Renewable Energy and Electricity
(Updated 24 October 2013)
- There is unprecedented interest in renewable energy, particularly solar and wind energy, which provide electricity without giving rise to any carbon dioxide emission.
- Harnessing these for electricity depends on the cost and efficiency of the technology, which is constantly improving, thus reducing costs per peak kilowatt.
- Utilising electricity from solar and wind in a grid requires some back-up generating capacity due to their intermittent nature. Policy settings to support renewables are also generally required to confer priority in grid systems and also subsidise them, and some 50 countries have these.
- Utilising solar and wind-generated electricity in a stand-alone system requires corresponding battery or other storage capacity.
- The possibility of large-scale use of hydrogen in the future as a transport fuel increases the potential for both renewables and base-load electricity supply.
Technology to utilise the forces of nature for doing work to supply human needs is as old as the first sailing ship. But attention swung away from renewable sources as the industrial revolution progressed on the basis of the concentrated energy locked up in fossil fuels. This was compounded by the increasing use of reticulated electricity based on fossil fuels and the importance of portable high-density energy sources for transport – the era of oil.
As electricity demand escalated, with supply depending largely on fossil fuels plus some hydro power and then nuclear energy, concerns arose about carbon dioxide emissions contributing to possible global warming. Attention again turned to the huge sources of energy surging around us in nature – sun, wind, and seas in particular. There was never any doubt about the magnitude of these, the challenge was always in harnessing them.
Today we are well advanced in meeting that challenge. Wind turbines have developed greatly in recent decades, solar photovoltaic technology is much more efficient, and there are improved prospects of harnessing tides and waves. Solar thermal technologies in particular (with some heat storage) have great potential in sunny climates. With government encouragement to utilise wind and solar technologies, their costs have come down and are now in the same league as the increased costs of fossil fuel technologies due to likely carbon emission charges on electricity generation from them.
Demand for clean energy
There is a fundamental attractiveness about harnessing such forces in an age which is very conscious of the environmental effects of burning fossil fuels and sustainability is an ethical norm. So today the focus is on both adequacy of energy supply long-term and also the environmental implications of particular sources. In that regard the near certainty of costs being imposed on carbon dioxide emissions in developed countries at least has profoundly changed the economic outlook of clean energy sources.
A market-determined carbon price will create incentives for energy sources that are cleaner than current fossil fuel sources without distinguishing among different technologies. This puts the onus on the generating utility to employ technologies which efficiently supply power to the consumer at a competitive price.
Sun, wind, waves, rivers, tides and the heat from radioactive decay in the earth's mantle as well as biomass are all abundant and ongoing, hence the term "renewables". Only one, the power of falling water in rivers, has been significantly tapped for electricity for many years, though utilization of wind is increasing rapidly and it is now acknowledged as a mainstream energy source. Solar energy's main human application has been in agriculture and forestry, via photosynthesis, and increasingly it is harnessed for heat. Electricity remains a niche application for solar. Biomass (eg sugar cane residue) is burned where it can be utilised. The others are little used as yet.
Turning to the use of abundant renewable energy sources other than large-scale hydro for electricity, there are challenges in actually harnessing them. Apart from solar photovoltaic (PV) systems which produce electricity directly, the question is how to make them turn dynamos to generate the electricity. If it is heat which is harnessed, this is via a steam generating system.
If the fundamental opportunity of these renewables is their abundance and relatively widespread occurrence, the fundamental challenge, especially for electricity supply, is applying them to meet demand given their variable and diffuse nature*. This means either that there must be reliable duplicate sources of electricity beyond the normal system reserve, or some means of electricity storage (see later section).
Policies which favour renewables over other sources may also be required. Such policies, now in place in about 50 countries, include priority dispatch for electricity from renewable sources and special feed-in tariffs, quota obligations and energy tax exemptions.
The prospects, opportunities and challenges for renewables are discussed below in this context.
This load curve diagram shows that much of the electricity demand is in fact for continuous 24/7 supply (base-load), while some is for a lesser amount of predictable supply for about three quarters of the day, and less still for variable peak demand up to half of the time; Some of the overnight demand is for domestic hot water systems on cheap tariff. With overnight charging of electric vehicles it is easy to see how the base-load proportion would grow, increasing the scope for nuclear and other plants which produce it.
Most electricity demand is for continuous, reliable supply that has traditionally been provided by base-load electricity generation. Some is for shorter-term (eg peak-load) requirements on a broadly predictable basis. Hence if renewable sources are linked to a grid, the question of back-up capacity arises, for a stand-alone system energy storage is the main issue. Apart from pumped-storage hydro systems (see later section), no such means exist at present on any large scale.
However, a distinct advantage of solar and to some extent other renewable systems is that they are distributed and may be near the points of demand, thereby reducing power transmission losses if traditional generating plants are distant. Of course, this same feature sometimes counts against wind in that the best sites for harnessing it are sometimes remote from population, and the main back-up for lack of wind in one place is wind blowing hard in another, hence requiring a wide network with flexible operation.
Non-hydro renewable power capacity reached 480 GWe at the end of 2012.
Rivers and Hydro Electricity
Hydro-electric power, using the potential energy of rivers, is by far the best-established means of electricity generation from renewable sources. It supplies over 16% of world electricity (99% in Norway, 58% in Canada, 55% in Switzerland, 45% in Sweden, 7% in USA, 6% in Australia) from 990 GWe capacity (end of 2012). Global installed capacity was about 970 GWe at the end of 2011. Half of this is in five nations: China (212 GWe), Brazil (82.2 GWe), USA (79 GWe), Canada (76.4 GWe), and Russia (46 GWe). Apart from those four countries with a relative abundance of it (Norway, Canada, Switzerland and Sweden), hydro capacity is normally applied to peak-load demand, because it is so readily stopped and started. This also means that it is an ideal complement to wind power in a grid system, and is used thus most effectively by Denmark (see case study below). In 2010, hydro supplied about 3500 GWh (40% capacity factor), underlining its generally peak use.
Hydropower using large storage reservoirs is not a major option for the future in the developed countries because most major sites in these countries having potential for harnessing gravity in this way are either being exploited already or are unavailable for other reasons such as environmental considerations. Growth to 2030 is expected mostly in China and Latin America. China has commissioned the $26 billion Three Gorges dam, which will produce 18 GWe, but it has displaced over 1.2 million people.
The chief advantage of hydro systems is their capacity to handle seasonal (as well as daily) high peak loads. In practice the utilisation of stored water is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.
Run-of-river hydro systems are usually much smaller but have potentially wider application. Some short-term pondage can help them adapt to daily load profiles, but generally they produce continuously, apart from seasonal variation in river flows. Small-scale hydro plants under 10 MWe represent about 10% of world capacity, and most of these are run-of-river ones.
Pumped storage is discussed below under: Renewables in relation to base-load Electricity Demand.
Utilization of wind energy has increased spectacularly in recent years, with annual increases in installed capacity of around 20% in recent years. The 38 GWe increment in 2010 represented an investment of EUR 47 billion (US$ 65 billion), and it was followed by a 41 GWe increase in 2011 and 45 GWe in 2012. This brought total world wind capacity to 283 GWe, with tens of thousands of turbines now operating. However, all this has to be backed up with conventional generating capacity, due to low (20-30%) utilization and intermittency. (see later sections on this aspect.)
Wind turbines of up to 6 MWe are now functioning in many countries, though most new ones are 1-3 MWe. The power output is a function of the cube of the wind speed, so doubling the wind speed gives eight times the energy potential. In operation such turbines require a wind in the range 4 to 25 metres per second (14-90 km/hr), with maximum output being at 12-25 m/s (the excess energy being spilled above 25 m/s). While relatively few areas have significant prevailing winds in this range, many have enough to be harnessed effectively and to give better than a 25% capacity utilisation.
Where there is an economic back-up which can be called upon at very short notice (eg hydro), a significant proportion of electricity can be provided from wind. The most economical and practical size of commercial wind turbines is now about 2 MWe, grouped into wind farms up to 200 MWe. Depending on site, most turbines operate at about 25% load factor over the course of a year (European average), but some reach 33%.
