What is the Lifespan of Geothermal Heat Pumps?

Scott Sklar, President, The Stella Group
April 14, 2010 | 17 Comments

A reader writes: I was talking with a friend the other day and he said that he heard that closed-loop systems only last about 10 years because the ground temperature equalizes after several years as a result of so much heat being removed. He said that after 10 years the ground no longer maintains its average ground temperature of 55 degrees F. I have never heard of that before and scientifically that doesn't make any sense to me. I'm guessing you've come across this argument before?

Here is my response to this question: Even in this shaky economy, ground-coupled or geothermal heat pump sales are steadily increasing in the United States. Over 1,000,000 geothermal (or ground-source) heat pumps are used in U.S. residential, commercial and government buildings. And each year, U.S. homeowners install approximately 50,000+ geothermal heat pumps. WaterFurnace, based in Indianna was founded in 1983, Earthlinked, based in Florida was founded in 1980 and Oklahoma-based ClimateMaster is over 50 years old. None of the residential systems that these companies have installed have this kind of saturation problem.

Dan Ellis, ClimateMaster, CEO and one of the early leaders in the industry says that this issue is confronted “only in larger commercial systems with a dense ground heat exchanger array and with unbalanced seasonal loads, (ie more heat rejection than extraction or vice versa).”

He continues, “In these situations the center of the ground-coupled heat exchanger (GHX) array begins to act as thermal storage. This is a well-known design aspect that good GHX design software considers. The software looks out over long time periods to allow for any impact. The size of the GHX is adjusted to keep within design parameters over the long time period. Alternatively, the load can be balanced with supplemental heat rejection or in many others ways.”

Last summer, I had a geothermal heat pump ductless HVAC system installed at my Arlington, Virginia home. And I specify ground-coupled (geothermal) heat pumps systems for my clients striving for net-zero or zero energy buildings. I want to emphasize that even for larger commercial systems, geothermal heat pump systems and hybrids with solar thermal, are the most energy efficient systems for heating and cooling buildings.

Overall, I am a big fan.

Scott Sklar, founder and president of The Stella Group Ltd., in Washington, DC, is the Chair of the Steering Committee of the Sustainable Energy Coalition and serves on the Boards of Directors of the Sustainable Buildings Industry Council, the Business Council for Sustainable Energy, and the Renewable Energy Policy Project. The Stella Group, Ltd., a strategic marketing and policy firm for clean distributed energy users and companies using renewable energy.

17 Comments

Jon Pierce
January 19, 2013

Yes
Sweedish diagrams of dry soil storage of solar cisterns of water, had, CARDIOD 3-d shape of the stored earth heat around the cistern which was only insulated top side

John Porritt
May 7, 2010

The key to the efficient use of energy is storage and, to me, the single most difficult issue. Wind and solar would work really well if batteries were, say ten times as good as they are now. I know in the 19th century they used to drag ice blocks up from lakes and bury them in ground pits insulated with straw and they had cool houses and ice cream through the summer. Also, I recently read an article about a company that was storing heat in massive containers of gravel.

So, carrying winter cold to summer is possible using ice but storing summer heat for winter seems a more difficult nut to crack. In Quebec, where we are, there is a limited amount of A/C required relative to heat will cause the geo environment to progressively cool. Therefore, any views on whether it would pay to keep dumping heat into the ground using Solar and/or Geothermal even if the A/C is not required. Another thought I have is that we have a swimming pool that we stop using when the water gets to about 75F so would there be any payback in pumping the pool water through the loop for a couple of weeks at the end of the season.

I guess what I am asking is, can the ground beneath our feet be used economically as a storage medium?

