Intermittent power source
Encyclopedia
An intermittent energy source is any source of energy that is not continuously available due to some factor outside direct control. The intermittent source may be quite predictable, for example, tidal power
, but cannot be dispatched
to meet the demand of a power system. Examples of intermittent sources include wind and solar power. Effective use of intermittent sources in an electric power grid usually relies on using the intermittent sources to displace fuel that would otherwise be consumed by non-renewable power station
s, or by storing energy in the form of renewable pumped storage, compressed air or ice, for use when needed, or as electrode heating for district heating schemes.
The storage of energy to fill the shortfall intermittency or for emergencies is part of a reliable
energy supply. The capacity of a reliable renewable energy
supply, can additionally be fulfilled by the use of latency measure
s and backup or extra infrastructure and technology
, using mixed renewables to produce electricity above the intermittent average
, which may be utilised to meet regular and unanticipated supply demands.
The penetration of intermittent renewables in most power grids is low, but wind for example generates 11% of electric energy in Spain
and Portugal
, 9% in the Republic of Ireland, and 7% in Germany
. Wind provides nearly 20% of the electricity generated in Denmark
, however this percentage forces Denmark to import and export large amounts of energy to and from the EU grid, to balance supply with demand.
The use of small amounts of intermittent power has little effect on grid operations. Using larger amounts of intermittent power may require upgrades or even a redesign of the grid infrastructure.
The extent to which the intermittency of solar-generated electricity is an issue will depend to some extent on the degree to which the generation profile of solar corresponds to demand. For example, solar thermal power plants such as Nevada Solar One
are somewhat matched to summer peak loads in areas with significant cooling demands, such as the south-western United States. Thermal energy storage systems can improve the degree of match between supply and consumption. The increase in capacity factor of thermal systems does not represent an increase in efficiency, but rather a spreading out of the time over which the system generates power.
Wind-generated power is a variable resource, and the amount of electricity produced at any given point in time by a given plant will depend on wind speeds, air density, and turbine characteristics (among other factors). If wind speed is too low (less than about 2.5 m/s) then the wind turbines will not be able to make electricity, and if it is too high (more than about 25 m/s) the turbines will have to be shut down to avoid damage. While the output from a single turbine can vary greatly and rapidly as local wind speeds vary, as more turbines are connected over larger and larger areas the average power output becomes less variable.
According to a study of wind in the United States, ten or more widely-separated wind farms connected through the grid could be relied upon for from 33 to 47% of their average output (15–20% of nominal capacity) as reliable, baseload
power, as long as minimum criteria are met for wind speed and turbine height. When calculating the generating capacity available to meet peak demand, [ERCOT] (manages Texas grid) counts wind generation at 8.7% of nameplate capacity.
Because wind power is generated by large numbers of small generators, individual failures do not have large impacts on power grids. This feature of wind has been referred to as resiliency.
Wind power is affected by air temperature because colder air is more dense and therefore more effective at producing wind power. As a result, wind power is affected seasonally (more output in winter than summer) and by daily temperature variations. During the 2006 California heat storm
output from wind power in California
significantly decreased to an average of 4% of capacity for 7 days. A similar result was seen during the 2003 European heat wave
, when the output of wind power in France, Germany, and Spain fell below 10% during peak demand times.
According to an article in EnergyPulse, "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."
Nuclear plants have an additional disadvantage; for safety, they must instantly shut down in a power failure, but for nuclear-physics reasons, they can’t be restarted quickly. For example, during the Northeast Blackout of 2003
, nine operating U.S. nuclear units had to shut down and were later restarted. During the first three days, while they were most needed, their output was less than 3% of normal. After twelve days of restart, their average capacity loss had exceeded 50 percent.
Mark Z. Jacobson
has studied how wind, water and solar technologies can be integrated to provide the majority of the world's energy needs. He advocates a "smart mix" of renewable energy
sources to reliably meet electricity demand:
Mark A. Delucchi and Mark Z. Jacobson argue that there are at least seven ways to design and operate renewable energy systems so that they will reliably satisfy electricity demand:
Technological solutions to mitigate large scale wind energy type intermittency exist such as increased interconnection (the European super grid
), Demand response
, load management
, diesel generators (in National Grid
), Frequency Response / National Grid Reserve Service type schemes, and use of existing power stations on standby. Studies by academics and grid operators indicate that the cost of compensating for intermittency is expected to be high at levels of penetration above the low levels currently in use today Large, distributed power grids are better able to deal with high levels of penetration than small, isolated grids. For a hypothetical European-wide power grid, analysis has shown that wind energy penetration levels as high as 70% are viable, and that the cost of the extra transmission lines would be only around 10% of the turbine cost, yielding power at around present day prices. Smaller grids may be less tolerant to high levels of penetration.
Matching power demand to supply is not a problem specific to intermittent power sources. Existing power grids already contain elements of uncertainty including sudden and large changes in demand and unforeseen power plant failures. Though power grids are already designed to have some capacity in excess of projected peak demand to deal with these problems, significant upgrades may be required to accommodate large amounts of intermittent power. The International Energy Agency (IEA)
states:
"In the case of wind power, operational reserve is the additional generating reserve needed to ensure that differences between forecast and actual volumes of generation and demand can be met. Again, it has to be noted that already significant amounts of this reserve are operating on the grid due to the general safety and quality demands of the grid. Wind imposes additional demands only inasmuch as it increases variability and unpredictability. However, these factors are nothing completely new to system operators. By adding another variable, wind power changes the degree of uncertainty, but not the kind..."
UK academic commentator Graham Sinden, of Oxford University, argues 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. The main value of wind, (in the UK, 5 times the capacity credit value) is its fuel and CO2 savings. Wind does not require any extra back-up, as is often wrongly claimed, since it uses the existing power stations, which are already built, as back-up, and which are started up during low wind periods, just as they are started up now, during the non availability of other conventional plant. More spinning reserve, of existing plant, is required, but this again is already built and has a low cost comparatively.
Hydroelectric power is usually extremely dispatchable and more reliable than other renewable energy sources. Many dams can provide hundreds of megawatts within seconds of demand (see Ffestiniog power station
). The exact nature of the power availability depends on the type of plant.
In run-of-the-river hydroelectricity
, power availability is highly dependent on the flow of the river, making this type of generation mostly suitable only at locations where flow levels are controlled by upstream dams.
In conventional hydroelectric plants, there is a reservoir and a one-way generator. The water flow through its turbines can be adjusted frequently to meet changing demand throughout the day by running the generator when demand is high and not running it when demand is low.
Pumped-storage hydroelectricity
can make an even more significant contribution to peaking ability of the grid. These just move water between reservoirs and are powered by power from the grid when demand is low and put power back into the grid when demand is high. There also exist combined pump-storage plants that use river flow as well as extra pumping when demand is low, such as the 240 MW Lewiston Pump-Generating Plant.
Direct pumped-storage does not contribute any net generation to the grid, in fact, it increases the fuel used by other power plants because there is inefficiency in the turbine/generator. The economic benefit of pumped-storage plants lies only in increasing the capacity of the grid. This type of plant works well on a grid with many nuclear or renewable energy plants because the fuel is very cheap or essentially free, so it costs very little to keep them running at high power during the night when demand is low. Both pump-storage plants and natural flow hydro plants can help allow for intermittency of other plants by running at higher capacity for short times, but assistance is limited by the total capacity of the hydroelectric plant.
Conventional power plants (as well as nuclear plants) use water for cooling, and water shortages during hot summer months have occasionally resulted in periods when output has had to be curtailed, notably in France in 2006
.
Conventional power plant failures can remove large amounts of capacity from the grid suddenly, resulting in blackouts.
Every year or two (depending on the plant), the plant must be shut down for planned outages for about a month. This is typically done in the spring or autumn (fall) when electricity demand is lower, as such, on a national scale power output from nuclear increases corresponding with demand during the peak summer and winter months. This change in output commonly occurs on a yearly basis.
