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Geothermal Heat Pumps
Jamais Cascio, 9 Dec 05

heatpump.jpgIt's a little odd to think about, but you're probably standing on one of the best possible resources for home heating and cooling.

Although temperatures in the atmosphere can vary considerably over the course of a year (or even a day), the temperature underground remains fairly constant. At about six feet under, the soil measures from 45 degrees to 75 degrees fahrenheit, depending upon latitude. And this consistency, it turns out, can be a resource for keeping one's home warm in the winter and cool in the summer.

Geothermal heat pumps move heat from one place to another via the circulation of a refrigerant fluid. They have a number of advantages over traditional heating and cooling systems, including low noise and essentially no maintenance. Most importantly, they use significantly less energy than traditional gas, electric or oil-based heating & cooling systems. According to the US Environmental Protection Agency (PDF), geothermal heat pumps cost 30-40% less per month than traditional methods (and probably even less with current fuel prices). Moreover:

Greenhouse gas emissions associated with the use of geothermal heat pumps are 55 to 60 percent lower than those from standard air-source heat pumps. In most areas of the United States, GHPs had the lowest CO2 emissions and the lowest overall environmental impact of any space conditioning technology evaluated by EPA in its study.

It's not quite a renewable system, although the power requirements are sufficiently low as to make geothermal heat pumps a viable pairing with solar and/or wind generation. But how does a geothermal heat pump work?

...a geothermal heat pump doesn't create heat by burning fuel, like a furnace does. Instead, in winter it collects the Earth's natural heat through a series of pipes, called 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. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms.
In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool - by drawing heat from the interior, not by blowing in cold air.

This doesn't come cheap, at least not up front. A geothermal heat pump must be professionally installed, and costs average $7,500 or more for a typical home. That said, the California Energy Commission's Consumer Energy Center website claims that geothermal heat pumps are extremely low-maintenance ("virtually worry-free"), and save enough on utility bills that the investment can be recovered in as little as two years.

For more information, hit the Geothermal Heat Pump Consortium website.

(Thanks, JJ!)

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Comments

Geothermal heat pumps may be even better when combined with radiant heating systems.

Low temperature differential heat pumps might make it possible to use the temperature differences between the floor and ceiling of a room (up to 10 degrees F) to produce work.

Energy is all around us. We just don't have the eye to see it yet.


Posted by: gmoke on 9 Dec 05

Thanks Jamais!

This is definitely one of the best ideas for home heating and cooling around. I've been wondering why it's not better known - it seems simple enough. And the up-front cost amounts to only a few years worth of oil heating bills at current prices, not to mention summer air-conditioning bills!

Anyway, good to hear there's a consortium supporting this now!


Posted by: Arthur Smith on 9 Dec 05

It should be noted that GHP's work best in wet soils where energy transport is most efficient.

And while it is an efficient means of heating a house, you are cooling the ground. I live in Southern Ontario where heating demand is high and have heard second or third hand of instances where GHP's have cooled a local area (around the loop) enough to affect the systems own efficiency by over cooling the area around the loop. It strikes me that doing this on a large scale could be a bad thing.

I think this is an excellent technology in the right setting, what about coupling it to a solar thermal panel, or using it (similar to gmoke's post above) to move extra heat from an attic into a living space. If I recall, the largest cost associated with a GHP is the buried loop (often a few hundred feet of PVC pipe drilled into the ground inside a well casing) so using an alternate source of heat could drastically reduce costs...


Posted by: Ian Miller on 9 Dec 05

Heh, reminds me of the jokes about offering to sell people the proverbial swampland in Florida. So does this mean that wet swampland is really quite useful for habitation after all, with its high thermal conductivity?


Posted by: sanman on 9 Dec 05

Well, i live in a house with warm watter and heating provided by geothermal heat pup. The city i live in has a public infrastructure for it. Within 5 years we will have the (as far as i can tell) first geothermal power plant. We're europe and probably the worlds most renewable/energywise city anyways it seems. Electric cars and bikes are subventioned and i could recharge my eletro car for free if i would like to still, i rely on commuterrail and tramway (excellent service quelity) so i don't need one but the possibility alone is golden :-)


Posted by: Dominique on 10 Dec 05

What about the wurrums (that's what we call worms in Scotland)?
It seems to me that this technology is robbing energy from the soil ecosystem.
Yes, there is an ecosystem in that 'dirt' (as you Americans like to call it).


