9 ms·

Long-term thermal storage is something I've been fascinated with the last year or so. Heat loss inside of dirt is so incredibly slow it's hard to wrap your hea

Long-term thermal storage is something I've been fascinated with the last year or so. Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground! Obviously there is incredible potential to this even if you just keep the energy as heat. The amount of electricity we use on heating and air conditioning is huge. If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling. Really excited by companies looking into this and wish them the best of luck!

profsummergig 1y ago
> you only need to go down 10 ft or so before you hit the average annual temperature Is this because of geothermal energy leaking upwards? If so, it's not the dirt, it's the geothermal energy.
- vasco 1y ago
  Had the same thought, we'd have to put a thermometer inside a 10ft cube full of dirt for science.
  - profsummergig 1y ago
    > a 10ft cube full of dirt suspended in air
- werdnapk 1y ago
  Yes, the you can put "thick" insulation over top of any buried plumbing and the exposed bottom will gain geothermal heat from the below and it can prevent freezing.
- adverbly 1y ago
  Its a bit of both, but its primarily due to the high insulation. There are 2 gradients: The surface gradient is what I mentioned about and its quite steep(only a few meters to drop tens of degrees). After that, you reach approximately the average annual surface temperature, but do continue to get small drops due to the geothermal gradient. The geothermal gradient is relatively shallow - you need to go down a thousand meters to see tens of degrees drop.
- wcoenen 1y ago
  > Is this because of geothermal energy leaking upwards No. The heat energy comes from the sun. Power flux from geothermal is measured in milliwatts per square meter, while the sun can provide more than a kilowatt during the day. So real geothermal heating is negligible at the surface. That's why the temperature a few feet down equals the average annual temperature at the surface. The only reason people call this "geothermal" is because marketing people realized that this sounds more impressive than "ground source heat pump". It really should not be called "geothermal", because that's something very different. Real geothermal involves extremely deep drilling (not feasible for residential use) or unusual geology.
  - westurner 1y ago
    Geothermal energy: https://en.wikipedia.org/wiki/Geothermal_energy https://en.wikipedia.org/wiki/Geothermal_energy Geothermal heating > Extraction (GCHE, GHX) || Ground source heat pump (GSHP) https://en.wikipedia.org/wiki/Geothermal_heating https://en.wikipedia.org/wiki/Geothermal_heating GSHP: Ground source heat pump: https://en.wikipedia.org/wiki/Ground_source_heat_pump https://en.wikipedia.org/wiki/Ground_source_heat_pump Heat pump: https://en.wikipedia.org/wiki/Heat_pump https://en.wikipedia.org/wiki/Heat_pump #Types : > Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps. Heat pump > Types: - SAHP: Solar-assisted heat pump; w/ PV - acronym for a heat pump with TPV thermophotovoltaic heat to electricity: - acronym for a heat pump with thermoelectric heat to electricity: - TAHP: Thermoacoustic heat pump - ECHP: Electrocaloric heat pump Electrocaloric effect > Electrocaloric cooling device studies: https://en.wikipedia.org/wiki/Electrocaloric_effect#Electrocaloric_Cooling_Device_Studies https://en.wikipedia.org/wiki/Electrocaloric_effect#Electroc... GCHE, GHX: Ground-coupled heat exchanger: https://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger https://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger Acronyms! From https://www.google.com/search?q=Ground-coupled+heat+exchanger+acronym https://www.google.com/search?q=Ground-coupled+heat+exchange... : HGHE: Horizontal Ground Heat Exchanger: a GCHE installed horizontally e.g. in trenches VGHE: Vertical Ground Heat Exchanger: GCHE installed vertically e.g. in boreholes or piles. PGHE: Pile Ground Heat Exchanger: A specific type of GCHE that is integrated into the structural foundation piles of a building. Solar chimney or Thermal chimney: https://en.wikipedia.org/wiki/Solar_chimney https://en.wikipedia.org/wiki/Solar_chimney OTEC: Ocean Thermal Energy Conversion: https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversio... and the ecological salinity gradient: FWIU archimedes spiral turbines power some irrigation pumps in Holland at least. Is there an advantage to double/helical archimedes spirals in heat pumps if/as there is in agricultural irrigation? Screw turbine: https://en.wikipedia.org/wiki/Screw_turbine https://en.wikipedia.org/wiki/Screw_turbine Noiseless double-helical Achimedes spiral wind turbine on a pivot