Warm and Fuzzy on Geothermal?

This is a guest post by Tom Murphy. Tom is an associate professor of physics at the University of California, San Diego. This post originally appeared on Tom's blog Do the Math.

The Earth started its existence as a red-hot rock, and has been cooling ever since. It’s still quite toasty in the core, and will remain so for billions of years, yet. Cooling implies a flow of heat, and where heat flows, the possibility exists of capturing useful energy. Geysers and volcanoes are obvious manifestations of geothermal energy, but what role can it play toward satisfying our current global demand? Following the recent theme of Do the Math, we will put geothermal in one of three boxes labeled abundant, potent, or niche (puny). Have any guesses?

The Physics of Heat

Thermal energy is surprisingly hefty. Consider that putting a room-temperature rock into boiling water transfers to it an equivalent amount of energy as would hurling it to a super-sonic speed! We characterize the amount of heat an object can hold by its specific heat capacity, in Joules per kilogram per degree Celsius (or Kelvin, since one degree of change is the same in either system). Tying some energy concepts together, the definition of a kilocalorie (4184 J) is the amount of energy it takes to raise 1 kg (1 liter) of water 1°C. So we can read the specific heat capacity straight away as 4184 J/kg/K. This is a rather large heat capacity, on the scale of things. As a rule of thumb, 1000 J/kg/K is a marvelously convenient universal number for most substances: it works for wood, air, rock, etc. Liquids tend to be higher (typ. 2000 J/kg/K), and metals tend to be lower (150–500 J/kg/K). Rocks—relevant for geothermal energy—range from about 700–1100 J/kg/K, and although I would be happy enough to use the convenient 1000 J/kg/K for crude analysis, I will be somewhat more refined and use 900 J/kg/K for rock in this post—although I feel silly for it.

As an example, to heat a 30 kg dining room table by 20°C, we need to supply 600,000 J. Just multiply specific heat capacity by the mass and by the temperature change. A 1000 W space heater could do it in ten minutes (600 seconds), if all of its energy could be channeled directly into the table.

The next property to understand is thermal conductivity: how readily heat is transported by a substance. Differing thermal conductivity is why different materials at the same temperature feel like different temperatures to our touch. It’s because high thermal conductivity materials (metals) slurp heat out of our hands much faster than plastic or wooden objects would. Copper has a thermal conductivity of 400 W/m/K, while stainless steel has an abysmally low value (for a metal) around 15 W/m/K—which is one reason why stainless steel is the preferred metal in kitchens: we can tolerate holding the spoon or pot handle even when another part of the item is quite hot. Plastics are around 0.2 W/m/K, and foam insulation tends to be around 0.02 W/m/K. Rock falls between 1.5–7 W/m/K, with 2.5 W/m/K being typical.

How do we apply thermal conductivity? Imagine a flat panel of stuff with area, A, and thickness, t. Using the Greek letter kappa (κ) to represent thermal conductivity, the rate at which thermal energy flows across the panel given a temperature difference ΔT across it is κAΔT/t, which comes out in Watts.

Sources of Heat

Two sources contribute to the Earth’s heat. The first, contributing 20% of the total, derives from gravity. As proto-planetary chunks fell together under the influence of gravity, the kinetic energy they carried (converted gravitational potential energy) ended up heating the clumps that stuck together. If this were the only contributor, Earth’s center would have cooled significantly below its present levels by today. The other 80% of heating is the gift that keeps on giving: long-lived radioactive nuclei given to us by ancient supernovae (as with most of the other elements comprising Earth and ourselves). Specifically—in order of significance to heating—we have 232Th, 238U, 40K, and 235U, with half-lives of 14, 4.5, 1.25, and 0.7 billion years, respectively. Ironically, one can view the radioactive contribution as gravitational in origin also! This is because supernovae result from fusion losing the fight to gravity, and the heavy elements are created in the resulting gravitational collapse.

In total, the radioactive decay produces about 7×10−12 W/kg; in the mantle. The mantle occupies 85% of the volume of the Earth at an average density about 5 times that of water, having a mass of about 4.5×1024 kg. Multiply these together to get 34 TW of heat flow in steady state. If radioactivity is 80% of the story, this implies 42 TW total. Averaging over the area of the Earth, we get 0.08 W/m². Because of the decaying nature of radioactive materials, the heat generation was much higher a few billion years back, making Earth a more geologically active place (e.g., more volcanoes).

We can work up another estimate of the total geothermal heat flow by observing that the temperature gradient in the crust is 22°C/km. This gradient can be used as the ΔT/t part of the thermal conduction heat flow rate, κAΔT/t. Taking a square meter for A and 2.5 W/m/K for κ, we calculate a geothermal “loading” of 0.055 W/m². Indeed, Wikipedia reports a land-based heat flow of 0.065 W/m² while the ocean (due to thinner crust and thermally greedy water) averages 0.1 W/m².

Compared to Human Use

Using the Wikipedia value of 0.065 W/m² over land, multiplying by land area yields 9 TW. Humans use 13 TW currently. So if we managed to catch every scrap of land-based geothermal flow (and could use it efficiently), we would not fully cover our present demand. Needless to say, we’re not remotely capable of doing this.

Diffuse Use vs. Hotspots

Naturally, some places are better than others for tapping into geothermal energy. A map of the continental U.S. in heat flow (below) reveals that the west has more flow than the east. A similar map for North America (including oceans) can be found on the SMU website. On a large regional basis, some spots in the U.S. dip down to 0.03 W/m², while some of the better regions reach up beyond 0.1 W/m².

Note that Yellowstone, in northwestern Wyoming, is the hottest zone.

Even so, we’re talking thermal gradients that are at most in the neighborhood of 35°C/km. In order to produce electricity in a heat engine, we are stuck with a maximum thermodynamic efficiency of (ThTc)/Th, where “h” and “c” subscripts refer to absolute temperatures of the hot and cold reservoirs, respectively. At 1 km depth, this amounts to only 10% (and in practice we tend to only get about half of the theoretical maximum efficiency). One needs to drill at least 3 km down before being able to take advantage of steam (at 27% max efficiency). A depth of 5 km reaches 38% maximum theoretical efficiency—so perhaps 20% practical efficiency. Making a 1 GW electricity plant operating on the steady-state geothermal flow would require canvassing an area 200 km on a side buried 5 km deep even in the better regions having 0.1 W/m². Realizing that we’re stuck with thermodynamic inefficiency, a geothermal network covering every scrap of land area on the globe would get less than 2 TW of power at 20% end-to-end efficiency.

So rather than mess with the pathetically impractical commonplace thermal gradients for the purpose of electricity production, we look to hotspots like the Yellowstone region, or places where hot springs and geysers can be found at the surface. Indeed, The Geysers in California hosts 1.5 GW of installed geothermal electricity, but the power output has declined by almost a factor of two in recent decades (it is possible to draw out heat faster than it is replaced by conduction).

Part of the Geysers plant, in Calif. Source: EERE

The U.S. has about 3 GW of geothermal electricity installed, out of the worldwide total of 10 GW. Surprising to me, Iceland has just 0.6 GW installed, but this is 30% of their national electricity production. Another surprise to me was that the Philippines also derive about 30% of their electricity from geothermal sources, amounting to 1.9 GW.

I don’t have any handy back-of-the-envelope way to estimate the abundance of hotspots. Out of the 9 TW of diffuse land-based heat flow, I might guess that something like 1% (90 GW) may be available in the form of geyser-like surface steam. In short, these are rare beasts.

Direct Use

Rather than try to generate electricity, we could use direct heat from geothermal, or use the thermal mass of the ground as a push-point for heat pumps. The latter should not be called geothermal, since it is not tapping into the geothermal heat flow. As such, I will ignore it here and return to it at some later time together with a discussion of heat pumps for controlled climate applications.

The difficulty with extracting heat from the ground is that the gradient is rather small. For instance, hot water in the home generally wants to be about 45°C. This requires drilling 1.5 km (about a mile) down to get this warm—certainly impractical for individual homes. It could possibly be effective for communities or cooperatives that distribute hot water to a number of houses/businesses. Using geothermal energy for home heating faces similar distribution challenges.

