UK Renewable Energy Storage Options

Energy storage technologies span a diverse spectrum, each catering to specific duration and scalability requirements. Of the renewable energy storage options in the UK that will help deliver a clean grid, there is a large range of durations. Put simply, that means there’s lots of variation in how long a storage technology can maintain its full capacity power output.

There are three scales of storage options for the future of UK renewable energy and all will play a part in the energy transition, meeting the demands of different domestic, local, area, regional and national levels. Just as the road network needs all scales to operate, so does the energy storage portfolio.

Very Short-Duration Storage

Supercapacitors and flywheels are best for ultra-short durations of energy discharge, generally of up to four hours. This form of storage is ideal for rapid response and stabilising the grid. These can generate synthetic inertia, which is crucial for black-starting the grid during a nationwide power outage.

Other examples include cryogenic storage, otherwise known as Liquid Air Energy Storage, which is fairly expensive and complex but without geographical limitations. There are also flow batteries, but their dirty secret is that they tend to involve swimming pools full of concentrated acid.

All types of batteries have environmental issues related to mining, refining, processing and disposal.

Short-duration Energy Storage

Battery Energy Storage Systems (BESS) such as lithium-ion, dominate this segment. These offer storage durations from minutes to several hours, crucial for balancing supply and demand fluctuations.

Batteries played their part maintaining the UK grid in summer 2024, when they stepped in after an interconnector failed between the UK and Norway. The technology kept the grid balanced and prevented the lights from going off.

Most people have heard about lithium ion batteries, described by tech billionaire Elon Musk as ‘the new oil’. However, a major disadvantage of lithium is that there isn’t enough of it (or of cobalt and other esoteric metals) in the earth’s crust to support the planet. It is much better for applications where its weight, bulk and energy density are at a premium: portable equipment and transportation.

There are other options to lithium such as other lithium- and sodium-based chemistries, and in lead-acid. Each has its advantages and disadvantages. However, these minerals are just as—if not more—scarce than lithium and cannot maintain storage on a massive scale.

It’s important to note that batteries tend to quote their efficiencies as “gross” rather than round trip. The difference is the cooling, power conversion etc. These grid connected batteries’ actual round trip (i.e. grid-to-grid) efficiency is 42–68% depending on scale, on day 1; by year 5–8 their heat losses have tripled and so efficiency drops. CAES efficiencies are quoted as grid-to-grid.

Long-duration Energy Storage

This includes technologies like compressed air energy storage, pumped hydroelectric storage and thermal storage systems. These can store energy for extended periods, ranging from several hours to days or even months. As a result, these systems are essential for meeting sustained energy demands during periods of low renewable generation.

CAES

CAES has some geographical limitations but potential locations are widespread because salt caverns exist worldwide. Salt caverns are the perfect massive size to meet the demand required and expected in the future and because of their size are cheaper per unit of power generated compared to smaller alternatives.

It comes in two versions: diabatic (traditional) and adiabatic (such as Storelectric). Compressing air to a typical 70 bar (~30x car tyre pressure) heats it to ~605°C, but the air needs to be close to ambient to be stored in salt caverns without damaging them. The heat must therefore be extracted.

However, expanding the air to regenerate the electricity cools it to below -150°C. To avoid freezing the equipment, traditional CAES puts the heat back in by burning gas: inefficient (42–50% round trip) and polluting (50–70% of the emissions of an equivalent sized CCGT). Adiabatic CAES extracts the heat of compression, stores it separately and puts it back in during expansion, increasing efficiency to 60–70% and eliminating emissions; hybrid technologies are possible.

Pumped hydro

Pumped hydro involves using excess renewable energy to pump water uphill and then when demand is high but grid generation low, releasing this water to spin a turbine and generate electricity. The technology is ~98% of installed capacity, and ~75% efficient (higher numbers as some plants rely on in-flowing water). However, it often requires the flooding of one or two valleys, is considerably dearer than CAES and has fewer potential locations that tend to be very remote from both supply and demand.

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