Five technologies from the five broad technology categories: electrochemical; mechanical; chemical; electrical and thermal.
Pumped hydro has been used as a storage to support the grid since as far back as the 1890s, almost since the start of electricity distribution itself.
As the system undergoes its most fundamental transition from centralised to decentralised, storage is playing an increasingly important role, with requirements from the sub-second level up to months and even seasons.
More than 30 storage technologies are available currently and more are appearing apace with the advance of new techniques and materials, particularly at the nano level, and with the evolution of the business model.
Here we offer (a non-exhaustive) five energy storage technologies to watch – one each from the five broad technology categories: electrochemical, including solid and liquid batteries; and mechanical, from pumped hydro to flywheels to gravity, the most diverse; chemical; electrical; and thermal.
Mechanical: Pumped hydro storage
What: Energy storage with pumped hydro systems has been widely implemented around the world, with over 160GW of installed capacity and comprising over 90% of the world’s energy storage for the grid. Such systems require water cycling between two reservoirs at different levels with the ‘energy storage’ in the water in the upper reservoir, which is released when the water is released to the lower reservoir.
Why: Pumped hydro is a low cost and well understood form of mechanical energy storage. It is able to deliver benefits on scales from minutes to days and can support a range of grid services. It is a challenge is to find appropriate sites but older facilities may be readily modernised and can also form part of hybrid facilities with the siting of floating or nearby renewables.
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Where: Numerous projects are under way, with the installed capacity projected to double to 325GW by 2050. Loch Ness in Scotland, famously known for its so-far mythical ‘monster’, is set to be the site for the 450MW ‘Red John’ pumped hydro scheme at a cost of £550 million ($664 million) – one of three currently under way on the Scottish lochs by clean energy developer ILI Group.
Thermal: Molten salt storage
What: Thermal storage in essence involves the capture and release of heat or cold in a solid, a liquid or air and potentially involving changes of state of the storage medium, e.g. from gas to liquid or solid to liquid and vice versa. Several large scale technologies are being developed, including molten salt and liquid air, while hot water and storage heating offer potential for residential flexibility.
Why: Thermal storage offers a long lifetime option for long term storage on scales from days to months, with the proviso that certain options may be limited by the need for large underground storage caverns. As such it can offer a low cost option for systems with high penetration of weather dependent renewable energy.
Where: Molten salt is currently the most used in the power sector, due to its advanced technological readiness and application with concentrated solar power plants, with an installed capacity of over 21GWh worldwide. The fourth phase under development at the Mohammed Bin Rashid Al Maktoum Solar Park about 50km south of Dubai in the UAE is set to include a molten salt plant with what is believed to be the world’s largest thermal storage capacity of 13.5 hours.
Electrochemical: Flow batteries
What: Batteries are the most widely used form of energy storage, ranging from tiny pen batteries up to utility scale installations stacking tens of cells. As a technology they are also the broadest, with availability in numerous combinations of materials with differing characteristics and typically for energy applications up to about 112 hours of storage capacity. New developments are focused on improving their performance and lifetime; for example, to improve the range of electric vehicles.
Why: Flow batteries with liquid electrolytes offer a greater flexibility to independently tailored power and energy ratings for a given application, rather than other technologies when it comes to storing electrical energy. However, depending on the type, their applicability can be limited by the electrolyte storage requirements. Also, most flow batteries on the market today use vanadium – a rare, expensive and toxic transition metal.
Where: Scientists at the DOE Pacific Northwest National Laboratory have created a new symmetry-breaking design for a molecule that could substantially improve the voltage, electrochemical stability and solubility of electrolytes used in flow batteries. In cells these demonstrate more than 90% capacity over 6,000 cycles, projecting more than 16 years of uninterrupted service at one cycle per day.
Electrical: Supercapacitors
What: In a supercapacitor the charge is stored physically, with no chemical or phase changes taking place as in batteries or thermal storage technologies. Thus, the process is fast and highly reversible and the discharge-charge cycle can be repeated virtually without limit. Different capacitor technologies with different types of cells, electrolytes and system designs are available in a wide variety of capacitance and nominal voltage values.
Why: Supercapacitors provide the fastest response of storage solutions, in the range from a few milliseconds to minutes. Due to the relatively high specific energy combined with low internal resistance and the up to one million cycling capability without significant wear out, they are suitable for a wide variety of applications from regenerative braking to uninterruptible power supply systems and FACTS devices for managing power flows on transmission lines.
Where: Supercapacitors are widely installed in wind turbines on both land and sea to smooth the intermittency from gusting and other wind variabilities. As the North Sea offshore wind capacity grows, supercapacitors will be a key to optimise not only the power output but also – with their long lifetime and limited maintenance requirements – the O&M costs.
Chemical: Green hydrogen
What: Energy storage with hydrogen, which is still emerging, would involve its conversion from electricity via electrolysis for storage in tanks. From there it can later undergo either re-electrification or supply to emerging applications such as transport, industry or residential as a supplement to or replacement for gas.
Why: Green hydrogen is expected to become a major component of the future energy system and many powerto-gas projects are emerging across Europe and elsewhere. Hydrogen storage offers potential mainly for longer term applications of days and longer and several large demonstrations employing salt caverns are on the way. Further experience needs to be gathered on how this particular storage system could interact with wind generation and the gas and electricity networks.
Listen on Enlit.world: Energy Transitions Podcast: Lessons from a Danish hydrogen pilot project
Where: Southern California Gas Company (SoCalGas) is partnering with GKN Hydrogen and the US National Renewable Energy Laboratory to demonstrate green hydrogen as a megawatt-scale, clean energy storage resource and identify potential commercial use cases. The three-year project set to launch by year-end 2022 envisions connecting 500kg of hydrogen storage capacity to a renewables-powered electrolyser and fuel cell at NREL’s Flatirons Campus near Boulder, Colorado to produce and store green hydrogen for subsequent reconversion back to renewable electricity.
Want more energy storage insight. Try these from our sister site Enlit.world:
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