Sustainable Extraction

When drawing heat out of a region in the ground, that region will cool relative to its surroundings if heat is extracted at a rate faster than the nominal flow—leading to a depletion of thermal capacity. The replacement heat must ultimately come primarily from radioactive decay. Let’s ask how much rock volume needs to supply thermal energy for one house on a sustainable basis.

The average American household used 80 thousand cubic feet of natural gas in 2001 (apologies for old data and Imperial units). The gas is predominantly used for heating of one form or another: house, water, and food. 80,000 cf translates to about 800 Therms of energy per year, or 2700 W of continuous thermal power. Using our number from before that the mantle generates 7×10−12 W/kg, the average American home would need a rock mass of 4×1014kg, or a cubic volume 5 km on a side at a crustal density of 3.3 times that of water.

Can you believe this? All that volume for one house! This does not mean that the collection network needs to be this large. After all, heat is flowing from deeper down all the time. In this context, the average house needs to intercept an area 200 meters on a side at 0.065 W/m². Still quite a large outlay of piping 1.5 kilometers deep.

Thermal Depletion

What if we cheat and use a smaller collection network, relying on conduction to fill in with surrounding heat? How long will our resource be useful? I’ll spare you the derivation, but the recharge time via thermal conduction is proportional to density times specific heat capacity divided by thermal conductivity. Most importantly, it scales as the square of the dimension (think radius of the depleted zone). Using numbers for an egg (typical food will have values like: ρ ≈ 1000 kg/m³; κ ≈ 1 W/m/K; cp ≈ 2000 J/kg/K; R ≈ 0.02 m), I get a timescale of 800 seconds, or about 13 minutes. This is how long it would take an egg to cool down (or heat up in boiling water). Not to bad, as estimates go. Using numbers for rock, I get a one year time constant when R ≈ 5 m. Crudely speaking, this means we’d have access to a yearly “sustainable” volume—recharging in summer, for instance—around 500 cubic meters, holding 45 GJ (cpρVΔT) of thermal energy at a ΔT of 30°C. Used over a year, this provides something like 1400 W of average power—about half of the typically desired amount.

The danger is that once you try to go larger scale than this, the depletion volume gets larger, and the time to recharge scales up accordingly. Fundamentally, thermal depletion is a dimensional problem. You can draw out energy according to volume, but it is recharged according to area. So the problem is dimensionally stacked to come up short, leading to thermal depletion. This analysis deals with straight conduction. An underground fluid flow would change the story, and developed geothermal sites usually have this feature.

Damn the Depletion!

Still, if we don’t care about sustainable use of geothermal, we can just keep drilling new holes to deplete one region after the other. In this sense, we could evaluate the thermal endowment in the upper 5 km of crust under land. The average temperature in this layer is about 60°C above the surface value, so that each cubic meter (3300 kg) contains 180 MJ of thermal energy. Summed over 1.4×1014 m², we get about 1026 J. This is 250,000 years of our global appetite. A quarter-million years might seem close enough to indefinite that we’re willing to call it sustainable. Truly it is a substantial endowment. It’s the practical considerations that hold us off from rushing into this resource.

The energy derived is mostly useful for heat, being inefficient at producing electricity. It won’t fly our planes or drive our cars. And it’s buried under kilometers of solid rock, making it very difficult to access. Each borehole only makes available the heat in its immediate surroundings—unlike drilling for oil or natural gas, where a single hole may access a large underground deposit. So my guess is that we’ll burn every tree and fossil fuel on the planet before we start drilling through ordinary rock to stay warm. In other words, there is little incentive to dig deep for heat. By the time we run out of the easier resources—having burned every scrap of wood not bolted down—are we going to be left in a state to drill through rock at a massive scale?

In short, even though the thermal energy sitting under our feet is enormous in magnitude, it does not strike me as a lucky find. No one is racing to dig in. Perhaps it is simpler to say that it’s economically excluded, at present. And will it ever be cheaper to drill? For me, this falls into a category similar to space resources. Sure, they exist, but getting to them means that they might as well not be there, for practical purposes.

Geothermal In Perspective

Abundant, potent, or niche? Hmmm. It’s complex. On paper, we have just seen that the Earth’s crust contains abundant thermal energy, with a very long depletion time. But extraction requires a constant effort to drill new holes and share the derived heat among whole communities. Consider that two-thirds of our fossil energy goes up as waste heat, and often in cold environments. Waste is an appropriate word, in this context. But distributing the heat into useful places is a practical challenge to which we seldom rise.

Once we move to the steady flow regime, we get 9 TW across all land. This might qualify it as potent, except that practical utilization of the resource fails to deliver. For one thing, the efficiency with which we can produce electricity dramatically reduces the cap to the 2 TW scale. And for heating a home, we saw that we would need to capture zones well over 100 meters on a side. Recall that in similar fashion, the 1200 TW scale for wind dissipation was knocked way down to a handful of terawatts to account for the practically extractable portion—but still leaving it in the potent category. So realistically, steady-state geothermal fails to deliver, and lands in the “niche” box.

Clearly, geothermal energy works well in select locations (geological hotspots). But it’s too puny to provide a significant share of our electricity, and direct thermal use requires substantial underground volumes/areas to mitigate depletion. All this on top of requirements to place lots of tubing infrastructure kilometers deep in the rock (do I hear EROEI whimpering?). Even dropping concerns about depletion, the practical/economic challenges do not favor extraction of geothermal heat on a large scale. So geothermal is not giving me that warm, fuzzy feeling I seek. It’s certainly not riding to the rescue of the imminent liquid fuels crunch.

We’ll see nuclear fusion next week.

Hi Tom, thanks for another fascinating article.

There is an interesting story about the age of the Earth centered on heat flow and radioactivity. Prior to the scientific boom of the 19th Century, what was believed about the age of the Earth was based largely on the Bible and in particular on the calculations of Bishop Ussher who determined that the age of Earth was 4003 BC:

Ussher provides a slightly different time in his "Epistle to the Reader" in his Latin and English works:[4] "I deduce that the time from the creation until midnight, January 1, 1 AD was 4003 years, seventy days and six hours."


One of the first modern day estimates was provided by Lord Kelvin who based his calculations on heat loss and provided a range from 20 to 400 million years. Kelvin did not know about natural radioactivity - the heat within. Ironically, it is this very natural radioactivity that provided the tools to properly quantify the age of Earth - but it wasn't that simple. Early radiometric dates for crustal rocks varied enormously - that is because they give dates for events that occurred after Earth formed. It was not until U-Pb dates for meteorites were obtained that an age of 4550 million years became established as a close estimate in the mid 1950s. Wikipedia provide a good account:


Good post. There is an unfortunate ambiguity in the term 'geothermal'--it can be the kind discussed here, or it can be the much shallower kind, basically using the earth to store heat in the summer and store 'cool' in the winter (or, to put it the other way, to draw cool from in the summer and to draw relative warmth from in the winter.)

This form is more widely available and would seem to avoid the problems presented here of rapid draw down of the source. But it has it's own disadvantages of high up-front cost and dependence on electric power to cycle fluid through the system...Will there be a follow up article on this kind of geothermal?

The high up-front investment in digging or drilling I don't dispute, but I do know that there are ways to take good advantage of the ground's interseasonal Thermal Mass (if we are to distinguish it from Earth's 'Core Heat' ..), which can operate without electricity, and without compressor-based heat pump technology as well. As with so many of these things, it always involves which trade-offs you are willing to make, but it is eminently possible, and is even in use to this day.

It's striking to me that many of those bodies which died in the freezing cold might very well have never had their tissues freeze again once they were buried, since even at 6 feet below ground, the temps at most inhabited latitudes never see anything close to 32 deg F.

We built very Low-tech Cool Tubes into the foundation of our house in Stoneham in 1980, and natural Convection drew in enough earth-warmed air to keep our pipes from ever freezing, and downgraded the power of any of the cracks in the building from drawing the coldest winter air inside.

This is mentioned in the article, and differentiated from heat mining. It's just using the thermal mass of the ground as a push-point for heat pumps.

Well, yes and no. I'd question the dimunitive use of "just". Considering the amount of energy we currently use heating homes and other buildings in winter and cooling them in summer, the potential for avoided energy consumption in geothermal ballasting is enormous. (~half of our 12 TW of primary energy use? A big number anyway. Too lazy to look it up just now.)

