assume 20% inflation in electricity tariffs to get a 10 yr payback. I am planning on doing it as much as an interesting experiment as for its economic value.

 

The other issue is that MCS installers are not used to thinking like this and in a sellers' market mostly want to throw in standard systems at maximum profit rip-off.

@sharpener - I have just gone through your economics analysis (better late than never) and the payback calculation is the only way to approach this of course. As you intimated before, the MCS wallahs might insist on an oversized HP so the extra cost would mainly lie with the thermal store. Having had one myself and encouraged many others to do so, I know that in general they are intrinsically fine bits of kit so water based thermal storage is a good way to go; certainly good enough for anyone going down the heat pump route to give it serious consideration. Off-peak energy is mainly renewable too, so this system is kind to the grid and the planet and your wallet.

[JamesPa]

28 minutes ago, Originaltwist said:

@sharpener - I have just gone through your economics analysis (better late than never) and the payback calculation is the only way to approach this of course. As you intimated before, the MCS wallahs might insist on an oversized HP so the extra cost would mainly lie with the thermal store. Having had one myself and encouraged many others to do so, I know that in general they are intrinsically fine bits of kit so water based thermal storage is a good way to go; certainly good enough for anyone going down the heat pump route to give it serious consideration. Off-peak energy is mainly renewable too, so this system is kind to the grid and the planet and your wallet.

This is all rather interesting.  Even if you don't have an oversized heat pump, your heat pump is oversized (relative to the load) most of the time, typically by a factor of 2.  So given that storage volume is likely to be the limiting factor, perhaps this has legs even in more modest implementation.

 

For example an 8kW heat pump for an 8kW design  load will have 4kW to spare for most of the year.  If the cheap electricity period is 6hrs that's 24kWhrs, about 400 l of water heated through 50C I think Thats as much volume as many people can accommodate.  Not sure however how to use the stored heat once it gets below the flow temperature for the heating, at this point you need a water to water heat pump to extract the energy.

 

Nevertheless there would seem to be milage in sacrificing energy efficiency for energy cost which, as you point out, is likely to be environmentally friendly given that low cost energy is offered when there is spare capacity.

Edited April 30 by JamesPa

[JohnMo]

Another consideration if you went this route would be choice of heat pump. Nothing complex is required as you would operate on a fixed flow temp from the heat pump, it wouldn't need modulation either. So a cheaper model would be fine. Maybe save £2k on heat pump, spent £1k on big thermal store and mixing valve.

[sharpener]

20 minutes ago, JamesPa said:

For example an 8kW heat pump for an 8kW design  load will have 4kW to spare for most of the year.  If the cheap electricity period is 6hrs that's 24kWhrs, about 400 l of water heated through 50C I think Thats as much volume as many people can accommodate.  Not sure however how to use the stored heat once it gets below the flow temperature for the heating, at this point you need a water to water heat pump to extract the energy.

 

Yes, on your figures 24kWh is 24 x 860 = 20,650 litre-degrees.

 

But in my analysis I was assuming 55C max (as that will in parallel charge the HW tank to 50), and 30C min (as the lowest temp at which the rads will give any useful heat to the bedrooms).

 

So exactly half i.e. 12 kWh thermal with a 400 l tank. Hence Cosy looks good bc you get two bites at the cherry every 24h and the 1300 - 1600 is when there will be relatively warm air outside as well.

 

Am now a bit more hopeful I can get it to work as I have at last discovered an MCS installer who understands the concept (and has given me a reasonable quote). Rather than others grudgingly saying they will follow the Vaillant schematics at my expense if I really really want. But as upthread the main reason is it will avoid needing to replace the HW cyl, the TS is actually a bit cheaper so the marginal cost is just the cost of 1 pump, 2 extra valves and some pipework.

[Originaltwist]

35 minutes ago, JamesPa said:

This is all rather interesting.  Even if you don't have an oversized heat pump, your heat pump is oversized (relative to the load) most of the time, typically by a factor of 2.  So given that storage volume is likely to be the limiting factor, perhaps this has legs even in more modest implementation.

 

For example an 8kW heat pump for an 8kW design  load will have 4kW to spare for most of the year.  If the cheap electricity period is 6hrs that's 24kWhrs, about 400 l of water heated through 50C I think Thats as much volume as many people can accommodate.  Not sure however how to use the stored heat once it gets below the flow temperature for the heating, at this point you need a water to water heat pump to extract the energy.

 

Nevertheless there would seem to be milage in sacrificing energy efficiency for energy cost which, as you point out, is likely to be environmentally friendly given that low cost energy is offered when there is spare capacity.

