Interest in sources of energy that provide alternatives to conventional fossil and nuclear fuel has been growing steadily over the past twenty years. In particular, emphasis has been placed on the development of renewable sources of energy. Renewable Energy is the term used to cover those energy flows that occur repeatedly in the environment and can be harnessed for human benefit. The ultimate sources of most of this energy are the sun, gravity and the earth's rotation.
Today few would perceive a future without the Renewables contributing to our energy provision and many believe that renewable energy will make a substantial contribution to our energy supplies in the longer term. This interest has been stimulated by concerns over the use of conventional energy technologies and their environmental impacts. With little or no net emissions of polluting gases the Renewables are seen as part of a solution to these problems.
The technologies to harness these renewable energy flows are many and various. Brief descriptions of some of the renewable energy technologies that have been considered candidates for development and deployment are presented below.
Wind power is an intermittent resource, which is strongly influenced by geographical effects such as the local terrain. The amount of wind energy is a strong function of the wind speed, the instantaneous power in the wind increasing as the cube of the wind speed.
Wind power has been harnessed by Man for over 2,000 years and is one of the most promising renewable energy sources for electricity generation. There are two basic design configurations — horizontal axis machines and vertical axis machines. Horizontal axis designs are at a more advanced stage of development and the evidence is increasing that they are also more cost-effective. Apart from the need to demonstrate adequate lifetimes, there is no doubt about the technical feasibility of harnessing wind power.
The existing technology offers a range of power ratings from a few kilowatts up to several megawatts. The technology is well established, with over 20,000 grid connected machines in operation world-wide. Current development work is concentrating on reliability, the further reduction of cost and noise levels, aspects of the electrical connection into the grid and overall performance.
There are limitations on the availability of land for wind turbine sites due both to physical constraints—such as the presence of towns, villages, lakes, rivers, woods, roads and railways—and institutional constraints such as the protection of land areas designated as being of national importance. Offshore there is potentially a very large wind resource but it will require additional technology development before it can be effectively exploited and it will cost more than onshore wind (see also Wind Turbines).
Hydro power comes from the energy available from water flowing in a river or in a pipe from a reservoir. Evidence of the use of hydro power as a source of energy has been found in primitive devices from the first century BC. During the Industrial Revolution, small-scale hydro power was commonly used to drive mills and various types of machinery. The first large-scale hydro scheme in the UK was built in Scotland in 1896.
Hydroelectric technology can be regarded as being fully commercialized. Turbine plant, engineering services and turnkey systems are sold by many organizations world-wide.
Tides are caused by the gravitational attraction of the moon and sun acting on the oceans of the rotating earth. The relative motions of these bodies cause the surface of the oceans to be raised and lowered periodically. In the open ocean, the maximum amplitude of the tides is about 1 m. Towards the coast, the tidal amplitudes are increased by the shelving of the sea bed and funnelling of estuaries. For example, in the Severn Estuary the maximum amplitude is about 11 m.
The energy obtainable from a tidal scheme varies with location and time. The available energy is approximately proportional to the square of the tidal range, and the output changes not only as the tide ebbs and floods each day but can vary by a factor of four over a spring-neap cycle. However, this output is exactly predictable in advance. Extraction of energy from tides is considered to be practical only at those sites where the energy is concentrated in the form of large tides and in estuaries where the geography provides suitable sites for tidal plant construction. Such sites are not commonplace, but a considerable number have been identified in the UK, which probably has the most favorable conditions in Europe for generating electricity from the tides. This is the result of an unusually high tidal range along the west coast of England and Wales, where there are many estuaries and inlets, which could be exploited.
A tidal barrage is a major construction project built across an estuary, consisting of a series of gated sluices and low-head turbine generators. Several locations around the world have been studied as potential barrage sites, but relatively few tidal power plants have been constructed. The first and largest (240 MW) tidal plant was built in the 1960s at La Rance in France, and has now completed more than 25 years of successful commercial operation.
Ocean waves are caused by the transport of energy from winds as they blow across the surface of the sea. The amount of energy transferred depends upon the speed of the wind and the distance over which it acts. As deep ocean waves suffer little energy loss, they can travel long distances if there is no intervening land mass. Therefore the western coastline of Europe has one of the largest wave energy resources in the world, being able to receive waves generated by storms throughout the Atlantic.
