Wind power grid integration

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Large-scale use of wind power raises many questions in integration into the existing electric power grid. Wind power is an intermittent energy source which must be used when available. If a large fraction of a system's energy is to come from wind power, provisions must be made to supply load during days with low wind. These provisions might take the form of spinning reserve already allocated in the system, start-up of stand-by power plants, or interconnections to other areas that can take up the load.

Because grid operational strategies are designed for traditional dispatchable energy sources like coal, integrating wind energy into the utility grid can be problematic. Within the power grid, there must be balance between load and generation, economic and policy incentives, cost-effective storage, and robust and distributed control.[1]

Collection and transmission network[edit]

Typical components of a wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position

In a wind farm, individual turbines are interconnected with a medium voltage (usually 34.5 kV) power collection system and communications network. At a substation, this medium-voltage electrical current is increased in voltage with a transformer for connection to the high voltage electric power transmission system.

A transmission line is required to bring the generated power to (often remote) markets. For an off-shore plant this may require a submarine cable. Construction of a new high-voltage line may be too costly for the wind resource alone, but wind sites may take advantage of lines installed for conventionally fueled generation.

One of the biggest current challenges to wind power grid integration in the United States is the necessity of developing new transmission lines to carry power from wind farms, usually in remote lowly populated states in the middle of the country due to availability of wind, to high load locations, usually on the coasts where population density is higher. The current transmission lines in remote locations were not designed for the transport of large amounts of energy.[2] As transmission lines become longer the losses associated with power transmission increase, as modes of losses at lower lengths are exacerbated and new modes of losses are no longer negligible as the length is increased, making it harder transport large loads over large distances.[3] However, resistance from state and local governments makes it difficult to construct new transmission lines. Multi state power transmission projects are discouraged by states with cheap electricity rates for fear that exporting their cheap power will lead to increased rates. A 2005 energy law gave the Energy Department authority to approve transmission projects states refused to act on, but after an attempt to use this authority, the Senate declared the department was being overly aggressive in doing so.[2] Another problem is that wind companies find out after the fact that the transmission capacity of a new farm is below the generation capacity, largely because federal utility rules to encourage renewable energy installation allow feeder lines to meet only minimum standards. These are important issues that need to be solved, as when the transmission capacity does not meet the generation capacity, wind farms are forced to produce below their full potential or stop running all together, in a process known as curtailment. While this leads to potential renewable generation left untapped, it prevents possible grid overload or risk to reliable service.[4]

Generator characteristics and stability[edit]

Induction generators, often used for wind power projects, require reactive power for excitation so substations used in wind-power collection systems include substantial capacitor banks for power factor correction. Different types of wind turbine generators behave differently during transmission grid disturbances, so extensive modelling of the dynamic electromechanical characteristics of a new wind farm is required by transmission system operators to ensure predictable stable behaviour during system faults (see: Low voltage ride through). In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators. Doubly fed machines—wind turbines with solid-state converters between the turbine generator and the collector system—generally have more desirable properties for grid interconnection. Transmission systems operators will supply a wind farm developer with a grid code to specify the requirements for interconnection to the transmission grid. This will include power factor, constancy of frequency and dynamic behaviour of the wind farm turbines during a system fault.[5][6]

Capacity factor[edit]

Worldwide installed capacity 1997–2008, with projection 2009–13 based on an exponential fit. Data source: WWEA

Since wind speed is not constant, a wind farm's annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 20–40%, with values at the upper end of the range in particularly favourable sites.[7] For example, a 1MW turbine with a capacity factor of 35% will not produce 8,760 MWh in a year (1 × 24 × 365), but only 1 × 0.35 × 24 × 365 = 3,066 MWh, averaging to 0.35 MW. Online data is available for some locations and the capacity factor can be calculated from the yearly output.[8][9]

Unlike fueled generating plants, the capacity factor is limited by the inherent properties of wind. Capacity factors of other types of power plant are based mostly on fuel cost, with a small amount of downtime for maintenance. Nuclear plants have low incremental fuel cost, and so are run at full output and achieve a 90% capacity factor. Plants with higher fuel cost are throttled back to follow load. Gas turbine plants using natural gas as fuel may be very expensive to operate and may be run only to meet peak power demand. A gas turbine plant may have an annual capacity factor of 5–25% due to relatively high energy production cost.

