Fossil fuels being the main source of climate-changing emissions are known to many. Over the first decade of the 21st century, the world has seen an exponential increase in the use of renewable energy resources, which means the era of renewables has arrived. Renewable energy refers to replenishable or inexhaustible power source, namely solar energy, wind energy, geothermal energy, biofuels, tidal and hydro-power. As science has confirmed that the increase of greenhouse gases that causes global climate change is in proportion of fossil fuel consumption, different countries have to deliver carbon-reduction promises in the international framework. Emissions reduction can be achieved through increasing energy efficiency and using more renewable energy resources. Recently, Greenpeace International has released a report on global electricity market development that shows renewables have become the world’s new favourite. In 2010, the market share of renewables was the largest in history. Since the end of 1990s, the development pace of wind and solar power has surpassed that of all the other power-generating technologies. The world has added the total installed capacity of renewables by 430 million kilowatts, equivalent of 45% of China’s current installation power capacity. Last year, globally the use of renewables grew by 87 million kilowatts, surpassing the development of coal-fired power for the first time.
In fact, unlike fossil fuels and nuclear energy, most of the renewables are not that controversial. It is only the availability of appropriate technologies which is a matter of wider discussions. The following sections will briefly introduce and evaluate major types of renewables.
WIND POWER
Wind energy (normally electrical power) is obtained from the wind using wind turbines. As with other forms of renewables, it occurs naturally and repeatedly in the environment. When the Earth is irradiated by the Sun, the ground absorbs some of this radiation that heats the ground and warms the air above. Hot air rises in convection currents. The uneven heating of the earth's surface causes winds. For example, if the Sun's rays fall on land and sea, the land heats up more quickly. This results in the air above the land moving upwards more quickly than that over the sea (hot air rises). As a result, the colder air over the sea will rush in to fill the gap left by the rising air. It is processes like these that give rise to high and low pressure areas, and thus to winds.
Wind power is the most economical energy with huge potential for large-scale development because the technology required is relatively simple. Producing no atmospheric emissions and pollutants, wind is truly a clean energy source and incurs no cost to drive the turbo generators. Unlike fossil fuels, there is no need to exploit, drill or transport any materials to the power stations. Nowadays, wind farms are able to generate electricity to be transmitted to customers through the normal grids.
Wind turbine
Today, large wind turbines are used to generate electricity. A wind turbine is usually made up of one or more aerodynamic blades with components such as wind wheel, generator head, rear wing, moving rotor and the tower. Wind wheel is the major component that changes wind to electricity. It looks like the propeller of an aeroplane. The blades of these wind turbines are about 30 metres long. The longer the diameter of the rotor is, the larger the swept area becomes and hence the generation capacity. The world’s largest offshore wind turbine is installed in Moray Firth, Scotland. It is 170-metre high with 63m long blades and is designed to generate 5MWe, as part of a 200-turbine project that aims to supply 20% of Scotland’s energy demand.
Wind turbines are usually grouped together in wind farms. A 20-turbine wind farm can generate enough electricity (about 1MW) for a small town. Turbines can produce between 500kW and 1MW of electricity. Factors affecting performance of a wind turbine include the windiness of the site, availability and turbine management.
(1) The windiness of the site: The power available from the wind is a function of the wind speed. Therefore, if the wind blows at twice the speed, its energy content will increase eight-fold. In practice, turbines at a site where the wind speed averages 8m/s can produce around 80% more electricity than those where the wind speed is 6m/s. Thus, the economic viability of a wind farm project is determined by windiness and its consistency.
(2) Availability: This is the capability to operate when the wind is available – an indication of the turbine’s reliability. Availability is typically 98% or more for modern European machines.
(3) Turbine management: Turbines in wind farms must be carefully arranged to gain maximum energy from the prevailing wind.
Therefore, the ideal siting of wind farm is large areas of flat land, on a smooth-top hill with a flat, clear exposure, free from excessive turbulence and obstructions such as large trees, houses or other buildings or those near coasts, onshore or offshore, that are subject to prevailing winds. For example in the Finnish Phyatunturi Arctic Test Wind Turbine site, a turbine generates 220kW at wind speed of 13m/s. The estimated annual production of the wind farm is 600 MWh. Small-scale wind power particularly suits remote off-grid locations where conventional methods of supply are expensive or unfeasible. However, owing to the noise emitted from wind turbines during operation and concerns about visual pollution (wind turbines destroying natural scenery), it may not be acceptable for turbines to be installed in any communities. (Porteous, 2008)
Generation cost
With technological advancement, the cost of wind power generation has decreased significantly over the last decade and compares favourably with conventional energy generation. Power generation costs are determined by (i) the installed costs of the plant (including interest during construction); (ii) operation and maintenance costs; (iii) energy productivity; (iv) cost of capital; and (v) the capital repayment period, although fuel cost (i.e. natural wind) is supposedly free.