Wave energy is still in the RD&D phase. Currently there are two types of device known to be operating in Europe. The Norwegians have developed a tapered channel device (Tapchan) but the concept is limited to use in areas where there is a small tidal rise and fall and having suitable shoreline topography. In the UK the Government has funded work on a 75 kW oscillating water column device incorporating a Wells air turbine developed by the Queens University, Belfast. This device is now connected to the grid on Islay in the Inner Hebrides.
World-wide, installed devices are limited to experimental plants of less than 100 kW, including oscillating water column devices incorporated in sea defence breakwaters.
Photovoltaic (PV) materials generate direct current electrical power when exposed to light. Power generation systems using these materials have the advantage of no moving parts and can be formed from thin layers (1 to 250 microns) deposited on readily available substrates such as glass. To date, the photovoltaic effect has been widely exploited where the low power requirements, good solar resource and simplicity of operation outweigh the high cost of PV systems. Current applications include consumer goods, such as calculators and watches and, on a larger scale, power systems for lighting and water pumping in developing countries and in remote areas with no grid supply. Other applications include powering of "professional" systems such as remote telecommunications facilities and cathodic protection of pipelines.
There is world-wide interest in developing PV systems for future power generation because of the huge potential renewable resource available and the environmental benefits offered by a technology which avoids the emissions and pollution associated with fossil-fuelled plant. However, PV is still a relatively young technology. Much research and development will be necessary if world-wide system costs (modules and associated components) are to be reduced to acceptable levels and significant new markets are to be established.
PV could contribute to electricity supply in two ways—through the use of central PV generating plant (PV power stations) or through building integrated systems where PV units would be located in the facades of domestic and commercial buildings. Building integrated systems could supply power for use inside the buildings for applications such as appliances, air conditioning and lighting, with any excess available for export to the grid (see also Solar cells).
Active solar thermal systems consist of solar collectors, which transform solar radiation into heat, connected to a heat distribution system. Due to the nature of the UK climate, such heating systems are best suited to applications at temperatures below 100°C. High temperature applications, such as thermal solar power for electricity generation, are not practical in the UK.
There is a developed technology and an existing small market for systems to supplement the heating energy demands of buildings. This market is served by a small number of manufacturers and installers, but many of the installers see solar heating as a secondary activity associated with another business, such as central heating installation.
Passive solar design (PSD) aims to maximize free solar gains to buildings so as to reduce their energy requirements for heating or cooling and lighting. It is most effective when used with energy efficiency measures as an integral part of energy-conscious design of new buildings. However, some PSD features, such as conservatories and roof space collectors, can be retrofitted.
The concept of PSD is not new. However, its potential energy benefits, as distinct from its use for aesthetic or health reasons, have only recently become a focus of attention. To maximize these benefits in terms of the heating requirements of a building, PSD seeks to orientate and arrange glazed surfaces so as to make full use of shortwave solar radiation for heating interior spaces and to avoid heat loss resulting from siting windows on shaded walls. To cool buildings it uses solar heated air to assist natural convection, thus providing natural ventilation and cooling. For lighting, it uses glazing to reduce the need for artificial lighting whilst still maintaining a comfortable environment. More complex approaches such as mass walls, atria or conservatories are basically extensions of these simple design principles. Effective use of PSD depends on sympathetic interior design and on grouping buildings to minimize shading and gain protection from prevailing winds (see also Solar Energy).
There is a large amount of heat just below the earth's surface— much of it stored in low permeability rocks such as granite. This source of geothermal heat is called "hot dry rock" (HDR). Attempts to extract the heat have been based on drilling two holes from the surface. Water is pumped down one of the boreholes, circulated through the naturally occurring, but artificially dilated, fissures present in the hot rock, and returned to the surface via the second borehole. The superheated water or steam reaching the surface can be used to generate electricity or for combined heat and power systems. The two boreholes are separated by several hundred meters in order to extract the heat over a sizeable underground volume. A typical HDR power station would produce about 5 MW of electricity and be expected to operate for at least 20 years.
The engineering of the underground "heat exchanger" has turned out to be a formidable technical problem, which has not yet been satisfactorily solved after more than ten years of intensive research in the UK, the USA and elsewhere. Because of the technical difficulties, there are no commercial HDR schemes in existence anywhere in the world.
Geothermal Aquifers extract heat from the earth's crust through naturally occurring ground waters in porous rocks at depth. A borehole is drilled to access the hot water or steam, which is then passed through a heat exchanger located on the surface. If the temperature of the hot fluid exceeds about 150°C it can be used for generating electricity; otherwise it can be tapped as a source of warm water. In the UK, there are very few sources with temperatures above 60°C and the resource would be exploitable mainly in district heating systems or industrial processes.
