Geothermal heat pump systems (or more correctly “ground-sourced heat pump systems”) are generally a good consideration for many projects, but animal care facilities add some complication.
Ground sourced heat pump systems work by pumping a closed loop of fluid (usually water and ethylene glycol) through a series of either vertical or horizontal wells to absorb or reject heat to the soil depending on whether it is winter or summer. The benefit of this system is that ground temperatures are fairly stable and the heat pumps can be optimized to a 55 degree loop temperature.
Ground-sourced systems work particularly well for residential and small commercial applications because the number of wells is low (typically two or three for a house). The absorption and rejection of heat has little impact on the surrounding soil temperatures and the system is relatively low cost for the benefit of heating and cooling from the earth due to the limited number of wells. This works because residential buildings have very low fresh air rates and are usually very well insulated by today’s standards.
There are two problems with animal care facilities that affect the benefit of ground sourced heat pump systems: the required number of wells and the low cost of natural gas.
Animal care facilities have high fresh air dilution rates. At a dilution rate of 30% outside air, relative to total air flow in the building, over 80% of the HVAC utility costs will be related to processing that fresh air on cold or hot days. Consequently, the insulation of the building has minimal effect on the economics of the cost as compared to the fresh air. The increased capacity created by the fresh air requirements drives the number of GSHP wells up by a factor of as much as four. Animal care facilities are typically in the range of one well for every 350 to 400 square feet. This drives the construction cost up proportionally, such that on an equivalent area basis, the animal care facility ground sourced system will be four times that of a residence.
The second problem is that natural gas prices have dropped in half over the past 10 years. The return-on-investment calculation for a heat pump system in heating climates is performed by comparing the cost of the heat pump system to a more conventional gas heating/electric cooling system over the lifetime of the equipment (typically 15 to 20 years). In performing these calculations, heat pump vendors often use the “worst case” scenario in predicting high future fuel prices because it makes the heat pump system appear to have a return on investment of less than 15 years.
The heat pump systems have a high installation cost but only moderate electrical operating costs for the pumps and compressors. A conventional system has about half the installation cost of a heat pump system. In cooling climates, the electrical consumption of the GSHP system is much less than the electrical consumption of a conventional air conditioning system.
Careful calculations are required to establish payback periods for this type of system on an animal care facility, but we would typically expect about a 12 to 18 year payback.
Copyright 2020 C. Scott Learned and Design Learned, Inc.
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