[srobert (OP)]

I've been looking at property and I've come across a couple where heating is provided by a ground source heat pump. As far as I understand it it works by circulating water through pipes buried just a couple of meters below ground (not deep enough for geothermal). The point is that the temperature a couple of meters down is a little above the ambient temperature. However I presume the temperature under ground is still lower than the home you are trying to heat. So in terms if the thermodynamics of the system how does this work?

[CliffordK]

A classic heat pump works like the opposite of a refrigerator or air conditioner.

Freon liquid is allowed to evaporate to create a cool gas phase.  This is then passed trough a warming coil outside the house, to warm the freon gas to ambient temperature. 

The now "warm" gas is then recompressed to form a liquid, essentially concentrating the heat, and making it much hotter.  This now hot liquid is passed through the heat exchanger inside the house.  Then the whole process is repeated.

Since one is essentially extracting and concentrating ambient heat, this whole process is more efficient than using ordinary electric "resistance heating".  Perhaps up to 5 times more efficient.

The problem is that the air outside is usually coldest when one wants to heat the house, and warmest when one wants to cool the house.  And, when the outside air temperature drops below freezing, the system efficiency essentially drops to that of a "resistance heater".  In fact, sometimes the systems will just use an axillary resistance heater.

Somewhere around 10 feet down, the earth stays at more or less the average annual temperature.  So, it tends to be warmer than the ambient temperature in the winter, and cooler than the ambient temperature in the summer.  Thus, it provides a better exchange medium than air.

You can have two types of systems.  If your house is on well water, you can have an "open system" in which the well water is pumped out of the ground, through the system, and discarded.  This would be particularly good if you have a shallow well with an ample water source, and minimal risk of lowering the water table, and adequate water disposal provisions.  A deeper well would require more energy to extract the water (but it may be slightly warmer coming out of the ground).  And, obviously one would be concerned with lowering the water table in arid locations, or places where there is little replenishment of the water table.

A closed loop system is what you are describing, where a deep trench is dug, and the water lines (or heat exchange media lines) are run underground.  I would assume you loose some efficiency with localized heating/cooling of the soil.  But, that would depend on the area of the heat exchange media, depth, and etc.  Some places with a deep freeze can freeze down a few feet of depth, and would need the exchange medium to be buried quite deep. 

One could presumably modify the system to use two wells, and inlet and an outlet well which might increase efficiency somewhat, however, typically one wants a one-way flow of well water to avoid contaminating the groundwater.

I assume you can monitor the wintertime inlet temperatures that should be at least 50 degrees or so.

[Geezer]

A classic heat pump works like the opposite of a refrigerator or air conditioner.

Maybe not so much the opposite of a refrigerator - more that some parts of the refrigerator are not in the expected places.

The bit that evaporates the refrigerant to absorb heat from inside the refrigerator is moved outside the house so that it can absorb heat from an external source like ambient air. The bit that gives up the heat as the refrigerant condenses is exposed to the ambient air inside the house as it is with a domestic refrigerator.

[MikeS]

srobert

As you rightly point out it is not geothermal.  It is solar.  The heat collector pipes cover a considerable area and although there may not be much heat available per square metre there is considerable heat available if the area is large enough.  The heat pump concentrates that heat.

[Geezer]

As you rightly point out it is not geothermal.  It is solar.

Depending on location it may be predominantly solar, but there is a geothermal component too. The Earth is exothermic.

[MikeS]

As you rightly point out it is not geothermal.  It is solar.

Depending on location it may be predominantly solar, but there is a geothermal component too. The Earth is exothermic.

True but insignificant.

Obviously I was generalising.  It used to be thought that a significant portion of the heat was geothermal but was later realised to be solar.  Under normal conditions the geothermal component is insignificant.  Geothermal is only cost effective in very limited areas of the Earth.

(Example geothermal energy when available can be used to drive power plants but if over used can sometimes lead to minor earthquakes.)

[Geezer]

The heat pump concentrates that heat.

Strictly speaking, it doesn't. Heat is a form of energy. In the SI system it has units of joules. There is no change in the amount of energy, and the energy didn't somehow get "stronger".

What the heat pump does is collect energy (heat) from one place and transport that energy to another place, a heat-sink inside the house.

[Geezer]

As you rightly point out it is not geothermal.  It is solar.

Depending on location it may be predominantly solar, but there is a geothermal component too. The Earth is exothermic.

True but insignificant.

How insignificant is it?

[MikeS]

The heat pump concentrates that heat.

Strictly speaking, it doesn't. Heat is a form of energy. In the SI system it has units of joules. There is no change in the amount of energy, and the energy didn't somehow get "stronger".

What the heat pump does is collect energy (heat) from one place and transport that energy to another place, a heat-sink inside the house.

I used "concentrate" in the sense that a ground heat pump takes low quality heat (low temperature) from a large volume or area and concentrates it into a much higher temperature in a lower volume or area.  As you say the total amount of energy remains the same.  Overall entropy increases as the heat pump is powered by electricity.

[MikeS]

As you rightly point out it is not geothermal.  It is solar.

Depending on location it may be predominantly solar, but there is a geothermal component too. The Earth is exothermic.

True but insignificant.

How insignificant is it?

Insignificant enough not to be considered.

Quote
"Ground source heat pumps are also known as "geothermal heat pumps" although, strictly, the heat does not come from the centre of the Earth, but from the Sun." " Ground source heat pumps harvest heat absorbed at the Earth's surface from solar energy. The temperature in the ground below 6 metres (20 ft) is roughly equal to the mean annual air temperature at that latitude at the surface."
http://en.wikipedia.org/wiki/Geothermal_heat_pump

[Geezer]

I used "concentrate" in the sense that a ground heat pump takes low quality heat (low temperature) from a large volume or area and concentrates it into a much higher temperature in a lower volume or area.  As you say the total amount of energy remains the same.  Overall entropy increases as the heat pump is powered by electricity.

