Re:Unintended consequences? (Score 2, Informative)

I can't help but wonder about what would happen if a sufficient number of people in an area used heat pumps, long term.

This is one of the questions I address in my book, Sustainable Energy - without the hot air (available from amazon, and for free (pdf) on the web). The chapter on "Smart Heating" highlights heat pumps. They are definitely going to be one of the pillars of the post-fossil-fuel future. With a typical suburban population density, there is indeed not enough ground area for everyone to get all their heating out of the ground, if they are not careful to put heat back at other times of year. If people suck too much heat, without putting it back, then the ground will gradually become frozen. There is a similar potential problem with ground source air-conditioning, where people use the heat pump the other way round a lot, dumping heat in the ground (or sucking cold, if you like). Eventually the ground warms up, and the A/C doesn't work so well. This has happened in central London. There are two fixes: (1) ensure that annual heat sucking matches annual heat dumping (by matching winter heating to summer A/C, and perhaps even adding solar hot water panels to the roof, to get extra heat to dump during the summer); or (2) use a heat pump but with a different heat source, for example the air. As already noted, air-source heat pumps are not great if you have -40 degree winters. But in many countries with mild winters (eg Britain) I think air-source heat pumps are the best choice for green heating. Sustainable Energy - without the hot air also has an appendix on heat pumps in which the relevant equations are worked out from first principles. David MacKay Cambridge