like a pinwheel: Liam F1 average output with 5m/s wind: 1500 kWh/yr (4.11 kWh/day); Weight: ~100 kg / ~220 lbs; Diameter: 1.5 m / 4.92 ft What about CO2 and heat pumps? Would a CO2 heat pump make sense? Absorption Heat pump (AHP) https://en.wikipedia.org/wiki/Absorption_heat_pump https://en.wikipedia.org/wiki/Absorption_heat_pump Adsorption Heat pump (AHP) CO2-Sorption Heat Pump: a Adsorption Heat pump (AHP) that uses CO2 as the adsorbate. NISH: Nano-Ionic Sorption Heat Pump; with e.g. sustainable hydrogels Is it better to just recover waste heat from other processes; in a different loop? LDES heat pump Supercritical CO2 heat pump Aerogels don't require supercritical drying anymore, There's also buoyancy. The pyramid builders may have used buoyancy in a column of heated bubbly water to avoid gravity, in constructing the pyramids as a solar thermohydrodynamic system with water pressure.
Aurornis 1y ago
> Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground! That’s not entirely insulation. Some of the heat flows upward toward the surface during winter and some warmth flows downward during summer. > If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling. Geothermal heating and cooling already exists. It’s semi-popular in some areas. It can be expensive to install depending on your geology and the energy savings might not compensate for that cost for many years. Modern heat pumps are very efficient even if the other side is exposed to normal outdoor air, so digging deep into the earth and risking leaks in the underground system isn’t an easy win.
- deleted 1y ago
  [deleted]
- afiori 1y ago
  the parent was not talking about geothermal, but storage which would come as interesting to store winter cold for the summer, I have no idea whether it would work. Based on some guesses and uninformed searches if a house spends 200MJ on cooling and there is a 20 C delta between winter temperatures and desired cooling temperature and assuming a specific heat capacity of ~800 J/(kg*K) you would need 12.5 tons of rock as battery which would be around 6~8 m³ which sound very small. I am sure that there are hundreds of complex factors at plays (eg rain water and aquifers reheating the battery during spring) but it came out to be a far smaller number than I would have guessed.
  - lazide 1y ago
    That’s because 200 MJ is a small amount of cooling. It’s only about 55 kWh.
    - afiori 1y ago
werdnapk 1y ago
10ft below ground is enough to take advantage of geothermal heat. You don't have to go "very far" to reach warmer soil in winter because the soil PLUS the snow on top is pretty much just insulating the deeper ground from the cold air. Start getting into permafrost though where the cold is more constant and that cold layer gets deeper.
- mrgaro 1y ago
  10ft is definitely not enough for practical use. In order to heat a rural house with a heatpump connected to geothermal you need in order of 200-300ft deep hole, at least here in Finland.
  - Ekaros 1y ago
    For ground source heat pumps you have two approaches. Either you have deep hole. Or you have a large field. In later case not as much depth is needed, but you do need much larger area.
    - mrgaro 1y ago
      Good point and very true!
- thijson 1y ago
  At a certain depth the temperature curve is 6 months delayed from the surface. So it's getting to its warmest during the winter, and coldest during the summer. At a deeper depth it's pretty constant year round.
jfengel 1y ago
Does the slow heat transfer interfere with attempting to use that heat? I can imagine that there's a lot of total energy in the dirt 10 feet down. But once you've tapped the energy near your well, how long does it take to replenish? How long until the immediate vicinity reaches equilibrium with the surface?
- pfdietz 1y ago
  Slow heat transfer (and the absolute need to minimize capex) drives the system to operate on very long (seasonal) time scales.
- abeppu 1y ago
  ... and similarly doesn't it mean the pile is slow to absorb heat when your PV installation is trying to dump energy into it?
  - progbits 1y ago
    You don't need to store it in the dirt. You use the dirt as insulation, and store in something like molten salt or whatever which can be pumped up to surface and has good thermal conductivity to then extract the heat. At least that's my understanding of all these systems.
    - ted_dunning 1y ago
      Read the article. He is talking about storing the heat in the dirt and he gives good economic reasons for that.
    - chairmansteve 1y ago
      The point of the article is that storing in the dirt is much simpler and cheaper than molten salt.
  - nyeah 1y ago
    It means the heat stays near the hot pipes for quite a while.
shrubble 1y ago
