Variable renewable energy

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 partially unavailable. Intermittency refers to the changes of the variability of daily solar radiation according to the scale considered Intermittency and variability of daily solar irradiation. J.M. Vindel, J. Polo. Atmospheric Research. .
• Dispatchability is the ability of a given power source to increase and decrease output quickly on demand. The concept is distinct from intermittency; dispatchability is one of several ways system operators match supply (generator's output) to system demand (technical loads).[2]
• Penetration in this context is generally used to refer to the amount of energy generated as a percentage of annual consumption.[3]
• Nominal power or nameplate capacity refers to the maximum output of a generating plant in normal operating conditions. This is the most common number used and typically expressed in Watt (including multiples like kW, MW, GW).
• Capacity factor, 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, practically the minimum power within a longer period, usually expressed as a percentage of the nominal power.[4]
• Firm capacity is the amount of power that can be guaranteed to be provided as base power.
• Non-firm capacity is the amount of power above the firm capacity that is usually to be sold at higher price on the spot market.

Conventional hydroelectricity, biomass and geothermal are completely dispatchable as each has a store of potential energy; wind and solar production are typically without storage and can be decreased, but not dispatched, other than when nature provides. Between wind and solar, solar has a more variable daily cycle than wind, but is more predictable in daylight hours than wind. Like solar, tidal energy varies between on and off cycles through each day, unlike solar there is no intermittentcy, tides are available every day without fail. Biofuel and biomass involve multiple steps in the production of energy – growing plants, harvesting, processing, transportation, storage and burning to create heat for electricity, transportation or space heating. In the combined power plant used by the University of Kassel to simulate using 100% renewable energy, wind farms and solar farms were supplemented as needed by hydrostorage and biomass to follow the electricity demand.[5]

Wind power

Day ahead prediction and actual wind power

Wind power forecasting is the least accurate of all of the variable renewable energy sources. Grid operators use day ahead forecasting to determine which of the available power sources to use the next day, and weather forecasting is used to predict the likely wind power and solar power output available. Although wind power forecasts have been used operationally for decades, as of 2019 the IEA is organizing international collaboration to further improve their accuracy.[6] The variability of wind power can be seen as one of its defining characteristics.[7]

Erie Shores Wind Farm monthly output over a two-year period

A wind farm in Muppandal, Tamil Nadu, India

Over the entire year more than 20 percent of South Dakota's electricity is generated from wind 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 then the wind turbines will not be able to make electricity, and if it is too high 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.[8][9][10][11]

• Intermittence: Regions smaller than synoptic scale (the size of an average country) have mostly the same weather and thus around the same wind power, unless local conditions favor special winds. Some studies show that wind farms spread over a geographically diverse area will as a whole rarely stop producing power altogether.[9][10] However this is rarely the case for smaller areas with uniform geography such as Ireland,[12][13][14] Scotland[15] and Denmark which have several days per year with little wind power.[16]
• Capacity factor: Wind power typically has a capacity factor of 20–40%.[17][18]
• Dispatchability: Wind power is "highly non-dispatchable".[19] MISO, which operates a large section of the U.S. grid, has over 13,000 MW of wind power under its control and is able to manage this large amount of wind power by operating it as dispatchable intermittent resources.[20]
• 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.[18][21]
• Variability: Site dependent.[22] Sea breezes are much more constant than land breezes.[8] Seasonal variability may reduce output by 50%.[23]
• Reliability: A wind farm has high technical reliability when the wind blows. 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 shutdown of wind turbines is predictable via weather forecasting. The average availability of a wind turbine is 98%, and when a turbine fails or is shut down for maintenance it only affects a small percentage of the output of a large wind farm.[24]
• Predictability: Although wind is variable, it is also predictable in the short term. There is an 80% chance that wind output will change less than 10% in an hour and a 40% chance that it will change 10% or more in 5 hours. Predictability increases as weather forecasts become better.[25]

Wind generates about 16% (EWEA – 2011 European Statistics, February 2012) of electric energy in Spain and Portugal,[26] 9% in Ireland,[26] and 7% in Germany.[27] Wind provides around 40% of the annual electricity generated in Denmark[28] (up from 20% in 2005);[29][30] to meet this percentage Denmark exports surpluses and imports during shortfalls to and from the EU grid, particularly Norwegian Hydro, to balance supply with demand.[31]

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.[32]

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 wave output from wind power in California significantly decreased to an average of 4% of capacity for seven days.[33] 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.[34] Heat waves are partially caused by large amounts of solar radiation.