It is rare that nuclear power plants adjust their power output to correspond with demand on a daily basis because pressurized water reactors (PWR, which are the vast majority of nuclear power plants) use a chemical shim
in the moderator–coolant to control their power level. (Boiling water reactors (BWR), however, can use a combination of control rod
s and recirculation water flow speed to control their power level, and so in markets such as Chicago
, Illinois
where half of the local utility's fleet is BWRs it is common to load-follow
although less economic to do so.)
In the UK one of the key criteria for determining the amount of required spinning reserve is the possible loss of Sizewell B, a 1.2 GW nuclear power plant.
At one point in the fall of 2007, out of 16 nuclear power stations in the UK, seven were offline due to a combination of planned and unplanned outages.
In USA and UK these diesels have usually been purchased for other reasons e.g. for emergency standby, in water works, hotels, hospitals, etc. and in some cases for electricity substations - e.g. Cuyahoga Falls, USA (10 × 1.6 MW Caterpillar) and Tregarron Mid Wales UK (3 × 1.6 MW Caterpillar), but can be readily used to automatically synchronize and feed into the grid.
In the UK 500 MW of such plant is routinely started within a few minutes; this is perfectly acceptable to the engines' service life in a scheme operated by National Grid called National Grid Reserve Service. It has been established that there is 20 GW of such diesel plant in the UK and it has been pointed out that there is no technical reason why this quantity could not be brought into the Reserve Service scheme to assist handling very rapid changes in renewable output, whilst conventional plant is started or indeed stopped.
Storage of electrical energy results in some lost energy because storage and retrieval are not perfectly efficient. Storage may also require substantial capital investment and space for storage facilities.
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.
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.
The level of penetration of intermittent variable sources is significant for the following reasons:
Renewable electricity supply in the 20-50+% penetration range has already been implemented in several European systems, albeit in the context of a integrated European grid system:
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. Discussion of acceptable or unacceptable penetration figures should be treated and used with caution, as the relevance or significance will be highly dependent on local factors, grid structure and management, and existing generation capacity.
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."
High penetration scenarios may be feasible in certain circumstances:
using HVDC cables, indicates that the entire European power usage could come from renewables, with 70% total energy from wind at the same sort of costs or lower than at present. This proposed large European power grid has been called a "super grid
."
The model deals with intermittent power issues by using base-load renewables such as hydroelectric and biomass for a substantial portion of the remaining 30% and by heavy use of HVDC to shift power from windy areas to non-windy areas. The report states that "electricity transport proves to be one of the keys to an economical electricity supply" and underscores the importance of "international co-operation in the field of renewable energy use [and] transmission."
Dr. Czisch described the concept in an interview, saying "For example, if we look at wind energy in Europe. We have a winter wind region where the maximum production is in winter and in the Sahara region in northern Africa the highest wind production is in the summer and if you combine both, you come quite close to the needs of the people living in the whole area - let's say from northern Russia down to the southern part of the Sahara."
indicates that it would be feasible to accommodate 42% (of demand) renewables
in the electricity mix. This acceptable level of renewable penetration was found in what the study called Scenario 5, provided 47% of electrical capacity (different from demand) with the following mix of renewable energies:
The study cautions that various assumptions were made that "may have understated dispatch restrictions, resulting in an underestimation of operational costs, required wind curtailment, and CO2 emissions" and that "The limitations of the study may overstate the technical feasibility of the portfolios analyzed..."
Scenario 6, which proposed renewables providing 59% of electrical capacity and 54% of demand had problems. Scenario 6 proposed the following mix of renewable energies:
The study found that for Scenario 6, "a significant number of hours characterized by extreme system situations occurred where load and reserve requirements could not be met. The results of the network study indicated that for such extreme renewable penetration scenarios, a system re-design is required, rather than a reinforcement exercise." The study declined to analyze the cost effectiveness of the required changes because "determination of costs and benefits had become extremely dependent on the assumptions made" and this uncertainty would have impacted the robustness of the results.
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:
claimed that at 20 percent penetration, additional standby generators to compensate for wind in Colorado would cost $8 per MWh, adding between 13% and 16% to the $50–$60 cost per MWh of wind energy.
The Union of Concerned Scientists
conducted a study of the costs to increase the renewable penetration in Colorado to 10% and found that for an average residential bill "customers of municipal utilities and rural electric cooperatives that opt out of the solar energy requirement" would save 4 cents per month, but that for Xcel Energy
customers there would be additional cost of about 10 cents per month. Total impact on all consumers would be $4.5 million or 0.01% over two decades.
In evidence to the UK House of Lords Economic Affairs Select Committee, National Grid 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 notes that "increasing levels of such renewable generation on the system would increase the costs of balancing the system and managing system frequency."
A 2003 report by Carbon Trust and the UK Department of Trade and Industry (DTI) found that the costs for reinforcement and new build of transmission and distribution systems to support 10% renewable electricity in the UK by 2010 would be £1.6 to £2.4 billion. The study classified "Intermittency" as "Not a significant issue" for the 2010 target. The same 2003 study found that achieving 20% renewable electricity in the UK by 2020 would cost £3.2bn to £4.5bn in transmission and distribution system construction and reinforcement. The study classified "Intermittency" as a "Significant Issue" for the 2020 target.
has no intermittency.
states that:
On December 10, 2007 Patrick Moore
, co-chair of the Clean & Safe Energy Coalition - a pro-nuclear group funded by the Nuclear Energy Institute
- wrote: Mr. Moore is a co-founder and former leader of Greenpeace
, but he has not been involved with Greenpeace since 1986.
which via expanded federal loan guarantees in the proposed 2011 budget is supporting a nuclear renaissance
.
Australian researchers at the University of New South Wales claim to have solved the energy storage problem for solar and wind power with the development of vanadium redox batteries
. (U.S. patent issued in 1986).
Some renewable electricity sources have identical variability to coal-fired power station
s, so they are base-load, and can be integrated into the electricity supply system without any additional back-up. Examples include:
Furthermore, supporters argue that the total electricity generated from a large-scale array of dispersed wind farm
s, located in different wind regimes, cannot be accurately described as intermittent, because it does not start up or switch off instantaneously at irregular intervals. With a small amount of supplementary peak-load plant, which operates infrequently, large-scale distributed wind power can substitute for some base-load power and be equally reliable.
Hydropower
can be intermittent and/or dispatchable, depending on the configuration of the 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 will be variable on a seasonal or annual basis (dependent on rainfall and snow melt).
Amory Lovins
suggests a few basic strategies to deal with these issues:
Moreover, efficient energy use
and energy conservation
measures can reliably reduce demand for base-load and peak-load electricity.
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. In the UK, one summary of other studies indicated that if assuming that wind power contributed less than 20% of UK power consumption, then the intermittency would cause only moderate cost.
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.
The Pembina Institute
and the World Wide Fund for Nature
state in the Renewable is Doable plan that resilience is a feature of renewable energy:
Tidal power
Tidal power, also called tidal energy, is a form of hydropower that converts the energy of tides into useful forms of power - mainly electricity....
, but cannot be dispatched
Dispatchable generation
Dispatchable generation refers to sources of electricity that can be dispatched at the request of power grid operators; that is, generating plants that can be turned on or off, or can adjust their power output on demand...
to meet the demand of a power system. Examples of intermittent sources include wind and solar power. Effective use of intermittent sources in an electric power grid usually relies on using the intermittent sources to displace fuel that would otherwise be consumed by non-renewable power station
Non-renewable resource
A non-renewable resource is a natural resource which cannot be produced, grown, generated, or used on a scale which can sustain its consumption rate, once depleted there is no more available for future needs. Also considered non-renewable are resources that are consumed much faster than nature...
s, or by storing energy in the form of renewable pumped storage, compressed air or ice, for use when needed, or as electrode heating for district heating schemes.
