Intermittent energy source

Intermittent power sources are sources of electric power generation that may be variable or intermittent, primarily sources of renewable energy such as wind and solar generated electricity. The variable nature of power generation from intermittent sources has raised concerns about the ability of electricity grids to absorb intermittent power and the economic implications. However, in small to moderate amounts, integration of intermittent power sources has little effect on grid operations.

Several studies have demonstrated the technical feasibility of integrating intermittent power at levels substantially higher than is common in most countries (from 15-30% penetration), and at least three countries have more than 20% wind penetration. Relatively few changes to large grids are normally required and the associated system costs are moderate. International groups are studying much higher penetrations (30-75%, corresponding to up to 20% of national electricity consumption) and preliminary conclusions are that these levels are also technically feasible.

Methods to manage wind power integration range from those that are commonly used at present (e.g. demand management) to potential new technologies for grid energy storage. Improved forecasting can also contribute as the daily and seasonal variations in wind and solar sources are to some extent predictable.

Intermittency and renewable energy

Two forms of intermittent renewable energy, wind and solar electricity generation, present challenges due to the short timeframe of changes in generation and, in many cases, the limited correlation with demand cycles.

Solar will vary greatly throughout the course of a diurnal cycle, and both may also be subject to a wide range of variation profiles based on season and location. In addition, day-to-day power generation may vary significantly (due to prevailing winds or cloud cover) with predictability estimated by weather services. Similarly, the ability of operators to control output for both is generally limited to curtailment or storage: power output can be decreased or stored, but generally not increased at will. Curtailment and storage of output are common features in electrical grids and for wind and solar installations, though reducing power sold to the grid may substantially affect project economics. Proponents of high penetrations of variable sources argue that a spot pricing or demand response will be required whereby pricing or demand is adjusted inversely with the variable output of the intermittent sources, and that this has an inherently low cost.

Hydropower can be intermittent and/or dispatchable, depending on the configuration of physical plant. Typical hydroelectric plants in the dam configuration may have substantial storage capacity, and be considered dispatchable. Run of the river hydroelectric generation will typically have limited or no storage capacity, and be intermittent on a seasonal or annual basis (dependent on rainfall and other factors). Hydroelectric dams have limits to dispatchability, since storage is finite and there are often environmental and regulatory requirements that regulate minimum and maximum release into the water system. Hydrostorage is also used in many locations to manage supply and demand, but true hydrostorage is not an energy source: it is a mechanism used to store excess generating capacity for use during times of greater demand.

Terminology

Several key terms are useful for understanding the issue of intermittent power sources. These terms are not standardized, and variations may be used. Most of these terms also apply to traditional generating plant.

• Nominal or nameplate capacity: This is the most common number used for referring to the normal maximum output of a generating source. For example, a wind turbine may be referred to as a 1.5 MW turbine, or a 120 MW wind farm.
• Capacity or average capacity factor: the average expected output of a generator, usually over an annual period. Generally stated as a percentage of the nameplate capacity. For wind, capacity factors are generally between 25-40% of the nameplate capacity, depending on the local wind resource.
• Capacity credit: generally, the amount of output that may be statistically relied upon during periods of peak demand. This figure will depend on the correlation of local production and demand features, and will not be directly comparable between different grids. Crucially, the correlation between different generating facilities is a key factor in determining overall system reliability. If different wind farms, for example, can separately be relied upon to produce 10% of peak demand, but they have low correlations between them because they are located far apart, their combined capacity contribution will be higher than 10% due to diversification. Estimates of the capacity credit for wind vary from 10-30%, although 10% is used in some locations as a baseline figure (due to lack of sufficient historical data, for example).
• Penetration or peak penetration refers to the nominal capacity over estimated peak demand in the system grid (for example, of wind-generated power) in percentage. As noted below, this figure does not provide much information beyond the scale of the amount of (potential) wind generation to peak demand. Most large systems have wind penetration of substantially less than 5%, although Denmark has approximately 44%, Spain 26%,Portugal 24%, and Germany 23%. At much higher penetrations, more appropriate measures and corrections to the gross penetration have been proposed to better characterize the issue of intermittency. Penetration may also be used to refer to the amount of energy generated as a percentage of annual consumption.