Five days of hourly output of five wind farms in Ontario

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."[35]

In Ontario, Canada, the Independent Electricity System Operator has been experimenting with dispatchable wind power to meet peak demands. In this case a number of wind generators are deliberately not connected to the grid, but are turning and ready to generate, and when the need for more power arises, they are connected to the grid. IESO is trying this as wind generators respond to sudden power demands much faster than gas-powered generators or hydroelectricity generators. [36]

Solar power

Daily solar output at AT&T Park in San Francisco

Solar power is more predictable than wind power and less variable – while there is never any solar power available during the night, and there is a reduction in winter, the only unknown factors in predicting solar output each day is cloud cover, frost and snow. Many days in a row in some locations are relatively cloud free, just as many days in a row in either the same or other locations are overcast – leading to relatively high predictability. Wind comes from the uneven heating of the earth's surface,[37] and can provide about 1% of the potential energy that is available from solar power. 86,000 TW of solar energy reaches the surface of the world vs. 870 TW in all of the world's winds.[38] Total world demand is roughly 12 TW, many times less than the amount that could be generated from potential wind and solar resources. From 40 to 85 TW could be provided from wind and about 580 TW from solar.[39]

Dish Stirling

Seasonal variation of the output of the solar panels at AT&T park in San Francisco

Intermittency inherently affects solar energy, as the production of renewable electricity from solar sources depends on the amount of sunlight at a given place and time. Solar output varies throughout the day and through the seasons, and is affected by dust, fog, cloud cover, frost or snow. Many of the seasonal factors are fairly predictable, and some solar thermal systems make use of heat storage to produce grid power for a full day.[40]

• Intermittency: In the absence of an energy storage system, solar does not produce power at night or in bad weather and varies between summer and winter. When intended to produce electricity only for peak air conditioning loads in the summer, there is no intermittency; in the winter can be complemented with wind power for peak loads.
• Capacity factor Photovoltaic solar in Massachusetts 12–15%.[17] Photovoltaic solar in Arizona 19%.[41] Thermal solar parabolic trough with storage 56%.[42] Thermal solar power tower with storage 73%.[42]

The impact of intermittency of solar-generated electricity will depend on the correlation of generation with 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 like the small Spanish Gemasolar Thermosolar Plant can improve the match between solar supply and local consumption. The improved capacity factor using thermal storage represents a decrease in maximum capacity, and extends the total time the system generates power.[43][44][45]

Tidal power

Types of tide

Tidal power is the most predictable of all the variable renewable energy sources. Twice a day the tides vary 100%, but they are never intermittent, on the contrary they are completely reliable. It is estimated that Britain could obtain 20% of energy from tidal power, only 20 sites in the world have yet been identified as possible tidal power stations.[47]

Historically grid operators use day ahead forecasting to choose which power stations to make up demand each hour of the next day, and adjust this forecast at intervals as short as hourly or even every fifteen minutes to accommodate any changes. Typically only a small fraction of the total demand is provided as spinning reserve.[51]

Some projections suggest that by 2030 almost all energy could come from non-dispatchable sources – how much wind or solar power is available depends on the weather conditions, and instead of turning on and off available sources becomes one of either storing or transmission of those sources to when they can be used or to where they can be used.[39] Some excess available energy can be diverted to hydrogen production for use in ships and airplanes, a relatively long term energy storage, in a world where almost all of our energy comes from wind, water, and solar (WWS). Hydrogen is not an energy source, but is a storage medium. A cost analysis will need to be made between long distance transmission and excess capacity. The sun is always shining somewhere, and the wind is always blowing somewhere on the Earth, and during the 2020s or 2030s it is predicted to become cost effective to bring solar power from Australia to Singapore.[52]

If excess capacity is created, the cost is increased because not all of the available output is used. For example, ERCOT predicts that 8.7% of nameplate wind capacity will be reliably available in summer[53] – so if Texas, which has a peak summer demand of 68,379 MW[54] built wind farms of 786,000 MW (68,379/0.087), they would generate, at a 35% capacity factor,[55] 2.4 million MWh per year – four times use, but might be sufficient to meet summer peaks. In practice it is likely that there are times with almost no wind in the entire region, making this not a practical solution. There were 54 days in 2002 when there was little wind power available in Denmark.[56] The estimated wind power installed capacity potential for Texas, using 100 meter wind turbines at 35% capacity factor, is 1,757,355.6 MW.[57] In locations like British Columbia, with abundant water power resources, water power can always make up any shortfall in wind power.[58]

Wind and solar are somewhat complementary. A comparison of the output of the solar panels and the wind turbine at the Massachusetts Maritime Academy shows the effect.[59] In winter there tends to be more wind and less solar, and in summer more solar and less wind, and during the day more solar and less wind. There is always no solar at night, and there is often more wind at night than during the day, so solar can be used somewhat to fill in the peak demand in the day, and wind can supply much of the demand during the night. There is however a substantial need for storage and transmission to fill in the gaps between demand and supply.

