Are Geothermal Heat Pumps a good idea?

Electrically powered Geothermal Heat pumps deliver 3 or more times as much heat energy as they consume in electricity.  This seems like a fine deal.   I'm not so sure.  Here is a cut at how they compare to a current technology natural gas furnace on greenhouse gas emissions and on fuel cost for heating:


Greenhouse Gas Emissions:

How much greenhouse gas is produced in providing 100,000 BTU of heat to the house with a geothermal heat pump vs a gas furnace? 

Geothermal Heat Pump:
To produce 100,000 BTU of heat for the house, a geothermal heat pump system operating at a COP of 3 uses 9.76 KWH.

If generated at a coal fired power plant, this will produce 19.5 lb of Carbon Dioxide **


Gas Furnace:

To produce the same 100,000 BTU of heat for the house, a 90% efficient gas furnace will burn 1.11 Therm of gas.
This will generate 13.2 lbs of Carbon Dioxide **

So, the geothermal heat pump produces almost 1.5 times as much greenhouse gas as an up-to-date gas furnace.  It might be argued that some geothermal heat pumps do better than a COP of 3, but it would have to have a COP of 4.5 to just breakeven with the gas furnace.  The little data I have seen on actual field measurements of geothermal heat pump COPs would make me think that 3 might be pretty close.  

Update: this is a recent paper that provides measured COPs for 3 modern residential GSHP installations -- the measured values were 3.7, 3.4, and and 3.4 for the three homes. 



Update 3/2012:  NG is becoming more common for generating electricity in the US -- if you do the calculation above for electricity generated from NG, the CO2 emissions for the heat pump drop from 19.5 lbs to 12.5 lbs -- so, a bit less than an NG furnace. 
Another thing to consider if you live in  a climate that requires significant space cooling in the summer, geothermal heat pumps are very efficient air conditioners, so this is a point in favor of the GSHP. 

Of course, if you now use electric resistance heating, any heat pump will be a significant gain in both energy used and emissions.



Fuel Cost:

Fuel costs costs depend greatly on what gas and electricity sell for in your area.  Just for comparison, if your rates are 10 cents per KWH for electricity and $1 per therm for gas, it comes out this way:

Geothermal Heat Pump:
At 10 cents per KWH, the 9.76 KWH to produce 100,000 BTU would cost  $0.98


Modern Natural Gas Furnace:

At $1 per therm, the 1.1 therm to produce 100,000 BTU would cost $1.1


The geothermal heat pump saves about 11% -- your rates might be much different, so just ratio the cost of electricity and gas up or down be the ratio of your rate to the rate used above. 


If the same comparison is done for other fuels (electricity, oil, ...), the results tend to be more in favor of the heat pump.  Paritucularly for heating with electricity, the GSHP has much better efficiency and much lower emissions.


The Wikipedia entry appears to reach about the same conclusion...  (see Environmental Impact)




An Alternate(?)


The question in my mind is, would you be better off to spend the extra money that would go into the geothermal heat pump system on more insulation, better weather sealing, more efficient windows, and passive or active solar heating?  All of these measures save energy AND dramatically reduce greenhouse gas emissions.


Well designed solar homes can achieve 70% energy reductions, and do this without going "over the top" on either cost or appearance.  Here is one Example:  Building America Van Geet Home

Extra costs for improving insulation and weather sealing can (at least to some extent) be made up for in a simpler and cheaper HVAC system.



** Calculation Details:

The basic reason that heat pumps do not do so well on greenhouse gas emissions even though they are around 300% efficient is that the electricity to run them comes (largely) from coal fired power plants that are only about 30% efficient, and that burn a very high carbon fuel (coal). 


Geothermal Heat Pump:


Coefficient Of Performance (COP) = 3.0

Electricity supplied by coal fired power plant (about 50% of electricity in the US)


Electricity for 100K BTU = (100000 BTU)/((3412 BTU/kwh)(3COP)) = 9.76 kwh

According to the greenhouse gas calculator at:
The coal fired plant that provides the 9.76 kwh of electricity will put out 19.5 lbs of carbon dioxide.

Natural Gas Furnace:


Gas Furnace Efficiency  = 90% (typical new condensing gas furnace or boiler)

Gas for 100K BTU = (100000BTU)/((100000BTU/therm)(0.9 efic))  = 1.11 Therm

Using  the same carbon calculator as above, this 1.1 therm will generate 13.2 lbs of carbon dioxide.


Maybe I've got this wrong?  Please let me know if you think so.





More here in this  Home Energy article...

The link above seems to have some problems with the images, and actually is more complete on the Wayback Machine...



Updated March 2012