No part of our climate operates as an isolated, static system, but rather a constantly shifting series of temperature variations. In virtually all parts of the country, the outside temperature rarely stays in the optimal range of 55-75 degrees for very long, forcing us to employ energy to create insulated, indoor micro-climates for ourselves. In fact, in the average American home this is the largest single energy usage throughout the year. In 2005, Americans used 5.18 quads (quadrillion BTUs) of energy to heat and cool their homes at a collective cost of $82 billion, according to the Energy Information Administration (EIA).
The ground beneath us, however, holds an extremely stable temperature regardless of seasonal weather conditions above. Past the first 4-6 feet of earth, the soil and rock below are always an average of +/- 55 degrees Fahrenheit (depending on latitude) which makes it a prime candidate for both depositing waste heat and taking excess heat back to the surface. This is the central concept of how geothermal works: using the large thermal mass of the earth to help balance the temperature inside building environments.
Geothermal heating and cooling starts with a deep hole drilled hundreds of feet into the ground called a well. Depending on the size of the building, numerous wells may be required but an average home can usually need only one. A long, continuous pipe is fed down the well and back up again, forming a loop at the top—a pipe that will eventually be filled with water. This is called a “vertical closed loop” system as opposed to “horizontal closed loop” or “open loop” systems. Unlike a well drilled for drinking water, the hole is not drilled to extract anything material from the earth, but rather to allow the ground to bring the column of water to its ambient temperature.
So now we have a long pipe full of water at 50 degrees…what does that do for us? Well in the winter, the ground is considerably warmer than the air. By circulating the water through the depth of the well we can warm it up before running it through a heat pump that extracts the heat, sending cooler water back down into the well to be heated again. The heat can be exchanged directly to the water of a radiant floor system, or used to heat coils in a duct with air blowing over them (replacing a furnace.) The system works constantly, extracting a few degrees of heat to maintain the temperature inside.
In the summer, the same system can work in reverse. Instead of extracting heat, the heat pump can take the heat from the air and push it into the geothermal water (leaving cooler air behind). The water then circulates down the well, but the same 50 degrees that was warmer than the air in the winter is now cooler than the air, allowing the excess heat to seep into the ground with colder water coming back up. This replaces the noisy condensing unit of a common air conditioning system.
The beauty of the system is that in either case, the only thing that is really using power is the heat pump. Gone are the deliveries of oil, propane or money spent on natural gas—all of which bring significant energy expenditures and emissions of their own to reach a house. Geothermal is not, by any means, restricted only to residential applications. Any type of building can utilize geothermal for heating and cooling with larger buildings simply needing to add more wells. Geothermal also does not have to be an all-or-nothing endeavor. Especially on urban sites, wells can be drilled with the space available to supplement another heating and cooling system.
Given that the earth is doing most of the heavy lifting when it comes to changing the temperature of the water, the Department of Energy claims that a well-installed system can reach efficiencies of 300-600%. Overall, the geothermal heat pump will use 25-50% less energy than a conventional heating and cooling system and up to 72% less than an electric-resistance heating system (electric baseboard) with air conditioning according to the EPA which labels geothermal one of the most efficient heating and cooling technologies available today. They also last for a while. Most well systems will be built to last up to 50 years (much longer than any furnace or AC condensing unit).
Geothermal heating and cooling presents an opportunity to displace an enormous amount of fossil fuels. According to the EIA, about 8 million households rely on heating oil as their primary heat source and purchased over 4.6 billion gallons in 2008. Given that only 2 gallons of heating oil come from every 42-gallon barrel of crude, that alone counts for over 96.6 billion gallons of oil pulled out of the ground. If a geothermal system was combined with a small array of PVs or a small-scale wind turbine, heating and cooling a home could require no fossil fuels at all.
The geothermal market is starting to pick up some steam, with shipments of geothermal heat pumps in the U.S. more than tripling from 2001 to 2009. The only deterrent is usually the upfront cost—not of the heat pumps themselves, but of the well. Drillers can charge anywhere from $10,000-30,000 depending on the required depth and soil composition, but when coupled with Federal and State level tax credits, financial payback can happen within 5-10 years making it a great deal all around.
As we look for ways to reduce our nation’s energy consumption, geothermal heating and cooling can be a cornerstone of an efficiency policy. Especially in the Northeast, where demand for heating is the highest in the country, geothermal should become the standard, primary choice for both renovations and new construction. As a renewable resource that does not suffer from the intermittent nature of solar and wind power, geothermal can provide significant, national energy reductions if deployed on a larger scale.
Image Credit: drillingsupply.info