Ground-source heat pumps
by Chris Woodford. Last updated: April 26, 2015.
No matter how well insulated your home seems to be, heat will always leak out on a cold day, making you a little bit colder every minute. On days like this, wouldn't it be great if we could throw nature into reverse so our homes actually got hotter? What if you could flick a switch and make the heat flow in the opposite direction? It sounds absurd, but you can actually buy a home heating system that does just that! Heat pumps literally suck heat in from the ground or the air outside to warm up your home. Even though this simple idea is at least a century old, it's still very much an unexploited technology. Let's take a closer look at how it works!
Photo: This eco house produces all its own energy from solar panels, passive solar (large areas of glass windows), and a heat pump. The house is literally sucking in heat from the ground underneath it! Photo courtesy of US Department of Energy.
How does heat energy get inside Earth?
It's hard to think of Earth as a hot place, especially if you live in somewhere like Alaska or Greenland. But that's because we scrabble around on the surface of the planet, exposed to changing seasons and cold winds. Underground, it's a very different story. Volcanoes and geysers hint at how much energy is being generated by nuclear reactions firing off deep inside our planet, where even solid rocks turn to a hot bubbling soup. Geologists have estimated that there's about 42 million megawatts of this geothermal energy trapped inside Earth, which is equivalent to the energy made by 25,000 large power plants!
Unfortunately, we can't easily access this kind of geothermal energy, except from a relatively few places on Earth (think of Iceland or Yellowstone National Park). Even so, we can extract heat stored in the ground pretty from much anywhere on the planet. How come? Instead of tapping into geothermal energy, most heat pumps make use of the simple fact that the ground is generally warmer than the atmosphere above it in winter because of the energy it systematically receives, year-round, from the Sun. (That's why it's often better to refer to them as ground-source heat pumps rather than geothermal heat pumps.) In the top 3m (10ft) or so of Earth's surface, the temperature stays a steady 10–20°C (50–60°F). So, even if you don't live on top of a volcano or have a geyser in your backyard, not far below ground level, there's still a huge reservoir of heat just waiting to be tapped.
Photo: There's lots of geothermal energy trapped inside Earth. Geysers like this occur when rainwater and melted snow drips down through hot rocks then blasts back up again as boiling steam. Picture by Robert Blackett, Utah Geological Survey, courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
How can you move heat?
Virtually all the heating systems people use in buildings work by generating heat energy where it's needed. Electric radiators convert electrical energy into heat by passing electric currents through thin wires called filaments or elements (similar to those in an electric toaster). Gas and oil central heating boilers burn energy-rich fuels with oxygen from the air in a process called combustion. The energy released in this way is used to heat hot water that circulates through the radiators in our homes.
Heat pumps work in a totally different way. Instead of creating heat, they simply pick it up and move it from one place to another—usually from the ground under your home into the building itself. If that sounds weird, just think for a moment about the refrigerator or air conditioner unit in your home. A refrigerator also works by moving heat from one place to another: it uses a cooling fluid to absorb heat from the chiller compartment, move it outside, and then release it into the room using radiator fins on the back of the case. An air conditioner works in a broadly similar way but cools down an entire room (or an entire building). We can think of a heat pump as a bit like a refrigerator or an air conditioner working in reverse: instead of moving heat from the inside to the outside, it moves heat from the outside to the inside—even in winter!
Chart: You might think heat pumps would be used only in really cold countries; Sweden and Switzerland account for a quarter of the world's heat-pump capacity between them. But the technology can be used pretty much anywhere. Half of worldwide installed heat pump capacity is in the United States (where there are over a million separate units), just over a third is in Europe (where Sweden dominates), and the remainder is in Asia. Statistics from Geothermal Heat Pumps: Overview of Market Status: February 2009 (Executive Summary), US Department Energy (see references below).
How heat pumps work
In a central heating system, there's a continuous circuit of water pipes, starting at the boiler, flowing through all the radiators in turn, and going back to the boiler again. When cold water flows into the boiler, it gets heated up very rapidly. A pump circulates it through the radiator pipes, where it gives off its heat and cools, before flowing back to the boiler to pick up more heat and go round the loop again.
A heat pump system is very similar but, instead of the boiler, the source of heat is the ground underneath your home. Water (usually mixed with antifreeze) is forced through a long string of plastic pipes buried in the ground outside or directly underneath the building. The water picks up some of Earth's heat and flows into the building, where the energy is removed by a heat exchanger and delivered through air ducts, cooling the water in the process. The cooled water then passes back outside to pick up some more heat from the ground.
Photo: A heat pump on the campus of Georgia Tech. Photo courtesy of US Department of Energy.
You can see then that a heat pump system really consists of three separate parts:
- Ground loop: The network of plastic pipes, filled with water, that collects heat from Earth.
- Heat pump: A simple water pump that moves the water around a loop.
- Air duct: Also known as the air handler, this extracts the heat from the water and uses a fan to blow it through your home.
The only energy you have to supply to the system is the electricity you need to power the pump and the air blower unit. All the energy that actually heats your home is coming from the ground.
Types of ground-source heat pumps
Ground-loop pipes need to extract as much heat as possible and there are four different ways to achieve this. Different methods work best for different types of buildings depending on where they're located. For ordinary-sized homes, the simplest and commonest installation involves running the ground-loop pipes at a relatively shallow depth (about 1.5–2m or 4–6 ft) through a relatively large horizontal area next to your house. Office buildings and schools, which need to collect much more heat, typically use a smaller horizontal area of pipes but run them vertically much deeper (about 30–120m or 100–400ft). The heat doesn't necessarily have to come from soil or rock. If you live near a large pond or lake, you can use that as your heat source instead of the ground. The pipes run horizontally about 2.5m (8ft) below the water surface to prevent them from freezing up.
Artwork: Three types of closed-loop heat pump system:
1. A horizontal loop, where the pipes run near the ground surface but over a large area; 2. A vertical loop has much deeper pipes recovering more heat from deeper, warmer ground; 3. A horizontal loop using a lake or pond as its heat source.
These three types of system all use closed loops of pipes (so the same fluid is contained within and flows through the heating system at all times, like the coolant in a refrigerator). There's also an alternative design, known as an open-loop system, where you pump the water out from your house into an open, underground well and then pump it back out again. The water has to be very clean for you to be able to do this, however.
Heat pumps can warm your home in winter, but they have another huge advantage too: run them in reverse in summer and they can extract heat from your home and sink it back into the ground—working just like air conditioners, only much more cheaply. They can also supply you with free hot water in summer and roughly half-price hot water in winter. Because the pipes are buried underground, the ground-loops are virtually maintenance free and (unlike air conditioners) practically impossible to damage or vandalize. The manufacturers are so confident in their products that heat pumps typically carry warranties of 25–50 years. Customers like them too: according to the US Department of Energy, user satisfaction ratings are about 90 percent!
How good are heat pumps?
In a word: superb! According to the US Department of Energy, they give annual energy savings of 30–70 percent in heating costs in winter and 20–50 percent in cooling costs in summer. That means they pay for themselves in 2–10 years. They're about 50 percent more efficient than gas furnaces (gas central-heating boilers) and 75 percent more efficient than oil furnaces (oil boilers). All round, they're good for your pocket—and great for the planet. (Having said that, they do need to be properly installed. One UK study published in 2010 found that 80 percent of heat pumps were badly installed and massively underperforming—sometimes consuming more energy than they actually produced.)