China leads the field with over 62 GWe installed, USA has 47 GWe, Germany has 29 GWe, Spain has 21.5 GWe, and India 16 GWe at the end of 2011. World total then was 238 GWe.
Potentially the world’s largest wind farm is that planned by Forewind for the Dogger Bank in the North Sea, costing some £30 billion. Stage 1 is 2.4 GWe, followed by 4.8 GWe, to give 7.2 GWe, which Forewind says will supply some 25 billion kWh/yr to the UK grid at projected 40% annual capacity factor.
With increased scale and numbers of units, generation costs decreased but have now stabilised. They are still greater than those for coal or nuclear, and allowing for backup capacity and grid connection complexities adds to them. Wind is intermittent, and when it does not blow, back-up capacity such as hydro or quick-start gas is needed. When it does blow, and displaces power from other sources, it may reduce the profitability of those sources and hence increase prices.
One approach to mitigate intermittency is to make hydrogen by electrolysis and feed this into the gas grid. E.On is building a pilot plant to produce up to 360 m3/hr of hydrogen at Falkenhagen, Germany, to feed into the Ontras gas grid, which can function with 5% hydrogen. It has been suggested that all electricity from wind might be used thus, greatly simplifying electrical grid management. Vattenfall at Prenzlau in Germany is also experimenting with hydrogen production and storage from wind power via electrolysis.
However, government policies in many countries ensure that power from wind turbines is able to be sold (see Appendix). The Global Wind Energy Council claimed that world capacity of 121 GWe at the end of 2008 would produce 260 TWh per year (ie 24.6% capacity factor). Applied to the 238 GWe at the end of 2011 that is 511 TWh/yr. Wind is projected to supply 3% of world electricity in 2030, and perhaps 10% in OECD Europe. In the World Energy Outlook 2011 New Policies Scenario, 1304 GWe of new wind capacity would be added by 2035, balanced by 397 GWe retired to then.
Wind turbines have a high steel tower to mount the generator nacelle, and have rotors with three blades up to 50m long. Foundations require a substantial mass of reinforced concrete. Hence the energy inputs to manufacture are not insignificant. Also siting is important in getting a net gain from them. In the UK the Carbon Trust found that small wind turbines on houses in urban areas often caused more carbon emissions in their construction and fitting than they saved in electrical output (CT 7/8/08).
Bird kills, especially of raptor species, are an environmental impact of wind farms. In the USA half a million birds are killed each year, including 83,000 raptors (hawks, eagles, falcons etc) according to reports of a peer-reviewed published estimate in Wildlife Society Bulletin. A similar estimate comes from the US Fish & Wildlife Service. Other figures are based on 2.1 fatalities per turbine per year. There is particular concern regarding birds covered by the US Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act, which make bird fatalities illegal.
New wind farms are increasingly offshore, in shallow seas. The UK has 3300 MWe wind capacity offshore, more than the rest of the world combined as of early 2013. The London Array, 20 km offshore Kent, has 175 turbines of 3.6 MWe, total 630 MWe, on a 245 km2 site and claims to be the world's largest offshore wind farm.
Solar energy is readily harnessed for low temperature heat, and in many places domestic hot water units (with storage) routinely utilise it. It is also used simply by sensible design of buildings and in many ways that are taken for granted. Industrially, probably the main use is in solar salt production – some 1000 PJ per year in Australia alone (equivalent to two-thirds of the nation's oil use). It is increasingly used in utility-scale plants, mostly photovoltaic, and by mid-2013 some 15 GWe of utility-scale solar power was installed, 3.1 GWe of this in China, and 2.9 GWe in each of Germany and USA.
Three methods of converting the sun's radiant energy to electricity are the focus of attention.
Photovoltaic (PV) systems
The best-known method utilises light, ideally sunlight, acting on photovoltaic cells to produce electricity. Flat plate versions of these can readily be mounted on buildings without any aesthetic intrusion or requiring special support structures. Solar photovoltaic (PV) has for some years had application for certain signaling and communication equipment, such as remote area telecommunications equipment in Australia or simply where mains connection is inconvenient. Sales of solar PV modules are increasing strongly as their efficiency increases and price falls, coupled with financial subsidies and incentives. In 2012 world installed PV capacity reached the 100 GWe milestone, with 30.5 GWe installed that year. In the World Energy Outlook 2011 New Policies Scenario, 553 GWe of new solar PV capacity (and 81 GWe of CSP) would be added by 2035.
Thin-film PV modules using silicon or cadmium telluride are at least 20% less costly than crystalline silicon-based ones, but are less efficient. Even working on 1 kilowatt per square metre in the main part of a sunny day, intensity of incoming radiation and converting this to high-grade electricity is still relatively inefficient – typically 10% in commercial equipment or up to 30% in more expensive units. But the cost per unit of electricity - at least ten times that of conventional sources, limits its unsubsidised potential to supplementary applications on buildings where its maximum supply coincides with peak demand.
More efficiency can be gained using concentrating solar PV (CPV), where some kind of parabolic mirror tracks the sun and increases the intensity of the solar radiation up to 1000-fold. Modules are typically 35-50 kW. In Australia a 2 MWe demonstration plant followed by a 102 MWe dense-array CPV power station is planned by Silex SolarSystems for Mildura in Victoria, with A$ 125 million government support promised. Anticipated cost of power is under 15c/kWh. Silex claims 34.5% conversion efficiency, with a target of 50%. In the USA Boeing has licensed its XR700 high-concentration PV (HCPV) technology to Stirling Energy Systems with a view to commercializing it for plants under 50 MWe from 2012. The HCPV cells in 2009 achieved a world record for terrestrial concentrator solar cell efficiency, at 41.6%. CPV can also be used with heliostat configuration, with a tower among a field of mirrors.
In 2011 several Californian plants planned for solar thermal changed plans to solar PV – see mention of Blythe, Imperial Valley and Calico below.
Many solar PV plants are connected to electricity grids in Europe and USA, and now China. The OECD IEA reported 23 GWe of solar PV capacity in 2009, 17 GWe of this in Europe. Japan has 150 MWe installed and the USA had over 200 MWe of utility-scale PV at end of 2010. China's 200 MWe Golmud solar park was commissioned in 2011 and is claimed to produce 317 GWh/yr (18% capacity factor). The 100 MWe Perovo solar park in Ukraine was commissioned in 2011 also, with 15% capacity factor claimed. EdF has built the 115 MWe Toul-Rosieres thin-film PV plant in eastern France. There is a 97 MWe Sarnia plant in Canada. In Italy, SunEdison plans to build a 72 MWe solar PV plant near Rovigo, for $342 million. India’s 214 MWe Gujarat Solar Park was commissioned in 2012 and aims for eventual 1000 MWe capacity. The Indian government announced the 4 GWe Sambhar project in Rajasthan in 2013, expected to produce 6 TWh/yr, ie capacity factor of 17%. The initial 1 GWe, assumed to be PV, is expected to operate from 2016. MidAmerican’s Antelope Valley plants in California comprise a 579 MWe development with Sunpower as EPC contractor and due to be complete at the end of 2015. Its panels will track the sun, giving 25% more power. MidAmerican Solar owns the 550 MWe Topaz Solar Farms in San Luis Obispo County, Calif., and has a 49% interest in the 290 MWe Agua Caliente thin-film PV project under construction by First Solar in Yuma County, Arizona. Many PV plants are over 20 MWe, and quoted capacity factors range from 11% to 27%.
