Sustainable Energy Without the Hot Air: Warming up with heat pumps

Heating a home is a major contributor to our energy consumption. In Sustainable Energy Without the Hot Air, David MacKay stated how we need to take more innovative measures to make heating our houses more efficient and require less electricity. Using heat pumps in homes and offices is key to this. The following is an extract from the chapter Smarter Heating.

How do heat pumps work?

Like district heating and combined heat and power, heat pumps are already widely used in continental Europe, but strangely rare in Britain. Heat pumps are back-to-front refrigerators. Feel the back of your refrigerator: it’s warm. A refrigerator moves heat from one place (its inside) to another (its back panel). So one way to heat a building is to turn a refrigerator inside-out – put the inside of the refrigerator in the garden, thus cooling the garden down; and leave the back panel of the refrigerator in your kitchen, thus warming the house up.

What isn’t obvious about this whacky idea is that it is a really efficient way to warm your house. For every kilowatt of power drawn from the electricity grid, the back-to-front refrigerator can pump three kilowatts of heat from the garden, so that a total of four kilowatts of heat gets into your house. So heat pumps are roughly four times as efficient as a standard electrical bar-fire. Whereas the bar-fire’s efficiency is 100%, the heat pump’s is 400%. The efficiency of a heat pump is usually called its coefficient of performance or CoP. If the efficiency is 400%, the coefficient of performance is 4.

Heat pumps can be configured in various ways. A heat pump can cool down the air in your garden using a heat-exchanger (fig. 1), in which case it’s called an air-source heat pump. Alternatively, the pump may cool down the ground using big loops of underground plumbing (many tens of metres long), in which case it’s called a ground-source heat pump. Heat can also be pumped from rivers and lakes.

Fig. 1 A heat pump

Some heat pumps can pump heat in either direction. When an air source heat pump runs in reverse, it uses electricity to warm up the outside air and cool down the air inside your building. This is called air-conditioning. Many air-conditioners are indeed heat-pumps working in precisely this way. Ground-source heat pumps can also work as air-conditioners. So a single piece of hardware can be used to provide winter heating and summer cooling.

People sometimes say that ground-source heat pumps use “geothermal energy,” but that’s not the right name. As we saw in Chapter 16, geothermal energy offers only a tiny trickle of power per unit area (about 50mW/m2), in most parts of the world; heat pumps have nothing to do with this trickle, and they can be used both for heating and for cooling. Heat pumps simply use the ground as a place to suck heat from, or to dump heat into. When they steadily suck heat, that heat is actually being replenished by warmth from the sun.

Limits to growth

Because the temperature of the ground, a few metres down, stays sluggishly close to 11◦C, whether it’s summer or winter, the ground is theoretically a better place for a heat pump to grab its heat than the air, which in midwinter may be 10 or 15◦C colder than the ground. So heat-pump advisors encourage the choice of ground-source over air-source heat pumps, where possible. (Heat pumps work less efficiently when there’s a big temperature
difference between the inside and outside.)

However, the ground is not a limitless source of heat. The heat has to come from somewhere, and the ground is not a very good thermal conductor. If we suck heat too fast from the ground, the ground will become as cold as ice, and the advantage of the ground-source heat pump will be diminished.

In Britain, the main purpose of heat pumps would be to get heat into buildings in the winter. The ultimate source of this heat is the sun, which replenishes heat in the ground by direct radiation and by conduction through the air. The rate at which heat is sucked from the ground must satisfy two constraints: it must not cause the ground’s temperature to drop too low during the winter; and the heat sucked in the winter must be replenished somehow during the summer. If there’s any risk that the natural trickling of heat in the summer won’t make up for the heat removed in the winter, then the replenishment must be driven actively – for example by running the system in reverse in summer, putting heat down into the ground (and thus providing air-conditioning up top).

Conclusion

We should replace all our fossil-fuel heaters with electric-powered heat pumps; we can reduce the energy required to 25% of today’s levels. Of course this plan for electrification would require more electricity. But even if the extra electricity came from gas-fired power stations, that would still be a much better way to get heating than what we do today, simply setting fire to the gas. Heat pumps are future-proof, allowing us to heat buildings efficiently with electricity from any source.

Nay-sayers object that the coefficient of performance of air-source heat pumps is lousy – just 2 or 3. But their information is out of date. If we are careful to buy top-of-the-line heat pumps, we can do much better. The Japanese government legislated a decade-long efficiency drive that has greatly improved the performance of air-conditioners; thanks to this drive, there are now air-source heat pumps with a coefficient of performance of 4.9; these heat pumps can make hot water as well as hot air.

Another objection to heat pumps is “oh, we can’t approve of people fitting efficient air-source heaters, because they might use them for air-conditioning in the summer.” Come on – I hate gratuitous air-conditioning as much as anyone, but these heat pumps are four times more efficient than any other winter heating method! Show me a better choice. Wood pellets? Sure, a few wood-scavengers can burn wood. But there is not enough wood for everyone to do so. For forest-dwellers, there’s wood. For everyone else, there’s heat pumps.

Read more about how to reduce your energy consumption and play your role in saving the planet in ‘Sustainable Energy Without the Hot Air’ available for purchase as a paperback where all good books are sold, or you could download the PDF for £/$ 0.00 (yes, really nothing to pay).

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