Geothermal Power Generation: Potential and Limits
April 19th, 2010
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Geothermal power generation – the use of heat from within the earth to generate electricity should not be confused with geothermal heat pumps. While heat pumps are great for heating homes or commercial structures, they are net consumers of power. Geothermal electricity, on the other hand, is used to generate power from heat within the earth, normally used to produce steam. It has been touted as a major source of “Green” energy.
In locations where there is significant thermal activity near the surface of the earth, geothermal energy works great. In places like Iceland, parts of California, Alaska and parts of the Mediterranean areas exist that can be used to generate significant amounts of electricity economically. Geothermal energy is not completely free of environmental impacts. The water that is brought up from depths may contain a variety of toxic minerals and also can release gases like CO2 and sulfur dioxide. However, when done properly, the impacts can be manageable and are often fairly small.
Geothermal energy is also not necessarily truely “renewable.” In areas where near-surface hot rocks exist, extracting thermal energy faster than it is replenished by sources deeper in the earth can lead to the area being cooled to the point where the thermal reserve has been depleted and is no longer sufficient to generate power. Whether or not this is a concern depends on the nature of the local geothermal feature.
The potential for extracting energy from deep boreholes into the earth was recognized as early as the mid 1800’s. In the early 20th century, inventor Nicola Tesla believed that this would be the primary means of generating electricity in the future.
Unfortunately, most of the world simply does not have much near-surface geothermal activity and even in areas where it does exist, the features are limited. In order for geothermal energy to be a major energy source in the world, it will need to be made available in more areas and with a greater capacity. This presents a problem.
What is required:
While there are areas of the world where geothermal activity near the earth’s surface exists, these are not terribly common. In geologically stable areas, located away from plate boundaries or volcanic hot spots, there is generally no significant geothermal activity near the earth’s surface. Such areas include most of Europe, the eastern half of North America, much of Latin America and Asia. Even in areas that do show seismic or volcanic activity, geothermal reserves may be few and far between.
Unless nature has provided such geothermal activity, there is no choice but to drill down to levels where heat is sufficient to provide reasonable amounts of energy. Although it is theoretically possible to extract useful energy from temperatures bellow the normal boiling point of water (such as by a low temperature sterling engine, ammonia-based engines, or other low boiling point working fluids), doing so generally provides poor thermal efficiency and does not provide for large amounts of power generation.
As a general rule, when it comes to thermal energy sources, the higher the temperature the better. In order to get a reasonably good energy return, temperatures of at least 120 degrees C and ideally more than 200 degrees C are required. Higher temperature also reduces the volume of fluid required to produce a given amount of energy.
About one statue mile down will get you a temperature level of about 60 degrees C above the surface temperature. Two miles down will provide about 120 degrees C above the surface temperature. Thus about two miles, or about 3.2 kilometers would be about the minimum for decently high temperatures. Ideally, even deeper, to a depth of five kilometers or more would be desirable to run a modern high pressure steam turbine. Even deeper and combined cycle power generation would become an option.
Larger energy projects will obviously require larger volumes of fluid flow than small energy products. Depending on the type of geothermal energy system, more than one borehole may be required to begin with. For example, many systems have a separate borehole for water injection and steam recovery. Two boreholes of a medium diameter drilled down a few kilometers may provide enough steam to operate a very small geothermal power plant of a few megawatts. However, if infrastructure level generation is required, it larger boreholes and many more of them will be needed.
Therefore, realistically, geothermal energy in most areas is going to require multiple boreholes drilled to a depth of roughly a few thousand meters or 10-15 thousand feet, minimum.
The task of deep drilling:
There is a limit to how far down you can go with standard conventional drilling methods. It depends on the geology of the area, but standard drilling methods are generally limited to anywhere from a few hundred to a little over a thousand meters before a variety of problems become insurmountable and more extreme measures need to be adopted. There are techniques which have been developed for dealing with the requirements of ultra-deep boring, but they are anything but simple and easy.
By convention any borehole drilled deeper than 10,000 feet is considered “deep.“ While there are established methods of drilling this deep, only a few boreholes in the world have achieved 10,000 feet in depth or more. In most cases, such boreholes were drilled over the course of many years. The most common use for deep boreholes is oil and gas exploration and recovery. Despite the scale of the industry and maturity of the technology, the cost of drilling even a relatively shallow well (under half a kilometer) is tens of millions of dollars
The single largest problem is that as the depth increases, the pressure on the rock begins increases. When the pressure is high enough, the rock begins to act with a high degree of “plasticity.” Although we normally think of rock as being solid and immobile, at depths of a thousands of meters, the pressure of the mass of material above is high enough to squeeze the material into any void. Thus, any hole drilled to such depths will literally “close up” due to the plasticity of the rock. To deal with this problem, the borehole must be kept under enormous positive pressure. This is achieved by a system which pumps drilling fluid or “slurry” into the hole and maintains high pressure on the slurry at all times.
