The World's Greenest Battery

Medium-duration energy storage: The key to a low-carbon future

Sarah Keeley


The electricity systems we have developed over the last century are now facing urgent need for redesign. The threat of climate change, an increasing share of intermittent renewable energy sources like solar and wind and the rapid adoption of electric vehicles all pose challenges to existing electricity systems, globally. Energy storage has become an essential part of the new electricity mix, providing flexible power supply, reducing costs and ensuring reliable services for consumers. For a low-carbon future, storage will be needed at all points of the electricity system and will require energy storage over a vast range of discharge times – from less than one-tenth of a second to more than a year. 

No single set of technologies can deal with this huge range. In fact, there are four different ranges: 

  1. Very-short duration storage (less than five minutes), handled best by flywheels, supercapacitors and possibly large inductors 
    Short-duration storage (five minutes to four hours), dominated by electrochemical batteries and demand-side response actions 
    Medium-duration storage (four to 200 hours), where thermo-mechanical solutions are the main options  
    Long-duration storage (over 200 hours), achieved mainly storing bio-mass, bio-gas, ammonia, other synthesised fuels and/or hydrogen. 

At present, approximately 97% of installed energy storage is long-duration storage with most of it coming from pumped hydro plants around the world. However, these types of facilities are often costly, can take years to build and lack flexibility in regard to location. In the shorter-duration energy storage market, lithium-ion dominates but for stationary power applications that need to deliver power for many hours at a time, it’s not the answer. 

Current and emerging medium-duration storage technologies  
Flow batteries 
Flow batteries have a number of different chemistries such as Vanadium redox, iron and zinc. According to the Energy Storage Association they are most suitable for medium-sized medium-capacity projects. Chemicals are mechanically pumped across electrodes which pushes electricity through the system. These batteries have shown promise for medium-duration storage, able to discharge continuously for four or more hours. At present their prices are relatively high but these are expected to fall over time. Unlike lithium-ion, power and capacity can be scaled independently so the storage can be tailored more closely to the use case, which results in solutions being more cost-effective. While some of the chemicals used can be environmentally hazardous, much of the system can be recycled when the system is decommissioned. 
Thermo-mechanical energy storage (TMES) 
Major progress in the development of medium-duration thermo-mechanical energy storage systems has been made in recent years, sparking interest in this technology for commercial and industrial applications. Life expectancies in the range of 20–30 years, low costs, a low environmental impact and flexibility regarding sites make thermo-mechanical energy storage a promising option for medium-duration storage of electricity. Cheesecake Energy’s eTanker storage system is one instance of a thermo-mechanical energy storage system. Some TMES technologies such as traditional compressed air energy storage (CAES) require specific geographical features like a salt cavern for storing compressed air; and others, like liquid air require large-scale installations to be viable, whereas eTanker can be installed anywhere, is easily moved from one location to another and has a medium-scale that makes it applicable in a far wider variety of customer use cases.  
Medium-duration storage can deliver significant value in some widespread and increasingly important applications, including: 
EV Charging 
Fleet depots and motorway service areas can see prohibitively high costs to upgrade their grid connections to meet the demands of EV charging at scale. These upgrades can also take months to years to implement, causing uncertainty and unwanted delays. Medium-duration energy storage can overcome these constraints by managing the capacity available from the existing grid connection, filling up the store when EV charging demand is low and making that energy available when needed by the vehicles. This can more than double the power available for charging, unlocking full electrification and giving fleet operators and individuals the ability to choose the zero-emission solution for their journeys.  
Microgrids (sometimes also called mini-grids) are local energy networks that can be large or small but that have some of the characteristics of a “national” grid, in so far as they balance multiple sources of power and multiple loads. It is becoming increasingly common for microgrids to exist at different scales on an electricity system from a village in India to an industrial park in England to a US city. Medium-long-duration storage systems are a very powerful tool for microgrid developers to employ to balance renewable generation sources with variable loads. 
Shifting Solar 
Many organisations require significant proportions of energy use outside of the peak solar production hours around either side of solar noon. In some cases their peak demand can coincide with peak electricity prices in the 4-7pm band. Industrial operations such as mines and cement works often have a need for 24 hour constant base load. Solar power is often far cheaper than conventional energy generation. Power Purchase Agreements are becoming more commonplace, where solar is combined with storage to provide cheap solar power precisely when needed.  
As the cost of emerging technologies like flow batteries and thermo-mechanical energy storage falls further, storage will become increasingly competitive, and the range of services it can provide will only increase. Medium-duration energy storage can provide a range of key energy services in an affordable manner and has the potential to drive serious electricity decarbonisation and help transform the entire energy sector. 
Cheesecake Energy is developing medium-duration, medium-scale energy storage for applications across the electrical ecosystem. To learn more about our technology, click here. If you have a project in mind, get in touch with us today. 

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Cheesecake Energy Ltd is registered in England and Wales. Company number 10317962. 
Registered address: Ingenuity Centre, Triumph Road, Nottingham, Nottinghamshire, NG7 2TU

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