A South Korean consortium has commissioned 42 MWe PV capacity at two plants in Bulgaria, which are expected to produce 61 GWh/yr (16.5% capacity factor), their cost being EUR 154 million (EUR 3667/kW). Research continues into ways to make the actual solar collecting cells less expensive and more efficient. In some systems there is provision for feeding surplus PV power from domestic systems into the grid as contra to normal supply from it, which enhances the economics. The 2000 MWe Ordos thin-film solar PV plant is planned in Inner Mongolia, China, with four phases – 30, 100, 870, 1000 MWe – to be complete in 2020. Over 30 others planned are over 100 MWe, most in India, China, USA and Australia. Serbia plans a 1 GWe solar PV project costing EUR 1.3 billion which is expected to deliver 1.15 TWh/yr to Enerxia Energy from 2015, a 13% capacity factor, without any feed-in tariff. (That output at EUR 50/MWh would return EUR 57.5 million pa. After EUR 20 million pa maintenance, it is less than 3% pa return on capital.)
In recent years there has been high investment in solar PV, due to favourable subsidies and incentives. In 2011 Italy saw 9000 MWe of solar PV installed, and Germany 7500 MWe of solar. In Germany, solar PV capacity reached 32.4 GWe at end of 2012 (7.6 GWe installed during the year) and generated 28 billion kWh, increasing 45% over 2011, but apparently only 11% capacity factor. In Italy, feed-in tariffs range form EUR 15 to 27 c/kWh, depending on size, giving a 2011 cost to consumers of nearly EUR 6 billion. In the World Energy Outlook 2011 New Policies Scenario, 553 GWe of new solar PV and 81 GWe of CSP capacity would be added by 2035. Solar PV capacity at the end of 2011 was 67 GWe.
Solar thermal systems, concentrating solar power (CSP)
Solar thermal systems need sunlight rather than the more diffuse light which can be harnessed by solar PV. A solar thermal power plant has a system of mirrors to concentrate the sunlight on to an absorber, the energy then being used to drive turbines – concentrating solar thermal power (CSP). About 2.55 GWe of CSP capacity worldwide (end of 2012), three quarters of this in Spain, supplies a proportion of the solar electricity. More CSP is under development.
The concentrator may be a parabolic mirror trough oriented north-south, which tracks the sun's path through the day. The absorber is located at the focal point and converts the solar radiation to heat in a fluid such as synthetic oil, which may reach 700°C. The fluid transfers heat to a secondary circuit producing steam to drive a conventional turbine and generator. Several such installations in modules of up to 80 MW are now operating. Each module requires about 50 hectares of land and needs very precise engineering and control. These plants are supplemented by a gas-fired boiler which generates about a quarter of the overall power output and keeps them warm overnight.
A simpler CSP concept is the Fresnel collector using rows of long narrow flat (or slightly curved) mirrors tracking the sun and reflecting on to one or more fixed linear receivers positioned above them. The receivers may generate steam directly.
In mid-2007 Nevada Solar One, a 64 MWe capacity solar thermal energy plant, started up. The $250 million plant is projected to produce 124 million kWh per year and covers about 160 hectares with 760 mirrored troughs that concentrate the heat from the desert sun on to pipes that contain a heat transfer fluid. This is heated to 390°C and then produces steam to drive turbines. Nine similar units totaling 354 MWe have been operating in California as the Solar Energy Generating Systems. More than twenty Spanish 50 MWe parabolic trough units including Andasol 1-3, Alvarado 1, Extresol 1-2, Ibersol and Solnova 1-3, Palma del Rio 1-2, Manchasol 1-2, Valle 1-2, commenced operation in 2008-11. Andasol, Manchasol and Valle have 7.5-hour heat storage.
Other US CSP parabolic trough projects under construction include Abengoa's Solana in Arizona, a 280 MWe project with six-hour molten salt storage enabling power generation in the evening and early morning. It has a 778 ha solar field and started operation in 2013. The $2 billion cost is offset by a $1.45 billion loan guarantee from the US Department of Energy. Abengoa's 280 MWe Mojave Solar Project near Barstow in California also uses parabolic troughs in a 715 ha solar field and is due on line in 2014. It has a $1.2 billion federal loan guarantee.
In 2010 California approved construction of the $6 billion, 968 MWe Blythe CSP plant by Solar Trust, the US arm of Solar Millennium at Riverside, Calif., using parabolic trough technology in four 250 MWe units occupying 28.4 sq km and funded partly by US Dept of Energy. The company has a $2.1 billion loan guarantee and a 20-year power purchase agreement with SC Edison, from 2013. However, this has now become a solar PV project, apparently due to difficulty in raising finance. Also in California, Imperial Valley (709 MWe), and Calico (663 MWe) are Stirling engine systems (see below), though the new owners of Calico are switching 563 MWe of it to PV, and Imperial Valley is re-permitting for PV. Abu Dhabi commissioned its 100 MWe Shams parabolic trough CSP plant in 2013; it cost $600 million.
Another form of this CSP is the power tower, with a set of flat mirrors (heliostats) which track the sun and focus heat on the top of a tower, heating water to make steam, or molten salt to 1000°C and using this both to store the heat and produce steam for a turbine. California's Solar One/Two plant produced 10 MWe for a few years. An 11 MWe Spanish power tower plant PS10 has 624 mirrors, each 120 m2 and produces steam directly in the tower. A 20 MWe version PS20 is adjacent, and by 2015 Spain expects to have 2000 MWe of CSP operating. The 500 MWe Guzman CSP plant at Palma de Rio was opened in 2012. Power production in the evening can be extended fairly readily using gas combustion for heat.
The US Department of Energy awarded a $1.37 billion loan guarantee to BrightSource Energy to build the 392 MWe Ivanpah Solar Power complex in the Mojave Desert of California. It comprises three CSP Luz power towers which simply heat water to 550°C to make steam, using 300,000 heliostat mirrors in pairs each of 14 m2 per MWe, in operation from 2013 as the world's largest CSP plant. The steam cycle uses air-cooled condensers. There is a back-up gas turbine. The company is seeking some A$ 450 million Australian government support for a similar two-tower, 250 MWe plant with gas-fired evening function in Australia. BrightSource plans a similar 500 MWe plant nearby in the Coachella Valley.
Using molten salt in the CSP system as the transfer fluid which also stores heat, enables operation into the evening, thus approximating to much of the daily load demand profile. Spain's 20 MWe Gemasolar (formerly Solar Tres) plant has 2500 mirrors/ heliostats, each 115 m2 and molten salt storage, claiming to be the world's first near base-load CSP plant, with 63% capacity factor. Its cost is reported to be $33,000 /kW. Spain's 200 MWe Andasol plant also uses molten salt heat storage, as does California's 280 MWe Solana. The salt used may be 60% sodium nitrate, 40% potassium nitrate with melting point 220°C. Andasol stores heat at 400°C and requires 75 t of salt per MW of heat. Its condensers require 5 L/kWh for cooling. Spain's Gemasolar employs 6250 tonnes of salt. In Nevada the 110 MWe Crescent Dunes plant is under construction, with 10-hour heat storage. In Colorado the 2x100 MWe SolarReserve plant in San Luis Valley will use molten salt.
Another CSP set-up is the Solar Dish Stirling System which uses reflectors to concentrate energy to drive a stirling cycle engine. A Tessera Solar plant of 709 MWe is planed at Imperial Valley in California. The system consists of a solar concentrator in a dish structure with an array of curved glass mirror facets which focus the energy on the power conversion unit's receiver tubes containing hydrogen gas which powers a Stirling engine. Solar heat pressurizes the hydrogen to power the four-cylinder reciprocating Solar Stirling Engine and drive a generator. The hydrogen working fluid is cooled in a closed cycle. Waste heat from the engine is transferred to the ambient air via a water-filled radiator system. The stirling cycle system is as yet unproven in these large applications, however.
With solar input being both diffuse* and interrupted by night and by cloud cover, solar electric generation has a low capacity factor, typically less than 15%, though this is partly addressed by heat storage using molten salt. Power costs are two to three times that of conventional sources, which puts it within reach of being economically viable where carbon emissions from fossil fuels are priced.
