Let's imagine that we have a heat pump which isn't going to draw from the grid for 4-hours out of the 24. And we'll make the maths easy by assuming that the house has a daily heat loss of 100kWh.
We'd need a water cylinder capable of storing 4/24 x 100kWh which comes to 16.7kWh
Now suppose your heat-pump is such that it can deliver water at 50°C max, and that you're prepared to allow your radiator temperature to fall to 35°C by the end of the 4-hour period. Your Thermal store will exhibit a 15°C drop whilst delivering that 16.7kWh.
The calculation:
1 watt of power for 1 sec = 1 Joule
Thus 1kWh = 1000 watts x 60 secs x 60 mins and therefore 1kWh = 3,600,000J (3,600 kJ)
16.7kWh = 60,120,000J (60,120kJ)
We need to find the mass of water which will hold that energy.
Energy required = Specific Heat Capacity x Temp loss x mass in grams where the Specific Heat Capacity of water is 4.184J/gram/°C
Change the formula around: Mass (grams) = Energy / (Spec Heat x Temp loss)
= 60,120,000 / (4.184 x 15°C) = 957935 grams
Convert to kilograms = 957.935Kg
1Kg of water is 1 litre capacity, so we need a tank holding almost 1000 litres 1 litre of water occupies a cube with each side being 100mm.
Conclusion:
Assume that this tank is a cylinder with an internal diameter of 800mm (about the largest domestic hot water cylinder you could buy!)
Volume of a cylinder : V=πr²h
Change around the formula to find height (h)
h = V/(πr²)
Our cylinder would then need to be 1.98m tall.
Add at least 40mm of high quality insulation around it, plus the stainless steel strong enough to contain 1 metric-tonne of water. That gives you a tank almost a meter across and the height of the ceiling in an average home.
Feel free to play around with my figures. I just wanted to make sure that the required formulas were presented in this Topic.
Their firm proposal is to implement a process to be called Consumer Led Flexibility (CLF).
That will allow 3rd parties to act as Load Controllers. Households who sign up to do so will have their heat-pumps turned off during the early-evening demand peak as required by national statistics.
The Load Control Agent will then credit the householder's Energy Account for providing that demand-reduction.
It's therefore worthwhile considering how a house might remain heated during that period by storing energy in advance.
I'll add some first hand data here, albeit a single data point made through observation.
Conveniently, our house recorded a daily heat loss of around 100kWh on the coldest day last winter, so fits nicely with @Transparent's example.
Last year we ran our heat pump on the Octopus Cosy tariff, and operated a switch off policy for the 3h peak window between 4-7pm. The temperature drop experienced was 3-4C in that 3 hour window, and was pretty unpleasant without some alternative form of heat. A 4h switch off may translate to a 5C heat loss for us (21C down to 16C).
In terms of water volumes, our system with 12 radiators (some quite large), lengthy (over 10m) 28mm primaries and a 50L volumiser has a total system volume of just under 200L (190L by my estimates). Substantial thermal mass, but nowhere near the 1000L required to store sufficient heat by @Transparent's calculations to help offset that heat loss.
Of course this only applies on the coldest days of the year, and during the milder months of spring and autumn, heat loss will be significantly less and a lengthy switch off may be practical without losing so much heat as to be intolerable, but I do not believe the above strategy is a practical strategy for most homeowners for the coldest 3 months of the year. We could certainly tolerate a 3h switch off now whilst OATs are still in double digits.
I should note that we were able to operate such a strategy last winter, even if pretty intolerable, mainly because we were able to recover that 3-4C of heat loss pretty quickly due to having a massively oversized heat pump in our property. A correctly sized heat pump would simply not have the capacity to recover a 3-4C heat loss on the coldest days of the year in a reasonable amount of time. See here for a recent discussion of set backs and recovery thereof:
In my limited experience, it doesn’t seem like a viable national strategy to me.
This post was modified 3 hours ago 5 times by Old_Scientist
Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.
In my limited experience, it doesn’t seem like a viable national strategy to me.
I have to agree with that. Although our house keeps heat quite a bit better than yours, I think, I got fed up of operating on one of the modes that my heat pump will operate in (a mode with a fairly high degree of room influence) because of the temperature swings, so early on switched to pure weather comp which for my house is much more stable.
The swings with which I got fed up were after an hour to two switched off, so three hours would not be tolerable.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
The SSES Team has not yet suggested any particular time-frame for which HPs would be switched off... and this would probably be a decision to be made by Elexon as their preferred administrator of the scheme.
However, I think it's a fair assumption that it would need to be 3hrs-ish due to the nature of the demand peak.
I have raised a good many objections. Many of these fell outside the scope of the recent consultation subject of Governance, and DESNZ requested that I send those in anyway. I did so in a 15-page separate document.
Sticking with the proposal of it being heating appliances which are switched off, I assume there is concensus here that a heat-pump would be less efficient as it is switched back on? It would have to 'run harder' in order to make up for the heat lost from the dwelling during the off-period.
Let's assume it was an ASHP which was normally operating with a COP of 3.0 (yes, I realise many can't achieve that!) is it possible to provide a reasonable estimate of the extra electricity it would consume during the time taken to raise the temperature back to the normal setting?
That's a critical calculation for the CLF Scheme to be viable for the typical consumer.
The 'compensation' he gets paid for having taken part in Demand Reduction must cover not only the electricity which would normally have been used during the off-period, but also the extra that is needed to recover the temperature whilst running less efficiently.
Let's assume it was an ASHP which was normally operating with a COP of 3.0 (yes, I realise many can't achieve that!) is it possible to provide a reasonable estimate of the extra electricity it would consume during the time taken to raise the temperature back to the normal setting?
It depends entirely on how quickly you want to recover!
If for example you want to recover in the same time that you switched off for (eg switch off for 3 hrs recover for 3 hrs), then you need to operate at twice the delivered power (which is unlikely to be possible unless the heat pump is well oversized, given that this will presumably happen at min temps).
Power output from a radiator (sensible assumption IMHO because its the majority case) is proportional to (Trad-Troom)^1.3. So if Trad is say 45C and Troom is 20C you need to raise Trad to 20+1.7*25=62C for the duration. Thats 17C higher, making it roughly 50% less efficient (very, very, very, very rough rule of thumb 3% per degree).
So during the recovery period you will consume 3* as much electricity as you 'saved' (2* as much because of twice the energy plus a further 1* as much because of the loss of efficiency.
So if its a fairly typical 7kW house operating at a COP of 3 it will normally run at 2.3kW and during recovery it will, in principle, be drawing ~6kW (which is only going to work if the heat pump is a well oversized 12kW model).
Not sure how much this strategy will help with peak management TBH unless there is a very significant element of randomisation of timing.
In reality what will happen is a much slower recovery probably spread over 12hrs or more, but it will likely seem quicker because the air temperature heats up before the fabric does and its an asymptotic recovery curve.
It would be better to give people fixed periods during which they were incentivised not to use their heat pump, corresponding to high demand periods. They could then plan accordingly by adjusting their WC curves to suit the reduced operating hours, as they do at present. Forced switch off could then be reserved for true emergencies.
Im coming to the conclusion that using switch off for demand management, other than in an emergency/very exceptional case, is a non starter.
Do you really think the switch off will be for 3 hrs, or is it more likely to be either an hour (which is the typical duration for the Octopus saver session) or even 10 mins when everyone puts the kettle on during the cup final.
This post was modified 17 minutes ago 2 times by JamesPa
