Long-duration energy storage: get on with it Contents

Appendix 3: Glossary

Technical term


Black start

Black start is the process of restarting the grid after a partial or total shutdown, which would require isolated facilities being restarted and gradually connected to each other to form an interconnected electricity system again. This requires individual facilities in strategic locations, which are capable of switching themselves back on in case of a power outage, which the National Grid can procure “black start” services from to restart the grid if needed.


Curtailment in renewable energy is the deliberate reduction in output below what could have been produced in order to balance energy supply and demand or due to transmission constraints. Wind generation in the UK is often curtailed.


Reduction or elimination of carbon dioxide emissions from a process, such as manufacturing or energy production.


Shifting energy demand towards electricity—for example, electric cars replacing fossil fuel cars, or heat pumps replacing gas boilers


The process of using electrical currents to drive chemical reactions—particularly the splitting of water into hydrogen and oxygen. Machines that do this are called electrolysers.

Explicit trading

Explicit auction is when the transmission capacity on an interconnector is auctioned to the market separately and independently from the marketplaces where electrical energy is auctioned. Since the UK has left the EU’s electricity market, it must bid for transmission capacity in this way, and it has been argued that this is a less efficient form of trading, as traders have to bid for capacity before they know the price of electricity that will be sold.262


Inertia in power systems is currently provided by rotating turbines in thermal electricity generators. If there is a sudden change in system frequency, such as a power plant outage, the turbines keep spinning due to their inertia, which slows down the change in frequency of electricity on the grid while stability is restored, so it is important for controlling the frequency of the alternating electrical current. Renewables do not have the same property of spinning turbines that rotate in sync with the electricity frequency.


In renewable electricity, this refers to electricity generation that is not continuous but subject to periodic stopping. Wind and solar are both intermittent.

Load factor

The load factor is the effective percentage of theoretical maximum generating capacity that is used over a given period of time. A power facility with a maximum, or nameplate capacity of 100 MW might only generate 80 MWh in an hour if it is not always operating at full capacity, so the load factor would be 80%.


No-regret actions are cost-effective now and under a range of future climate scenarios and do not involve hard trade-offs with other policy objectives.263 A no-regrets level of investment in hydrogen storage would be an amount of infrastructure which is useful under a range of different assumptions about what the future energy mix and price of renewables would be, which can be determined through modelling.

Phase-change materials

In thermal storage, phase-change materials are materials that can absorb or release heat by changing phase—for example, from solid to liquid (absorbing heat as they melt).

Reactive power

Reactive power services allow the grid to maintain safe voltages (the voltage is the amount of power transferred by a given current of electricity), and generators or other electricity system assets that can help maintain voltage control across the network are said to absorb or generate reactive power.


“Unabated” use of fossil fuels colloquially refers to fossil fuels that are used without any carbon capture and storage methods to reduce their emissions.

262 Frontier economics, ‘Brexit and interconnectors: a £45m problem?’: https://www.frontier-economics.com/uk/en/news-and-insights/articles/article-i8192-brexit-and-interconnectors-a-45m-problem/ [accessed 10 January 2024]

263 Climate change, ‘Examples of ‘no-regret’, ‘low-regret’ and ‘win-win’ adaptation actions’ (November 2012): https://www.climatexchange.org.uk/research/projects/examples-of-no-regret-low-regret-and-win-win-adaptation-actions/ [accessed 10 January 2024]

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