Large CSP schemes in North Africa, supplemented by heat storage, are proposed for supplying Europe via high voltage DC links. One proposal is the TuNur project based in Tunisia and supplying up to 2000 MWe via HVDC cable to Italy. A related and more ambitious one is Desertec, with estimated cost of EUR 400 billion, networking the EU, Middle East and North Africa (MENA) with 20 transmission lines of 5 GW each, to provide 15% of Europe's electricity and much of that in MENA by 2050. The Desertec Foundation was set up in 2009 as an NGO to promote the Desertec concept. The Desertec Industrial Initiative GmbH (Dii) “Desert Power” is a Europe-based consortium founded in 2009 to advance the grand vision and work towards creation of a market for desert power in EU and MENA. It comprises 55 companies and institutions and is active in Morocco, Algeria and Tunisia. The first Dii-fostered project is to be the Ouarzazate 500 MWe CSP plant in Morocco, with its first 160 MWe phase operable in 2014. In mid-2013 the Desertec Foundation left the Dii consortium. Bosch and Siemens had left it in 2012.
The Mediterranean Solar Plan (MSP) targets the development of 20 GWe of renewables by 2020, of which 5 GWe could be exported to Europe. Total investment would be of the order of EUR 60 billion. The OECD IEA's World Energy Outlook 2010 says: The quality of its solar resource and its large uninhabited areas make the Middle East and North Africa region ideal for large-scale development of concentrating solar power, costing 10 to 13.5 c/kWh ... in 2035. Solar power could be exported to Europe (at transmission costs of 2 to 5 c/kWh) and/or to countries in sub-Saharan Africa. The report projects that the actual CSP generation cost in North Africa could be the same as EU wholesale electricity price in 2035 – about 10 c/kWh.
CSP boost to fossil fuel power, hybrid systems
Solar energy producing steam can be used to boost conventional steam-cycle power stations. Australia's Kogan Creek Solar Boost Project will be the largest solar integration with a coal-fired power station in the world when it is operational in 2013. A 30-hectare field of Areva Solar's compact linear Fresnel reflectors at the existing Kogan Creek power station will produce steam which will be fed to the modern supercritical 750 MWe coal-fired power station, helping to drive the intermediate pressure turbine. The solar boost at 44 MW (peak sunshine) will add 44 million kWh annually, about 0.75% of output, for $105 million – equivalent to $19,000/kW of base-load capacity. The 2000 MWe Liddell coal-fired power station has a 2 MWe equivalent solar boost (9 MW thermal addition).
In the USA the federal government has a SunShot initiative to integrate CSP with fossil fuel power plants as hybrid systems. Some $20 million is offered for two to four projects. The US Department of Energy says that 11 to 21 GWe of CSP could effectively be integrated into existing fossil fuel plants, utilizing the turbines and transmission infrastructure.
While CSP is well behind solar PV as its prices continue to fall and utilities become more familiar with PV. However, CSP can provide thermal storage and thus be dispatchable and it can provide low-cost steam for existing power plants (hybrid set up). Also, CSP has the potential to provide heating and cooling for industrial processes and desalination.
Solar updraft tower
Another kind of solar thermal plant is the solar updraft tower, using a huge chimney surrounded at its base by a solar collector zone like an open greenhouse. The air under this skirt is heated and rises up the chimney, turning turbines as it does so. The 50 MWe Buronga plant planned in Australia was to be a prototype, but Enviromission's initial plans are now for two 200 MWe versions each using 32 turbines of 6.25 MWe with a 10 square kilometre collector zone under a 730 metre high tower in the Arizona desert. Thermal mass – possibly brine ponds – under the collector zone means that some operation will continue into the night. A 50 kWe prototype plant of this design operated in Spain 1982-89. In China the 27.5 MWe Jinshawan solar updraft tower is under construction.
A significant role of solar energy is that of direct heating. Much of our energy need is for heat below 60oC, eg. in hot water systems. A lot more, particularly in industry, is for heat in the range 60-110oC. Together these may account for a significant proportion of primary energy use in industrialised nations. The first need can readily be supplied by solar power much of the time in some places, and the second application commercially is probably not far off. Such uses will diminish to some extent both the demand for electricity and the consumption of fossil fuels, particularly if coupled with energy conservation measures such as insulation.
With adequate insulation, heat pumps utilising the conventional refrigeration cycle can be used to warm and cool buildings, with very little energy input other than from the sun. Eventually, up to ten percent of total primary energy in industrialised countries may be supplied by direct solar thermal techniques, and to some extent this will substitute for base-load electrical energy.
Where hot underground steam can be tapped and brought to the surface it may be used to generate electricity. Such geothermal sources have potential in certain parts of the world such as New Zealand, USA, Philippines and Italy. Global installed capacity was about 11.7 GWe at the end of 2012, including 2600 MWe in California, 1900 MWe in Philippines and 1200 MWe in Indonesia, and in 2012 geothermal produced at least 72 billion kWh worldwide. In Japan 500 MWe of capacity produces 0.3% of the country's electricity. In New Zealand 420 MWe produces over 7% of the electricity, and Iceland gets most of its electricity from 200 MWe of geothermal plant, and also most of its district heating. Mexico has 958 MWe geothermal, Italy 843 MWe and Nevada 47 MWe. Lihir Gold mine in Papua New Guinea has 56 MWe installed, the last 20 MWe costing US$ 40 million – about the same as annual savings from the expanded plant. Geothermal electric output is expected to triple by 2030. The largest geothermal plant is The Geysers in California, producing about 1000 MWe, but diminishing. See also Geothermal Energy Association website.
There are also prospects in certain other areas for hot fractured rock geothermal, or hot dry rock geothermal – pumping water underground to regions of the Earth's crust which are very hot or using hot brine from these regions. The heat – up to about 250°C – is due to high levels of radioactivity in the granites and because they are insulated at 4-5 km depth. They typically have 15-40 ppm uranium and/or thorium, but may be ten times this. The heat from radiogenic decay* is used to make steam for electricity generation. South Australia has some very prospective areas. The main problem with this technology is producing and maintaining the artificially-fractured rock as the heat exchanger. Only one such project is operational, the Geox 3 MWe plant at Landau, Germany, using hot water (160ºC) pumped up from 3.3 km down (and maybe should be classed as conventional geothermal). It cost EUR 20 million. A 50 MWe Australian plant is envisaged as having 9 deep wells – 4 down and 5 up.
Ground source heat pump systems or engineered geothermal systems also come into this category, though the temperatures are much lower. The 1997 Geoscience Australia building in Canberra is heated and cooled thus, using a system of 210 pumps throughout the building which carry water through loops of pipe buried in 352 boreholes each 100 metres deep in the ground. Here the temperature is a steady 17°C, so that it is used as a heat sink or heat source at different times of the year. See 10-year report (pdf).
Harnessing the tides with a barrage in a bay or estuary has been achieved in France (240 MWe in the Rance Estuary, since 1966), Canada (20 MWe at Annapolis in the Bay of Fundy, since 1984) and Russia (White Sea, 0.5 MWe), and could be achieved in certain other areas where there is a large tidal range. The trapped water can be used to turn turbines as it is released through the tidal barrage in either direction. Worldwide this technology appears to have little potential, largely due to environmental constraints.
However, placing free-standing turbines in major coastal tidal streams appears to have greater potential, and this is being explored.
Currents are predictable and those with velocities of 2 to 3 metres per second are ideal and the kinetic energy involved is equivalent to a very high wind speed. This means that a 1 MWe tidal turbine rotor is less than 20 m diameter, compared with 60 m for a 1 MWe wind turbine. Units can be packed more densely than wind turbines in a wind farm, and positioned far enough below the surface to avoid storm damage. A 300 kW turbine with 11 m diameter rotor in the Bristol Channel can be jacked out of the water for maintenance. Based on this prototype, early in 2008 the 1.2 MWe SeaGen twin turbine was installed in Strangford Lough, Northern Ireland, billed as the first commercial unit of its kind the world’s largest grid-connected tidal stream turbine. It produces power 18-20 hours per day and is operated by a Siemens subsidiary. The next project is a 10.5 MWe nine-turbine array off the coast of Anglesey. An 86 MWe tidal turbine project in Pentland Firth, between Orkney and the Scottish mainland has been approved, and MeyGen’s initial 9 MWe demonstration array of six turbines is expected on line in 2015, using Atlantis and Andritz technology. The first Atlantis 1MWe prototype was deployed at the European Marine Energy Centre at Orkney in 2011, and a 1 MWe Andritz Hydro Hammerfest prototype is also deployed there.
