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Line artwork illustrating the concept of a ground-source heat pump that extracts heat from the ground under a building.

Heat pumps

Must we burn the whole Earth to keep warm? Looking at the last couple of hundred years of human history, you might think that's where we're heading. The Industrial Revolution—and all that's happened since—was powered by combustion: burning coal, then oil and natural gas, to drive everything from steam engines and power plants to cars, boats, and planes. The consequences, apart from pollution, include a rapidly warming planet that could eventually prove too hot to handle. There's lots of talk about renewable energysolar cells, wind turbines, and things like energy harvested from the sea—but it's taking time to come onstream. There is another way of producing renewable energy that most people don't know about at all. Heat pumps "mine" the warmth buried just underground and ferry it into buildings, a bit like air conditioners working in reverse. Simple, efficient, and effective, and widely used in Scandinavia for decades, they could provide much of our home heating needs in the future. How exactly do they work? Let's take a closer look!

Artwork: The basic concept of a heat-pump: it extracts heat from the ground (or the air or water outside) and pumps it into a building. The only energy you need to make this happen is the power to drive the pump.

Contents

  1. Where does the heat inside Earth come from?
  2. What is a heat pump?
  3. Types of heat pumps
  4. Advantages and disadvantages of heat pumps
  5. Who invented the ground-source heat pump?
  6. Find out more

Where does the heat inside Earth come from?

Car engines and power plants make useful energy by combustion—the process of burning a fuel like coal or gas with oxygen from the air to release chemical energy locked inside it. But you don't necessarily have to burn a fuel to make heat energy. As I'm typing these words, I'm sitting by a window with sunlight streaming in and warming my legs (my body is soaking up and storing passive-solar energy). The same thing is happening right across Earth: when the Sun shines on the land, all that energy has to go somewhere, so it warms the ground underneath.

Although we're used to the idea that temperatures vary from day to day, and day to night, it's much less obvious that the temperature a little way underground is both more constant and, in winter at least, significantly higher than it is at ground level: in the top 5m (16ft) or so of Earth's surface, the temperature stays a steady 10–20°C (50–60°F). [1] In other words, there's solar energy "buried" underground that we can "mine" with a heat pump—a machine that extracts energy from the ground (or sometimes from water or even air) and pumps it inside a cold building to heat it up.

Geothermal or ground source?

One quick note about terminology. Although these machines are sometimes called "geothermal heat pumps," that's a slightly misleading description. Geothermal energy is the heat that comes from deep inside Earth that we can see spurting to the surface in steamy geysers or bubbling lagoons in places like Iceland; we can harness such wild, extreme energy only in a relatively small number of very specific locations, typically using geothermal power plants to turn it into electricity. Heat pumps, on the other hand, are tapping into more modest amounts of solar energy that have warmed the Earth from the outside in—and we can tap into that pretty much anywhere. To avoid confusion, it's better to talk about ground-source or geo-exchange pumps and reserve the word geothermal for the geysers and lagoons.

Hot geysers are a sign of geothermal energy

Photo: Geysers like this occur when rainwater and melted snow drips down through hot rocks inside Earth then blasts back up again as boiling steam. Geologists have estimated there's about 70–80 gigawatts of this geothermal energy inside our planet, which is equivalent to the energy made by about 70–80 really big nuclear power plants! [2] Ground-source heat pumps aren't tapping into this "deep heat," however, but into gentler warmth nearer the surface that comes from the Sun. Picture of "Old Faithful" Geyser in Yellowstone National Park by Carol M. Highsmith. Credit: Gates Frontiers Fund Wyoming Collection within the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.

What is a heat pump?

An air-source heat pump outside a building

Photo: An air-source heat pump, seen from the outside, looks much like an air conditioner. Photo by Molly Rettig courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL) (photo id #136954).

