It's even better than that. In addition to the factor of n from ratio of volume to surface area, there's also a factor of n from the increased thermal resistance of the mass of the storage volume (the temperature gradient from the surface to the center goes as 1/n). So, the thermal time constant of the object scales as n^2.

This very favorable scaling is why natural geothermal retains heat even though the input energy was delivered gradually over as much as millions of years.

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fy20 5 hours ago | parent | next [-]

I've always wondered why we don't build homes with a buried tank of water used as heat storage. In the summer it can be heated with solar thermal to around 90c, and in the winter heat can be drawn out and go through radiators or underfloor heating, with a mixer valve. You just need a few pumps and valves, not even a heat pump is needed.

If you assume a modern house with a heat load of 1800kWh per year (fairly standard for a new build medium sized home where I live, in Northern Europe) that means you'd need a tank roughly 50m3, or 10,000 gallons for Americans. In terms of insulation you'd need around 50cm of XPS foam, and it would be buried a meter below ground.

It's nothing terribly complicated in terms of construction or engineering. Of course you'd pay more upfront, but then your heating bills would be practically zero. In warmer climates it would be much simpler, you could probably get away without burying it.

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Ndymium 5 hours ago | parent | next [-]

This is essentially what a ground source heat pump system is. Except instead of a sealed water tank you just make a tall hole that fills with water and the sun will warm it for you during the summer automatically.

1800 kWh is very little. We use around 12000 kWh and our neighbours' new house uses around 8000 kWh annually and most of that is heating. I'm not sure how many houses can hit 1800.

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nandomrumber 3 hours ago | parent | next [-]

A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that use a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through the seasons.

https://en.wikipedia.org/wiki/Ground_source_heat_pump

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Maxion 3 hours ago | parent | prev [-]

Heath energy required != electricity requirement.

A modern house in Finland needs around 15-24kWh a year of heat energy if it's well insulated. On the higher end for big + northern houses, and less if you're smaller and further south.

Some get this energy by burning wood, others with heat pumps, and some with direct electricity.

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nandomrumber 3 hours ago | parent [-]

24kWh is 1kW drawn continuously for 24hrs.

That can’t possibly heat any home for an entire year.

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shellfishgene 2 hours ago | parent | next [-]

I think they mean per square meter of living space.

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sl-1 2 hours ago | parent | prev [-]

I think MWh is meant, otherwise it makes no sense

	▲	hdgvhicv an hour ago \| parent [-]
		My 90sqm bungalow in the U.K. uses about 15MWh a year for heating - 1500 litres of oil, almost all in winter. Peak load is about 2.5kW over a day (60kWh)

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kitd 2 hours ago | parent | prev | next [-]

I can't find the link now, but there was an episode of Grand Designs here in the UK (a show detailing private individuals developing interesting or unusual homes) where the owner was building a passively heated house based on an idea by his architect father.

The ground beneath the footprint of the house was insulated around the sides to a depth of about 2m, effectively extending the thermal mass of the house into the ground. After construction, it took about 2 years (IIRC) to warm to a stable level, but thereafter required little to no energy to stay at a comfortable temperature year round.

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elric 3 hours ago | parent | prev | next [-]

50m³ is huge. IMO that would be an engineering challenge that would probably impact the sability of the foundation if not done right.

Ground source heat pumps are expensive because of the buried piping, I imagine this would be even more costly.

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badc0ffee 4 hours ago | parent | prev | next [-]

Something like that was attempted south of Calgary, in Canada: https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community

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mzhaase 3 hours ago | parent | prev [-]

Its kind of done. Active heating systems often have the intake air go through the foundation so it heats up in summer and cools down in winter reducing both heating and cooling costs.

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kragen 4 hours ago | parent | prev [-]

Billions, mostly.