Interesting thought about spare capacity. Here's your idea modelled but with a smaller Dt as it's hard to get the tank below 25C - but yes, 400 litres hits the spot.

[joe90]

interesting reading, also if you have PV, excess can be used with an immersion to make even greater savings.

[TerryE]

This is all a lot easier if you are doing a new build that is optimised for heat storage.  In our case we have a warm slab -- that is a concrete raft foundation where the insulation is on the outside of the raft so we have ~70 tonnes of concrete inside our thermal envelope and we have 30cm cellulosic filler in our twin-wall timber frame.  Whilst this doesn't have as good an R-value as PIR, it has a far higher specific heat and so a high decrement-delay time constant.  What this means is that we can dump the heat into the slab pretty much anytime the energy is cheapest.  The house cools about 1°C / day in the winter without any heating.

 

We use a 2.88 kW heater to heat our UFH and no other fixed heating.  We seem to be be paying about 4-10p / kWh this month for our heat on the Octopus Agile tariff because we only draw this down when the price is cheap.  It was even cheaper in Feb / Mar and we had a few days where we were paid to heat the house.   

 

We don't have an ASHP, because I can't make an investment case for installing one.  Our system has no expensive moving parts; no ASHP to maintain or replace.  Just the electricity running costs.

Edited April 30 by TerryE

[joe90]

@TerryE build is a good example of spending money on insulation and good design rather than “kit” which will have a lifespan shorter than the house itself. 

[JohnMo]

Good insulation and 100mm plus of concrete in the floor can make energy use and storage can easy task - but you have to embrace it.

 

I currently have next to no need to heat the house, but the garden room still needs heat, so just use the floor of the house as a buffer. Heat pump puts out 30 Deg water for 15 to 20 an hour to garden room fan coil, house UFH runs at the same time.  It's cheap, it's there, zero additional plumbing no energy goes to waste. Currently should be getting CoP of 4+, so between E7 and battery the heat energy is being produced at 4p (or less) per kWh.

[SteamyTea]

3 hours ago, JamesPa said:

Not sure however how to use the stored heat once it gets below the flow temperature for the heating

Blend in power from the HP or a secondary resistance heater if you have some spare PV generation.

 

I think trying to power a house purely from stored energy is difficult and requires demand side management, so much better to take a pragmatic hybrid route.

If it was easy to 'batch charge' the power requirements, then we would already be doing it, not as if any of this is new technology.

 

(I use the term 'power' to mean any sort of energy per unit time i.e. thermal, electrical, compressed air etc)

[Originaltwist]

I think a lot of things have changed to make off-peak storage in water viable:

High temperature heat pumps enable a wider Dt and so more heat stored in smaller tanks.

Off-peak electricity becoming even cheaper thanks to the electric car era.

Unstopable wind and nuclear power has to be dumped

Better insulated homes need far less energy.

PV more common, enabling DHW top up with an energy diverter

 

Combine all the above and a 1,000 litre tank works - a few years ago you'd be looking at a swimming pool.

[TerryE]

A 1,000 ltr tote buried in hole in the garden wrapped in say 400mm EPS 100 storing water at 60°C would leak heat at about 50W.  If you don't want to "waste" that heat, then stick a greenhouse or shed on top.  This should be good to store around 40 kWh heat for dumping into UFH.  Just an off the wall idea.  

[joe90]

28 minutes ago, TerryE said:

A 1,000 ltr tote buried in hole in the garden wrapped in say 400mm EPS 100 storing water at 60°C would leak heat at about 50W.  If you don't want to "waste" that heat, then stick a greenhouse or shed on top.  This should be good to store around 40 kWh heat for dumping into UFH.  Just an off the wall idea.  

I love thinking outside the box, but at what cost, you have already proved a house can be virtually cost negative by building smarter. @TerryE I can’t remember if you have PV ? If you don’t and add it your costs can be lower still.

[TerryE]

12 hours ago, joe90 said:

@TerryE I can’t remember if you have PV ?

 

Nope. We are in the "sustainable" core of our village and the planners like every new change to be sympathetic to the street scene.  So PV on our street-facing roof (SSE aspect) was considered a no-no. 

 

There's no point in having an additional heat battery for us as our whole house intrinsically works as its own heat battery, but my thought is that this approach might be more appropriate for a retrofit or more traditional build-style.  Any form of heat battery should allow you to time-shift heat generation and storage from periods where the unit price of electricity is low and decouple this from consumption demand. 

 

Edited May 1 by TerryE