Some tidal stream generators are designed to oscillate, using the tidal flow to move hydroplanes connected to hydraulic arms sideways or up and down. A prototype has been installed off the coast of Portugal.
Another experimental design is using a shroud to speed up the flow through a venturus in which the turbine is placed. This has been trialled in Australia and British Colombia.
A major pilot project using three kinds of tidal stream turbines is being installed in the Bay of Fundy's Minas Passage, about three kilometers from shore. Some 3 MWe will be fed to the Canadian grid from the pilot project. Eventually 100 MWe is envisaged. The three designs are a 10m diameter turbine from Ireland, a Canadian Clean Current turbine and an Underwater Electric Kite from USA.
Tidal power comes closest of all the intermittent renewable sources to being able to provide a continuous and predictable output, and is projected to increase from 1 billion kWh in 2002 to 35 billion in 2030 (including wave power).
Harnessing power from wave motion is a possibility which might yield significant electricity. The feasibility of this has been investigated, particularly in the UK. Generators either coupled to floating devices or turned by air displaced by waves in a hollow concrete structure (oscillating water column) are two concepts for producing electricity for delivery to shore. Other experimental devices are submerged and harness the changing pressure as waves pass over them. The first commercial wave power plant is in Portugal, with floating rigid segments which pump fluid through turbines as they flex at the joints. It can produce 2.25 MWe. Another – Oyster – is in UK and is designed to capture the energy found in nearshore waves in water depths of 12 to 16 metres. Each 200-tonne module consists of a large buoyant hinged flap anchored to the seabed. Movement of the flap with each passing wave drives a hydraulic piston to deliver high-pressure water to an onshore turbine which generates electricity. The 315 kW demonstration module being tested in the Orkney Islands is expected to have about a 42% capacity factor.
Numerous practical problems have frustrated progress with wave technology, not least storm damage.
Ocean Thermal Energy
Ocean thermal energy conversion (OTEC) has long been an attractive idea, but is unproven beyond small pilot plants up to 50 kWe. It works by utilising the temperature difference between equatorial surface waters and cool deep waters, the temperature difference needing to be about 20ºC top to bottom. In the open cycle OTEC the warm surface water is evaporated in a vacuum chamber to produce steam which drives a turbine. It is then condensed in a heat exchanger by the cold water. The main engineering challenge is in the huge cold water pipe which needs to be about 10 m diameter and extend a kilometre deep to enable a large water flow. A closed cycle variation of this uses an ammonia cycle. The ammonia is vapourised by the warm surface waters and drives a turbine before being condensed in a heat exchanger by the cold water. A 10ºC temperature difference is sufficient.
Beyond traditional direct uses for cooking and warmth, growing plant crops particularly wood to burn directly or to make fuels such as ethanol and biodiesel has a lot of support in several parts of the world, though mostly focused on transport fuel. More recently, wood pellets for electricity generation have been newsworthy. The main issues here are land and water resources. The land usually must either be removed from agriculture for food or fibre, or it means encroaching upon forests or natural ecosystems. Available fresh water for growing biofuel crops such as maize and sugarcane and for processing them may be another constraint.
Burning biomass for generating electricity has some appeal as a means of utilising solar energy for power. However, the logistics and overall energy balance usually defeat it, in that a lot of energy - mostly oil based - is required to harvest and move the crops to the power station. This means that the energy inputs to growing, fertilising and harvesting the crops then processing them can easily be greater than the energy value in the final fuel, and the greenhouse gas emissions can be similar to those from equivalent fossil fuels. Also other environmental impacts can be considerable. For long-term sustainability, the ash containing mineral nutrients needs to be returned to the land. Three 660 MWe units of Drax, Britain’s largest coal-fired power station, are being converted to burn wood, most of it imported (like the coal of higher heat value that it replaces). No CO2 emissions are attributed to the actual burning, on the basis that growing replacement wood balances out those emissions, albeit in a multi-decade time frame. Unlike coal, the wood needs to be stored under cover
In Australia and Latin America sugar cane pulp is burned as a valuable energy source, but this (bagasse) is a by-product of the sugar and does not have to be transported. In the EU in 2010 over 11 million tonnes of wood pellets were used. In 2012 Europe imported 4.36 million tonnes of pellets and in 2015, 15 million tonnes import is projected, two thirds from North America (as in 2012). The pellets are made from sawmill residues preferably, but also forest residues and low-value timber. UK demand is expected to reach 11 million tonnes by 2015, equivalent to twice that amount of fresh wood.
By 2030 biomass-fuelled electricity production was projected to triple and provide 2% of world total, 4% in OECD Europe, as a result of government policies to promote renewables. However, such projections are increasingly challenged as the cost of biofuels in water use and pushing up food prices is increasingly questioned. In particular, the use of ethanol from corn and biodiesel from soybeans reduces food production and arguably increases world poverty. The cost in subsidies is also increasingly questioned: in the OECD US$ 13-15 billion is spent annually on biofules which provide only 3% of liquid transport fuel.
In 2008 about 100 million tonnes of grain (enough feed nearly 450 million people) was expected to be turned into fuel. This includes a legislated 40% of the US corn crop, aided by heavy subsidies. Meanwhile basic food prices have risen sharply, leading the UN Food & Agriculture Organisation in mid 2012 to call for the USA to halt its biofuel production to prevent a food crisis.
In recent years there has been discussion as to whether nuclear power can be categorised as “renewable”. In the context of sustainable development it shares many of the benefits of many renewables, it is a low-carbon energy source, it has a very small environmental impact, similarities that are in sharp contrast to fossil fuels. But commonly, nuclear power is categorised separately from ‘renewables’. Nuclear fission power reactors do use a mineral fuel and demonstrably depletes the available resources of that fuel.
In the future nuclear power will make use of fast neutron reactors. As well as utilizing about 60 times the amount of energy from uranium, they will unlock the potential of using even more abundant thorium as a fuel. In addition, some 1.5 million tonnes of depleted uranium now seen by some people as little more than a waste, becomes a fuel resource. In effect, they will ‘renew’ their own fuel resource as they operate. The consequence of this is that the available resource of fuel for fast neutron reactors is so plentiful that under no practical terms would the fuel source be significantly depleted.
‘Renewables’, as currently defined, would offer no meaningful advantage over fast neutron reactors in terms of availability of fuel supplies. Most also tend to make very large demands on resources to construct the plant used for harnessing the natural energy – per kilowatt hour produced, much more than nuclear power.
Centralised state utilities focused on economies of scale can easily overlook an alternative model - of decentralized electricity generation, with that generation being on a smaller scale and close to demand. Here higher costs may be offset by reduced transmission losses (not to mention saving the capital costs of transmission lines) and possibly increased reliability. Generation may be on site or via local mini grids.
Renewables in Relation to Base-load demand
It is clear that renewable energy sources have considerable potential to meet mainstream electricity needs. However, having solved to problems of harnessing them there is a further challenge: of integrating them into the supply system. Obviously sun, wind, tides and waves cannot be controlled to provide directly either continuous base-load power, or peak-load power when it is needed, so how can other, controllable sources be operated so as to complement them?
If there were some way that large amounts of electricity from intermittent producers such as solar and wind could be stored efficiently, the contribution of these technologies to supplying electricity demand would be much greater – see following subsection.
There is some scope for reversing the whole way we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Today's peak-load equipment could be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power at short notice when they were unable to. This is essentially what happens with Denmark, whose wind capacity is complemented by a major link to Norwegian hydro (as well as Sweden and the north German grid).
