7 Renewable and non-renewable energy supplies
Energy resources can be considered in a completely different way from their energy density — whether or not they are renewable. Some energy sources incorporate energy released comparatively recently from the Sun and are replenished naturally over a timescale of days to tens of years. Therefore solar, wind and wave energy resources, being continually available, are renewable energy supplies. Other examples of renewables are geothermal and tidal energy sources.
Other potential energy resources are legacies of solar power received and converted into stored energy in the geological past; coal is a good example. Coal seams are replenished naturally over a timescale of millions of years. Once coal began to be exploited faster than its rate of creation it became non-renewable and its use at current levels cannot be sustained.
Which energy sources mentioned so far are renewable, and which are nonrenewable on the scale of a human lifetime?
All fossil fuels are slowly being renewed by the death, burial and decay of present plant and animal life, but at an extremely slow rate compared with the pace of human economic activity. The Earth's natural systems may eventually replace all the fossil fuels that humanity has already used, but how and when that might become possible is impossible to judge.
Likewise, only extremely slow geological processes renew fissionable nuclear fuels. The instability of naturally radioactive isotopes such as uranium and thorium, which formed in stars much larger than the Sun during the moment of their destruction as supernovae, results in them gradually diminishing with time. Replenishing the ores of uranium and thorium by geological processes takes hundreds of million years. So, nuclear fuels too are non-renewable. Even the hydrogen isotope deuterium, which is potentially a vast source of energy from thermonuclear fusion, is a 'one-off' legacy resource and is finite. But even so its potential vastly outstrips that of all other non-renewable energy sources.
The distinction between renewables and non-renewables is one of timescale and energy concentration, but it is critical for human society. Think of renewable energy resources as income, and non-renewable energy resources as inheritance. We 'spend' the Earth's energy resources constantly for cooking, travelling, heating or cooling buildings, manufacturing and in many other ways. At present, modern industrial societies generate energy mostly from fossil fuels, thereby depleting an inheritance accumulated from millions of years of 'banked' solar energy and internal heat. Much less energy is currently generated from day-to-day energy 'income', i.e. from renewables: the global energy 'accountancy' has become unbalanced and unsustainable.
Setting aside the environmentally damaging effects of burning fossil fuels, such as atmospheric pollution and global warming, sooner or later present energy generating policy will deplete our stock of fossil fuel. To stay solvent in energy terms over the long term leaves no choice other than to transfer society's day-to-day energy supply to renewable sources, or return to a low-energy society. The other alternative is harnessing energy from nuclear fission or fusion, but that too cannot last indefinitely. This book aims to provide a scientific basis to understanding some of the decisions that will need to be made to enable humanity to stay 'solvent' in energy terms. Yet you will no doubt be well aware that such decisions are not those of individuals, but are presently dominated by economic and political factors, irrespective of the scientific facts.