Timeshifting domestic electic energy demand using a domestic heat store (for a post gas world)?

michaels

With discussion of how we decarbonise domestic energy consumption I was wondering whether there might be mileage in combining ASHP with some form of heat store - perhaps a large well insulated thermal mass - water or iron - buried under every house.


The ashp could be used to 'charge up' the heat store when ever the combination of time of day electricity prices and outside temperature made it most cost effective and the store could then provide daily heating and hot water needs - or even cover multi-day usage if the supply mix was more renewables based and thus variable.

What I need is someone (Martyn?) to do the maths on whether the numbers add up in terms of how much thermal mass is needed.

WE use a humungus 32k kwh of gas per annum with a peak day of about 20kwh. Not sure how much leccy we would need to replace this if using an ashp - this would be on the coldest winter day, is it reasonable to assume a factor of 2x or ashp output in these conditions? The input is natural gas used, which is obviously not 100% of the energy is actually heating the house or water.

Other times of year the gain factor will be better and in the summer it is possible free PV could be running AC with the 'waste' heat being stored as hot water for a win-win-win.

So I think the heat capacity of water is about 4.2MJ and of Iron about 3.7MJ so who can do the maths on what size store with say a 10 degree temp delta from 40-50c would be needed to provide a whole day of domestic heat energy?

Thanks

michaels

MY calcs suggest about 2,500 litres of water could store one peak day of household heating/hot water energy need with a delta of 10 degrees which sounds fine and that even 2 weeks of storage would not be ridiculously large.

EricMears

michaels wrote: »

MY calcs suggest about 2,500 litres of water could store one peak day of household heating/hot water energy need with a delta of 10 degrees which sounds fine and that even 2 weeks of storage would not be ridiculously large.

I haven't checked the arithmetic but express no doubts about the accuracy of the 2,500 litres.

However, 14 days of storage would presumably be 14 x 2,500 which is 35,000 litres. I think I'd regard storing 35 tonnes of water in a single household as "ridiculously large". Indeed, I have often been ridiculed for my own storage of 3,500 litres of rainwater !

mnbvcxz

I'm not sure if this is the article I once read but the buried insulated milk tanker cylinders sound similar....
http://m.earth.org.uk/milk-tanker-thermal-store.html

The builditsolar website tends to have lots of similar ideas too.

I generally get the impression that underfloor heating the thermal mass in a floor that's being built anyway remains the mostly unrivalled peak of practicality for.this sort of apication. Always hope.that the 100 dollar a kw battery will actually turn up.

zeupater

Hi

A little like a domestic version of ... this civic development ... that I came across a few years back, but you do need a serious amount of summer daytime cooling to grab the heat to store in the first place ..

Offices etc, particularly ones for public sector employees (they do tend to get decent workspaces!) with plenty of bodies, computers & solar gain to fight against, may make sense, however, if it's a standard home with average windows etc and you're paying for it yourself, then it's likely not to show a reasonable return unless it went really mass market & ancillary things like borehole costs dropped dramatically (that's significantly closer to £zero than they are now!) ...

HTH
Z

Ben84

It's an interesting idea. Your most practical option for tackling gas use in your home however is likely to be already commonplace technologies though - insulation and draft proofing. Any future changes in energy supply or heating appliances will benefit from this too by making the installation smaller. Anyone with plans to generate or store energy on site should be weighing up the savings of a smaller system against the cost of energy saving changes.

michaels

Ben84 wrote: »

It's an interesting idea. Your most practical option for tackling gas use in your home however is likely to be already commonplace technologies though - insulation and draft proofing. Any future changes in energy supply or heating appliances will benefit from this too by making the installation smaller. Anyone with plans to generate or store energy on site should be weighing up the savings of a smaller system against the cost of energy saving changes.

I believe climate change remediation plans require the UK to switch from using gas to renewable energy for heating houses within the next 15-20? years.

Given that most of the current housing stock will still be in place and there is probably a maximum level of efficiency measures that can be taken there is likely to remain a sizeable hot water and heating requirement to be switched from gas. The only way I can see currently of getting that RE into homes is via the electricity grid (I suppose another option would be using remote RE to produce hydrogen but ignoring that we are talking electricity).

Heat pumps, probably mostly air source, are the obvious answer but the drawbacks are that these do not support any form of time shifting within the day let alone over several days so would put more pressure on the peak supply which is already likely to be under pressure at certain times due to RE intermittency. The performance is worst when outside temps are lowest - that well known winter high pressure light winds no sunshine low temperatures scenario we always talk about.


Traditional storage heaters (perhaps augmented via heat pumps for increased efficiency could obviously help with the 'time of day' peaks and troughs but adding additional (low loss) heat storage at the point of demand could be used to cover longer periods of supply fluctuation and also by regulating demand put less pressure on the grid.

As you mention the key is the economics, would this local storage make more sense than other storage technologies done at grid scale. For me the advantage of storing the energy as heat where it will be used as heat is that there are potentially fewer transformations required and that it is possible the storage options using existing (low) tech could be implemented very cheaply.