Case study: West Denmark
West Denmark (the main peninsula part) is the most intensely wind-turbined part of the planet, with 1.74 per 1000 people - 4700 turbines totaling 2315 MWe, 1800 MWe of which has priority dispatch and power must be taken by the grid when it is producing. Total system capacity is 6850 MWe and maximum load during 2002 was 3700 MWe, hence a huge 81% margin. In 2002, 3.38 billion kWh were produced from the wind, a load factor of 16.8%. The peak wind output was 1813 MWe on 23 January, well short of the total capacity, and there were 54 days when the wind output supplied less than 1% of demand. On two occasions, in March and April, wind supplied more than total demand for a few hours. In February 2003 during a cold calm week there was virtually no wind output. Too much wind is also a problem - over 20 m/s output drops and over 25 m/s turbines are feathered. Generally, a one metre/second wind change causes a 320 MWe power change for the whole system.
However, all this can be and is managed due to the major interconnections with Norway, Sweden and Germany, of some 1000 MWe, 600 MWe and 1300 MWe respectively. Furthermore, especially in Norway, hydro resources can normally be called upon, which are ideal for meeting demand at short notice. (though not in 2002 after several dry years). So the Danish example is a very good one, but the circumstances are far from typical.
Case study: Germany
The 2006 report from a thorough study commissioned by the German Energy Agency (DENA) looked at regulating and reserve generation capacity and how it might be deployed as German wind generation doubled to 2015. The study found that only a very small proportion of the installed wind capacity could contribute to reliable supply. Depending on time of year, the gain in guaranteed capacity from wind as a proportion of its total capacity was between 6 and 8% for 14.5 GWe total, and between 5 and 6% for 36 GWe total projected in 2015. This all involves a major additional cost to consumers.
Case study: UK
The performance of every UK wind farm can be seen on the Renewable Energy Foundation web site. Note particularly the percentage of installed capacity which is actually delivering power averaged over each month.
However, if hydro is the back-up and is not abundant then it will be less available for peaking loads. If gas is the back-up it this may be the best compromise between cost and availability. If brown coal generating plant is the back-up and thus has to be run at lower output to accommodate the wind-generated input then the CO2 emissions per kWh increase, eroding or even eliminating any emission advantage from wind. In Germany brown coal plants need to be run inefficiently to back up wind capacity and this has both cost and CO2 implications (see below).
In practical terms non-hydro renewables are therefore able to supply up to some 15-20% of the capacity of an electricity grid, though they cannot directly be applied as economic substitutes for most coal or nuclear power, however significant they become in particular areas with favourable conditions. Nevertheless, they will make an important contribution to the world's energy future, even if they cannot carry the main burden of supply. The Global Wind Energy Council expects wind to be able to supply between 10.8 and 15.6% of global electricity by 2030.
A 2005 report on wind energy in Germany by E.On, the country's largest grid operator, pointed out that: "As wind power capacity rises, the lower availability of the wind farms determines the reliability of the system as a whole to an ever increasing extent. Consequently the greater reliability of traditional power stations becomes increasingly eclipsed. As a result, the relative contribution of wind power to the guaranteed capacity of our supply system up to the year 2020 will fall continuously to around 4%. In concrete terms, this means that in 2020, with a forecast wind power capacity of over 48,000 MW in Germany (Source: DENA grid study), 2,000 MW of traditional power production can be replaced by these wind farms." Hence "traditional power stations with capacities equal to 90% of the installed wind power capacity must be permanently online in order to guarantee power supply at all times." Wind energy cannot replace conventional power stations to any significant extent.
In some places pumped storage is used to even out the daily generating load by pumping water to a high storage dam during off-peak hours and weekends, using the excess base-load capacity from low-cost coal or nuclear sources. During peak hours this water can be used for hydro-electric generation. Relatively few places have scope for pumped storage dams close to where the power is needed, and overall efficiency is less than 80%, but about 21 GWe pumped storage is in service in the USA and 38 GWe in Europe*
Energy storage with compressed air (CAES) in geological caverns is being trialed, using gas-fired or electric compressors. When released (with preheating) it powers a turbine, up to 300 MWe, with overall about 70% efficiency.* CAES capacity can even out the production from a wind farm and make it partly dispatchable.
As described above in the solar thermal subsection, some CSP plants use molten salt to store energy overnight. Spain's 20 MWe Gemasolar claims to be the world's first near base-load CSP plant, with 63% capacity factor. Spain's 200 MWe Andasol plant also uses molten salt heat storage, as does California's 280 MWe Solana.
The largest utility-scale electricity storage so far is a 4 megawatt sodium-sulfur (NaS) battery system to provide improved reliability and power quality for the city of Presidio in Texas. The 4 MWe set-up was energized early in 2010 to provide rapid back-up for wind capacity in the local ERCOT grid. Another 4 MWe NaS system was commissioned in May 2013. The $18 million Yerba Buena Battery Energy Storage System Pilot Project in San Jose, California, was set up by PG&E with $3.3 million support from the California Energy Commission. PG&E operates a similar 2 MWe battery system near its Vaca-Dixon solar plant in Solano County, California. Sodium-sulfur batteries are widely used elsewhere for similar roles. Lower-cost lead-acid batteries are also in widespread use at small utility scale, with banks of up to 1 MWe being used to stabilise wind farm power generation. A 0.5 MWe Purewave Storage Management system with 1280 advanced lead-acid batteries was commissioned in September 2011 at PNM's Mesa Del Sol, Albuquerque New Mexico, by S&C Electric Co. The GS Batteries are capable of up to 4000 deep discharge cycles.
Following a two-year study by the California Public Utilities Commission, the state in 2010 passed legislation requiring 1325 MWe of electricity storage (excluding large-scale pumped storage) by 2024. In 2013 it had 35 MWe total. The purpose of the legislation is to increase grid reliability, reduce peak capacity requirements, and enhance the utility of solar and wind inputs to it. The storage systems can be connected with either transmission or distribution systems, or be behind the meter.
Intermittency and grid management
Grid management authorities faced with the need to be able to dispatch power at short notice treat wind-generated power not as an available source of supply which can be called upon when needed but as an unpredictable drop in demand. In any case wind needs about 90% back-up, whereas the level of back-up for other forms of power generation which can be called upon on demand is around 25%, simply allowing for maintenance downtime.
The OECD IEA's World Energy Outlook 2010 considered the likely integration costs of variable renewables in 2035 under three headings: interconnection costs, balancing costs and maintaining supply adequacy. For the EU these together, for onshore and offshore wind, CSP and large scale PV, amounted to 1.63 c/kWh, and for USA 1.73 c/kWh.
Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is claimed that wind generation output can be predicted with 90% certainty 24 hours ahead. This means that it is possible to deploy other plant more effectively so that the economic value of that wind contribution is greatly increased.
The high cost of wind power plus the need for substantial back-up gives rise to very high system generating costs by present standards, but in some places it may be the shape of the future.
Modeling done by the UK National Grid Corporation shows the effect of wind's unreliability on the required plant for achieving the 20% UK renewables target:
|Contribution from wind
% of 400 TWh
|Wind capacity GWe
||Conventional capacity GWe
||Spare capacity GWe
Thus, building 25 GWe of wind capacity, equivalent to almost half of UK peak demand, will only reduce the need for conventional fossil and nuclear plant capacity by 6.7%. Also, some 30 GWe of spare capacity will need to be on immediate call continuously to provide a normal margin of reserve and to back up the wind plant's inability to produce power on demand - about two thirds of it being for the latter.
Ensuring both secure continuity of supply (reliably meeting peak power demands) and its quality (no voltage drops etc) means that the actual potential for wind and solar input to a system is severely limited. Doing so economically, as evident from the above UK figures, requires low-cost back-up such as hydro, or gas turbine with cheap fuel. For the UK, with little interconnection beyond its shores, a 20% renewables target is difficut.
