# Are ground source heat pumps reversible, for heat storage?

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Descriptions of ground source heat pumps (GSHP's) talk about extracting heat from the ground for residential heating, but I haven't heard of putting heat back into the ground as thermal energy storage using these systems. Does anyone do that? Would it be efficient - or at least cost effective - where an oversupply of say, solar energy during warmer seasons can be used?

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Posted (edited)
5 minutes ago, Ken Fabian said:

Descriptions of ground source heat pumps (GSHP's) talk about extracting heat from the ground for residential heating, but I haven't heard of putting heat back into the ground as thermal energy storage using these systems. Does anyone do that? Would it be efficient - or at least cost effective - where an oversupply of say, solar energy during warmer seasons can be used?

Have you any references for this?

I had thought that the layer of the Earth used for heat pumps is considered as an effectively infinite reservoir (unlike the air) at a constant 50oF over the whole land area of the globe. Deep bore vertical systems are even warmer of course.

This means that any amount of heat you extract or add is negligible in comparison and does not change the temperature.

Edited by studiot

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Posted (edited)

Studiot, GSHP's take heat from the surrounding ground and has to reduce it's temperature.

Quote

As the GSHP is removing heat from the ground (for heating) or adding it (for cooling), the operation of the system can affect its lifespan. For example, if heat is extracted from the ground more quickly than it is replenished by solar or geothermal heating (or groundwater flow), the effectiveness of the system will diminish over time. (British Geological Survey)

I would read that as being replenished by solar warming of the soil surface. I recall reading that summer heat from the surface slowly replenishes the heat that was taken out over winter, and calculation of the ideal depth of the ground pipes is based on that. The source cited above mentions GSHP being used for summer cooling, which does return heat to the ground - but I am not aware of it used intentionally as thermal energy storage, eg. diverting excess solar PV through summer; I would expect raised ground temperatures should improve winter effectiveness for heating.

I live in a warm climate, where this technology is not in  common use and am not familiar with it - although air-source heat pumps are becoming common for efficient electric hot water systems. Those are becoming a popular and cost effective alternative to passive solar hot water for households with solar PV especially, using solar electricity that otherwise would go back to the grid, although also popular for those without rooftop solar who might otherwise use inefficient resistance type water heaters.

Edited by Ken Fabian

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3 minutes ago, Ken Fabian said:

Studiot, GSHP's take heat from the surrounding ground and has to reduce it's temperature.

It’s a matter of how much. Heating air takes relatively little energy, as compared with a larger volume of ground, so the temperature drop is correspondingly smaller. Plus, the ground is coupled to its surroundings. Any significant drop in temperature would result in heat flow in from the area around the GSHP.

similarly, if you raised the temperature, heat would flow out. Any ability to store energy would be short-term.

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Posted (edited)
38 minutes ago, swansont said:

It’s a matter of how much. Heating air takes relatively little energy, as compared with a larger volume of ground, so the temperature drop is correspondingly smaller. Plus, the ground is coupled to its surroundings. Any significant drop in temperature would result in heat flow in from the area around the GSHP.

similarly, if you raised the temperature, heat would flow out. Any ability to store energy would be short-term.

The essential nature of GSHP's is that (where they are reliant on summer warming of the ground, which I believe most are) the ground stores heat accumulated through summer which is used in winter - I would not call that short-term storage. During the summer phase the flow of heat would be outwards from the pipework, diminishing with distance but would not be lost except to the surface - or where groundwater flows are involved, lost to those, in which case it would not be suitable for the kind of use as storage I am talking about. The heat becomes part of the surrounding geothermal heat that replenishes what is taken by the pipework in winter. I don't see a lot of room for outright loss of that heat even through loss to the surface - because during the summer the temperature gradient through to the surface would be less, and closer to the surface, reversed.

