DOI: 10.1615/AtoZ.r.renewable_energy

There are two distinct classes of energy supplies defined according to their source:

  1. energy obtained from "finite sources" (e.g., fossil fuels, uranium ores) which should be treated as stores of capital value, and

  2. energy from "renewable sources" (e.g., sunshine, falling water, wind, crops, waves, tides) treated as flows with value as income.

Renewable energy may be defined as "energy obtained from the continuing and repetitive currents of energy passing through the environment" [Twidell and Weir (1986)]. Note that renewable energy passes in the environment whether or not it is utilized, whereas the finite sources represent a potential requiring external action for the release of energy, which is usually initiated by man.

The renewable flows of energy emanate from three distinct primary sources in the natural environment:

  1. The Sun (and hence Solar Energy for solar energy thermal conversion, power from the wind for Wind Turbines, waves, photosynthesis for biomass and biofuels, photoconversion as in Solar Cells and Hydro Power).

  2. Geothermal heat, Geothermal Systems

  3. Tides, in tidal power.

In addition, it is becoming common to include energy obtained from continuously accumulating wastes as a second form of renewable energy, e.g., landfill gas, waste incineration.

The total flows of renewable energy are very large [Sorensen (1979)]. Table 1 quantifies these flows as potentially available for use: (a) at the Earth's surface, and (b) as per capita values assuming a world population of 6 billion (the population in 2000 AD). The same table includes equivalent present day use of fossil and nuclear fuels. Per capita energy demand varies greatly within and between countries from about 10 kW/capita consumed continuously for the extravagant rich in the USA to 0.1 kW for the urban poor in Central Africa.

Table 1. Energy flows (a) for the whole earth, (b) averaged per person assuming a world population of 6 billion

*Compare this prediction with present national averages: 10 kW USA, 5 kW UK, 0.3 kW India.

Important interpretations can be made from Table 1:

  1. Renewable energy flux is about 10,000 times more per capita than the energy required for a modern high-tech, energy efficient, society; i.e., 20,000 kW/capita potentially available from renewables to supply the 2 kW/capita needed for all forms of heat, electricity, transport, manufacturing etc, assuming efficient methods. There is no shortage of energy.

  2. The generally available renewable fluxes of sunshine, wind and biomass are extremely large.

  3. The less generally available renewable fluxes of hydro, wave and geothermal, depend much on the local situation where particular fluxes may be unusually intense and therefore harnessable.

  4. As a rule of thumb, the power flux density for commercial renewable energy devices is about 1 kW/m2 for solar-sunshine and wind technologies, and about 10 kW/m2 for hydraulic devices. For fossil fuel and nuclear energy plant the power flux densities are several orders of magnitude larger than from renewables. Consequently, the capture area of renewable energy plant, and hence size of structures, is significantly larger per unit of primary power produced than for fossil and nuclear plant. However, the latter require very large distribution networks, not essential for renewables. If whole systems are compared, the land areas and labor required for renewable energy, fossil fuel or nuclear supplies are similar.

  5. The relatively large structures per unit of power, make visual intrusion and environmental impact particular challenges for renewables.

  6. The dispersed nature of renewables provides a bias towards local use, with smaller distribution systems required as compared with centralized fossil and nuclear supplies.

  7. Renewable energy is generally free at source in the environment and so capital costs dominate the economics.

  8. Renewable energy sources are varied and dispersed, depending on the climatic, topological, geographical and settlement characteristics of each area. Consequently the scale of applicability for a particular method is usually between 100 km to 1000 km, so requiring regional (not national) assessment and development.

  9. Development of renewables depends critically on institutional factors, such as planning, finance, environmental impact and institutional policy for sustainability.

  10. Renewable energy from natural sources is intrinsic to the environment and therefore does not cause chemical or radioactive pollution and does not increase atmospheric CO2 concentrations. This is in marked contrast to fossil fuels and nuclear power, which create external costs of pollution, waste disposal and security, and for fossil fuels, climate change. The economic value of renewable energy is therefore significantly enhanced by this benefit of low external costs, which should be credited within the prices and taxes of commercial energy supplies.

  11. There is a strong ecological argument that links the ecological evolution of the Earth with the abstraction of carbon dioxide from the Atmosphere, so producing life-supporting, oxygen containing, air. The resulting atmosphere has a control function on the Earth's temperature, regulating climate. The abstracted carbon is contained in fossil carbonates and coal, peat, oil and methane. Within the last 100 years mankind's use of fossil fuels has released previously bound carbon to significantly increase CO2 and other gases with the ability to cause climate change. This, and other forms of pollution from finite energy sources, including radioactive waste, is a principle reason for encouraging the implementation of renewable supplies of energy (see also Greenhouse Effect).