According to a 2007 Stanford University study published in the Journal of Applied Meteorology and Climatology, interconnecting ten or more wind farms can allow an average of 33% of the total energy produced to be used as reliable, baseload electric power, as long as minimum criteria are met for wind speed and turbine height.[10][11]

Penetration[edit]

Wind energy "penetration" refers to the fraction of energy produced by wind compared with the total available generation capacity. There is no generally accepted "maximum" level of wind penetration. The limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for storage or demand management, and other factors. An interconnected electricity grid will already include reserve generating and transmission capacity to allow for equipment failures; this reserve capacity can also serve to regulate for the varying power generation by wind plants. Studies have indicated that 20% of the total electrical energy consumption may be incorporated with minimal difficulty.[12] These studies have been for locations with geographically dispersed wind farms, some degree of dispatchable energy, or hydropower with storage capacity, demand management, and interconnection to a large grid area export of electricity when needed. Beyond this level, there are few technical limits, but the economic implications become more significant. Electrical utilities continue to study the effects of large (20% or more) scale penetration of wind generation on system stability and economics.[13][14][15][16]

At present, a few grid systems have penetration of wind energy above 5%: Denmark (values over 19%), Spain and Portugal (values over 11%), Germany and the Republic of Ireland (values over 6%). The Danish grid is heavily interconnected to the European electrical grid, and it has solved grid management problems by exporting almost half of its wind power to Norway. The correlation between electricity export and wind power production is very strong.[17]

Intermittency and penetration limits[edit]

Diagram of the TVA pumped storage facility at Raccoon Mountain Pumped-Storage Plant

Electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and seasonally. Annual variation also exists, but is not as significant. Related to variability is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be "scheduled". Wind power forecasting methods are used, but predictability of wind plant output remains low for short-term operation.

Because instantaneous electrical generation and consumption must remain in balance to maintain grid stability, this variability can present substantial challenges to incorporating large amounts of wind power into a grid system. Intermittency and the non-dispatchable nature of wind energy production can raise costs for regulation, incremental operating reserve, and (at high penetration levels) could require an increase in the already existing energy demand management, load shedding, or storage solutions or system interconnection with HVDC cables. At low levels of wind penetration, fluctuations in load and allowance for failure of large generating units requires reserve capacity that can also regulate for variability of wind generation. Wind power can be replaced by other power stations during low wind periods. Transmission networks must already cope with outages of generation plant and daily changes in electrical demand. Systems with large wind capacity components may need more spinning reserve (plants operating at less than full load).[18][19]

A series of detailed modelling studies which looked at the Europe wide adoption of renewable energy and interlinking power grids using HVDC cables, indicates that the entire power usage could come from renewables, with 70% total energy from wind at the same sort of costs or lower than at present. Intermittency would be dealt with, according to this model, by a combination of geographic dispersion to de-link weather system effects, and the ability of HVDC to shift power from windy areas to non-windy areas.[20][21]

Pumped-storage hydroelectricity or other forms of grid energy storage can store energy developed by high-wind periods and release it when needed.[22] Stored energy increases the economic value of wind energy since it can be shifted to displace higher cost generation during peak demand periods. The potential revenue from this arbitrage can offset the cost and losses of storage; the cost of storage may add 25% to the cost of any wind energy stored, but it is not envisaged that this would apply to a large proportion of wind energy generated. Thus, the 2 GW Dinorwig pumped storage plant adds costs to nuclear energy in the UK for which it was built, but not to all the power produced from the 30 or so GW of nuclear plants in the UK.

