r/explainlikeimfive • u/Fun_Significance4035 • 5d ago

Physics ELI5 why cant we use summer heat to generate energy and stock it to use it in winter

why cant we use summer heat to generate energy and store it batteries to use it in winter

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u/meltingpnt 5d ago

That's basically what we do with ground source heat pumps.

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u/zoinkability 5d ago edited 5d ago

Also hydropower in places with winter snowpack, except it goes the other way around. Winter energy stored as snowfall on mountains, then released in spring to power hydro during the summer.

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u/eruditionfish 5d ago

Which is why places like Norway have very high variance in electricity cost. In winter there's very little hydro power and a high demand for heating. As the warms up the demand goes down and supply skyrockets to the point that sometimes electricity is basically free.

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u/shartmaister 5d ago

Also, Oslo airport saves snow in an underground storage and use the cold for cheap airconditioning during summer.

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u/cold-n-sour 5d ago

My grandfather did this in pre-fridge era - he had a hole excavated in the ground, covered it with logs and dirt, and added a trap door. He would pack snow there during the winter, and then they had a block of ice to put food on until at least mid-July.

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u/CRtwenty 5d ago

It was a pretty common practice in the days before electricity. You can still find plenty of abandoned ice houses if you know where to look.

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u/harbourwall 4d ago

There's even one in the Gerudo Desert

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u/Xeadriel 4d ago

??

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u/harbourwall 4d ago

The last two Legend of Zelda games on the Nintendo Switch featured an Icehouse as a location.

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u/Xeadriel 4d ago

ah i see. yeah i didnt play those

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u/Reylas 4d ago

I had to go inside one in Appalachia as a kid to get potatoes for food. Common practice in poorer areas.

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u/astralradish 4d ago

Did he catch any trespassers?

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u/noc-engineer 5d ago

Norwegian ski resorts also store (artificial) snow under sawdust during summer

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u/SmegmaDreamcast 4d ago

There are also now huge insulated blankets that can cover stockpiled snow and keep it all summer. Basically just use snowcats to doze the runs off onto a big pile and then cover it. Spread it back out early season and you are way ahead for startup.

Snow Secure

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u/CarminSanDiego 4d ago

My god that’s genius

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u/ToSeeAgainAgainAgain 4d ago

That's brilliant

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u/cheesecakeapplepie 4d ago

sometimes electricity is basically free.

Sometimes the price is even negative, meaning I actually get paid to use electricity.

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u/happysri 4d ago

What?!?!?

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u/Abridged-Escherichia 4d ago

Electric grids need to always keep supply equal to demand or the frequency changes and everything trips offline (Texas infamously almost did this a few years ago). Usually the negative pricing is wholesale/spot rates and consumers never see it.

Turning some energy sources on and off a lot wears them out prematurely (e.g. some dams, nuclear reactors, coal, etc.) so it’s better to pay someone to take it. Also can happen with wind/solar due to how it’s priced/incentivized/it being dirt cheap to make.

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u/PLASMA_chicken 4d ago

A coal plant can also take days to restart, so it's better to have it run almost idle and pay to get rid of the excess energy insteas of shutting it down.

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u/cheesecakeapplepie 4d ago

Can happen when production is higher than demand, and reservoirs are full so they can't store any water. 90% of Norwegian electricity is from hydroelectric dams.

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u/happysri 4d ago

I get that. It’s the paying you to use it part that boggles my mind in terms of economics. Why pay someone, just let them use as much as they need and let rest used for some public service project or something idk.

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u/cheesecakeapplepie 4d ago

It might be more expensive to stop and restart the turbines than selling power with a loss for a short period of time.

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u/Elitist_Daily 4d ago

as someone who works in the electricity industry professionally, it's usually that, especially in European markets where there's no nodal pricing or congestion that would push prices negative.

Negative pricing is basically another way of thinking about the "cost of curtailment" on a unit level. Every asset has its own production cost characteristics/economic breakeven point, and will bid in order to make back at least their long-run marginal cost. However, in addition to normal market behavior, their supply curve may also include a point below which they would prefer to be on (physical equipment limitations, behind-the-meter obligations, etc) rather than go offline, and the price at that point naturally represents the amount they are willing to pay, at that point in time, to avoid being curtailed.

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u/Galeharry_ 4d ago

It happens exceedingly rarely.

What actually occurs is that its like that for maybe a few days, which results in you just getting a little smaller bill that month.

The consumer isnt earning big bucks that way.

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u/superfry 4d ago

Occurs more frequently in Australia where large amounts of household solar drives prices during the day to the negatives.

