In the last chapter, we learned that electrification could shrink transport’s
energy consumption to one fifth of its current levels; and that public trans-
port and cycling can be about 40 times more energy-efficient than car-
driving. How about heating? What sort of energy-savings can technology
or lifestyle-change offer?
The power used to heat a building is given by multiplying together
|power used||=||average temperature difference × leakiness of building|
|efficiency of heating system|
Let me explain this formula (which is discussed in detail in Chapter E)
with an example. My house is a three-bedroom semi-detached house built
about 1940 (figure 21.1). The average temperature difference between the
inside and outside of the house depends on the setting of the thermostat
and on the weather. If the thermostat is permanently at 20 °C, the average
temperature difference might be 9 °C. The leakiness of the building
describes how quickly heat gets out through walls, windows, and cracks,
in response to a temperature difference. The leakiness is sometimes called
the heat-loss coefficient of the building. It is measured in kWh per day
per degree of temperature difference. In Chapter E, I calculate that the
leakiness of my house in 2006 was 7.7 kWh/d/°C. The product
average temperature difference × leakiness of building
is the rate at which heat flows out of the house by conduction and ventilation.
For example, if the average temperature difference is 9 °C then the
heat loss is
9 °C × 7.7 kWh/d/°C ≅ 70 kWh/d.
Finally, to calculate the power required, we divide this heat loss by the
efficiency of the heating system. In my house, the condensing gas boiler
has an efficiency of 90%, so we find:
|power used||=||9 °C × 7.7 kWh/d/°C||=||77 kWh/d.|
That’s bigger than the space-heating requirement we estimated in Chapter
7. It’s bigger for two reasons: first, this formula assumes that all the heat is
supplied by the boiler, whereas in fact some heat is supplied by incidental
heat gains from occupants, gadgets, and the sun; second, in Chapter 7 we
assumed that a person kept just two rooms at 20 °C all the time; keeping
an entire house at this temperature all the time would require more.
OK, how can we reduce the power used by heating? Well, obviously,
there are three lines of attack.