Edited by Ken Fabian
improve clarity

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Posted (edited)

If this is being or were to be done - storing excess energy eg solar PV but other sources also, as ground heat during summer for winter - then I would think the efficiency of the system would improve if the electricity to heat part is not done with resistance heating, but uses air source heat pump type water heaters. Cost effectiveness would still be a question, although GSHP's are already one of the most cost effective forms of heating.

Edited by Ken Fabian

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On 1/5/2019 at 3:54 PM, studiot said:

Have you any references for this?

I had thought that the layer of the Earth used for heat pumps is considered as an effectively infinite reservoir (unlike the air) at a constant 50oF over the whole land area of the globe. Deep bore vertical systems are even warmer of course.

This means that any amount of heat you extract or add is negligible in comparison and does not change the temperature.

Ground water temps here where I live are 70 degrees fahrenheit, are you speaking of a world wide average?

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5 minutes ago, Moontanman said:

Ground water temps here where I live are 70 degrees fahrenheit, are you speaking of a world wide average?

Yes and they can reach boiling in certain tributories of the Amazon and places like Iceland if there is local tectonic activity.

I should have been more specific and specified stable ordinary ground free of such.

23 hours ago, Ken Fabian said:

Studiot, GSHP's take heat from the surrounding ground and has to reduce it's temperature.

Discussion goes better if you read what I wrote more thoroughly.

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4 minutes ago, studiot said:

Yes and they can reach boiling in certain tributories of the Amazon and places like Iceland if there is local tectonic activity.

I should have been more specific and specified stable ordinary ground free of such.

Discussion goes better if you read what I wrote more thoroughly.

There is no local tectonic activity, the water table here is ~70 degrees, might be 68 to 72, I'd have to look it up to be sure but it has to do with the average air temps from what I've been told...

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32 minutes ago, Moontanman said:

There is no local tectonic activity, the water table here is ~70 degrees, might be 68 to 72, I'd have to look it up to be sure but it has to do with the average air temps from what I've been told...

Interesting.

I think there is some fault movement and minor quaking in N. Carolina generally.

And far down is that water table?

Is the water warmer further down?

But of course if the water is already that warm how would Ken store more heat in it?

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Posted (edited)
2 hours ago, studiot said:

Discussion goes better if you read what I wrote more thoroughly.

We seem to be talking about different things and I may not have been clear enough. I am talking about the kind of GSHP's where pipework is buried in the ground nearby (1 or 2 metres under) and relies on seasonal ground warming to replenish the heat. I had thought it was the most common sort of GSHP but I could be wrong there.

The British Geological Survey link supports what I said about this type of system - that efficiency declines if heat is not replenished quickly enough, which I (legitimately I think) take to mean GSHP's use lowers the temperature of the ground being used as thermal mass (when used to extract heat). I don't see how drawing heat out could have no affect on local ground temperatures radiating out from the pipework.

This kind of buried pipework -

Yes, there are variants that make use of geothermal heat that have nothing to do with seasonal ground warming - heat replenishment by local volcanic/hydrothermal sources, hot rock (heat from natural radioactive decay) or underground water, but for those relying on that seasonal warming of the ground the potential to deliberately add more heat over summer for use in winter is there. I think it might reduce the overall size and extent of underground pipework, reducing construction costs, to have seasonal warming supplemented this way. Note I am asking about inclusion of diverted energy to this type of thermal storage being done and if it would be beneficial - not insisting it is.

Edited by Ken Fabian

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There's this sort of thing.

Not sure surface layer is enough though. Might need to drill or go with a tank.

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17 hours ago, studiot said:

Interesting.

I think there is some fault movement and minor quaking in N. Carolina generally.

And far down is that water table?

Is the water warmer further down?

But of course if the water is already that warm how would Ken store more heat in it?

Not very far, less than a meter in many areas, we live on a cretaceous sand pile. You can dig a well with a water hose, lots of shark teeth and small bones are often brought to the surface when drilling a well... The groundwater is usually anoxic and at one time if you dug a hole would would flow out under pressure, still does in a few places like boiling springs lake.  I use to scuba dive and in the bottom of some lakes there would often be what looked like a sand fountain as water flowed up through the sand... If you go deep enough you run into a layer of marl which is mined for concrete and graveling roads...