It doesn't even necessarily require heat pumps to exploit it. The heat store can be engineered for a ~6 month lag behind ambient. In winter, it delivers warmish water from heat stored over the summer. In summer, it delivers coolish water from "cold" stored over the winter.

It works best as a district utility in new developments. A bit over a year ago, I published an article at Energy Pulse (here) proposing an "energy and shipping canal" for the Mexicali and Imperial Valley area. Part of what I envisioned was a corridor of new "eco-housing" developments around artificial salt water lakes and marshes along the canal route. (I know, utopian geo-engineering stuff, much derided here.) It wasn't in the article, but I've been playing with a building code that would go along with these developments. One of the things that the building code would require -- along with sewer, water, power, and com fiber to all homes -- would be a "thermal ballast" water loop. Two parallel pipes, one for delivery of water from the district store to homes, the other returning the water from homes to the district store. Coupled with other requirements for insulation and passive solar features, it would give the developments near zero energy consumption for heat and AC.

My irrational instinct is that the earths core keeps the atmosphere on. leave it alone.

I wondered the same thing. What unforeseen consequences would there everntually be from taking large amounts of energy out of the earth and pumping it (eventually) into the air? Does anything important depend on having the heat under there--even indirectly? Obviously, what we're doing now has all sorts of issues--but I wonder what knock-on effects this might have.

This has been my basic objection to OTEC at scale since I was in my teens. I see lesser problems with geothermal.

I also object to the sun delivering all that radiative output to the inner planets. One of them might maintain enough water to harbor life forms, which could evolve and eventually cause all types of problems for the ecosystems there, until they exhausted all the natural mineral concentrations and increased the entropy of the place. Heaven help us all if they figure out warp drive before they do all that and make it out of their little solar system stuck out there in the "boonies" of the galaxy.

Earths radius; 6371 km.

Deepest drill hole ever; Odoptu OP-11 Well; 12.345 km.

percent of earth penetrated; 0.193%

Even if you could draw heat from twice as far down as your borehole, you've done nothing to the mantle; as the Earth's crust is 30 to 50 km thick (on land at least).

I haven't verified the math, but the crust is supposed to make up less than 1% of the volume of the planet.

You will have to find something else to worry about. Fortunately, since you come here it won't be hard to do so ;-).


Deepest drill hole ever; Odoptu OP-11 Well; 12.345 km.

Odoptu, is a land based rig drilling into an offshore firld, with a TVD, (Total vertical depth) of less than 2000m. The number you quote ie the MD (measured depth) Exxon at Odoptu have drill these super long wells as extended reach wells, and would be reaching something like 9 to 10km offshore, but they are not very deep as in TVD.

Odoptu is offshore Sakhalin Is Russia, where the sea freezes from Dec to May, one of the reasons to drill from the beach.

Some numbers regarding surface energy:
- Solar energy provides about 240 W/m2 averaged over the entire surface the earth.

- Geothermal heat averaged over the surface is about 0.1 W/m2.

- World primary energy usage is about 15 TW (15*10^13 W), averaged over the surface of the earth (51*10^13 m2) this gives 0.3 W/m2.

- A doubling of CO2 greenhouse gas (GHG) adds approx. an additional 3 W/m2 averaged over the entire surface the earth.

Humans already release 3x more energy into the atmosphere as geothermal energy does.
Human energy released into the atmosphere is dwarfed by the daily averaged solar energy influx.
Human energy released into the atmosphere is dwarfed by the secondary effect of energy usage: rising GHG forcing.

Releasing a bit more geothermal energy to the atmosphere will not globally effect climate. Doubling CO2 levels adds 10x all the geothermal energy.

So if one worries about the influence of extracting geothermal energy on climate then adding all those GHG's through burning fossil fuels should be really worrying.

the concern [apparently unfounded] was the effect on the magnetosphere

"...taking large amounts of energy out of the earth and pumping it (eventually) into the air?"

The geothermal heat produced by the earth's natural radioactive decay already conducts to the surface and radiates 'into the air' around the globe, and has been for some 4 billion years. Nobody can stop this or even slow it down. The question then is whether some of that heat can be economically captured and put to good use while producing little or no emissions, or should it be allowed to continue conducting 'into the air' untouched.

Irrational maybe, but not necessarily wrong. I think it's now considered mainstream that the magnetic field from the inner dynamo keeps the solar wind from stripping the atmosphere to some degree.

Still, there's no scenario under which humans could significantly remove the planet's core heat, so rest easy.

I'll mention in passing that there's sufficient geothermal potential in hotspots to supply the entire human race with heat and electricity indefinitely. We just need to scale the human population down to the requisite size.

Great series of posts, kudos to the author.

Realistically we can only tap the heat in the upper few kilometers, transfer from the deep earth is only be very slow processes of condiction and convection. Now it turns out plate tecktonics depends upon mantle convection, which depends upon the interior being hot enough to promote convection (and perhaps the plates being hot enough at depth to be sufficiently plastic). And the recycling of a lot of stuff important to the biosphere depends upon plate tecktonics.

I am sure my concern is somewhat exaggerating the danger but in theory any change in the heat gradient brings up heat from below deeper in the core even if we source from the crust.

The top of the crust boundary condition on temperature when compared to the heat of the core isn't much different from absolute zero. So what happens at the surface (or the upper few kilometers) can have almost no effect deep down.

it would strip it all off. its what we have over places like Mars AIUI.

This article is very timely for where I live, so I'd like to first thank the author for writing it.

I live in Taiwan, and we just had a presidential and national assembly election a few days ago (Saturday, Jan 14). Of the two leading presidential candidates (there was a tiny third party that got just 2%), one was decidedly anti-nuclear and vowed to shut down all our nuke power plants, making up the loss of electricity with unspecified "alternative power." The other (incumbent) candidate says he'll gradually shut down our old generation II reactors, but keep the new generation III reactor which is set to open next year.

The incumbent won, so we'll keep the nukes for now. But the election did spur some (mostly uninformed) discussion about alternative power. Wind was considered to be the prime focus, because "it works in Denmark." Sort of. But I very much doubt it would work for Taiwan. We have a subtropical climate, very little wind most of the year, but some in winter. We are a small, crowded island, and I don't know where we could site wind towers other than on concrete platforms offshore. We do not have nearly enough hydro-electric power to provide the requisite backup for when the wind doesn't blow (which is most of the time). Given our climate, peak demand for electric power is in summer, when we are practically windless. But we do have lots of wind during typhoons - at which time the windmills would have to be shut down to avoid damage.

So on to geothermal. That's been touted by our politicians as the BIG SOLUTION No. 2, just next to wind. You see, we have some hot springs, so it's assumed that we are "rich in geothermal." But we aren't. There is just one volcano on Taiwan, and it hasn't been active for tens of thousands of years. Most of our mountains are the result of faulting. We are not another Iceland, or even like the Philippines (which has some active volcanoes, and very little electrical power demand because it's poor). Taiwan is relatively rich, power consumption per person is high, and I don't see how we can run the air-conditioners, high-speed railway, Taipei subway system, and microchip factories on geothermal. Or wind.

Solar is the last "solution." Actually, I have some modest hope for solar. It is pretty sunny here, especially in the south. But the urban jungle where most of the population lives doesn't look very well suited to rooftop solar. Your average apartment building is rather space-constrained. Some people do have rooftop solar hot water heaters. Solar photo-voltaic panels are not a common sight, though Taiwan does manufacture them.

Brasil, which is arguably considerably more tropical has some economically viable wind. Taiwan has some pretty impressive topography IIRC, which is why your hydro isn't bad. I wonder if WT at higher elevations might be useful. But, I doubt it could be a major power source, but it might help along the margins. Also ocean thermal gradient seems to be having a bit of a comeback.

A volcano need not be active in any recent time period for there to be a good geothermal resource a couple miles down a hole.


There's one application of geothermal capture that might have more universal utility especially in colder climates. Some years ago I saw a story about a rather low tech method of geothermal heating. A project done for a nursing home under construction in Georgia. Shallow wells (200'?) were drilled and plastic piping was u-tubed down these several holes. Rather simple after that: water was circulated around and the heat was captured via a liquid heat pump (I think). Cheap: a water well drill rig, two hands and several thousand $'s of piping. I don't think they anticipated it would supply all the heat required during the winter but would economically supplement that demand.

