Geothermal Heat Pump System Save Energy
at Senior Living Facility
The objective of this project was to design and build a facility to meet the space comfort demands of senior living while keeping energy usage and costs at a minimum.
Geothermal closed loop heat pump systems take advantage of the earth's ability to store energy and transfer that energy for use in space conditioning and supplemental domestic water heating for a building. The soil acts as an insulator so that while outside air temperatures may be extreme, the ground temperature at depths greater than 9.1 m (30 ft) will closely match the annual average air temperature. The closed loop piping will transfer heat from the ground to the heat transfer fluid for the heat pump to heat the building in winter and transfer heat to the ground from the heat pump when air conditioning in summer. In commercial applications, an additional benefit is that the geothermal heat pump system may supply heat to one area of a building while cooling a different area at the same time.
In addition to the energy savings from geothermal heat pumps and the use of a portion of the heat rejected to assist in domestic water heating, the HVAC design included heat recovery units. These units transfer heat from the exhaust air to the outside ventilation air. This reduces the energy required to condition the ventilation air before distribution throughout the facility.
Shepherd's Meadows is a four-story, 5,420 m2 (58,345 ft2) building providing senior assisted living. In addition to 66 individual apartments, the facility includes a commercial kitchen and dining area to serve residents.
91 geothermal heat pumps provide 460,596 W (131 Tons) of cooling and 429,233 W (1,464,960 Btu/h) of heating. Each heat pump has an average COP of 3.0. 136 vertical geothermal wells of 45.7 m (150 ft) depth serve the heat transfer loop piping system supplying the heat pumps.
Each of two 5.6 kW (7.5 hp) pumps circulate 12.6 L/s (200 gpm) of an environmentally safe heat transfer fluid throughout the loop system.
Two of the larger heat pumps send some rejected heat to supplement the domestic water heater.
Each of three energy recovery units transfer approximately 75% of the heat from 1298 L/s (2,750 CFM) exhaust air to the same amount of outside ventilation air.
No boilers or cooling towers are used in the HVAC system.
Space heating and cooling and domestic water heating is accomplished with electricity separately metered. The average monthly space conditioning electric usage of the facility from September 1996 through April 1997 was 16.04 MJ/m2 (0.414 kWh/ft2). The average monthly water heating electric usage from September 1996 through April 1997 was 7.17 MJ/m2 (0.185 kWh/ft2).
In general geothermal heat pump systems save more than 25% in energy usage over PTAC's coupled with conventional air handlers, both using electric heat and DX cooling.
By saving USD 1347 in energy cost per month, the difference in the higher installed cost for the geothermal system over the alternate system described above was paid for in 6.5 weeks.
The installed cost of the HVAC system, including wells, was USD 583,450 or USD 107.65/m2 (USD 10.00/ft2). The installed cost of a similar type facility with the alternate system described above was USD 107.32/m2 (USD 9.97/ft2).
This project qualified for the local electric utility's rate of USD 10.286/GJ (USD 0.037/kWh) for space conditioning for the months of October through May. The same rate applies to the domestic water heating year round.
Geothermal average monthly energy cost (September 1996 through April 1997):
· Domestic Water Heating USD 0.071/m2 (USD 0.0066/ft2) Space Conditioning USD 0.178/m2 (USD 0.0165/ft2) Total USD 0.249/m2 (USD 0.0231/ft2)
· Comparative system average monthly energy cost: Water heating and space conditioning USD 1.72/m2 (USD 0.16/ft2)
· Energy cost savings of Geothermal over comparative system USD 1.47/m2 (USD 0.1369/ft2)
Aapex Engineering Inc.
Baker, Bednar & Associates Inc.
The Middleton Corporation
John A. Turley