Geothermal Energy - Underground heat storage

Can it ever replace fossil fuels or other renewable energy sources ?
Will it work near the equator where the temperature gradient is not as steep compared to cold countries like Canada ? What would be the temperature at 500 feet below ground in say Los Angeles or Phoenix ?

Our airport’s new terminal uses geothermal energy to meet more than 50% of their heating and cooling needs. They drilled about 600 geothermal wells. Unfortunately they never provided any more details other than the below.

Geothermal Radiant Floor Heating and Cooling** – This system uses heat that has been extracted from the building during warmer months, and stored 500 feet underground, to heat the floors of the main terminal areas via radiant slabs and tubing in colder months. This creates a steady year-round temperature in the building, reducing our energy consumption overall.*

Our terminal is so warm that I break into an instant sweat the moment I come through the door. Can anyone shed more light on how they store the hot air in the wells and how this will work in winter when it drops to -40F on some days. Won’t they run out of the stored heat really fast ?
I am pretty sure it is co-generation thing where they have a traditional HVAC working in tandem.

There are a couple different types of geothermal heating. Direct use is where you tap into actual hot water or magma heat. Places like Iceland or little pockets here and there where you have hot springs. That’s what a lot of people think when they hear geothermal. That type is actually quite rare, and only exists in a handful of places on the planet.

The main type of geothermal heating is ground (or lake) source heat pumps. These don’t actually gather or store warm-to-the-touch heat, but extract small amounts of heat from a very large surface area (multiple wells hundreds of feet deep) and concentrate them into a smaller one (the house). The temperature of the ground they extract heat from is actually cold to the touch, but they use a condenser like on a refrigerator. Nothing in the fridge is as warm as the coils on the back of the fridge, but that heat was extracted from the cold space inside it.

The temperature hundreds of feet down is fairly uniform, so geothermal heating should work most anywhere. The amount of heat is pretty much limitless, so you won’t run out of heat to extract. BUT, the infrastructure needed to extract it can be expensive, and of course you need energy to power the system. I installed a geothermal system in my old house. It used an electric pump to circulate fluid through 4 back yard wells, and an electric fan extracted and distributed the heat from a big condenser and forced air system in the basement; it looked like a furnace.

There are some semi-passive systems which use little or no energy to circulate air or water, but these seem to be mostly one-off designs and are neither mainstream nor well-researched (aka I don’t know if they even work). So, current geothermal technology still relies on a power source to extract additional energy, so it’s not a complete replacement.

Overall it cost me just as much to heat with geothermal as it would have with natural gas. That was mainly based on the price of electricity at the time (which was high), and the low cost of gas. Some people get the impression that switching to geothermal eliminates a big percentage of costs; it didn’t for me.

Thank you so much for that detailed explanation.

I’m a BIG proponent of passive geothermal. Another way to look at it is, the ground is pretty constant around 55F. So in the summer, you have cheap access to 55 degree material to cool the air to whatever temp you wish. In winter, you can cheaply warm to 55, and only need supplemental energy (gas, solar, electricity…) to heat it up another 15-20 degrees or so to be comfortable.

Of course, it is easier/cheaper to install in some areas than others. Expensive if near surface bedrock.

But the main points against it are expensive installation costs (requires drilling as opposed to conventional forced air furnace/AC units) and the artificially low price of fossil fuels.

Passive geothermal sounds awesome.
At what average depth would we reach that constant 55F ?
Who absorbs the difference between actual production costs and the selling price for fossil fuels ?
I hope it is not subsidized by the government.

That temperature depends on where you are. 55F sounds about right for the northern US, but it’ll be warmer closer to the Equator, and cooler further away. Get far enough north (or south), and the ground temperature is below freezing, and you have permafrost.

That’s not really geothermal, though. That’s all heat that came from the Sun. It’s just averaging it out over the course of the year. Which does, usually, result in a more comfortable temperature for humans than the extreme, but it’s not a new energy source.

You can get a similar effect by putting a lot of water in and around your house. Water works the same way as earth for this, except you need less of it, because water has a much higher specific heat.

Not an expert, but it is pretty shallow. I believe 4-6’. Basically just below how far you need to sink fenceposts to prevent heaving in winter. They can either lay the piping out as a field, running back and forth, which requires a clear site pre-construction - or as a deep shaft in tighter spots. Drilling can be pricey.

In terms of the costs of fossil fuels, I’m also talking about externalities such as pollution, environmental degradation, Co2… You could even factor in military costs of maintaining access to the Middle East.


That’s not quite accurate. Potential exists in a lot of places, especially if we drill deeper. We humans have made quite a habit of living near places that have high geothermal potential, for good or ill. Well, except Australia.

But having said that, it’s never going to be more than a few percent of our global energy budget. 10% at most, if we really worked at it.

I meant the resource is only close to the surface and therefore cheap & easy to develop in a handful of places. Yes if we want to spend the money we could drill and eventually get to where it’s hot. But a big part of how well these projects works is the efficiency of the design and installation costs.

For my installation the drilling came to about 50% of the cost. Horizontal trenching is much cheaper if the space exists. You can do that yourself, but to drill you need to hire a drill rig which starts expensive and only gets really expensive the deeper you go.

There was a story on NPR (?) this morning talking about this very thing. The place in the story that installed the heat pump kind of system had to drill 23 wells to 400’. It was a large, old, building an the cost was over $1 million. Their gas bill went from $1200 to under $60 per month. The story said that you drill to bedrock. That depth varies depending on location. They also talked about regional systems where a neighborhood would share wells. Sounds promising but far off, quite expensive to install and a long time to recoup the cost.

If a neighborhood shares wells, will they not need more wells?

I looked that story up. They made the switch to geothermal at the correct time: they needed to replace their whole heating system anyway. If you’ve got an existing system that’s still running ok it’s harder to justify the high cost of a new geo system.

A couple things the story didn’t go into details on: they say the heat pump uses “very little” electricity, but don’t say what “very little” means. They also don’t mention how much power the circulating pumps use. So yes the gas bill dropped dramatically that’s expected (in my house I had no gas bill at all). But they didn’t mention how much their electric bill went up. Mine basically doubled. It can still make sense if the gas-to-electric cost ratios are favorable. Running my system now is probably cheaper than it was 12 years ago due to the price changes.