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Is a Ground-Source Heat Pump a Renewable Energy System?

Grouping ground-source heat pumps with solar water heaters and photovoltaic systems for tax credits has led to confusion

Posted on Jul 24 2012 by Allison A. Bailes III, PhD, GBA Advisor

Here's another rant that goes in my “drives me crazy” bin of articles. I'm in good company, too. Another article that ran at Green Building Advisor recently discussed making the choice between an air-source heat pump and a ground-source (a.k.a. “geothermal”) heat pump. At the end of the article, Peter Yost mentioned that ground-source heat pumps,“have been given quite the green ‘pass’ or ‘seal of approval’ because they are portrayed as using a ‘renewable’ energy source, and that makes me crazy.”

So there's your answer to the question I asked in the title: No! Ground-source heat pumps are not a renewable energy source. Let's all go out and have a productive day now.

Can a heat pump be classified as an energy source?

OK, I'll say a bit more. Remember that article I wrote about how your air conditioner works? And the one before that about how air-source heat pumps get heat out of cold air to heat your home in winter? A ground-source heat pump does exactly the same thing, with one difference.

In an air-source heat pump or air conditioner, you're pulling heat from the outside air and putting it into the home (heating mode) or putting heat from the home into the outside air (cooling mode). The diagram below shows what's going on. The heat exchange with the outside is going on in the part labeled as the condenser coil. In an air-source heat pump or air conditioner, that coil surrounds the noisy metal box that sits outside your home. The fan inside the condensing unit pulls outdoor air across that coil, and the refrigerant running through the coil either gives up heat to the outside (cooling mode) or picks up heat from the outside (heating mode).

The only substantial difference with a ground-source heat pump is that you're using the ground (or a body of water) instead of the outside air. The heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. in a ground-source heat pump isn't a coil but a loop of pipe carrying the working fluid. That loop of pipe can be horizontal or vertical (see the photo of a well, above), but its job is simply to exchange heat with the ground. It's doing the job of the condenser coil above. (Actually, there are two closed loops and three heat exchanges going on, but I've taken the liberty of simplifying the process here, which doesn't change anything regarding our original question.)

I've focused on air conditioners and heat pumps here, but the same analysis applies to refrigerators, which are just another kind of heat pump that uses the refrigeration cycle. All of these need an input source of energy to run the refrigerant through the system. In an air-source heat pump, you also need a blower to move air across the evaporator coil and through the duct system and another fan in the condensing unit. In a ground-source heat pump, the fan in the condensing unit is replaced with a pump for the working fluid in the ground loop.

So, is a refrigerator a renewable source of energy?

A poor choice of terminology

I think the main source of the confusion about ground-source heat pumps and renewable energy is the unfortunate use of the term “geothermal” in connection with these devices. When you hear the word “geothermal,” you think of lava or geysers, of volcanoes blowing their tops. You think of beautiful Icelandic maidens in steaming pools of hot water surrounded by snow. (Or is that just me?) You think of heat engines being driven and doing useful work by harnessing the heat from within the earth.

But that's not what ground-source heat pumps are or do. They're just like your regular air conditioner or heat pump except they use the ground instead of the air as the source or sink for heat. They still use electricity to power the pump that moves the working fluid through the loops. They still use electricity for the blower that moves the air through the duct system. They're still using even more electricity to run the compressor, which is the pump for the refrigeration cycle. All that electricity generally comes from outside the home, often from a power plant that burns coal or natural gas. The last I heard, most folks don't consider those fuels renewable.

Ah, but now that you mention Iceland, Allison, I hear you thinking, isn't a ground-source heat pump doing the same thing they do when they pump heat from their numerous hot springs and use it in buildings? They're using pumps to move the heat, too, just like a ground-source heat pump does? Well, yes and no. It appears at first to be the same thing, and we do indeed call the heat they're using a “renewable” source of heat. The difference, though, is that they're pumping high-grade heat that comes in at a temperature high enough to be used directly. They don't have to “concentrate” it through the use of a refrigeration cycle as a ground-source heat pump does.

I don't use the term “geothermal heat pump.” It's too confusing. Even Thomas Friedman, the columnist for the New York Times who's written some well-received books about the environment, is confused about this. He wrote in a column a few years ago that he's using renewable energy in his home because he installed a “geothermal” heat pump.

Really now. If a ground-source heat pump is a renewable energy source, then so is a refrigerator. Can you see why this drives me crazy?

Allison Bailes of Decatur, Georgia, is a RESNET-accredited energy consultant, trainer, and the author of the Energy Vanguard blog.