Obviously much cheaper than a full blown deep geothermal recovery project. More importantly: it could be done in many areas of the country. I don't recall if they planned to make use of the system for cooling. In Houston we don't have a great demand for heating except for short periods during the winter. But our constant subsurface temp is around 74 F. I wonder if such a system would supplement our high AC bills.

Geothermal Heating & Cooling
Geothermal Heating & Cooling is more cost-effective in Georgia than Houston. The equipment used is water-source heat pumps (WSHPs).

In climates like Houston, the big pay-off, if there is sufficient land to distance the wells, is to replace cooling towers used for chillers. The water savings can pay for such a system in 2-3 years.

Maybe I am wrong, but it seems to me that the 'heat pump' concept for HVAC uses some of the subsurface constant temperature as a basis for its deployment. Further, I would hazard a guess that solar heating of the soil has much to do with the constant 74 deg in Houston.

All in all the article is disappointing. Not in the sense of being wrong, or poorly written. It was well written and informative. I just didn't want to hear what it said.


There goes one of my heart held 'cures' for the energy dilemna. And there are so few of those!

Ah, well. Life goes on, at least for a while. The 'outer space' solutions reviled above though may have more merit, since 'outer space' will last a bit longer than Earth's residual geothermal reservoirs.


The heatpump solutions depend upon a longterm balance between heat removed and heat put into the underground thermal reservoir. If you tried it in say Alaska, you'd never return the heat you removed during the heating season, and after a few years the heat reservoir would freeze. Likewise if you tried it in Brasil or Saudi Arabia, also dumping heat into the reservoir, but not removing it, it would heat up until it became worthless.

Off-season, you run the process in reverse. If the dominant season is summer, you run cooling towers in the winter to meet long term balance. If the dominant season is winter, you run solar thermal in the summer to inject heat. It's thermal storage with ambient ground temps helping the storage medium on the margin.

What does that do to the ecomomics of it. Rather than getting a benefit during the offseason, you are expending energy to recharge the thermal reservoir.

You get off-season benefit to the extent you need heating or cooling in the non-dominant season (in other words your winter heating loads offset your summer cooling loads and vice versa). You are needing to move only a fraction of the difference between heating and cooling loads (since they offset each other and natural flows will move a major fraction in most climes). The economics (for large projects) in summer-dominated areas are purportedly good to use cooling towers since it shifts the rejection temperatures to ambient air at non-peak times/seasons (which is below ground temp). I have to say I am less familiar with the economics in the other direction.

Retd. engineer near me (Borders, UK) did just that with boreholes and heat pump. Through ex-glacial sand and gravel and intercepted several layers of moving water. He and his son back-filled round the vertical tubing carefully by hand to maximise contact! Tells me he gets 4:1 energy 'out' to energy 'in', so he just makes a bit more than 100% use of the energy going in to the power station. We don't need a/c in our summers.

This is how it's being done in individual house heating projects in The Netherlands. These systems rely on low quality (i.e. not very hot) heat at a relatively constant temperature and a heat pump to upgrade the heat so it can be used to heat the home and provide hot sanitation. In winter the system extracts heat from underground water layers and in summer it pumps excess heat back down the hole.

Alternatively one can lay a shallow (below the frost layer, say 1 to 2 meters deep) network of flexible piping (like a radiator) horizontally under a large surface of the garden. Again the goal is to access a relatively constant source of energy: soil that doesn't change much in temperature over the year. This is a very low cost solution.

Hi Rockman!
Providing part of your house heating with the method you mentiond is widespread in Sweden. I think its called downhole heatexchanger in English. Tapping to much heat from your ground water can drain the hole. If you buy a solarheater you can also heat your downhole in the summer. But the groundwater move so i don't know how effective it is.

One problem with the method is that it takes about 15% electric energy to drive the heatpump. Which is takes energy from the electric grid.

One well/hole plus a heatpumpinstalation cost between 100000-20000 dollars. The heatpump are supposed to work up to 20 years.

Swedish wikipedia states these figures:
Drill hole diameter = 127 mm (5 ")
Maximum energy to be tapped per meter of hole and year = 140 kWh
Downhole Depth 200 m. Effective water-hole depth = 190 m
Possible outlets heat energy = 190 mx 140 kWh = 26,600 kWh.

You need to space holes about 15 meters from each other.



milo - Thanks for the info. Good to hear someone out there has some common sense. Unfortunately for all the decades we had access to cheap energy it allowed us to be wasteful. Y'all were forced to be more efficient long ago and that should be a big help as we stumble down the PO trail.

Milo + Rockman, Drake Landing is a relatively new community in Canada, just south of Calgary, Alberta, that utilizes a district solar-ground exchange / storage loop to heat the homes in winter. Each house and garage has an array of solar thermal panels (hundreds of them) that collect summer heat and pump it into the ground via a community collection system to an array of 144 wells drilled to 37 metres and placed below a park in an area 35 metres in diameter. The array is covered with thick insulation, sand etc. The heat storage builds to 80 degrees C by the end of summer, and is extracted in winter and distributed back to the houses where a heat exchanger and blower distributes it to the interiors.


My only knocks on this project are that it required large public subsidies to offset a portion of the high capital costs, and that it assumes the typical form of inefficient suburban sprawl far outside of a major urban centre where public transit doesn't reach, hence extraordinarily high vehicle kilomtres driven per person, which tends to cancel some of the energy gains of the heating and storage system.

However, it works. Today's temperature with the wind chill was minus 29 degrees C.

What an elegant district heating and storage design. I would like to see a cost breakdown, both capital and levelized cost. Solar water thermal is relatively cheap, and I would think so would be the 144 37M wells when drilled all at the same spot? Rockman?

It seems to me a future powered by variable sources like solar and wind, this approach completely solves the technical aspect of scaling up energy storage for the portion of energy required for space/water heating (and cooling?). In the higher latitudes solar received power can vary 40:1 from summer to winter solstices, but this underground water storage approach works in year round time frames, bringing the higher latitudes to the solar table. Just for purposes of demonstration, some quick numbers show that a body of underground water the size of a smallish lake, say 1 million M^3, with a 60degC rise handles all the residential and commercial space and water heating needs of the US.

Elegant is an appropriate word to describe this solar-geothermal district heating system. According to their web site they are entering their fourth year of operation, and are approaching 90% 'solar fraction.' I think their success story is not attributable the technology alone, which is rather old hat now (solar thermal panels have been around long before the Grateful Dead got lazy and started playing country rock), but to the design which puts these two elements together in a very feasible way to address the crucial issue of heat storage in a cold climate

I have no data to back this up, but it strikes me as possible to try this system farther north where summer days are longer, knowing that the 'solar fraction' may well be lower on average due to the longer winters. I would suggest the building standards be rewritten to emphasize superior energy efficiency with superinsulation.

Financing is still an issue, and it may take a few more hits of triple-digit oil in the next few years and much higher demand / prices for natural gas to make it affordable by comparison. But splitting the capital cost down into fractions and making them integral with mortgages, and distributing system ownership equally among all users in a non-profit company covering an entire community may help assuage these concerns a little.

I hope they utilize such a system in more urban settings with low rise apartments and attached single-family row houses, because transportation and urban form are the other side of the urban energy issue. Breaking it down to an affordable single home system is also important. But these will require maintaining generous government energy grants, something that appears increasingly infeasible to Canada's prime minister who seems to be hell bent on securing a directorship on a Big Oil board when he leaves office.

One other thing I thought I'd bring up. Due to the big local debate on geothermal, I read up on it. One of the things I found out about was that the city of Basel, Switzerland, had a big geothermal experimental project going on under the city. The type of geothermal they were attempting is called "hot dry rock", where they pump down water though pipes in what would normally be impermeable rock that is hot enough to generate steam. The process tends to cool the rock, and perhaps that explains why it generated numerous small earthquakes. The quakes were severe enough to damage several buildings, resulting in millions of dollars in lawsuits and the project had to be shut down.

On a more positive note, Australia thinks there is potential to generate power from hot dry rock in some parts of the Outback. It's a desert region, very few people, so earthquakes aren't a big issue as far as humans are concerned, though the kangaroos might not like it. The power generated would hopefully be sufficient to make it feasible to export it via high-voltage lines to urban areas. I think they have one hot-spot in South Australia that looks particularly promising. However, it's still just talk - I'd like to see it working before judging how practical it is.

