Green Energy An introduction to the different solutions for replacing coal, oil and gas or avoiding the emission of greenhouse gases Updated 26 September, 2007
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Reference

Renewable Resources

The solar energy available is enormous. Every hour enough energy arrives at the Earth to meet our current energy demands for a whole year. Simple but effective applications of solar energy use it as it arrives. For example it can be focused on a container to heat water, it can stream through windows to heat buildings, or it can provide hot water by heating solar thermal panels on roofs.

The two main methods of converting solar energy to electricity are either to focus it on a boiler to create high temperature steam, which can drive a turbine, or to convert it directly into electricity using solar cells.

More indirect uses of solar energy extract the energy after it has been converted into wind or waves, or after it has raised water to be dropped as rain and then to drive hydroelectric schemes.

Biomass Schemes burn plant material which may be grown for the purpose. This is renewable as the carbon generated in burning the fuel came from the atmosphere as the plants were grown.

Two other sources of renewable energy do not depend on the heat arriving from the sun. Tidal energy arises from the gravitational attraction of the moon and sun. It can be extracted most easily by taking energy from tidal flows into and out of estuaries.

Geothermal energy takes energy from the hot interior of the earth.

The Current State for Large Scale Renewable Power Generation

Hydroelectric Power is a well established technology, which is very economic in appropriate cases, though construction costs can be high.

Geothermal energy is cost effective where it is easily available as in Iceland

Wind Power is now a well established technology, and the least expensive of the remaining technologies for electrical power generation. However its costs have not yet reached parity with fossil fuel generation and so its use relies on subsidies. As such subsidies are available installations of wind turbines is proceeding. Denmark generates 20% of its power by these means. The use of wind power to generate more than 20% of the required peak power brings a reducing benefit as it needs to be backed up by other plant to allow for the unreliability of the wind.

Biomass as a source of energy has a long history. Humans have used wood fires from prehistoric times. Its use as a method of generating motor fuel from sugar cane or corn is well established. Brazil meets a significant proportion of its motor fuel requirements from Ethanol generated from sugar cane.

Elsewhere the generation of Biodiesel is being encouraged. The main source in Europe is rapeseed and in the USA maize. However in 2007 a paper in Atmospheric Chemistry and Physics by respected authors reported that using biodiesel from these sources generated significantly more nitrogen dioxide than had been assumed. As nitrogen dioxide is a much more potent greenhouse gas than carbon dioxide the effect on global warming is calculated to be greater than from the use of fossil fuels.

Where crops are grown solely for the purpose of converting solar energy into fuel, the efficiency of conversion is typically about 0.5%, so a large area of land is needed to support a modest power station. A drive to increase the use of Biofuels is likely to put more pressure on vulnerable natural habitats such as forests, and lead to an undesirable growth in the use of artificial fertilisers.

Other Ideas

The other methods of generating mains power are still in the early stages of introduction, or are no more than ideas.

Generating electricity directly from Solar Energy, for example by focusing sunlight on a collector and generating electricity from the resulting high temperature, is likely to become economic if fuel costs increase. The alternative of direct conversion to electricity by Solar Cells is less likely to be successful for large scale power generation, as there is a fundamental limit on the proportion of solar energy which solar cells can convert to electricity. Currently production of solar cells worldwide is at about 80 Mwatts, and each cell needs to operate for about 4 years before it has generated
the energy used in its production. A large array of solar cells is unlikely to cost less than a smaller array of mirrors. A third method for large scale energy generation uses a very tall chimney and surrounds the base over a wide radius with a light transparent roof. The air under the roof is heated and drawn to the chimney. Energy is extracted by turbines at the base of the chimney, while much of the space under the roof could be used for agriculture. The energy available increases exponentially with the height of the chimney. The Prototype Manzanares Solar Chimney system in Spain generated 50 kW. A single Solar Tower power plant with a collector area of 7 km in diameter and a chimney 1 km high built and operated in an area with an annual global solar radiation of 2.3 MWh/m² might generate between 700 and 800 GWh per year. Thus a small number of Solar Tower power plants can even replace a large nuclear power station. However as with all solar powered systems the energy supply is only available during the day.

Generating electricity from Biomass grown for this purpose requires large areas of land for limited energy production, and is labour intensive.

Tidal Energy seems attractive. It is estimated that tidal schemes in selected places around the UK could generate at least 20% of the current UK electricity demand. The energy is intermittent but schemes in different locations with tides 3 hours apart can complement each other. Some schemes work by damming estuaries, making the water turn turbines as it enters and leaves. Trapping water with dams interferes with shipping and wild life.

Tidal Currents can be used to generate energy without requiring dams. Rotors can be installed where islands or other constrictions generate fast horizontal tidal flows.

In principle Ocean Currents could be a major source of energy. The gulf stream moves at 4 km/hour in some places, and a turbine anchored in it could generate continuous power. However there are obvious difficulties in getting the energy ashore.

Getting energy from Ocean Thermal Energy uses the fact that deep ocean temperatures are much less than surface temperatures. Unfortunately the energy that can be generated from the relatively small temperature difference is probably too low to be viable.

Small Scale Generation

Where power is generated for local use, the economics change.

Solar cells are widely used for powering installations such as motorway telephones where they save the cost of cabling mains electricity.

Solar Thermal Panels for water heating are reasonably competitive with electricity, when installed in suitable buildings.

