1. Background

[GLOSS]Renewable energy sources[/GLOSS] (RES) can be defined as: energy sources that are derived from the sun or other natural processes. Those energy sources are replenished over relatively short time periods, and therefore can be considered as “unlimited” in supply – non-depletable forms of energy. In other words, RES is a collective noun for energy, which is obtained from other than fossil fuels; this includes the energy obtained by processing [GLOSS]sustainable biomass[/GLOSS] (i.e. excluding peat), [GLOSS]solar energy[/GLOSS], [GLOSS]geothermal energy[/GLOSS], [GLOSS]wind energy[/GLOSS], [GLOSS]hydro energy[/GLOSS], energy from ocean thermal energy, ocean wave and tidal energy, and the biodegradable portion of waste.

Of these, biomass and waste are the most important for combustion engineers. It must be noted that non-biodegradable waste material can be used for energy generation, although it is not renewable energy source. Other RES are becoming of importance in relation to hybrid systems. In the following, the various energy sources and the technologies applied will be introduced.

2. Biomass Energy

[GLOSS]Biomass[/GLOSS] is renewable fuel from organic origin, including organic waste, residues from agriculture and energy crops, as well as the organic part of waste. Energy from biomass is versatile: it can produce thermal and electrical energy usually in large and medium scale industrial units (power plants) or transport fuel.

Solid biofuel

Solid biofuels (generally called biomass) are solid renewable energy sources from living things. They are to be distinguished from solid fossil fuels which are also of biological origin but which are non-renewable. Solid biofuels include wood, straw, energy crops, organic wastes. Energy from solid biofuels is generally released in fluidised bed or grate boilers producing electricity and/or heat. However, there is an increasing development towards the co-firing of solid biofuels and waste fuels with solid fossil fuels – various types of pulverised coal – in power generation boilers.

Liquid biofuel

Liquid biofuels are liquid renewable energy sources from living things. They are to be distinguished from fossil liquid fuels which are also of biological origin but which are non-renewable. Liquid biofuels are transport fuels, primarily biodiesel and bioethanol/ETBE: two key liquid fuels processed from agricultural crops and other renewable feedstock.

Biogas

Biogas is the product of organic material solid and/or liquid) decomposition
([GLOSS]anaerobic digestion[/GLOSS]), composed mainly of methane and carbon dioxide. Biogas is produced in large quantities in landfill sites, and it has potential as a fuel to generate electricity or to provide process heat.

3. Solar Energy

Solar Photovoltaic

Direct conversion of solar radiation into electricity by photovoltaic cells.

Solar Thermal

Conversion of the suns energy into useful heat, captured by fabric of building or solar collector. Solar thermal heating is applied to water, air or structural materials. Conversion of light to heat can be achieved through passive systems (e.g. solar energy through windows, solar buildings) or active systems (mechanically transferring heat by means of a working fluid such as oil, water or air). Solar energy can be utilized in low temperature heating and cooling) or electricity generation.

Solar buildings

Passive and active utilization of the suns energy in buildings Solar buildings are those that employ technologies, design techniques, materials and equipment which utilise solar energy. Heat from the sun, daylight and the wind can all be used to enhance comfort in buildings and reduce energy consumption.

Photochemical conversion

The conversion of sunlight into chemical fuels, using biological or chemical processes, e.g. artificial photosynthesis.

4. Geothermal Energy

Geothermal energy comes from the natural heat of the earth stored in rock and water. This heat is within the earth and can be extracted through boreholes driven into a hot aquifer or injecting cold water through hot dry rock at depths shallow enough to be economically feasible. Low enthalpy resources (50°C to 150°C) can be used for heating purposes: large base load demands such as district heating, horticulture, recreational uses such as spas. Medium and high enthalpy resources (> 150°C) are used for electricity production.

