What it is:
To simplify a fairly complex process, geothermal makes use of the consistent temperatures of earth and rock. As Gerald Hartford, vice president and managing director for Syska Hennessy Group's San Diego office, points out, "We use the earth as a thermal mass, and we either extract heat from it or reject heat to it." During the winter, a geothermal system absorbs the earth's heat and moves it inside; in the summer, it moves indoor heat underground.
There are a few ways geothermal can be used: horizontal or vertical loops (closed loops) and geothermal lakes/ponds (open loops). (Closed-loop systems can also be used in lakes or ponds as well.) Heat pumps, loop pumps, loop fluids, and piping make up a geothermal system, with the majority of the material and equipment located indoors.
How it works:
Horizontal loops are used if land is suitable for drilling or trenching. Piping is placed vertically in narrow, horizontal trenches. If conditions don't support trenching, vertical boreholes/loops are used, making use of pipe that goes into the ground, does a U-turn, and comes back up, explains Greg Allen, sustainable design strategist at HOK, Toronto. "They're usually made of plastic and are grouted to keep a thermal contact with the surrounding medium." Heat is either soaked up or dispersed by a fluid circulating within the plastic pipe. That fluid is then circulated into a building's heat pumps and used for heating/cooling.
Less commonly used, geothermal lakes/ponds use horizontal loops of pipe in water-bearing earth. Instead of loop fluid, water is pumped into the heat pump, where heat is extracted; water is discharged back to its original source or into a return well. Hartford points out that using a manmade body of water on your site is easier than tapping into a publicly owned body of water. "You'll have a maze of regulatory criteria to follow with a public source," he says.
Geothermal has a high coefficient of performance (COP). "A coefficient of performance stands for how many units you can get out vs. 1 unit of energy in. For geothermal, COP ranges from 3 to 4. So, if we have 1 unit of electrical energy in and we convert that to BTUs, we get three or four times that in BTUs out," says Hartford.
The systems are anywhere from 25- to 40-percent more efficient than other HVAC systems.
Obviously, geothermal makes use of a natural heating/cooling source: the earth.
Site affects whether or not geothermal is an option. "If there is poor conductivity and no water contact, or if it's solid granite, the cost of drilling becomes much more expensive," says Allen.
Geothermal can be high maintenance. "You've got to put heat pumps all over the building. Each has a refrigerant compressor and a filter; they have a lifespan of around 10 years, and then they have to be replaced," says Hartford.
The heat pumps can cause some setbacks. "They can't typically handle the ventilation air load. Over the years, people have used gas-fired make-up air-handling units to supply ventilation air. With that method, you're defeating the whole purpose of geothermal. The best and most effective way: Use a water-to-water geothermal heat pump, creating a closed loop of heating or cooling water, and use that as the tempering element to facilitate the outside air," says Hartford.
Geothermal isn't common practice yet. "The people that design the systems oftentimes fail to understand the heat transfer efficiency of the earth," says Hartford. He says that digging a "test bore" - a 200-foot-deep hole where a closed-loop line is placed, along with instrumentation to determine thermal conductivity - is a way to combat this problem.
Where it works:
Geothermal works in almost all of continental North America. "It's just going to be more efficient and effective in different parts," says Hartford.
One of the most important factors to consider: Do you have the land area for geothermal? "It takes up quite a bit of ground. If you have vertical boreholes, they need to be spaced so far apart," Hartford points out. He says that underneath parking lots and football or soccer fields are good places to consider (although, with parking lots, you'll have to tear them up if you ever need to do repair work).
Schools and office buildings are good candidates for geothermal. "If you need really close control of your supply air temperature and humidity, geothermal just isn't applicable," says Hartford.
And, ground condition doesn't matter as much as you might think. "People worry about what the ground is made of: rock, black dirt, sand, etc.," says Hartford. "If you can determine the thermal conductivity of the hole, then you don't care if there's water down there or if it's moist soil or dry soil."