This is called PAHS, passive annual heat storage and has been tried in some alternative energy housing. You put pvc pipes into a hill of dirt that is covered by a plastic sheet or other waterproof membrane; during hot summer months you use a small fan to put heat into the pile; during winter the heat moves from the dirt to the house.
- ted_dunning 1y ago
  This is definitely not what the OP is talking about. He is talking about electrically heating very large amounts of dirt to temperatures of 600C or more. Your PVC tubing approach is talking about 50 times smaller swings.
Cthulhu_ 1y ago
Underground heat storage isn't new nor anything startuppy though, we're well beyond the "companies looking into it" stage. This page [0] mentions it's been around commercially since the 90's and experimentally since the (19)30's, and interest started in the 70's. But depending on your definition of this, it's been around for hundreds if not thousands of years. People used to cut ice out of frozen lakes and store it in underground basements for year-round cooling. And in arid climates they have windcatchers [1] and other techniques where they store the nighttime cool for usage during the day, or these [2] to store or even create ice, all without using electricity. [0] https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag... [1] https://en.wikipedia.org/wiki/Windcatcher https://en.wikipedia.org/wiki/Windcatcher [2] https://en.wikipedia.org/wiki/Yakhch%C4%81l https://en.wikipedia.org/wiki/Yakhch%C4%81l
- cyberax 1y ago
  The problem here is the density. Air conditioners are rated in tons of cooling, with even the small ACs rated at at least 1 ton a day. So even a small house can easily melt through its weight in ice during summer.
  - deleted 1y ago
    [deleted]
teeray 1y ago
> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later We already do, in a way: septic tanks
SomeHacker44 1y ago
This reminds me of the interesting articles over the last few years about the heat from the London metro system over the last century saturating the ground so the tubes have become extremely hot.
The_Fox 1y ago
With regards to wrapping your head around heat loss- this winter some work was done on our property while there was snow on the ground. A bunch of snow got covered in dirt. In the spring, maybe three weeks after all the snow on the ground had melted, I moved the pile and was surprised to find all the snow still frozen. It had been under maybe 18 inches of dirt. I was pretty surprised to see it.
beambot 1y ago
Wouldn't the thermal conductivity/insolution that makes this so appealing be a liability when you want to extract useful heat to use?
- zharknado 1y ago
  Yes, the article addresses this explicitly. The key criterion to make it viabile is steady long-term energy consumption for the winter months. Otherwise the cost to rapidly extract the heat gets too high and wrecks the economics. The application here is big, slow annual oscillations. Slow charge, slow discharge.
blackoil 1y ago
Tech Ingredients did a video on this last week. https://youtu.be/s-41UF02vrU?feature=shared https://youtu.be/s-41UF02vrU?feature=shared
- pfdietz 1y ago
  This video has nothing to do with the OP.
dylan604 1y ago
> 4 months of winter buffered by 10 ft of ground! I'm sorry, but you write this as if that's nothing. Making a 10 foot hole is a massive amount of energy being spent. It's a massive amount of weight as 1 cubic yard of dirt is roughly one ton. In 10 cubic feet, that's roughly 3.5 tons. I say this as someone that moved 6 cubic feet of dirt by myself with a shovel and a wheelbarrow. So to think of 10 feet of dirt as a slow insulator would have to be one of the worst insulators out there.
- pfdietz 1y ago
  > Making a 10 foot hole is a massive amount of energy being spent. If the dirt has a density of 2, then lifting out a 10 meter cube would require 55 kWh of energy, which would cost a few dollars.
coryrc 1y ago
It's already done and proven to work further North than anywhere in the USA: https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community We (USA) could have 80% of our Northern homes off fossil fuel and electric heat for less cost if we were a little more forward thinking and willing to work together. But after nearly two decades they're decommissioning because the one-off components needed too much NRE to refurbish. If we all adopted this it'd be cheaper than what we pay today and zero greenhouse gas emissions. It'd finally make living in the temperate climates more climate-friendly than the warmer latitudes.
- V__ 1y ago
  > if we were a little more forward thinking and willing to work together it's really depressing to read this and deep down immediately know: well so that's never going to happen then.
- cogman10 1y ago
