1890. For all the fanfare, the difference between the new and the old is
really quite disappointing!

Notice that the building power consumptions, per unit floor area, are
in just the same units (W/m2) as the renewable powers per unit area that
we discussed on pages 43, 47, and 177. Comparing these consumption and
production numbers helps us realize how difficult it is to power modern
buildings entirely from on-site renewables. The power per unit area of
biofuels (figure 6.11, p43) is 0.5 W/m2; of wind farms, 2 W/m2; of solar
photovoltaics, 20 W/m2 (figure 6.18, p47); only solar hot-water panels
come in at the right sort of power per unit area, 53 W/m2 (figure 6.3, p39).

Improving the coefficient of performance

You might think that the coefficient of performance of a condensing boiler,
90%, sounds pretty hard to beat. But it can be significantly improved upon,
by heat pumps. Whereas the condensing boiler takes chemical energy
and turns 90% of it into useful heat, the heat pump takes some electrical
energy and uses it to move heat from one place to another (for example,
from outside a building to inside). Usually the amount of useful heat
delivered is much bigger than the amount of electricity used. A coefficient
of performance of 3 or 4 is normal.

Figure E.13. Ideal heat pump efficiencies. Top left: ideal electrical energy required, according to the limits of thermodynamics, to pump heat out of a place at temperature Tin when the heat is being pumped to a place at temperature Tout = 35 °C.
Right: ideal electrical energy required to pump heat into a place at temperature Tin when the heat is being pumped from a place at temperature Tout = 0 °C. Bottom row: the efficiency is conventionally expressed as a “coefficient of performance,” which is the heat pumped per unit electrical energy. In practice, I understand that well-installed ground-source heat pumps and the best air-source heat pumps usually have a coefficient of performance of 3 or 4; however, government regulations in Japan have driven the coefficient of performance as high as 6.6.