4.2.2 Renewable sources of energy
This section focuses on a few general issues concerning the future exploitation of renewable sources of energy. It seems inevitable that these relatively clean energy sources will see extensive development during the 21st century. Only hydropower has been exploited close to capacity so far. Investment and environmental disruption remain barriers for further hydro schemes, both on a large scale in developing countries and on a small scale anywhere. This is true for many of the renewable energy sources, though some countries have fostered major renewables industries (e.g. wind power, offshore in Denmark and onshore in Germany).
Ironically, the geographic distribution of renewable energy potential is just as uneven as that of fossil and nuclear fuels, so that some countries have a far greater endowment than others. The UK, for example, is well-placed for wind, wave and tidal power, but much less so for hydro, solar and geothermal resources. In future such global inequalities in energy resources will have to be smoothed out (as today) by international trade, probably of generated electricity.
Another widespread difficulty with many renewable sources is intermittency of supply combined with unpredictability; the fact that the amount of electricity generated often varies on short timescales. Some sources are unpredictable (wind and wave energy), others are cyclical (tidal, solar), and others are more constant (geothermal). In designing an electricity supply system that can guarantee to meet demand, a delicate balancing act would be required. It is unlikely that renewable sources alone could provide a stable electricity supply, especially as most countries will not have the full range of renewable options. Thus, one of the most important features of future energy supply systems will be integration, that is, the ability to manage a complex mix of different energy sources as predicted for 2025 in Figure 18 to produce a stable power supply that can track the demand curve.
Sources that are designed to supply electricity continuously (mainly nuclear, tidal, waste, biomass and geothermal) will be harnessed to provide the base load, i.e. the minimum continuous demand for electricity between about 3.00 and 6.00 am during summer nights. Other sources can be brought on-stream to cope with periods of increased demand, which varies throughout the year and also during each day. The maximum demand, or peak load, occurs only briefly around 18.00 pm on winter evenings, for about 300 hours in a year. The brief, increased demand then can be satisfied by reliable, rapid-response power stations using oil or hydropower from pumped storage schemes. Demand during daytime hours, winter and summer, is mainly satisfied by power stations that burn coal and natural gas, supplemented by renewable sources that are less predictable (wind and wave) or which vary during the year (solar and hydropower). Although coal and natural gas still dominate the schematic electricity management scheme shown in Figure 18, their overall use is reduced from that in the early 21st century by the deployment of alternative energy sources.
Research at Oxford's Environmental Change Institute suggests that if intermittent renewable sources contributed a significant proportion of electricity (>20%) — and they will have to for a sustainable future with minimal global warming (Figures 9 and 12) — substantial backup generation capacity would be needed. Historically, this has been in the form of fossil-fuel power stations, including the most expensive 'spinning reserve', where turbines are actually rotated without generating but ready to supply electricity instantly. Various measures, however, can reduce the need for this fossil-fuel backup:
- distributing installations (e.g. wind turbines) widely, on the premise that the wind is always blowing somewhere
- using a range of different intermittent energy sources, especially those that are partially complementary (e.g. sunny weather often means light winds, and vice versa)
- matching energy sources to periods of high demand (e.g. winter demand is matched by increased wind and wave potential)
- replacing fossil-fuel backup power stations with other storable energy sources.
This latter point highlights the last major issue with renewables — storage. Whereas fossil fuels are easily transportable storehouses of energy, renewable sources have to be harnessed and the energy used immediately. If some of the surplus energy from renewables produced at times of low demand (e.g. solar power in the summer, or night tides) could be stored ready for release when demand rose, many of the problems of renewable supply could be solved. The Dinorwic pumped storage scheme is the only example of such a storage scheme in the UK at the time of writing (2005), but there is potential for similar schemes in the form of tidal reservoirs. Of course, such geographically fixed means of storing surplus energy are most suited to local and at most regional energy planning. Another possible solution does offer a means of transporting stored energy. It involves electrolysis of water to form more easily transportable hydrogen and oxygen, and either the use of fuel cells to recombine them, releasing electrical energy, or burning hydrogen as a fuel. These processes form part of a futuristic economic system based on hydrogen.