Comment by fy20
16 hours ago
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.
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.
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
You can also recharge your geothermal well or ground heat collection field by heating the outgoing thermal collection liquid with either cheap electricity (rooftop solar?) or direct solar heat collection. I think this will be a growing thing as the earliest mainstream ground source heat wells start to be a few decades old. Many of them are sized so that they don't fully recover during the summer, so the heat output slowly drops.
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.
24kWh is 1kW drawn continuously for 24hrs.
That can’t possibly heat any home for an entire year.
6 replies →
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.
I remember that one, but also can't find a link. Wasn't the heat from the people living there, and from appliances etc. enough to keep it at a comfortable temperature?
I'd be interested to see the ventilation system.
I love ideas like this.
I'd like to see the stats on temperature levels over a lifetime.
> 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.
Because building houses is already expensive, and that would add significant amount, pushing it into "can't afford it in the first place". And zero ability to realistically service it means anything going wrong might make whole investment moot.
On top of that, any investment like that competes with "why not just put the money into low risk fund"
> 1800kWh per year
now factor in losses for months now factor the fact the energy you're using for heating is one you're not using for... energy or selling
also is that heat or energy ? Because if that's "what power heat pump used", multiply that by 3-4
It's just... expensive to do it like this. Expensive enough that most people that could did the math and it wasn't mathing
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.
> I've always wondered why we don't build homes with a buried tank of water used as heat storage
Skip the tank completely. Use the ground directly. This is what geothermal heating does.
Drill a deep hole and drop tubes into it. Use a heat pump to pump heat into or out of the ground. There is so much easily accessible thermal mass in a borehole that you don’t need to deal with a giant underground water tank
I'm not sure if the 1800kWh is correct here. I'm guessing it's one of these two:
- You're talking about what heat pumps use in electricity. However, the system would store heat. If a heat pump uses 1 kWh to get 3 kWh of heat into the house, a heat based storage system needs to store the 3 kWh.
- You're confusing gas & electricity. 1800 m3 in gas would be about correct. However, that's about 9,5 kwh per m3 in heat.
There are interesting heat storage methods though, there is a long term basalt heat storage system in 'Ecodorp Boekel' in The Netherlands. It uses solar to heat during the summer and heats the homes with that in winter.
Due to size though, it only really works in 'collective' communities. The bigger the size, the more heat it can store per size.
Something like that was attempted south of Calgary, in Canada: https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community
Sounds like it didn’t go so well: In 2020, the system started showing signs of deterioration resulting in significant maintenance issues. System components, knowledge, and technical expertise for repairs were becoming increasingly challenging to find. In response to system failures, the Drake Landing Solar Company added redundancies to the system to be sure that homes in the community were receiving heat… In 2024, a decommissioning process for the Drake Landing Solar Community began, where the majority of the 52 homes were converted to natural gas-fired furnaces.
Nowadays, solar thermal collectors are completely obsolete except in very niche applications. Solar PV is so cheap that it’s literally cheaper (not to mention much less maintenance) to wire a bunch of PV panels to a resistance hot water heater than it is to directly heat the water with thermal collectors!
And if you use resistance heaters, you can get to much higher temperature with practical systems.
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.