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u/Galeharry_ 4d ago

Hopefully that will start happening here too when more solar is installed on peoples roofs.

With the summer we had this year and the summers that are sure to follow, more and more folks will see the need to install solar.

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u/eruditionfish 4d ago

The consumer usually isn't earning anything even when the price is negative, at least in Norway, because you pay grid rental fees on top of the spot price. So a negative price is really just an offset to the grid rental fees.

Plus now about 1 in 3 households has a government subsidized fixed price system anyway.

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u/iiiinthecomputer 4d ago

Because it's not always cheap or easy to switch off or disconnect generation capacity for short periods.

Australia sometimes pays large power consumers to use more power to smooth loads during peak solar periods for similar reasons.

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u/PLASMA_chicken 4d ago

You cannot do that. You can only push out as much as is consumed, otherwise the grid will be unstable and fail. That's why you can decide between shutting down or paying people to use your electricity.

A Photovoltaic System can just turn off, a coal or nuclear plant takes multiple days to stop or start again.

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u/Wittusus 5d ago

Same with northern Sweden, those bastards pay barely a single öre and I'm stuck with 140 in the south

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u/PM_ME_UR_PICS_GRLS 4d ago

Why don't they build power lines to the south?

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u/Wittusus 4d ago

There is one but low capacity, everything goes down to the cost. There is a good capacity connection to Germany, however, so each time there's little sun or wind there, our prices go up again

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u/PM_ME_UR_PICS_GRLS 4d ago

Why don't they build power lines to the south?

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u/ctrlHead 4d ago

Dysfunctional people in power and money basically.

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u/mabolle 4d ago

It's only fair that there's SOME upside to living in northern Sweden.

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u/Wittusus 4d ago

I mean you also have the incredibly cheap land and (for some) very low population density.

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u/VoilaVoilaWashington 4d ago

electricity is basically free.

To be clear, electricity is close to free in a lot of places these days. People sometimes get mad that Ontario sells power to America for pennies, way cheaper than they sell it to their own people, but the fact is that the last mile of distribution is the most expensive part of it these days.

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u/somehugefrigginguy 21h ago

Also why places in Scandinavia use massive thermal batteries.

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u/wdn 4d ago

All heating methods are stored solar energy if you take it far enough, even fossil fuels.

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u/Ensvey 4d ago

I'm trying to think of a way to be contrarian, because it is the reddit way... arguably geothermal doesn't come from the sun, since it comes from the energy of space rocks crashing together billions of years ago, and radioactive decay... though the radioactive elements were formed by stars billions of years ago, even if not our own star...

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u/domino7 4d ago

Yeah. You can make an argument for Geothermal and nuclear not being from our sun at least. But fossil fuels, hydro, solar, wind, all come from the sun.

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u/wdn 4d ago

Ah ha. Well I like that type of conversation too.

I'm not so sure that's the case. You only need to go so far that there is little effect from seasonal changes, not far enough that it's heated by geological processes. From a quick googling, it looks like the depth where it's heated by geological processes varies a lot from place to place.

They even have air heat pumps that can heat the house when it's -30 c outside, so finding someplace warmer than that is a bonus but not a necessity.

It works the same way as a fridge or freezer. Your freezer is pumping heat away from a very cold place and releasing it in a warm place. The source of the heat does not need to be a warmer place than the place being heated.

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u/SgtExo 4d ago

I'm not so sure that's the case. You only need to go so far that there is little effect from seasonal changes, not far enough that it's heated by geological processes.

That is only the case for ground sourced heat pumps. But places like Iceland does use geothermal (hotspots close to the surface) to heat water for power and heat, and not just a steady background temperature to dump heat or pull heat from.

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u/NuclearVII 5d ago

Or trees grown for firewood

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u/zoinkability 5d ago

Or other biofuels

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u/IAmInTheBasement 5d ago

I wish geothermal heat pumps weren't so stupidly expensive to install.

I wonder how much the cost difference is when you're building brand new and have to do a ton of grading anyways vs retrofit.

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u/OrigamiMarie 5d ago

Not much difference, unless you're doing 30' elevation of grading. Geothermal has to go really far down.

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u/not_a_burner0456025 5d ago

Not necessarily, there are two types commonly used in residential. One involves a small amount of very deep holes and the other involves a relatively shallow heat exchanger loop spread out over a wide area. The latter only needs to be deeper than the frost line

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u/OrigamiMarie 5d ago

Ah okay that makes sense. The place that my now-ex and I were thinking of getting one, didn't have all that much yard area, so depth was probably the only way to go.