A report entitled “Wind Power in the UK” by the Sustainable Development Commission outlines the average costs of wind power generation. The average generation costs of onshore wind power are around 3.2 pence per kilowatt hour (p/kWh) and around 5.5p/kWh for offshore. If the cost of carbon to society and the environment is counted towards the electricity generation costs, the price of wind power will be even lower since it produces no harmful emissions. Every modern wind turbine can save more than 400 tonnes of carbon emissions a year. (Porteous, 2008) (BWEA, 2009)
Status of wind power
In 2010 alone, wind power, inter alia, has added installed capacity by over 35 million kilowatts (up 24.6%), a scale that nuclear power reached only after running for a decade. China is now the largest producers and consumers of wind power, representing 22.4% of the world’s total capacity of 199,523 MW last year, followed by the USA (20.2%) and Germany (13.7%). (BP, 2011) China is also the world’s leading manufacturer and exporter of aerodynamic blades for wind turbines.
Geographically, as the UK is on the edge of Atlantic Ocean, it has one of the best wind resources in Europe. Offshore wind farms in coastal waters are being developed because winds blowing across the sea are often stronger than those inland. Nevertheless, a study by the Renewable Energy Foundation has indicated that England and Wales are not windy enough to allow large turbines to work at the rates claimed for them. Wind turbines in Cornwall, expected to be the most efficient, operated at only 24.1% of capacity, on average, while turbines in mid-Wales on average achieved 23.8%, those on Yorkshire Dales 24.9%, and those in Cumbria 25.9%. Turbines in Scotland fared much better. Southern Scotland achieved 31.5%, Caithness, Orkney and Shetland 32.9%, and offshore (North Hoyle and Scroby Sands) 32.6%. (Clover, 2006)
The largest offshore wind farm, set to produce 1GW of electricity, is built 20km off the Kent and Essex coast, in the Thames Estuary. It has received subsidies of approximately £160million per year. The main weakness of wind energy is that it is unstable and often intermittent. In the case of Thames Estuary wind farm, if the installed capacity of the 400-plus turbines is 1.3 GW, then the average output will only be 390MW even with a generous load factor of 30%. This would actually provide 5kW to 78,000 homes, just enough to power an electric kettle and a toaster. If there is a high pressure system above southeast England (being so frequent), there will even be zero output from these wind farms. The annual theoretical savings of carbon would only be 1.46 Mt which reduces global levels by a farcical 0.005%. (Porteous, 2008)
SOLAR POWER
Same as wind power, solar power is a clean, renewable and infinite energy source. It is already being harnessed in many parts of the world and it has the potential to provide several times the current world energy consumption if properly exploited. The ground receives only 47% of energy from the sun as most of it has dismissed in the atmosphere. Scientists said if solar energy is collected in full for 40 minutes, it is sufficient for the world to consume for a year. Solar can be used to directly produce electricity, for heating and even for cooling. It’s common practice these days to power small devices, like calculators, using very small solar cells. Photo voltaic is also used to provide electricity in areas with no power grid. Architects are also using photovoltaic cells increasingly as a design feature.
The term ‘solar power’ means to convert sunlight directly into thermal or electrical energy. There are two basic types of solar power, being ‘photovoltaic’ and ‘solar thermal’.
Solar photovoltaic
Solar photovoltaic (PV) involves the generation of electricity from light through the use of a semi-conductor material that can be adapted to release electrons, the negatively charged particles that form the basis of electricity. All photovoltaic cells have at least two layers of such semi-conductors, one positively charged and another one negatively charged. When light shines on the semi-conductor, the electric field across the junction between these two layers causes electricity to flow, generating DC (direct current). The stronger the light is, the greater the flow of electricity becomes. On both a large and a small scale photovoltaic can deliver power to the electrical grid, or stand on its own.
For instance, solar roof tiles or slates can replace conventional roofing materials. Flexible thin film modules can even be integrated into vaulted roofs, while semi-transparent modules allow for an interesting mixture of shading and daylight. Photovoltaic cells can also be used to supply peak power to the building on hot summer days when air conditioning systems need most energy, thus helping to reduce the maximum electricity load.
Solar thermal refers to concentrating sunlight into a single line or point on large mirrors. The heat created there is used to generate steam, which is hot and highly pressurised, and to power turbines, which generate electricity. In sun-drenched regions, solar thermal power plants can guarantee large shares of electricity production.