In a March 2004 report Eurelectric and the Federation of Industrial Energy Consumers in Europe pointed out that "Introducing renewable energy unavoidably leads to higher electricity prices. Not only are production costs substantially higher than for conventional energy, but in the case of intermittent energy sources like wind energy, grid extensions and additional balancing and back-up capacity to ensure security of supply imply costs which add considerably to the end price for the final consumer." "Reducing CO2 by promoting renewable energy can thus become extremely expensive for consumers," though both organisations fully support renewables in principle. The economic disadvantage referred to will also be reduced as carbon emission costs become factored in to fossil fuel generation.
Because wind turbine output is so variable, for planning purposes its potential output is discounted to the level of power that can be relied upon for 90% of the time. In Australia that figure comes to 7% of installed wind capacity, in Germany it is 8%, which is all that can be included as securely available (ie 90% of the time).* On the 90% availability basis, other technologies can be counted on for much higher reliability, and hence the investment cost per kilowatt reliably available is much less.
A 2006 report by the UK Energy Research Council looked at the system implications and costs of intermittent inputs from renewables whose variability was uncontrollable. It found that intermittent sources meeting up to 20% of electricity demand need not compromise reliability, but was likely to have a significant cost. The report looked at system balancing impacts, in managing fluctuations from system balancing reserves, and reliability impacts which affected ability to meet peak demand and also required a greater system margin (15-22% higher). It costed the former at £2-3/MWh and the latter at £3-5/ MWh - total £5-8 (0.5p to 0.8p/kWh) with 20% wind input.
Nuclear power plants are essentially base-load generators, running continuously. Where it is necessary to vary the output according to daily and weekly load cycles, for instance in France, where there is a very high reliance on nuclear power, they can be adapted to load-follow. For BWRs this is reasonably easy without burning the core unevenly, but for a PWR to run at less than full power for much of the time depends on where it is in the 18 to 24-month refueling cycle, and whether it is designed with special control rods which diminish power levels throughout the core without shutting it down. So while the ability on any individual PWR reactor to run on a sustained basis at low power decreases markedly as it progresses through the refueling cycle, there is considerable scope for running a fleet of reactors in load-following mode. Generation III plants have more scope for load-following, and as fast neutron reactors become more established, their ability in this regard will be an asset.
If electricity cannot be stored on a large scale, the next logical step is to look at products of its use which can be stored, and hence where intermittent electricity supply is not a problem.
System Integration Costs of Intermittent Renewable Power Generation
Power generation technologies generally compete with each other both in regulated and deregulated markets to supply electricity through a ‘merit order’ based on availability and marginal cost of production for any given period. Fossil fuel, nuclear, biomass and hydro power generators can all to varying degrees supply electricity ‘on demand’, in other words supply from these sources can be called upon or adjusted to meet demand. The feed-in of renewable energy, mostly wind and solar, is intermittent due to the uncertainty of meteorological conditions. In grid management terms they are not dispatchable. Therefore the energy system needs backup capacity from the on-demand-sources to bridge periods with high or low generation from renewables. The targeted rapid increase of power supply from intermittent renewable sources in many countries will present a fundamental challenge to the smooth functioning of many electricity supply systems.
Wind and solar power are the forms of renewable power that are expected to grow most rapidly. They accounted for 35% of EU renewable capacity in 2009, a percentage that the IEA in World Energy Outlook 2011 expects to increase to 55% in 2015 in its central 'new policies' scenario. By 2030 the IEA expects wind and solar to constitute 34% of total EU electrical capacity, up from 11% in 2009. However, the intermittency of wind and solar generation meant that the amount of electricity supplied from wind and solar capacity in 2009 was less than 5% of the total; 133 TWh was generated by EU wind turbines in 2009 which equated to a capacity factor of 20%. In the same year, nuclear had a capacity factor of 73%; thus to generate an equivalent amount of potential power to nuclear on the basis of these load factors, it is necessary to install three or four times as much wind capacity*. But that is not the main problem.
Wind and solar power supply is largely governed by wind speed and the level of sunlight, which can only loosely be related to periods of power demand. It is this feature of intermittent renewable power supply that results in the imposition of additional costs on the generating system as a whole, which will implicitly be paid for either by other generators, consumers or taxpayers.
The IEA disaggregates these system costs into three components:
- Adequacy costs: the cost of ensuring that the power system has sufficient capacity to meet peak loads.
- Balancing costs: the cost of ensuring that the power system can respond flexibly to demand changes at any given time.
- Interconnection costs: the cost of linking sources of supply to sources of demand.
Required adequacy and balancing capacity for intermittent renewables
Additional back-up capacity is needed to meet demand in good time when meteorological conditions result in insufficient wind and solar power generation. The adequacy and balancing capacity must itself have a high degree of availability, ie, it should be from a dispatchable source. This reserve or backup capacity is most likely to be needed during periods of high demand and lack of wind and solar, for instance on a calm winter’s evening. In such a situation it can be imagined that significant levels of dispatchable backup capacity would be needed to ensure security of supply.
Estimates that have been made of the required level of reserve capacity indicate relatively low levels in practice. The IEA estimates that for every 5 megawatts of variable renewable capacity installed, about 1 megawatt of other flexible capacity is needed on a global basis in order to ensure system adequacy*. Such a global estimate would appear to rely on certain assumptions regarding the degree of interconnectedness of the power supply system, the mix of renewable technologies, access to storage, the 'smartness' of the grid, etc. It appears likely that in some countries and regions, the required backup capacity would need to be significantly higher than 20%.
Estimates of the wind/solar reserve requirement to date have largely been made on the basis of relatively low levels of renewables penetration (ie, less than 20%). Given the absence of experience with maintaining reliable electricity supply from systems with a high proportion of intermittent renewables in regions of major demand, such results should be treated with caution. The reserve capacity ratio is expected to increase exponentially in proportion to the reliance of the system on intermittent power generation*. Germany is currently planning on increasing renewable power supply to 35% of the total by 2020 and 50% by 2030**. The EU as a whole is targeting a 20% renewable contribution to power supply by 2020. The adequacy and balancing capacity requirements for such high levels of intermittent renewable penetration can only be estimated theoretically.
In World Energy Outlook 2010 the IEA estimates that adequacy and balancing costs for intermittent renewable power supply range from 0.18-0.8c/kWh of intermittent renewable power supply in the USA and from 0.13-0.65c/kWh in the EU in the New Policies Scenario in 2035*. The New Policies Scenario envisages that intermittent renewable power supply would constitute 13% of the 2035 total power supply in the US and 22% in the EU; these levels of penetration are relatively modest and the adequacy and balancing costs could be expected to increase significantly at higher levels of penetration.
IEA estimates* of generating (busbar) costs of wind in the USA and EU vary between 7.0-23.4c/kWh (solar is even more costly), so these adequacy and balancing cost estimates are equivalent to no more than 11% of the levelised wind generating cost. However, the costs of backup capacity clearly depend on the type of backup capacity envisaged. Pumped storage is often cited as an ideal renewable form of flexible backup but is relatively expensive (between 1.1-23.2 c/kWh); gas turbines would be generally cheaper, but still quite expensive given that they would be operating only for part of the time and therefore suffer low load factors (probably less than 20%). Gas turbines would also emit greenhouse gases as it would likely be prohibitively expensive to install CCS for sources with such low load factors. It is possible that where supply reliability cannot be guaranteed, many electricity users placing a premium on reliability (eg, hospitals) will invest in expensive local generating capacity (typically diesel generators).
Interconnection costs for wind and solar
The third category of intermittent renewable integration cost is grid interconnection. Wind and solar farms are ideally sited in areas that experience high average wind speeds and high average solar radiation respectively. These sites are often, even typically, distant from areas of electricity demand. Transmission and distribution networks will often need to be extended significantly to connect sources of supply and demand - this is a current challenge in UK and North Germany. The IEA estimates these costs at 1.2 c/kWh in the USA and 0.9 c/kWh in the EU, which again must be recovered either from other producers, consumers or taxpayers.
