Jan. 19, 2012
When geothermal systems were first explained to me, I understood that the system was very similar to air cooled heat pumps. Only instead of using the ambient air temperature as the medium for heat exchange through the compressor, the constant temperature of the ground provided a much lower delta from inside to outside cooling temperature and therefore would result in higher energy efficiencies and lower utility costs.
That concept is true. But as a designer watching a number of systems being installed in both residential and institutional projects over the last several years, the nuances of operating geothermal systems are very different from conventional heat pumps.
Like some conventional systems, heat pumps can contain an electric resistance coil to provide a heating boost in extreme temperatures. Many geothermal systems have electric resistance — running them is akin to turning on your toaster — the efficiencies you expect from your geothermal system are tossed aside when the coil is activated. Haven't we all heard from various sources that the best way to run our systems is to turn down the thermostat to goose bump range when we're not home?
Programmable thermostats exist to help us manage that concept if we fail to manually adjust those temperatures. But geothermal systems can't be turned up and down like your typical gas furnace where your air is running over a flame. So, if you swing the temperatures in too wide a pattern — say 62 to 68 degrees in unoccupied to occupied mode — you are signaling your system to run that resistance coil to provide the comfort boost. With a geothermal system, it's much better to narrow the temperature swing to a couple of degrees to avoid calling on the resistance mode.
In addition, in two projects in the past year, owners have told me that in extreme temperatures, their ground loops ran out of heat. Think of the loop field as a giant heat sink — the pipes run through the field and trade heat and cold back and forth from the ground temperature to the fluid circulating in the piping. Now, in extreme cold, the system can pull all of the heat out of the loop field and suddenly wouldn't be able deliver heat to the system.
One of my clients said, when this happened to him, it took a few hours for the loop field to build up enough heat reserves to deliver heat to his home. If you have a resistance coil, and you weren't watching the system, you might not know that this could happen and just pay the premium until the capacity built up in the ground again. But before I heard of this happening, I never imagined that you could exhaust the heat from the ground. I think I thought the supply was infinite.
Finally, geothermal systems are all electric, so if you are conscious of your carbon footprint, you must consider the source of your electricity. Electricity as a fuel source, unless it's generated from PVs on site, has a higher carbon footprint than natural gas, in part, due to the efficiency losses through distribution. If you don't have on site renewables, you can also opt to purchase off site clean power and that would make the issue of carbon footprint moot.
You must be asking if, given my experience with geothermal systems in operation, I would still include them in my designs. The answer has two parts. If I were designing a system for heating only, I'd probably recommend a high efficiency boiler (and natural ventilation for cooling). That would require getting past our cultural dependence on air conditioning. But if the project I'm working on requires both heating and cooling, I would choose geothermal but would spend time helping my clients understand that they require a different operating protocol.
By Lois Vitt Sale, chief sustainability officer at Wight & Co.
Read more about geothermal.