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Is a geothermal heat pump right for you?

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Geothermal heat pump with installer Patrick RyanI’ve tried it all: caulking cracks, blowing in insulation, replacing drafty windows and—I’m especially proud of this one—installing a mail-slot cover so airtight it could be used in a space shuttle docking module. Yet my home heating bill remains an object of fear and loathing. After years of trying low-tech solutions, I’m drawn to a high-tech solution, and the one I keep hearing about is the geothermal heat pump.

Heat pumps, of whatever variety, give you the giddy feeling of breaking the laws of physics. The gas boiler in my basement is 80 percent efficient; burning gas unavoidably heats the exhaust gases as well as the water for the radiators. An electric heater is almost 100 percent efficient. But a heat pump can be more than 100 percent efficient. A 1000-watt electric heater emits 3400 BTU of heat in an hour, but the same amount of electricity, used to run a heat pump, might transfer 15000 BTU of heat into the house.

The trick is that a heat pump moves heat rather than generating it. It’s basically just an air-conditioner or refrigerator. During summer, it cools the house; during winter, it runs in reverse, warming the house by refrigerating the outdoors. The outdoors barely notices (unless you’re in an urban area thick with heat pumps)—it’s like spitting in the sea, which scarcely affects the sea, but does affect the spitter. No matter how cold it gets out there, there’s always some heat to harvest, although the task gets harder the colder it gets. By avoiding the production of new heat, the pump evades the naive efficiency limit. It still takes energy to run, but the energy is used mechanically (to circulate and compress a refrigerant) rather than thermally.

Most heat pumps scavenge heat from the outside air. A geothermal one gets it from the ground. In fact, the word "geothermal" is deceptive. These devices don’t tap into geysers or volcanic hotspots. At the depths we’re talking about, the ground is heated not by Earth’s interior but by the sun. "This energy is just solar energy—it’s just another form of it," explains Patrick Ryan of Ryan Energy Technologies in Union, N.J. A better term is "ground-source" heat pump, but "geothermal" has stuck because it sounds so much sexier.

As sunlight warms the surface, heat diffuses downward—a slow process that mutes the daily and seasonal temperature variations. In dry soil, the annual temperature swings are halved with every three feet of depth, so the subsurface temperature quickly approaches the average surface temperature, around 55 degrees Fahrenheit here in New Jersey. That’s already pretty close to a typical thermostat setting, so a ground-source heat pump has less work to do than an air-source pump does and therefore consumes a half or third as much energy.

Two weekends ago, Ryan showed me one of his projects in Whitehouse Station, in the rural western part of N.J. (Yes, there is a rural western part of New Jersey.) The system consists of a horizontal lacework of pipes buried eight feet under a large meadow where the owners are now planting an apple orchard. Water circulates through them and reaches a temperature of 45 degrees. It doesn’t get all the way up to the ground temperature of 55 degrees, because it is flowing too quickly to come to equilibrium with the ground.

Flowing into the basement, the water enters a heat exchanger, where the water pipe wraps around a refrigerant line. The water heats up the refrigerant, cools off to 40 degrees, and flows back outside to warm up again. The pump mechanism then compresses the refrigerant, raising its temperature without adding any heat. Finally, a fan blows air through the refrigerant coil and into the ducts of the house. Another coil preheats domestic hot water, saving money on that, too. In the summer, it is the air that warms the refrigerant that warms the water, sucking heat out of the house and pushing it into the ground. To watch Ryan explaining the system, see the camera-phone video I’ve posted to YouTube.

Unfortunately, the economics aren’t quite as wondrous as the technology. In 2009 Ryan gave me an estimate for my house. Our property is too small for a horizontal loop, so it would have taken three 400-foot wells down into the Triassic-era sandstone under our house. Another complication was that we’d have had to replace our steam heating with forced air. Heat pumps don’t do steam: the temperature difference is too large. The total cost was so outlandish that I use it to this day as a party joke: $70,000, plus the costs of acquiring the permits and repaving our driveway after the drillers had dug it up. A state rebate would have covered $10,000, and a Federal tax credit a third of the remainder, but no bake sale was going to cover this one.