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Jul 24, 2012 7:53 AM ET

active or passive
by shane claflin

A passive system utilizes existing heat. In an active system, heat is generated. Passive is a more accurate term for something that is "renewable". The heat pump would be renewable if powered by a solar PV system. minus the refrigerant as a pollutant.

Jul 24, 2012 8:20 AM ET

Edited Jul 24, 2012 8:22 AM ET.

Shane, pollutant? So is the
by aj builder, Upstate NY Zone 6a

Shane, pollutant? So is the rest of the machine then, the metal, the PVC plastic covered wiring. Doesn't matter what label you "actively" attach. The least polluting is no home and no inhabitant. All the rest is closer to being similar than not.

This site is getting polluted.

Jul 24, 2012 8:53 AM ET

Response to Shane Claflin
by Martin Holladay, GBA Advisor

I don't think I agree with your definitions of "passive" and "active" -- namely that "a passive system uses existing heat" and "in an active system, heat is generated."

As applied to solar equipment, "passive" usually refers to design elements that don't require electricity, pumps, or blowers to operate (for example, south-facing glazing and interior thermal mass). "Active" usually refers to equipment that includes pumps or blowers that use electricity.

Jul 24, 2012 1:56 PM ET

I have rarely taken so much
by Lloyd Alter

I have rarely taken so much abuse as when I tried to do a jargon watch of geothermal vs GSHP. Commenters wrote "This strikes me as uninformed drive-by "journalism" if you can call it that."

I love the refrigerator analogy. It makes it all so clear.

Jul 24, 2012 2:59 PM ET

Renewable or non-renewable? I think...neither.
by Dustin Harris

Seems to me that a GSHP is a way of leveraging, of getting more heat or cool for the joule. It seems less akin to PV than it is to insulation: one is a way of producing energy (and thus the label of "renewable" makes sense) while the other is a way of getting more use out of the energy from whatever source.

Jul 24, 2012 5:08 PM ET

by shane claflin

Thanks for clarifying Martin, I was groggy this mornin. I was thinking more along the lines of passive energy. Something that doesn't require an input to produce heat or electricity. Solar PV, I would call a passive system, and solar hot water to capture heat, even with the pumps? is it semi-passive?

Jul 24, 2012 5:11 PM ET

response to AJ
by shane claflin

The metal and plastics are recyclable. I think the refrigerant ends up as a hazardous waste. I could be wrong, I was wrong once.

Jul 24, 2012 6:18 PM ET

Heartily agree, Allison.
by James Morgan

GSHPs are wildly oversold: a greenwash product which would not exist in the market without tax credits. Hardly a promising sign of sustainability.

Jul 24, 2012 6:38 PM ET

More on passive and active
by Martin Holladay, GBA Advisor

I have never heard the adjective "passive" applied to a photovoltaic system. A solar hot water system is an example of an active solar system -- clearly it isn't passive like south-facing glazing or a dark-colored slab floor.

Jul 24, 2012 7:48 PM ET

So is ground source more efficient than air source
by Danny Kelly

Good article but still wondering about the big question. Most "geothermal" salesman will tell you about the COP being twice that as an air source heat pump. I believe there are still federal and state tax credits for these systems as well - is that warranted?

Jul 25, 2012 5:18 AM ET

Edited Jul 25, 2012 5:19 AM ET.

Response to Danny Kelly
by Martin Holladay, GBA Advisor

Good data on the COP of installed residential ground-source heat pump systems are surprisingly rare. Manufacturers report COP data that omits electricity used by pumps, and this reported data is misleading.

The best data I know of come from two researchers: Rob Aldrich of Steven Winter Associates, who measured the COP of a residential system in Connecticut (COP = 3.5), and from Andy Shapiro, who measured the COP of three residential systems in Vermont (average COP = 2.75). The data were reported in the April 2008 issue of Energy Design Update.

Several researchers have calculated that a ground-source heat pump installation usually costs more to install than ductless minisplit air-source heat pumps plus a PV system to provide all of the electricity necessary for the air-source heat pumps. Even if the air-source heat pumps have a slightly lower COP than the ground-source heat pump, you still end up ahead.

Jul 25, 2012 8:20 AM ET

just because
by Robert Brown

I can't let this go, I'll note for the record on this one more time;

1. air source is better, in almost all cases.
2. Still, the energy you are getting from GSHP IS renewable when you are heating. you are pulling energy from the ground. energy returns to the ground via natural means. When you are REJECTING heat, it's an air conditioner removing unwanted thermal energy from your home.
3. The refrigerator is pulling energy from its source... the stuff in the box... and rejecting it to your house. bad analogy.
4. the problem with the renewable energy you get in heating mode GSHP, is that the parasitic losses are high in the pumps that geo MFGs like to use because apparently they have no idea how to pump water efficiently. I say that with a little snark, but only a little... most geo guys are not high grade hydronicists, they are mostly air guys. so the ratio of energy gained compared to energy used isn't significantly enough better than air source (current gen tech at least) to justify the cost differentials. In some cases it can even be worse. Some are starting to use ECM pumps, which will reduce this problem. They also have to redesign their heat exchangers to be less restrictive....