William Fitch
April 17, 2010

To Danleywolfe:

I don't know if anybody addressed the "water compression" problem you were having or not. You are correct that water is basically non compressable. In all HP's air or ground, there is a gas, usually Freon (current flavor of the week is 410A) which is compressed and changes from gas to liquid, liquid to gas, cyclical in nature that allows for the heat extraction and deposition through a coaxial heat exchanger.
Despite one mention above geo heat pumps usually do better in cooling mode, over all. The simplest way to think of it is look at the temperature in the house, lets say 70 DegF desired target for heating season and 75 DegF target for cooling season. Now, at those times, where is the average ground temp in relation to each target temp..??.. In my neck of the woods, PA, a reasonable average heating season ground temp is 50 DegF. This is 20 DegF below the target temp desired in the house or you can look at it as 20 degrees working against me in my goal. Now, in AC mode, a reasonable average ground temp for cooling season is 65 DegF (for me, Horiz ground loop). This is 10 degrees COOLER than the target temp I am shooting for in the house, or I have 10 degrees working in my favor for the goal. In short, the ground is working more in favor of the HP during cooling season than in heating season. As you move Southward and the ground temps get warmer, you will reach a balance point in relation to the ground temps and target temps. There are many other factors which enter into this whole "dynamic" but such goes way beyond this discussion.
Additionally, as someone mentioned above, you should have a de-superheater installed on the geo unit to preheat the water especially in cooling season. If you are offsetting an electric water heater, the end energy gains/savings will be GREATLY enhanced.

.....Bill

bob freeston
April 16, 2010

Heat gain or loss in a loop field over time is by latitude. The potential for gain is in the southern U S, the potential for loss is in the northern U S. A properly designed system should have no problem. All this applies to closed loop systems. Open loop systems don't have this potential. I converted to GSHP 5 years ago drawing from an existing high volume water well and dumping to a large dry well. Summer efficiency is enhanced because the system dumps heat first to my hot water system for "free" hot water. In the winter the system is producing hot water at low cost. In both cases it feeds to an electric resistance tank at 100 degrees F. The electric boosts it to 120. The Canadians are doing something interesting with at least two 45 house developments in central Canada that collect solar thermal all summer and pump the heat to a central bore field in bed rock. The area stores vast amounts of heat which is pumped out all winter. The houses just have small booster heaters for late winter.

V. Bruce Stenswick
April 16, 2010

I heat my house in a Minneapolis suburb with a GSHP. I am a big proponent. First, in the niche of residential heating retrofits in a northern climate, you really only have two choices that give you a path to zero carbon emissions, GHSP's and pellet stoves. On a large scale GSHP's are probably a better choice. Here is an idea to help alleviate the up front costs. Use a PACE mechanism to fund the loop field. A system for a house like mine should cost about $18000, with half of that for the loop field. Sell bonds and loan homeowners the money for the loop field, tack on 2% to cover defaults, so someone with a house like mine would get $9000 and have to pay back $9180. Have them pay $50 per year extra on their taxes, with a baloon of 1/2 the balance when they sell. Subsequent homeowners take over on the same terms until the loan is paid off.

As for clean or dirty, I am fortunate enough to be able to buy all of my electricity through a green energy program, "WindSource".

Gary McCallum
April 16, 2010

GSHP's are a great way to save energy mostly in regards to instalations in older buildings as a simple way to improve effciency. Any new structures should try to attain passavehaus standard so there is no need for overly complicated mechanical solutions in heating and cooling. Unless of course one wants to build a monster house. In those cases just build to the passivehaus insulation standard and reduce the size of your GSHP by a 70% minimum.
danlywolf otherwise its more like a nine year payout which is not great, You can do better managing your own portfolio.
If you can afford a portfolio you can afford to help make this planet a better place. I'm a builder and I hear that argument all to often and mostly from people whose wealth leaves them not wanting anything but more. Not investing in OUR future will soon have its consiquences.