As an example, say you use electricity directly to provide heat, efficiency is high so approximately 1kwh of leccy will give 1kwh of useable heat. Using a heat pump the same 1kwh will give between 1 and 5 kwh depending on the air temp, most heat will be wanted when it is coldest so lets say on average 1kwh gives 2kwh. Even if some sort of daily storage is enabled it is likely that demand will peak when RE generation is lowest and potentially prices are highest.

Instead with longer term bigger stores let us assume that 'heat is stored when temps are higher and RE is abundant and that each kwh of electricity can store 4kwh of heat energy. Further assume that to get the heat back also requires anther 1kwh to return the 4kwh stored so 2kwh get us 4kwh i.e. the same 200% efficiency as using the heat pump and short storage suggested above. The gain comes from the store being replenished when RE is abundant and cheap and drawn down when supplies are tight and unit prices are high.


Assuming a house uses 10k kwh of heat per annum. With a heat pump/night store arrangement this might require 5k units at 20p/unit = £1000 (200% efficiency). With the longer term store we need the same 5k units (2.5k to fill the store at 400% efficiency and 2.5k to return the heat when needed again at 400% efficiency) but these might now be sourced when supply was at its highest and unit rates were only 10p giving an annual cost of £500.

Say we want a 6 year pay back no investment this would mean the long term store would need to be less than £3k more than a daily store which might be possible considering it is just a large insulated tank of water.

I need a diagram but basically the options are:
1) ASHP feeds heat into hot water tank and 'night store heaters' on a daily cycle
2) ASHP feeds heat into large thermal store and a separate heat pump (if required, an old fashioned heat exchanger might be adequate if the heat store is hot enough) returns this heat into hot water and space heating as required.

michaels

zeupater wrote: »

Hi

A little like a domestic version of ... this civic development ... that I came across a few years back, but you do need a serious amount of summer daytime cooling to grab the heat to store in the first place ..

Offices etc, particularly ones for public sector employees (they do tend to get decent workspaces!) with plenty of bodies, computers & solar gain to fight against, may make sense, however, if it's a standard home with average windows etc and you're paying for it yourself, then it's likely not to show a reasonable return unless it went really mass market & ancillary things like borehole costs dropped dramatically (that's significantly closer to £zero than they are now!) ...

HTH
Z

Thanks for the link, it took me to this:
https://www.icax.co.uk/banking_on_iht.html

Whilst the economics may make better sense now offsetting cooling in the summer, I envisage a time when we no longer use domestic gas for heating and hot water and power generation is from intermittent RE and thus we can use heat storage in conjunction with ashp to handle this intermittency at demand side with the minimum number of energy storage media transfers as we go straight from electricity being used when it is being generated at lowest cost to heat being stored where it will be used.

antrobus

michaels wrote: »

With discussion of how we decarbonise domestic energy consumption I was wondering whether there might be mileage in combining ASHP with some form of heat store - perhaps a large well insulated thermal mass - water or iron - buried under every house.....

My mind boggles at the cost of digging a ruddy big hole under any house in the UK. It would almost certainly be cheaper to knock the house down and rebuild.

michaels wrote: »

I believe climate change remediation plans require the UK to switch from using gas to renewable energy for heating houses within the next 15-20? years.....

Are you sure?

Something like 85% of UK households have gas central heating. That's over 22 million households. It's gonna take a lot longer than 15-20 years to get them all converted. And not much point even starting until you've got masses of surplus renewable capacity.

Martyn1981

antrobus wrote: »

Are you sure?

Something like 85% of UK households have gas central heating. That's over 22 million households. It's gonna take a lot longer than 15-20 years to get them all converted. And not much point even starting until you've got masses of surplus renewable capacity.

Hi. It's not so much about ending GCH, more the end of new/replacement GCH boilers. The government did start a consultation on this issue for non-residential boilers. I think they were looking at a date of 2035 for the ending of GCH boilers being installed, but of course the existing boilers might last another 20yrs.

I don't think any solid announcements have been made regarding residential boilers, most likely because the news will make a massive fuss about it, and people won't focus on the date for change, but more likely get hysterical about the date itself as when we all die from hypothermia.

There would still be lots of options, perhaps even GCH boilers but running on bio-gas (see Ecotricity's idea of producing 90% of UK domestic needs from grass), or from excess RE generation, or from a higher mix of H2 in there, or even H2 to run fuel cells and then heat pumps.

But priority one, is increased efficiency. Reduce the heating need considerably, and the problem gets much easier. As I was once told, reduce your heat loss by 50% and you might reduce your 'additional' heating needs by 75%+, since we get some heating from solar gain, electricity consumption, DHW consumption, people and pets.

michaels

Here is a bit more on the decarbonisation of domestic heat.

https://www.theguardian.com/environment/2018/nov/22/uk-hydrogen-heating-2050-emissions-targets-gas-boilers-electric-climate-change

The preferred solution seems to be ashp supplemented by H2 boilers to address the intermittency/peak demand issue that leccy without storage and grid supplementation can't support. My 'solution' was to use local medium term 'heat stored as heat' instead.

Obviously insulation is a large part of the solution but we are not going to wholesale replace the existing housing stock which leaves the question of what level of efficiency can be achieved with the existing stock?


I would have thought most homes could incorporate 2 cubic metres of storage somewhere?