  Yeah, district heating/cooling would make us so much more efficient. It wouldn't even take a whole lot of space or land to implement. You could stick it right next to a community water tower.
- sokka_h2otribe 1y ago
  Uhh, I think a big reason why solar thermal is more meh these days is that solar panels are so cheap and heat pumps are reasonable
  - defrost 1y ago
    The article is talking of the revival of [solar {...} thermal] storage precisely because solar panels are so cheap and heat pumps are reasonable. The {...} is the counter intuitive step of solar -> electric -> heat storage for six months -> electricity for later. The secret sauce is increased heat deltas, not just heating dirt directly using sunny daytime temperatures but really cranking up the heat of an underground mass to 500C + (IIRC - I skimmed the article some hours ago).
cyberax 1y ago
> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling. This is literally what ground-loop heatpumps are doing. The ground loop is used as an energy source in winter, and since water is always at 0C, the heat pump efficiency can always be around 500%. And vice versa in summer.
dgacmu 1y ago
Dirt has an r-value of between 0.25 and 1 per inch according to some quick googling, so 10ft would be maybe R-60. Surprisingly, that's only equivalent to about 10" of polyiso rigid foam. What this project is really taking advantage of is the super cheap thermal mass. Dirt has about a quarter of the specific heat of water, but it is, literally, dirt cheap, and much easier to keep in place than a liquid.
- ted_dunning 1y ago
  You can also have a temperature cycle of 600C with dirt. Water is limited to 50-70C so the dirt loses a factor of 4 in specific heat, but wins back a factor of 10 due to delta T. The net is dirt wins by a factor of 2.5.
  - pfdietz 1y ago
    The thermal conductivity of rocks (and, I presume, soils) also typically declines as temperature increases, falling by a factor of 2 or 3 at 600 C.
tenthirtyam 1y ago
I really can't help but think there might be a fundamental problem here. If heat loss in dirt is so incredibly slow, and we use a heat pump to extract that heat, then wouldn't it be equally slow to replace the heat extracted? (absent heat injection, i.e. reversing the heat pump's operation in summer) Has anyone looked at the subsurface ground temperatures after days, weeks, months, even years of heat pump operation? I do seem to remember seeing one article on the subject showing that after one winter the subsurface temperature had declined enough to materially affect the heat pump's COP. But the timescale didn't extend to multiple years. edit: found this one: https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/Mohammadzadeh.pdf https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/M... In one of eight cases studied where heat flow is unidirectional (cooling load only) over a 20 year timescale the authors find: "the mean ground temperature ... increased from 21.87 °C to 26.18 °C, ... . This significant rise could have a potential impact on the performance of the system in the later years of its operational life." The other 7 cases showed weaker or negligible long term variation. An additional graph shows COP variation over 15 years and the worst case shows a decline of perhaps 10% (just eyeballing it). Surprisingly, some cases showed a long term improvement in heating COP - presumably the injection of summer heat into the soil made for warmer soil than just sunshine and natural diffusion? So my takeaway: "it depends." :-)
- pfdietz 1y ago
  There are no heat pumps involved in the OP idea.
  - tenthirtyam 1y ago
    Oh! You're right, I hadn't realized. But... wouldn't it be more efficient with a heat pump and exploit the 2-4x COP? Ok, maybe I get it now: heat pump wouldn't work at the high temps they are achieving. Thx.
    - pfdietz 1y ago
      Why are you optimizing for efficiency rather than overall cost?
IAmBroom 1y ago
You are confusing the concepts of insulation and reservoirs. When 5 of the 6 semiaxes of possible heat flow have no temperature gradient, the temperature becomes much more stable. Insulation would be if a 10 ft radius dirt ball maintained a stable temperature all year round - which would surprise me, although dirt does have some insulative value. However, wet dirt is really not very insulative - try sleeping on the ground sometime.
- pfdietz 1y ago
  The thermal time constant of a 3d object scales as the square of the linear dimensions of the object, so time constants measured in years are quite achievable. The dirt here will not be wet in most of its volume -- the initial charging will bake out all volatiles. The hottest part of the pile will become something like dry brick. It's like storing energy in ziggurats.