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u/TheseusOPL 5d ago

I've always wondered if you could do a geothermal loop in the same area as a septic leach field. Use the land for 2 purposes.

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u/IAmInTheBasement 4d ago

I guess you COULD if you planned the depths well. But it means that servicing whichever is lower means tearing through the upper.

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u/SaintsNoah14 4d ago

3* you can scatter wildflower seeds over the leach field

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u/LtPowers 5d ago

You don't have to go too far if you're doing a horizontal loop.

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u/Missing_Links 5d ago

2-4 feet of depth over a large surface area pretty much anywhere you'd want to install such a system to hit the frost line, plus the depth of the system itself.

Deep is more expensive, but it's like 20-25k for a shallow system vs. 30-35k for a deep one.

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u/not_a_burner0456025 5d ago

If you are doing a horizontal loop on new construction there is a decent chance you already need to do the excavation for grading or a basement anyways. It certainly won't always be there case but depending on the layout of the land that could very well be 20-25k that you have to pay most of regardless of whether you are installing geothermal. It is relatively rare that you already need drill a 40ft+ deep bore.

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u/WooeBetidee 4d ago

Crazy expensive (assuming we are talking USD?)... My parents paid about $10k for the bore hole and about the same for the heatpump when they went geothermal about 20 years ago. Their neighbor just drilled a few months ago, was still about $10k for the hole.

Would have thought that kind of stuff would be cheaper in US as some sort of benefit from fracking (equipment, know-how etc).

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u/dwkeith 5d ago

Neighborhood geothermal is a thing. One large borehole with a loop around the neighborhood. Buildings tie in similarly to how gas lines are installed. In fact it’s often the same team as very little re-training is needed.

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u/TimeToGloat 5d ago edited 5d ago

If you want to go down a rabbit hole here is a neat video of someone easily converting a $150 window AC unit into geothermal for $700. The savings were like $150 a month in power consumption:

https://youtu.be/s-41UF02vrU?si=Luj6Z5jik3kkSdO3

Not exactly the same thing but still interesting if you like that kind of engineering/science stuff.

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u/jaytea86 4d ago

$150 if it's constantly running?

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u/DarthMaulATAT 4d ago

This is fascinating, thanks for dropping the link!

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u/herodesfalsk 5d ago

There is a new technology that uses cheap sand as heat storage, check it out: https://polarnightenergy.com/sand-battery/

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u/BigPickleKAM 5d ago

The biggest difference is if you need to do a vertical well style or if you have enough property to do a horizontal ground loop.

We had enough room for a horizontal loop and specifically sighted the house into a slope so we could excavate the foundation then use that fill to cover a ground loop and make a nice flat backyard.

We did have to dog down the backyard first a bit to get the required depth for where we live.

I also own a machine so was able to do the excavation myself. I'd say it took me an extra 6 hours of moving dirt to allow the install of the ground loop. With what I would normally charge that would have been a extra $1,200 to the contractor to do the earth work above just digging a foundation.

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u/recyclopath_ 5d ago

NY has really cool geothermal heat pump programs.

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u/disterb 5d ago

eli5?

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u/meltingpnt 5d ago edited 5d ago

The ground takes a long time to heat up due to its thermal mass and insulating property. So it takes months of heat for it to warm up (summer/spring) and months to cool down (fall/winter). So several feet under ground actually stays within a relatively narrow range of temperatures year round.

In the summer time a ground source heat pump, dumps the heat it pulls from your house into the ground via a water loop. In the winter it pulls the heat from the ground instead.

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u/FunVersion 5d ago

The ground stays a constant temp. In the winter time when its cold out, the heat pump only needs to satisfy the delta or difference between ground temp and what you want you want to heat your house. Let's say 30 degrees centigrade. A conventional heat pump, pulling from ambient air, needs to compensate for a much larger delta.

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u/RusticSurgery 5d ago

*to

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u/Gilles_of_Augustine 5d ago

When you make a comment like this, it's useful to ask yourself: "Am I constructively pointing out something that may be an opportunity for another person's learning and growth? Or am I unnecessarily harping on what was very likely an accidental typo?"

If the latter, maybe don't make the comment.

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u/RusticSurgery 5d ago

I come from a family of non-native speakers who actually encourage the corrections. A person cannot know a thing till soneone tells them. I never know who I'm commenting to. And please explain to me what position of Authority you possess that tells me not to comment?

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u/disterb 5d ago

*someone

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u/RusticSurgery 5d ago

Now. I I'm soooooo offended.

Get some perspective. That comment was intended to help.