What’s more, it was hard to know how much I’d save. Although heat pumps use less energy, they use electrical energy, which, BTU for BTU, is several times more expensive than natural gas. A rough estimate for my net savings was a third, in which case the payback period would have been longer than my life expectancy. The heat-pump maker ClimateMaster has another savings-calculator here.

Others confirm that retrofitting an old house for geothermal heat makes sense only under limited circumstances. Tom Mandel in Teaneck, N.J., one of the first homeowners in the state to give it a go, estimates he paid $55,000 and saves $400 a month. So he doubts he’ll ever recoup the expense. He adds that his house takes longer to heat up than before and requires an extra boost from an electric heater when the outside temperature drops below 20 degrees.

Geothermal can make sense for new construction, especially in areas where there’s no gas hookup. Alan Sexstone, who responded to a Twitter request I put out for geothermal users, says he decided on a geothermal heat pump when he and his wife built a house south of Morgantown, W.Va., 11 years ago. The system comprises three 100-foot wells and cost about $10,000 to install—about twice as much as propane or electric heat, but low enough that the system will almost certainly pay for itself. Like Mandel, Sexstone says the system works well down to about 20 degrees, below which he needs to supplement it with a wood stove.

Another homeowner I heard from through Twitter built a house two years ago near Boulder. He paid $50,000 (before tax credits) for his geothermal system and estimates he saves $1000 a year. In his case, the savings partly reflect the lower effective electricity rate he gets because of his solar array.

There is irony here, because geothermal should actually make more sense for old construction than for new. New houses tend to be better insulated, which reduces their heating costs and hence the potential for savings. But this fact has to be set against the extra costs of retrofitting and the larger size of the average new house.

Even If I had the money, I’d probably be better using it to button up my whole house as tight as that docking module. After all, the ultimate way to heat your house is not to heat it. Some superinsulated houses stay warm by the body heat of their inhabitants. That’s hard to pull off with an old house like ours, but in an upcoming post I’ll tell the story of a nearby homeowner who has nearly done it.

I also plan to revisit the topic of geothermal heat pumps (and of space shuttles—I’m going down to the Endeavour launch next week), so please let me know your experiences here or on Twitter.

Geothermal installer Patrick Ryan. Photo by George Musser.





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Comments 30 Comments

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  1. 1. JamesDavis 8:33 am 04/18/2011

    Because of the cost and the low heat input, that is just not good for about 99% of Americans.

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  2. 2. RobLL 12:13 pm 04/18/2011

    Nice article. It is difficult for the non-specialist to gather information on subjects like this. Some factors are price of electricity, average temperatures winter and summer, total heat or cooling needed for a given house. We use an air heat pump here in the Pacific NW, where seldom do we need to heat or cool more than 20 degrees F. Also electricity is only 4.5 cents per KW. House has a lot of thermal mass, and better than code insulation, I would consider spending a few/several thousand dollars upgrading insulation but doubt if I could find anyone available who knows more about it than myself – without having to spends most of the money for consultants.

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  3. 3. Soccerdad 4:13 pm 04/18/2011

    Does it bother anyone else that the government is willing to pay well off homeowners tens of thousands of dollars to help them save money on their utility bill? This is green energy policies at work, weather they are paying rich homeowners or corporations like GE, it takes money from taxpayers and gives it to a chosen few.

