Excellent primer on Geothermal.

"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."

http://physics.ucsd.edu/do-the-math/2012/01/warm-and-fuzzy-on-geothermal/

Hat tip: The Oil Drum.

Should be a drum of extra-virgin organic olive oil, silly!

But yes, TOD is on my regular reading list, and in particular their news summary every couple of days at:

http://www.theoildrum.com/section/drumbeat

is very good and new postings are flagged up on Twitter at:

@TheOilDrum

Rgds

Damon

Read the paper and you'll find that geothermal can only ever be a niche product.

<blockquote><cite>Posted By: gcar90</cite>We've had geothermal electricity for over 100 years now, since 1911:

<a rel="nofollow" href="http://www.reuk.co.uk/Larderello-Worlds-First-Geothermal-Power-Station.htm">http://www.reuk.co.uk/Larderello-Worlds-First-Geothermal-Power-Station.htm</a>

So anyone that knows this should ask the question why, after a century of development, there's only a few GWe of geothermal in the world.</blockquote>

The main problem seems to be that geothermal energy is impractical to exploit in many of the heavily populated regions of the planet. It's easy enough to exploit in areas that are volcanically active, but hard elsewhere. Years ago, when I was still living in Cornwall, there was an attempt to exploit geothermal energy at Rosemanowes quarry, a couple of miles away from me at the time. The project drilled some deep boreholes and after a few years did manage to get some hot water out, but not hot enough to be useful. I believe they are still looking at using the technique in the area, but after 30 odd years there's still no real prospect of a viable geothermal power station being built. The reason has to do with the UK being a long way from any volcanic activity - the depth of old rock above the mantle here is such as to make drilling for geothermal heat sources an expensive proposition.

<blockquote><cite>Posted By: dickster</cite>For some reason, Southampton have managed to use geothermal to heat various buildings for quite a few years now.

No volcanoes anywhere near?</blockquote>

But they are only pulling out low grade heat, not anything hot enough to be useful for a power station (which was the specific question).

The Hot Rocks project 30 years ago was doing the same, heating water to a high enough temperature for heating, but not anything like hot enough for power generation (and I believe there are a few more geothermal district heating systems still being looked at in Cornwall).

If you want steam for a power station then at the moment it seems you have to be in or very close to a volcanically active region, just to get high enough temperatures to make it viable.

<blockquote><cite>Posted By: gustyturbine</cite>JS,
I was in the area a while ago. We had a meeting with a guy at the Eden project. They have geothermal plans for sure. The meeting was very informative. It has some issues though,
Gusty.</blockquote>

Sounds like an interesting application, as my guess is that some of those domes need a fair bit of heating in winter and this would be a great way to reduce their energy bill.

The hot rocks people concluded that the heat source they were tapping into in the Cornish granite wasn't wholly geothermal, I believe, but came in part from nuclear decay from radionuclide's in the granite - in effect it's low grade nuclear energy. I remember going down Wheal Jane mine when it was still open and it was incredibly hot down at the bottom, and that mine wasn't very deep at all, something like 450 m I believe, and nowhere near deep enough for true geothermal energy to have cause the temperature rise.

It would be a really, really good idea if people read the paper before commenting. Then perhaps they wouldn't need to comment.

Except perhaps to say thank you.

Posted By: wookeyIt would be even better if posters summarised what they were linking to, so we didn't all have to read it individually.

barely anything different in it to when I was taught about it 15 years ago or so. unsurprisingly really.

It essentially concludes that for power generation it's a niche resource only viable in a few places. For space heating the resource is there, but is mostly impractical, and likely to be far too expensive to extract (both money and eroei) and by the time we'd be looking at it we ought to mostly be in passive houses anyway.

An interesting bit about the depletion of the 1.5GW californian plant that's obivously been extracting heat faster than it can be replenished.

ah, you meant this bit?

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.

that's potentially a useful rule of thumb guide.

I don't quite understand the extraction rate bit though, because surely that depends on the spacings and dimensions of the pipework used to input and extract the heat.

I also don't quite get why he's given a figure for extraction rate averaged over the year, when he talks about recharging in summer. Actually, I've just done the sums, and that figure is defintely the extraction of 45GJ averaged over an entire year. So the answer to his problem with it being half the typically desired amount is blatently obvious - don't extract for 6 months of the year, when you're recharging it, then extract at double the rate for the time when you actually do need the energy... problem solved isn't it?

