Energy choices- the big picture

 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.