Thank you Tom!

I knew the potential of geothermal is grossly hyped, but I did not realize that the potential is so very poor in terms of the feasibility of utilizing it.

But I do think there are potential ways to "cheat" a little in many possible cases and extract some useful energy-for instance there are numerous large deep pools of brine found by geologists looking for oil, and there are probably many such pools in places where there is no potential for oil.

It might be possible to tap such a deep brine aquifer with a few wells and pump the cooled brine down hole after bringing it to the surface to capture the heat;and if the brine can move freely in its underground home, this would solve the problem of needing a huge system of deep plumbing.

A city of large town located over a shallow aquifer(one not subject to depletion for some reason or another) in a climate with both a pronounced heating and cooling season should potentially be able to make a very large groundwater heat pump system work in conjunction with a district heating and cooling system.Such a system properly designed would not deplete the thermal capacity of the aquifer.

Sometime back I read a piece about a geothermally heated and cooled greenhouse constructed for experimental purposes by a (?) university someplace in the northern part of the US..The system basically consisted of excavating a large pit underneath a large greenhouse and embedding a large number of small diameter u shaped pipes in the pit(before backfilling) with both ends above ground at either end of the greenhouse(erected after the pit was finished);one set of ends being connected to a manifold and a fan, so that air could be forced through all the pipes and emerge at the far end, and flow back through the greenhouse to the manifold end.

Apparently this worked well enough that the project was considered to be a technical success in terms of both cooling and heating the greenhouse.

If anybody can locate this and post a link to it, the audience here would enjoy reading it.

Ghung, you are a gentleman and a scholar without a doubt!


It's too bad we don't have the Campfire anymore!

There are so many ideas worth a freewheeling discussion!

It's interesting how these folks claim that condensation and evaporation (phase change) is improving the efficiency of these installations quite a bit. I'm considering such a system for my greenhouse, except that the site I've selected is only a couple of feet above the water table (bottom land), and I fear the tubes would flood. I also have questions about mold growth in the system. Needs further examining...

Only a couple of feet above the water table is good! Water is a much better conductor and thermal store than dirt. The tubes would need to be water tight, of course, but that shouldn't be a problem.

mac - Back in the late 70's the govt had a program giving big tax incentives to operators that drilled deep dry holes in Texas and La. Thus the millions spent to drill those wells were sunk cost and didn't play into the economics. So a number of pilot projects were attempted. I don't remember the details but even with most of the well costs already being expensed and the govt tax break the projects never worked economically. You don't have to drill much deeper than 14,000' in S La. to find big fat salt water sands above the boiling point. I think one of the problems was dealing with the salt concentrations especially after the water cooled off.

Your article seems to be based on the assumption that we use geothermal energy at a rate sustainable in perpetuity. Yet most geologists consider it as a one off heat mining operation. If we compare the amount of heat energy in the top few kilometers of the rust, we get a pretty decent number. I'd be willing to bet its several times larger than what is available from fossil fuels. So if it were affordable, it could buy a few centuries worth of BAU like living...

This is treated at length in the article, and is even larger than you think.

Appreciating your typo.. makes me wonder if the Tribe of Pangeaea would like to get a tee-shirt for their citizens entitled "Crust Never Sleeps" .. or is that just another entitlement?

One aspect of geothermal power not mentioned is that putting geothermal power installations in "hot spots" puts them in danger.

The article mentions Iceland and the Philippines as countries heavily reliant on geothermal power: one with a boreal cold climate, and the other tropical. What do they have in common? Recent major volcanic eruptions. Iceland had significant eruptions in 2010 and 2011, neither of which damaged geothermal installations, but that was probably more good luck than good management. The Philippines had a really major eruption in 1991 when Mount Pinatubo took its place as the second largest eruption in the 20th century. Clarke Air Base, 25km away, was effectively destroyed, and Subic Bay Naval Base, 75km away, sustained major damage. No geothermal power plants were in the affected area, but the area of damage depends on wind direction. And the Philippines has four other major volcanoes, a number of minor volcanoes, plus numerous "extinct" volcanoes such as Mount Arayat, for a total of 23 volcanoes on Luzon alone.

So we might conclude that good spots for geothermal power are not very good for long periods without eruptions. So they are not good places for major long-term capital projects.

I think that most volcanoes, which are not classified as active have an eruption probability per year of less than one in a thousand. That means a few percent risk that a facility might be heaviliy damaged by an eruption in its lifetime, probably not even the leading risk factor. Obviously you would like to spread your risks out geographically, so one eruption doesn't take out too much of your power supply.

FYI, if memory serves, and unless the plant is broken Nicaragua also has a substantial fraction of national electricity provided by geothermal from a plant on a volcano in the middle of lake Managua. Given that the mean time between eruption even of moderately active volcanoes is substantially higher than the mean life of a powerplant, I am not sure this is a big issue, even where the area being tapped is actively volcanic.

Yes, that makes sense. The same as it makes sense not to worry about building a nuclear power station in a tsunami-prone area when a really big tsunami only occurs every thousand years or so.

I assume you never drive or ride in an automobile since you might be killed in an accident?

Approximately 28,000 killed, and tens of thousands injured by the earthquake, and tsunami. Zero injured or killed by radiation. Why is it so obvious we should eliminate nuclear power? Almost all of the news coverage is about, “the nuclear disaster”, when that is a very minor part of the earthquakes damage.

Thousands of square kilometers uninhabitable for generations to come, billions upon billions to contain and cleanup the mess and a legacy for some future generation to decommission.

All that so we want cheap power now. I'm sure the future generations will be pleased with our gift from the past.

And this is equivalent to the risks of geothermal near a moderately active volcano, how again?

"Thousands of square kilometers uninhabitable for generations to come..."

The Fukushima nuclear disaster was certainly bad enough, but I think the quoted comment is a slight exaggeration. The evacuated zone around the nuclear plant (and the zone where human habitation is prohibited) is based on an area within 20 km (or 12 miles) of the plant. A circle with a 20 km radius has an area equal to about 1256 square km (or 452 square miles). However, the nuclear plant in question is right on the Pacific, along a reasonably straight coastline running north-south. As I see it, this would mean that almost half of the exclusion circle falls in the Pacific ocean where no humans reside anyway. So almost half of the potential exclusion area has no real effect on where people may live, leaving maybe 700 square km (252 square miles) of Japan as uninhabitable due to the accident (i.e. very unfortunate, but not really a matter of "thousands of square km").

As for the duration of the this exclusion area, the claim that this will be "for generations to come" may also be a bit too much. Many of the estimates I've seen (from various sources) say "decades" and others specify 20 years (not even one generation). There may be guesswork and predictions involved with putting a number on this, but consider that the cities of Hiroshima and Nagasaki were entered by American troops and others within a month or so of the atomic bombings. And during the 1950's both cities were completely rebuilt. As far as I know, both of these once massively irradiated cities are now fully inhabitable in all areas, and have been for some time.

Obviously *no* permanent structures should be built on tsunami-prone coasts, and they should be set off-limits to human habitation. All people living there should be evacuated immediately. Likewise all flood plains; and areas at risk of tornadoes or wild fires. ... Or, you know, not....

In regard to Iceland there are ongoing small quakes in the region of the country where they are injecting water for their geothermal program (as I look there were about a dozen in the last six hours). Click on the Hengill region at the Icelandic Met Office for a detailed closeup or the Reykjanes Peninsula for a more regional look. I think that the largest was about 1.8. They have had some up to about a 3, but the largest recorded was, IIRC, a 4.3 at the Geysers when they were doing some reinjection work.

Given the fact that this comes up pretty much everywhere there are geothermal powerplants, I think we can take it as given that they are causing the quakes. I tend to come down on the side of them releasing quakes and relieving pressure in areas that would be seismically active anyway, rather than increasing the likelihood of destructively sized quakes, although I'd be the first to admit that's uninformed speculation.

Can't we increase the 'efficiency' of this system by utilizing previously drilled holes eg mines & oil wells. Or fluids from deep underground eg hot oil? Circulating water through an exhausted oil reservoir?

It seems odd to drill holes for heat between all the oil wells. How hot is oil when it comes out, anyway?

Industry guys?

