1) Geothermal
2)Solar pv and solar thermal
3) Tidal and wave
4)Wind
5)Nuclear
6)people
7)?

Why only 1 and 2b? Aren't all of these “low-carbon” heat sources in the sense that they're all energy sources which can be used for heating (probably best via heat pumps in the case of sources which primarily produce electricity) which cause small carbon emissions in operation and probably have embodied carbon emissions of the same order of magnitude of most large-scale energy sources?

I'm kinda glad they frown on it!! Thinking of a few suspicious characters I know, and the thought of them fussing with fission or fusion within a short distance of my place turns me quite cold

good one!

Low carbon should include locally grown, sourced, processed and burnt biomass.

Nice, net low carbon solution....

Oh, wait, although you said low carbon, you really meant polution in general? Why are we still talking about carbon emissions on their own?

Do we mean low carbon or carbon neutral through it life cycle. Also low/no carbon does not automatically mean problem free. Geothermal has the negative effect of lowering the ground temperature significantly if used to intensively.

Jonti

Hi Paul - an old controversy between us

But how does that solar radiation (summer) get down there to replace the heat extracted (winter)?
Cos that downward heat flow doesn't happen 'naturally' - at a depth somewhere between 1.5 and 3m ground temp remains steady all yr round, at annual surface mean temp i.e. 'half way' between summer and winter av air temp. At that depth, heat is neither being added nor lost but the ground hangs at an equilibrium temp.

That is, until GSHP starts extracting heat from that stable-temp zone, as if it were an endless resource of heat. Leaving aside heat replacement by groundwater flow, assuming dry ground or ground with static water content, the GSHP must lower the temp of that hitherto stable-temp zone as it extracts heat.

The proposition is that the heat extracted is replenished from the surface by solar input, and the ground temp restored to its hitherto (pre-GSHP) stable temp.

What will persuade solar-originated surface heat to flow downward in large quantity, to such depth? Yes, in any ground there is such annual ebb and flow of heat, but 'naturally' only into the shallow layers of ground. What can cause that flow to reach much deeper, in order to replenish heat extracted by GSHP from deep down?

Can only be establishing an 'unnatural' delta-t gradient extending from surface all the way down to the extraction zone. The deep-ground end of the gradient must get lowered all winter long, so that when summer comes and the surface adopts its usual temps, a steep temp gradient then exists, and heat flows.

If that's the mechanism then GSHP does indeed mean that the ground it's extracting from does drop and drop in temp all winter - it does refrigerate the ground - and the summer replenishment cannot (in just one annual cycle) be to the same equilibrium temp that was previously established over many annual cycles.

There's a tiny contribution from that source (upward heat flow) - it's very weak by the time it's distributed over the earth's whole surface, to be dissipated to space.

Posted By: Cliff PopeOr is there a difference between "geothermal" and "ground-source"? Item (1) on the list is geothermal.

Yes there is! True geothermal uses hot rocks (which may be hot due to friction or to nuclear activity - the former case in places like Iceland, the latter case in places such as Cornwall with granite). Ground-source, also known as ground-exchange, uses surface heat which is predominantly solar in origin, even to a depth of a couple of hundred metres. There is, of course, a tiny amount of heat coming upwards from the Earth's core, but, in most places, this is tiny and can be neglected.

Posted By: fostertomIf that's the mechanism then GSHP does indeed mean that the ground it's extracting from does drop and drop in temp all winter - it does refrigerate the ground - and the summer replenishment cannot (in just one annual cycle) be to the same equilibrium temp that was previously established over many annual cycles.

My empirical experience contradicts this assertion Tom. Of course, during the heating season, the ground temperature does drop, but my experience has been that the water input temperature at the start of the next season is back to where it was, or even slightly higher (as we're having a run of hot summers).

Paul in Montreal.

p.s. there was a paper I found which demonstrated real climate change by measuring the deep ground (around 100m) temperature and observing that it was rising as the annual average air temperature was rising too

Posted By: Paul in Montreal

Posted By: JontiGeothermal has the negative effect of lowering the ground temperature significantly if used to intensively.

Are you sure? For properly sized systems, I don't think there's any evidence of this ... though in parts of the US where the loops were undersized, the ground ended up warming up (most of the usage was A/C). In my location with 10x more heating degree days than cooling degree days, there has been no noticeable shift in ground temperature. Computed insulation values show that there's at least 7x more solar radiation received in a year than is taken out as heat in winter even with the assumption that the radiation has to be restricted to the small area of land the vertical loop is located in.

Or do you mean real geothermal, where hot rocks are used to raise steam? In that case, you could be correct. What many people call geothermal is really "ground exchange".

Paul in Montreal.

Paul,

firstly, I defined my comment with the term 'too intensively'. Secondly, to suggest that by extracting heat from the ground actually raises the temperature as you do defies logic but then if it is true you have just solved the heating problems of the world

The more we extract, the warmer the ground gets so the more you will be able to extract sounds good if true but is it better to raise the temperature than it is to sink it?

