Variable renewable energy

Variable renewable energy (VRE) is a renewable energy source that is non-dispatchable due to its fluctuating nature, like wind power and solar power, as opposed to a controllable renewable energy source such as hydroelectricity, or biomass, or a relatively constant source such as geothermal power or run-of-the-river hydroelectricity.

The International Energy Agency says that there has been too much attention on issue of the variability of renewable electricity production. This issue only applies to certain 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. But at high levels of market penetration it requires careful analysis and management, and additional costs may be required for backup or system modification. 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.

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.

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