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  4. 4. benjid 12:04 am 04/19/2011

    Thanks for the interesting article. Ground source heat pumps are the most efficient option in my area, except for passive solar achieved with good SE facing windows. I’m amazed at the high cost in your area. I live in ND, constructed a 2600 sq ft home in 2002, and had a ground source heat pump installed. I thought you might be interested in my numbers. The system has 8 vertical loops 150 feet deep. My system has a Desuper for supplementing my hot water heater and puts out 55,000 BTU/hr. It cost $17,000 turnkey, which included all ducting. The heat pump was $5400 and the loop system was $8800. Some examples of KW comsumption are 2007: 8800 KWh, 2008: 9760 KWH, 2009: 9410 KWH. These are annual totals of heat, hot water, and cooling combined. The system does not need any supplemental heat even in our extreme ND winters. This company actually offered a hydronic version also, which was designed for floor heat. I have the forced air version because of the extra cost of floor heat. My father just had a ground source heat pump installed in 2010 and his system cost $16000 to put into an existing home. These dollar figures are retail, rebates and tax incentives were not included. Maybe there’s not enough competition where you are? Incidentally, the system has worked flawlessly since installed. For me personally this system has come close to paying for the extra cost already.

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  5. 5. benjid 12:11 am 04/19/2011

    Sorry for the lengthy response, but just also wondering how a ground source system in a more moderate climate like NJ can’t handle the lower winter temps by itself. Including my own I know 3 systems here in ND and none of them require a supplemental heat source. Maybe in that part of the country they skimp on the capacity of the system (length of buried pipe and size of compressor)?

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  6. 6. bma24 9:31 pm 04/20/2011

    I wonder about the technical competence of the author. He states that geoexchange systems use solar energy. How could heat radiated by the sun penetrate down a few hundred feet? Not possible. The heat comes form the earth’s core.
    the author also picked the worst case economic scenario – a detached house. The best application for geoexchange is in multiunit buildings where economies of scale generate much quicker paybacks.

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  7. 7. angiras 10:22 am 04/21/2011

    There is a simple way to get heat from the Earth in areas with significant ground water circulation. It essentially uses a heat PIPE to deliver heat to the heat pump.
    Weld closed a large diameter, 8", stainless steel pipe, 10 or 20 feet long, depending on the depth of the groundwater, drill a vertical hole of the same diameter and lower the pipe, sealed-end first into the hole. Then weld a number of turns of tubular coil around the top end which protrudes above the ground. Weld the top closed, incorporating a valve. Pump out the large tube to form a vacuum inside and insert some ammonia. Attach the ends of the tubular coil to the input of your heat pump.

    The heat at depth will evaporate the ammonia which will condense on the walls of the tube at the top releasing the heat. The condensed ammonia droplets will naturally run back down the walls of the tube to be recirculated forever. Adding ‘fins’ to the large tube at the bottom would help collect more heat.

    If the circulation of the ground water passing by the lower end of the column is insufficient it will freeze and no more heat will be delivered.

    This only works one-way, for heating, not for cooling, but would be more efficient than a buried-grid field ground-water system because there is no energy needed to pump the fluid collecting the heat.

    A similar solution might also be used in the proposed high temperature commercial geothermal plants, where the heat source is believed to be too hot for conventional submerged pumps.

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  8. 8. FarmerFrank 8:53 pm 04/21/2011

    Your comments are about right for Iowa too. We put in a heat pump in our new home in 2009. Supplemental resistance heat has come on once, when we’d been gone for a while and had set the thermostat back. I don’t understand the need for supplemental heat at such moderate temps as some are quoting. For a 24K BTU unit, we have 2 250′ loops.

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  9. 9. benjid 11:24 pm 04/21/2011

    Are my prices in line with Iowa too? I have propane backup, but only because my geotherm is on off-peak so the electric rate is cheaper. If the utility is not controlling the electricity, the propane has never come on. If I’m being controlled, I let it get down to 60 before the propane would kick in. Then when they turn the power to the geotherm back on again, it easily brings the whole house temp back up by itself even in the dead of winter.