The following summaries consider most of the large variety of renewable energy sources and supplies. Unless otherwise stated, information should be referenced to Twidell and Weir (1986) from which it has been updated.

Solar irradiation

Solar radiation received onto a surface (irradiation) is entirely described and enumerated by physics. The Sun at 5800 K radiates as a thermodynamic black body between 0.3 to 2.5 μm (short wave radiation). Fortunately harmful ultraviolet radiation around 0.3 μm is normally removed by ozone in the upper atmosphere. A maximum of about 30 MJ/(m2 day) of bright sunshine is incident on Earth with maximum irradiation of 1 kW/m2 or less. Globally, a thermal equilibrium is reached as the surface radiates upwards day and night in long wave radiation, 9 to 11 μm. Molecular scattering of air, aerosol, particulates and clouds, together with reflections, produce diffuse shortwave radiation of between 10% of total solar irradiation on a clear day, to 100% on a cloudy day. The latitude, longitude, geometry and manoeuvrability of the receiving surface greatly affects the irradiation received; thus in higher latitudes during winter the vertical components of solar irradiation on walls may be larger than the horizontal component on roofs. Likewise in higher latitudes during summer, the integrated daily solar irradiation on a horizontal surface may be more than in the Tropics.

Solar thermal utilization

The use of solar irradiation for direct heating, or as heat into heat engines producing mechanical and electrical power, is reviewed fully in Solar Energy. If the solar heat is collected and then transferred elsewhere for use, the system is "active". If collected for immediate use, as through a window, the system is "passive." Applications include water heating, building heat, drying, distillation, desalination and power production from heat engines (see ref ISES).

Solar cell, photovoltaic, electricity production

Perhaps the most distinctive property of solar radiation is the production of electricity (with no moving parts, pollution or noise) from its incidence onto certain semiconductor junctions, see Solar Cells. Such power installations are most economic for small applications giving valued "service" (e.g., pocket calculators, watches, parking meters), for remote sites (e.g., rural hospitals, telecommunications, and water pumping off the grid), and for peak power (e.g., midday city office electricity). World production of solar cells is now 50 MW/y capacity, increasing at about 10%/y with falling costs.


The photons of solar radiation received at the Earth's surface have energy between about 3 eV (0.4 μm, ultraviolet) and 0.6 eV (2 μm, near infrared). Within this energy range photons may excite electrons in semiconductors and absorbing molecules, so producing electrical current in photovoltaic devices and chemical change in photosynthetic substrates. This general process is named photoconversion, and can be interpreted as the attempt by mankind to emulate and develop the principles of photosynthesis in natural plants. The only commercial development of photoconversion to date is the manufacture of semiconductor solar cells for electricity generation (see Solar Cells), however in the laboratory hydrogen and other fuels can be produced directly from solar radiation incident on photochemicals.


This name is usually restricted to the generation of shaft power from falling water for direct mechanical tasks, e.g., milling, or, predominantly, electricity. The other hydraulic supplies are tidal and wave power. Today, a world total hydroelectric generating capacity of 550 GW, with an average capacity factor of 42%, generates over 2,000 TWh/y. The world potential exploitable resource using conventional engineering is about 16,000 TWh/y, although environmental and financial criteria may limit this to about 8,000 TWh/ y, i.e., 4 times the present generation. Of the major areas with hydro potential only North America, Western Europe and Japan have harnessed 50% or more of the technical potential. Hydro-power has always been sought-after as high quality power and is by far the most established renewable supply of electricity. Its capital intensive construction usually has government finance at low interest rates for about 20 years; thereafter, with the capital paid, its long life time of at least 100 years provides very cheap power. Grid connected utility hydroelectricity may be used as base load supply, for rapid response to meet peak demand and, less occasionally, as pumped storage. The scale of economic plant is from about 2 kW capacity for a single premises, perhaps autonomous generation with an electronic load controller, to 10 GW as a major national grid supply.

If water of flow rate Q, density p (1000 kg/m3), falls through a height H (the head), then the maximum power available is P, where, with acceleration due to gravity g:

Losses are due to: (i) pipe friction, presented as a loss of head height of perhaps about 5% in large schemes and less than 30% in small schemes, (ii) turbine efficiency, usually around 90% for all but very small schemes, (iii) electricity generator efficiency usually about 95%, and (iii) electrical transmission losses around 30%. (For more detail see Hydro Power.)