As physicist Amory Lovins has said:

The variability of sun, wind and so on, turns out to be a non-problem if you do several sensible things. One is to diversify your renewables by technology, so that weather conditions bad for one kind are good for another. Second, you diversify by site so they're not all subject to the same weather pattern at the same time because they're in the same place. Third, you use standard weather forecasting techniques to forecast wind, sun and rain, and of course hydro operators do this right now. Fourth, you integrate all your resources — supply side and demand side..."[60]

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:[61]

The energy transition to renewables ensures less power outages. In 2016, the number of minutes in Germany (13 min) was almost half as many as in 2006.

Mark A. Delucchi and Mark Z. Jacobson identify seven ways to design and operate variable renewable energy systems so that they will reliably satisfy electricity demand:[62]

1. interconnect geographically dispersed, naturally variable energy sources (e.g., wind, solar, wave, tidal), which smoothes out electricity supply (and demand) significantly.
2. use complementary and non-variable energy sources (such as hydroelectric power) to fill temporary gaps between demand and wind or solar generation.
3. use “smart” demand-response management to shift flexible loads to a time when more renewable energy is available.
4. store electric power, at the site of generation, (in batteries, hydrogen gas, molten salts, compressed air, pumped hydroelectric power, and flywheels), for later use.
5. oversize 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.
6. store electric power in electric-vehicle batteries, known as "vehicle to grid" or V2G.
7. forecast the weather (winds, sunlight, waves, tides and precipitation) to better plan for energy supply needs.[62]

Jacobson and Delucchi say that wind, water and solar power can be scaled up in cost-effective ways to meet our energy demands, freeing us from dependence on both fossil fuels and nuclear power. In 2009 they published “A Plan to Power 100 Percent of the Planet With Renewables” in Scientific American. A more detailed and updated technical analysis has been published as a two-part article in the refereed journal Energy Policy.[63]

An article by Kroposki, et al. discuses the technical challenges and solutions to operating electric power systems with extremely high levels of variable renewable energy in IEEE Power and Energy Magazine.[64] This article explains that there are important physical differences between power grids that are dominated by power electronic based sources such as wind and solar and conventional power grids based on synchronous generators. These systems must be properly design to address grid stability and reliability.

Renewable energy is naturally replenished and renewable power technologies increase energy security because they reduce dependence on foreign sources of fuel. Unlike power stations relying on uranium and recycled plutonium for fuel, they are not subject to the volatility of global fuel markets.[65] Renewable power decentralises electricity supply and so minimises the need to produce, transport and store hazardous fuels; reliability of power generation is improved by producing power close to the energy consumer. An accidental or intentional outage affects a smaller amount of capacity than an outage at a larger power station.[65]

The International Energy Agency says that there has been too much attention on issue of the variability of renewable electricity production.[66] The issue of intermittent supply applies to popular renewable technologies, mainly wind power and solar photovoltaics, and its significance depends on a range of factors which include the market penetration of the renewables concerned, the balance of plant and the wider connectivity of the system, as well as the demand side flexibility. Variability will rarely be a barrier to increased renewable energy deployment when dispatchable generation is also available. But at high levels of market penetration it requires careful analysis and management, and additional costs may be required for back-up or system modification.[66] Renewable electricity supply in the 20-50+% penetration range has already been implemented in several European systems, albeit in the context of an integrated European grid system:[61]

In 2011, the Intergovernmental Panel on Climate Change, the world's leading climate researchers selected by the United Nations, said "as infrastructure and energy systems develop, in spite of the complexities, there are few, if any, fundamental technological limits to integrating a portfolio of renewable energy technologies to meet a majority share of total energy demand in locations where suitable renewable resources exist or can be supplied".[67] IPCC scenarios "generally indicate that growth in renewable energy will be widespread around the world".[68] The IPCC said that if governments were supportive, and the full complement of renewable energy technologies were deployed, renewable energy supply could account for almost 80% of the world's energy use within forty years.[69] Rajendra Pachauri, chairman of the IPCC, said the necessary investment in renewables would cost only about 1% of global GDP annually. This approach could contain greenhouse gas levels to less than 450 parts per million, the safe level beyond which climate change becomes catastrophic and irreversible.[69]