The slurry does more than just keep the borehole under pressure. While maintaining a pressure of thousands of PSI, the slurry must also flow down to the drill bit and back up, in order to carry the spoil from the drilling. It also provides some level of lubrication to the drilling system. Various systems have been developed which allow the slurry to be pumped down the drill pipe and around the bit and then back up to the surface, while always maintaining the high pressure. A number of specialized valves, pumps, regulators and recirculates are employed to do this.
In some circumstances, the slurry also turns the drill bit (known as a “mud motor). The system used at the world’s deepest borehole, the Kola Superdeep Borehole, used a system where the drill pipe did not actually spin, as in a conventional drilling system. Instead, slurry pumped down the hole acted as a hydraulic fluid to turn the drill bit. This helped with the problem of torsion on the extremely long drill pipe, although it also introduced its own challenges.
Of course, as drilling progresses, additional lengths of drill pipe must be added, and as even the best drill bits wear out, they must be brought up and replaced. Doing this cannot interrupt the enormous pressure of the drilling fluid or the hole would close back up. Therefore, a specialized gasket system known as a blowout preventer is used to allow for the insertion and removal of lengths of drill pipe, without ever breaking the seal and compromising the pressure. Blowout preventers come in a number of designs depending on the pressure they need to contain and whether they’re intended to cap holes being actively drilled or ones which have already been drilled to completion.
There are a number of other complications that also arise. The slurry must be cleaned and reused and failure to handle the slurry properly could leave behind bits of drill spoil that would act as an abrasive and further reduce drill bit lifetime. Heat is also a problem. Not only does heat increase as depth increases, but the friction of drilling produces heat which is not easily dissipated when in a deep borehole, and the pumping and compression do not help either. In order to keep the drill bit cool, the slurry must be refrigerated before it is pumped down the borehole.
If this is not complex enough, there is another problem which is presented by such deep drilling and the use of such high pressures. The high pressure fluid is kept in the borehole by the external pressure of the rock at deep depths, but these pressures do not exist at the lesser depths, where the rock is more porous and the high pressure fluid can easily leak out of the borehole and make pressure containment impossible. If fluid were simply pumped in at the surface, it would not be contained by the upper portion of the hole. There’s only one way to deal with this and that is to line the upper part of the borehole with pipe, which is pounded in one length at a time, to keep it sealed tight. Failure of the blowout preventer or the associated plumbing could result in the hole being lost. Even after drilling is complete, the borehole must always be kept at extremely high pressure or it could close up and be lost.
Therefore, the process involves the following, (extremely simplified) basic steps:
- Drill to the approximate limits of conventional unpressurized spinning-pipe drilling
- Use a powerful hydraulic system to pound lengths of pipe down the hole you have created
- Cap the hole, begin pumping in pressurized fluids and check that it has maintained a seal
- Move onto the second phase of boring, now using a high pressure fluid method
- As depth increases, increase fluid pressure and introduce refrigeration as needed
Risks and Complications:
Of course, there is a high degree of risk associated with drilling to such depths. While seismic data and geological theory provide some insight into what is located under miles of crust, to some degree, this is an educated guess. It’s not unusual for deep boreholes to encounter unexpected materials, potentially halting the project in its tracks. The Kola Superdeep Borehole encountered several unexpected anomalies at depth, including large amounts of hydrogen gas. The second deepest borehole, the Bertha Rogers hole in Oklahoma had to stop after encountering an unexpected deposit of molten sulfur. Nobody is entirely sure whether such areas of molten sulfur are common or whether it was just an oddity that the the Bertha Rogers hole ran into. Given the lack of exploration at such depths, there’s no way of being certain of exactly what a borehole may encounter when drilled in an area that has not previously been drilled to extreme depths.
At extreme depths, heat and pressure increase stress on the drill bit, mud motor and pipe. This reduces the lifespan of the bit and requires that it be replaced more and more frequently. Each time the bit needs to be replaced, the entire length of drilling pipe, totaling thousands of feet, must be removed from the hole, one pipe at a time. Finally, the bit must be removed and replaced before the whole process can be restarted, reattaching the drill pipe to send the bit back down to resume drilling.