The small quakes are called the seneca guns, I've heard and felt them many times, they are usually written off as offshore methane clathrate releases...

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On 1/7/2019 at 3:04 PM, Endy0816 said:

There's this sort of thing.

Not sure surface layer is enough though. Might need to drill or go with a tank.

None of those appeared to be precisely what I had in mind, despite many elements in common. I was just thinking some GSHP systems - even pre-existing ones - look suitable to be used as heat storage. The advantages I see would be in making do with less pipework and excavations, but it does sound like there are a lot more ways of doing ground source heating and seasonal energy storage than I first thought.

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On 1/7/2019 at 1:33 AM, Ken Fabian said:

1)

We seem to ﻿be talking about different things and I may not have been clear enough. I am talking about the kind of GSHP's where pipework is buried in the ground nearby (1 or 2 metres under) and relies on seasonal ground warming to replenish the heat. I had thought it was the most common sort of GSHP but I could be wrong there.

The British Geological Survey link supports what I said about this type of system - that efficiency declines if heat is not replenished quickly enough, which I (legitimately I think) take to mean GSHP's use lowers the temperature of the ground being used as thermal mass (when used to extract heat). I don't see how drawing heat out could have no affect on local ground temperatures radiating out from the pipework.

This kind of buried pipework -

2)

Yes, there are variants that make use of geothermal heat that have nothing to do with seasonal ground warming - heat replenishment by local volcanic/hydrothermal sources, hot rock (heat from natural radioactive decay) or underground water, but for those relying on that seasonal warming of the ground the potential to deliberately add more heat over summer for use in winter is there. I think it might reduce the overall size and extent of underground pipework, reducing construction costs, to have seasonal warming supplemented this way. Note I am asking about inclusion of diverted energy to this type of thermal storage being done and if it would be beneficial - not insisting it is.

2)

Nor was I, I was responding to Moon's useful input.

1)

This is all words, in particular what do you mean by "local ground temperatures" and "heat is not replenished quickly enough"

So here are some proper engineering calculations.
I have taken my figures for calculation from a Leeds University study and appended their PDF.
I am sorry I have lost the web reference so my thanks if anyone can find it.
This study is interesting in that it was primarily aimed at utilising excavations for piling.
The technique could also be used to cool the building, but as the following calculations show, longer term heat storage (months) is difficult because the heat would dissipate into the surrounds in a shorter time scale.

So using the following symbols

A = area in square metres
Q = power flow in watts per square metre
L = distance in metres
K = Soil thermal conductivity or heat transfer coefficient in watts per metre-degree.
$\theta$ = temperature in degrees C

So the controlling equation of heat flow is equation 1 in the Leeds article

$K = \frac{Q}{A}*\frac{L}{\theta }$

or rearranging

$\theta = \frac{Q}{{KA}}L$

Differentiate with respect to distance give the temperature gradient in degrees per metre.

$\frac{{d\theta }}{{dL}} = \frac{Q}{{KA}}$

Rearranging

$Q = KA\frac{{d\theta }}{{dL}}$

Now let us put some numbers into this.
I was quoted that I needed 100 metres of buried pipe for a GS heat pump so taking this figure, and considering 3 sides of a box around the pipe 1 metre from the pipe.
That is considering the faces 1 metre below and to each side of the box, but not the top since that is in contact with the upper soil, which we are discounting, we find that the box has sides 2 metres so the face area is 3 *2*100 = 600 square metres. Or A = 600.

Taking an average values of 3 Watts/metre-degree for K from the Leeds table and limiting the temperature gradient to 2oC per metre thus making the pipe 2oC colder than the interface areas we find that

$Q = KA\frac{{d\theta }}{{dL}} = 3*600*2 = 3600$  watts

So with minimal temperature drop this system can sustain nearly 4 kilowatts continuous transfer in or out of the soil.