The storage of energy to fill the shortfall intermittency or for emergencies is part of a reliable
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
energy supply. The capacity of a reliable renewable energy
Renewable energy
Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...
supply, can additionally be fulfilled by the use of latency measure
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
s and backup or extra infrastructure and technology
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
, using mixed renewables to produce electricity above the intermittent average
Fault-tolerant design
In engineering, fault-tolerant design is a design that enables a system to continue operation, possibly at a reduced level , rather than failing completely, when some part of the system fails...
, which may be utilised to meet regular and unanticipated supply demands.
The penetration of intermittent renewables in most power grids is low, but wind for example generates 11% of electric energy in Spain
Wind power in Spain
Spain is the world's fourth biggest producer of wind power, after China, the United States and Germany, with an installed capacity of 19,959 megawatts at the end of 2010, a rise of 1,609 MW for the year...
and Portugal
Wind power in Portugal
In December 2010, there was 3,937 MW of wind power nameplate capacity installed in Portugal. The major wind turbine manufacturers in the Portuguese market are Enercon, Vestas and Gamesa...
, 9% in the Republic of Ireland, and 7% in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. Wind provides nearly 20% of the electricity generated in Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
, however this percentage forces Denmark to import and export large amounts of energy to and from the EU grid, to balance supply with demand.
The use of small amounts of intermittent power has little effect on grid operations. Using larger amounts of intermittent power may require upgrades or even a redesign of the grid infrastructure.
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 power plants.- Intermittency can mean the extent to which a power source is unintentionally stopped or unavailable, but intermittency is frequently used as though it were synonymous with variability.
- Variability is the extent to which a power source may exhibit undesired or uncontrolled changes in output.
- DispatchabilityDispatchable generationDispatchable generation refers to sources of electricity that can be dispatched at the request of power grid operators; that is, generating plants that can be turned on or off, or can adjust their power output on demand...
or maneuverability is the ability of a given power source to increase and/or decrease output quickly on demand. The concept is distinct from intermittency; maneuverability is one of several ways grid operators match output (supply) to system demand. - Nominal or nameplate capacityNameplate capacityNameplate capacity, also known as the rated capacity, nominal capacity, installed capacity or maximum effect, refers to the intended technical full–load sustained output of a facility such as a power plant, a chemical plant, fuel plant, metal refinery, mine, and many others.For dispatchable power,...
, or maximum effect refers to the normal maximum output of a generating source. This is the most common number used and is typically expressed in megawatts (MW)WattThe watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
. - Capacity factorCapacity factorThe net capacity factor or load factor of a power plant is the ratio of the actual output of a power plant over a period of time and its potential output if it had operated at full nameplate capacity the entire time...
, average capacity factor, or load factor is the average expected output of a generator, usually over an annual period. Expressed as a percentage of the nameplate capacity or in decimal form (e.g. 30% or 0.30). - Capacity credit: generally, the amount of output from a power source that may be statistically relied upon, expressed as a percentage.
- Penetration in this context is generally used to refer to the amount of energy generated as a percentage of annual consumption.
- Firm capacity the amount of power that can be guaranteed to be provided as base power
- Non-firm capacity the amount of power above the firm capacity that is usually to be sold at higher price on the spot marketSpot marketThe spot market or cash market is a public financial market, in which financial instruments or commodities are traded for immediate delivery. It contrasts with a futures market in which delivery is due at a later date...
Solar energy
Intermittency inherently affects solar energy, as the production of electricity from solar sources depends on the amount of light energy in a given location. Solar output varies throughout the day and through the seasons, and is affected by cloud cover. These factors are fairly predictable, and some solar thermal systems make use of heat storage to produce power when the sun is not shining.- Intermittency: In the absence of an energy storage system, solar does not produce power at night.
- Capacity factor Photovoltaic solar in Massachusetts 12-15%. Photovoltaic solar in Arizona 19% Thermal solar parabolic trough 56% Thermal solar power towerSolar power towerThe solar power tower is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors to focus the sun's rays upon a collector tower...
73%
The extent to which the intermittency of solar-generated electricity is an issue will depend to some extent on the degree to which the generation profile of solar corresponds to demand. For example, solar thermal power plants such as Nevada Solar One
Nevada Solar One
Nevada Solar One is a concentrated solar power plant, with a nominal capacity of 64 MW and maximum capacity of 75 MW spread over an area of 400 Acres. The projected CO2 emissions avoided is equivalent to taking approximately 20,000 cars off the road annually. The project required an investment of...
are somewhat matched to summer peak loads in areas with significant cooling demands, such as the south-western United States. Thermal energy storage systems can improve the degree of match between supply and consumption. The increase in capacity factor of thermal systems does not represent an increase in efficiency, but rather a spreading out of the time over which the system generates power.
Wind energy
- Intermittence: A single wind turbine is highly intermittent. Theoretical arguments often claim that a large wind farm spread over a geographically diverse area will as a whole rarely stop producing power altogether, however this is in contradiction to the observed variability in total power output of wind turbines installed in Ireland and Denmark.
- Capacity Factor: Wind power typically has a capacity factor of 20-40%.
- Dispatchability: Wind power is "highly non-dispatchable".
- Capacity Credit: At low levels of penetration, the capacity credit of wind is about the same as the capacity factor. As the concentration of wind power on the grid rises, the capacity credit percentage drops.
- Variability: Site dependent. Sea breezes are much more constant than land breezes.
- Reliability: A wind farm is highly reliable (although highly intermittent). That is, the output at any given time will only vary gradually due to falling wind speeds or storms (the latter necessitating shut downs). A typical wind farm is unlikely to have to shut down in less than half an hour at the extreme, whereas an equivalent sized power station can fail totally instantaneously and without warning. The total shut down of wind turbines is predictable via weather forecasting.
According to a study of wind in the United States, ten or more widely-separated wind farms connected through the grid could be relied upon for from 33 to 47% of their average output (15–20% of nominal capacity) as reliable, baseload
Base load power plant
Baseload is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements...
power, as long as minimum criteria are met for wind speed and turbine height. When calculating the generating capacity available to meet peak demand, [ERCOT] (manages Texas grid) counts wind generation at 8.7% of nameplate capacity.
Because wind power is generated by large numbers of small generators, individual failures do not have large impacts on power grids. This feature of wind has been referred to as resiliency.
Wind power is affected by air temperature because colder air is more dense and therefore more effective at producing wind power. As a result, wind power is affected seasonally (more output in winter than summer) and by daily temperature variations. During the 2006 California heat storm
2006 North American heat wave
The 2006 North American heat wave spread throughout most of the United States and Canada beginning on July 15, 2006, killing at least 225 people. That day the temperature reached 117 °F in Pierre, South Dakota, with many places in South Dakota that hit well into the 120s. A 130 degree...
output from wind power in California
Wind power in California
Wind power in California has been an area of considerable activity for many years. California was the first U.S. state where large wind farms were developed, beginning in the early 1980s. By 1995, California produced 30 percent of the entire world's wind-generated electricity...
significantly decreased to an average of 4% of capacity for 7 days. A similar result was seen during the 2003 European heat wave
2003 European heat wave
The 2003 European heat wave was the hottest summer on record in Europe since at least 1540. France was hit especially hard. The heat wave led to health crises in several countries and combined with drought to create a crop shortfall in Southern Europe...
, when the output of wind power in France, Germany, and Spain fell below 10% during peak demand times.
According to an article in EnergyPulse, "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."
