It is anticipated that the harnessing of renewable energy will expand rapidly, in the light of widespread concern over global warming and the remedial actions that governments are taking. Despite such good intentions, however, there is every indication that, overall, renewables will still make only a modest contribution in 2020. It is vital, therefore, that society continues to develop the various technologies and gains experience in their operation as a step towards growth later in the 21st century.
Combustion technology – agricultural and forestry waste, municipal solid waste, energy crops – may make a useful contribution to energy supplies in many countries. Common factors that will limit its take-up will be the low cost of competing fossil fuels (unless carbon taxes are introduced), resource availability, the capital cost of constructing facilities and, in some instances, public opposition to the siting of these facilities. In the case of agricultural and forestry waste, the scope for expanding operations is strictly limited by the resource available and by the cost and the amount of energy consumed in collection. A further factor to be considered is that much of this biomass when left in situ decays and helps to enrich the soil. All new landfill sites will be expected to have gas collection and combustion facilities. It should be noted, however, that the recycling of consumer products is an important and growing trend that will reduce the amount of waste going into landfill and consequently will decrease the amount of combustible gas that is generated. Schemes for growing energy crops will face competition from those wishing to use the land for agricultural purposes or buildings. Moreover, there is a public perception that turning over agricultural land to energy crops is not a good idea. Similar conversion of traditional forest or wild land is also likely to meet some opposition, due to the concern over loss of biodiversity and damage to the eco-system through soil degradation and depletion of essential minerals. In addition, there is a strong case for planting more forests to sequester carbon dioxide, rather than cutting them down in infancy to bum. In Europe, the situation is made more complex by the controversial Common Agricultural Policy that effectively determines agricultural land use, but which will probably be modified as further countries join the European Union.
Energy crops may also be grown to produce bio-fuels (methanol, ethanol, biodiesel), as well as for direct combustion to generate electricity. The economic incentive for alcohols as a petrol extender depends on the competing cost of petroleum. If fuel cells assume a significant role in road transportation, then methanol is a candidate fuel. In the foreseeable future, however, it would appear that methanol will be manufactured from natural gas rather than from energy crops.
A final word of caution regarding energy derived from biomass. Renewable bioenergy is not necessarily the same as sustainable energy. Careful account must be made of the input of fossil fuel in the form of fertilizers, and also of petroleum for the machinery to harvest and convey the biomass to the processing plant. Furthermore, there may be environmental and social impacts in the growing and harvesting of crops that outweigh the renewable benefits.
Recalling that it takes hundreds of large wind turbines to replace one major power station, it is doubtful that wind energy, despite showing rapid growth in percentage terms, will become more than just a minor contributor to overall electricity generation. A particular problem with on-shore wind farms is that of gaining planning permission for construction. Experience has shown that nearby residents often form pressure groups to oppose the erection of large wind turbines, power lines and pylons in their ‘backyard’. Off-shore wind farms are not so open to objection and significant numbers of turbines are being installed off the North Sea coast. Nevertheless, enthusiasts in the UK who advocate the building of many thousands of such turbines over the next decade have been taken to task in a report from the Royal Academy of Engineering. This emphasizes the severe engineering problems to be faced, as well as the impracticality and high costs, in harnessing wind energy to meet most of the UK target of 10% electricity from renewable energy by 2010 (note, existing hydroelectric supplies are not counted). By contrast, wind power is ideal for many isolated communities provided that there is a grid to provide back-up. Otherwise, it is necessary to install battery storage and this adds significantly to the cost. An alternative is to have a hybrid system that comprises a wind turbine and a petrol-driven generator.
Marine-based technologies (tidal flows, wave energy, ocean thermal energy) will have less of an impact. For example, it is unlikely that the capital investment required to build major tidal barrage schemes will be forthcoming in the next 20 years and, in any event, there are very few suitable sites. Tidal barrages are not necessarily confined to rivers with large tidal ranges, they can also be set up in shoreline lagoons that flood. Many potential sites are available around the coast of Britain. Tidal marine currents present an opportunity and there may be a few of these constructed to generate electricity, probably under government stimulus to help encourage renewables rather than as a result of direct commercial competition with fossil – or nuclear-generated electricity. Wave energy also requires substantial capital investment if it is to be implemented on a large scale. No doubt some small wave-energy machines will be built and demonstrated, but they are very unlikely to provide a significant source of global electricity in the near future.
Finally, there are the solar technologies. Solar heating of buildings and domestic hot water olfers many opportunities, as discussed above. Solar photovoltaic (PV) generation of electricity also has made important strides in recent years as the efficiency of silicon PV cells has improved and their cost has fallen. Building – integrated PV panels appear to be the most economic way forward. By 2020, this technology may well be widespread in sunny climates, particularly in countries where fossil fuels have to be imported. Polymer PV materials and dye-sensitized photoelectrochemical cells are at an early stage of development, but success in these ventures could lead to a dramatic fall in the cost of such electricity. This is an exciting area of research that should be pursued vigorously. Moreover, solar panels based on these new materials could be brought to market rapidly, building on the skills and experience of the existing PV industry.