Intermittency: solar energy

Main article: Solar power

Intermittency inherently affects solar energy, as the production of electricity from solar sources depends on the amount of light energy in a given location. At current penetration levels, solar energy presents few issues for integration into existing electricity grids, and discussion of potential issues is highly theoretical. In principle, electric grids with high proportions of solar energy will also suffer from intermittency: electricity produced is not dispatchable and cannot be increased on demand.

The extent to which the intermittency of solar-generated electricity is an issue will also depend on the degree to which the generation profile of solar corresponds to demand cycles. In locales where air conditioning is a driver of demand and corresponds to periods of high sunlight, the intermittency may be less problematic. This may also correspond to locations with high solar production possibilities. For example, solar thermal power plants designed for solar-only generation (such as Nevada Solar One) are ideally matched to summer noon peak loads in prosperous areas with significant cooling demands, such as the south-western United States. Using thermal energy storage systems, solar thermal operating periods can even be extended to meet base-load needs. [2]

A variety of renewable energy sources used in combination can help to overcome intermittency. Stormy weather, bad for direct solar collection, is generally good for wind power and small hydropower plants; dry, sunny weather, bad for hydropower, is ideal for photovoltaics.[3]

Intermittency: wind energy

Main article: Wind power

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 factors). While the output from a single turbine can vary greatly and rapidly, as more turbines are connected over larger area, the slower and 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. 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). 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, and oncern 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).

A wind farm in Muppandal, Tamil Nadu, India

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 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.

Proponents argue that since a conventional dispatchable plant can be and is routinely cycled, operators 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 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 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.

Economic impacts of variability

Estimates of the cost of wind energy may include estimates of the "external" costs of wind variability, or be limited to the cost of production. All electrical plant has costs that are separate from the cost of production, including, for example, the cost of any necessary transmission capacity or reserve capacity in case of loss of generating capacity. Many types of generation, particularly fossil fuel derived, will also have cost externalities such as pollution, greenhouse gas emission, and habitat destruction which are generally not directly accounted for. The magnitude of the economic impacts is debated and will vary by location, but is expected to rise with higher penetration levels. At low penetration levels, costs such as operating reserve and balancing costs are believed to be insignificant.

Intermittency may introduce additional costs that are distinct from or of a different magnitude than for traditional generation types. These may include:

• Transmission capacity: transmission capacity may be more expensive than for nuclear and coal generating capacity due to lower load factors. Transmission capacity will generally be sized to projected peak output, but average capacity for wind will be significantly lower, raising cost per unit of energy actually transmitted.
• Additional operating reserve: if additional wind does not correspond to demand patterns, additional operating reserve may be required compared to other generating types, resulting in higher capital costs for additional plants. Contrary to statements that all wind must be backed by an equal amount of "back-up capacity", intermittent generators contribute to base capacity "as long as there is some probability of output during peak periods." Back-up capacity is not attributed to individual generators, as back-up or operating reserve "only have meaning at the system level."
• Balancing costs: to maintain grid stability, some additional costs may be incurred for balancing of load with demand. The ability of the grid to balance supply with demand will depend on the rate of change of the amount of energy produced (by wind, for example) and the ability of other sources to ramp production up or scale production down. Balancing costs have generally been found to be low.
• Storage, export and load management: at high penetrations (more than 30%), solutions (described below) for dealing with high output of wind during periods of low demand may be required. These may require additional capital expenditures, or result in lower marginal income for wind producers.

An official at Xcel energy claimed in December 2006 that at 20 percent penetration, additional standby generators to compensate for wind would cost $8 per MWh, adding between 13% and 16% to the $50-$60 cost per MWh of wind energy. Estimates from other sources have been lower, perhaps reflecting matters specific to that company's operating conditions.