Edited by studiot

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Are you talking about using ground water directly as a heat sink? Groundwater moves, it is generally not static, this effect should have an effect on direct ground water use or sealed buried pipes...

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On 1/11/2019 at 2:01 AM, Moontanman said:

Are you talking about using ground water directly as a heat sink?

Moontanman - I am not. What I've been talking about would have sealed pipework and would be unsuited to places with groundwater flows. There are examples of heat storage that utilise static underground aquifers, some using the natural water and others with separate fluid, usually glycol mix . But it does sound like flowing groundwater has it's own advantages for ground source heating and cooling if you have it.

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On 1/10/2019 at 11:50 PM, studiot said:

This is all words, in particular what do you mean by "local ground temperatures" and "heat is not replenished quickly enough"

Studiot - seasonal ground storage of heat (without insulation) is being done, although references I've found are to deeper boreholes rather than the near surface type systems I had been thinking about. There are larger installations, but they do seem to be marketing towards individual households also. These appear to be systems designed and managed for a net zero exchange of heat over each year; they can freeze the ground they are in if too much heat is taken out. Long running imbalances will accumulate.

The descriptions I found were with boreholes at varying depths - mostly below 150m and as deep as 500m, with holes between 4 and 5 metres apart, so each effectively operates within a heat mass within 2 - 3m from the heat exchanger. That is not a lot more than trench type GSHP systems.

It looks like your calculations are based on unidirectional flow of heat in a column of earth.  I think the volume of material with boreholes - radially outwards from linear borehole -  would increase at pi squared of the distance. Near surface trench type systems might be more sheetlike, so volume doubles with distance, plus the sides.  Either way I would expect the temperature gradient - and flow rates - to drop off as distance increases - (but not stop entirely). Nor is the flow in one direction - what happens when the flow is reversed? Whilst any underground heat store with significantly raised working temperatures loses heat, I expect the rate it loses it should slow over time within the most utilised volume and (I think) may involve some 'pre-heating' that remains unrecoverable. I think the nature of these systems is that they do successfully - and efficiently - store heat seasonally, but they need to be designed and managed according to their characteristics.

I don't know what design changes might occur if lots of added heat were factored in; deeper trenches maybe? A secondary set of pipework at a different depth? I just don't see problems with technical feasibility of 'enhanced' heating being incorporated into these systems.

------------------------------

(not necessarily the most current or best sources - better ones would be welcome) -

Edited by Ken Fabian

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13 hours ago, Ken Fabian said:

Moontanman - I am not. What I've been talking about would have sealed pipework and would be unsuited to places with groundwater flows. There are examples of heat storage that utilise static underground aquifers, some using the natural water and others with separate fluid, usually glycol mix . But it does sound like flowing groundwater has it's own advantages for ground source heating and cooling if you have it.

---------------------------------

Studiot - seasonal ground storage of heat (without insulation) is being done, although references I've found are to deeper boreholes rather than the near surface type systems I had been thinking about. There are larger installations, but they do seem to be marketing towards individual households also. These appear to be systems designed and managed for a net zero exchange of heat over each year; they can freeze the ground they are in if too much heat is taken out. Long running imbalances will accumulate.

The descriptions I found were with boreholes at varying depths - mostly below 150m and as deep as 500m, with holes between 4 and 5 metres apart, so each effectively operates within a heat mass within 2 - 3m from the heat exchanger. That is not a lot more than trench type GSHP systems.

It looks like your calculations are based on unidirectional flow of heat in a column of earth.  I think the volume of material with boreholes - radially outwards from linear borehole -  would increase at pi squared of the distance. Near surface trench type systems might be more sheetlike, so volume doubles with distance, plus the sides.  Either way I would expect the temperature gradient - and flow rates - to drop off as distance increases - (but not stop entirely). Nor is the flow in one direction - what happens when the flow is reversed? Whilst any underground heat store with significantly raised working temperatures loses heat, I expect the rate it loses it should slow over time within the most utilised volume and (I think) may involve some 'pre-heating' that remains unrecoverable. I think the nature of these systems is that they do successfully - and efficiently - store heat seasonally, but they need to be designed and managed according to their characteristics.

