Operating a home on thermal storage

Terraformer · 2024-07-23 13:21:03

I feel this needs its own topic

Calliban wrote:

A typical washer uses 0.79kWh per cycle and is used 200x per year.
https://checkappliance.co.uk/how-much-e … chine-use/
This UK company manufactures a hot fill washing machine that they claim reduces electric energy consumption by 64%. This reduces the electrical energy consumption to 288Wh per cycle or 158Wh per day (average).
https://ebac.com/washing-machines/e-care-8kg-hot-fill
A dryer uses somewhat more power. But it isn't difficult to dry clothes on a line. My wife and I use a combination of outside drying on a line and an indoor airer.
A typical fridge uses 166kWh/year in the UK. That is 454Wh/day.
https://www.homebuilding.co.uk/advice/h … fridge-use
A 40W-equivalent LED bulb uses about 5W of power. Some 5 of these bulbs, running 8h aday, will use 200Wh of energy per day.
Total = 812Wh. If we can take the fridge off of baseload by including thermal storage, the total is reduced to 358Wh.

It definitely seems to be feasible without needing much modification of off the shelf products. I'd suggest a chest freezer be considered also, due to its advantages in retaining cold air when opened and the space to produce ice. Also a spin dryer would add very little to the demand whilst removing quite a bit of the water.

Extractor fans should be considered, especially if drying on an indoor rack.

We should build up a list of COTS products that would be useful in achieving this.

kbd512 · 2024-07-23 14:03:31

Terraformer,

How about a venturi built into the home which uses natural convection, from baseboard to ceiling, to assist with drying clothes?

That might be real useful to have because it doesn't require a power source. Natural air movement does the job for you.

Calliban · 2024-07-23 16:51:57

For the purpose of this discussion, I am going to state figures for the average UK dwelling. This is quite a bit smaller than the average US dwelling. It is inappropriate to talk about an average US dwelling, because climatic conditions vary too much. Any discussion relevant to the US needs to look at individual states.

According to this site, the average UK dwelling uses 12,037kWh/year on space heating.
https://www.ovoenergy.com/guides/energy … do-you-use

I have used government statistics to provide a breakdown of energy end-use for dwellings.
https://assets.publishing.service.gov.u … _copy.xlsx

Assuming that space heating is 12,037 total energy use, the breakdown is as follows:

Space heating: 64.54% - 12,037kWh
Water heating: 16.95% - 3,161.25kWh
Cooking: 2.69% - 501.69kWh
Lighting: 2.92% - 544.58kWh
Appliances: 12.89% - 2,403.99kWh
Total: 100% - 18,650kWh

This looks about right. I know that an average house uses between 3000 and 5000kWh electricity per year.

Lets assume that half of space heating takes place over the 90 days of winter, with the other half used in autumn and spring (180 days). During the winter we need 66.87kWh of heat per day. Let's assume we use a heat pump to provide that heat. We then store heat in water, so we still have heat if the power goes down. I am going to assume a 40°C water storage temperature. How much water do we need to store, assuming a room temperature of 20°C?

V = 66.87 x 3600kJ/((40-20)x4.18) = 2880 litres

To store 3 days worth of space heat, we would need 8,646 litres of warm water. That is a cubic tank just over 2m aside. I think this would fit in most dwellings.

For household hot water stored at 80°C, we would need a volume of 0.87m3 for 1 week of supply. I think we would heat input water to 40°C using the heat pump and then use a reisistance heater to heat it up to 80°C.

Here is a list of energy hungry appliances in the home.
https://www.comparethemarket.com/energy … st-energy/

I calculate that collectively, they account for 11.24kWh of electricity demand. That is almost the entirety of electricity demand.

Last edited by Calliban (2024-07-23 17:26:36)

tahanson43206 · 2024-07-23 17:51:46

For Calliban re opportunity of this new topic ....

Is there any chance a publication in the UK might be interested in publishing a piece on how to implement the ideas you and Terraformer (and occasionally other members) have been discussing?

I don't know if a single article would pay back the investment, but a movie made from the article might.

What I'm wondering is whether the potential for income might help to justify the investment?

Beyond the article is the potential to create a business to install systems for paying customers.