A heat pump is like an air conditioner or refrigerator working in reverse. A refrigerator extracts heat from a metal box (the chiller compartment where you store your food) by circulating a very cold fluid through a convoluted pipe that loops around inside it. The fluid expands and picks up heat, cooling down the box and warming up by an equal amount, before getting pumped outside and round the back of the refrigerator. There, it's compressed (squeezed into a smaller volume) so it gives up its heat to your kitchen, cooled, and pumped back inside the refrigerator so it can repeat the process again. Hey presto, heat gets pumped from the inside of the box to the outside, making the chiller compartment cooler by making your kitchen warmer. A heat pump is very similar only instead of removing heat from a cool chiller box, it takes it from the warm soil and rock beneath your home or garden; and instead of releasing that heat into your kitchen, it releases it more generally into your home, usually through an underfloor heating system.

The magic thing about a heat pump is that it can put much more heat into your home than the energy you use to make it spin around. That sounds like a horrible violation of the famous law of physics known as the conservation of energy (which says that you can't create or destroy energy), but it isn't. The pump doesn't create any heat: it simply moves heat out of the ground. The relatively modest amount of electricity that powers the pump enables it to move a much bigger amount of heat energy into your home. That's why heat pumps are so efficient: in effect, they give you most of your heat energy for free.

You can probably see from this that we'd need four key bits to make ourselves a heat pump:

  1. There's the pipe that goes into the ground to extract heat. That's called the ground loop.
  2. The pump that forces fluid down through the loop and back up again.
  3. Just like in a refrigerator, there's a compressor that effectively squeezes the heat out of the fluid when it's been pumped inside your home.
  4. There's something that releases that heat so you can feel the benefit of it inside your home.
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Types of heat pumps

You'll often hear people talk about "ground-source heat pumps" (which pick up heat buried underground) because those are the most common—and they're the most common because, all round, they're the most practical and efficient. Soil and rock underground is at a fairly constant temperature from day to day, so getting heat out of it is predictably efficient.

We can also use heat pumps to extract heat from water (something like a lake might seem cold but, as it's hotter than absolute zero, it still contains heat we can pull out) or even the air. Water- and air-source pumps (as these are called) are much less common than ground-source ones. That's because most of us don't have lakes in our backyard (ruling out water-source pumps) and because air tends to fluctuate in temperature much more than the ground, making it less efficient as a source of heat (in other words, making ground-source pumps more attractive). It's worth noting that it's possible to have hybrid heat pumps that take heat both from the ground and the air.

Artwork comparing three different kinds of closed-loop heat pumps work: horizontal loop, vertical loop, and horizontal loop using a lake or pond

Artwork: Three common types of ground-source heat pump system based on closed loops: 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.

Closed loop, ground-source

Ground-source heat pumps use a closed loop of pipe and, because it extracts a certain amount of heat, it has to come into contact with a certain amount of ground to do so. There are two ways of achieving that. Either the loop can run deep underground in a relatively small area (so it's a vertical pipe) or it can run much closer to the ground (about 1.5–2m or 4–6 ft below the surface) over a much larger area (making it a horizontal pipe). [3] Generally speaking, the bigger your building, the more heat you need, and the bigger or deeper the loop needs to be to extract that heat. Some heat pump loops are really huge. Vertical loops can run 100m (330ft) below ground (the length of an athletic sprint track) or sometimes even deeper, while horizontal loops might stretch just over a meter deep and as far as 200m (650ft) across (they tend to be longer than vertical loops because, being closer to the surface, they're likely to be cooler during winter). Horizontal pipes are sometimes installed as a "slinky" (curved or coiled) to save space. [4]

How does a ground-source heat pump work?

In practice, most ground-source heat pumps actually have two closed loops and a heat exchanger that passes heat from one to the other. So there's a very large loop that goes down into the ground filled with water and anti-freeze, and a much smaller loop that takes heat from this loop and passes it on to the heating system.

How a ground-source heat pump uses a heat exchanger.