Jul 25, 2012 8:31 AM ET

Missing the Point - Wrong Analogy
by Chris Williams

I think this article is slightly missing the point, the heat pump itself IS NOT an energy source, just like a solar PV panel by itself, is not an energy source. However, where it's getting it's energy from IS an energy source. Solar PV = Sun, WaterSource Heat Pump = Ground. Oil Furnace = Oil.

Saying the ground is not an energy source is like saying an oil tank filled with 400 gallons is not an energy source, it doesn't make any sense. Is the air an energy source, yes. It has energy in it.

Regarding the ground source vs air source debate, let me gather some data and get back to you. Main issues when comparing the technology is 1) LCOE. Air source lasts for 15 years, Geo 30 years. Over the life of the system, air source BTUs are much more expensive 2) Any technically compacity that will make an air source unit better (compressor, fans) will also be applied to geo, because it's the same technically. If you get air source with a COP of 5, geo will be 7+, always.

Jul 25, 2012 8:33 AM ET

How Are you Defining Better
by Chris Williams


How are you defining better for air-source?


Jul 25, 2012 10:47 AM ET

Response to Chris Williams
by Martin Holladay, GBA Advisor

I understand that the COPs you cite are for illustrative purposes only. But these COPs ("air source with a COP of 5, geo will be 7+, always") are totally fanciful and unrealistic.

Like I said, measured COPs of installed ground-source heat pump systems seem to be in the range of 2.75 to 3.5.

Jul 25, 2012 11:08 AM ET

Forest for the trees
by Martin Orio

Groundsource heat pumps are a proven technology that extracts stored solar energy from the ground in winter and store solar gain in your property in winter. It is true that based on how they work they are not as easy to "classify".
They are no less a "renewable" then solar PV or thermal. Those systems use circulators to move solar energy.
So does a groundsource (aka geothermal) heat pump.
Further, a GSHP installed to ASHRAE 90.1 standards (not always done sadly), is the most efficient form of heating (operating SYSTEM COPs of up to 5.3), and EERs (not SEERs) of up to 31.7 currently available.
Although they do, by definition need to be connected to the earth, they are fully 2xs as efficient annually as their air-to-air cousins.
People who install a geothermal heat pump properly reduce there personal carbon footprint by 50% on average, and create an even baseload for the kilowatts derived from PV, wind, biomass, etc. as well as the dirty fossil burning technologies that currently provide for some of our electric demand. They just use as little as 20% of what the same electric investment would be for straight resistance heat.
Also, GSHPs used for cooling don't create a heat island by dumping solar gain back into the hot summer air, instead they store it underground (nature's clean battery) to be used the following winter for heating.
Solar, wind, hydro, it's all good. Classify it as you might, but don't discount geo! Use those green kilowatts to power the most effective and efficient space conditioning!!!

Jul 25, 2012 12:58 PM ET

Back-up Heat
by Harold Simansky

One point not to be lost is how the efficiency drops for air source heat pumps as the outside temperature becomes colder. Once the temp goes below 20 or so, depending on the model, you will need some sort of back-up heating system. For many it will be electric (ugh) for some maybe it will be a fireplace and for a few it may be an old oil system. At that point, the nod starts going to GSHP.

Jul 25, 2012 1:28 PM ET

Response to Harold Simansky
by Martin Holladay, GBA Advisor

Your information is out of date.

Mitsubishi Mr. Slim Hyper-heat (P series) is rated down to -13 degrees F. These models exhibit 100% of rated heating capacity at 5°F and 87% at -4°F.

Jul 25, 2012 2:19 PM ET

What should GSHP systems be called?
by Joshua Kresge

I can see how this drives you crazy. You have valid points. Heat Pumps are not sources of energy. The Sun is a source of energy, freely moving water is a source of energy.

The ground however is source of stored energy which can be harvested by using a system - that system was unfortunately labeled a Geothermal Heat Pump System a long time ago. The name is the #1 source of confusion.

"Geothermal? Cool lava in my backyard to generate heat" (Joe Homeowners first impression).

This whole debate could give a class of law students a whole semester of expensive & useless discussion.

The ground is a solar panel, a very big one. Its energy comes from the sun. The sun is renewable source of energy. Harnessing that energy is our mission - geothermal heat pump SYSTEMS extract energy from the ground in heating mode with the help of electricity (its source could be renewable or nonrenewable).

