GEOEXCHANGE SYSTEMS
The New HVAC Technology
Notes
So often when new technologies develop, the application of the
technology swiftly outgrows the original intention. The result many times
is that the original terms for equipment linger long after the terms become
inappropriate. (Remember calling a refrigerator an "Ice Box".) A prime
example is GeoExchange technology for conditioned living space.
Within this relatively obscure technology are several terms which are
inappropriate:
- Ground Source Heat Pump
A Ground Source Heat Pump utilizes the earth for both a source of heat, and
a heat sink. In the winter, the earth is used as a source of heat; in the
summer the earth is used as a heat sink. Therefore, the more appropriate
term is Ground Exchange Heat Pump, or more simply GeoExchange Heat Pump
(GHP).
- Geothermal Heat Pump
Geothermal is an inappropriate term because it can be confused with
geothermal sources. There are several installations in the U.S. where raw
natural geothermal heat is used to produce electrical energy. Examples of
geothermal energy are geysers and naturally occurring hot springs.
- Water Source Heat Pump
Water Source Heat Pump is actually a very accurate term for almost all
GeoExchange Heat Pumps. Unfortunately, using the term creates confusion
with systems that use water as the primary conditioning medium, such as
large commercial buildings that use chilled water for air conditioning.
Also, technically the "water" used is almost always a mixture of an
antifreeze liquid and water (closed loop systems).
- Energy Efficiency Ratio (EER)
Energy Efficiency Ratio (EER) for a refrigerant type air conditioner (or
heat pump in cooling mode) is defined as Btu/h divided by kilowatt input.
The EER (and its cousin SEER) is used by manufacturers to provide an
efficiency benchmark of a particular unit. However, it must be understood
that the EER is only valid for a given set of parameters. These parameters
include outside temperature and relative humidity for air source equipment.
As outside temperatures rise above the manufacturer's parameter (typically
95°
F), the actual EER drops off significantly from the "rated" value.
Typically, an EER can drop 20% if outside temperature increases to
110°F. (Also, available BTU output of the unit drops dramatically as
temps increase over 95°F.) For this reason, it is inappropriate to
compare a GeoExchange Heat Pump to a conventional refrigerant type air
conditioner, or to an air source heat pump.
- Coefficient of Performance (COP)
The Coefficient of Performance (COP) is a useful benchmark for comparing
heating units. It is defined as the BTU/hr output divided by kilowatt
input, times a conversionfactor of 0.293. The conversion factor yields a
result that is dimensionless and represents a true efficiency. Typical
values for heat pumps are 3.0 — 4.0. As is the case with the EER
number, the COP is also inappropriate for comparisons between GHP's and air
source heat pumps. The reason is the same as for the EER number. That is,
the COP for air source heat pumps is based upon a given set of parameters.
These parameters include outside air temperature. Again, as outside air
temperature gets colder, the actual COP lowers. With a GHP, outside air
temperature has no effect on COP. This argument is noted in the 2000 ASHRAE
Handbook, S7.33: "The normal procedure for evaluating COP does not provide
a workable method for comparing efficiencies of dissimilar energy streams.
Instead, each energy stream should be valued on the same energy basis..."
Graphics for this article provided by:
Geothermal Heat Pump Consortium
http://www.ghpc.org
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