Here's how it works:

  1. The ground loop (shown here as a coiled slinky) is filled with a working fluid containing propylene glycol (or another antifreeze) that absorbs heat from the ground.
  2. The working fluid is pumped into a heat exchanger (dark gray) and flows around the red-blue closed loop.
  3. Inside the heat exchanger, the working fluid gives up its heat to a different fluid, a volatile (easily evaporating) refrigerant (shown in light blue), which flows around a second closed loop, entirely inside your home. The refrigerant fluid boils and evaporates to form a low-temperature, low-pressure vapor.
  4. The vapor passes into a compressor unit (red), which increases its temperature and pressure.
  5. After exiting the compressor, the vapor gives up its heat (to your space heating radiators) and warms your home. It may also heat your hot water using a separate heat exchanger. By giving up its heat, the vapor condenses back to a liquid at lower temperature and high pressure.
  6. The liquid passes through an expansion unit that reduces its pressure, turning it back into a low-pressure, low-temperature liquid ready for the cycle to repeat.
  7. Meanwhile, the fluid from the other loop returns to the ground to pick up more heat.

Open loop, water source

If you're trying to extract heat from something like a lake, pond, or river, there's no reason why one of the loops can't be open. In other words, you could pump water (containing heat) from the lake itself, extract the heat with a heat exchanger and a second loop, then return the water (cooled somewhat, because you've removed heat) back to a different part of the lake some distance away (so there's enough of a temperature difference to make heat extraction efficient). A system like this is called an open-loop or sometimes a surface-water heat pump. If the lake water isn't suitable for pumping (perhaps because it's too silted up), it's possible to sink a closed-loop into it instead and extract heat in the same way as we extract it from the ground.

Air source

Heat pumps can also take heat from the air outside a building and pump it inside—so they're effectively working like air conditioners in reverse, with the primary purpose of heating a building rather than cooling it, but using much the same principle. During winter, air temperatures are much more variable than ground temperatures (the air heats up and cools down far more quickly and changes more erratically than either the surface or deep level ground beneath your home), so air-source heat pumps tend to be less efficient and less dependable than ground- or water-source ones. Obviously, it's much easier to extract useful heat from the ground than from the windy air above it, which could be ten degrees colder.

The two parts of an air-source heat pump, either side of a wall

Photo: A simple air-source heat pump has two parts, which fit either side of a wall a bit like a basic air-conditioning unit. Picture by Dennis Schroeder courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

How does an air-source heat pump work?

Short answer: As I said above, almost exactly like a refrigerator! Where a refrigerator picks up heat from inside the cooling compartment and dumps it in your kitchen, a heat pump picks up heat from outside your home and dumps it inside—using almost exactly the same process.

How an air-source heat pump uses a volatile fluid, an evaporator, and a compressor to move heat inside your home

Here, quite simplified, is how it works. The whole process is based on circulating a volatile (easily evaporating) fluid around a closed loop, one part of which is inside your home and the other part is outside.

  1. Outside your home, the pump sucks in air (containing heat)—and the working fluid evaporates: it turns from a low-pressure, low-temperature liquid into a low-temperature, low-pressure gas.
  2. The gas gets pumped into the compressor unit (there's one at the bottom of your refrigerator, round the back). The compressor squeezes the gas so its pressure and temperature increase.
  3. As the gas passes through the inside of your home, it gives off much of its heat, warming your home, and cools back down into a high-pressure, somewhat lower temperature liquid.
  4. The liquid is allowed to expand through an expansion unit (equivalent to the expansion valve in your refrigerator), so it turns back into a low-temperature, low-temperature liquid.

Advantages and disadvantages of heat pumps

Advantages

Heat pumps have a lot of things going for them. Widely used all over the world for decades, they're tried-and-tested machines with relatively few moving parts, so they're long lasting, reliable, very quiet, and very low maintenance. Other than the electricity needed to power the pump, they consume no fuel, so they produce little or no carbon emissions (none at all if the pump is powered 100 percent renewably by something like photovoltaic solar panels or a wind turbine) and no pollution (there's also no risk of carbon monoxide poisoning, as there is with any form of combustion heating). No fuel means no fuel to have delivered, pay for, move around, or store securely (unlike with boilers that burn solid fuel such as biomass, oil, or coal).