So is a GHP on its own a renewable energy system? No - its piece of metal. Is a GHP hooked up to a ground loop and electric & pumps a renewable energy system? It can be PART of one.
If the electricity used in the system were generated from a renewable source (like hydroelectric) the overall system could be considered "renewable", thus we'd have an expanded renewable energy system. If its electricity supply is a coal fired power plant than the overall system is technically non-renewable. Thats my take on it, but like Shane I have been wrong before.

But enough about academic interpretations - lets get to the real point here.

The point is that some believe the GHP industry has benefited from being portrayed as using a ‘renewable’ energy source.... and I agree. And whether or not you think it fits any definition the fact is that consumers think GREEN SAVE THE PLANT when they see geothermal portrayed as renewable. The message at our micro-level is incorrect, but the overall premise that geothermal is a more “green” system is correct. (by green I mean higher efficiency, lower impact on the environment, less expensive to operate).

Lets try something new here - lets propose solutions instead of the usual everyone bash everyone else's product and beliefs. Geothermal Heat Pump is not the best name for a ground source heat pump system because of the confusion it creates - if you could rebrand "Geothermal Heat Pump System" to something else what would it be?

Jul 25, 2012 3:29 PM ET

Response to Joshua Kresge
by Martin Holladay, GBA Advisor

Engineers and GBA have been calling these systems ground-source heat pumps for a long time.

Jul 25, 2012 3:44 PM ET

Response to Robert Brown
by Allison A. Bailes III, PhD, GBA Advisor

Rob, I disagree. A refrigerator and a GSHP do exactly the same thing. They use electricity to move heat. They also both do it with about the same coefficient of performance (COP). The only difference is where the heat is coming from and going to. The refrigerator, in fact, is a better analog than Iceland's pumping of heat from the ground, which is a true geothermal source.

Jul 25, 2012 3:53 PM ET

Response to Chris Williams
by Allison A. Bailes III, PhD, GBA Advisor

Yes, the ground can be a source (or sink) of heat. So can the air. The main reason it's not a renewable source of energy is because the input electricity has to do a lot of work through the refrigeration cycle to turn the heat into something usable.

Jul 25, 2012 4:14 PM ET

Response to Martin
by Joshua Kresge

I'm aware that most in the industry correctly call them Ground-Source Heat Pumps - but if you were to re-brand the entire idea to avoid confusion for home owners could you come up with a new name that made sense and appealed to the masses? My point is that GSHPs need to be presented and sold as something completely different than just another heat pump - after all the equipment might be similar but the customer experience and expectations are entirely different.

It wouldn't be "Renewable Heating Plant" or "Lava Comfort Generator" but it might be an "EcoPump System" or "GeoHeater" or "EarthPump" or "Heat PipeLine" or "Earth Radiator" or ????????

Jul 25, 2012 4:29 PM ET

Response to Joshua Kresge
by Martin Holladay, GBA Advisor

You wrote, "My point is that GSHPs need to be presented and sold as something completely different than just another heat pump."

Well, you are thinking like a marketer. I'm thinking like an engineer. I like to call things by their proper names.

Your marketing efforts have a high hurdle, unfortunately, because in the eyes of many energy experts, a GSHP is just another heat pump -- although one that costs much more than equipment that makes more sense.

Jul 25, 2012 4:35 PM ET

About claims that GSHPs take advantage of solar hear
by Martin Holladay, GBA Advisor

Many GSHP marketers claim that their systems are a type of renewable energy system because they gather solar heat from the soil. This is silly.

After all, the range of temperatures for the soil and air on this planet are usually in the range of -65°F to 140°F. That's a good thing. If the temperature of the earth's soil and air were near absolute zero (−459.6°F), we'd all be in trouble. Every house on the planet, whether heated by an oil furnace or solar energy, benefits from this fact.

However, we don't usually call an oil furnace a "renewable energy system" just because it benefits from the fact that it doesn't have to operate in an absolute-zero environment. We take it for granted that the air and soil around our houses are temperate.

So let's not allow GSHP manufacturers to make a big deal about the fact that the sun warms the soil. Of course it does! The sun also warms the air, and my roof. Thank goodness for that.

A GSHP is a heating device operated by electricity.

Jul 25, 2012 4:51 PM ET

Edited Jul 25, 2012 4:53 PM ET.