Heat pumps

Exploiting naturally stored solar energy and some intrinsic heat from the earth itself already using heat exchangers installed some 100m underground. Mostly using electricity or fuel as the necessary driving energy input.

5. Wind Energy

Wind energy is the conversion of air movements into heat and power. Electrical energy can be generated using wind-turbines. The energy can be extracted from the wind by transferring the momentum of passing air to rotor blades. The energy is concentrated into a single rotating shaft. The power of the shaft can be used in many ways: for example, large modern turbines convert it electricity. Wind – a form of solar energy – is a low-density source of power. The quantity of energy developed is the product of the density of the air and the cube of the wind speed.

6. Hydro Energy

Hydro energy in general is produced from the movement of a mass of water: streams, rising and falling of tides through lunar and solar gravitation, wave energy, energy of sea currents. The power plant can be classified by the produced electrical energy as large-scale hydro (> 10 MW) and small scale hydro (< 10 MW).

7. Tidal Energy

Tidal energy exploits the natural rise and fall of coastal tidal waters caused principally by the interaction of the gravitational fields of the Sun and the Moon. Some coastlines, particularly estuaries accentuate this effect creating tidal ranges of up to 11 m. The modern version of a tide mill is a semi-permeable barrage built across an estuary, allowing floodwaters to fill an impounded basin via a series of sluices.

Tidal energy can be exploited in two ways: by building semi-permeable barrages across estuaries with a high tidal range and by harnessing offshore tidal streams. Barrages allow tidal waters to fill an estuary via sluices and to empty through turbines. Tidal streams can be harnessed using offshore underwater devices similar to wind turbines.

At high water the sluice gates are closed, creating a head of water on the ebb tide. Releasing the water through a series of conventional bulb turbines generates electricity. In future schemes the energy yield would be enhanced by pumping water into the estuary on the flood tide (‘flood pumping’), thereby increasing the volume of water released through the turbines on the ebb tide. A variant is tidal stream (or marine current) technology, which aims to exploit the strong tidal currents which are found in shallow seas, particularly where natural constrictions exist, such as around headlands or between islands. Tidal stream technology is in its infancy. Devices similar to submerged wind turbines would be used to exploit the kinetic energy in tidal currents. Only one of these devices exists – a 5kW machine that has been operated in Japan since 1990.

8. Wave Energy

Waves, particularly those of large amplitude, contain large amounts of energy. Wave energy is in effect a stored and concentrated form of solar energy, since the winds that produce waves are caused by pressure differences in the atmosphere arising from solar heating.

The classification of wave energy devices with respect to the shoreline is: shoreline devices, nearshore devices and offshore devices.

Shoreline devices have the advantage of relatively easier maintenance and installation and do not require deep-water moorings and long underwater electrical cables. The less energetic wave climate at the shoreline can be partly compensated by the concentration of wave energy that occurs naturally at some locations by refraction and/or diffraction. The three major classes of shoreline devices are the oscillating water column (OWC), the convergent channel (TAPCHAN) and the
Pendulor.

Nearshore devices are situated in shallow waters (typically 10 to 25 m water depth). Again the OWC is the main type of device.

Offshore devices are situated in deeper water, with typical depths of more than 40 m.

Estimated contribution of RES by European Commission, [1].

Mtoe – million tons of oil equivalent.

Keywords:

Biofuel, biomass, energy production, fuel, geothermal, hydro, renewable energy sources, RES, solar, sustainable, tidal, wave, wind.

Source:

[1] Energy for the future: renewable sources of energy. White Paper for a Community Strategy and Action Plan. European Commission. Communication from the Commission. COM(97)599 final, 26/11/1997.

[2] AGORES (A Global Overview of Renewable Energy Sources), the Official European Commission Web Site for Renewable Energy Sources
(http://www.agores.org/SECTORS/default.htm)

[3] ATLAS Project official Web Site. European Network of Energy Agencies (EnR), http://europa.eu.int/comm/energy_transport/atlas/