There is, of course, some potential overlap within these sub‐sectors, and some activities could fall within more than one of the three main categories. For example, Additional Energy Sources could potentially fall within the Renewable Energy sector, but has been include within Emerging Low Carbon because it includes mainly new energy sources and those that are still in development (such as hydrogen fuel cells). Alternative Fuels include potential substitutes for current carbon fuels including nuclear power, biofuels, biomass, hydrogen, electricity powered vehicles etc. It has been treated separately from Alternative Fuels for Vehicles, in part because of the high market values of each sector, and in part due to the different nature of use of the different fuels (for example, biodiesel for heating and vehicle propulsion). Biofuels appear under both Alternative Fuels and Alternative Fuels for Vehicles. They are not double counted but are used differently in both sub‐sectors – for example, biodiesel can be used in heat generating equipment, within Alternative Fuels, or in cars and trucks, within Alternative Fuels for Vehicles. A full explanation and rationale for the categorisation approach is available from the authors. .