Penetration

Penetration is most frequently cited in terms of nameplate (nominal maximum) capacity of wind to peak demand, generally uncorrected for actual production. Penetration may also be referred to as a percentage of annual production (or demand), which takes into account the actual or expected output of electricity.

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.

There is no generally accepted maximum level of penetration, as each system's capacity to compensate for intermittency differs, and the systems themselves will change over time. For most systems worldwide, existing penetration levels are significantly lower than practical or theoretical maximums; for example, a UK study found that "it is clear that intermittent generation need not compromise electricity system reliability at any level of penetration foreseeable in Britain over the next 20 years, although it may increase costs." As of 2006, Denmark has more than 40% penetration and at least two other countries (Portugal and Germany) have penetration levels (nominal to peak demand) of more than 20%.

• As the fraction of energy produced by wind ("penetration") increases, different technical and economic factors affect the need for grid energy storage facilities, 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, hydropower or other 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), although mixed wind/diesel systems have been used in isolated communities with success at relatively high penetration levels.
• In jurisdictions where the price paid to producers for electricity is based on market mechanisms, compensation to producers per unit is higher when they produce when demand is high and production low. The profitability of wind farms will therefore be higher if their production schedule coincides with these periods. If wind represents a significant portion of supply and wind farm output is highly correlated, overall revenues could be lower. In economic terms, the marginal revenue of the wind sector as penetration increases may diminish.
• If wind and other generating sources significantly exceed demand and mechanisms to export, store or otherwise divert this energy 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. In some 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).
• Although penetration is generally stated in terms of nameplate capacity (peak output) over peak demand, at higher penetrations of wind generation, penetration may be better measured as peak wind output over low demand plus export and storage. Variations on this approach may more accurately capture the likelihood of "excess" supply during periods of high wind output, and the ability of the system to economically absorb additional wind.
• Electricity demand is variable but generally very predictable on larger grids; errors in demand forecasting are typically no more than 2% in the minutes-hours-day ahead timeframe. Depending on the demand profile and location, local weather conditions - particularly temperature - may be the primary driver of demand, and the sensitivity of demand to prediction errors may be well understood. Wind energy production can also be forecast, but there is considerably less experience predicting wind speeds, and the time frame of forecasts and sensitivity factors less well understood. At present, error rates for predicting wind production at important timeframes for grid operators (hours and day-ahead) are significantly higher than for demand predictions.
• The maximum proportion of wind power allowable in a power system will thus depend on many factors, including the size of the system, the attainable geographical diversity of wind, the conventional plant mix (coal, gas, nuclear, hydroelectric) and seasonal load factors (heating in winter, air-conditioning in summer) and their statistical correlation with wind output. For most large systems the allowable penetration fraction (wind nameplate rating divided by system peak demand) is thus at least 15% without the need for energy storage. In addition, the interconnected electrical system may be much larger than the particular country or state (e.g. Denmark, California) being considered. A study published in October, 2006, by the Ontario Independent Electric System Operator (IESO) found that "there would be minimal system operation impacts for levels of wind capacity up to 5,000 megawatts (MW)," which corresponds to 17% of projected peak load (nameplate wind capacity over peak load); at the time of publication, Ontario had only 300 MW of installed wind capacity. While there are both practical and theoretical upper limits (as with any type of electric power generation), these upper limits are frequently many times higher than existing installed capacity.
• Wind power generation tends to be higher in the winter and at night (due to higher air density), so the appropriateness of wind power in high concentrations may crucially depend on the prevalence of air conditioning in a given jurisdiction. Wind power may be weakest in the hot summer months, and particularly during the day when air conditioning demand is highest. Conversely, systems where heat is electrical may be well-suited to higher penetration of wind power.

Geographic diversity

The variability of production from a single wind turbine can be high. Combining any additional number of turbines (for example, in a wind farm) results in lower statistical variation, as long as the correlation between the output of each turbine is imperfect, and the correlations are always imperfect due to the distance between each turbine. Similarly, geographically distant wind turbines or wind farms have lower correlations, reducing overall variability. Since wind power is dependent on weather systems, there is a limit to the benefit of this geographic diversity for any power system.