Small Wind Power installations can be useful particularly in isolated locations, to reduce requirements for mains electricity.

Small Water Turbines are being installed for example in old water mills.

A significant issue where electricity is being generated locally in variable quantities is whether it is possible to export excess energy, when available, to the mains grid, and get paid for this energy. The small scale user would most like to be allowed just to run their electricity meter backwards, however this is not a fair representation of what the energy is worth to the electricity supplier.

Ideas under Development

The current Solar Cells made from Silicon are expensive, and cheaper alternatives are being researched and developed. At present these alternatives are less efficient than the 15% conversion of solar energy into electricity achieved by silicon solar cells, but should be significantly cheaper. The alternatives include electrically conductive plastics, a cell using a dye called phthalocyanine to mimic the action of chlorophyl, and a Cadmium Sulphide cell coated with very fine nanotubes.

The Thermal Chip currently being launched by ENECO Inc can convert heat into electricity using a design of semiconductor diode. Efficiencies of 30% are claimed. The heat could come from solar energy, or from fuels where it is an alternative to a fuel cell. The device also works in reverse. If electricity is supplied it delivers refrigeration.

Nuclear Energy

Fission Energy

Current State:

Nuclear Reactors are in widespread use. They generate about 70% of the electricity used in France. However very few new nuclear power stations have been built since the Chenobyl and Three Mile Island accidents.

In the UK nuclear power stations generate about 20% of all the electricity used. The earliest Power Stations have been closed after operating for about 45 years, and 13 remain operational.

Four of these are of the Magnox design which was developed by the United Kingdom Atomic Energy Authority. These were commissioned between 1966 and 1971, and are due to be closed between 2006 and 2009, unless their operators convince the regulatory authorities that it is safe to extend their life. Their total output capacity is 2,240 MWe, or about 4% of the total UK generating capacity.

Seven power stations have Advanced Gas-cooled Reactors (AGRs). These were commissioned between 1977 and 1989 and have a total output capacity of 8300 MWe. If these are kept in operation for 40 years they will close between 2017 and 2029.

The final power station is a Pressurised Water Reactor PWR) commissioned in 1995, which might be decommissioned in 2035.

If the existing nuclear capacity is to be replaced by new nuclear power stations, the process will be needed to start immediately, as the process of design, gaining planning approval and construction is likely to take many years.

Cost:

As nuclear reactors require so little fuel, the cost of electricity is dependent mainly on the cost of construction and final decommissioning of the plant. There is also the cost of storing the radioactive residue. Currently the cost of decommissioning is very considerable. This makes the overall cost of nuclear energy greater than that of fossil fuel at recent fuel prices. However currently Uranium is mined from ores containing about 1 percent of the element, and only a limited amount of such ore has been found. If Uranium had to be obtained from ores and other repositories such as sea water where it is in much lower concentrations, the energy to extract it may approach the energy released when it is used, so the long term future of fission reactors is doubtful.

Advantages:

Nuclear reactors do not generate any greenhouse gases. There is enough nuclear fuel to provide power for a reasonable period in the future. The industry is relatively safe, with only one accident that caused significant loss of life in the last 50 years.

Disadvantages:

The spent fuel and the materials that surround the reactor core are highly radioactive and must be kept away from living beings for many years - however after about 500 years it is no more radioactive than the original Uranium ore. It is politically very difficult to get agreement on burial sites for this material. Reactors can create material for making nuclear weapons, and so there is concern that nuclear energy assists more states to obtain nuclear arms. Plutonium is fairly poisonous. The concentration of radioactive material at a power station could make it a target for terrorists.

Fusion Energy

Power Available:

The amount of power that could be generated by fusion is even greater than that available from fission.

Current State:

Experimentation continues but fusion reactors remain a long term prospect. The Joint European Torus (JET) located in Culham, Oxfordshire, can produce energy - it has generated 16 MW for a second, though it required more energy than this to run. In June 2005 there was international agreement to build a preproduction reactor ITER in Cadarache, France. This is planned to begin operation in 2016 and is designed to generate 500 MW for 500 seconds at a time. If this is successful prototype power stations may be built.

Cost:

As no viable fusion reactor design exists, their costs are unknown. There have been design studies which indicate that such plants may be economically viable.

Advantages:

Fusion reactors do not generate any greenhouse gases. The fuel is very widely available. The main products of fusion are not radioactive. The reactors are inherently safe, they contain very small quantities of fuel at any time, and it is difficult to create the conditions for energy generation. So any disturbance in their operation is likely to stop the reaction very rapidly.

Disadvantages:

The materials that surround the reactor core will become radioactive and must be kept away from living beings for many years. However the materials being irradiated can be chosen to limit the problems with this radioactivity.

Storing the Gases

A number of schemes are being worked on for trying to store Carbon Dioxide, so called sequestration, rather than releasing it to the atmosphere. The main alternatives being considered are:

• Biological: Increasing the growth of plants to trap the carbon. The storage can be long term if the plant material is retained in the soil.
• Geological: Storing the Gas underground, for example in the reservoirs where natural gas or oil has been extracted.
• Mineralisation: Converting it to a solid form such as a carbonate, which can then be used as a building material.
• Oceanic: Storing compressed and liquid carbon dioxide at the bottom of the oceans, where it should be maintained in a liquid state by the local pressure and temperature.

A coal fired plant in which the Carbon is sequestered may be an acceptable and economic alternative to nuclear energy.

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