At any time during the process, a malfunction of the blowout preventer or any other compromise to the seal of the borehole can result in the entire project failing and much of the hole closing up. This also complicates the adding and removal of pipe sections, as they must be fed through the blowout preventer without breaking the seal.
In the event that the drill pipe breaks or part of the bit breaks off, it will need to be retrieved from the hole. This presents its own problems, which depend on depth. A grappling bit on the end of drill pipe or a system of cables may be used to pull broken sections back to the surface. Like all other things at such depths, this is no easy task.
Thus while geothermal power generation does work well in some locations, these areas are limited and most of the best locations have already been developed.
This entry was posted on Monday, April 19th, 2010 at 12:25 am and is filed under Bad Science, Enviornment, Good Science, Politics. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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April 19th, 2010 at 1:54 am
I’ve sometimes wondered if it might be possible to mine the mantle, perhaps drilling from a site on the deep ocean floor. I haven’t bothered to do any research on the subject though… still, if it’s at all possible, I’m sure there’s a great wealth of minerals down there.
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April 19th, 2010 at 4:19 am
Well that’s interesting. It hadn’t occured to me that the water would carry contaminants to the surface, because I had assumed that the heat exchange would occur through pipe, not that the process water would be in direct contact with the rock.
I had also wondered about why lower boiling point fluids would not work, but a Pop Mechanics article about a low temperature geothermal plant in Alaska, clued me in that low temperature is not a problem thermodynamically, as long as you have a heat sink that’s substantially cooler. That’s a problem for most places that aren’t Alaska.
http://www.popularmechanics.com/science/environment/4245896?series=15
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April 19th, 2010 at 4:26 am
Shafe said:
Enhanced geothermal uses multiple boreholes. At least one that pumps water in and at least one that extracts water. Between these boreholes there exists a vast “web” of fractures created in the hot rocks with hydraulic fracking or some other method of fracturing.
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April 19th, 2010 at 4:45 am
Buzzo, there have been attempts made to develop laser-based drilling with direct diode lasers that would not need to line the borehole with expensive casings and would be at least a factor 10 faster than rotary drilling. It is as of yet very much unproven, but it could make enhanced geothermal a LOT more practical if you live in the right place. Right now geothermal looks pretty crappy unless you have a resource nearly as good as Iceland.
The relation between drilling cost and drilling depth is not quite as bad as exponential in drilling depth(e.g. see the wellcost lite model) but way worse than linear. You can see why quite easily; in order to change a drill bit you’ve got to retract the entire drill from the hole and dismantle each section as you go; the deeper you go the hotter it gets and the more you’ll spend on cooling or replacing drills which wear out faster at higher temperature.
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April 19th, 2010 at 7:13 am
Soylent said:
“It is as of yet very much unproven”
Yeah.. that would be the issue. Getting to the point of vaporizing rock with lasers and having the whole system work properly is not exactly a small task. It may someday work, but I would not count on it.
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April 19th, 2010 at 9:18 am
DrBuzz0, this article has made the movie ‘Armageddon’ much much more entertaining. Thank you.
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April 19th, 2010 at 9:45 am
Very much like solar heating for domestic hot water is cost effective, geothermal heat pumps can save a great deal of energy to heat a home.
This is probably the best use of this source of energy and the one that can be most widely deployed.
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April 19th, 2010 at 10:41 am
drbuzz0 said:
No-no; you don’t vaporize rock, you heat it at fast enough rates to spall it through thermal stress.
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April 19th, 2010 at 10:44 am
I should also note that it works in the lab; now you’ve got to make the laser small enough to fit on the end of a drill-shaft and able to operate under nasty conditions.
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April 19th, 2010 at 11:49 am
Soylent said:
Be that as it may, it still does not make the process easy. It might make it a bit less difficult, but there’s still the issue of keeping the hole under tremendous pressure and of feeding thousands of lengths of drill pipe down there.
Drilling down thousands and thousands of meters is simply never going to be an easy or simple task.
Shafe said:
Yes, it is possible to use lower temperatures, but there’s always lower efficiency. Any thermal engine has better efficiency when there is a higher temperature difference. This is why most steam electric plants run at temperatures above 300 degrees Celsius.
If you are talking about turning this into a very very major power source and extracting multiple megawatts per borehole then high temperatures like that are required.
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April 19th, 2010 at 11:50 am
An excellent review of the potential and challenges of geothermal power Steve, and a good summary of the problems of deep drilling. I think most drilling is done with downhole motors (mud or electric) rather than rotary drillstrings now though, in particular all directional drilling.