It's not all solar!
by Allison A. Bailes III, PhD, GBA Advisor

Thanks for pointing out the issue with the source of the heat for GSHPs, Martin. Indeed, much of what's stored at the surface does originate from the solar energy hitting the Earth, but a huge majority of the heat within our planet is left over from the formation of the Earth, which was due to gravitational energy. That energy is slowly dissipating over time, but we're at pretty much a steady-state temperature because that loss is being replaced by the heat from radioactive decay inside the Earth. Those two sources are where the true geothermal energy comes from.

Jul 25, 2012 4:57 PM ET

Sun warming soil
by Joshua Kresge

Martin - correct me if I'm wrong here but isn't the whole purpose of a ground loop to take advantage of the constant temperature of the ground? (Which is constant because of the sun, my house isn't heated on a winter night by the sun). Isn't a geothermal heat pump more efficient than a regular heat pump throughout the winter because of the warmer climate in the earth as compared to the outside air? Are you telling us that air source heat pumps are superior to geothermal heat pumps in operation or the cost of geothermal never makes sense?

It seems as if you are calling GEO b.s. because it uses electricity - is it not a real solution? Is it not a better option for homeowners burning fossil fuels like propane and oil?

I truly respect your experience and opinion here, just trying to get a complete understanding.

Jul 25, 2012 4:58 PM ET

Edited Jul 25, 2012 4:59 PM ET.

Thanks for the correction, Allison
by Martin Holladay, GBA Advisor

I appreciate your point, Allison -- but I assume that the additional facts that you provided aren't enough for you to give the GSHP industry a free ride on their "geothermal" claims...

Jul 25, 2012 5:58 PM ET

Edited Jul 25, 2012 6:00 PM ET.

Response to Joshua Kresge
by Martin Holladay, GBA Advisor

Sometimes it makes sense to heat with electricity -- especially if the electricity is produced by wind turbines, PV arrays, or hydro. It makes less sense to heat with electricity when the electricity is generated by a coal plant. If you live in an area of the country with coal generation, then natural gas heat makes more sense than electric heat.

On average, ground-source heat pumps have a slight efficiency edge over air-source heat pumps, but not enough of an edge to justify their very high installation cost -- in most cases. If you can install one inexpensively, however, it might be a good choice.

Jul 25, 2012 10:19 PM ET

by Ron Keagle

I would divide geothermal heat into two categories:

1) Heat below room temperature.
2) Heat above room temperature.

Item #2 is found in natural earth sources similar to hot springs or molten lava. Item #1is found in all the rest of the earth, including frozen ground. Both are geothermal.

Item #2 can be used as is, and it is renewable.

Item #1 is also renewable, but it requires compressing its mass to raise its temperature above room temperature. The compressing requires energy. That energy could be provided by a renewable or non-renewable source.

So I would say that the term “geothermal heat pump” is just fine. What is needed is a term that distinguishes the two different categories of geothermal heat.

Jul 26, 2012 6:51 AM ET

Edited Jul 26, 2012 7:08 AM ET.

Terminology and some new data
by Matt Davis

The question of whether a source of energy is renewable should be based on whether the energy removed can be replenished over relatively short time scales. Whether additional energy is required for utilization goes to the question of performance, efficiency, and return on investment, but not renewability. The thermal energy in the ground to depths of several hundred feet is largely attributable to earth surface temperatures (solar energy). In systems that are properly designed and account for local conditions, the thermal energy extracted from the shallow subsurface is renewable.

Regarding measured COPs, we have some unpublished (i.e. take it or leave it) data from 5 residential installations in New England this past spring. The COP calculations include the circulating pump as a non-thermal component but do not include hot-water generation. The average daily COPs range from 2.6 to 3.8. The lower COPs are older installations with single stage heat pumps. Newer installations with two-stage heat pumps with both closed and open loops have higher COPs, and the measured values line up well with manufacturer specifications. Open loop systems that use domestic water wells as the energy source have much lower installation costs, increasing the rate of return on investment.

Jul 26, 2012 7:10 AM ET

Average COPs
by Martin Holladay, GBA Advisor

Your average COPs of 2.6 to 3.8 are close to the figures I quoted (2.75 to 3.5). Sounds about right.

So I guess that (according to your logic) air-source heat pumps are also renewable energy systems?

Jul 26, 2012 7:35 AM ET

Renewable Heat
by Matt Davis

Yes, and it's not just my logic that leads to that conclusion. The UK has a new Renewable Heat Incentive in which both air source and ground source heat pumps are recognized as renewable energy systems. However, because of uncertainty in actual COPs, they are not yet providing financial incentives for air source heat pumps. Incentives for ground source heat pumps will initially be provided only for commercial systems that can demonstrate favorable COPs.