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  10. 10. gmusser 11:06 am 04/22/2011

    You might equally well ask how heat from Earth’s interior could warm the shallow subsurface. Both solar heat and interior heat move through the crust by conduction and the physics is well understood. The geothermal heat gradient is roughly 1 degree C per 30 meters of depth, but this gradient is added onto an oscillating surface boundary condition determined by solar heating.

    The geothermal heat flux is, on average, about a tenth of watt per square meter, compared to about 300 W/m^2 of solar heating. If not for the sun, the surface and immediate subsurface would reach an equilibrium temperature of about 30 K. You have to go down several kilometers before the interior heat becomes comparable to solar heating.

    I did not "pick" the worst-case economic scenario. It picked me. The question I posed is whether a geothermal heat pump is viable for a homeowner. Of course it works in some settings, but it doesn’t seem to be visible for me or other owners of detached houses who would need to retrofit the system.

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  11. 11. FarmerFrank 12:45 pm 04/22/2011

    Your prices are in the same ballpark. How the utilities use rates for peak-shaving vs. incentives to go all electric can get bizarre. The air-to-air heatpumps in this area use either propane or natural gas as back-up for extreme weather. Geographical differences in soil conditions can make a big difference in heat transfer efficiencies.

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  12. 12. bma24 11:15 am 04/25/2011

    "The geothermal heat gradient is roughly 1 degree C per 30 meters of depth, but this gradient is added onto an oscillating surface boundary condition determined by solar heating."!?!
    Put another way, the sun only heats the surface when it shines, and there is no insulating blanket of snow, and when the surface is not cooled at night through convection and radiation?
    How about some empirical evidence to counter your position:
    1. There are existing 20+ storey buildings located in densely populated urban areas with wells drilled below the building’s foundations. That soil never receives any heat from the sun.
    2. go down a mine that is located in the North, below a lake (e.g. salt mine near Goderich Ontario under Lake Huron) the temperature reaches 70 degrees year round once you descend below 500 feet.

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  13. 13. gmusser 8:41 pm 04/25/2011

    My "position"? This isn’t my position, but well-established planetary physics. I invite you to consult any geophysics textbook — look for the chapter on heat transfer.

    Link to this
  14. 14. Graychild 8:54 am 05/6/2011

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  15. 15. MightyDrunken 8:27 am 05/12/2011

    Hi bma24,

    you are not thinking in the right terms. Continental crust is about 20 miles thick. Therefore the heat flow near the surface from the hotter core is low, about 65 mW/m2.
    In your sky scrapper example the soil is heated by the sun. Not directly by sunlight but by the warmth of the atmosphere caused by the sun. The temperature difference may be low but the distance is so much smaller that solar heating will be a significant amount compared to geological heating.
    Your 2nd example does not say much as many places around the world have hotspots.

    Link to this
  16. 16. Grumpyoleman 3:42 pm 05/23/2011

    I cut my electric bill by over 1/3 by joining an electric co-op. I know it’s creeping socialism, but it works.

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  17. 17. PGracey 11:42 pm 05/29/2011

    Retrofitting an old house is much more difficult than including a system in a new one. For those circumstances it perhaps pays to do as the old-timers would do; heat only the most necessary parts of the house. The coming end to our binge on fossil fuels will make us do this eventually so we might as well get started now.

    My gas floor heater in my older duplex here in So. Calif. died half a dozen years ago, and I replaced it with a small air to air portable heat pump that drew on my attic crawl space for its thermal exchange. I found that even with the high cost of electricity locally I was paying only a little more than what my winter gas bill had been. The sunny climate helps of course, and the crawl space is tapped directly for its heat with a box fan whenever it rises five degrees F above the living space to help keep the cost low.

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  18. 18. thull6047 4:10 pm 06/10/2011

    Think out of the box. I used my leach bed and septic system to get better access to more heat. Where the snow melts first so to speak. Kept the system closed and tried not to pump water very far not up and down hundreds of ft. Could pull 15 degrees out of the water and not stop fermentation process. Heated my pool in the summer with waste heat from air conditioning the house. You can multiply the efficiencies by preheating hot water as well. My system cost was under $10,000 and savings were near $5000 a year. I did this in OH 27 years ago!!

