Sustainability and policy - Climate Change: Biological and Human Aspects - page 464

T 1 is heat discarded in degrees Kelvin andT2 T 2 is heat input in degrees Kelvin. Efficiency
trends for generation have improved. Energy generation that relies on a boiler or the
thermal expansion of gases is restricted by the Carnot cycle that relates the temperature
(in Kelvin) of the burning fuel to the temperature of the outgoing coolant (see above).
Because of engineering constraints it is difficult to have commercial boilers operating
much above 550 ◦ C (823 K), whereas coolant temperatures (in temperate countries)
tend to be a little above the average environmental temperature and so are around
10-20 ◦ C (283-293 K). This gives a theoretical maximum practical efficiency of
around 65%. This applies to both fossil- and nuclear-power generation. Such early
20th-century power stations had an efficiency of around 25% while at the century's
end, in the more developed nations, efficiencies of new generating plants had risen to
30-37%. One trick around this limitation is not to transform the fuel energy solely into
another form (electricity) but to give the station's heat output direct to the consumer.
So, instead of the station inefficiently generating electricity for consumers' electric
heaters, heat from the plant goes straight to their houses. This is the theory behind
combined heat and power stations. Another way around this limitation is not to use
a boiler at all but some alternative way of transforming energy. Hydroelectric plants
capture the energy of falling water while solar photovoltaic cells capture that of the
Sun and both do so with far higher efficiencies than power stations. However, there
are other limitations, here, respectively, the availability of water and sunshine, as well
as limitations of cost. These costs can change with time, as the downward trend in
the cost of photovoltaic cells demonstrates.
Aside from the cost of new, more-efficient generating plants, there is the cost
(and indeed the carbon-energy cost) of discarding older, less-efficient plants. To take
an extreme illustration, consider what might happen if a new technology became
available tomorrow that doubled efficiency in construction and running in terms of
both financial costs and energy costs (in terms of oil and cement). It would actually
probably be less efficient in both these terms to discard recently built plants that used
the less-efficient, old technology if the old plants had not been running long enough
to pay off their construction costs (in terms of energy and money).
There is therefore a very real time lag involved in introducing new energy-efficient
technology. Regardless of development time, new efficient technologies, even if
theoretically practical, cannot be introduced instantaneously.
This same time-delay factor also applies to introducing energy-consuming tech-
nology with improved efficiency. A motor car needs to have paid off its construction
energy and finance costs before it is replaced by a more efficient car. True, we only
tend to think of such factors in financial terms, but the principle applies equally to
the energy costs of technology, be it the energy used to refine (or recycle) the metal
or the actual carbon in the said technology (plastics and so forth).
Again, as with energy generation, efficiency trends in consumption have also tended
to improve, especially since 1950, and progress was marked in the years following
the 1973 and 1980 oil crises (see Figure 8.1a). However, these efficiency gains in
consumption have been confounded in a number of ways.
First, we actually might use the improved efficiency savings to maintain consump-
tion comparable to old inefficient levels rather than lower consumption itself. This
is known as Jevons' paradox (or Jevons' effect) because it was first articulated by