Nuclear power
Nuclear power plants are intermittent in that they will sometimes fail unexpectedly, often for long periods of time. For example, in the United States, 132 nuclear plants were built, and 21% were permanently and prematurely closed due to reliability or cost problems, while another 27% have at least once completely failed for a year or more. The remaining U.S. nuclear plants produce approximately 90% of their full-time full-load potential, but even they must shut down (on average) for 39 days every 17 months for scheduled refueling and maintenance. To cope with such intermittence by nuclear (and centralized fossil-fuelled) power plants, utilities install a “reserve margin” of roughly 15% extra capacity spinning ready for instant use.Nuclear plants have an additional disadvantage; for safety, they must instantly shut down in a power failure, but for nuclear-physics reasons, they can’t be restarted quickly. For example, during the Northeast Blackout of 2003
Northeast Blackout of 2003
The Northeast blackout of 2003 was a widespread power outage that occurred throughout parts of the Northeastern and Midwestern United States and Ontario, Canada on Thursday, August 14, 2003, just before 4:10 p.m....
, nine operating U.S. nuclear units had to shut down and were later restarted. During the first three days, while they were most needed, their output was less than 3% of normal. After twelve days of restart, their average capacity loss had exceeded 50 percent.
Solving intermittency
The combination of diversifying variable renewables by type and location, forecasting their variation, and integrating them with despatchable renewables, flexible fueled generators, and demand response can create a power system that has the potential to meet our needs reliably. Integrating ever-higher levels of renewables is being successfully demonstrated in the real world:
In 2009, eight American and three European authorities, writing in the leading electrical engineers' professional journal, didn't find "a credible and firm technical limit to the amount of wind energy that can be accommodated by electricity grids". In Fact, not one of more than 200 international studies, nor official studies for the eastern and western U.S. regions, nor the International Energy AgencyInternational Energy AgencyThe International Energy Agency is a Paris-based autonomous intergovernmental organization established in the framework of the Organisation for Economic Co-operation and Development in 1974 in the wake of the 1973 oil crisis...
, has found major costs or technical barriers to reliably integrating up to 30% variable renewable supplies into the grid, and in some studies much more.
Mark Z. Jacobson
Mark Z. Jacobson
Mark Z. Jacobson is professor of civil and environmental engineering at Stanford University and director of the Atmosphere and Energy Program there...
has studied how wind, water and solar technologies can be integrated to provide the majority of the world's energy needs. He advocates a "smart mix" of renewable energy
Renewable energy
Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...
sources to reliably meet electricity demand:
Because the wind blows during stormy conditions when the sun does not shine and the sun often shines on calm days with little wind, combining wind and solar can go a long way toward meeting demand, especially when geothermal provides a steady base and hydroelectric can be called on to fill in the gaps.
Mark A. Delucchi and Mark Z. Jacobson argue that there are at least seven ways to design and operate renewable energy systems so that they will reliably satisfy electricity demand:
- (A) interconnect geographically-dispersed naturally-variable energy sources (e.g., wind, solar, wave, tidal), which smooths out electricity supply (and demand) significantly.
- (B) use complementary and non-variable energy sources (such as hydroelectric power) to fill temporary gaps between demand and wind or solar generation.
- (C) use “smart” demand-response management to shift flexible loads to a time when more renewable energy is available.
- (D) store electric power, at the site of generation, (in batteries, hydrogen gas, compressed air, pumped hydroelectric power, and flywheels), for later use.
- (E) over-size renewable peak generation capacity to minimize the times when available renewable power is less than demand and to provide spare power to produce hydrogen for flexible transportation and heat uses.
- (F) store electric power in electric-vehicle batteries, known as "vehicle to grid" or V2G.
- (G) forecast the weather (winds, sunlight, waves, tides and precipitation) to better plan for energy supply needs.
Technological solutions to mitigate large scale wind energy type intermittency exist such as increased interconnection (the European super grid
European super grid
The European super grid is a possible future super grid that would ultimately interconnect the various European countries and the regions around Europe's borders – including North Africa, Kazakhstan, Ukraine, etc...
), Demand response
Demand response
In electricity grids, demand response is similar to dynamic demand mechanisms to manage customer consumption of electricity in response to supply conditions, for example, having electricity customers reduce their consumption at critical times or in response to market prices...
, load management
Load management
Load management is the process of balancing the supply of electricity on the network with the electrical load by adjusting or controlling the load rather than the power station output...
, diesel generators (in National Grid
National Grid
-Electric power transmission systems:*National Grid , the electricity transmission network of Great Britain.*National Grid , the electricity transmission network of Malaysia....
), Frequency Response / National Grid Reserve Service type schemes, and use of existing power stations on standby. Studies by academics and grid operators indicate that the cost of compensating for intermittency is expected to be high at levels of penetration above the low levels currently in use today Large, distributed power grids are better able to deal with high levels of penetration than small, isolated grids. For a hypothetical European-wide power grid, analysis has shown that wind energy penetration levels as high as 70% are viable, and that the cost of the extra transmission lines would be only around 10% of the turbine cost, yielding power at around present day prices. Smaller grids may be less tolerant to high levels of penetration.
Matching power demand to supply is not a problem specific to intermittent power sources. Existing power grids already contain elements of uncertainty including sudden and large changes in demand and unforeseen power plant failures. Though power grids are already designed to have some capacity in excess of projected peak demand to deal with these problems, significant upgrades may be required to accommodate large amounts of intermittent power. The International Energy Agency (IEA)
International Energy Agency
The International Energy Agency is a Paris-based autonomous intergovernmental organization established in the framework of the Organisation for Economic Co-operation and Development in 1974 in the wake of the 1973 oil crisis...
states:
"In the case of wind power, operational reserve is the additional generating reserve needed to ensure that differences between forecast and actual volumes of generation and demand can be met. Again, it has to be noted that already significant amounts of this reserve are operating on the grid due to the general safety and quality demands of the grid. Wind imposes additional demands only inasmuch as it increases variability and unpredictability. However, these factors are nothing completely new to system operators. By adding another variable, wind power changes the degree of uncertainty, but not the kind..."
Denmark
A November 2006 analysis found that "wind power may be able to cover more than 50% of the Danish electricity consumption in 2025" under conditions of high oil prices and higher costs for CO2 allowances. Denmark's two grids (covering West Denmark and East Denmark separately) each incorporate high-capacity interconnectors to neighbouring grids where some of the variations from wind are absorbed.Capacity credit, fuel saving and need for extra back-up
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.UK academic commentator Graham Sinden, of Oxford University, argues 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. The main value of wind, (in the UK, 5 times the capacity credit value) is its fuel and CO2 savings. Wind does not require any extra back-up, as is often wrongly claimed, since it uses the existing power stations, which are already built, as back-up, and which are started up during low wind periods, just as they are started up now, during the non availability of other conventional plant. More spinning reserve, of existing plant, is required, but this again is already built and has a low cost comparatively.
Hydroelectricity
Ffestiniog power station
The Ffestiniog Power Station is a 360 MW pumped-storage hydroelectricity scheme near Ffestiniog, in Gwynedd, north-west Wales, United Kingdom. The power station at the lower reservoir has four water turbines, which can generate 360 megawatts of electricity within 60 seconds of the need arising. ...
). The exact nature of the power availability depends on the type of plant.
In run-of-the-river hydroelectricity
Run-of-the-river hydroelectricity
Run-of-the-river hydroelectricity is a type of hydroelectric generation whereby a considerably smaller water storage called pondage or none is used to supply a power station. Run-of-the-river power plants are classified as with or without pondage...
, power availability is highly dependent on the flow of the river, making this type of generation mostly suitable only at locations where flow levels are controlled by upstream dams.
In conventional hydroelectric plants, there is a reservoir and a one-way generator. The water flow through its turbines can be adjusted frequently to meet changing demand throughout the day by running the generator when demand is high and not running it when demand is low.
Pumped-storage hydroelectricity
Pumped-storage hydroelectricity
Pumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...
can make an even more significant contribution to peaking ability of the grid. These just move water between reservoirs and are powered by power from the grid when demand is low and put power back into the grid when demand is high. There also exist combined pump-storage plants that use river flow as well as extra pumping when demand is low, such as the 240 MW Lewiston Pump-Generating Plant.