I figured that, but we should try to be precise about these things, otherwise we will confuse ourselves, and our readers.

The term "low quality heat" is frequently used, but it's meaningless. There is is only heat (energy), and it has no variation in "quality". A substance can be hotter, or colder, so that we have more or less heat per mass of the substance.

Entropy may have increased, but the important point is that work had to be done by the pump to transport the energy from a cooler place to a warmer place.

[Geezer]

"Ground source heat pumps are also known as "geothermal heat pumps" although, strictly, the heat does not come from the centre of the Earth, but from the Sun." " Ground source heat pumps harvest heat absorbed at the Earth's surface from solar energy. The temperature in the ground below 6 metres (20 ft) is roughly equal to the mean annual air temperature at that latitude at the surface."

The mean annual air temperature at that latitude has to take into account geothermal radiation at that latitude.

What would you call "insignificant", and who would determine that the value should not be considered?

[MikeS]

I used "concentrate" in the sense that a ground heat pump takes low quality heat (low temperature) from a large volume or area and concentrates it into a much higher temperature in a lower volume or area.  As you say the total amount of energy remains the same.  Overall entropy increases as the heat pump is powered by electricity.

I figured that, but we should try to be precise about these things, otherwise we will confuse ourselves, and our readers.

The term "low quality heat" is frequently used, but it's meaningless. There is is only heat (energy), and it has no variation in "quality". A substance can be hotter, or colder, so that we have more or less heat per mass of the substance.

Entropy may have increased, but the important point is that work had to be done by the pump to transport the energy from a cooler place to a warmer place.

"Ground source heat pumps are also known as "geothermal heat pumps" although, strictly, the heat does not come from the centre of the Earth, but from the Sun." " Ground source heat pumps harvest heat absorbed at the Earth's surface from solar energy. The temperature in the ground below 6 metres (20 ft) is roughly equal to the mean annual air temperature at that latitude at the surface."

The mean annual air temperature at that latitude has to take into account geothermal radiation at that latitude.

What would you call "insignificant", and who would determine that the value should not be considered?

That's what I thought I said
"I used "concentrate" in the sense that a ground heat pump takes low quality heat (low temperature) from a large volume or area and concentrates it into a much higher temperature in a lower volume or area."

I agree and that's what I said
"Overall entropy increases as the heat pump is powered by electricity."

"Geothermal radiation"What is that? Geothermal is geothermal and solar radiation is just that but I don't know what you mean by geothermal radiation.

To the best of my knowledge geothermal ground heat (exothermic) is generally not applicable to the design of most (domestic) ground heat pump systems.  I guess any extra heat it does produce is a bonus. 

"Averaged over the earths surface, the (geothermal) heat energy flow is 0.06 Watts per square meter (500 times less than incoming solar energy flux)"
http://zebu.uoregon.edu/1996/ph162/l18.html

[Geezer]

Thanks for the data re. solar versus geothermal. My objection to your original statement was that it implied geothermal energy made no contribution. It does make a small contribution, and the amount can vary significantly depending on location.

Regarding heat: Heat is energy. Energy cannot be destroyed or created. If it could be "concentrated" we would be able to solve the Earth's energy problems quite easily. I'm not trying to be pedantic - this is a very important point that is based on at least 200 years of scientific experiment.

Geothermal radiation is what prevents the Earth from turning into a blob of molten lava. We are sitting on top of an extremely large fission reactor (as in nuclear power station) that continuously produces a large amount of energy (heat) as the atomic particles within it decay.

The only way the Earth can maintain thermal equilibrium is by radiating this energy into the Earth's atmosphere.

[CliffordK]

The cause of Earth's heat is still being debated.  It is probably from multiple sources including primordial heat, friction in the core, and radioactive decay (not necessarily fission, but simple alpha/beta/gamma decay).

The surface temperature more or less matches the lower troposphere temperature, as you would expect at the junction between a heat sink and the atmosphere.

Atmospheric warming may be best thought of blocking the radiation of heat from the Earth's core.  In a sense, one can drill a borehole core, recording temperatures, and read previous surface temperatures like rings on a tree, and calculate the number of inches a year that the heat is rising.  Again, that would be more or less warm temperatures blocking the rising of the heat, and cool temperatures radiating more heat, and this effect being distributed deep down.

So, for the heat pump.  Is the distinction between geothermal and average atmospheric temperatures important?  Probably not that much.  One might ask that if one is taking heat out of the earth, where is that heat being replaced from.  While the ground temperature may be dependent on atmospheric temperatures, you would probably find that that heat is actually being replaced from a geothermal source from below.

I suppose that distinction might be important if you were on a small lot, and chose to use ground below your foundation for an exchange medium.

If you get deep enough, the ground temperature should be fairly constant...  and deeper than that, it should be warmer. 

However, I wonder if there would be a level where there would be a 6 month delay in the propagation of heat.  So, you would pick up July warming in December, and pick up December cooling in July.

[syhprum]

There is an old myth that a thaw drives the frost deeper which is of course another way of saying there is a delay.

[MikeS]

Geezer

My original answer was meant to be a practical, helpful and straightforward answer to srobert who had viewed properties with ground heat pumps.

quote Geezer
"We are sitting on top of an extremely large fission reactor."

Radioactive decay yes but fission?

[Geezer]

Fission can be spontaneous, although its role in heating the Earth is probably very small. I should probably have stopped at atomic decay.