278 Energy andtransport for thefuture
Energy conservation and efficiency in buildings
If we turn lights off in our homes when we do not need them, if we turn
down the thermostat by a degree or two so that we are less warm or if
we add more insulation to our home, we are conserving or indeed saving
energy. But are such actions significant in overall energy terms? Is it
realistic to plan for really worthwhile savings in our use of energy?
To illustrate what might be possible, let us consider the efficiency
with which energy is currently used. The energy available in the coal,
oil, gas, uranium, hydraulic orwind power isprimary energy. It is either
used directly, for instance as heat, or it is transformed into motor power
or electricity that in turn provides for many uses. The process of energy
conversion, transmission and transformation into its final useful form
involves a proportion of the primary energy being wasted. For example,
to provide one unit of electrical power at the point of use typically requires
about three units of primary energy. An incandescent light bulb is about
three per cent efficient in converting primary energy into light energy;Thermodynamic efficiencies
When considering the efficiency of energy use, it can be important to
distinguish between efficiency as defined by the First Law of Thermo-
dynamics and efficiency as defined by the Second Law. The second
particularly applies when energy is used for heating.
A furnace used to heat a building may deliver to heating the building
say eighty per cent of the energy released by full combustion of the fuel,
the rest being lost through the pipes, flue, etc. That eighty per cent is
a First Law efficiency. An ideal thermodynamic device delivering 100
units of energy as heat to the inside of a building at a temperature of
20 ◦C from the outside at a temperature of 0◦C would only require just
under seven units of energy. So the Second Law efficiency of the furnace
is less than six per cent.
Heat pumps (refrigerators or air conditioners working in reverse)
are devices that make use of the Second Law and deliver more energy
as heat than the electrical energy they use.^12 Although typically their
Second Law efficiencies are only about thirty per cent, they are stillable
to deliver more heat energy than the primary energy required to generate
the electricity they use. Because of their comparatively high capital and
maintenance costs, however, heat pumps have not been widely used. An
example of their substantial use is their contribution to district heating
in the city of Uppsala in Sweden where 4 MW of electricity is employed
to extract heat from the river and deliver 14 MW of heat energy.