Posted By: SteamyTeaHave I ever mentioned Thermal Inertia I=root(kpc), it answers all the questions.http:///forum114/extensions/Vanillacons/smilies/standard/wink.gif" alt="" title="" >

but if you halved the spacings of the heat extraction pipes & doubled the surface area of the pipes in contact with the thermal mass, then you should still be able to extract the same overal quantity of heat at double the rate.

or actually, maybe it's more complex than that, but there must surely be a level of decreased spacings / increased surface area at which the extraction rate could be doubled.

So you could extract the same overall volume of heat, just at double the rate of extraction, so over 6 months rather than a year.

Gavin, I think what they try to achieve is a balance, input=output, all the Thermal Inertia does is determine that rate for a given size and shape. As long as the extraction rate does not exceed the replenishment rate of a season (assuming inter-seasonal here) then the method of heat extraction is really irrelevant. What would matter is the rate that heat is needed, or what sort of building it is used in. That is really what sets the size of the thermal store in all cases.
When calculating this sort of thing I find it easier to work out the energy amounts from temperature differences rather than try and work out the temperatures from the change in energy amounts. It is what Absolute Zero was invented for.

Posted By: SeretGSHP is geothermal of course

All depends where you are, but generally it is considered solar energy.

No, you need to be hundreds of metres down to be getting any significant portion of geothermal anywhere in the UK, AFAIK, and any ground-water movement in that column would be far more significant thermally.

Rgds

Damon

Posted By: SteamyTeaGavin, I think what they try to achieve is a balance, input=output, all the Thermal Inertia does is determine that rate for a given size and shape. As long as the extraction rate does not exceed the replenishment rate of a season (assuming inter-seasonal here) then the method of heat extraction is really irrelevant. What would matter is the rate that heat is needed, or what sort of building it is used in. That is really what sets the size of the thermal store in all cases.
When calculating this sort of thing I find it easier to work out the energy amounts from temperature differences rather than try and work out the temperatures from the change in energy amounts. It is what Absolute Zero was invented for..

I appreciate this, but as far as I can see this isn't what they've done, they've simply made a mistake and calculated total heat stored, then the average heat output per hour from the year possible from that store.

My point is that in an inter seasonal store, it would only be drawing that heat for 6 months at most, more likely 4 months, not all 12 months, so providing you size and space the extraction tubes accordingly you ought to be able to extract it at double or triple the rate given, which instantly puts it into the useful energy storage category as it pretty much matches the heat demand for a very well insulated house (eg a 4kW heat pump).

Posted By: SeretHaving said that I'm not a geologist, I'm just trotting out the what I've been told, and am quite happy to admit that I could be talking out of my borehole.

I'm afraid what you've been told is incorrect. This is why, really, GSHP systems should not be called geothermal (as they're not), but they should be called ground-exchange heating systems. As I said, the borehole temperatures are equal to the annual average air temperature at that location. Where I am, it's about 5C. In the southern US, the temperatures are more like 20C - and both locations (as we're not near any close-surface volcanic activity) have about the same core earth heat input.

For my particular system, I calculated that the incident solar radiation over a year (assuming it doesn't extend beyond my property boundary - which is only 25 feet by 110 feet) - is at least 7x more than the heat I remove per year for heating.

Of course, there are locations (such as Southampton) where there are "hot rocks" relatively close to the surface, and these, indeed, can be used for true geothermal applications.

Paul in Montreal.

Posted By: Paul in Montreal

I'm afraid what you've been told is incorrect. This is why, really, GSHP systems should not be called geothermal (as they're not), but they should be called ground-exchange heating systems. As I said, the borehole temperatures are equal to the annual average air temperature at that location. Where I am, it's about 5C. In the southern US, the temperatures are more like 20C - and both locations (as we're not near any close-surface volcanic activity) have about the same core earth heat input.

You'd expect that though. The different air temperature will mean that the heat transfers out of the crust at a different rate. I couldn't tell you whether that would account for the whole difference, but it would factor.

I might look into it a bit more though. From a quick skim through some related papers it does state that what affects the performance of the borehole is predominantly the thermal conductivity of the grout and the surrounding rock. And incidentally DamonHD the same paper (Claesson, Hellstrom 2011) confirms that axial heat transfer is negligible in "long boreholes", although it doesn't state what it classes as being "long" for GSHP boreholes. But I haven't found anything which can tell me what proportion of the actual energy stored in the first couple of hundred metres of rock comes from above and below. Certainly the performance of the borehole is dominated by geological properties, and there seems to be no mention in the papers I've seen of solar inputs but I wouldn't be surprised at all if the energy input was substantially solar, so perhaps it's best thought of as a kind of hybrid "geosolar" system.