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  19. 19. blk 12:19 pm 06/16/2011

    We use a geothermal heat pump for our sole source of heating. We live in Minnesota. If it can work for us, it can work for the vast majority of the residents of the United States, who will have much lower heating requirements.

    In much of the country air conditioning actually consumes more energy than heating, and ground-source air conditioning works extremely well.

    The only problem I have with it is the noise. Because the temperature of the heated air is relatively low, it has to push more air through the system, and that makes it noisier. It’s not like a wind tunnel, but it generally runs more of the time than a gas-fired furnace and the fans run faster. Still, the total cost of running the system is half what heating with gas was.

    This past winter we added even more insulation, and as the article author notes, that made a big difference. The system ran much less often and was much quieter.

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  20. 20. doddstacy 2:57 pm 07/5/2011

    I’ve just discovered your blog, so my comment is dated. I assume in reply 10 of this comment thread your calculation of a 30 K earth surface temperature, if not for the sun (or other radiant input flux), is the temperature required to radiate a flux of about 0.1 W/m^2 to space, thus dissipating the "geothermal" flux. The existing boundary condition however is that the (average) earth surface temperature is as required to radiate away both the large (average) solar input flux and the (very much smaller) geothermal flux. While solar input is the first order determinant of the surface boundary average temperature, that does not mean that it is the first order source of the energy extracted by the heat pump in winter.

    The argument about whether a heat pump’s energy source (for heating mode) is "earth," "geothermal," or "solar" is semantic. The energy source that interacts seasonally with the heat pump is simply a thermal mass of soil, hopefully large enough to undergo an acceptably small seasonal bulk temperature cycle. The boundary conditions for that thermal mass are set collectively by three factors (neglecting moisture): the relatively constant "geothermal" conductive heat flux from below, the oscillating "solar" conductive heat flux from above – determined by seasonal solar heat flux and the thermal diffusivity of the soil above – and finally by the seasonal energy removal by the heat pump ground loop. A shallow source loop will favor "solar" supply, a deeper loop favors "geothermal" supply. A transient thermal system analysis carried out over a typical year’s cycle would establish whether the total energy input to the thermal mass was primarily from above or below.

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  21. 21. 1stop 4:44 am 07/14/2011

    I am from 1 Stop Air Conditioners & it’s my opinion that the human race has simply become lazy & we have fallen into a comfort zone where we have been relying & depending so much on energy sources such as electricity since its early invention that we have become complacent and have not really made any effort or discovered any real new form or technology to provide efficient & affordable power. We are only now forced to come up with alternate power sources as a result of the growing costs of electricity and the impact it has on our planet and of course demand.
    The oil companies claim to pour billions into this market and to date I have not heard of any major breakthrough’s.
    We have the most powerful energy source at our finger tips the sun but we cannot harness its energy effectively or efficiently & I find this hard to understand.
    I hope we find a solution & find one fast.

    Link to this
  22. 22. Charlie42 10:44 am 07/30/2011

    We retrofitted to a geothermal heat pump in Phoenix. The system uses our swimming pool (which is in the ground, after all) as a heat source/sink. We added a cooling tower to cool the circulating water on hot summer days. The house stays cool in the summer – even when it’s 118F outside – and nice and warm during our mild winters. The cooling tower helps keep the pool water cool in midsummer. The circulating water warms the pool in spring and fall, adding at least a month to our swimming season at each end. Total current draw to cool our 2400 sf house is around 11 amps. The local power company (APS) paid half the capital cost of the system so our out of pocket was very reasonable. I would recommend a similar system to anyone in our situation: we needed a new AC and we already had a pool.