profsummergig 1y ago

> you only need to go down 10 ft or so before you hit the average annual temperature Is this because of geothermal energy leaking upwards? If so, it's not the dirt, it's the geothermal energy.

vasco 1y ago
Had the same thought, we'd have to put a thermometer inside a 10ft cube full of dirt for science.
- profsummergig 1y ago
  > a 10ft cube full of dirt suspended in air
werdnapk 1y ago
Yes, the you can put "thick" insulation over top of any buried plumbing and the exposed bottom will gain geothermal heat from the below and it can prevent freezing.
adverbly 1y ago
Its a bit of both, but its primarily due to the high insulation. There are 2 gradients: The surface gradient is what I mentioned about and its quite steep(only a few meters to drop tens of degrees). After that, you reach approximately the average annual surface temperature, but do continue to get small drops due to the geothermal gradient. The geothermal gradient is relatively shallow - you need to go down a thousand meters to see tens of degrees drop.
wcoenen 1y ago
> Is this because of geothermal energy leaking upwards No. The heat energy comes from the sun. Power flux from geothermal is measured in milliwatts per square meter, while the sun can provide more than a kilowatt during the day. So real geothermal heating is negligible at the surface. That's why the temperature a few feet down equals the average annual temperature at the surface. The only reason people call this "geothermal" is because marketing people realized that this sounds more impressive than "ground source heat pump". It really should not be called "geothermal", because that's something very different. Real geothermal involves extremely deep drilling (not feasible for residential use) or unusual geology.
- westurner 1y ago
  Geothermal energy: https://en.wikipedia.org/wiki/Geothermal_energy https://en.wikipedia.org/wiki/Geothermal_energy Geothermal heating > Extraction (GCHE, GHX) || Ground source heat pump (GSHP) https://en.wikipedia.org/wiki/Geothermal_heating https://en.wikipedia.org/wiki/Geothermal_heating GSHP: Ground source heat pump: https://en.wikipedia.org/wiki/Ground_source_heat_pump https://en.wikipedia.org/wiki/Ground_source_heat_pump Heat pump: https://en.wikipedia.org/wiki/Heat_pump https://en.wikipedia.org/wiki/Heat_pump #Types : > Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps. Heat pump > Types: - SAHP: Solar-assisted heat pump; w/ PV - acronym for a heat pump with TPV thermophotovoltaic heat to electricity: - acronym for a heat pump with thermoelectric heat to electricity: - TAHP: Thermoacoustic heat pump - ECHP: Electrocaloric heat pump Electrocaloric effect > Electrocaloric cooling device studies: https://en.wikipedia.org/wiki/Electrocaloric_effect#Electrocaloric_Cooling_Device_Studies https://en.wikipedia.org/wiki/Electrocaloric_effect#Electroc... GCHE, GHX: Ground-coupled heat exchanger: https://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger https://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger Acronyms! From https://www.google.com/search?q=Ground-coupled+heat+exchanger+acronym https://www.google.com/search?q=Ground-coupled+heat+exchange... : HGHE: Horizontal Ground Heat Exchanger: a GCHE installed horizontally e.g. in trenches VGHE: Vertical Ground Heat Exchanger: GCHE installed vertically e.g. in boreholes or piles. PGHE: Pile Ground Heat Exchanger: A specific type of GCHE that is integrated into the structural foundation piles of a building. Solar chimney or Thermal chimney: https://en.wikipedia.org/wiki/Solar_chimney https://en.wikipedia.org/wiki/Solar_chimney OTEC: Ocean Thermal Energy Conversion: https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversio... and the ecological salinity gradient: FWIU archimedes spiral turbines power some irrigation pumps in Holland at least. Is there an advantage to double/helical archimedes spirals in heat pumps if/as there is in agricultural irrigation? Screw turbine: https://en.wikipedia.org/wiki/Screw_turbine https://en.wikipedia.org/wiki/Screw_turbine Noiseless double-helical Achimedes spiral wind turbine on a pivot like a pinwheel: Liam F1 average output with 5m/s wind: 1500 kWh/yr (4.11 kWh/day); Weight: ~100 kg / ~220 lbs; Diameter: 1.5 m / 4.92 ft What about CO2 and heat pumps? Would a CO2 heat pump make sense? Absorption Heat pump (AHP) https://en.wikipedia.org/wiki/Absorption_heat_pump https://en.wikipedia.org/wiki/Absorption_heat_pump Adsorption Heat pump (AHP) CO2-Sorption Heat Pump: a Adsorption Heat pump (AHP) that uses CO2 as the adsorbate. NISH: Nano-Ionic Sorption Heat Pump; with e.g. sustainable hydrogels Is it better to just recover waste heat from other processes; in a different loop? LDES heat pump Supercritical CO2 heat pump Aerogels don't require supercritical drying anymore, There's also buoyancy. The pyramid builders may have used buoyancy in a column of heated bubbly water to avoid gravity, in constructing the pyramids as a solar thermohydrodynamic system with water pressure.