The 150 MW Andasol solar power station is a commercial parabolic trough solar thermal power plant, located in Spain. The Andasol plant uses tanks of molten salt to store solar energy so that it can continue generating electricity even when the sun isn't shining.[1]

Construction of the Salt Tanks which provide efficient thermal energy storage[70] so that output can be provided after the sun goes down, and output can be scheduled to meet demand requirements.[71] The 280 MW Solana Generating Station is designed to provide six hours of energy storage. This allows the plant to generate about 38 percent of its rated capacity over the course of a year.[72]

An intermittent energy source is any source of energy that is not continuously available for conversion into electricity and outside direct control because the used primary energy cannot be stored. Intermittent energy sources may be predictable but cannot be dispatched to meet the demand of an electric power system.

The use of intermittent sources in an electric power system usually displaces storable primary energy that would otherwise be consumed by other power stations. Another option is to store electricity generated by non-dispatchable energy sources for later use when needed, e.g. in the form of pumped storage, compressed air or in batteries. A third option is the sector coupling e.g. by electric heating for district heating schemes.

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.[73][74]

The penetration of intermittent renewables in most power grids is low, global electricity production in 2014 was supplied by 3.1% wind, and 1% solar.[75] Wind generates roughly 16% of electric energy in Spain and Portugal,[26] 15.3% in Ireland,[76] and 7% in Germany.[27] As of 2014, wind provides 39% of the electricity generated in Denmark.[77][78][79] To operate with this level of penetration, Denmark exports surpluses and imports during shortfalls to and from neighbouring countries, particularly hydroelectric power from Norway, to balance supply with demand.[31] It also uses large numbers of combined heat and power (CHP) stations which can rapidly adjust output.[80]

The intermittency and variability of renewable energy sources can be reduced and accommodated by diversifying their technology type and geographical location, forecasting their variation, and integrating them with dispatchable renewables (such as hydropower, geothermal, and biomass). Combining this with energy storage and demand response can create a power system that can reliably match real-time energy demand.[81] The integration of ever-higher levels of renewables has already been successfully demonstrated:[82][61]

A research group at Harvard University quantified the meteorologically defined limits to reduction in the variability of outputs from a coupled wind farm system in the Central US:

The problem with the output from a single wind farm located in any particular region is that it is variable on time scales ranging from minutes to days posing difficulties for incorporating relevant outputs into an integrated power system. The high frequency (shorter than once per day) variability of contributions from individual wind farms is determined mainly by locally generated small scale boundary layer. The low frequency variability (longer than once per day) is associated with the passage of transient waves in the atmosphere with a characteristic time scale of several days. The high frequency variability of wind-generated power can be significantly reduced by coupling outputs from 5 to 10 wind farms distributed uniformly over a ten state region of the Central US. More than 95% of the remaining variability of the coupled system is concentrated at time scales longer than a day, allowing operators to take advantage of multi-day weather forecasts in scheduling projected contributions from wind.[83]

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 the British 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[84][85][86] 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,[87] 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.[88] Smaller grids may be less tolerant to high levels of penetration.[73][89]

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..."[8]

With sufficient energy storage, highly variable and intermittent sources can supply all of a regions electrical power. For solar to provide half of all electricity and using a solar capacity factor of 20%, the total capacity for solar would be 250% of the grids average daily load. For wind to provide half of all electricity and using a wind capacity factor of 30% the total capacity for wind would be 160% of the grids average daily load.

A pumped storage facility would then store enough water for the grids weekly load, with a capacity for peak demand i.e.:200% of the grid average. This would allow for one week of overcast and windless conditions. There are unusual costs associated with building storage and total generating capacity being six times the grid average.

As of 2019 interconnectors and hydrogen are predicted to be used much more for export of VRE.[52]

All sources of electrical power have some degree of variability, as do demand patterns which 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.

The capacity of a reliable renewable power supply, can be fulfilled by the use of backup or extra infrastructure and technology, using mixed renewables to produce electricity above the intermittent average, which may be used to meet regular and unanticipated supply demands.[90] Additionally, the storage of energy to fill the shortfall intermittency or for emergencies can be part of a reliable power supply.