My long and fruitless campaign to introduce the terminology “geocoupled heatpumps” goes on… please, DV8 2XL, it’s not a source of energy.
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April 19th, 2010 at 1:08 pm
“standard drilling methods are generally limited to anywhere from a few hundred to a little over a thousand kilometers”
I’m assuming that “kilometers” should be “meters” there… because a thousand kilometers? That’s REALLY deep.
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April 19th, 2010 at 1:57 pm
Great article. Geothermal energy can save a great deal of money when heating a home. Good to see it applied on a larger scale.
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April 19th, 2010 at 2:04 pm
Sigivald said:
Noted and corrected.
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April 19th, 2010 at 3:07 pm
How about using it just for heating without bothering about electrical generation?
Considering how much of our energy use is for heat, pumping out large quantities of 60C water for a heating system might be feasible.
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April 19th, 2010 at 3:24 pm
metatron said:
The big killer in all district heat systems is distance. Iceland can pull it off because they have good shallow sources just about anywhere they drill, so they don’t have to pipe the geothermal fluid over great distances. In the rest of the world, it is best to make electricity on site at the wellhead because it can be transmitted farther will less loss, less expensively.
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April 19th, 2010 at 4:36 pm
metatron said:
Deep borehole geothermal heating does exist as an alternative to shallower heat-pump based systems. Boreholes which are deep but not as deep as steam generating drill holes can be used to produce heat for structures. However, it’s not necessarily any better than heatpump based heating or even other types of heating. There’s no free lunch here. You still have to force the fluid down a borehole and pull it up again, even losing some of it to fissures in the ground and fluid friction.
Well pumps are some of the biggest energy users for residential customers – and that’s just for shallow water-drawing wells. So forcing water down one borehole and up another is going to potentially be kilowatts of power for even a reasonably small single-user application.
That does not mean it can’t come up ahead and be a descent source of heat. In some circumstances, where the ground temperature is warm enough at not too far down it can work out decently or better. It’s just not a free ride and more often than not it’s not worth the expense.
This is another reason why power generation generally means you want to have temperatures over 200 C. You can’t afford to expend energy pumping fluids down a hole and back up only to have a small rise in temperature.
In general there’s no free lunch here. The one circumstance where you get a freeby is circumstances where there is a pre-existing hot spring. Then there’s no need to pump water or bother with any of the energy expenditure.
In Steamboat Springs Colorado hot water that is reasonably low in mineral content (low enough that it won’t foul the pipes right away) comes out of the ground by the thousands of gallons an hour. It can be used to keep structures warm and then still has enough heat left over to heat outdoor pools and finally run under the sidewalk to melt snow before being discharged.
In a few other scattered locations there are hot springs which provide enough hot water to keep the structures in the area heated with no need for fuel.
In Japan there are a number of hot spring sites where the water is used for hot baths. In some cases, it comes out of the ground scalding hot – too hot for baths. It is therefore used to heat buildings and power steam rooms and only after giving up much of its heat to those uses is it piped to baths.
In these circumstances, you can get heat for free. However, they’re very very few and far between.
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April 20th, 2010 at 12:37 pm
drbuzz0 said:
Nice explanation, but I’m wondering why it takes so much energy to pump the water back up again?
Taking an object from x down to y and back up to x shouldn’t result in the loss of any potential energy (- some losses from friction) back in South Africa I toured a gold mine, and the water they brought back up from the deep shaft cooling systems was HOT.
DV82XL said:
What prevents you from drilling a well right under your customers? Then distance is no issue.
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April 20th, 2010 at 12:41 pm
metatron said:
Cost and the suitability of the local geology.
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April 20th, 2010 at 5:12 pm
metatron said:
Raising water any distance is going to require a significant amount of energy. At my parents house, they have a private well that goes down only about 150 feet, if not less. The well pump needs a dedicated 220 volt 20 amp circuit.
In theory if you force the water down it should displace an equal amount of water that comes back up, but in an injection well system, it’s actually a lot more complicated than that. The water is actually forced through porous rock and drawn up another pipe.
For 60 C some place like most of the US, Northern Europe, Australia (places where there’s no significant shallow geothermal activity) you still have to force the water down a shaft of at least a half to three quarters of a statute mile then into the surrounding rock strata and back up another equally deep shaft.
Much of the energy from the pressurization is lost because you are still pulling water up against its weight on one side. At those kind of distances you get almost no help at all from convection. The flow of the system has to be pretty rapid or much of the heat will be lost to the surrounding rock on the way up and the water temperature will just equalize to that of the surrounding rock.
