A municipal complex gets GeoExchange® via a pond
Struggling with an HVAC system composed of various technologies of ages up to 25 years in their main municipal headquarters building, the City of Fort Collins, Colorado installed a complete geothermal heat pump retrofit. The new system will provide a carbonless heating and efficient cooling system for over twice that long, and was designed to be expandable for additional workspaces.
Tighten the building first-
As is fully appropriate in such cases, a complete makeover with a boost of thermal envelope protection took place before the installation. There was new insulation and windows to keep future heating and cooling loads lower. This cuts the size of the geo system needed and therefore the cost of obtaining perpetual free fuel from the site’s adjacent lake.
What is most often an underground heat exchanger configuration of either vertical-drilled boreholes a with a grouted, U-bend pipe loop or a series of horizontally laid and buried pipes took a different path. Metallic (sandwiched) metal plates that circulate closed loop water brought from and back to the building’s heat pumps, served as the system’s heat exchanger.
Underwater can be cheaper to install-
There are two advantages here. First, drilling or excavating for a terrestrial ground loop is a higher cost than sinking prefabbed racks of metal plates to the bottom of a lake or pond of adequate size for the heating/cooling load. Second, the closed loop water circulating through the metallic plates has greater thermal conductivity than high density polyethylene pipe (typically encased in grout or buried in dirt). The rate of heat transfer between the pond and the metallic plate exchanger is greater because the liquid medium of pond water makes its thermal exchange convectively. The greater the temperature difference between the closed loop and the pond, the more active will the pond water’s convection be to reach toward equilibrium with the circulated loop water.
The incoming closed loop water is routed around the ceiling zone of all interior spaces where individual heat pumps use it to heat or cool different zones as needed. It then returns to the pond.
The size and temperature of some pond loops is cool enough in summer that the closed loop can stay close to 50°F. This enables ducted air cooling over coils, rather than the use of the heat pump itself, saving even more electricity.
What about payback?
Before considering a retrofit investment with geothermal heat pump technology, many citizens concern themselves with tax credits (that municipal operations do not qualify for). They also want to know about how long it will take for the investment to “pay back” its costs with savings. To that, this writer asks, “when was the last time that your replacement refrigerator, car, or house did that?”
—Bill Martin (with help from Major Geothermal, Wheat Ridge, CO)