Jul 26, 2012 7:42 AM ET

Response to Matt Davis
by Martin Holladay, GBA Advisor

During the winter, when a heat pump lowers the temperature of the outdoor air, you are calling it a renewable energy system. I wonder, though: during the summer, when a heat pump raises the temperature of the outdoor air, is it still a renewable energy system? Or does the change in direction of the heat flow alter the characteristics of the equipment?

Jul 26, 2012 8:40 AM ET

You guys are killing me! PV
by aj builder, Upstate NY Zone 6a

You guys are killing me!

PV provides energy. Heat pumps use energy to move heat.

GSHP and ASHP are both energy using heat pumps that move heat. Using the word renewable with these devices is nuts.

I still have my perpetual motion engine for sale. Send $10,000 cash to Aj builder ny.

Jul 26, 2012 8:49 AM ET

Ground coupled heat pumps
by Ed Lohrenz

ALLISON: Geothermal heat pumps (aka: geoexchange systems, ground source heat pumps, ground coupled heat pumps (GCHP), earth energy heat pumps, etc.) use electricity, whether it is produced by burning dirty coal, nuclear energy, wind generators, hydro-electric, or whatever, to gain access to a renewable energy source…the earth. The earth is a renewable source of energy. The energy stored in the earth gets there from several sources, primarily solar energy, but also energy from the deeper earth (true geothermal energy source), and also, energy recovered from waste heat sources (waste heat from air conditioning buildings, refrigeration systems, solar energy stored in the earth, waste energy from combined heat and power plants, etc.). In fact, the key difference between a GCHP system and other heat pumps (refrigerators, air conditioners, air source heat pumps) is that the earth STORES energy. An air source heat pump dissipates energy to the air outside and it blows away, and if the air is warm enough, it can extract energy from it…but only if it’s warm enough…if the air gets cooler the air source heat pump runs less and less efficiently till it finally reaches the point where electric heat is as efficient. A refrigerator, though the actual equipment works on exactly the same principle as a GCHP, does not take advantage of the renewable energy stored in the ground. It’s not the heat pump that is renewable…it’s the energy source…the ground.

A common misperception about a GCHP, even among well-educated people, is that since it does not produce power that can be used to run lights and equipment, it’s not producing renewable energy. Photo-voltaic cells simply convert solar energy to electricity. Wind is created by the impact of thermal energy (sunshine) on the earth. Hydro-electric power is created when thermal energy evaporates water, creating rain that flows down rivers creating electricity when it flows through a generator. A GCHP simply uses thermal energy that is stored in the ground (solar energy and some energy from the deep earth) directly instead of producing electricity. Using this energy directly is actually more efficient than converting energy. The only reason a heat pump is needed to use it in our homes and buildings is to upgrade the energy…increasing the temperature from normal ground temperature (45 to 70F) to something warm enough to keep us comfortable. That does not mean that the energy stored in the ground is not renewable (as long as the sun is shining – and if it’s not we have other issues!)

You also suggest in your response to Chris that “the main reason it’s not a renewable source of energy is because the input electricity has to do a lot of work through the refrigeration cycle to turn the heat into something useable.” Are you suggesting that because a piece of equipment (a heat pump) is needed to gain access to the energy in the ground that we shouldn’t use it? That’s like saying power generated by the wind is not renewable because we need a piece of equipment (the wind generator) to gain access to the electricity that is generated by it. Have you ever considered the energy that goes into maintaining a wind generator? Should that not be included in the equations?

So is a refrigerator a renewable source of energy? ABSOLUTELY IT IS – IF you are circulating a source of renewable thermal energy through it for it to work with! Is the refrigerator itself a source of renewable energy? It’s a tool we can use to gain access to renewable energy exactly like a wind generator or a photo-voltaic cell.

ROBERT: you say “air source is better, in almost all cases”. It depends on the climate you are working in. If the air temperature you are using as a heat source or sink is within the efficient operating range of the equipment you are using, an air source heat pump can be as efficient as a GCHP. The problem with that comment is that there are many places where people need heating and cooling that the air temperatures simply are not within the efficient operating range of the air source heat pump. I’m from an area where the temperature occasionally gets down to -40. Not many air source heat pumps out there that like those temperatures. The ground in my area is plus 45F all year round. That is well within the efficient operating range of a heat pump.

You also suggest that “geo guys are not high grade hydronicists”. This is like saying those builders are not high grade architects. There are good and bad designers in any field of expertise, including yours I’m sure.