Direct pumped-storage does not contribute any net generation to the grid, in fact, it increases the fuel used by other power plants because there is inefficiency in the turbine/generator. The economic benefit of pumped-storage plants lies only in increasing the capacity of the grid. This type of plant works well on a grid with many nuclear or renewable energy plants because the fuel is very cheap or essentially free, so it costs very little to keep them running at high power during the night when demand is low. Both pump-storage plants and natural flow hydro plants can help allow for intermittency of other plants by running at higher capacity for short times, but assistance is limited by the total capacity of the hydroelectric plant.
Conventional power stations
Once a conventional power station has come offline it may stay that way for more than a week.Conventional power plants (as well as nuclear plants) use water for cooling, and water shortages during hot summer months have occasionally resulted in periods when output has had to be curtailed, notably in France in 2006
2006 European heat wave
The 2006 European heat wave was a period of exceptionally hot weather that arrived at the end of June 2006 in certain European countries. The United Kingdom, France, Belgium, Netherlands, Luxembourg, Italy, Poland, the Czech Republic, Hungary, Germany and western part of Russia were most affected....
.
Conventional power plant failures can remove large amounts of capacity from the grid suddenly, resulting in blackouts.
- Capacity factor: Base load coal plant 70–90%
Gas-fired generation
Gas-fired plants are typically very reliable and dispatchable. These kinds of plants also often have the ability to quickly vary their output to adjust to the frequent jumps and changes in consumer demand. Thus these are very good as peaking units. These benefits are weighted against the high price of gas when deployed in the grid.- Capacity factor: about 60%.
Nuclear power
Nuclear power is considered a base load power source, in that its output is nearly constant and other types of plants are adjusted with changes in demand. This is done because output changes can only be made in small increments, and because of small fuel costs - there is little marginal cost between running at a low power and a high power, therefore it is cheapest for the system to run the nuclear plants at high power.Every year or two (depending on the plant), the plant must be shut down for planned outages for about a month. This is typically done in the spring or autumn (fall) when electricity demand is lower, as such, on a national scale power output from nuclear increases corresponding with demand during the peak summer and winter months. This change in output commonly occurs on a yearly basis.
It is rare that nuclear power plants adjust their power output to correspond with demand on a daily basis because pressurized water reactors (PWR, which are the vast majority of nuclear power plants) use a chemical shim
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
in the moderator–coolant to control their power level. (Boiling water reactors (BWR), however, can use a combination of control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s and recirculation water flow speed to control their power level, and so in markets such as Chicago
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...
, Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...
where half of the local utility's fleet is BWRs it is common to load-follow
Load following power plant
A load following power plant is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day. Load following plants are typically in-between base load and peaking power plants in efficiency, speed of startup and shutdown, construction cost, cost of electricity...
although less economic to do so.)
- Intermittence: Unplanned outages worldwide caused power losses varying from 3.1% and 1.4% of capacity between 1995 and 2005. Over that same period reactors worldwide encountered an average of 1.1 to 0.6 SCRAMScramA scram or SCRAM is an emergency shutdown of a nuclear reactor – though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads...
s per 7,000 hours critical (about a year of operation.) An automatic SCRAM is a protective measure that shuts the reactor down suddenly for safety reasons. - Capacity factor: U.S. average 92%. Worldwide average varied between about 81% to 87% between 1995 and 2005.
In the UK one of the key criteria for determining the amount of required spinning reserve is the possible loss of Sizewell B, a 1.2 GW nuclear power plant.
At one point in the fall of 2007, out of 16 nuclear power stations in the UK, seven were offline due to a combination of planned and unplanned outages.
Diesel engine generation
Small high-speed diesels are very commonly used within large power grids throughout North America and Europe. France uses about 5 GW of such diesels to cover the intermittency of their nuclear stations; these are all in private hands - at small scales factories and the like - with their usage being triggered semi-randomly by a special tariff - - which encourages these users to start their diesels.In USA and UK these diesels have usually been purchased for other reasons e.g. for emergency standby, in water works, hotels, hospitals, etc. and in some cases for electricity substations - e.g. Cuyahoga Falls, USA (10 × 1.6 MW Caterpillar) and Tregarron Mid Wales UK (3 × 1.6 MW Caterpillar), but can be readily used to automatically synchronize and feed into the grid.
In the UK 500 MW of such plant is routinely started within a few minutes; this is perfectly acceptable to the engines' service life in a scheme operated by National Grid called National Grid Reserve Service. It has been established that there is 20 GW of such diesel plant in the UK and it has been pointed out that there is no technical reason why this quantity could not be brought into the Reserve Service scheme to assist handling very rapid changes in renewable output, whilst conventional plant is started or indeed stopped.
Compensating for variability
All sources of electrical power have some degree of unpredictability, and demand patterns 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 large amounts of highly variable power generation may require changes to existing procedures and additional investments.Operational reserve
All managed grids already have existing operational and "spinning" reserve to compensate for existing uncertainties in the power grid. The addition of intermittent resources such as wind does not require 100% "back-up" because operating reserves and balancing requirements are calculated on a system-wide basis, and not dedicated to a specific generating plant.- Some coal, gas, or hydro power plants are partially loaded and then controlled to change as demand changes or to replace rapidly lost generation. The ability to change as demand changes is termed "response." The ability to quickly replace lost generation, typically within timescales of 30 seconds to 30 minutes, is termed "spinning reserve."
- Generally thermal plants running as peaking plantsPeaking power plantPeaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...
will be less efficient than if they were running as base loadBase load power plantBaseload is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements...
. - Hydroelectric facilities with storage capacity (such as the traditional dam configuration) may be operated as base load or peaking plants.
- In practice, as the power output from wind varies, partially loaded conventional plants, which are already present to provide response and reserve, adjust their output to compensate.
- While low penetrations of intermittent power may utilize existing levels of response and spinning reserve, the larger overall variations at higher penetrations levels will require additional reserves or other means of compensation.
Demand reduction or increase
- Demand responseDemand responseIn electricity grids, demand response is similar to dynamic demand mechanisms to manage customer consumption of electricity in response to supply conditions, for example, having electricity customers reduce their consumption at critical times or in response to market prices...
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 have been widely created in the American, British and French systems for the use of these systems, such as favorable rates or capital cost assistance, encouraging consumers with large loads to take them off line or to start diesels whenever there is a shortage of capacity, or conversely to increase load when there is a surplus. - Certain types of load control allow the power company to turn loads off remotely if insufficient power is available. In France large users such as CERN cut power usage as required by the System Operator - EDF under the encouragement of the EJP tariff.
- Energy demand managementEnergy demand managementEnergy demand management, also known as demand side management , is the modification of consumer demand for energy through various methods such as financial incentives and education...
refers to incentives to adjust use of electricity, such as higher rates during peak hours. - Real-time variable electricity pricing can encourage users to adjust usage to take advantage of periods when power is cheaply available and avoid periods when it is more scarce and expensive.
- Instantaneous demand reduction. Most large systems also have a category of loads which instantly disconnect when there is a generation shortage, under some mutually beneficial contract. This can give instant load reductions (or increases). See National Grid Reserve Service
- Diesel generators, originally or primarily installed for emergency power supply are often also connected to the National Grid in the UK to help deal with short term demand supply mismatches.
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:- Pumped storage hydropower is the most prevalent existing technology used, and can substantially improve the economics of wind power. The availability of hydropower sites suitable for storage will vary from grid to grid. Typical round trip efficiency is 80%. See also: Pumped-storage hydroelectricityPumped-storage hydroelectricityPumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...
- Thermal energy storageThermal energy storageThermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. They can be employed to balance energy demand between day time and night time. The thermal reservoir may be maintained at a temperature above or below that of the...
stores heat. Stored heat can be used directly for heating needs or converted into electricity. - Ice storage air conditioningIce storage air conditioningThermal energy storage using ice is practical because of the large heat of fusion of water. One metric ton of water, one cubic metre, can store 334 million joules or 317,000 BTUs...
