Heat pumps
Last updated: June 12, 2008.
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.
Let's take a closer look at how they work.
Photo: This eco house produces all its own energy from
solar panels, passive solar glazing, and a heat pump.
Photo courtesy of US Department of Energy.
Geothermal: heat energy 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!
Getting access to all that hot stuff is
easier than you might think. In the top 3m (10ft) or so of Earth's
surface, the temperature stays a steady 10-20°C (50-60°F).
So, not far below ground level, we have 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!
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 geothermal 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.
Four types of ground loop
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.
Other advantages
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 and 75 percent more
efficient than oil furnaces. All round, they're good for your
pocket—and great for the planet.
Further reading