Solar capacity
In 2010, the installed solar capacity was 39777.8 MW, an increase of 72.6% from 2009. By 2015, the total installed capacity of solar thermal power plants will have passed 50,000 MW, according to projections. By 2020, additional capacity would be rising at a level of almost 45,000 MW each year, and the total installed capacity of solar thermal power around the world could reach almost 500,000 MW – enough to power more than 30 million homes. (BP, 2011) (Greenpeace International, 2009)
Instability of solar energy suppliesA photovoltaic system does not need bright sunlight to operate. It also generates electricity on cloudy days, with its energy output proportionate to the density of the clouds. Owing to the reflection of sunlight from clouds, days with a few clouds can even result in higher energy yields than days with a completely clear blue sky.
Even on a cloudy day, when the light is coming from many angles at once, a vacuum tube collector can be very effective. The absorber inside the vacuum tube absorbs radiation from the sun and heats up the fluid inside, just as in a flat solar panel. Additional radiation is picked up from the reflector behind the tubes. Whatever the sun angle, the round shape of the vacuum tube allows the sun to reach the absorber directly.
Multi-purpose energy source
Solar thermal technologies on the market now are efficient and highly reliable, providing solar energy for a wide range of applications, from domestic hot water and space heating in residential and commercial buildings, to swimming pool heating, solar-assisted cooling, industrial process heat and desalination of drinking water. (Greenpeace International, 2009)
HYDRO-POWER
Hydro-electric energy is water energy which is clean and renewable, but the facts that building a power-generating dam usually requires large-scale deforestation downstream and migration of indigenous inhabitants to elsewhere arouse much controversy.
Moving water contains an enormous store of natural energy, whether the water is part of a running river or waves in the ocean. The amount of power involved can be seen from the destructive force of a river breaking its banks and causing floods or of tall waves breaking on shallow coastlines. Water energy can be harnessed and converted to electricity and the generation of hydro-electric power does not produce greenhouse gas emissions. It is a renewable energy resource because water is constantly replenished through the Earth’s hydrological cycle. All a hydroelectric system needs is a permanent source of running water, like a creek or river. Unlike solar or wind energy, it can produce power continuously, 24 hours a day, except in drought periods.
River powerIn 2005, 19% of the world's electricity was produced by hydro-power plants, according to the US Energy Information Administration. Hydro-power harnesses the potential energy of impounded water (i.e. water running downstream) in a storage dam used to power a water turbine. The greater the drop in elevation, the faster the water flows, and the more electricity that can be produced. Pipeline friction losses, turbine and alternator inefficiencies combine to make overall efficiency of the order of 90% for large installations (over 5MWe) but these can be much lower in smaller installations. (Porteous, 2008)
Small-scale hydropower is an environmentally benign energy source with large growth potential. Small-scale hydro-systems can produce plenty of electricity without needing the large dams. Classified as ‘small’, ‘mini’ or ‘micro’ depending on how much electricity they produce, small hydro systems capture the river’s energy without diverting too much water away from its natural flow. In the UK, hydro-power is very much a local opportunity which can supply isolated communities very effectively. There is currently a small (less than 3%) hydro-power installed capacity, but 22 new projects totalling 9 MWe are proposed in the Fifth Non-Fossil Fuel Order.
Unfortunately, the dams that go with large scale hydropower can drown ecosystems as in the cases of the Three Gorges Project in China and the dam project in Amazon rainforest. Water needs of downstream communities, farmers and ecosystems should also be taken into account. In addition, hydro-power projects can be unreliable during prolonged droughts and dry seasons when rivers dry up or reduce in volume.
Wave power
Wave power is the harnessing of energy in the ebb and flow of the tides to generate electricity. This is an ‘income’ source of energy, which takes two forms:
(a) Barrages with conventional hydroelectric turbines installed in special sluices, enabling incoming and outgoing tides to generate power. The suitable tidal locations are severely restricted to certain types of shorelines.
(b) Floating power stations which rise and fall with the tide and have wave-powered flats or paddles driving water turbines for power generation. This method is restricted to the availability of sites as for barrages. (Porteous, 2008)
The first tidal power plant with a capacity of 240,000 kW was built in La Rance by France in 1960 while the first commercial wave power station was built on the island of Islay, Scotland, and known as ‘Limpet 500’ as it feeds 500 kW of electricity into the island’s grid since 2000. Today, the world’s largest ‘tidal current’ 1.2 MW unit is installed at Strangford Lough, Northern Ireland in 2008. Promoters, ‘Marine Current Turbines’, is planning a follow-on 10 MW unit which is claimed to achieve a tidal capacity of up to 500 MW by 2015. (Porteous, 2008)
Wave power as a renewable energy does not emit any greenhouse gases, release any sewage or cause any pollution. However, obtaining the equivalent amount of energy as from 1 kg of coal requires 1 tonne of sea water falling through 3 km, according to engineering assessments. Therefore, the captial costs of harnessing wave power may be quite high. This would involve large-scale wave plants in near-shore or off-shore environments, which is a technology under development. These large-scale on-shore wave power generating stations could face similar problems to those encountered by some wind farm projects, where opposition focuses on the aesthetic and noise impact of the machinery on the environment. Some may be concerned if the wave turbines will trap or kill many fish or birds.
