Hacker News Reader

Comment by phreeza | Hacker News Reader

I am always a bit bearish on geothermal, because the energy flow through the earths crust is just so damn low. Per surface area, it is about 3 orders of magnitude less than solar irradiation, which means that the circumstances in which it really makes sense to exploit geothermal are those where you can effectively harvest flows from a much larger area, most likely due to convection of water or magma. My understanding is that it will never make sense for e.g. every house in a suburban setting to have their own heat probe and pull energy from that, they will be competing with their neighbors and effective energy gained will be negligible.

View on HN

It’s often less about the heat flow than simply the amount of heat contained in that volume of rock.

For your example: A 1/2 acre home is 2023 m2, 1kg of rock is ~2000j/degrees Celsius, 1 cubic meter of rock is ~2500 kg, down 1k = ~2000 j * 2500 * 2023 * 1000 / 60 / 60 / 1000 ~= 2,800,000 kWh per degC. If you’re talking 1kw of heat on average from that rock you only drop 1 degree after 300 years.

Of course 1km is a fairly deep, but if you’re using a heat pump chances are you’re averaging much less than 1kw over the entire year.

Thanks that is a great point I hadn't considered. I suppose strictly speaking it is not renewable when you use it like that, but still pretty clean.

This is totally irrelevant to anything in the article, except in the sense that both pertain to thermal energy and the earth. They have nothing else in common. You can't run a factory, a computer, or a car off a ground-source heat pump, because heat pumps consume energy; they don't produce it.

Most of that post had nothing to do with heat pumps, however digging 1km down for a single family home is generally a poor method of extracting energy from the earth as it can be as little as 6C and is only 25C on average above ambient temperature.

Much higher ground temperatures are of course useful to create electricity, but such systems are best centralized not used for single family homes. Also, as a centralized system it’s volume is arbitrary. In the north that 6-25C can still make heat pumps vastly more efficient, but you’re extracting energy and cooling that down over time. Which is why it is relevant to the discussion.

I agree with most of that.

The great thing about geothermal is that it's 24/7 dispatchable without expensive added storage. That would make it a great companion to solar and wind energy in place of natural gas. Even 5-10% of total power on an absolute basis from geothermal could be more valuable than it appears by adding stability to the grid without fossil fuels. Drilling into geothermal would make sense if it were cheaper than adding grid-scale battery storage for nighttime use.

The energy flow from the Earth's core is small in a percentage sense, but keep in mind that humanity's energy use is actually tiny when measured on planetary or cosmic scales. Here's the total solar surface area we'd need, for scale:

https://www.axionpower.com/knowledge/power-world-with-solar/

Cover much of New Mexico with solar PV and you could power all of global industrial civilization (ignoring storage).

Sounds like the systems would work best where you have both heating and cooling loads throughout the year.

Though usually you’re pumping more heat out of the ground than in. There must be a perfect place for these systems where it’s well balanced.

Yeah, per-house is probably never going to make sense, but larger scale operations where they can use hydraulic fracturing techniques to expand the surface area could be the solution. They call these Enhanced Geothermal Systems, and basically they frack the rock between two well bores to try to maximize connectivity and surface area.

I’d say more than half of single family houses in Sweden are heated by geothermal heat pumps, so it’s _very_ viable.

But not as a source of electricity right?

is surface area a fair comparison? A deep well should be able to extract all along its length

How does a deep well extract all along its length? My belief was the production well will experience a gradient (so the bottom of the well will have the temperature you need), and the temperature will drop as you get closer to the surface (which also contributes to the calcite scaling problem geothermal can experience).

Also, I think a lot of wells add concrete casing (or metal, as indicated in the article) around portions of the well, which would prevent extraction around those zones of the well.

assuming you can reach a working temperature differential why not continue deeper with the well? From that threshold depth onwards you can extract heat along a line segment that extends to the ultimate depth (which is pressumably dictated by engineering limitations)

caveat: thinking like a physicist, not an engineer :-)

Reply on news.ycombinator.com