These were my thoughts as well; retasking the miles of played-out well bores, especially the horizontal/fracked holes being punched these days. Besides the environmental/plugging requirements, what are the problems involved with using these as thermal sources or sinks? I'm sure that many of these plays may be geologically unsuitable, but still....

Ghung - Here you go...great minds think alike: "Several companies already have begun to recognize this “stranded” heat energy resource and are beginning to harness this energy for electrical power generation in operations use."

From: http://www.aapg.org/explorer/2010/11nov/emd1110.cfm

I doubt its a lot of energy. Does this make sense for wells that are close to the grid, or does it only make sense for wells that are far from a powerline?

If there is a well then there is bound to be a grid nearby? Those old wells with the ja-knikker pumps (pumpjacks) need electricity right?

Sure, the locations of these former oil wells may be far from ideal for geothermal use, but you could also look at it as a very valuable head start, to be complemented by new drilling and new wells as needed. And I have read that the USA is the most intensively drilled region on earth, so the use of former oil and gas wells should be especially important for American geothermal considerations.

One type of proposal I've seen suggests using pairs of nearby former oil wells (or perhaps 1 former oil well matched with 1 newly drilled well) as the input and output conduits for water that gets heated to produce electricity. Rather than try to explain such a scheme any further, here is a link to several geothermal animations showing concepts for pairs of wells being used to produce electricity:


There are 4 animations at the link above, and the 3rd one on EGS (Enhanced Geothermal Systems) is especially interesting. It advocates fracking of target rock formations to create a subsurface network of fractures through the heated rock layer. And this fracking is continued until the openings are interconnected, such that water can flow between the 2 wells. This same animation also states that "induced seismic" activity can be useful for determining the extent of the fracture network. That sounds a little bit like using deliberate earthquakes to help with geological analysis (...very small earthquakes no doubt...right??).

Here in little 'ol New Zealand geothermal provides 13% of our electricity - the sector is undergoing something of a re-expansion with a series of plants on the drawing board:
However it seems unlikely that it will rise to more than 20% of our supply anytime soon. It is very important though in our renewables mix - we have had recent rapid expansion of wind, and geothermal represents a good baseload back up to that (as does hydro, a long as it rains!). Currently we get about 75% of our electricity from renewables, although that is factored to rise to 90% by 2025.

Depleting the earth's core sounds like a horrible idea, even if it were possible to extract 250,000 years of energy, the solidification of the interior and killing the magnetic field that keeps the surface from being irradiated isn't worth it. Unless we want a dead planet like mars of course :). Instead of wasting that heat on space heating and killing most of the life on earth, why not use solar gain buildings with lots of thermal mass and insulation a la www.earthship.org ?

menaus - The earth loses thermal energy to the atmosphere many thouands of time greater than man could ever hope to capture. The interior of the earth is constantly heated by the decay of radioactive particles. Comparably to trying to raise the salinity of the oceans by your peeing at the beach. LOL.

Here's everything anyone would every want to know about geothermal: http://www1.eere.energy.gov/geothermal/maps.html

I don't know if 250KY would be enough at max geothermal extraction to cause those problems. I do think that it is presumptious of us to think that we, as a species, will be anything like we are today in 250KY! Think about how different we are than we were 250,000 years ago; heck, think about how different we were 125,000 years ago. At which time Neanderthals were competing with HSS [modern man], and by some accounts breeding with us to produce whatever it is we are today.

No. If there are any homo sapiens around then, they will be significantly changed from ourselves. And, they will not be burning fossil fuels. My guess is that by then they will be adapted to an ice age that will begin when the impact of AGW wears off. And one that will be made worse, and lengthened, by the 19th-21st centuries. If they even know about us, they may well curse us for our idiocy. If, as I said, there are any homo-saps left.

As for peeing at the beach, I have heard from some AGW deniers that there is no way we could possibly have sufficient impact on the air and sea to make any appreciable difference. They are probably wrong. How long are we able to keep peeing before we change the acidity of the oceans? (I don't think our urine is salty enough to make much difference, but the acid part might).


zap - I think many AGW deniers are typical of many folks who can't grasp magnitude. You peeing at the beach is obviously different than dumping trillions of tons of urine into the oceans. But along those lines: with each breath Zap releases GHG to the atmosphere. Collectively, how much GHG does mankind produce by breathing every year? I've searched but never found anyone making the estimate of how much an average human emits daily. Perhaps insignificant..perhaps not. I have no sense of the magnitude. But whatever that amount is you have to multiply it by 2.5 trillion to get the yearly output. I did see an interesting "fact" last week: 80%+ of all GHG comes from utility companies producing electricity. Again, no idea if that makes sense magnitude wise.

We have to recognize that there are both sources and sinks for CO2. Human respiration recycles carbon that was biologically sequestered as food. If it hadn't been made into food, it would have oxidized before long by another process biological or otherwise. The whole problem with argumentation from purity (doing anything that releases CO2 is a crime against humanity or whatever), is that our actions can cut both ways CO2 wise. If we grew biomass and buried it deeply enough that it wouldn't oxide for a long long time, I'm doing something that is carbon negative. If I have to burn a ton of carbon in order to sequester two tons, I'm carbon negative. So geo-goodness doesn't require that we don't do anything that releases CO2 (like breathing), but that the overall balance of all our activities is zero or negative.

But you could estimate that by the human power consumption, something like 3000Kcal per day per person. Multiply by seven billion, and divide by how many Kcals per kilogram of a typical hydrocarbon.

The 80% in that news report was of sources reporting to the EPA, which ignores mobile sources and small-scale sources.

Rock: Apparently the human body consumes about 550 liters of oxygen per day. At 1.49 grams per liter a person consumes 785.95 grams of oxygen per day. The molecular weight of oxygen is 32 so therefore 24.686 mols of oxygen. When combined with Carbon at a molecular weight of 44 the average person emits 1.086 KG CO2 per day. My best guess is about 2.7 Billion metric tons of CO2. This is approximately 8% of the contribution from fossil fuels and cement as at 2010.

I probably got it wrong but there you have it.

S - Mucho thanks. I was just hoping for some sense of scale and your number seems reasonable. So now I have one more reason to not take up jogging: increased respiration produces more GHG. So us couch potatos are doing our part to save the planet...YEAH US!!!

Of course, one has to examine the carbon inputs as well. For example, grass fed livestock consume carbon in their grazing, which has been pulled from the atmosphere. They also generate methane, though, which can complicate the picture.

How much carbon is pulled from the atmosphere for grains and beans, versus the amount of carbon based fuel used to plant, cultivate, harvest, process, transport, and cook? If the balance is carbon negative, then that would need to be subtracted from the amount of CO2 exhaled.

Hehe, nurturing couch potatos to sequester carbon and fix global warming. That's a fantastic idea! But what happens when a couch potato dies?


Well you can always sequester them underground in an extra wide coffin!

Ok. But don't get tempted to harvest seam-gas from that area at a later stage. That would nullify all previous efforts.

To get a sense of whether an argument about the carbon cycle has merit one has to know how the carbon cycle works and how fossil fuels fit into this picture.

In short (and simplified):
There are three major buffers of carbon: biological matter (trees, grass, soils, animals), atmosphere and ocean. Carbon atoms are constantly being redistributed between these buffers. Trees grow taking carbon out of the atmosphere, fallen leafs rot releasing carbon into the atmosphere, oceans absorbing carbon as e.g. plankton grows and releasing carbon as plankton dies. These buffers have been about as large as they have for many many thousands of years (because extracting carbon from these buffers as ocean sediment and releasing carbon from volcanoes takes a long time). This can be seen in the relatively constant levels of CO2 over the last million year (180 ppm during glacials, 260 ppm during interglacials).

Because of this constant amount of carbon in the three buffers humans and animals breathing out CO2 is carbon neutral: the carbon we're built from out comes from plants or the ocean and the plants or the ocean will absorb it again after we breath the carbon out. The carbon exhaled was already in the carbon cycle, hence breathing does not contribute to the rising levels of CO2 in the atmosphere.

