by Karl Driessen
Some people may have heard about harnessing the heat stored in the earth’s crust, in places like California, Nevada, or Iceland. Such geothermal fields are often located around places with volcanic activity. When you see hot springs or geysers, there is definitely something geothermal going on. And this can be tapped for clean energy: the Geysers, located north of San Francisco, has a capacity to generate 900 megawatts of power without fossil fuels, using the steam coming out of the ground.
But geothermal heat in Forest Hills? Really? It turns out that there are some neighbors that are using the heat stored in their backyards to heat and cool their homes. In fact, I am one of those, as some people in the neighborhood found out when they noticed a big drill rig in our backyard, wondering if perhaps we were hoping to strike oil.
With gas furnaces being phased out in new construction in some jurisdictions, our construction plans prompted the question of whether we could “cut the gas pipe” and decarbonize our heating and cooling. Was it even possible to do this in DC? How long would it take? How much would it cost? A whole bunch of unknowns when we started.
No volcanoes here. Residential geothermal doesn’t require the boiling temperatures used by geothermal power plants. Instead, it leverages the near-constant temperature of the ground underneath us (around 50 degrees Fahrenheit). A device called a heat pump extracts heat from the ground in winter and uses it to warm the house. Conversely, in summer the heat pump removes heat from the house and buries it in the ground.
How does that work? In a nutshell, a heat pump is like a refrigerator. Most of us will have noticed that the back of a refrigerator can get quite warm, while inside, milk and vegetables are kept cold. The neat trick of a heat pump used for heating and cooling is that this process can be reversed by flipping the so-called reversing valve. If it is hot inside, dump the heat outside; if it is cold, dump the heat inside.
A popular and economical type of heat pump exchanges the heat with the surrounding air. This looks very similar to a standard air conditioning compressor, with the added functionality of generating heat in winter.
Geothermal systems instead use water-source heat pumps, exchanging heat through a closed loop with coolant buried deep into the ground. For more details, check out the Rocky Mountain Institute explainer on geothermal heat pumps.
Why in the world would anyone go to such depths (literally!) to install a geothermal system? There are a few important advantages. It is environmentally friendly, as it does not produce greenhouse gases. It is very efficient in heating and cooling, using up to 50 percent less energy. This is because it moves heat, it doesn’t burn fuel to generate heat. While it is relatively costly to install, in the long run it is cost effective because of its low energy use and the expected lifetime of the infrastructure (50 to 100 years or more) and equipment. It is also less noisy than outdoor compressors.
By the way, the Inflation Reduction Act has made geothermal installations eligible for a 30 percent tax credit, helping offset the high initial costs.
Our geothermal journey has not been short. We had received a couple of quotes before Covid-19 struck. When we picked up the thread in 2022, one geothermal contractor had stopped doing business in DC, and the other – a local HVAC contractor – saw the driller it worked with retire. It took some time to find a drilling company – most regional companies choose not to operate in the District because the typical urban lot size is too small. Obtaining the soil boring permit took a few months; perhaps a bit longer than usual because the application was pending right at the time that the DC Department of Consumer and Regulatory Affairs (DCRA) was being split up into the Department of Building (DOB) and the Department of Licensing and Consumer Protection (DLCP).
Drill, baby, drill – for environmental sustainability! The permit kicked off a noisy multi-day drilling spree (thank you again neighbors for your forbearance!) to prepare for the installation of the geo loops. Given the size of the drill rig – think fire engine – access to the property from the alley was not easy, but the truck just managed to squeeze in. An enormous frack tank was placed in the backyard to collect any groundwater that might erupt during the drilling. (Thankfully, that did not happen! Other neighbors apparently weren’t so lucky and hit a spring that is now used to water their garden.)
The crew proceeded to drill wells between 240 and 320 feet deep, and inserted pipes through which the coolant will circulate. This video shows the drill grinding away, with a crewmember shoveling away the rock dust that comes up.
Next up is the fusing of the loops into one long circuit, and then bringing those coolant-filled lines into the house to connect to the heat pump.
Urban residential geothermal is a reality, and the District could include it as part of its Climate Ready DC plan to reduce greenhouse gas emissions. Our lot was barely suitable for geothermal, but it is possible to scale up geothermal heat pumps to heat and cool entire neighborhoods, as is done for example in a 400-home development in Texas.
In Washington DC, much of this would involve retrofitting existing housing when natural gas distribution is terminated. I can imagine Washington Gas being re-baptized Washington Gas & Geo, with geo wells drilled 20 feet apart in every alley and street.