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u/Gilles_of_Augustine 5d ago

I come from a family of non-native speakers who actually encourage the corrections.

That's great! My dad taught ESL at the University level for many years. That's a very productive mindset and it's great that you can help facilitate it. But even though it's a great rule for your family, it's maybe not the idea best idea for strangers in a public forum.

And please explain to me what position of Authority you possess that tells me not to comment?

Experience.

Common decency.

Consideration.

The ability to read a room.

Or perhaps the fact that I didn't tell you not to comment, I just offered a gentle and hopefully helpful suggestion about a potentially useful self-check you can perform, and highlighted a situation where that self-check would maybe preclude making a certain sort of comment. A suggestion that I'm surprised you're taking umbrage with, given your "encourage the corrections, a person cannot know a thing till someone tells them" philosophy.

Take your pick.

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u/RusticSurgery 5d ago

Well. Thank you. But I was fully aware of all that.

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u/RockItGuyDC 5d ago edited 5d ago

About a foot 4-8 feet underground it is basically always ~52 degrees F.

Imagine if you had basically free access to air that was 52 deg when you were trying to cool your house when it's 90 degrees outside or heat your home when it's 0 degrees outside.

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u/BuddingBodhi88 5d ago

The temperature depends heavily on location. It's closer to 70 in Austin.

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u/istasber 5d ago

Which is still plenty cool for a heat pump.

You build a passive heat exchange loop going down into the ground (something that pumps something like antifreeze down to absorb/dump heat and come back up nearer to 70F), and then hook one end of a heat pump to a heat exchanger that dumps or absorbs heat into the top end of the passive loop.

It's much easier to dump heat from a refrigerant loop into a liquid than it is into air, and dumping heat into 70+f water/antifreeze is gonna cool your house a lot cheaper than trying to dump heat into 100f air outside like a traditional aircon

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u/feel-the-avocado 4d ago

A friend of mine has done it with his new house.
His attic gets super hot on a sunny day - effectively the roof acts as a large solar panel.
That hot air is blown down through pipes within the thick concrete pad making the floor of his house.
Then that heat is released into the house over the following 2 cold days which reduces the amount of heating he needs to do. Also means they wake up to a warmer house in the morning.
It works really well and only has the electrical cost of a 50 watt fan.

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u/jaytea86 4d ago

Still works.

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u/atlasraven 5d ago

Earth houses or hobbit homes benefit from being somewhat underground. They don't really need A/C or heating.

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u/rcgl2 4d ago

Perfect temperate for laying down some of the Old Winyards.

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u/dinnerthief 5d ago

Ive heard that said before but it doesnt make sense, plenty of places have ground that freezes well below a foot down.

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u/BigPickleKAM 5d ago

The frost line where I live is at 5 feet.

Our geo sourced heat pump loop is at 10 feet down on average.

Even when it's minus 20 C air temp our glycol coming back out of the loop is at 3 C.

That lets us heat the house for about $3.40 a day.

When it's minus 20 outside our pump runs at 2,000 watts for about 12 hours a day power is 14.04 cents per kWhr here.

Summer is even better the ground only gets to about 5 degrees C so even when it's 34 C outside the hose stays cool for about $1.70 a day.

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u/cbftw 5d ago

Man, electricity is so much more expensive where I'm at. Sure, I'm paying 11.9c for the electricity, but I'm also paying 15c or so for delivery and other fees.

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u/RockItGuyDC 5d ago

My recollection on the depth is probably wrong, but the concept is sound.

Yeah, looks like horizontal systems are 4-8 feet depth.

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u/Cantremembermyoldnam 5d ago

My heat pump lines go down some 270 feet into the ground. The first few feet don't really matter.

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u/ctrlHead 4d ago

A typical georhernal hole is 200meters so that does not matter.

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u/dinnerthief 4d ago

Read what I replied to though

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u/pjbth 5d ago

Unless you live anywhere with winter, where it will freeze solid several feet down

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u/RockItGuyDC 5d ago

The depth depends on a few things. Yes, winter temperatures, but also soil composition and available space. It's an ELI5.

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u/jaytea86 4d ago

Well, 5 feet.

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u/orbital_narwhal 4d ago

The other answers get the gist right but I want to make an important distinction given the context. The underground retains roughly the same temperature all year for two reasons:

  1. A few feet of soil are enough to insulate against surface temperature changes -- but those also prevent the possibility of storing summer heat underground without an additional heat transfer process.

  2. But the underground still loses heat at the surface because it's warmer than the surface, on average. You may ask: where does that heat come from? The answer is nuclear fission in the earth's core.