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  23. 23. stuartrivchun 12:43 am 10/13/2011

    I completely agree with you Ground source heat pumps can be applied to a wide range of both domestic and commercial applications, ranging from heating a small residential property with a swimming pool to heating and cooling a large commercial building a ground source heat pump can help to reduce running costs. GSHP is beneficial for the environment as well as its installations is helping me in saving money,energy, operating cost. For further information in respect of Geothermal Heat Pump you can visit http://www.egshpa.com/

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  24. 24. RickH5253 10:06 pm 11/23/2011

    I installed a geothermal heatpump in the fall of 2010. The system was sized to provide a 70 deg F temperature rise over the outside ambient temperature. Based on my observations, the system achieved its design target. We didn’t need supplemental heat until the outside temperature dropped to 5-10 below zero overnight.

    The system cost $51,500, or ~ $36000 after the Federal Tax Credit. I estimate we avoided buying and burning ~1000 gallons of heating oil. Based on a recent quote, that amount of oil would cost about $3800 this winter. We spent an extra $800 in electricty to run the system last winter. So, we can expect to save ~$3000 running the system this winter…giving us a breakeven period of 12 years.

    Of course this system should also increase the value of our house by some amount. I think a conservative, reasonable value for this might be 5 times the cost savings or ~$15K. Factoring this in, the breakeven period is reduced to ~ 7 years.

    This is definitely a long term committment. But, it comes with additional benefits that may be more emotional than clearly financial: we have reduced our carbon footprint and we have reduced our impact on the global petroleum supply/demand equation that is enriching many countries that are not exactly friendly to the US.

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  25. 25. moesatriani 10:59 am 03/21/2012

    “Ground source heat pumps are the most efficient option in my area, except for passive solar achieved with good SE facing windows. I’m amazed at the high cost in your area.” ~benjid

    That’s the same for me too. I love my heat pump . I am saving a ton of money.

    Link to this
  26. 26. lylaburns123 11:31 am 09/17/2012

    My husband just put in some geothermal heat pumps in our house. I don’t know much about them but he says they are supposed to be pretty good. Hopefully we will have a good experience with them. Thanks for the post!

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  27. 27. BARTDRAPER 9:29 pm 10/10/2012

    We are planing to put a 5 ton sys. in a moble home in about the center of washington state water table is about 6 foot in the winter they tell me i need 5 horizontal runs @ 250 foot of this 3/4 i.d. hdpe tube times 5 , no pipe is to be any closer to the other than 3 foot , that’s a lot of digging , if i use the coiling methed more like thay use in the pond and my tubing is in the water table is the heat from the earth going to keep up ?

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  28. 28. sesmith 10:58 am 12/29/2012

    We put in a GSHP in our 170 year old house in central NY last year. Payback over oil will be less than 7 years. Our system is designed so that 2 degrees F. is the temp that aux heat will be necessary to help the system heat the house. In reality, it takes temps of several degrees below zero overnight for the aux to start kicking in for a while by morning. This is all in the design. It could be designed for much colder temperatures, but there is a price point where it doesn’t make sense. GSHP’s have been used successfully in Alaska, so I don’t understand why aux heat would be necessary for temps in the 20′s in NJ unless the units are undersized for the building or designed and installed less than optimally.

    Too bad the prices seem to be so high in NJ. I’d get a couple more estimates. I’d also look at some of the newer air source units. They might make more sense in NJ where it’s more temperate than here.

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  29. 29. rickgoulette 8:13 pm 12/29/2012

    Dry-air systems do not understand what they are they are doing. Do not do businees with them. If you would like to see a screwed up system please contact me.

    Link to this
  30. 30. hjames1623 2:37 pm 03/15/2013

    What an interesting read! I’ve been looking for someone that does Phoenix heating and cooling but now I think I should look into geothermal heat. It’s so cool and efficient. Thanks for sharing! http://www.hoffmanrefrigeration.com

    Link to this

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