While wind power is variable, it is reliable in the sense that simultaneous failure of a large number of units (and the associated loss of generation capacity) is unlikely. It is highly improbable that a large number of wind turbines could fail simultaneously; concentrated transmission systems may be more likely points of failure. This should be contrasted with large single-source generators (nuclear, fossil, or other), which can go off-line in short periods of time; all power systems incorporate some reserve to compensate for such "single-source" losses of generating capacity.

Multiple wind farms spread over a wide geographic area and gridded together produce power more constantly and with less variability than smaller installations. Wind output can be predicted with some degree of confidence using weather forecasts, especially from large numbers of turbines/farms. The ability to predict wind output is expected to increase over time as data is collected, especially from newer facilities.

Compensating for variability

As noted, all sources of electrical power have some degree of unpredictability, and demand patterns (while relatively predictable) routinely drive large swings in the amount of electricity that suppliers feed into the grid. Wherever possible, grid operations procedures are designed to match supply with demand at high levels of reliability, and the tools to influence supply and demand are well-developed. The introduction of a power source in large amounts, however, that has a random element, relatively large differences between peak and trough output, and that may not be well-matched to demand cycles may require changes to existing procedures and additional investments.

Operational reserve & peak demand reduction

At times of high or increasing demand where wind output may simultaneously be falling, a number of solutions are either commonly used today or potentially feasible. Since all managed grids already have existing operational and "spinning" reserve, the amount of incremental reserve required for wind may be inconsequential. Contrary to perceptions, the addition of intermittent resources such as wind does not require 100% "back-up" or reserve plant; operating reserves and balancing requirements are calculated on a system-wide basis, and not dedicated to specific generating plant.

• Because conventional powerplants can drop off the grid within a few seconds, for example due to equipment failures, in most systems the output of some coal or gas powerplants is intentionally part-loaded to follow demand and to replace rapidly lost generation. The ability to follow demand (by maintaining constant frequency) is termed "response." The ability to quickly replace lost generation, typically within timescales of 30 seconds to 30 minutes, is termed "spinning reserve." Nuclear power plants in contrast are not very flexible and are not intentionally part-loaded. A power plant that operates in a steady fashion, usually for many days continuously, is termed a "base load" plant. Generally thermal plants running as "peaking" plants will be less efficient than if they were running as base load. Hydroelectric facilities with storage capacity (such as the traditional dam configuration) may be operated as base load or peaking plants, and complement high levels of wind penetration.
• In practice, as the power output from wind varies, part-loaded conventional plants, which must be there anyway to provide response (due to continuously changing demand) and reserve, adjust their output to compensate; they do this in response to small changes in the frequency (nominally 50 or 60 Hz) of the grid.
• Energy Demand Management or Demand-Side Management refers to the use of communication and switching devices which can release deferrable loads quickly, or absorb additional energy to correct supply/demand imbalances. Incentives can be created for the use of these systems, such as favorable rates or capital cost assistance, encouraging consumers with large loads to take advantage of renewable energy by adjusting their loads to coincide with resource availability. For example, pumping water to pressurize municipal water systems is an electricity intensive application that can be performed when electricity is available. Real-time variable electricity pricing can encourage all users to reduce usage when the renewable sources happen to be at low production.

Storage and demand loading

At times of low or falling demand where wind output may be high or increasing, grid stability may require lowering the output of various generating sources or even increasing demand, possibly by using energy storage to time-shift output to times of higher demand. Such mechanisms can include:

• Long-term storage of electrical energy involves substantial capital costs, space for storage facilities, and some portion of the stored power will be lost during conversion and transmission. The percentage retrievable from stored power is called the "efficiency of storage." The cost of compensating for the variability of wind has been studied extensively at low to medium penetrations, but would be expected to rise with higher penetration levels; the increase in costs with significantly higher penetration may be non-linear as the variability becomes more significant at higher levels, and particularly if storage needs to be purpose-built for wind. Pumped storage hydropower is the most prevalent existing technology used, and can substantially improve the economics of wind power..See also: Grid energy storage
• In energy schemes with a high penetration of wind energy, secondary loads, such as desalination plants and electric boilers, may be encouraged because their output (water and heat) can be stored. The utilization of "burst electricity", where excess electricity is used on windy days for opportunistic purposes greatly improves the economic efficiency of wind turbine schemes. An ice storage device has been invented which allows cooling energy to be consumed during resource availability, and dispatched as air conditioning during peak hours. Various other potential applications are being considered, such as charging plug-in electric vehicles during periods of low demand and high production; at present, the scale at which such technologies are employed is relatively low.
• In Colorado, a test facility financed with the cooperation of the National Renewable Energy Laboratory will produce hydrogen from wind power that will be used for electricity production during peak hours. This hydrogen could also be used in hydrogen vehicles. Production costs are estimated at $8 per kilogram (roughly the equivalent of one U.S. gallon of gasoline), or approximately "three times as expensive as using gasoline". This cost estimate appears to be based on retail gasoline prices of approximately $2.65 per gallon, and without consideration of pollution externality; no public cost breakdown is available. Although this cost may not be considered economically competitive at present, the costs may come down in future as the technology is proven. For a grid with "excess" wind power, any additional marginal revenue from producing hydrogen, for example, may still improve the economics of wind and hence allow for higher penetrations.
• One solution currently being piloted on wind farms and in other applications, is the use of rechargeable flow batteries as a rapid-response storage medium [4]. Vanadium redox flow batteries are currently installed at Huxley Hill wind farm (Australia), Tomari Wind Hills at Hokkaido (Japan). A further 12 MWh flow battery is to be installed at the Sorne Hill wind farm (Ireland) [5]. The supplier concerned is commissioning a production line to meet other anticipated orders.

Complementary power sources and matching demand

• Electricity produced from solar energy could be a counter balance to the fluctuating supplies generated from wind. In some locations, it tends to be windier at night and during cloudy or stormy weather, so there is likely to be more sunshine when there is less wind.
• In some locations, electricity demand may have a high correlation with wind output, particularly in locations where cold temperatures drive electric consumption (as cold air is denser and carries more energy).
• The allowable penetration may be further increased by increasing the amount of part-loaded generation available. Systems with existing high levels of hydroelectric generation may be able to incorporate substantial amounts of wind, although high hydro penetration may indicate that hydro is already a low-cost source of electricity; Norway, Quebec, and Manitoba all have high levels of existing hydroelectric generation (Quebec produces over 90% of its electricity from hydropower, and the local utility, Hydro-Québec, is the largest single hydropower producer in the world). The US Pacific Northwest has been identified as another region where wind energy is complemented well by existing hydropower, and there were "no fundamental technical barriers" to integrating up to 6000 MW of wind capacity. Storage capacity in hydropower facilities will be limited by size of reservoir, and environmental and other considerations.
• Existing European hydroelectric power plants can store enough energy to supply one month's worth of European electricity consumption. Improvement of the international grid would allow using this in the relatively short term at low cost, as a matching variable complementary source to wind power. Excess wind power could even be used to pump water up into collection basins for later use. In practice, Denmark's system is well-integrated with the hydro-electric dominated Norwegian system, and Norwegian hydropower is used to balance fluctuations and shortfalls in Denmark; on occasion, Denmark exports electricity to Norway when generation is higher than demand (thereby increasing stored hydropower). Increased wind penetration may raise the value of existing peaking or storage facilities and particularly hydroelectric plants, as their ability to compensate for wind's variability will be under greater demand.