Wave power supporters argue that it should be a combination of on-shore generation and near-shore generation (using a different technology) which focused on meeting local or regional needs. These plants could also be designed as part of harbout walls or water-breaks, performaing a dual role for the communities. (Porteous, 2008)
Around the UK, there could be sufficient recoverable wave power to generate enough power in excess of domestic electricity demands. The World Energy Council estimates that wave power could produce two terawatts of energy each year. This is twice the world’s current electricity production, and is equivalent to the energy produced by 2,000 large oil, gas, coal and nuclear power stations. Some research suggests that less than 0.1% of the renewable energy within the world’s oceans could supply more than 5 times the global demand for energy. The total renewable energy within the world's oceans, if it could all be economically harnessed, would satisfy the present world demand for energy more than 5,000 times over. But until now, harnessing wave power was only a theoretical possibility. In fact, the technology is still under development, and it is too early to estimate how soon it will significantly contribute to the global energy picture. (Greenpeace International, 2006)
Current status of hydroelectricity
As of December 2010, the world’s total production of hydroelectricity amounted to 775.6 Mtoe, an increase of 5.3% over 2009. Among others, China is the largest producer of hydroelectricity. In 2010, it produced 163.1 Mtoe, which represented 21% of the world’s total. However, the UK produced only 0.8 Mtoe, a plunge of 32.4% compared with 2009. (BP, 2011)
GEOTHERMAL POWER
Geothermal power refers to the use of energy from the earth’s interior conducted to the surface in some areas of the globle. The core of the earth is as boiling hot as 5,500°C whereas the average temperature within 3m on the earth’s surface throughtout the year is around 10-16°C. Geothermal energy can usually be found and extracted in areas where igneous rocks, produced in the processes of vulcanicity, are in a molten or partly moten within 10km of the earth’s surface. This kind of energy is useful if it is available in superheated water or steam form. Italy, USA, Iceland, Russia, New Zealand, Taiwan and Japan all have suitable geothermal energy fields.
Other than electricity generation in conventional power plants, geothermal energy is used for heating up (underground) water which forms hot springs and geysers, etc. Water heated by geothermy can be used for bathing, cooking, supplying domestic heating and melting snow on the roads. Heating can be brought to the ground and into buildings with underground water pumps. Thanks to the stable undergound temperature, geothermal energy can provide both heating in winter and cooling in summer. According to the International Geothermal Association, in 2010, the world’s total installed capacity of geothermal power was 10906.2 MW, of which the USA, the Philippines and Indonesia are top three users in the world. (BP, 2011)
Geothermal power does not release any greenhouse gas or cause pollution. It does not emit noise and is very reliable. The annual rate of utilisation of geothermal power stations reaches 90% while that of fossil-fuel power stations just 65-75%. Nonetheless, even if there are abundant geothermal energy sources in many countries, the use of this renewable energy or so-called ‘hot rocks’ up to 10km under the earth’s crust remains not significantly developed. The major barriers are the high costs incurred in the extraction of geothermal energy and the low thermal efficiency in power generation because a total of 30% of geothermal heat has to be used to drive the turbines. As with other renewable energy sources, economic feasibility is important and it is essential to do a careful evaluation as the money spent on prestigious projects may be more effectively applied in domestic energy saving through improved house insulation. (Porteous, 2008)
ENERGY CRISIS EMERGING AGAIN
In the past, energy demand of human beings were small and natural resources abundant. That is why humans have been enjoying low-cost energy resources. For example, oil was formed afters tens of millions of years. Given the R/P ratio (Reserve/Production ratio) and the present rate of use, obviously the consumption of oil doubles every ten years. In 1950, the global oil reserve was sufficient for 100 years of use. All fossil fuels will be exhausted someday. Today, experts estimate that, among all primary energy resources, the global oil reserve can support us for another 40 years, natural gas for 60 years, coal for 200 years and uranium for 70 years. To avoid a potential energy crisis in the future, it is imminent to explore and promote the extensive use of substitute and renewable energies.
Solar, wind, geothermal and hydroelectric powers described in this chapter demonstrate more potential for development. Although developing these renewable energies is being restrained by the current technology levels that cannot deliver them at low costs and ideal energy conversion efficiencies, a major breakthrough in technological development is highly likely in the near future.
Read the full chapter in Kennet, M. and Mak, W. (2011) Green Economics and Climate Change. London: Green Economics Institute.
Email me at winstonkm.mark@googlemail.com
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