Burning fossil fuels, however, is releasing a lot of carbon into the carbon cycle that hasn't been part of the cycle for many millions of years. It increases the pressure on the buffers. The biological matter buffer cannot grow much (land area is limited) so the atmosphere and the ocean will have to hold most of the extra carbon, and this is observed: rising CO2 levels in the atmosphere and ocean acidification (CO2 absorbed in water results in carbonic acid).

menaus - This might help: To give some sense of scale the volume of the entire earth: 1,083,210,000,000 km3

The crust, including both continental and oceanic areas, is less than 1% of the earth's volume. Continental crust is only 29% of the crustal area. So now were down to 0.3%. Continental crust averages 30 km thick. But a geothermal mining well would be drilled deeper than 4 km. So now we're down to 0.04%. So if many millions of geothermal wells were drilled around the globe on every bit of land surface they would be accessing less than 0.04% of the earth's heat flow.

IOW I don't think the core has much to worry about.

Actually you could capture a good part of the heat flow. But of the stored heat (which on human -even civilizational timescales) is what matters, it is even less (because the temperature of the upper crust is at least ten times lower than the temperature of the deep earth).

Hi Tom,

Thanks for a great post. Somehow, I had missed the fact that California geothermal production has been dropping. Is the capacity factor dropping also? Do you know of a good source for a long-term annual series for California geothermal net electricity production and net capacity?


Just a point is it necessary to get high grade heat for it too be useful the up cast shaft at the mine where I used to work used to be like an oven. The exacter fan was 2,000 HP. In the winter the water in the air used to condense and it looked as if there was steam coming out. I often wondered why it couldn't have been ducted into a couple of acres of green houses.cheap free heating.

Yep, see EIA. Going back to 1990, net production peaked in 1991 at 117% of 2010, the lowest value since 1990 was in 1995 at 91% of 2010. Nameplate capacity was highest in 2010 at 2924MW in 2010, and lowest at 2652 in 1991. Capacity factor was highest in 1991 at 58%. I would guess that most of the capacity factor reduction below base-load fossil is related to daily and seasonal reject temperature variation which makes a bigger difference due to the lower temperature heat source. I know that depletion is something of an issue as I worked on providing the power supply for 2000HP of water pumping for groundwater recharge to one of the larger CA operations which was losing 20MW (~10%) of capacity annually after many years of operation. Another one of the operations I'm familiar with added 6MW of net capacity (23%) to the existing generators by increasing the cooling fan array size a few years back.

" I know that depletion is something of an issue..."

The issue at Geysers is depletion of the ground water, not the geothermal heat, as the operation has taken out far more water than has been put in.

I believe that's an accurate description of the issue at the complex I was talking about as well. They bought a hay farm in a nearby but hydrologically distinct basin for the water rights and are now doing recharge.

As mentioned upthread there had been high hopes for hot dry rock or HDR geothermal in parts of the Australian outback. There was talk of baseload power stations producing hundreds of megawatts that would justify new transmissions lines. So far zilch.

What were thought to be the best sites was where sandstone overlays granite basement rocks trapping the heat. The water would be looped as it is in short supply in the desert and it acquires some radon passing through the fracked granite. Since the steam is not that hot at 250C (I believe) the surface heat exchanger needs a mixed working fluid of water and ammonia (Kalina cycle) to beat the Carnot efficiency rule.

To tell the truth I'm not really sure why nothing has happened with HDR geothermal as there seems to be a deliberate silence. I know there have been problems with the underground water flow and they have asked the Australian federal government for more money. Those who enthused about gigawatts of HDR baseload power now look a bit foolish. Many of the supporters are opposed to nuclear fission but the heat source in the granite is radioactive decay. That seems somewhat hypocritical.

Links for two of the HDR players

"Many of the supporters are opposed to nuclear fission but the heat source in the granite is radioactive decay. That seems somewhat hypocritical.

Since the radioactive decay in deep rock is widely dispersed and distributed vs. the highly concentrated stuff humans insist on using, I don't see the hypocrisy. As they say, the poison is in the dose,,, but that's what we do.

I believe CANDU uses natural concentrations of uranium for fuel as an anti-proliferation measure. They also have substantially more intelligent scram procedures than U.S. plants (or some I am told by certain nuke folks).

I have been following Geodynamics HDR for the last 10 years. At the beginning it seemed promising, but over the years, it has become quite apparent just how difficult and expensive it is.

The share price says it all: http://au.finance.yahoo.com/echarts?s=GDY.AX#symbol=gdy.ax;range=5y;comp...

I think the problems are mainly economic and practical. You have to fracture the heat containing rock, and pump fluid down one well and up another. How much does it cost for well prep? What sort of issues with scaling do you have to deal with? How long before the volume accessible by the fluid has lost its heat content? All of these things potentially increase the cost. What about environmental dangers. Does fracking the rock create an earthquake hazard? What about toxic stuff disolved in the produced water? Better make sure you don't spill it, etc.

I have no doubt, that if geothermal was our only energy option (say the planet had no fossil fuels, no wind, and no sun, and no N fuel), that we would get our energy that way -and we would consider the price a bargain -even if it were say $5.00 per KWhour.

In re HDR in particular and any geothermal resource in general: In order to use heat energy one has to collect it and send it into the hot side of a practical heat engine. The practical limitations on what can be done to collect the resource are massive and are well understood both to pure science types and to practical HVAC engineers. But they are totally ignored in this blog. There are three ways that heat flows. Conduction, convection, and radiation. In the interior of a solid, only conduction happens. It has been happening for eons without benefit of humans. We have no ideas as to how to make it happen faster. Neither pure science ideas, nor 'practical' ideas from practicing engineers. Radiation happened only in a vacuum, not a solid, except in a few special crystals, which are only present at very low concentrations in the earth, so there is no wriggle room from it. Only convection is left. Convection happens in fluids, i.e. liquid and gas. One must arrange to have a fluid circulating between the interior of the hot rock and the hot input port of a heat engine. In the Geysers in California, nature has provided fissures and porous rock and water (a fluid) feeding into the fissures from a natural source. No such situation exists in Australia. So it must be provided in the project plan using funds provided in the project budget. Generally. the idea is to drill holes (wells) in the rock and put pipes in the holes. It is easy to compute how many kilometers of holes and pipe are needed per megawatt of design power output given any given geometry of wells and piping. Tedious but that's what computers are for. I don't have any special knowledge of the geology of central Australia, but I suspect that the shills for this idea made the mistake of hiring honest mining engineers to flesh out their story using some of their seed money from initial investors. I suspect the engineers could not "make the numbers work" in the way the shills wanted, even though they tried many different detailed ideas of holes and piping. For each different idea set there was a different killer cost, so there was never a clear conclusion that it could not work by a logical proof. And that's the way the real world is. As you put more and more known facts into the mix the 'solution' becomes more and more expensive.

I am quite confident in my opinion that 'depletion' of this resource will never be a problem. On the contrary, this resource is so difficult to capture that it is hardly worthy of being called a resource. If there are valid engineering studies that show that my opinion is valid, the people who know of them are either shills or people who signed non-disclosure agreements with the shills. So the shills live for another round of making presentations.

The presence of groundwater in conjunction with geothermal hotspots is not exactly unusual on the Pacific Rim. The types of rock fissures you are talking about are fairly usual, which is why most places you can drill a well and produce cool drinking water. In the geographic hotspots where the water is hot, it can be reinjected after producing power (although a small amount of water will be lost and makeup water will eventually be required). There is significant additional capacity which is economic at present power prices in CA, but is stranded due to transmission construction timeframes. I'm on the side of the meter where we are supposed to be building the transmission and buying the power, not the side where they are supposed to make the power, so I don't have much reason to misrepresent this.

Note that when I started this job 13 years ago it took roughly 18 months to license a sub-transmssion project in CA, and we are now looking at 6-7 years.

People fell in love with HDR because it offered low carbon power 24/7, not intermittent like wind and solar. Investors and governments offered money for a few years then became disillusioned. I hope somebody makes a TV documentary on it if the plant ends up rusting away in the desert. A timeline is here

Even if they could get it to work nobody will build a 300km transmission line without years of assured electricity supply. I wonder also when the 'hotspot' cools and they have to drill more wells kilometres away from the generating station. That's got to worsen EROEI with long insulated pipes. Drill rigs use a lot of diesel the very thing we're trying to eliminate.

Radiation happened only in a vacuum

Actually, it can happen through a fluid, such as Earth's atmosphere, though there are normally some losses. I don't see how it would apply to EGS, so this is only a minor note.