Phil Manke
April 16, 2010

To Underground Energy; Appreciated points. An aditional concern might be that further heat is generated at the power plant in either cooling or heating mode from production of the electricity, unless the GSHP is PV powered, which takes a chunk of PV, and may not be efective in heating mode. It has been stated in past submissions that GSHP is an energy efficiency technology, and not a renewable technology. Large thermal well systems have caused earthquakes. Local area desication may be a result of smaller systems. The large adoption of these systems may be from the push by power companies in order to retain the umbilical to the grid. Even electric co-ops with a mission statement to serve their patrons actually have a bottom line of selling more electric power. They also keep their per-diem charge up to deflate grid tie performance, while giving lower pricing to large users.

Mark Worthington
April 16, 2010

Please allow me to interject some clarity and facts into the discussion. First of all, the title of this article is misleading, as the life expectancy of the heat pump itself is typically 15-20 years. Because the author is referring to the phenomenon of long-term ground temperature increase in the vicinity of a ground heat exchanger (GHX), the title should refer to Ground Source Heat Pump (GSHP) systems. The GHX and the heat pump are two components of a GSHP system.

Next we need to consider some properties of closed-loop GSHP systems. The first is that the heat pump unit generates heat, and this affects the efficiency of the system differently in heating mode vs cooling mode. This means that if it is winter and you need 4 units of heating, your GSHP system will abstract 3 units from the ground and the heat pump itself will provide 1 unit of heat to deliver a total of 4 units of heat to the building. GSHP systems are inherently less efficient at cooling because cooling a building requires that you abstract heat from the building and dump it into the ground. Thus, in cooling mode, your GSHP system must remove 4 units of heat from the building and add 5 units of heat to the ground, accounting for the heat generated by the heat pump.

The next thing to consider is the climate in which the GSHP system is installed, and the thermal loads that will be generated by the building in question. In a perfect world, your GSHP system would remove exactly as much heat from the ground in winter as it adds in summer. This would avoid the phenomenon of long-term heat accumulation in the subsurface. Due to the fact that the heat pump itself heat to the ground in cooling mode, as discussed above, GSHP systems in many parts of the United States will add more heat to the ground in summer than they remove in winter, resulting in a net addition of heat and a long-term increase in temperature, and a corresponding decrease in system efficiency. Out of space for now.

danley wolfe
April 16, 2010

Matthew - I looked at www.egggeothermal.com and have one question. Since when is wster a compressible fluid? The web site says: "...geothermal heat pump doesn't create heat by burning fuel ... it collects the Earth's natural heat ... a loop installed below the surface of the ground or submersed in a pond or lake... Fluid circulates through the loop and carries the heat to the house.... an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature...The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system." Water or antifreeze do not gain heat through compression (gases gain heat through compression). I was told a furnace is needed to top the heat using water/ethylene glycol antifreeze as the heat transfer fluid. Is the heatpump at egggeothermal.com using air as the heat transfer fluid; if so they will need a lot of heat exchanger pipe in the loop since heat transfer to a gas is order of magnitude less efficient than liquid. I had local HVAC quote me a system at $24,000 including furnace replacement which gives a payout of around 4-5 years if you get the government kickback; otherwise it was more like 9 year payout which is not great, you can do better managing your own portfolio.

Matthew Blank
April 15, 2010

All,

There is a book coming out in September of this year, it is strictly on geothermal heating and cooling. It is a professional book called Geothermal HVAC, Green Heating and Cooling. It's being published by McGraw-Hill and authored by Jay Egg, co-author Brian Howard. The book will dispel the common misconceptions surrounding geothermal HVAC. This book will be the first, entire book solely dedicated to Geothermal HVAC. The author of the book's company's website is loaded with a ton of great information on everything Geothermal. Here it is: www.egggeothermal.com
Check out the site for some great information.

Matthew

William Fitch
April 15, 2010

Hello:

An interesting statement and as mentioned is probably only a potential problem in the deep South and for multi-borehole based systems. Anywhere the cooling load is even close to the ballpark of the heating load, a ground heating problem caused by AC will only benefit the HP during heating season. Additionally, AC is an easier task for a Geo heat pump than heating, demanding less juice BTU for BTU so that works over time in the systems favor as well...

