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Wind Power Intermittency and Penetration Limits
IntermittencyElectricity 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. 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 energy demand management, load shedding, or storage solutions. 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. Pumped-storage hydroelectricity or other forms of grid energy storage can store energy developed by high-wind periods and release it when needed.[16] 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 wind energy. Peak wind speeds may not coincide with peak demand for electrical power. In California and Texas, for example, hot days in summer may have low wind speed and high electrical demand due to air conditioning. In the UK, however, winter demand is higher than summer demand, and so are wind speeds.[17][18][19] Solar power tends to be complementary to wind;[20][21] on most days with no wind there is sun and on most days with no sun there is wind. A demonstration project at the Massachusetts Maritime Academy's shows the effect. A combined power plant linking solar, wind, bio-gas and hydrostorage is proposed as a way to provide 100% renewable power.[22] The 2006 Energy in Scotland Inquiry report expressed concern that wind power cannot be a sole source of supply, and recommends diverse sources of electric energy. [23] A report from Denmark noted that their wind power network was without power for 54 days during 2002.[24] 
Wind-generated power is a variable resource, and the amount of wind-generated electricity produced at any given point in time by a given plant will depend on wind speeds and turbine characteristics (among other factors). While the output from a single turbine can vary greatly and rapidly, as more turbines are connected over larger and larger areas, the slower and less variable the aggregate rate of change becomes, and as the number of units need to replace a single large conventional unit is very high, these cannot all fail simultaneously. As compared to many other types of electricity generation, wind is not normally dispatchable - it cannot be turned on at will by human or automatic dispatch to meet increased demand. Variability may be a more accurate term to describe wind's generation profile than intermittency, which implies an alternating presence or absence (generation that is either on or off).[6] In discussions of the pros and cons of wind power, the issue of variable power output may be termed intermittency or variability without distinction between the two terms. Of great concern is the performance of wind power during heat waves, because that is typically the yearly peak electricity demand for most temperate to hot climates. A study during the 2006 California heat storm revealed that output from wind power in California significantly decreased to less than 5% during peak demand. [7] A similar result was seen during the 2003 European heat wave, when the output of wind power in Germany fell to 10% during peak demand times, resulting in importing a peak of around 2,000 MW of electricity.[8] It has been said that, "the development and expansion of well-functioning day-ahead and real time markets will provide an effective means of dealing with the variability of wind generation." [9] However, the experience of Denmark, which has one of the greatest percentages of wind power utilisation in the world, would suggest otherwise. The ICE report on the Danish experience stated that wind power was so variable that Denmark exported most of its wind power, rather than use it itself. In addition, in 2002 the entire system had a total of 54 days without usable power generation. The report concluded that it would be difficult for "island" grid countries like Britain to use a large percentage of wind power in combination with slower reacting thermal power stations.[10] A similar report by the Renewable Energy Foundation confirms the problems experienced in Denmark and goes on to indicate that the UK may experience a rise in CO2 emissions through using wind power:
In the UK serous academic commentators such as Graham Sinden, of Oxford University argue that this issue of capacity credit is a "red herring" in that the value of wind generation is largely due to the value of displaced fuel, not any perceived capacity credit - it being well understood by the wind energy proponents that conventional capacity will be retained to "fill in" during periods of low or no wind. [11] 
Critics also argue that the economics of wind energy may be challenged when wind production is high at times of low demand. Due to the presence of other generating stations that are operated as base load (run as close to continuously as possible) or have minimum operating cycles, at high penetrations wind plants may contribute to the grid producing energy "surplus" to immediate local demand. As with other generating plant, wind energy output can be stored for future use, or may on occasion need to be curtailed or demand increased to compensate. While all of these solutions are commonly used to manage grids, wind "spilt" or curtailed generates no revenue, and prices for supply to the grid may be lower at times of high output, both of which may make both wind farms and dispatchable power plants less profitable. Energy storage used to arbitrage between periods of low and high demand always incurs some efficiency losses, though helps to mitigate demand peaks. Proponents argue that since a conventional dispatchable plant can be and is routinely cycled, operators may curtail its output rather than a wind plant. All conventional power plants have limits to the extent to which they can be cycled up and down and over different time periods and efficiency limits in certain circumstances. Although import and export capacity may be limited, surplus power may also be sold to neighbouring grids and re-imported at times of shortfall. Both shortfalls and surpluses of supply attributable to wind energy's variability will be less frequent at lower penetration levels. At low to medium levels of penetration (up to 15%), incremental regulation and operational reserve requirements are generally marginal, and demands for reduced supply (curtailment) infrequent. At lower levels (less than 5%), wind may simply be treated as "negative load" in the larger system or statistical "noise" in a large system. Very few grids have wind energy penetration above these levels. Many studies have considered penetration above these levels: a Minnesota study[12] 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. A diversity of renewable energy sources, each serving fewer and nearer users, would also greatly restrict the area blacked out if a grid connecting them failed. And when renewable energy sources do fail, they generally fail for shorter periods than do large power plants.[13] Penetration LimitsWind 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.[25] 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. At present, few grid systems have penetration of wind energy above 5%: Denmark (values over 18%), Spain and Portugal (values over 9%), 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.[26] 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.[27]
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Wind Power" |