Ice can be stored inter seasonally and can be used as a source of air-conditioning during periods of high demand. Present systems only need to store ice for a few hours but are well developed. - HydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
can be created through electrolysisElectrolysisIn chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
and stored for later use. NREL found that a kilogram of hydrogen (roughly equivalent to a gallon of gasoline) could be produced for between $5.55 in the near term and $2.27 in the long term. - Rechargeable flow batteries can serve as a large capacity, rapid-response storage medium. Main article: Flow batteries
- Some loads such as desalination plants and electric boilers, are able to store their output (water and heat.) These "opportunistic loads" are able to take advantage of "burst electricity" when it is available.
- Various other potential applications are being considered, such as charging plug-in electric vehicles during periods of low demand and high production; such technologies are not widely used at this time.
Storage of electrical energy results in some lost energy because storage and retrieval are not perfectly efficient. Storage may also require substantial capital investment and space for storage facilities.
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 correlationCorrelation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
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.
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
Wind power forecasting
A wind power forecast corresponds to an estimate of the expected production of one or more wind turbines in the near future. By production is often meant available power for wind farm considered...
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.
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; NorwayNorwayNorway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, QuebecQuebecQuebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, and ManitobaManitobaManitoba is a Canadian prairie province with an area of . The province has over 110,000 lakes and has a largely continental climate because of its flat topography. Agriculture, mostly concentrated in the fertile southern and western parts of the province, is vital to the province's economy; other...
all have high levels of existing hydroelectric generation (Quebec produces over 90% of its electricity from hydropower, and the local utility, Hydro-QuébecHydro-QuébecHydro-Québec is a government-owned public utility established in 1944 by the Government of Quebec. Based in Montreal, the company is in charge of the generation, transmission and distribution of electricity across Quebec....
, 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 6,000 MW of wind capacity. Storage capacity in hydropower facilities will be limited by size of reservoir, and environmental and other considerations. - The Institute for Solar Energy Supply Technology of the University of KasselUniversity of KasselThe University of Kassel, founded in 1970, is one of the newer universities in the state of Hesse. The university is in Kassel, and as of September 2010 has about 18,113 students...
, GermanyGermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
pilot-tested a combined power plantVirtual power plantA virtual power plant is a cluster of distributed generation installations which are collectively run by a central control entity....
linking solar, wind, biogasBiogasBiogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal dung, and kitchen waste can be converted into a gaseous fuel called biogas...
and hydrostoragePumped-storage hydroelectricityPumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...
to provide load-following power around the clock, entirely from renewable sources.
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.
- Integration with other grids can lower the effective concentration of variable power. 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. This effect is diminished if more neighboring grids also have high penetration levels of variable power.
- Integration of grids may decrease the overall variability of both supply and demand by increasing geographical diversity.
- Methods of compensating for power variability in one grid, such as peaking-plants or pumped-storage hydro-electricity, may be taken advantage of by importing variable power from another grid that is short on such capabilities.
- The capacity of power transmission infrastructure may have to be substantially upgraded to support export/import plans.
- Some energy is lost in transmission.
- The economic value of exporting variable power depends in part on the ability of the exporting grid to provide the importing grid with useful power at useful times for an attractive price.
Penetration
Penetration refers to the proportion of a power source on a system, expressed as a percentage. There are several ways that this can be calculated, with the different methods yielding different penetrations. It can be calculated either as:- the nominal capacity of a power source divided by peak demand; or
- the nominal capacity of a power source divided by total capacity; or
- the average power generated by a power source, divided by the average system demand.
The level of penetration of intermittent variable sources is significant for the following reasons:
- Power grids with significant amounts pumped storagePumped-storage hydroelectricityPumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...
, hydropowerHydropowerHydropower, hydraulic power, hydrokinetic power or water power is power that is derived from the force or energy of falling water, which may be harnessed for useful purposes. Since ancient times, hydropower has been used for irrigation and the operation of various mechanical devices, such as...
or other peaking power plantPeaking power plantPeaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...
s such as natural gas-fired power plants are more inherently capable of accommodating fluctuations from intermittent power. - 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.
Renewable electricity supply in the 20-50+% penetration range has already been implemented in several European systems, albeit in the context of a integrated European grid system:
In 2010, four German states, totaling 10 million people, relied on wind power for 43-52% of their annual electricity needs. Denmark isn't far behind, supplying 22% of its power from wind in 2010 (26% in an average wind year). The Extremadura region of Spain is getting up to 25% of its electricity from solar, while the whole country meets 16% of its demand from wind. Just during 2005-2010, Portugal vaulted from 17% to 45% renewable electricity.
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. Discussion of acceptable or unacceptable penetration figures should be treated and used with caution, as the relevance or significance will be highly dependent on local factors, grid structure and management, and existing generation capacity.
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."
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:
- 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 spreadSpark spreadThe spark spread is the theoretical gross margin of a gas-fired power plant from selling a unit of electricity, having bought the fuel required to produce this unit of electricity...
s 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.
- 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 response gas turbinesPeaking power plantPeaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...
and even faster diesels in the UK, France and US to control grid stability.
- 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 and by extending the scheme to domestic-sized loads. Energy can be stored by advancing deferrable domestic loads such as storage heaters, water heaters, fridge motors, or even hydrogen productionHydrogen productionHydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
, and load can be shed by turning such equipment off.
- Alternatively or additionally, power can be exported to neighboring grids and re-imported later. HVDC cables are efficient with 3% loss per 1000 km and may be inexpensive in certain circumstances. For example an 8 GW link from UK to France would cost about £1 billion using high-voltage direct currentHigh-voltage direct currentA high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
cables. Under such scenarios, the amount of transmission capacity required may be many times higher than currently available.
Penetration Studies
Studies have been conducted to assess the viability of specific penetration levels in specific energy markets.European super grid
A series of detailed modelling studies by Dr. Gregor Czisch, which looked at the European wide adoption of renewable energy and interlinking power grids the European super gridEuropean super grid
The European super grid is a possible future super grid that would ultimately interconnect the various European countries and the regions around Europe's borders – including North Africa, Kazakhstan, Ukraine, etc...
using HVDC cables, indicates that the entire European power usage could come from renewables, with 70% total energy from wind at the same sort of costs or lower than at present. This proposed large European power grid has been called a "super grid
Super grid
A super grid is a wide area transmission network that makes it possible to trade high volumes of electricity across great distances. It is sometimes also referred to as a "mega grid".-History:...
."
The model deals with intermittent power issues by using base-load renewables such as hydroelectric and biomass for a substantial portion of the remaining 30% and by heavy use of HVDC to shift power from windy areas to non-windy areas. The report states that "electricity transport proves to be one of the keys to an economical electricity supply" and underscores the importance of "international co-operation in the field of renewable energy use [and] transmission."
Dr. Czisch described the concept in an interview, saying "For example, if we look at wind energy in Europe. We have a winter wind region where the maximum production is in winter and in the Sahara region in northern Africa the highest wind production is in the summer and if you combine both, you come quite close to the needs of the people living in the whole area - let's say from northern Russia down to the southern part of the Sahara."
Grid study in Ireland
A study of the grid in IrelandRepublic of Ireland
Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...
indicates that it would be feasible to accommodate 42% (of demand) renewables
in the electricity mix. This acceptable level of renewable penetration was found in what the study called Scenario 5, provided 47% of electrical capacity (different from demand) with the following mix of renewable energies:
- 6,000 MW wind
- 360 MW base load renewables
- 285 MW additional variable renewables (other intermittent sources)
The study cautions that various assumptions were made that "may have understated dispatch restrictions, resulting in an underestimation of operational costs, required wind curtailment, and CO2 emissions" and that "The limitations of the study may overstate the technical feasibility of the portfolios analyzed..."