(th)

Last edited by tahanson43206 (2024-07-23 17:53:28)

Terraformer · 2024-07-24 05:07:14

Huh. Three of the most power hungry devices on that list are literally heating hot water. Then there's the hob, which for the most part is used to heat hot water. And the dryer, which heat hot air...

Three hours of TV a day is quite a lot though. A lot of people watch stuff on their tablets and laptops now.

I don't think the heating demand in even a somewhat more insulated house would be anywhere near that amount. British houses are really poorly insulated; even a few centimetres of cork board on the inside would improve things a lot. We let the perfect be the enemy of the good and then don't insulate. A lot of (most) houses here can't be brought up the passivhaus standards, so we just don't do anything, and older ones need breathable insulation to avoid damage instead of a lot of modern synthetic options. But there's still a lot that can be done.

Still find it crazy how much electricity we use...

Calliban · 2024-07-24 09:57:45

Rockwool is the most common and one of the cheapest insulation materials, at about £80/m3. In terms of cost, it is second only to straw, but unlike straw it is completely fireproof. It has thermal conductivity of 0.04W/m.K at room temperature. We could apply 1cm sandwich panels of rockwool to the outer surfaces of a house and render over them. They would provide the same insulation value as 25cm of brickwork, more than doubling the insulation value of the walls of a Victorian terrace. Cost ~£1/m2. So for a mid terrace with two 3m x 8m external walls, cladding cost would be negligible. The labour needed to remove the old render, attach the clad and render over it, would dominate the cost. If we could increase clad thickness to 1", materials cost would still be negligible. But the insulation value of the walls would quadruple.

Improving insulation value even for old Victorian houses is a better option than attempting to put in a powerful heat pump. The better insulated the house, the lower the effective radiator temperature can be and the more efficient the heat pump. Halving the heat load of the house will more than halve the amount of electricity consumed by a heat pump. And the capital cost of the heat pump will be commensurately lower. Given that space heating is two-thirds of the domestic energy budget, it really makes sense getting this right. It also makes it much easier to store the heat needed to keep the house warm if there is a power outage of several days. If we combine superinsulation with a high COP borehole heat pump, we can probably reduce space heating needs to 1000kWh per year for the average dwelling. And that doesn't need to be baseload, because we can store the heat.

Last edited by Calliban (2024-07-24 10:13:41)

Terraformer · 2024-07-24 10:18:44

And, of courses, one major benefit of external insulation is that the heavy thermal massing is within it. The house itself can store a large amount of heat, once it has warmed up. Perhaps there is a way we can speed the heat transfer along, helping to cool in summer and store heat in winter? A mid terrace might have 25cm of stone/brich on either party walls (the one I grew up in is 50cm narrower on the inside -- those walls are thick, whatever Max Brooks thinks in his zombie stories). At about 7m long by 6m tall, that's ~20 cubic metres. At 1.6MJ (~0.5kWh) per cubic metre, cooling down by 2 degrees would put 20kWh of heat into the house from just the party walls. It will take time for the house to cool. Which means also that once the walls are warm, heating energy will not be taken by them, only by the air and furniture.

Ventilation is important, and at 100L/s air exchange (good ventilation, rarely achieved) and freezing temperatures outside we're talking ~2kW of heating needed. I'd suggest shrinking our windows slightly to make it easy to put in vents... raising the ground floor window of a terrace by 20cm isn't going to screw it up super badly from an aesthetic standpoint, and means we can run pipes and cables into the house without needing any hatchet jobs. Recovering 75% of the heat would make it a far more manageable 500W heating. Or none, if there are 4 or 5 people in the house.

Last edited by Terraformer (2024-07-24 10:54:32)

Calliban · 2024-07-24 10:53:51

Terraformer wrote:

Huh. Three of the most power hungry devices on that list are literally heating hot water. Then there's the hob, which for the most part is used to heat hot water. And the dryer, which heat hot air...
Three hours of TV a day is quite a lot though. A lot of people watch stuff on their tablets and laptops now.

If we rework the most energy hungry appliances to use stored heat, then the total energy consumption of appliances won't change much. What will change is that 90% of the energy used no longer has to be on demand. It can be stored during times of plenty as heat.

1. For an oven / hob, this could be done using some sort of phase change material in a jacket around the inner surface of the oven. This where high quality aerogel insulation will likely pay for itself.

