Intermittent energy source: Difference between revisions

Revision as of 17:44, 27 March 2007

Revision as of 18:08, 27 March 2007

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{{main|Wind power}}

[[Image:BlueCircleSouthernCementBerrimaNSW.JPG|thumb|Cement works in [[New South Wales]], [[Australia]]. Energy-intensive process like this could utilize burst electricity from wind.]]

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 thingsfactors). While the output from a single turbine can vary greatly and rapidly, as more turbines are connected over larger area, the slower and often less variable the aggregate rate of change becomes. 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, unless the wind resources are continuous or predictable in nature. Variability may be a more accurate term to describe wind's generation profile than intermittency, which may imply an alternating presence or absence (generation that is either on or off), depending on the season and ___location profiles. 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.

As wind energy installations grow in absolute terms and as a proportion of existing output, critics have raised concerns about integrating wind energy into existing grids due to the variability of power output from wind. In combination with the output profile of existing power plants, concern has been expressed about the extent to which wind can be relied upon for output during periods of high demand, sometimes referred to as its base capacity factor. Proponents claim this is a misstatement or misunderstanding of the windpower case, which assumes little capacity benefit, and argue that the main economic benefit comes from fuel displacement rather than capacity replacement (overall energy output versus replacement of peak generating output).

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The annual capacity factor is the average output over a year divided by the turbine's nameplate capacity. The base capacity factor is primarily a function of the statistically reliable output of wind during the period of peak demand in a given area, as these are the times when the capacity to meet demand without threatening grid stability or shortages will be most critical.

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 requirementsbe stored at times of low demand or to sold to neighboring grids. As with other generating plant, wind energy output may on occasion need to be curtailed, energy stored for later use, or loaddemand 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 could make both wind farms and dispatchable power plants less profitable. Energy storage used to arbitrage between periods of low and high demand may be capital intensive and always incurs some efficiency losses.

Proponents argue that since largea conventional dispatchable plant can be and is routinely cycled, conventional dispatchable plantoperators may curtail its output rather than wind plant. All conventional powerplants 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 however.limited, Surplussurplus power may also be sold to neighbouring grids and re-imported at times of shortfall, although import and export capacity may be limited.

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. Very few grids have wind energy penetration above these levels. Some studies have considered penetration above these levels: a [[Minnesota]] study<ref>http://www.puc.state.mn.us/docs/windrpt_vol%201.pdf Minnesota study on wind penetration levels</ref> 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. The specific costs and issues will vary from region to region.

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At high penetrations, the expected peak production of wind-generated electricity may be more important; while there is no generally accepted measure of the relevant proportion, measures may include expected peak wind output minus firm export capacity over minimum demand levels (and possibly corrected for minimum base-load generation that cannot be economically shut down, such as nuclear). All of these figures should be treated and used with caution, as the relevance or significance (or any implied limits) will be highly dependent on local factors, grid structure and management, and existing generation capacity.

* As the fraction of energy produced by wind ("penetration") increases, different technical and economic factors affect the need for [[grid energy storage]] facilities, [[Energy demand management|demand side management]], grid import/export, and/or other management of system load. Large networks, connected to multiple wind plants at widely separated geographic locations, may accept a higher penetration of wind than small networks or those without storage systems or economical methods of compensating for the variability of wind. In systems with significant amounts of existing [[Pumped-storage hydroelectricity|pumped storage]], [[hydropower]] or other [[Peaking power plant|peaking power plants]], such as natural gas-fired power plants, this proportion may be higher. Isolated, relatively small systems with only a few wind plants may only be stable and economic with a lower fraction of wind energy (e.g. Ireland).

* In jurisdictions where the price paid to producers for electricity is based on market mechanisms, revenue for all producers per unit is higher when they produce when prices are higher. The profitability of wind farms will therefore be higher if their production schedule coincides with these periods (generally, high demand / low supply situations). If wind represents a significant portion of supply and wind farm output is highly correlated, overall revenues would be lower. In economic terms, the [[marginal revenue]] of the wind sector as penetration increases may diminish.

* Depending on the profile of other generation, strong wind generation at times of low demand may result in an excess of supply, which can harm grid stability, as certain generation types are not maneuverable. If mechanisms to export, store or otherwise divert this energy do not exist or are insufficient, wind turbines may have to curtail their output (for example, by changing the pitch of the turbine blades). This is a normal operating procedure that can be handled by turbine operators and control software. It reduces, however, the revenue generated by the wind plant and will affect the economic viability of wind production. Some jurisdictions - notably Germany - require grid operators to purchase from renewable sources first. In other cases, grid pricing procedures may allow for nil or negative prices, providing incentives to market participants to curtail production or increase load (for example, for storage). Excess supply events which may require curtailment can be expected to increase with wind penetration, which may also encourage the development of storage solutions.