What about some hard numbers? As we've already seen, the magic thing about a heat pump is that it moves several times more energy than it consumes; in other words, it's well over 100 percent efficient (300–400 percent is typical) compared to the very best condensing gas boilers, which are 90–99 percent efficient. One common measurement of the efficiency of heat pumps is called the coefficient of performance (CoP), and it's simply the amount of heat you get out of your system divided by the electrical energy you put into it to drive the pump (the percentage efficiency divided by 100). Typical CoP values are in the range 3–4. [5]

According to the US Department of Energy, heat pumps give annual energy savings of about 30–60 percent. They're several times more efficient than the best gas furnaces (gas central-heating boilers) and 75 percent more efficient than oil furnaces (oil boilers). That means they pay for themselves in 5–10 years (obviously depending on the type of system, what it's replacing, and other factors like how well your house is insulated to retain the heat the pump extracts) and last two to three decades (with the ground loop often warranted for as much as 50 years), so the long-term economic case for putting them in new buildings is very compelling. [6] And owners seem to love them: according to one detailed academic study by the Open University, 75 percent of those surveyed (85 percent of private householders; 58 percent of social-housing occupants) thought their heat pump was "better or much better" than their previous heating system.

Bar chart showing the growth in the total number of European ground-source heat pumps between 2005 and 2020.

Chart: Europe has seen a steady growth in energy produced using heat pumps. Statistics and data courtesy of European Heat Pump Association.

Heat pumps are especially popular in Europe. According to the European Heat Pump Association, by October 2021, a total of almost 17 million pumps had been installed in 21 European countries. In the 17 years between 2005 and 2021, the total number of installed pumps in the region went up almost 17 times. Having said that, Europe now covers a very large area and a huge diversity of different countries, and some are notably more enthusiastic than others. Three nations alone (France, Germany, and Italy) account for over half of all heat pump sales, while ten nations (those three plus Spain, Sweden, Finland, Norway, Poland, Denmark and the Netherlands) make up almost 90 percent of sales. It's also important to put these figures in context. There are roughly 200 million households in the 27 members of the European Union. Although the EHPA's statistics use a different definition of "Europe," you can see that heat pumps are still used by only a fraction of all European homes (perhaps 10 percent, as a rough estimate, based on these imperfect figures).

Takeup is rather lower in the United States. According to the International Energy Agency there were just 1.7 million heat pumps in the United States in 2020 (roughly a tenth as many as in Europe), with 40 percent installed in homes and the rest in commercial or other buildings.

Disadvantages

You need land where you can sink a loop and the construction process can be expensive and disruptive (which is another reason why heat pumps are better for new builds). In the UK, where I live, you generally don't need planning permission for residential heat pumps (air, water, or ground), unless you live in a listed (heritage) building or in a conservation area—but it's as well to check first.

Heat pumps don't produce such high temperatures as gas boilers and furnaces, so they're often used with low-temperature underfloor heating. Ordinary radiators are likely to produce significantly lower temperatures from a heat pump than you'd get with a conventional gas boiler, so they'll take longer to heat up your home from cold. Unlike electric and natural gas heating, which can be used in almost any building, heat pumps depend on the local geology and climate, so you need to factor that in; and obviously, you can't use them just for one apartment on the 93rd floor! Another important point is that heat pumps 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.

Some air-source heat pumps can be noisy—they are mechanical pumps with moving parts—but they aren't that different from air conditioners. One typical user quotes 45–55dB, which is about the same noise level as moderate traffic.

But all forms of heating have their pros and cons. Taken in the round, heat pumps are a very attractive long-term alternative (or addition) to green technologies like solar hot water panels, photovoltaic solar panels, and passive solar.

Who invented the ground-source heat pump?

William Thomson, Lord Kelvin, 1824-1907

Photo: Lord Kelvin's thermodynamics research—the science of moving heat—underpins modern heat-pump technology. Photo by T. & R. Annan & Sons courtesy of US Library of Congress.