Export & import arrangements with neighboring systems

• It is often feasible to export energy to neighboring grids at times of surplus, and import energy when needed. This practice is common in Western Europe and North America.
• This linking of systems means that the relevant "penetration area" for wind systems may be considerably larger than the political entity or grid operator's area, making penetration for wind even less an issue. Denmark's 44% penetration, in the context of the German/Dutch/Scandinavian grids with which it has interconnections, is considerably lower as a proportion of the total system.
• Since correlation between wind turbine outputs decreases with distance, the integration of grids may decrease the overall variability, as long as the wind turbines themselves are located at a considerable distance and not in the same weather system. For example, Germany's wind farms are predominantly located in the north of the country, reducing the value of the distance benefit due to the proximity to Denmark.
• If neighboring grids both have significantly high levels of wind generation, and correlations between turbines are greater than zero, the likelihood that both will experience periods of high or low wind output at the same time may reduce the value of export/import arrangements, and also depend on the generation profile of other sources.
• Transmission capacity for export may have to be substantially upgraded. Substantial transmission upgrades are already required within many countries, in some cases partly due to plans to significantly increase wind capacity.
• Ultimately, the ability to export at will presupposes the presence of sufficient demand in the export market at prices sufficient to justify the investment in wind capacity and any additional transmission capacity required. At small and moderate penetration levels this may be a reasonable assumption, but at very high penetration levels will need to be demonstrated in practice.

Maximum penetration limits

There is no generally accepted maximum penetration of wind energy that would be feasible in any given grid. Rather, economic efficiency and cost considerations are more likely to dominate as critical factors; technical solutions may allow higher penetration levels to be considered in future, particularly if cost considerations are secondary.

High penetration scenarios may be feasible in certain circumstances:

1. Power generation for periods of little or no wind generation can be provided by retaining the existing power stations. The cost of using existing power stations for this purpose may be low since fuel costs dominate the operating costs. The actual cost of paying to keep a power station idle, but usable at short notice, may be estimated from published spark spreads and dark spreads. As existing traditional plant ages, the cost of replacing or refurbishing these facilities will become part of the cost of high-penetration wind if they are used only to provide operational reserve.

2. The aggregate maximum rate of change of generation from a close to 100% wind scenario may be lower than the existing rate of change of total generation due to unscheduled power station outages (for example, in the UK). The aggregate rate of change of output of such a scenario may also be smaller than the rate at which power stations can be warmed / started and ramped up, and successive short term weather forecasts (from 12 hours for an initial forecast to 5 minutes for final balancing) may be sufficient to schedule sufficient running plant, warming plant and spinning reserve plant.

3. Automatic load shedding of large industrial loads and its subsequent automatic reconnection is established technology and used in the UK and US, and known as Frequency Service contractors in the UK. Several GW are switched off and on each month in the UK in this way. Reserve Service contractors offer fast reponse gas turbines and even faster diesels in the UK, France and US to control grid stability.

4. In a close-to-100% wind scenario, surplus wind power can be allowed for by increasing the levels of the existing Reserve and Frequency Service schemes whereby a rise in system frequency would automatically connect loads, and disconnect them later, and by extending the scheme to domestic-sized loads. This means energy can either be stored by advancing deferrable domestic loads such as storage heaters, water heaters, fridge motors, or even hydrogen production. This would still result in lower revenue for wind generation and involve additional costs, but is in theory technically feasible. Under a high-penetration wind scenario, the amount of load shedding and reconnection would likely be several times higher, presenting challenges for system stability.

5. Alternatively or additionally, power can be exported to neighboring grids and re-imported later. EHVDC cables are efficient at 3% loss per 1000 km and may be inexpensive in certain circumstances. Under such scenarios, the amount of transmission capacity required may be many times higher than currently available.

External links

• European Wind Energy Association, Large Scale Integration of Wind Energy in the European Power Supply: analysis, issues, and recommendations, December 2005
• New York Times article on wind intermittency, "It’s Free, Plentiful and Fickle"
• Ontario grid operator data on current and historical output from all generator sources, including wind, showing variability in wind output
• Power Point presentation showing how national generating systems are actually controlled in detail.
• ESB National Grid (Ireland), "Impact of Wind Power Generation In Ireland on the Operation of Conventional Plant and the Economic Implications", 2004
• The Costs and Impacts of Intermittency, UK Energy Research Council, March 2006
• Grid Integration of Wind Energy