Fossil generators’ system stability role could be fulfilled by renewables, says UKERC report

The ‘inertia’ that fossil fuel plants provide to the electricity system, which keeps the system in stable operation and allows it to ‘ride through’ disturbances, can largely be replicated by very fast response frequency control systems and synthetic inertia, according to a new report from the UK Energy Research Centre (UKERC). The report, Costs and Impacts of Intermittency, updates a 2006 assessment with a full survey of recent research since then.

The finding is important because large fossil-fuelled plants with heavy machinery were thought to have an important role in keeping the electricity system within frequency and voltage limits. Stability services have provided additional revenue for such plants and in some cases they have been kept from closure to perform the service.

The report does not suggest that no conventional generators are required. But it says that even when renewable energy levels are above 50% – which can happen already at times in the UK, when low demand coincides with high wind and solar levels -  “sufficient inertia-like resilience could be provided, typically through a combination of very fast response frequency control systems and synthetic inertia.” And although inertia is inherent in large fossil plants because there is a large rotating mass, researchers have found other sources. For example “there is considerable inertia in the rotating mass of wind turbines”, and it found studies suggesting that in concert they might provide as much inertia as a large fossil generator of the same rated power, albeit with additional control systems. The use of very fast-response assets like batteries could also help provide the service.

The finding was among several that suggested a high renewables penetration could be managed at relatively low cost.

Up to a renewables penetration of about 30%, the cost of the reserves required – to cover both short-term fluctuations and longer periods of low generation – is as low as £5/MWh or less, according to the UKERC report. Higher penetrations (measured either by the proportion of annual supply or of peak demand supplied by renewables) have fewer data sources available but the extra cost is likely to be £15-45/MWh depending on how flexible the existing system is.

The report notes that all types of power generator have characteristics that have to be accommodated by the system. If they are the incumbent plant, the system is generally designed around them and so the management costs are not obvious and may not be identified and recognised. But the system design, and the amount of flexibility it has, partly determines the cost of adding new types of generator, which is why the system cost of renewables varies.

At high penetrations UKERC found there was a possibility that generation would have to be curtailed on days of high wind and sun. But that varied depending on the structure of the industry, and in the UK and Europe analysis suggested that curtailment levels are very low until over 50% of electricity is from renewables.

What is the additional transmission and distribution costs of high renewables penetration? The study said there were few data sources for scenarios with higher than 30% renewables. Below 30%, costs are in the range of £5-£20/
MWh, and this cost varies more between studies or between scenarios than it does between different penetration levels.

There is a rebound effect as conventional generators are operated in less efficient ways to give priority to renewables, but that was low, the researchers found. Although it varied between systems, the extra emissions from running fossil plants flexibly were less than 10% of the emissions saved by using renewables.

Read the full report: The Costs and Impacts of Intermittency II – ukerc

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