Whom do we have to thank for this brilliant idea? The theory of heat engines (machines that can generate mechanical energy by heating fuel) largely began with French engineer Nicolas Sadi Carnot (1796–1832), who laid the foundations for the modern science of thermodynamics (how heat moves). British scientist Lord Kelvin (William Thomson, for whom the Kelvin temperature scale is named) built on Carnot's work and figured out the theory of heat pumps (how a machine can move heat energy from one place to another).

Kelvin's work was concerned with the theoretical movement of heat; efficient, practical heat pumps only really appeared in the early decades of the 20th century when electric power became widespread, giving us such indispensable inventions as the electric refrigerator and air conditioner. Mexican-born Swiss engineer Heinrich Zoelly invented the modern, electrically powered ground-source heat pump in 1912. After that, numerous inventors improved on the idea. Around 1930, Douglas Warner developed an air conditioning system using the ground as a heat sink (which is effectively a ground-source heat pump idea working in reverse), for which he was granted US Patent US 1,957,624: Air conditioning with ground cooling and solar heat in 1934. According to geologist David Banks, Robert C. Webber of Indianapolis developed one of the first, practical, working heat pumps around 1945, after experimenting with using waste energy from his refrigerator to heat his home. Taking the idea a step further, he buried a 152-m copper coil underground and used that to extract heat instead. He sold the rights to this idea to a group of local businesses in 1948, but I can find no record of a patent for it either in his name or that of the company (Webber Engineering Corporation).

How it works

The early heat pump I've chosen to illustrate here was patented by Marvin Smith and Emory Kemler of Muncie Gear Works, Inc. in the late 1940s. Smith and Kemler's 1940s water-well heat pump is not the world's first geothermal pump, but it's the earliest one for which I can find a decent diagram. Like most heat pumps, it can run as either a heater or a cooler. Here, I've colored and numbered to show how it works as a heat pump.

Although this diagram looks confusing at a glance, it's simpler than it appears: there are two separate fluid loops connected by a pair of heat exchangers (the red and blue boxes near the top). The lower loop removes heat from the water well at the bottom, while the upper loop carries that heat to whatever the pump is heating.

Early design for a ground-source heat pump taking heat from a deep water well, designed by Marvin M. Smith in 1946/1949

Artwork: An early ground-source heat pump designed by Marvin Smith and Emory Kemler, from US Patent #2,461,449: Heat pump using deep well for a heat source. Artwork courtesy of US Patent and Trademark Office.

In a bit more detail, here's how it works: (1) Heat is extracted from a deep water well, passes up through valves (2) to a heat exchanger (3), before returning through the circuit to a cooler, higher part of the same well (4). The heat exchanger passes the heat to a fluid in a closed loop, where it gives up its heat to the "receiver" (6, the room or whatever else is being heated), before passing back around the loop to pick up some more. You can read a detailed explanation in US Patent #2,461,449: Heat pump using deep well for a heat source.

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References

  1.    Figures for ground temperature vary around the world, and for various other reasons too. I've taken my 10–20 degrees from measurements in Nicosia, Cyprus quoted in [PDF] Measurements of Ground Temperature at Various Depths by Georgios Florides and Soteris Kalogirou.
  2.    According to the World Bank, global geothermal potential stood at 70–80 GW in 2017, of which about 15 percent is currently exploited. The US Department of Energy estimates that typical nuclear plants produce 1GW of power.
  3.    An Introduction to Thermogeology: Ground Source Heating and Cooling by David Banks, Wiley, 2009, p.128 quotes 1.2–2m.
  4.    Faber & Kell's Heating & Air-conditioning of Buildings by Doug Oughton, Steve Hodkinson, Taylor & Francis, 2008, p.388 quotes "up to 100m deep depending on geology, the energy output required from the system and the space available." Mechanical and Electrical Equipment for Buildings by Walter T. Grondzik et al, Wiley, 2011, p.370 quotes 120–200m (400–650ft) of pipe installed 1–2m (3–6ft) deep.
  5.    Heat Pumps by Eugene Silberstein, Cengage, 2015 quotes a range of COP values, roughly 3–6, for heat pumps operating in different heating and cooling modes.
  6.    Figures in this paragraph come from the US Department of Energy.

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