Tidal power

Tides are caused by the combined forces on the sea of: (i) the resultant gravitational pull of the Sun and the Moon, and (ii) the centrifugal force from rotation about the center of gravity of the Earth and the Moon. Both forces are very approximately in the Earth's equatorial plane, so the seas "bulge outwards" towards this plane. Because of the Earth's rotation on its axis, the "bulge" appears as a traveling deep-water wave causing coastal tides of semi-diurnal periodicity 12 h 25 min. In practice dynamical and local factors alter this basic model.

To understand tidal power applications, it is necessary to appreciate the deep-water wave characteristic of tides. In the open ocean the amplitude of the tide is about 1 m. However this increases dramatically if the tide travels as a deep-water wave up an estuary in a time to form a resonance with the following tide. The condition for this may be modeled and calculated. For a simple estuary of length L and depth h with a semidiurnal tide, the resonant condition is:

For example, the Severn Estuary in south west England meets this condition to give an enhanced tidal range of 10 to 14 m that could provide 10% of UK electricity. Considering all potentially harnessable sites worldwide, the total electricity production would have an average power of about 62,000 MW.

Tidal power from a basin of area A and tidal range R, is harnessed by constructing a barrier across the mouth of the estuary. For each tide of period T, the water of density ρ runs through turbine generators to give:

Refinements are made to allow for monthly variations in tidal range and other effects.

There are [Johansson et al. (1993)] only about 4 tidal power plants worldwide, ranked in power output as: France (La Rance) 240 MW giving 540 GWh/y; Canada (Annapolis) 18 MW, 30 GWh/y; China (Jiangxia) 3.2 MW, 11 GWh/y and others; Russia (Kislaya Guba) 0.4 MW.

Tidal stream power may be harnessed from a turbine in a similar way to wind power. If the turbine has 40% capture efficiency and the stream of maximum speed u has sinusoidal speed variation, the average electrical power per unit cross section of the turbine is 0.1ρu3. Advantages over wind are that the fluid density ρ is about 800 times greater than air and the fluid speed is regulated. However the difficult working environment, the rarity of sites and the usually low tidal stream speeds has deterred commercial interest so only historic mills and developmental machines have been constructed.

Wave power

For a surface wave of period T, wavelength λ, frequency ω and amplitude a, the total of the excess energy (potential and kinetic) in unit width and one wavelength is:

Separate analysis is needed to calculate the power carried forward with the group velocity of the dispersive wave. For unit width of wave front, this power is:

For an average Western Atlantic deep water sea wave of wavelength 100 m and amplitude 1.5 m, the power so calculated is 73 kW/m, corresponding with long term measurements.

Considerable research, especially in the UK, has quantified the large resource potentially available in wave power considering the many complications of variations in wave height, form and direction. However as yet there are no commercially available devices, nor indeed many developmental sea-going prototypes. Without doubt, the rigours and extremes of real sea conditions are daunting to such development.

There have been many proposals and trial devices to harness wave power. These may be categorized as: (i) surface floats activated by the wave surface, (ii) submerged floats rotating in the deep water wave, or (iii) pneumatic, with water forcing air through a turbine, as in an oscillating water column. All devices need a constant force of reaction, obtained by installation: (a) on shore, (b) on the sea bottom near shore, or (c) from the average reaction of a long spine floating off shore.

Wind power (see Wind Turbines)

The generation of electricity from the wind is a modern international growth industry. For air of density ρ, speed u, passing through a rotor of area A, the power in the wind is the kinetic energy passing per unit time:

Some kinetic energy remains in the wind for air to leave the rotor, so only a fraction Cp is converted to shaft power. Cp is the nondimensional Power Coefficient, with a maximum value of 16/27 (0.59) by the Betz criteria considering the linear momentum of the air stream. The speed of rotation for maximum power extraction relates to the other main nondimensional characteristic, the tip-speed ratio λ, equal to the ratio of the speed of the tip of the rotor blades to the unperturbed wind speed u. Most modern wind turbines for electricity production are 2 or 3 bladed on a horizontal axis, having Cp about 40% and λ about 8. Rotor diameter ranges from about 1 m (for battery chargers of about 50 W electricity capacity) to about 30 m (for commercial machines of about 500 kW capacity) and to 100 m (for very large developmental machines).