Aurornis 1y ago

> Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground! That’s not entirely insulation. Some of the heat flows upward toward the surface during winter and some warmth flows downward during summer. > If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling. Geothermal heating and cooling already exists. It’s semi-popular in some areas. It can be expensive to install depending on your geology and the energy savings might not compensate for that cost for many years. Modern heat pumps are very efficient even if the other side is exposed to normal outdoor air, so digging deep into the earth and risking leaks in the underground system isn’t an easy win.

deleted 1y ago
[deleted]
afiori 1y ago
the parent was not talking about geothermal, but storage which would come as interesting to store winter cold for the summer, I have no idea whether it would work. Based on some guesses and uninformed searches if a house spends 200MJ on cooling and there is a 20 C delta between winter temperatures and desired cooling temperature and assuming a specific heat capacity of ~800 J/(kg*K) you would need 12.5 tons of rock as battery which would be around 6~8 m³ which sound very small. I am sure that there are hundreds of complex factors at plays (eg rain water and aquifers reheating the battery during spring) but it came out to be a far smaller number than I would have guessed.
- lazide 1y ago
  That’s because 200 MJ is a small amount of cooling. It’s only about 55 kWh.
  - afiori 1y ago

werdnapk 1y ago

10ft below ground is enough to take advantage of geothermal heat. You don't have to go "very far" to reach warmer soil in winter because the soil PLUS the snow on top is pretty much just insulating the deeper ground from the cold air. Start getting into permafrost though where the cold is more constant and that cold layer gets deeper.

mrgaro 1y ago
10ft is definitely not enough for practical use. In order to heat a rural house with a heatpump connected to geothermal you need in order of 200-300ft deep hole, at least here in Finland.
- Ekaros 1y ago
  For ground source heat pumps you have two approaches. Either you have deep hole. Or you have a large field. In later case not as much depth is needed, but you do need much larger area.
  - mrgaro 1y ago
    Good point and very true!
thijson 1y ago
At a certain depth the temperature curve is 6 months delayed from the surface. So it's getting to its warmest during the winter, and coldest during the summer. At a deeper depth it's pretty constant year round.

jfengel 1y ago

Does the slow heat transfer interfere with attempting to use that heat? I can imagine that there's a lot of total energy in the dirt 10 feet down. But once you've tapped the energy near your well, how long does it take to replenish? How long until the immediate vicinity reaches equilibrium with the surface?

pfdietz 1y ago
Slow heat transfer (and the absolute need to minimize capex) drives the system to operate on very long (seasonal) time scales.
abeppu 1y ago
... and similarly doesn't it mean the pile is slow to absorb heat when your PV installation is trying to dump energy into it?
- progbits 1y ago
  You don't need to store it in the dirt. You use the dirt as insulation, and store in something like molten salt or whatever which can be pumped up to surface and has good thermal conductivity to then extract the heat. At least that's my understanding of all these systems.
  - ted_dunning 1y ago
    Read the article. He is talking about storing the heat in the dirt and he gives good economic reasons for that.
  - chairmansteve 1y ago
    The point of the article is that storing in the dirt is much simpler and cheaper than molten salt.
- nyeah 1y ago
  It means the heat stays near the hot pipes for quite a while.

shrubble 1y ago

This is called PAHS, passive annual heat storage and has been tried in some alternative energy housing. You put pvc pipes into a hill of dirt that is covered by a plastic sheet or other waterproof membrane; during hot summer months you use a small fan to put heat into the pile; during winter the heat moves from the dirt to the house.

ted_dunning 1y ago
This is definitely not what the OP is talking about. He is talking about electrically heating very large amounts of dirt to temperatures of 600C or more. Your PVC tubing approach is talking about 50 times smaller swings.

