Life-cycle Energy Densities and Spatial Footprints of Various Electrical Power Systems

What is it about?

The criticism made by Professor Ian Fells (Proc. Inst. Mech. Eng. Part A - J. Power Energy, 2002) and others that renewable energy technologies for electricity generation have a low energy density in comparison with fossil fuel or nuclear power stations has been examined. A range of conventional and renewable power generators were evaluated in order to determine their energy densities and spatial footprints on a life-cycle (‘cradle-to-gate’) basis. It is shown that the nuclear fuel cycle (both with diffusion and centrifuge enrichment) has the highest energy density amongst the power systems examined, with bioenergy plants having the lowest. The extensive land requirement for the cultivation of energy crops, coupled with a typical harvesting cycle of three years, lowers the energy density of typical bioenergy plants. This is consistent with the recent finding of Alderson, Cranston & Hammond (Energy, 2012) that the ‘ecological, or environmental, footprint’ of solid ‘first generation’ biofuels gives rise to potentially significant GHG emissions, and exhibited the highest land-take of the various power technologies that they examined. Onshore wind power exhibited a relatively promising energy density among the renewable energy systems studied, and had a greater energy density than that of its offshore counterpart. This can be explained by the greater energy inputs and land take than that associated with offshore installations. However, the energy density of wind farms is likely to rise as the efficiency of the turbine technology improves. Perhaps surprisingly, the energy density of the offshore wind farm falls below that of PV arrays. Renewables clearly produce ‘dilute electricity’ in the sense of having an energy density that is orders-of-magnitude less than conventional sources as suggested by Fells (2002). However, there are many other energy, environmental and economic factors that will determine the usefulness or otherwise of various electrical power systems in the transition towards a low carbon future.

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Why is it important?

A criticism that is often made of renewable energy technologies for electricity generation [such as bioenergy plants, solar photovoltaic (PV) cell arrays, wind turbines, and the like] is that they have a low energy density in comparison with fossil fuel or nuclear power stations. Fells (2002) suggested, for example, that if all the wind farms operating in the world in about the year 2000 were to be concentrated on the South Downs of England (a range of chalk hills, 670 km2 in area, and a protected National Park since 2011), then only 10% of UK electricity demand would be met. On a similar basis, he argued (Fells, 2002) that to replace Scotland's two nuclear power stations e Hunterston B in Ayrshire [a 960 MW plant consisting of two Advanced Gas-cooled Reactors (AGRs)] and Torness in East Lothian [a 1190 MW plant made up of two AGRs] e a total of 10,000 250 kW LIMPET-type wave power generators (i.e., shoreline oscillating water column devices) would be required of the type installed on the island of Islay (one of the Hebridean islands; off the north west of Scotland). In the case of biomass energy, Fells (2002) postulated that an area the size of the county of Kent (3736 km2 of the South East of England) would have to be covered in coppiced willow in order to replace half of the output from Dungeness B nuclear power station (a 1040 MW plant consisting of two AGRs, and located in the same county). Thus, renewables are said to produce ‘dilute electricity’ with an energy density that is orders-of-magnitude lower than conventional sources (Fells, 2002).

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