Geothermal power (see Geothermal Systems)

The interior of the Earth is hotter than the surface as a result of heat released by radioactive decay giving an average temperature gradient of about 30°C/km depth. The average outward heat flux is very small, 0.06 W/m2, and of no useful potential. However a few locations have unusual geothermal concentration that may be utilized. There are 3 types of manageable geothermal source: (i) hydrothermal aquifers where ground water percolates downwards and is heated in specific sites of concentrated heat flow for emission as hot water, wet steam or dry steam, (ii) hot igneous systems associated with subterranean molten lava, (iii) hot rocks, usually very large volumes of low thermal conductivity granite, of elevated temperature; the rock is drilled and shattered to allow forced water circulation for heat removal. However whatever type of system, temperatures of extracted water are lower than in thermal power stations, so the efficiency of thermal electricity generation is low. The most efficient use of the energy is for heating and drying (of which a total worldwide capacity of 23,000 MW (heat) is operational and planned), nevertheless there is substantial worldwide electricity generation (15,000 MWe operational and planned). Geothermoelectricity production occurs in about 20 countries, with the largest capacity in the USA (3 GW), Philippines (2 GW), Mexico (1 GW), Italy (0.9 GW), Japan (0.4 GW) and New Zealand (0.3 GW). Thermal use is mostly in Japan (3 GW), China (2 GW) and Hungary (1 GW). There are many thermal cycle and heat exchange systems used in these processes. (See also Geothermal Systems.)

Ocean thermal energy conversion (OTEC)

Tropical ocean surfaces are about 20°C hotter than deep water 1000 m below and so form a solar collector of enormous capacity. A heat engine can be operated between the surface temperature Th and the deep water at Th — ΔT. In thermodynamically ideal conditions of a Carnot engine and perfect heat exchangers, the mechanical power produced would be:

where the deep sea water has density ρ, specific heat capacity c and flow rate Q. As with all real heat engines and heat exchangers that have to function in nonequilibrium conditions, the efficiency achieved is less than half the hypothetical ideal, i.e., about 2% for the best OTEC. However the basic analysis explains the quadratic dependence on ΔT and the very large volume flow rates for deep water cooling. Practical systems have operated with both closed cycle and open cycle vapor turbines. Several countries maintain R&D activity for demonstration projects in tropical oceans, either for floating constructions or, more likely, for shore based plant on islands with immediate deep water, e.g., Nauru in the South Pacific. A secondary benefit is the supply of nutrients brought up with the cooling water for enhanced fish population. (See also Ocean Thermal Energy Conversion.)

Biofuels and biomass

Biomass is the organic material of plants and animals that reacts with oxygen in combustion and metabolism to produce heat and work. Since all biomass would decay naturally to emit CO2, utilizing biomass as fuel does not add to climate change gases in the environment and is preferable to consuming fossil fuel in which carbon has been removed from the atmosphere. About 250 × 109 tonne/y of dry matter biomass cycles in the natural environment, i.e., about 40 t/(person y) with an average heat of combustion of about 15 MJ/kg. This compares with about 27 MJ/kg for coking grade coal. Primary biomass may be converted to more readily used biofuels, e.g., charcoal, ethyl alcohol, methanol, methane. Photosynthesis is the primary process for capturing solar radiation to produce plant biomass at efficiencies between 1% (mature woodland) to 10% (optimum greenhouse with enhanced CO2). The potential maximum efficiency of laboratory emulated photosynthesis is 30%. Biomass is the dominant and most widespread form of renewable energy from domestic cooking to agro-industries; worldwide biomass supplies 15% of all utilized energy. The uses are

  1. Thermochemical (immediate combustion, pyrolysis, pretreatment);

  2. Biochemical (alcohol fermentation, anaerobic digestion for biogas of methane and CO2);

  3. Agrochemical (direct extraction of extrudates, e.g., oils).

Producer gas (a mix of CO, H2 and N2 with water vapor) is the thermochemical product of burning biomass in restricted air.

Biomass materials form the major part of waste from homes, agriculture, forestry and commerce. In effect such waste is an unstoppable flow in the human environment and hence may be considered a renewable resource. After removal of all recyclable materials, the waste may be biochemically digested for biogas or heat, or incinerated most economically for heating, e.g., district heating, and for electricity. Since payment is usually made for waste disposal, the economics of energy from waste is generally more favorable than for "natural" renewables.


ISES (International Solar Energy Society, with many national associations) see the journal "Solar Energy" from Pergamon Press, Oxford. UK; "Advances in Solar Energy" from American Solar Energy Society, Boulder, USA; "Sunworld" and "Sun at Work in Europe" from Franklin Co., Birmingham, UK

Johansson, T. B., Kelly, H., Reddy, A. K. N. and Williams, R. H. (1993) Renewable Energy—Sources for Fuels and Electricity, Earthscan, London, and Island Press, Washington D.C.

Sorensen, B. (1979) Renewable Energy, Academic Press, London.

Twidell, J. W. and Weir, A. D. (1986) Renewable Energy Resources. E. & F.N. Spon, London.

Most national governments and some regional authorities have information services supplying brochures, booklets and data concerning renewable energy. The International Energy Agency (the IEA) also has useful information. The European Commission Directorates DG XII (research) and DG XVII (energy) have considerable information on renewable energy developments.

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