In particular geographic regions, peak wind speeds may not coincide with peak demand for electrical power. In the US states of California and Texas, for example, hot days in summer may have low wind speed and high electrical demand due to air conditioning. Some utilities subsidize the purchase of geothermal heat pumps by their customers, to reduce electricity demand during the summer months by making air conditioning up to 70% more efficient;[23] widespread adoption of this technology would better match electricity demand to wind availability in areas with hot summers and low summer winds. Another option is to interconnect widely dispersed geographic areas with an HVDC "Super grid". In the USA it is estimated that to upgrade the transmission system to take in planned or potential renewables would cost at least $60 billion.[24] Total annual US power consumption in 2006 was 4 thousand billion kWh.[25] Over an asset life of 40 years and low cost utility investment grade funding, the cost of $60 billion investment would be about 5% p.a. (i.e. $3 billion p.a.) Dividing by total power used gives an increased unit cost of around $3,000,000,000 × 100 / 4,000 × 1 exp9 = 0.075 cent/kWh.

In the UK, demand for electricity is higher in winter than in summer, and so are wind speeds.[26][27]

A report from Denmark noted that their wind power network was without power for 54 days during 2002.[28] Wind power advocates argue that these periods of low wind can be dealt with by simply restarting existing power stations that have been held in readiness or interlinking with HVDC.[20]

Three reports on the wind variability in the UK issued in 2009, generally agree that variability of wind needs to be taken into account, but it does not make the grid unmanageable; and the additional costs, which are modest, can be quantified.[29]

One way to account for intermittency is through a regional connection. If there is a sudden decrease in generation in one wind turbine or wind farm, nearby wind turbines and farms may be able to assist the grid in recovering lost frequency. Frequency controllers can extract kinetic energy from the wind turbine immediately or seconds later. This newfound capability to provide grid support may be the key to wind power's future success, according to a report from a coalition of researchers from universities, industry, and government, supported by the Atkinson Center for a Sustainable Future.[1]

There is still need to address disparities in resources and consumption at differing locations. Investment to reinforce the connection between regional grids will offer a partial solution.[1]

Capacity credit and fuel saving[edit]

Many commentators concentrate on whether or not wind has any "capacity credit" without defining what they mean by this and its relevance. Wind does have a capacity credit, using a widely accepted and meaningful definition, equal to about 20% of its rated output (but this figure varies depending on actual circumstances). This means that reserve capacity on a system equal in MW to 20% of added wind could be retired when such wind is added without affecting system security or robustness. But the precise value is irrelevant since the main value of wind, (in the UK, worth 5 times the capacity credit value[30]) is its fuel and CO2 savings.

References[edit]