MARTIN HOLLADAY: Mitsubishi Mr. Slim may by rated down to -13F. What is the efficiency at that temperature, and what is the life-expectancy of the equipment at that temperature? You also suggest “sometimes it makes sense to heat with electricity…especially if the electricity is produced by wind turbines, PV arrays, or hydro” and it makes less sense when it is generated by a coal plant”. Have you thought how much better it would be, even if the COP of a GCHP is only 3.0, to use one third of the electrical energy to take advantage of the renewable energy stored in the ground under the building. Then you only need one third of the wind generators, PV arrays, hydro plants or coal fired generators…and one third of the power lines that environmentalists like to bitch and complain about.

Jul 26, 2012 9:07 AM ET

Edited Jul 26, 2012 9:46 AM ET.

Response to Ed Lohrenz
by Martin Holladay, GBA Advisor

You wrote, "A common misperception about a GCHP, even among well-educated people, is that since it does not produce power that can be used to run lights and equipment, it’s not producing renewable energy." I guess I plead guilty as charged. I have concluded that a ground-source heat pump is not producing renewable energy.

You wrote, "Have you thought how much better it would be, even if the COP of a GCHP is only 3.0, to use one third of the electrical energy to take advantage of the renewable energy stored in the ground under the building?" Of course anyone who chooses to heat with electricity should use a heat pump. If you can attain a COP of 3 -- and that goal is attainable with either an air-source or a ground-source heat pump, although actual COPs can, of course, be a little lower or higher -- then you can just barely achieve parity (assuming that the electricity comes from a fossil-fuel-burning power plant operating at an efficiency of 33%) with a furnace in your home that burns the fuel directly.

Jul 26, 2012 9:16 AM ET

Edited Jul 26, 2012 9:21 AM ET.

Ed, come back to earth. Laws
by aj builder, Upstate NY Zone 6a

Ed, come back to earth. Laws of thermodynamics my man. PV, windmills produce energy to do work from sun energy streaming to earth. That is where we start to consider a device to be sourcing renewable energy. These devices produce energy that can produce work.

Geothermal can also be tapped to produce work.

Heat pumps that move heat energy to heat or cool our homes NEED ENERGY PUT IN TO MOVE THE HEAT. They CANNOT produce work Ed.

This thread if harnessed might yield some heat energy but energy was put in to get it out.

Jul 26, 2012 9:34 AM ET

Response to Ed Lohrenz
by Allison A. Bailes III, PhD, GBA Advisor

You wrote, "A common misperception about a GCHP, even among well-educated people, is that since it does not produce power that can be used to run lights and equipment, it’s not producing renewable energy." I'd say that's not a misperception or misconception or misunderstanding or anything of the sort. A heat pump takes energy to move heat. That's not production in any sense of the word. Moving isn't the same as producing.

A heat engine, on the other hand, takes heat and does work. If you hook up a device to pull heat out of the ground, do some work with it or turn it into electricity, and then dump the waste heat to a lower temperature sink, then I'd call that a renewable energy source.

The key to this whole thing is the word pump.

Jul 26, 2012 9:46 AM ET

Edited Jul 26, 2012 9:51 AM ET.

Heat Pump Function
by Ron Keagle

My understanding is that a heat pump is acutally doing more than just moving heat, as would be the case with a circulator pump, for example.

Doesn't a heat pump actually "process" the heat by reducing its mass and raising its temperature?

I would conclude that the heat pump is not producing heat. But it is "processing" heat, and it is also moving heat. And the processing and moving are two different functions.

Jul 26, 2012 9:53 AM ET

Response to Martin Holladay
by Matt Davis

Martin, You seem to have taken the intractable position that GSHP systems are not renewable (which is the topic of this discussion), so entering into a dialog about the renewable attributes of ASHPs seems to be of little benefit. Cheers.

Jul 26, 2012 9:56 AM ET

Response to Ron Keagle
by Allison A. Bailes III, PhD, GBA Advisor

Yes, the heat pump does indeed process the heat through the refrigeration cycle. That's where the real pumping action occurs, and that's what takes the work. Heat doesn't have mass since it's a form of energy. (I'm ignoring Einstein's mass-energy equivalence here, as given in his famous equation E=mc^2, since we're not crossing that border.) But yes, a heat pump does raise the temperature. See my article from last week, The Magic of Cold, which explains the refrigeration cycle.

Jul 26, 2012 10:58 AM ET

MARTIN: Electricity is almost
by Ed Lohrenz

MARTIN: Electricity is almost always equated to the amount of GHG emissions from the source. A fossil fuel plant produces GHG's to produce electricity. What's the source of the energy. Try tracing back the source energy for oil or natural gas, including extracting gas and oil from the ground, process it, pipe it around the country, etc. That information is hard to find, and no one quotes figures on the GHG's from burning gas from the source. So if we consider only the GHG's that come into play when we burn gas, that's not entirely accurate. What about the energy to pump it to your home, extract it from the ground etc. So let's be fair and take into account the source of the gas and oil and then compare it to the source of electricity.

