The history of energy use has
been about developing sources which provided more concentrated forms of energy,
fuels with higher energy densities, usually delivering higher temperatures via
combustion, so as to drive machines for motive power or electricity generators.
Even for simple heating, we have preferred fuels with high energy density, so
as to take up less storage space. Ease
of access to the fuels of course modified our choices. In many parts of the
world, wood and biomass still represent large sources for heating and cooking,
despite being much bulkier than fossil fuels. However, where coal, oil or gas
are available, they tend to dominate since they can be stored and transported
more easily, and usually yield higher energy outputs per tonne. In terms of
electricity production, we have developed thermodynamic systems which convert
heat from fuels to rotary motion to drive generators, and the higher the
temperature that can be obtained, the more efficient the conversion system is overall. Combustion processes can be enhanced by
forced drafts and the steam that can be produced using this heat can have its
temperature raised in pressurised systems.
Some of the waste heat can be recycled and used to improve energy efficiency
further.
We have probably reached near the maximum thermodynamic conversion
efficiency possible with existing systems- supercritical stream generation,
combined cycle gas turbines, Combined Heat and Power plants and so on. That is
true whatever the heat source. Electricity generation using heat from nuclear
fission, or even fusion, to raise steam, is also thermodynamically limited. We
have also come up against other limits. The global reserves of fossil and
fissile fuel are finite: we are using up ever-depleting stocks. The energy conversion processes also have
problems. Harmful wastes are produced, such as toxic gases, acid emissions, and
long-lived radioactive materials. Operating
at high temperatures and pressures involves safety risks. Ever since we first started
burning fossil fuels, and indeed before then with wood, there have been
resultant health and environmental impacts. Some of these have been contained,
for example via flue gas scrubbers and the like, but it is hard to see how the
main product of fossil fuel combustion, carbon dioxide gas, can be dealt with,
other than by capturing it and storing it.
That can be done to some extent, at a price, but it is an inelegant
approach, something of a ‘botch’, with a range of risks. Can we be certain that
the vast amount of carbon dioxide that would be produced if we continued to
burn off our fossil resources will stay put for ever in geological strata?
It would be preferable not to burn off our fossil resources, so as to
avoid the linked health and environmental impacts and crucially to limit
climate change. Some look to nuclear energy as a better option. In some ways
that represent the ultimate step in our search for high energy density
fuels. Vast amounts of energy can be
produced by the fission or fusion of the atoms of suitable materials, so that
the fuel volume per unit of energy produced can be very small. Higher temperature fission reactors can have
higher energy conversion efficiencies and in theory fusion can generate very
higher temperatures, although we have yet to develop technologies to exploit
that. The down-side of nuclear are the costs and the risks. Fission has not
proved to be as cheap as was at one time hoped and few would venture estimates
for the costs of hypothetical fusion systems. What we can say, from experience
so far, is that whatever the technology, there will be unique risks with
nuclear systems, dealing with which will add to the costs. The costs of fission
will also rise as fissile reserves deplete. The fuel resource for fusion plants
may be cheaper and larger, but even so, there may be risks and costs, and the
question remains, do we want to continue down this path of ever increasing
energy density.
The standard response is to say yes of course, and in fact there is no
alternative. Anything else represents a retreat back to less efficient
systems. We abandoned water mills and
wind mills long ago, as soon as coal became widely available. We have to
continue in that direction. However, increasingly, environmentalists have asked
if that is really true. They say a different approach can be adopted, based on
using renewable energy resources, not simply substituting them for existing
energy sources, just plugged on to the same system of use, but as part of a
wider transition to a more sustainable approach to energy supply and use.
The main drawback of this approach is usually held to be the low energy
density of these diffuse energy sources. That means that large areas have to
use used for energy collection, so that there will be conflicts with other land
uses, as well as major environmental impacts. The sources are also often
variable, so that balancing systems have to be provided, adding to the
cost. Overall it’s claimed that
renewables cannot supply enough energy, reliably and cost- effectively, to meet
our needs. The simple response is that these problems and limitations can be
dealt with and are in any case less than those that face continued reliance on
conventional energy options.
It hardly seems necessary to reprise the problems with conventional
energy systems. Air pollution from fossil fuel combustion has reached epidemic
proportions in some new Asian cities, climate change driven by the resultant
carbon dioxide emissions threatens ever worse social and economic and heath
problems in the years ahead, while the risk of major nuclear disasters remains
a continuing concern. At the same time, the beginnings of an alternative
approach are emerging, with renewables supplying nearly a quarter of global
electricity. There are projections that this could expand to near 100% by 2050
and that heat and transport needs could also be met from renewables by then. However,
for that to happen would require resolution of what some see as unsurmountable
problems. Which is one reason why faith in the conventional approach, suitably
upgraded, remains strong.
While carbon capture and storage is sometimes seen as part of this
approach, as already indicated, sequestration of emissions from fossil fuel
combustion is a limited, short to medium term, option, unlikely to allow us to
use more than a fraction of the remaining fossil fuel reserves. By contrast it
is sometimes argued that nuclear energy can supply us with energy into the far
future, with fast neutron breeder reactors in effect extending the uranium
resource for perhaps centuries and the potential for fusion being effectively
unlimited. However, there are some major
problems with these options as will be explored in the next post in this
series.
It is possible that nuclear and renewables will co-exist for some while.
At present renewables supply more than twice as much energy as nuclear
globally, with nuclear growth stalled but renewables booming. That trend seems
likely to continue, although differing patterns may emerge around the world.
Some countries may still opt for a predominance of nuclear (Russia for
example), but, in most others, renewables are likely to dominate. For example,
they already supply ten times more electricity than nuclear in China, with the
output from wind projects alone being larger than that from nuclear. Although,
the advent of new technology may change the pattern in future, for the moment,
in most (but not all) countries and regions the nuclear options do not look to
be as promising as the renewable options.