Posted by: Ali on 10 Dec 05

Geothermal energy storage can be great. But storing the energy in a fluid maybe isn't so great, because of the limited heat capacity. One of the smarter ideas I've come across in this respect was to use stearine or parafine (candlewax) for storage. It undergoes a fase change at just the right temperature, so the heat capacity approaches infinity. Additional benefits are less energy leakage and therefore limited environmental impact on underground ecosystems, etc.
Here is a nice overview of the state of thermal storage technologies: Thermal Energy Storage; A State-of-the-Art (http://www.ntnu.no/em/dokumenter/smartbygg_rapp/Storage_State-of-the-art.pdf)


Posted by: Rikkert on 10 Dec 05

Although geothermal heating and cooling does indeed change the temperature of the soil and rock structure around the pipes, not to a large degree. Efficiency loss is minimal.
Coupled with this, if you heat a building in the winter, you pull heat out of the ground. If you need to cool it in the summer, you put heat back. The ground acts like a giant thermal battery.
One problem with trying to completely on renewable energy is that heating and cooling make up a very large portion of the total energy a home uses annually. By reducing the amount of energy need to heat or cool by 50% or more, that is a huge chunk.
Geothermal systems are especially cost effective in a project where you would already be digging a basement in a new construction.


Posted by: Josh Clements on 10 Dec 05

Geothermal heat pumps have been around for a long while, so I'm surprised that they are considered novel here. The ones that cause local ground cooling or heating tend to be the shallow horizontal variety, which is the least efficient way to go. Deep vertical open or closed-loop geothermal systems go a few hundred feet into the earth and are a good choice when already building a well, and do not cause localized effects as the sink is too large.

One thing not mentioned is the BATISO method of heating and cooling a building, which uses large concrete thermal masses in the ceiling as well as the floor for hydronic heating and cooling, coupled with a small ventilation system attached to a humidifier/dehumidifier to remove dampness or dryness as well as filter the air. In most temperate climates there would be enough cooling capacity in the cold water flow from an open-loop vertical system to cool a building entirely without a compressor, effectively a passive heat pump using the deep earth as a heat sink. In hot climates a small heat pump compressor would suffice.

The BATISO design further reduces the heating and cooling load of a building by 70-80% and due to its low-flow large-mass nature allows the heating or cooling source to be active at intermittent times with delays of 12 hours or more.

Such a building equipped with a heat pump could be easily heated or cooled by solar or wind power alone since the total electrical load would be in the range of 5-10% of resistance based heat and normal compressor based air conditioning. And since it wouldn't have to operate continually it would be a good place to use the dump load from a wind generator or off peak power from a utility at night.

Another way to heat such a building would be with wood, where a short hot fire would be transferred into the thermal mass for a long period. An external firehouse with a water jacket as a water store could heat such a building for up to five days with a single burn, even in very cold climates where the temperatures fall to -40c.


Posted by: Michael Slavitch on 10 Dec 05

This is all great. It would be even greater if the heat pump were driven by a small thermal engine, such as (of course) a stirling engine of the kind NASA likes so much for small amounts of space isotope power. The "waste heat" from such a small engine would simply add to the heat flow to the energy store, making the whole system far more efficient than a central power plant doing the same work.

All of the above can be labled "obvious to one skilled in the art". Too bad it isn't obvious to one and all.


Posted by: wimbi on 11 Dec 05

There seems to be a bit of confusion in some of the comments between what a heat pump is and what a heat exchanger is. Both can be useful for something like this.

A heat pump pulls heat energy the opposite direction from how it would normally travel--heat goes from cold to hot. A heat exchanger lets heat energy move the direction it normally would--from hot to cold--but does it much more effectively than circumstances would allow without the heat exchanger. A heat exchanger is a passive device, using no energy whatsoever.

The system in the article is a heat pump, but as a couple folks mentioned, a heat exchanger would also be useful (and less expensive, though perhaps the digging outweighs the cost of heat pump & its energy use anyway). Ground at 45 degrees would be perfet for cooling in summer, and in winter it could either be disconnected (for mild climates) or if outside air is colder than 45, it could assist the house's main heating system, so the heater would be working against a temperatute difference from room temp to 45F, not from room temp to the outside air's 10F, for instance.


Posted by: Jeremy Faludi on 12 Dec 05

Coming from Africa, where we have had a continual problem with depletion of forests for fuel wood, it would be wonderful if the clever people above could sacrifice their time/energy/resources and go and set up these (and other systems) so that we Africans would stop depleting the natural forests which would ultimately be beneficial to all mankind...However, the main uses for wood as fuel, is for cooking not necessarily heating,so what of this other fine idea of drawing power from Lava??...


Posted by: Dread on 13 Dec 05

Learn how to set up a heat pump for your home here.


Posted by: Bob from Seattle on 15 Dec 05



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