I am stating the results of a research institute based in Basel, Switzerland which I read about 5 years ago though I no longer have the article.

Paul, you cannot get round the logic that if you extract something it will reduce in amount comparison to if you do not extract it. This is true regardless of the fact that the underlying temp. is rising or not.

I made the point to show that no matter what you chose to do it will have some effect. It never ceases to amaze me that so many people here are so against one thing seeing only problems and then champion something else as though it has no problems. Ground source heat pumps are fine but if you use then to heat an entire city by extracting the heat from beneath that city then it will drop the relevant ground temperature and the sun will not reheat as it will be absorbed and reflected by the buildings and streets but not the ground.

Jonti

Posted By: JontiPaul, you cannot get round the logic that if you extract something it will reduce in amount comparison to if you do not extract it. This is true regardless of the fact that the underlying temp. is rising or not.

My assertion, supported by anecdotal evidence of my own system, is that the average temperature has not appeared to have changed in the seven years I've been running my system. The entering water temperature at the start of winter seems to be about the same year on year - if anything, it has risen a bit, though the past couple of years have been significantly warmer than average and last year, in particular, had a very wet (and hot) summer - which could account for transporting more energy into the ground.

As for an entire city dropping the ground temperature - my calculations showed (as I'd had this argument with someone local who said that it couldn't work if "everyone had a GSHP) that, on my small area of land in the City (110x25 feet), I was receiving about 7x the solar radiation per year as I was pulling out in heat. Of course, I also run my system in A/C mode in the summer - but the amount of heat I put back every year is about 1/10 of the amount of heat I take out in winter.

Posted By: Ed DaviesNo he doesn't. PiM says that the ground is heated when the loop is used for air conditioning so inserting, rather than extracting, heat.

Exactly.

Paul in Montreal.

Posted By: fostertomPaul, what has to be explained about your anecdotal evidence, is why ground temp doesn't fall progressively over the winter and why it fully recovers by solar input over the summer (without the boost of dumped a/c heat). Maybe you have ground water movement bringing in a continuous supply of heat, thus diluting the cooling over a wide area? If so, then your system is an exception, or an alternative model, and doesn't provide answers to the Q kicked off by Jonti above.

That could well be the case. The only reports I've seen of (vertical) loop temperature change was some in the southern US where (a) the ground is dry and (b) the loops were too short for the cooling load. (a) was mitigated to some extent by introducing "soaker hoses" to inject water into the ground - I've also seen the same suggestion for horizontal loop problems in dry areas. I'm on a hill and I wouldn't be surprised if there's ground water movement, though there didn't appear to be anything significant when the drilling was done. Actually, this is the case for both systems I have - both in quite different locations but, again, there's the possibility that water movement in the ground helps. I've seen mention of using pea gravel rather then cementitious grout is an advantage in "wet" holes - maybe for this very reason.

Paul in Montreal.

Posted By: JontiAs to the second part of you question I find the lowering of the ground temperature by over extraction is no different and therefore just as bad as global warming raising temperatures.

At the end of the day, though, where has the heat come from that was extracted out of the ground? Maybe with judicious ground cooling global warming could be counteracted? Just joking! As it takes energy to move the heat in the first place and this energy has come from fossil fuel burning in the first place (in all likelihood, except, perhaps, in my case where our electricity is close to 100% hydro).

Posted By: JontiCan you answer me how the sun heats up the ground that is below buildings?

If all the ground is covered, you're right ... but the heat in the building does also escape into the ground too, unless it's perfectly insulated. I would think that, in most places, there is some groundwater movement that transports the heat, unless you're in a very arid area - and it was these arid areas that did report some problems with the ground heating up due to the large A/C load and undersized loops.

My A/C usage, though, does not put back all the heat that we take out during winter, but maybe the movement of the fluid in the ground loop does help re-distribute the temperature gradient more rapidly than would be the case if we didn't run air conditioning.

Paul in Montreal.

Is it that complicated Jonti ?

If you abstract heat locally, and the ground temperature reduces, you increase the temperature difference to any surrounding bit of ground and heat flows to the cooler region - basic thermodynamics.

You cannot "insulate" the ground any more than you can insulate a building or a pipe - you can only slow down the rate of heat transfer.

Regards

Barney

<blockquote><cite>Posted By: Jonti</cite>Barney,

so why is it always cold inside my fridge?
Jonti</blockquote>

Because you keep supplying it with electricity to run the pump that pumps the heat out that constantly leaks in across the insulation?

Anyone who has travelled on the London underground in recent summers might agree with me that it would be an idea to encourage ground source heating for the buildings above.

Jonti, although your grasp of the principles seems a bit unconventional in some repects, you are IMO correct about the ground temperature reducing. The answer is that it does, but if you were to do the sums you would find it to be reduced by a smaller amount than you apparently expect to achieve the extra heat capture needed to replace the heat extracted.