The average American household used 80 thousand cubic feet of natural gas in 2001

2005 EIA data says approx 40,000 ft3 of gas used per household (46,555 billion ft3 divided by 111 million homes)
Table US3 - Total Consumption by Fuels Used, 2005

Anyone else run any of the other numbers independently? MIT's report on US potential from a couple of years ago said;


Based on growing markets in the United States for clean, base-load capacity, the panel thinks that with a combined public/private investment of about $800 million to $1 billion over a 15-year period, EGS technology could be deployed commercially on a timescale that would produce more than 100,000 MWe or 100 GWe of new capacity by 2050.

100 GWe in the US doesn't seem niche to me when all of coal's current generation capacity (including many old plants that are run at a very low duty cycle) is roughly 320 GW.

Not all homes use gas. Your average divides by all homes, not just gas users.

Geothermal produced 0.36% of U.S. electricity in 2010. 96% was in CA (83%) and NV (feeding CA). HI, ID, UT were the only other states with any production. 6.2% of power generated in CA was geothermal. CA would have significantly more but there are transmission constraints.

Still, the question stands - How can a potential of 100 GWe in the US be classified niche to me when all of coal's current generation capacity (including many old plants that are run at a very low duty cycle) is roughly 320 GW?

Great series of posts Tom

So if I am keeping score correctly, so far we have:


Solar Thermal (storage)
Nuclear (breeder)


Passive home heating
Hot water


Biofuels (?)

I would personally categorize wind, geothermal, and hydro as regional rather than potent or niche.

Toms definition here (in the context of his articles)

A better set of labels is “abundant,” “potent,” and “niche.” The last could also be thought of as “boutique,” in that it is cute, perhaps decorative, may serve some function, but will never be a heavy lifter. The “potent” label—formerly “useful”— is meant to indicate a source that could supply a healthy fraction (say over a quarter) of our global demand if fully exploited. We will never fully exploit any resource, so the numbers at least need to support ¼-scale before we can believe that it may play a major role.

I know, but I disagree that total global energy is the only reasonable measure of utility, or that today's energy consumption is the right ruler for 100% scale. Geothermal and hydro both have grid benefits beyond net energy. Also, if the resource is greater than 1/4 scale in broad geographic areas, I think that warrants a fourth category. as Tom notes, it is possible to reasonably disagree about the categorizations.

if the resource is greater than 1/4 scale in broad geographic areas, I think that warrants a fourth category

This is a good point. Our current energy distribution systems are not the only possibilities. Most renewable energy sources do tend to be regional in nature. This is certainly true of wind and hydro, but solar also to some extent.

if the resource is greater than 1/4 scale in broad geographic areas, I think that warrants a fourth category

Absolutely, and I would go so far as to apply that to anything that is 1/8th scale or higher, as there could be scenarios where 5 such technologies could combine for the total solution, or 3 could combine for 60% of the solution. The categories could be;

- Regionally abundant
- Regionally potent
- Regionally minor

Again, a number of minor resources could aggregate into potent or abundant.

I would have said tidal is regional too. Around the UK there is plenty of rise and fall to drive schemes, 20', 40' etc. Around here, squit all, a tenth of that. I've picked up some strong currents when diving here but nothing like what is needed for good generation.


Total tidal is bupkis, though, even where it exists at all. I guess you could call it regionally niche, where I would call the other three regionally potent.

"I would personally categorize wind, geothermal, and hydro as regional rather than potent or niche."

If you classify hydro as regional (instead of niche), it doesn't tell you anything about the ability of hydro to provide enough energy (quantity) to meet global demand - which is what I think Tom is trying to illustrate in his series of articles.

By classifying hydro as niche, it is immediately apparent that hydro can't provide the quantity required.

Abundant, Potent and Niche relate to quantity.
Regional relates to location.

Now that's not to say that regional isn't an additional useful overlay.
For example, hydro is abundant in the region of Norway, even though it's niche on a global scale.

I disagree, since technologies which are truly niche are unlikely to see significant use, while technologies which are regionally potent will. Further, since he and I both think that negawatts will be a significant factor, I think the 25% cutoff is too high. I'd be happier with an estimated potential production percentage of current consumption, rather than 3 categories. Qualitative factors should lead to both geothermal and hydro (both reduce the need for storage of other sources, provide grid regulation, and are dispatchable) taking a greater share of total generation than this percentage would indicate if compared directly to the potential of all renewable sources. This is all just quibbling, however, and I am happy he's doing what he's doing, although I think he needs to dig a little deeper on the analysis of some of the techologies.

"while technologies which are regionally potent will"

So hydro is now "regionally potent" :p

"I think the 25% cutoff is too high"

It doesn't matter too much as long as it is defined.
You seem to object to the word "niche" - again it doesn't matter too much as long as it is defined within the context in which it is being used.

At a global potential of approximately 10% of our current energy scale, my initial reaction is to throw hydro into the “niche” box with tidal, since my criterion is that a resource be theoretically able to meet a quarter of our demand to be labeled “potent”—which incidentally is in line with what oil, natural gas, and coal each deliver to us today: all are momentarily “potent” sources by this reckoning.

Relying on measuring energy sources per BAU keeps us trapped in BAU thinking.

This is a discussion about a series of articles examining the potential of alternative energy sources to replace fossil fuels ...... I don't think anyone here is at risk of being "trapped in BAU thinking"

That's basically what I've got. But we should add geothermal depletion (mining heat) to the abundant box. And biofuels (esp. algae) may deserve the potent slot. Glad someone's keeping score. In a few weeks, I'll finish individual topics and summarize.

In a few weeks, I'll finish individual topics and summarize.

Thanks, I look forward to it.

A bit of nit-picking to an otherwise excellent article.

The units of thermal conductivity should be W / m K (watts per meter kelvin, note space between "m" and "K" as mK is milli-kelvin)
or alternatively Wm-1K-1,
instead of watts per meter per kelvin.

So the heat transfer equation κAΔT/t comes out ok:

(W/(m K)) * m2 * K / m leaves one with just watts after the meters and kelvin cancel.

I'm not following: W/m/K is the same as Wm−1K−1, or Watts per meter per Kelvin.

It's a question of associativity of the "per" operator. I.e., does "W/m/K" mean "(W/m)/K" or "W/(m/K)"?

Personally, I'm with you, Tom. I prefer "W/m/K". My brain automatically parses it as intended. That's the usual way that expressions are parsed by compilers.

Right. I take my cues from the programming world. Order of operations in C, Python, Perl, Fortran, etc. all agree on what W/m/K means (12/4/3 = 1, not 9). But I guess I can see now how it could be read as ambiguous to someone less stamped by computerese.

Tom, great article. Great humor. Actually read all thru w only minor scanning.What a workout.
Conclusion: It's hilarious. We're screwed. LOL
So, what else is new?


Above in the comments you wrote that "we should add geothermal depletion (mining heat) to the abundant box.". That conclusion seems to be the important finding of the article, given the 250,000 year supply. There is an extremely brief discussion of economics, but given the 150:1 cost reduction solar has achieved in 30 years, that needs much more support to be convincing.

The article doesn't seem to really reflect the primary finding that geothermal is abundant. I get the sense that you started with one perception of geothermal, and when you finished writing the article you hadn't really "processed" the fact that your calculations really led you to a different result.

So, if you were to rewrite the article today, it would be a little different, right?

The abundant thermal resource was not so much an afterthought as a so-what. I am not jazzed about low grade energy that is extremely inconvenient to access. Most importantly, I think it does little to help us out of the energy crunch (liquid) I anticipate in the next few decades, so it gets something of a shrug from me.

low grade energy that is extremely inconvenient to access.

That sounds reasonable. Still, do we really know that? Your discussion was extremely short. The same thing was often said about wind and solar by people who's intuition told them that wind/solar wasn't useful because of "low density", and they were wrong. Wind power costs fell by 10:1 over the last 30 years. Solar power costs fell by 150:1. We know that geothermal heat can be extremely concentrated in places, just as hydro and wind can be.

Doesn't geothermal need real analysis before being dismissed?

it does little to help us out of the energy crunch (liquid)

This seems to be a different standard for evaluation than has been used so far. By this standard wind, solar and nuclear are also irrelevant. Most of your articles have taken an extremely long-term point of view. Why switch now?