Penetration refers to the proportion of a primary energy (PE) source in an electric power system, expressed as a percentage.[3] There are several methods of calculation yielding different penetrations. The penetration can be calculated either as:[102]

1. the nominal capacity (installed power) of a PE source divided by the peak load within an electric power system; or
2. the nominal capacity (installed power) of a PE source divided by the total capacity of the electric power system; or
3. the electrical energy generated by a PE source in a given period, divided by the demand of the electric power system in this period.

The level of penetration of intermittent variable sources is significant for the following reasons:

• Power grids with significant amounts of dispatchable pumped storage, hydropower with reservoir or pondage or other peaking power plants such as natural gas-fired power plants are capable of accommodating fluctuations from intermittent power more easily.[103]
• Relatively small electric power systems without strong interconnection may only be stable and economic with a lower fraction of wind energy (e.g. Ireland), although hybrid wind/diesel and solar/diesel systems have been used in isolated communities with success at relatively high penetration levels.[104][105]

Renewable electricity supply in the 20-50+% penetration range has already been implemented in several European systems, albeit in the context of an integrated European grid system:[61]

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."[102]

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

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

• Transmission capacity: transmission capacity may be more expensive than for nuclear and coal generating capacity due to lower load factors. Transmission capacity will generally be sized to projected peak output, but average capacity for wind will be significantly lower, raising cost per unit of energy actually transmitted. However transmission costs are a low fraction of total energy costs.[118]
• 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".[119]
• 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 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.

There are differing views about some sources of renewable energy and intermittency. The World Nuclear Association argues that system costs escalate with increasing proportion of variable renewables.[121] Proponents of renewable energy use argue that the issue of intermittency of renewables is over-stated, and that practical experience demonstrates this.[122] In any case, geothermal renewable energy has, like nuclear, no intermittency (but they both receive the energy from radioactive materials like uranium, thorium and potassium).

The U.S. 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.[123][124] Some renewable electricity sources have identical variability to coal-fired power stations, 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 Australia and the United States.
• Solar thermal electricity with overnight heat storage in molten salt, water or rocks.

Grid operators in countries like Denmark and Spain integrate large quantities of renewable energy into their electricity grids, with Denmark receiving 40% of its electricity from wind power.[125] For the month of February 2020, the grid in eastern Germany had an average of 85% power from wind and solar.[126]

Supporters say that the total electricity generated from a large-scale array of dispersed wind farms, located in different wind regimes, cannot be accurately described as intermittent, because it does not start up or switch off instantaneously at irregular intervals.[127] 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.[128]

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).[8]

Amory Lovins suggests a few basic strategies to deal with these issues:

"The variability of sun, wind and so on, turns out to be a non-problem if you do several sensible things. One is to diversify your renewables by technology, so that weather conditions bad for one kind are good for another. Second, you diversify by site so they're not all subject to the same weather pattern at the same time because they're in the same place. Third, you use standard weather forecasting techniques to forecast wind, sun and rain, and of course hydro operators do this right now. Fourth, you integrate all your resources — supply side and demand side..."[129]

Moreover, efficient energy use and energy conservation measures can reliably reduce demand for base-load and peak-load electricity.[14][130]

International groups are studying much higher penetrations (30-100% renewable energy), and conclusions are that these levels are also technically feasible.[131]

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:

Diversity and dispersal also add system security. If one wind turbine fails, the lights won't flicker. If an entire windfarm gets knocked out by a storm, only 40,000 people will lose power. If a single Darlington reactor goes down, 400,000 homes, or key industries, could face instant blackouts. To hedge this extra risk, high premiums have to be paid for decades to ensure large blocks of standby generation.[32]

• Brittle Power
• Control of the National Grid
• Cost per kWh of transmission in the UK
• Demand response
• Energy security and renewable technology
• Energy storage
• European super grid
• Geothermal heat pump
• High-voltage direct current (HVDC)
• List of power outages
• List of energy storage projects
• Load management
• Motor-generator
• National Grid Reserve Service
• Northeast Blackout of 2003
• Relative cost of electricity generated by different sources
• Economics of new nuclear power plants (for more cost comparisons)
• Spark spread: calculating the cost of back up
• Three-phase electric power
• Smart grid and V2G to override intermittency.
• Energy security and renewable technology
• List of energy storage projects

• Sivaram, Varun (2018). Taming the Sun: Innovation to Harness Solar Energy and Power the Planet. Cambridge, MA: MIT Press. ISBN 978-0-262-03768-6.

• 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