Impacts of wind and solar on base-load generators
The three categories of intermittent renewable integration costs are estimated by the IEA to vary approximately in sum from 1.1 to 1.7 c/kWh in the EU and 1.3 to 1.9 c/kWh in the USA. As footnoted above, these estimates are theoretical and pertain to the penetration of intermittent renewables forecast by the IEA in their ‘New Policies Scenario’ for 2035.*
* The figures given in the chart represent the higher values of the range of values given by the IEA. For some of the assumptions used please refer to earlier Footnote.
However, there is another category of costs that result from the operation of renewables and these can be described as the external costs borne by other power producers, in particular base-load power producers, as a result of intermittency. The structure of wind and solar levelised generation costs is characterised by high capital, significant O&M costs and zero fuel costs. As a result, the operating costs for these sources are very low and when power is generated they undercut and are able to displace all other sources of power in a utility’s merit order. In a situation of high levels of wind and solar power penetration and during periods of low demand, baseload generators will be displaced in the merit order. This is also required by legislation in many countries.
The impact of high levels of intermittent, low cost power will be to reduce the load factors of base-load power generators, and thereby increase their unit costs per kilowatt-hour. Given the high capital costs of nuclear, such an impact will significantly increase the levelised generation costs of nuclear. For example, a 15% decrease in the capacity factor of a nuclear power plant could increase its levelised cost by about 24%. In a situation similar to that targeted by Germany for 50% of supply to be renewable by 2030, renewable capacity when fully utilised would provide up to100% of supply. Inevitably there would be periods when a great deal of base-load capacity would be forced off-line.
Nevertheless, the path towards a higher share of renewables is widely forecast, even if not necessary, and the system will have to handle many challenges due to their intermittency.
The Hydrogen Economy
Hydrogen is widely seen as a possible fuel for transport, if certain problems can be overcome economically. It may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without normal burning.
Making hydrogen requires either reforming natural gas (methane) with steam, or the electrolysis of water. The former process has carbon dioxide as a by-product, which exacerbates (or at least does not improve) greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power.
With intermittent renewables such as solar and wind, matching the output to grid demand is very difficult, and beyond about 20% of the total supply, apparently impossible. But if these sources are used for electricity to make hydrogen, then they can be utilised fully whenever they are available, opportunistically. Broadly speaking it does not matter when they cut in or out, the hydrogen is simply stored and used as required.
A quite different rationale applies to using nuclear energy (or any other emission-free base-load plant) for hydrogen. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods and any not needed to meet civil demand being used to make hydrogen at other times. This would mean maximum efficiency for the nuclear power plants, and that hydrogen was made opportunistically when it suited the grid manager.
About 50kWh is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial.
Renewable energy sources have a completely different set of environmental costs and benefits to fossil fuel or nuclear generating capacity.
On the positive side they emit no carbon dioxide or other air pollutants (beyond some decay products from new hydro-electric reservoirs), but because they are harnessing relatively low-intensity energy, their 'footprint' - the area taken up by them - is necessarily much larger.
Whether Australia could accept the environmental impact of another Snowy Mountains hydro scheme (providing some 3.5% of the country's electricity plus irrigation) is doubtful. Whether large areas near cities dedicated to solar collectors will be acceptable, if such proposals are ever made, remains to be seen. Beyond utilising roofs, 1000 MWe of solar capacity would require at least 20 square kilometres of collectors, shading a lot of country.
In Europe, wind turbines have not endeared themselves to neighbours on aesthetic, noise or nature conservation grounds, and this has arrested their deployment in UK. At the same time, European non-fossil fuel obligations have led the establishment of major offshore wind forms and the prospect of more.
However, much environmental impact can be reduced. Fixed solar collectors can double as noise barriers along highways, roof-tops are available already, and there are places where wind turbines would not obtrude unduly.
APPENDIX: Government Support for Renewables Deployment
In an open market, government policies to support particular generation options such as renewables normally give rise to explicit direct subsidies along with other instruments such as feed-in tariffs, quota obligations and energy tax exemptions. In the EU, feed-in tariffs are widespread.
Corresponding to these in the other direction are taxes on particular energy sources, justified by climate change or related policies. For instance Sweden taxes nuclear power at about EUR 0.6 cents/kWh.
European Environment Agency figures in 2004 gave indicative estimates of total energy subsidies in the EU-15 for 2001: solid fuel (coal) EUR 13.0, oil & gas EUR 8.7, nuclear EUR 2.2, renewables EUR 5.3 billion.
The Global Wind Energy Council (2008) reported that "In the pursuit of the overall target of 21% from renewable electricity by 2010, the Renewable Electricity Directive 2001 gives EU Member States freedom of choice regarding support mechanisms. Thus, various schemes are operating in Europe, mainly feed-in tariffs, fixed premiums, green certificate systems and tendering procedures. These schemes are generally complemented by tax incentives, environmental taxes, contribution programs or voluntary agreements."
France has a feed-in tariff of EUR 8.2 c/kWh to 2012, which then will decrease.
Germany's Renewable Energy Sources Act gives renewables priority for grid access and power dispatch. It is regularly amended to adapt feed-in tariffs to market conditions and technological developments. For wind energy an initial tariff applies for up to 20 years and this then reduces to a basic tariff of EUR 5.02 c/kWh. The initial tariff is EUR 9.2 c/kWh for onshore wind and 15 c/kWh for offshore wind from January 2009. The combined subsidy from consumers and government totals some EUR 5 billion per year - for 7.5% of its electricity.
Denmark has a wide range of incentives for renewables and particularly wind energy. It has a complex 'Green Certificate' scheme which transfers the subsidy cost to consumers. However, there is a further economic cost borne by power utilities and customers. When there is a drop in wind, back-up power is bought from the Nordic power pool at the going rate. Similarly, any surplus (subsidised) wind power is sold to the pool at the prevailing price, which is sometimes zero . The net effect of this is growing losses as wind capacity expands.
Italy in 2008 legislated to provide EUR 18 c/kWh on a quota system for wind power.
Spain has different levels of feed-in tariffs depending on the technology used. A fixed tariff of EUR 7.32 c/kWh is one option, or a fixed premium of 2.93 c/kWh on the market price (but with a floor of 7.13 cents) is the other, as of 2008. The tariffs for renewables are adjusted every four years.
Greece has a feed-in tariff of 6.1-7.5 c/kWh, whereas the Netherlands relies on exemption from energy taxes to encourage renewables.
The UK has not used any feed-in tariff arrangement, but is to do so from 2010. Meanwhile a specific indication of the cost increment over power generation from other sources is given by the 4.5-5.0 p/kWh market value for the Renewables Obligation, by which utilities can cover the shortfall in producing a certain proportion of their electricity from renewables by paying this amount and passing the cost on to the consumer. In addition there is a Climate Change Levy of 0.43 p/kWh on non-renewable sources (at present including nuclear energy, despite its lack of greenhouse gas emissions), which corresponds to a subsidy.
Sweden subsidises renewables (principally large-scale hydro) by a tax on nuclear capacity, which works out at about EUR 0.67 cents/kWh from 2008. For wind, there is a quota system requiring utilities to buy a certain amount of renewable energy by purchasing certificates.
In Norway the government subsidises wind energy with a 25% investment grant and then production support per kWh, the total coming to NOK 0.12/kWh, against a spot price of around NOK 0.18/kWh (US$ 1.3 cents & 2 cents respectively).
In the USA the wind energy production tax credit (PTC) of 1.5 c/kWh indexed to inflation (now about 2.1 c/kWh) has provided incentive, though this expires every two years before being renewed by Congress.
Canada provides a production incentive payment of 1c/kWh for wind power, plus feed-in tariffs.
In Australia energy retailers are required to source specified quantities of power from new (non hydro) renewables. The obligation is tradeable and there is a fallback tax of AUD 4 c/kWh for retailers failing to comply.
In India ten out of 29 states have feed-in tariffs, eg 2.75 times the tariff for coal-generated power in Karnataka, plus a federal incentive scheme paying one third of the coal-fired tariff.
Small-scale PV input is encouraged by high feed-in tariffs, eg 48 c/kWh in Germany and 50 c/kWh in Portugal.
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