The use of aquifers is well established in certain geologically favored parts of the world, such as Iceland, Hungary, Italy, the USA and the Paris Basin of France. Some 6 GW of electrical generating capacity is currently installed overseas in several regions where both steam and water are produced at temperatures over 200°C. In addition, geothermal resources are used in many district and process heating schemes.
There are a number of ways in which biological systems can be used to produce energy. Agricultural wastes can be used to produce energy by combustion or by biological processing to produce liquid or gaseous fuels. It is also possible to grow crops specifically for energy purposes.
Agricultural and forestry wastes fall into two main groups, dry combustible wastes such as forestry wastes and straw, and wet wastes such as “green” agricultural crop wastes (i.e., root vegetable tops) and farm slurry. The former group of biofuels are utilized using thermal processes to give heat directly (via Combustion), or converted into a second fuel either gaseous (via gasification) or liquid (via Pyrolyis). The latter group of biofuels are best utilized via anaerobic digestion to produce methane (“biogas”).
Currently, very little use is made of these materials as sources of energy, despite the fact that frequently there is a cost associated with their clean disposal. For example, surplus straw now has to be ploughed into the soil and animal slurries must be contained to prevent water course adulteration. Conversion of these wastes into fuels can generally be accommodated within existing agricultural and forestry practice. Thus in future these wastes may be considered as additional income earners.
The use of fuels derived from agricultural and forestry residues could create markets which energy crops might then supply at a later date. In Denmark today, there are 50 MWe of straw burning plants and in the USA wood fuelled power stations total approximately 6,000 MWe.
Crops which may be grown to produce energy range from food crops grown for energy purposes to woody biomass. From these sources solid, liquid or gaseous biofuels may be derived. Many methods for the conversion of biomass are available, reflecting the diversity of the resource. The drier, lignin-rich materials (e.g., wood) are best suited to combustion, gasification or pyrolysis conversion processes. Wetter biomass can be converted through anaerobic digestion to a methane-rich biogas fuel. Other fermentation techniques produce liquid fuels such as Ethanol.
Well-developed systems of varying scale are available to provide direct heat or electricity from the crop. Conventionally, electricity is produced by burning wood in a boiler to generate steam that is fed to a turbine. More advanced technologies involving Gasification are ready for demonstration. They should allow electricity production at higher efficiency and lead to significant reductions in costs.
There are now significant opportunities for the production of energy crops. In Europe one important factor is of the reform of the Common Agricultural Policy, a central element of which is the reduction of food overproduction. New measures will result in some farm land becoming potentially available for non-food crops, including energy crops. The benefit of this approach is that such land will remain productive and not fall derelict. This will go some way to maintaining farm incomes and rural economies.
World interest in biomass is growing rapidly with major programs underway in Scandinavia, the USA and Europe. Significant energy crop enterprises already exist in Brazil and Sweden with development programs underway in Scandinavia, Europe and North America. Over 6,000 ha of short rotation coppice have been established in Sweden alone.
There are a number of so-called advanced conversion technologies that are being considered as alternatives to conventional steam raising plant for converting biofuels. Principal amongst these are pyrolysis, gasification and liquefaction. These thermochemical processes produce solid, liquid and gaseous intermediates from biofuels, which can be used to produce electricity and/or heat, or be upgraded to directly substitute for fossil fuels. The intermediates can be used in an engine to produce power, avoiding the use of a steam cycle. This gives a very significant increase in conversion efficiency with comparable capital and operating costs at the scales relevant to biofuels (less than 50 MWe). In this way, the economic viability of electricity production from energy crops, forestry wastes and straw is significantly improved.
These processes promise to be inherently less polluting than conventional incineration. By reducing the costs of pollution abatement and offering a more secure disposal route in the light of ever more stringent pollution legislation, the incorporation of these technologies in energy from waste schemes is likely to increase. For sewage sludge, gasification may prove to be the best option for both disposal and power generation as conventional incineration pushed to the limits is barely autothermic.
In the Scandinavian paper industry over 100 MWth of biomass gasification plant has been in operation for many years. Major advanced biomass gasification projects for power generation are now underway in Sweden, Finland and Hawaii.
An Assessment of Renewable Energy for the UK, ETSU R82—HMSO, ISBN 0-11-515348-9.
Renewable Energy Resources: Opportunities and Constraints, 1900-2020, World Energy Council.
Renewable Energy: Sources for Fuels and Electricity (1993) T. B. Johansson et al., Island Press.