A client is considering either a geothermal heat pump or an air-source heat pump with electric thermal storage as a backup, and we want to do a payback analysis. What is the difference in efficiencies of these heat pump options?
The performance of a heat pump depends on many factors. Probably the most basic is the difference between the temperature of the heat source and the temperature at which the heat is delivered. The larger this difference, the lower the efficiency. For the ground source heat pump, the temperature of the ground is warmer than the air during the coldest part of the winter and colder than the air during milder weather in the spring and parts of the fall. The website "Heat pump performance" (from IEA Heat Pump Programme Information Centre) explains the basic thermodynamics. Additional topics are covered at "About heat pumps."
Our library staff came up with several excellent citations:
- A new specification for ENERGY STAR qualified geothermal heat pumps went into effect December 1, 2009, which ENERGY STAR says "...will be over 45 percent more energy efficient than standard options." For more detail, compare the data in these two web pages:
Geothermal Heat Pumps Key Product Criteria and
Air-Source Heat Pumps and Central Air Conditioners Key Product Criteria
- Comparing Fuel Costs of Heating and Cooling Systems (476K Adobe® Acrobat® .pdf)
is a pretty good comparison study from Kansas (Kansas State University Engineering Extension. June 2003). See especially Table 4. Electric heating costs — $ per MBTU delivered for several appliances and performance levels.
- Another long study with good information is
An Information Survival Kit for the Prospective Geothermal Heat Pump Owner (229K Adobe® Acrobat® .pdf) by Kevin Rafferty, P.E., Geo-Heat Center, Oregon Institute of Technology. The following quote from this publication explains comparison problems:
"One of the most confusing aspects of geothermal heat pump technology is equipment ratings. These heating and cooling performance values are useful for comparing units of the same type (i.e., ASHP to ASHP or GHP to GHP). Unfortunately, the ratings used for different types of equipment (furnaces, ASHP, GHP) are not generally comparable, making comparisons difficult. As a result, it is useful to know what the ratings values include and what they don't.
"All heat pumps are rated by the American Refrigerant Institute (ARI). Results are published every six months in the Directory of Certified Applied Air Conditioning Products (for GHPs) and the Directory of Certified Unitary Products (for ASHPs).
"For water-source heat pumps (the type of heat pump used in all GHP systems), cooling performance is defined by an index called EER (Energy Efficiency Ratio). This is the cooling effect produced by the unit (in Btu/hr) divided by the electrical input (in watts) resulting in units of Btu/watt*hr. Electrical input includes compressor, fans and 'pumping' allowance (for the groundwater or ground loop).
"Heating performance is defined by the index called COP (Coefficient Of Performance). This is the heating affect produced by the unit (in Btu/hr) divided by the energy equivalent of the electrical input (in Btu/hr) resulting in a dimensionless (no units) value. Again, the COP includes an allowance for pumping.
"Both the COP and EER values for groundwater heat pumps are single point (valid only at the specific test conditions used in the rating) values only. This in contrast to the seasonal values (HSPF and SEER) published for air-source equipment. COP and EER are not the same as, or valid for use in comparison to, SEER and HSPF."
- You can compare heating systems using Western Area Power Administration Energy Services' Heating System Cost Calculator. This will give you a quick comparison of the end use cost for the homeowner.