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Geothermal heat pumps are an efficient source of space heating and cooling that can also provide hot water. The case studies presented here examine two approaches: a project with a centralized geothermal heat pump system and one with multiple geothermal heat pumps installed in each unit.
Case Study: Multi-Unit Housing Uses Centrally Controlled Geothermal Heat Pumps
Cambridge Cohousing, Cambridge, Massachusetts
Cambridge Cohousing is a residential project consisting of 41 units of housing on a narrow 1.5-acre site between the street and a railroad track. The housing units range from large, 3-story townhouses to 1-, 2-, and 3-bedroom flats. The project also includes communal facilities: a large kitchen, dining area, childcare and recreational facilities, a library, and shared gardens. It houses about 80 people and totals 63,000 square feet.
The entire project is heated and cooled from a central plant, consisting of eight 10-ton ground-source heat pumps. These heat pumps are served by coolant loops in three 1500-foot-deep wells on site. The heat pumps produce water at 120°F for circulation to individual air handlers in each dwelling unit. A gas boiler boosts the temperature further to provide domestic hot water and to increase the heating capacity of the system for very cold weather. During the summer, the heat pumps are switched to the cooling mode.
The housing units feature zoned controls with individual duct fans instead of a large blower. These fans collectively draw a maximum of 530 W, compared with 1200 W for a typical blower. The walls include R-19 insulation and aluminum-clad low-E windows. Altogether, these features cut the project's energy use by more than 50%: the project's total annual energy cost is $44,870, compared to a standard practice estimate of $92,000.
Case Study: Multi-Unit Housing Uses Individual Geothermal Heat Pumps
Takoma Village Cohousing, Washington, D.C.
This 4-story, 51,000-square-foot project consists of 43 one- to four-bedroom attached flats and townhouse units plus a 4800-square-foot shared building known as the "Common House." In addition to energy efficiency features, the decision was made to install individual heat pumps in each unit (and several in the Common House) for a few reasons: each unit's electric bill would accurately reflect its residents' comfort choices, each unit could change from heating to cooling cycles without consideration of the neighbors' choices, and centralized systems generally become cost-effective only for developments of over 100 units.
Programmable thermostats allow residents to control HVAC usage based on their own occupancy patterns. Townhouse units further benefit from free water preheating during the summer from the "desuperheater" component, which uses waste heat from the heat pumps for domestic hot water heating rather than rejecting it to the ground loop.
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