other hand we try to suck a flux bigger than 5 W/m2, we should expect that
we’ll be shifting the temperature of the ground significantly away from its
natural value, and such fluxes may be impossible to demand.
The population density of a typical English suburb corresponds to
160 m2 per person (rows of semi-detached houses with about 400 m2 per
house, including pavements and streets). At this density of residential
area, we can deduce that a ballpark limit for heat pump power delivery is
5 W/m2 × 160 m2 = 800 W = 19 kWh/d per person.
This is uncomfortably close to the sort of power we would like to deliver
in winter-time: it’s plausible that our peak winter-time demand for hot air
and hot water, in an old house like mine, might be 40 kWh/d per person.
This calculation suggests that in a typical suburban area, not everyone
can use ground-source heat pumps, unless they are careful to actively dump
heat back into the ground during the summer.
Let’s do a second calculation, working out how much power we could
steadily suck from a ground loop at a depth of h = 2 m. Let’s assume that
we’ll allow ourselves to suck the temperature at the ground loop down
to ΔT = 5 °C below the average ground temperature at the surface, and
let’s assume that the surface temperature is constant. We can then deduce
the heat flux from the surface. Assuming a conductivity of 1.2 W/m/K