Limits to growth of heat pumps

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 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).

Let's put some numbers into this discussion. How big a piece of ground does a ground-source heat pump need? Assume that we have a neighbourhood with quite a high population density - say 6200 people per km2 (160 m2 per person), the density of a typical British suburb. Can everyone use ground-source heat pumps, without using active summer replenishment? A calculation in Chapter E (p303) gives a tentative answer of no: if we wanted everyone in the neighbourhood to be able to pull from the ground a heat flow of about 48kWh/d per person (my estimate of our typical winter heat demand), we'd end up freezing the ground in the winter. Avoiding unreasonable cooling of the ground requires that the sucking rate be less than 12kWh/d per person. So if we switch to ground-source heat pumps, we should plan to include substantial summer heat-dumping in the design, so as to refill the ground with heat for use in the winter. This summer heat-dumping could use heat from air-conditioning, or heat from

Figure 21.12. How close together can ground-source heat pumps be packed?

area per person (m2)

Bangalore

37

Manhattan

39

Paris

40

Chelsea

66

Tokyo

72

Moscow

97

Taipei

104

The Hague

152

San Francisco

156

Singapore

156

Cambridge MA

164

Sydney

174

Portsmouth

213

Table 21.13. Some urban areas per person.

Table 21.13. Some urban areas per person.

roof-mounted solar water-heating panels. (Summer solar heat is stored in the ground for subsequent use in winter by Drake Landing Solar Community in Canada [www.dlsc.ca].) Alternatively, we should expect to need to use some air-source heat pumps too, and then we'll be able to get all the heat we want - as long as we have the electricity to pump it. In the UK, air temperatures don't go very far below freezing, so concerns about poor winter-time performance of air-source pumps, which might apply in North America and Scandanavia, probably do not apply in Britain.

My conclusion: can we reduce the energy we consume for heating? Yes. Can we get off fossil fuels at the same time? Yes. Not forgetting the low-hanging fruit - building-insulation and thermostat shenanigans - 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 -1 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.

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