Variable renewable energy
Variable renewable energy or intermittent renewable energy sources are renewable energy sources that are not dispatchable due to their fluctuating nature, such as wind power and solar power, as opposed to controllable renewable energy sources, such as dammed hydroelectricity or bioenergy, or relatively constant sources, such as geothermal power.
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.
Options to absorb large shares of variable energy into the grid include using storage, improved interconnection between different variable sources to smooth out supply, using dispatchable energy sources such as hydroelectricity and having overcapacity, so that sufficient energy is produced even when weather is less favourable. More connections between the energy sector and the building, transport and industrial sectors may also help.
Background and terminology
The penetration of intermittent renewables in most power grids is low: global electricity generation in 2021 was 7% wind and 4% solar. However, in 2021 Denmark, Luxembourg and Uruguay generated over 40% of their electricity from wind and solar. Characteristics of variable renewables include their unpredictability, variability, and low operating costs. These, along with renewables typically being asynchronous generators, provide a challenge to grid operators, who must make sure supply and demand are matched. Solutions include energy storage, demand response, availability of overcapacity and sector coupling. Smaller isolated 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.
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 or variability is the extent to which a power source fluctuates. This has two aspects: a predictable variability, such as the day-night cycle, and an unpredictable part. The term intermittent can be used to refer to the unpredictable part, with variable then referring to the predictable part.
- Dispatchability is the ability of a given power source to add output on demand. The concept is distinct from intermittency; dispatchability is one of several ways system operators match supply to system demand.
- Penetration is the amount of electricity generated from a particular source as a percentage of annual consumption.
- Nominal power or nameplate capacity is the theoretical output registered with authorities for classifying the unit. For intermittent power sources, such as wind and solar, nameplate power is the source's output under ideal conditions, such as maximum usable wind or high sun on a clear summer day.
- Capacity factor, average capacity factor, or load factor is the ratio of actual electrical generation over a given period of time, usually a year, to actual generation in that time period. Basically, it is the ratio between the how much electricity a plant produced and how much electricity a plant would have produced if were running at its nameplate capacity for the entire time period.
- Firm capacity or firm power is "guaranteed by the supplier to be available at all times during a period covered by a commitment".
- Capacity credit: the amount of conventional generation power that can be potentially removed from the system while keeping the reliability, usually expressed as a percentage of the nominal power.
- Foreseeability or predictability is how accurately the operator can anticipate the generation: for example tidal power varies with the tides but is completely foreseeable because the orbit of the moon can be predicted exactly, and improved weather forecasts can make wind power more predictable.
Wind power
File:Aralvaimozhy station.jpg|right|thumb|A wind farm in Muppandal, Tamil Nadu, IndiaWind-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.
- Intermittence: Regions smaller than synoptic scale, less than about 1000 km long, 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. This is rarely the case for smaller areas with uniform geography such as Ireland, Scotland and Denmark which have several days per year with little wind power.
- Capacity factor: Wind power typically has an annual capacity factor of 25–50%, with offshore wind outperforming onshore wind.
- Dispatchability: Because wind power is not by itself dispatchable wind farms are sometimes built with storage.
- 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. Seasonal variability may reduce output by 50%.
- 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.
- 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.
Solar power
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.- Variability: In the absence of an energy storage system, solar does not produce power at night, little in bad weather and varies between seasons. In many countries, solar produces most energy in seasons with low wind availability and vice versa.
- Capacity factor Standard photovoltaic solar has an annual average capacity factor of 10-20%, but panels that move and track the sun have a capacity factor up to 30%. Thermal solar parabolic trough with storage 56%. Thermal solar power tower with storage 73%.