.....Bill

Allen Gordon
April 15, 2010

In some regions that are cooler year around, such as the foothills of the rocky mountains near Denver, air conditioning in the summer, is not needed. In this case, the GSHP will not add heat back to the ground in the summer time. There would not be sufficient time in the shorter summer season to warm the ground before the GSHP system is called upon to provide heating. In these areas it makes sense to have an additional solar hot water system, with a lower energy requirement, to provide the primary heat source with the GSHP as a backup system. In the summer, the solar hot water system would provide heat back to the ground.

Mel Riser
April 15, 2010

The facts are they don't quit working, they get less efficient. There is data from the University of Oklahoma, that shows that units installed south of Oklahoma, or areas where the ground temps do not get extremely cold in the winter, over a ten year life span, heat up the ground and become less efficient.

It seems the ground cannot lose the heat and it residually builds up. This does not cause the units to stop working, only get less efficient. So if it is a SEER 27 unit, it might get reduced to 24 pr 20 over ten years do to the residual heat retention.

I am currently and investigator/advisor to a research project at the University of Texas in Austin to study this heat buildup and suggest ways/means to mitigate it, making GSHP's more efficient in the deep south.

The research project is being funded by the DOE, and will model the GSHP systems, add in the residual heat buildup and see how that affects efficiencies in the models. Then we will introduce various heat pump technologies designed to augment the BTU loss in the ground and suggest methods to prevent or mitigate the buildup.

This is a VERY researched problem by the University of Oklahoma Mechanical Engineering Department, and they have 20 years of data on GSHP's to prove it.

The issue is not whether they get less efficient over time in the deep south, but how we can set the systems up to make the ground heating less an issue, or how an active/passive system in winter might help the GSP lose the residual heat load it pumped into the earth.

Anyone wanting to discuss this research project more can contant me, Mel Riser, melvin.t.riser@nasa.gov, or the Professors at UT running the research project. Dr. Glenn Y Masada, masada@austin.utexas.edu or Dr. Tess Moon, Tess.Moon@engr.utexas.edu

a b
April 15, 2010

An alternative to geothermal (ground) water heating / cooling is to use the outside air to take in the needed energy. Air/ water heat pumps are simpler, cheaper and even reliable.

http://www.energie.pt/cat/cat.php?cid=2&cult=uk

Glenn Kenneally
April 15, 2010

Although always referred to as geothermal energy, what we are actually talking about here is solar energy stored in the ground, rather than energy from the earths core. In a correctly sized Heat Pump collector this energy is constantly being replenished by the suns energy and rainwater and therefore ground freezing should never occur.
It can however occur in poorly designed systems where the geothermal collector has been undersized. In this situation energy is being extracted from the ground faster than it can be replenished, and over time freezing of the ground will occur. A survey of the type of ground should be carried out before beginning to install the collector, as the energy output will vary from site to site. It may be as low as 10W/sqm for one project or as high as 25W/sqm at another, and if you try to extract more energy than available, ground freezing and ultimately high running costs will be the result.

ANONYMOUS
April 14, 2010

In comment #1 "CEA" writes: "Geothermal grew at nearly 26% during 2009."

Well, Geothermal capacity most certainly did not grow at 26% in 2009; its growth rate was in a modest single-digit range. This "26%" figure includes all projects ongoing in 2009. Some of these might not ever be completed and many of which won't be completed for years. You should only count generation actually brought on line in quoting a growth figure--not the full project pipeline.
Steven

Aaron Moline
April 14, 2010

Geothermal grew at nearly 26% during 2009. An impressive gain. The earth's crust will always be moving and generating heat (at least as long as we exists on this planet!). Geothermal continues to display how even the earth itself can provide us with energy. This potential should not be ignored.
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Scott Sklar

Scott, founder and president of The Stella Group, Ltd., in Washington, DC, is the Chair of the Steering Committee of the Sustainable Energy Coalition and serves on the Boards of Directors of the Sustainable Buildings Industry Council, the...