2. Instead of a kettle, we have a 100l stainless steel container filled with hot water, surrounded by several cm of rockwool and heated by a reistance heater on a thermostat. This could store a week's worth of boiling water.

3. I would personally drop the tumble dryer. Anytime it isn't raining, an outdoor line will dry clothes just fine. Damp linens can dry out indoors after doing most drying outside. So that is 4.88kWh gone from the appliance budget.

4. The dishwasher and washing machine need about 200Wh of mechanical energy each per cycle. All the rest os hot water, which can come from the house hot water tank.

5. Fridge & freezer use 130W of power on average, adding up to 3.12kWh per day. This is again storable heat.

Total baseload power needed for appliances: 400Wh (clothe and dishwasher) + 600Wh TV & laptop = 1kWh per day. Even this can be postponed. If you are drawing electric power from storage, you don't have to use the washing machine or dishwasher that day. You can wash dishes and clothes by hand in hot water. And you can read under a 1W chair lamp instead of watching TV. Lighting needs ~200Wh/day.

However, by my estimate, the appliances still use 5.36kWh of thermal energy. The cooker and water dispenser are resistance heater powered, so 3.52kWh/day. That leaves 1.84kWh of hot water needed per day for the washing machine and dishwasher. The water entering the hot water tank can be preheated to 30°C by the heat pump. A resistance heater then heats it from 30 to 60°C. The heat pump cuts about 1/3rd of the energy needed to heat it from 10 to 60°C. So producing 1.84kWh of hot water, requires an input of 1.23kWh of electricity.

For the average dwelling, some 3161kWh are already used to provide hot water. If we apply the same assumptions as above, we need 2108kWh of electricity each year, to provide this hot water. That is 5.77kWh electricity per day, in addition to the 1.23kWh needed to make hot water for appliances. So 7kWh/day in total. It would appear that water heating will come to dominate the energy budget of the house.

Summing up:
1. A well insulated house may need as little as 1000kWh/year (2.74kWh/day) for space heating.
2. The same house needs 7kWh/day for hot water heating.
3. About 1.2kWh/day for pure electrical appliance needs + lighting, although arguably, this could be reasonably cut to 0.2kWh/day during powercuts.
4. The hot water dispenser and cooker need 3.52kWh per day, but all of this can be stored as heat.
5. Refrigeration / freezing = 3.12kWh/day.

Total = 16.38kWh/day intermittent electricity + 1.2kWh/day baseload

So about 93% of household energy needs can be met by stored heat. The remaining 7% is for applications that require electrical power for mechanical power, light and electronics.

This does give me some hope about the prospects of adapting to an intermittent energy future. Looking at Gridwatch, wind power production is highly seasonal. But even in winter there are deep lulls in production often lasting several days. There will be some solar in the mix, along with some biomass and nuclear. If we can develop houses can store at a few days of heat and baseload electric power, a future of rolling blackouts will be much easier to adapt to.

PS. Washwater and appliance drainage water, should contain a most of the heat needed to heat it in the first place. It will probably still be at 25°C when it enters the drain. In a well insulated house, could drain water provide the heat source for the space heating heat pump?

Last edited by Calliban (2024-07-24 11:41:51)

Terraformer · 2024-07-24 11:37:51

Should also be noted that it's fairly straightforward to use shower drain heat to preheat cold water, reducing the quantity needed from the tank. I think off the shelf heat exchangers can recover about 2/3? With showers being the primary hot water use, this shrinks both storage needs and energy consumption.

SpaceNut · 2024-07-24 19:40:33

Most electrical items require a startup surge current and that is a problem when looking at just the average run power requirement.

A typical size of a hot water tank is 80 gallons for a small family and while you can do with a 40 gallon single the extra hot water comes in handy.

80 gallons is just over 300 liters.

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#1 2024-07-23 13:21:03

Operating a home on thermal storage

#2 2024-07-23 14:03:31

Re: Operating a home on thermal storage

#3 2024-07-23 16:51:57

Re: Operating a home on thermal storage

#4 2024-07-23 17:51:46

Re: Operating a home on thermal storage

#5 2024-07-24 05:07:14

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#6 2024-07-24 09:57:45

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#7 2024-07-24 10:18:44

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#8 2024-07-24 10:53:51

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#9 2024-07-24 11:37:51

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#10 2024-07-24 19:40:33

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