I don't know what design changes might occur if lots of added heat were factored in; deeper trenches maybe? A secondary set of pipework at a different depth? I just don't see problems with technical feasibility of 'enhanced' heating being incorporated into these systems.

------------------------------

(not necessarily the most current or best sources - better ones would be welcome) -

Quote

It looks like your calculations are based on unidirectional flow of heat in a column of earth.

All heat flow is unidirectional.

That is the Zeroth Law of Thermodynamics.

I can't see how my description can be interpreted as a column?

Did you understand my calculations, they were only an engineer's 'ballpark' type?

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7 hours ago, studiot said:

All heat flow is unidirectional.

That is the Zeroth Law of Thermodynamics.

In the very real sense that sometimes heat flows from the house, and sometimes it flows into the house.

It's a bit like saying water flows down hill.

That's true- except in a discussion about water pumps.

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11 hours ago, studiot said:

I can't see how my description can be interpreted as a column?

Did you understand my calculations, they were only an engineer's 'ballpark' type?

My maths skill are not great but what I don't understand is your choice to treat the flow as from a constant surface area of (3 sides of) an arbitrary box 1m x 1m around the pipe  - ie treated like a column with that effective cross section - rather than dealing with a volume and effective surface area which increases as the distance increases. The ground above does behave differently but probably should not be left out, and trench layout may make it effectively a sheet-like flattened pipe rather than a pipe; I don't expect the calculations to be within my abilities - but I don't think these will make seasonal heat storage ineffective or involve large, ongoing heat losses.

I am not convinced of the applicability of your calculations and I disagree that the conclusion that storing heat in uninsulated earth cannot work is demonstrated by them. I suppose I am going off the fact that borehole heat storage is being used and the claims for it's effectiveness seem sound. Also there is effective summer to winter heat storage in earth at the depths used for trench type GSHP's or they would not work. Systems like this often do operate in both directions so they do pump heat into the ground in summer - and if more heat is added, a significant amount of that heat will be available in winter and heating will be more efficient.

I do think these kinds of energy technologies will be increasingly important in a world where fossil fuel burning is being curtailed.

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I tried reading all the above including some of the math but it just made my head hurt. Look, here are a few thoughts from a person who works in air-conditioning, a lot. Unless you are talking a system I don't know, the ground link is effectively the heat rejection component of the refrigeration cycle. And that's it really.

The heat rejection component works equally as well as heat providing - if the temps work and if your refrigeration equipment is reversible. You'll still need a three pipe system if you want heating and cooling at the same time. The ground link would usually presume a heat rejection ground source at approx 16 degrees C. And it can return heat at the same temp personally I'd add a gas boiler but that's a personal choice -  For winter cycle I'd like a heat rejection supply  temp at say 30 degrees C.

But that's just me.

Edited by druS

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11 hours ago, Ken Fabian said:

I do think these kinds of energy technologies will be increasingly important in a world where fossil fuel burning is being curtailed.

Agreed 100%

9 minutes ago, druS said:

I tried reading all the above including some of the math but it just made my head hurt. Look, here are a few thoughts from a person who works in air-conditioning, a lot. Unless you are talking a system I don't know, the ground link is effectively the heat rejection component of the refrigeration cycle. And that's it really.

The heat rejection component works equally as well as heat providing - if the temps work and if your refrigeration equipment is reversible. You'll still need a three pipe system if you want heating and cooling at the same time. The ground link would usually presume a heat rejection ground source at approx 16 degrees C. And it can return heat at the same temp personally I'd add a gas boiler but that's a personal choice -  For winter cycle I'd like a heat rejection supply  temp at say 30 degrees C.

But that's just me.

Yes all agreed and well put, from a professional in the field. +1