Jul 26, 2012 2:07 PM ET

Edited Jul 26, 2012 2:54 PM ET.

Ed, you're way far from
by aj builder, Upstate NY Zone 6a

Ed, you're way far from planet Earth. Come back come back. This subject is difficult enough for the PhDs but we are close to having some chime in that the moon landing was faked.

Ron, seriously? We are not changing the mass of refrigerant, the mass is always the same. The pump is doing two things, circulating the refrigerant and compressing it so as to set it up to for two locations where we change it's phase from liquid to gas and back again.

Give this up all... we are so far from reality that we will next have to splain the Kennedy assassination.

Waht a massive thread, has me gasping for air and LOL. Of course I could be wrong on all my points too, but it sure has been a pleasure I tell yaa. Smiling in the ADKs.

Jul 26, 2012 4:53 PM ET

by Ron Keagle

AJ, your point is well taken that the mass is always the same. I meant to refer to the compression of the mass so it becomes more dense. But then we have to deal with the contention that heat does not have mass.

Jul 26, 2012 5:52 PM ET

Edited Jul 26, 2012 5:54 PM ET.

We got to quit while we are behind Ron
by aj builder, Upstate NY Zone 6a

We got to quit while we are behind Ron. You meant compression of the refrigerant gas which does have mass. Heat Ron is not mass. Check out Wikipedia for heat. As to refrigerant, somewhere amongst some engineering books from long ago I have a fluid dynamics textbook that would cross your eyes in pain if you are not up to snuff with calculus, differential equations and physics. At that level I would really have to toss bull here to explain any of it. Compressible fluids and incompressible fluid flow modeling mathematically is for the birds... especially since they fly with the aid of a compressible fluid we call air and some in the past called ether. Going in deep to design a heat pump system is an engineering specialty. And that is where Allison comes in.

Jul 26, 2012 7:11 PM ET

Heat Without Mass
by Ron Keagle


This is an interesting point about heat and mass. I can understand the concept of how the two are separate, but I have difficulty in seeing the practical example. Can you give me an example of the existence of heat without mass?

Heat in the form of radiation need not have mass, but heat in that form has no effect until it strikes mass. So tell me what heat without mass is like.

Jul 26, 2012 8:15 PM ET

To Geo or Not to Geo
by Joshua Kresge

A GSHP leverages a renewable energy that is the thermal energy of the earth.

The entire system is used to leverage a renewable energy source in an effective way that improves home comfort. A GSHP system is a more effective solution in terms of producing BTUs with a given input of energy.

The real question is when does it make sense for one to install a ground source heat pump as opposed to a regular heat pump OR other comfort system.

Could we come up with a formula or process that spit out the answer to "optimal comfort system" given the persons preferences? My feeling is that it can and should be done. Anyone interested in trying?

Jul 27, 2012 8:08 AM ET

Edited Jul 27, 2012 8:20 AM ET.

Heat as defined at Wikipedia
by aj builder, Upstate NY Zone 6a


Heat generated from the nuclear fusion in the Sun, and transported to Earth as electromagnetic radiation, is one of the driving forces of life on Earth.

Heat is energy transferred from one system to another by thermal interaction. In contrast to work, heat is always accompanied by a transfer of entropy. Heat flow is characteristic of macroscopic objects and systems, but its origin and properties can be understood in terms of their microscopic constituents.

Heat flow from a high to a low temperature body occurs spontaneously. This flow of energy can be harnessed and partially converted into useful work by means of a heat engine. The second law of thermodynamics prohibits heat flow from a low to a high temperature body, but with the aid of a heat pump external work can be used to transport energy from low to the high temperature.

In ordinary language, heat has a diversity of meanings, including temperature. In physics, "heat" is by definition a transfer of energy and is always associated with a process of some kind. "Heat" is used interchangeably with "heat flow" and "heat transfer". Heat transfer can occur in a variety of ways: by conduction radiation, convection, net mass transfer, friction or viscosity, and by chemical dissipation.

The SI unit of heat is the joule. Heat can be measured by calorimetry, or determined indirectly by calculations based on other quantities, relying for instance on the first law of thermodynamics. In physics, especially in calorimetry, and in meteorology, the concepts of latent heat and of sensible heat are used. Latent heat is associated with phase changes, while sensible heat is associated with temperature change.

Jul 27, 2012 10:48 AM ET

Origin of Ground Heat
by Ron Keagle

For heat pump extraction installations, I wonder how much of that geothermal heat originates from the sun warming the surface, versus originating from deep in the earth.

I understand that heat originating in the earth’s core is caused by radioactive decay.

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