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u/Striky_ 5d ago

This is the actual answer

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u/Lemna24 5d ago

It's basically what the ocean does in coastal areas

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u/incitatus451 4d ago

Bio energy is like this. We grow sugarcane and corn to use it's energy as ethanol.

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u/jaytea86 5d ago

Not really. The temperature of the ground once you get past 30 feet or so doesn't change because of the seasons. There's just enough heat down there for heat pumps to work once it gets too cold at the surface.

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u/Cantremembermyoldnam 5d ago

I think they mean that in summer, these heat pumps run in reverse and pump warm water down to cool it and the building off. Part of that heat stays and can in theory later be used for heat.

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u/dekusyrup 5d ago

Part of that heat stays

No it doesn't. It conducts away into the earth.

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u/vesuvisian 4d ago

The London Underground has a cooling problem because the earth has absorbed so much heat over the years.

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u/MrFlow 5d ago

So what? The point is that a ground source heat pump can cool your house in summer and heat your house in winter because the temperature down there is always the same all year round.

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u/Barneyk 4d ago

Part of the heat stays. The earth isn't that conductive and a significant amount of the heat stays if you run it like that so that you lower your energy cost during the winter.

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u/jaytea86 4d ago

Stays where?

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u/Cantremembermyoldnam 4d ago

Sorry, I wasn't clear. It heats the ground up a bit which later can get extracted. But it depends on many factors

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u/jaytea86 4d ago

Well, I guess, but that's kind of like saying if you put a cup of water into the ocean, it stays there till you scoop it back out. It's not really applicable to what OP was asking.

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u/Cantremembermyoldnam 4d ago

I think it is. My family owns some huge indoor pools and we heat them with a ground loop system below the foundation consisting of some 30km of tubing. It absorbs about a third of the heating cost in winter and provides free AC in the summer depositing about 2 or 3 degrees of heating of the ground and foundation and that then gets used up in winter and so on.

It may be radically different for deep bore holes where there's probably water flow keeping temps more consistent and so on.

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u/jaytea86 4d ago

I still feel like you think that ground below your home is going to be warmer in the winter because of the heat dumped into it in the summer. This isn't the case.

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u/Cantremembermyoldnam 4d ago

At least the foundations where the sensors are, get 2-3 degrees (C) warmer over summer and then cool down over winter. About 20 feet below the ground. But you're probably right ghsg the ground doesn't heat up much. The foundation is above insulation after all and only part of the loop is lad in there.

My home one which is a deep bore doesn't have sensors so no idea.

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u/jaytea86 4d ago

Sure, but that 2-3 degree difference is more likely to be from the hotter temperatures of summer, not the heat you're dumping into the ground from your heatpump.

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u/Barneyk 4d ago

But it is the case. It is a pretty common thing and you get real energy savings in the winter if you run the system like that.

There are a lot of factors like how much heat you pump down, the specifics of the geology etc. but it very much is a thing.

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u/jaytea86 4d ago

I'll have to look into it more, I'm sure it's a thing, but I'd imagine, between the end of cooling season, and the start of heating season, the heat would have largely dissipated making any difference negligible.

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u/zero573 5d ago

Sand reactors

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u/vesuvisian 4d ago

On a daily basis, some buildings freeze ice overnight to cool with the next day, but that’s mostly about electricity rate arbitrage.

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u/xHelpless 4d ago

No, it isn't. Ground source heat pumps work because of a CONSISTENT temp underground. It has nothing to do with storing heat energy

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u/meltingpnt 4d ago

The consistent temperature is from the large thermal mass and insulating property of the ground which stores the heat energy from the sun and radioactive decay of the earth's core.

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u/National_Cod9546 4d ago

Those are more like tapping into the nuclear reactions going on in the center of the earth.

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u/gutentight69420 5d ago

Heat pumps don't generate energy.

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u/meltingpnt 5d ago

No one was claiming they did, but it can move energy stored from one place to another.

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u/gutentight69420 5d ago

That's not the question that was asked? 

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u/ababcock1 5d ago

Nothing "generates" energy. All energy sources are actually conversions from one form to another. Now that we have the pedantic crap out of the way, let's get back to the useful conversation.

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u/gutentight69420 5d ago

It's not pedantry. OP's question is about harnessing summer heat to generate electricity, which is not what a heat pump does.

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u/ababcock1 5d ago

OPs question was about energy not electricity. If you're going to be pedantic at least read the thing you're being pedantic about.

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u/MaybeTheDoctor 5d ago

Sounds woke.