Scenario 6, which proposed renewables providing 59% of electrical capacity and 54% of demand had problems. Scenario 6 proposed the following mix of renewable energies:
- 8,000 MW wind
- 392 MW base load renewables
- 1,685 MW additional variable renewables (other intermittent sources)
The study found that for Scenario 6, "a significant number of hours characterized by extreme system situations occurred where load and reserve requirements could not be met. The results of the network study indicated that for such extreme renewable penetration scenarios, a system re-design is required, rather than a reinforcement exercise." The study declined to analyze the cost effectiveness of the required changes because "determination of costs and benefits had become extremely dependent on the assumptions made" and this uncertainty would have impacted the robustness of the results.
Canada
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 MW," which corresponds to a peak penetration of 17%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 externalitiesExternality
In economics, an externality is a cost or benefit, not transmitted through prices, incurred by a party who did not agree to the action causing the cost or benefit...
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
Operating reserve
In electricity networks, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply...
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. However transmission costs are a low fraction of total energy costs.
- Additional operating reserve: if additional wind does not correspond to demand patterns, additional operating reserve may be required compared to other generating types, however this does not result in higher capital costs for additional plants since this is merely existing plants running at low output - spinning reserve. 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.
Analyses of costs
Studies have been performed to determine the costs of variability. RenewableUK states:Colorado - Separate reports by Xcel and UCS
An official at Xcel EnergyXcel Energy
Xcel Energy, Inc. is a public utility company based in Minneapolis, Minnesota, serving customers in Colorado, Michigan, Minnesota, New Mexico, North Dakota, South Dakota, Texas, and Wisconsin. Primary services are electricity and natural gas...
claimed that at 20 percent penetration, additional standby generators to compensate for wind in Colorado would cost $8 per MWh, adding between 13% and 16% to the $50–$60 cost per MWh of wind energy.
The Union of Concerned Scientists
Union of Concerned Scientists
The Union of Concerned Scientists is a nonprofit science advocacy group based in the United States. The UCS membership includes many private citizens in addition to professional scientists. James J...
conducted a study of the costs to increase the renewable penetration in Colorado to 10% and found that for an average residential bill "customers of municipal utilities and rural electric cooperatives that opt out of the solar energy requirement" would save 4 cents per month, but that for Xcel Energy
Xcel Energy
Xcel Energy, Inc. is a public utility company based in Minneapolis, Minnesota, serving customers in Colorado, Michigan, Minnesota, New Mexico, North Dakota, South Dakota, Texas, and Wisconsin. Primary services are electricity and natural gas...
customers there would be additional cost of about 10 cents per month. Total impact on all consumers would be $4.5 million or 0.01% over two decades.
UK Studies
A detailed study for UK National Grid (a private power company) states "We have estimated that for the case with 8,000 MW of wind needed to meet the 10% renewables target for 2010, balancing costs can be expected to increase by around £2 per MWh of wind production. This would represent an additional £40million per annum, just over 10% of existing annual balancing costs."In evidence to the UK House of Lords Economic Affairs Select Committee, National Grid 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 notes that "increasing levels of such renewable generation on the system would increase the costs of balancing the system and managing system frequency."
A 2003 report by Carbon Trust and the UK Department of Trade and Industry (DTI) found that the costs for reinforcement and new build of transmission and distribution systems to support 10% renewable electricity in the UK by 2010 would be £1.6 to £2.4 billion. The study classified "Intermittency" as "Not a significant issue" for the 2010 target. The same 2003 study found that achieving 20% renewable electricity in the UK by 2020 would cost £3.2bn to £4.5bn in transmission and distribution system construction and reinforcement. The study classified "Intermittency" as a "Significant Issue" for the 2020 target.
Minnesota
Minnesota study on wind penetration levels and found that "total integration operating cost for up to 25% wind energy" would be less than $0.0045 per kWh (additional).Intermittency and renewable energy
There are differing views about some sources of renewable energy and intermittency. The World Nuclear Association argues that the sun, wind, tides and waves cannot be controlled to provide directly either continuous base-load power, or peak-load power when it is needed. Proponents of renewable energy use argue that the issue of intermittency of renewables is over-stated, and that practical experience demonstrates this. In any case, geothermal renewable energyRenewable energy
Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...
has no intermittency.
Views of critics of high penetration renewable energy use
The World Nuclear AssociationWorld Nuclear Association
The World Nuclear Association , formerly the Uranium Institute, is an international organization that promotes nuclear power and supports the many companies that comprise the global nuclear industry...
states that:
"Obviously sun, wind, tides and waves cannot be controlled to provide directly either continuous base-loadBase load power plantBaseload is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements...
power, or peak-load power when it is needed,..." "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." "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." "Relatively few places have scope for pumped storagePumped-storage hydroelectricityPumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...
dams close to where the power is needed, and overall efficiency is less than 80%. Means of storing large amounts of electricity as such in giant batteries or by other means have not been developed."
On December 10, 2007 Patrick Moore
Patrick Moore (environmentalist)
Patrick Moore is a former environmental activist, known as one of the early members of Greenpeace, in which he was an activist from 1971 to 1986...
, co-chair of the Clean & Safe Energy Coalition - a pro-nuclear group funded by the Nuclear Energy Institute
Nuclear Energy Institute
The Nuclear Energy Institute is a nuclear industry lobbying group in the United States.- Synopsis :According to its website, the NEI "develops policy on key legislative and regulatory issues affecting the industry. NEI then serves as a unified industry voice before the U.S...
- wrote: Mr. Moore is a co-founder and former leader of Greenpeace
Greenpeace
Greenpeace is a non-governmental environmental organization with offices in over forty countries and with an international coordinating body in Amsterdam, The Netherlands...
, but he has not been involved with Greenpeace since 1986.
Views of proponents of high penetration renewable energy use
The US Federal Energy Regulatory Commission (FERC) Chairman Jon Wellinghoff has stated that "baseload capacity is going to become an anachronism" and that no new nuclear or coal plants may ever be needed in the United States. This however is a minority viewpoint within President Obama's administrationPresidency of Barack Obama
The Presidency of Barack Obama began at noon EST on January 20, 2009 when he became the 44th President of the United States. Obama was a United States Senator from Illinois at the time of his victory over Arizona Senator John McCain in the 2008 presidential election...
which via expanded federal loan guarantees in the proposed 2011 budget is supporting a nuclear renaissance
Nuclear renaissance
Since about 2001 the term nuclear renaissance has been used to refer to a possible nuclear power industry revival, driven by rising fossil fuel prices and new concerns about meeting greenhouse gas emission limits. At the same time, various barriers to a nuclear renaissance have been identified...
.
Australian researchers at the University of New South Wales claim to have solved the energy storage problem for solar and wind power with the development of vanadium redox batteries
Vanadium redox battery
The vanadium redox battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy...
. (U.S. patent issued in 1986).
Some renewable electricity sources have identical variability to coal-fired power station
Coal-fired power station
A coal-fired power station produces electricity, usually for public consumption, by burning coal to boil water, producing steam which drives a steam turbine which turns an electrical generator...
s, so they are base-load, and can be integrated into the electricity supply system without any additional back-up. Examples include:
- Bio-energy, based on the combustion of crops and crop residues, or their gasification followed by combustion of the gas.
- Hot dry rock geothermal power, which is being developed in AustraliaGeothermal energy exploration in Central AustraliaGeothermal power in Australia is little used but growing. There are known and potential locations near the centre of the country that have been shown to contain hot granites at depth which hold good potential for development of geothermal energy...
and the United StatesGeothermal energy in the United StatesGeothermal energy in the United States continues to be an area of considerable activity. In 2010, the United States led the world in geothermal electricity production with 3,086 megawatts of installed capacity from 77 power plants; the largest group of geothermal power plants in the world is...
.
- Solar thermal electricity, with overnight heat storage in water or rocks, or a thermochemical store as with Nevada Solar OneNevada Solar OneNevada Solar One is a concentrated solar power plant, with a nominal capacity of 64 MW and maximum capacity of 75 MW spread over an area of 400 Acres. The projected CO2 emissions avoided is equivalent to taking approximately 20,000 cars off the road annually. The project required an investment of...
and Solar Tres.