Cthulhu_ 1y ago

Underground heat storage isn't new nor anything startuppy though, we're well beyond the "companies looking into it" stage. This page [0] mentions it's been around commercially since the 90's and experimentally since the (19)30's, and interest started in the 70's. But depending on your definition of this, it's been around for hundreds if not thousands of years. People used to cut ice out of frozen lakes and store it in underground basements for year-round cooling. And in arid climates they have windcatchers [1] and other techniques where they store the nighttime cool for usage during the day, or these [2] to store or even create ice, all without using electricity. [0] https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag... [1] https://en.wikipedia.org/wiki/Windcatcher https://en.wikipedia.org/wiki/Windcatcher [2] https://en.wikipedia.org/wiki/Yakhch%C4%81l https://en.wikipedia.org/wiki/Yakhch%C4%81l

cyberax 1y ago
The problem here is the density. Air conditioners are rated in tons of cooling, with even the small ACs rated at at least 1 ton a day. So even a small house can easily melt through its weight in ice during summer.
- deleted 1y ago
  [deleted]

teeray 1y ago

> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later We already do, in a way: septic tanks

SomeHacker44 1y ago

This reminds me of the interesting articles over the last few years about the heat from the London metro system over the last century saturating the ground so the tubes have become extremely hot.

The_Fox 1y ago

With regards to wrapping your head around heat loss- this winter some work was done on our property while there was snow on the ground. A bunch of snow got covered in dirt. In the spring, maybe three weeks after all the snow on the ground had melted, I moved the pile and was surprised to find all the snow still frozen. It had been under maybe 18 inches of dirt. I was pretty surprised to see it.

beambot 1y ago

Wouldn't the thermal conductivity/insolution that makes this so appealing be a liability when you want to extract useful heat to use?

zharknado 1y ago
Yes, the article addresses this explicitly. The key criterion to make it viabile is steady long-term energy consumption for the winter months. Otherwise the cost to rapidly extract the heat gets too high and wrecks the economics. The application here is big, slow annual oscillations. Slow charge, slow discharge.

blackoil 1y ago

Tech Ingredients did a video on this last week. https://youtu.be/s-41UF02vrU?feature=shared https://youtu.be/s-41UF02vrU?feature=shared

pfdietz 1y ago
This video has nothing to do with the OP.

dylan604 1y ago

> 4 months of winter buffered by 10 ft of ground! I'm sorry, but you write this as if that's nothing. Making a 10 foot hole is a massive amount of energy being spent. It's a massive amount of weight as 1 cubic yard of dirt is roughly one ton. In 10 cubic feet, that's roughly 3.5 tons. I say this as someone that moved 6 cubic feet of dirt by myself with a shovel and a wheelbarrow. So to think of 10 feet of dirt as a slow insulator would have to be one of the worst insulators out there.

pfdietz 1y ago
> Making a 10 foot hole is a massive amount of energy being spent. If the dirt has a density of 2, then lifting out a 10 meter cube would require 55 kWh of energy, which would cost a few dollars.

coryrc 1y ago

It's already done and proven to work further North than anywhere in the USA: https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community We (USA) could have 80% of our Northern homes off fossil fuel and electric heat for less cost if we were a little more forward thinking and willing to work together. But after nearly two decades they're decommissioning because the one-off components needed too much NRE to refurbish. If we all adopted this it'd be cheaper than what we pay today and zero greenhouse gas emissions. It'd finally make living in the temperate climates more climate-friendly than the warmer latitudes.