  1. ^ a b c Zehnder and Warhaft, Alan and Zellman. "University Collaboration on Wind Energy". Cornell University. Retrieved 17 August 2011. 
  2. ^ a b http://www.nytimes.com/2008/08/27/business/27grid.html?pagewanted=all&_r=0
  3. ^ Power System Analysis and Design.Glover, Sarma, Overbye/ 5th Edition
  4. ^ http://www.pressherald.com/news/there-is-a-problem-withwind-power-in-maine_2013-08-04.html?pagenum=full
  5. ^ Demeo, E.A.; Grant, W.; Milligan, M.R.; Schuerger, M.J. (2005). "Wind plant integration". Power and Energy Magazine, IEEE 3 (6): 38–46. doi:10.1109/MPAE.2005.1524619. 
  6. ^ Zavadil, R.; Miller, N.; Ellis, A.; Muljadi, E. (2005). "Making connections". Power and Energy Magazine, IEEE 3 (6): 26–37. doi:10.1109/MPAE.2005.1524618. 
  7. ^ Wind Power: Capacity Factor, Intermittency, and what happens when the wind doesn’t blow? retrieved 24 January 2008.
  8. ^ Massachusetts Maritime Academy — Bourne, Mass This 660 kW wind turbine has a capacity factor of about 19%.
  9. ^ Wind Power in Ontario These wind farms have capacity factors of about 28–35%.
  10. ^ "The power of multiples: Connecting wind farms can make a more reliable and cheaper power source". 21 November 2007. 
  11. ^ Archer, C. L.; Jacobson, M. Z. (2007). "Supplying Baseload Power and Reducing Transmission Requirements by Interconnecting Wind Farms". Journal of Applied Meteorology and Climatology (American Meteorological Society) 46 (11): 1701–1717. Bibcode:2007JApMC..46.1701A. doi:10.1175/2007JAMC1538.1. 
  12. ^ "Tackling Climate Change in the U.S" (PDF). American Solar Energy Society. January 2007. Retrieved 5 September 2007. 
  13. ^ The UK System Operator, National Grid (UK) have quoted estimates of balancing costs for 40% wind and these lie in the range £500-1000M per annum. "These balancing costs represent an additional £6 to £12 per annum on average consumer electricity bill of around £390." "National Grid's response to the House of Lords Economic Affairs Select Committee investigating the economics of renewable energy". National Grid. 2008. 
  14. ^ A study commissioned by the state of Minnesota considered penetration of up to 25%, and concluded that integration issues would be manageable and have incremental costs of less than one-half cent ($0.0045) per kWh."Final Report – 2006 Minnesota Wind Integration Study" (PDF). The Minnesota Public Utilities Commission. 30 November 2006. Retrieved 15 January 2008. 
  15. ^ ESB National Grid, Ireland's electric utility, in a 2004 study that, concluded that to meet the renewable energy targets set by the EU in 2001 would "increase electricity generation costs by a modest 15%" "Impact of Wind Power Generation In Ireland on the Operation of Conventional Plant and the Economic Implications" (PDF). ESB National Grid. February 2004. p. 36. Archived from the original on 25 June 2008. Retrieved 23 July 2008. 
  16. ^ Sinclair Merz Growth Scenarios for UK Renewables Generation and Implications for Future Developments and Operation of Electricity Networks BERR Publication URN 08/1021 June 2008
  17. ^ Dr. V.C. Mason (December 2008). "Wind power in Denmark" (PDF). Retrieved 9 October 2013. 
  18. ^ http://www.claverton-energy.com/is-wind-power-reliable-an-authoritative-article-from-david-millborrow-who-is-technically-experienced-and-numerate-unlike-many-other-commentators.html
  19. ^ http://www.claverton-energy.com/download/316/
  20. ^ a b Realisable Scenarios for a Future Electricity Supply based 100% on Renewable Energies Gregor Czisch, University of Kassel, Germany and Gregor Giebel, Risø National Laboratory, Technical University of Denmark
  21. ^ Effects of Large-Scale Distribution of Wind Energy in and Around Europe
  22. ^ Mitchell 2006.
  23. ^ "Geothermal Heat Pumps". Capital Electric Cooperative. Retrieved 5 October 2008. 
  24. ^ Wind Energy Bumps Into Power Grid’s Limits Published: 26 August 2008
  25. ^ U.S. Electric Power Industry Net Generation retrieved 12 March 2009
  26. ^ David Dixon, Nuclear Engineer (9 August 2006). "Wind Generation's Performance during the July 2006 California Heat Storm". US DOE, Oakland Operations. 
  27. ^ Graham Sinden (1 December 2005). "Characteristics of the UK wind resource: Long-term patterns and relationship to electricity demand". Environmental Change Institute, Oxford University Centre for the Environment. 
  28. ^ "Why wind power works for Denmark" (PDF). Civil Engineering. May 2005. Retrieved 15 January 2008. 
  29. ^ Wind Energy Variability and Intermittency in the UK
  30. ^ Dr Graham Sinden, Oxford Environmental Change Institute: The implications of the Em’s 20/20/20 directive on renewable electricity generation requirements in the UK, and the potential role of offshore wind power in this context. (Graham Sinden has published a number of papers looking at the effects of integrating variable/intermittent generation into the generation mix)