Furthermore, supporters argue that the total electricity generated from a large-scale array of dispersed wind farm
Wind farm
A wind farm is a group of wind turbines in the same location used to produce electric power. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other...
s, located in different wind regimes, cannot be accurately described as intermittent, because it does not start up or switch off instantaneously at irregular intervals. With a small amount of supplementary peak-load plant, which operates infrequently, large-scale distributed wind power can substitute for some base-load power and be equally reliable.
Hydropower
Hydropower
Hydropower, hydraulic power, hydrokinetic power or water power is power that is derived from the force or energy of falling water, which may be harnessed for useful purposes. Since ancient times, hydropower has been used for irrigation and the operation of various mechanical devices, such as...
can be intermittent and/or dispatchable, depending on the configuration of the 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 will be variable on a seasonal or annual basis (dependent on rainfall and snow melt).
Amory Lovins
Amory Lovins
Amory Bloch Lovins is an American environmental scientist and writer, Chairman and Chief Scientist of the Rocky Mountain Institute. He has worked in the field of energy policy and related areas for four decades...
suggests a few basic strategies to deal with these issues:
Moreover, efficient energy use
Efficient energy use
Efficient energy use, sometimes simply called energy efficiency, is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature...
and energy conservation
Energy conservation
Energy conservation refers to efforts made to reduce energy consumption. Energy conservation can be achieved through increased efficient energy use, in conjunction with decreased energy consumption and/or reduced consumption from conventional energy sources...
measures can reliably reduce demand for base-load and peak-load electricity.
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. In the UK, one summary of other studies indicated that if assuming that wind power contributed less than 20% of UK power consumption, then the intermittency would cause only moderate cost.
Methods to manage wind power integration range from those that are commonly used at present (e.g. demand management
Demand management
Demand management is a planning methodology used to manage forecasted demand.-Demand management in economics:In economics, demand management is the art or science of controlling economic demand to avoid a recession...
) to potential new technologies for grid energy storage
Grid energy storage
Grid energy storage refers to the methods used to store electricity on a large scale within an electrical power grid. Electrical energy is stored during times when production exceeds consumption and the stores are used at times when consumption exceeds production...
. Improved forecasting
Wind power forecasting
A wind power forecast corresponds to an estimate of the expected production of one or more wind turbines in the near future. By production is often meant available power for wind farm considered...
can also contribute as the daily and seasonal variations in wind and solar sources are to some extent predictable.
The Pembina Institute
Pembina Institute
The Pembina Institute is a Canadian not-for-profit think tank focused on developing innovative sustainable energy solutions. Founded in 1985, the Institute has offices in Calgary, Drayton Valley, Edmonton, Ottawa, Toronto, Vancouver, and Yellowknife....
and the World Wide Fund for Nature
World Wide Fund for Nature
The World Wide Fund for Nature is an international non-governmental organization working on issues regarding the conservation, research and restoration of the environment, formerly named the World Wildlife Fund, which remains its official name in Canada and the United States...
state in the Renewable is Doable plan that resilience is a feature of renewable energy:
See also
- Brittle PowerBrittle PowerBrittle Power: Energy Strategy for National Security is a 1982 book by Amory B. Lovins and L. Hunter Lovins, prepared originally as a Pentagon study, and re-released in 2001 following the September 11 attacks. The book argues that U.S. domestic energy infrastructure is very vulnerable to...
- Demand responseDemand responseIn electricity grids, demand response is similar to dynamic demand mechanisms to manage customer consumption of electricity in response to supply conditions, for example, having electricity customers reduce their consumption at critical times or in response to market prices...
- European super gridEuropean super gridThe European super grid is a possible future super grid that would ultimately interconnect the various European countries and the regions around Europe's borders – including North Africa, Kazakhstan, Ukraine, etc...
- Energy security and renewable technologyEnergy security and renewable technologyThe environmental benefits of renewable energy technologies are widely recognised, but the contribution thatthey can make to energy security is less well known. Renewable technologies can enhance energy security in electricity generation, heat supply, and transportation.-Energy security:Access to...
- High-voltage direct currentHigh-voltage direct currentA high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
(HVDC) - Northeast Blackout of 2003Northeast Blackout of 2003The Northeast blackout of 2003 was a widespread power outage that occurred throughout parts of the Northeastern and Midwestern United States and Ontario, Canada on Thursday, August 14, 2003, just before 4:10 p.m....
- List of power outages
- Calculating the cost of the UK Transmission network: cost per kWh of transmission
- Calculating the cost of back up: See spark spreadSpark spreadThe spark spread is the theoretical gross margin of a gas-fired power plant from selling a unit of electricity, having bought the fuel required to produce this unit of electricity...
- Load managementLoad managementLoad management is the process of balancing the supply of electricity on the network with the electrical load by adjusting or controlling the load rather than the power station output...
- National Grid Reserve Service
- Control of the National Grid
- Relative cost of electricity generated by different sources
- Economics of new nuclear power plantsEconomics of new nuclear power plantsThe economics of new nuclear power plants is a controversial subject, since there are diverging views on this topic, and multi-billion dollar investments ride on the choice of an energy source...
(for more cost comparisons)
- Economics of new nuclear power plants
- Motor-generatorMotor-generatorA motor-generator is a device for converting electrical power to another form. Motor-generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line...
- Three-phase electric powerThree-phase electric powerThree-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...
- Smart grid and V2G to override intermittency.
Further reading
These peer-reviewed papers examine the impacts of intermittency:- Dale, L; Milborrow, D; Slark, R; & Strbac, G, 2003, A shift to wind is not unfeasible (Total Cost Estimates for Large-scale Wind Scenarios in UK), Power UK, no. 109, pp. 17–25.
- Farmer, E; Newman, V; & Ashmole, P, Economic and operational implications of a complex of wind-driven power generators on a power system, IEE Proceedings A, 5 edn. vol. 127.
- Gross, R; Heptonstall, P; Anderson, D; Green, T; Leach, M; & Skea, J, 2006, The Costs and Impacts of Intermittency. UK Energy Research Centre, London http://www.ukerc.ac.uk/Downloads/PDF/06/0604Intermittency/0604IntermittencyReport.pdf
- Gross, R; Heptonstall, P; Leach, M; Anderson, D; Green, T; & Skea, J, 2007, Renewables and the grid: understanding intermittency, Proceedings of ICE, Energy, vol. 160, no. 1, pp. 31–41.
- Grubb, M, 1991, The integration of renewable electricity sources, Energy Policy, vol. 19, no. 7, pp. 670–688.
- Halliday, J; Lipman, N; Bossanyi, E; & Musgrove, P, 1983, Studies of wind energy integration for the UK national electricity grid, American Wind Energy Association Wind Worksop VI, Minneapolis.
- Holttinen, H, 2005, Impact of hourly wind power variations on the system operation in the Nordic countries, Wind energy, vol. 8, no. 2, pp. 197–218.
- Ilex & Strbac, G, 2002, Quantifying The System Costs Of Additional Renewables in 2020, DTI, urn 02/1620 http://www.berr.gov.uk/files/file21352.pdf
- Milligan, M, 2001, A Chronological Reliability Model to Assess Operating Reserve Allocation to Wind Power Plants, National Renewable Energy Laboratory, The 2001 European Wind Energy Conference http://www.nrel.gov/docs/fy01osti/30490.pdf
- Skea, J; Anderson, D; Green, T; Gross, R; Heptonstall, P; & Leach, M, 2008, Intermittent renewable generation and maintaining power system reliability, Generation, Transmission & Distribution, IET, vol. 2, no. 1, pp. 82–89.
External links
- Stationary Energy Storage…Key to the Renewable Grid
- 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
- Empowering Variable Renewables: Options for Flexible Electricity Systems
- Getting a (Firm) Grip on Renewables