V__ 1y ago
> if we were a little more forward thinking and willing to work together it's really depressing to read this and deep down immediately know: well so that's never going to happen then.
cogman10 1y ago
Yeah, district heating/cooling would make us so much more efficient. It wouldn't even take a whole lot of space or land to implement. You could stick it right next to a community water tower.
sokka_h2otribe 1y ago
Uhh, I think a big reason why solar thermal is more meh these days is that solar panels are so cheap and heat pumps are reasonable
- defrost 1y ago
  The article is talking of the revival of [solar {...} thermal] storage precisely because solar panels are so cheap and heat pumps are reasonable. The {...} is the counter intuitive step of solar -> electric -> heat storage for six months -> electricity for later. The secret sauce is increased heat deltas, not just heating dirt directly using sunny daytime temperatures but really cranking up the heat of an underground mass to 500C + (IIRC - I skimmed the article some hours ago).

cyberax 1y ago

> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling. This is literally what ground-loop heatpumps are doing. The ground loop is used as an energy source in winter, and since water is always at 0C, the heat pump efficiency can always be around 500%. And vice versa in summer.

dgacmu 1y ago

Dirt has an r-value of between 0.25 and 1 per inch according to some quick googling, so 10ft would be maybe R-60. Surprisingly, that's only equivalent to about 10" of polyiso rigid foam. What this project is really taking advantage of is the super cheap thermal mass. Dirt has about a quarter of the specific heat of water, but it is, literally, dirt cheap, and much easier to keep in place than a liquid.

ted_dunning 1y ago
You can also have a temperature cycle of 600C with dirt. Water is limited to 50-70C so the dirt loses a factor of 4 in specific heat, but wins back a factor of 10 due to delta T. The net is dirt wins by a factor of 2.5.
- pfdietz 1y ago
  The thermal conductivity of rocks (and, I presume, soils) also typically declines as temperature increases, falling by a factor of 2 or 3 at 600 C.

tenthirtyam 1y ago

I really can't help but think there might be a fundamental problem here. If heat loss in dirt is so incredibly slow, and we use a heat pump to extract that heat, then wouldn't it be equally slow to replace the heat extracted? (absent heat injection, i.e. reversing the heat pump's operation in summer) Has anyone looked at the subsurface ground temperatures after days, weeks, months, even years of heat pump operation? I do seem to remember seeing one article on the subject showing that after one winter the subsurface temperature had declined enough to materially affect the heat pump's COP. But the timescale didn't extend to multiple years. edit: found this one: https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/Mohammadzadeh.pdf https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/M... In one of eight cases studied where heat flow is unidirectional (cooling load only) over a 20 year timescale the authors find: "the mean ground temperature ... increased from 21.87 °C to 26.18 °C, ... . This significant rise could have a potential impact on the performance of the system in the later years of its operational life." The other 7 cases showed weaker or negligible long term variation. An additional graph shows COP variation over 15 years and the worst case shows a decline of perhaps 10% (just eyeballing it). Surprisingly, some cases showed a long term improvement in heating COP - presumably the injection of summer heat into the soil made for warmer soil than just sunshine and natural diffusion? So my takeaway: "it depends." :-)

pfdietz 1y ago
There are no heat pumps involved in the OP idea.
- tenthirtyam 1y ago
  Oh! You're right, I hadn't realized. But... wouldn't it be more efficient with a heat pump and exploit the 2-4x COP? Ok, maybe I get it now: heat pump wouldn't work at the high temps they are achieving. Thx.
  - pfdietz 1y ago
    Why are you optimizing for efficiency rather than overall cost?

IAmBroom 1y ago

You are confusing the concepts of insulation and reservoirs. When 5 of the 6 semiaxes of possible heat flow have no temperature gradient, the temperature becomes much more stable. Insulation would be if a 10 ft radius dirt ball maintained a stable temperature all year round - which would surprise me, although dirt does have some insulative value. However, wet dirt is really not very insulative - try sleeping on the ground sometime.

pfdietz 1y ago
The thermal time constant of a 3d object scales as the square of the linear dimensions of the object, so time constants measured in years are quite achievable. The dirt here will not be wet in most of its volume -- the initial charging will bake out all volatiles. The hottest part of the pile will become something like dry brick. It's like storing energy in ziggurats.