Here is a diagram of “Hot Deep Dry Rock” geothermal: http://www.tfcbooks.com/images/misc/geothermal-m.gif
The only difference if you only need 60 C for heating is it doesn’t need to be quite as deep, but it does still need to be pretty damn deep.
Incidentally: deep mines use tremendous amounts of power to pump out water and keep the tunnels dry. Just one of the pump stations in a South African mine can have four 2.4 megawatt pumps for a total power usage of 9.6 megawatts per pump station and some of the large mine complexes have multiple pump stations of that size.
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April 21st, 2010 at 1:23 am
drbuzz0 said:
That’s not an analogous situation. A conceptual model is a U-shaped tube full of water; any potential energy gained by water comming up one leg is lost by water going down the other. The energy it takes to pump water around is entirely in the form of friction which could be very significant if the bottom of the ‘U’ is in the form of porous rocks.
For geothermal heat-pumps you’re just interested in getting at the average yearly temperature, you could just as well choose to bury a long, meandering pipe in your yard at a few metres depth; but it’s actually cheaper in most cases to drill a couple of holes down to a few hundred meters, line them with pipe and connect them at the bottom(don’t ask me how they manage to do this step, but apparently it’s not too difficult). Another approach that is popular here in Sweden is the bottom of a nearby lake; 4 degrees C is not significantly worse than ~7 degrees C which is the yearly average here.
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April 21st, 2010 at 1:37 am
Soylent said:
Actually, when I think about it a little more, it’s not that important that the pipes be insulated from each other(it does help, because you “mine” heat from different volumes of earth if they are separated), so they probably just insert a pipe that is folded into a U-shape into a single borehole and make as many boreholes as they need.
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April 21st, 2010 at 2:36 pm
In the 1967 movie “The Graduate” (“and here’s to you, Mrs. Robinson”) a character has a word of advice for the newly graduated Dustin Hoffman. The word is “plastics”. I have a similar word for the author of this article.
It’s “barytes”.
In this case, it’s not the key to a bright future career that a college grad should consider taking up, but it is a key. It’s the key to how deep drilling is made feasible.
The article overstates the difficulties of deep drilling. I’m pretty sure it also understates the number of boreholes that are over 10,000 feet. I believe there have been multiple boreholes drilled to 18,000 feet or deeper in search of gas. And, IIRC, there are a number of producing gas wells at 12 – 14,000 feet.
The reason it’s feasible to drill this deeply is that dense powdered baryte minerals, when mixed into the drilling mud, can match the mud density to the density of the rock being drilled. So gravity does the pressurization automatically. At any depth, the static pressure in the drilling mud is the same as the static pressure in the surrounding rock, just by virtue of the column of heavy mud above it.
Blowout preventers are there for when oil or gas is encountered. Oil and gas have much lower density than rock, and if the borehole starts filling with them, the pressure in the upper regions of the well can shoot up. But they play little or no role in drilling operations at most times; they are not used for maintaining different working pressure levels at different depths of the bore, and are not needed for that. Gravity does it.
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May 6th, 2010 at 1:48 am
[...] Depleted Cranium » Blog Archive » Geothermal Power Generation … [...]
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October 12th, 2011 at 5:34 am
My name is John Chumari from Nairobi Kenya,Am a self made scientist, and i have over the years tried several different methods of generating electricity. Now I have succeded in bulding a generator that produces enough electricity to allow me to go off gring. the generator recycles the same electricity to run itself, and hence uses no fuel and emitts zero carbon. My only problem is that I dont know how to place its pictures online and I dont know how to show case this machine to the world cos even those who have seen it working refuses to beleive that its not a trick. I need someone to take me seriously, because this is real. am ready to demonstrate this machine to anybody or organization that realy cares about green energy. am not soliciting for funds or trying to sell the idea, I just want to show the world that it can be done cos I’ve done it.
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October 12th, 2011 at 7:07 am
John chumari said:
ROFL!
Didn’t think anyone would fall for the perpetual motion machine scam these days…
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October 12th, 2011 at 7:50 pm
Looks like it’s run of the mill spam to me, by someone who has yet to figure out what rel=nofollow does to comment spam (i.e. prevents Google from indexing it).
Although maybe it could be a 4-1-9 scam aimed at the Green movement.
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October 16th, 2011 at 9:54 pm
Very interesting insights regarding the drilling process. I can’t help but think of the movie “Armageddon” when they run the risk of experiencing blowouts that launch them into outer space. That’s one nice thing about drilling on Earth – GRAVITY!
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