The next entry in our Innovating to Net Zero Insight series is a little different, marking the first blog from one of our innovators. Over the coming months we’ll be inviting others developing solutions to improve the flexibility of our energy system to share with us how their technology works and what’s needed to maximise its potential.

First up is RheEnergise a provider of long-duration energy storage, headed by CEO Stephen Crosher.

Who we are

We are RheEnergise, a long-duration energy storage (LDES) solution provider. Our technology is a next-generation pumped hydro solution (High-Density Hydro) which is locationally flexible and rapidly scalable. Projects are quick to consent and construct, modular and repeatable, and low-cost. Our solution can provide 4-20 hours of storage, making High-Density Hydro a scalable and cost-effective solution for long-duration energy storage.

Pumped hydro has been around for over 100 years and works according to basic principles: when electricity is low-cost, it is used to pump water from a lower reservoir through pipes, into an elevated reservoir at the top of a hill. When electricity prices are high, the water is released from the upper reservoir, flowing back through the pipes into the lower reservoir via a turbine, generating electricity back to the grid. RheEnergise’s innovation is simple (see Figure 1): by using a fluid that is 2.5x denser than water in a pumped storage system, the vertical elevation requirement is reduced by a factor of 2.5. As a result, projects can be built on hills instead of mountains, and land requirements are significantly reduced. This opens up a huge number of sites globally. In the UK alone, we have identified over 6,500 potential sites.

Our vision for the future energy system

At RheEnergise, we envision a future energy system built on low-cost renewable energy sources, backed up by a range of energy storage technologies. The rapid decrease in the cost of wind and solar power has proven that such a future is possible, but also poses challenges due to the inherent variability of wind and solar. Flexibility is the key to realising a renewables-based electricity system.

There are various ways of providing grid flexibility:

• Demand-side response: allows us to align electricity demand with periods when renewable generation is abundant but relies on accurate forecasting and coordination of multiple individual actors and devices.
• Transmission interconnectors: are essential for sharing renewable electricity between regions with different weather patterns but are expensive and slow to build.
• Storage: provides a critical source of flexibility as it allows us to store renewable electricity when it is generated and use it when it is needed. UBS Investment Bank estimates that globally 9 TW of electricity storage will be required by 2050, which represents a 34x increase based on current levels.

Technological diversity will be key to meeting our energy storage needs, ensuring we don’t become over-reliant on a single technology from a single source. Batteries already play a critical role in short-term energy balancing, but we also need long-duration solutions. This need is particularly acute in Britain given the prevalence of wind energy in our electricity system, which leads to longer periods of electricity surplus and shortage due to the variable nature of wind patterns.

Aurora Energy Research predicts that Britain will need at least 18 GW of capacity with over eight hours storage duration by 2035. Having a broad portfolio of technological solutions for different applications will help to achieve this, while also providing essential ancillary services to the grid.

Bridging the gap with High-Density Hydro

High-Density Hydro can play an important role in bridging the peak power gap and the peak energy gap during periods of oversupply as well as undersupply. Our technology’s rapid response time means it can dispatch stored electricity at peak power output in a matter of seconds to meet peak demand. With a round-trip efficiency of 80% and low operating costs, High-Density Hydro is suitable for daily cycling which allows it to eat into the peak power gap by shifting demand from peak to off-peak times. Furthermore, as a scalable solution with an abundance of suitable sites, it can make a significant contribution to addressing the peak power gap.

Equally important, our solution can also help to bridge the peak energy gap due to its long discharge duration. Typically, short-duration solutions like lithium-ion batteries can only discharge at their maximum power output for up to four hours due to their high energy capacity costs (in £/MWh). This limits their ability to bridge the energy gap during prolonged periods of low renewable generation. To be cost effective, long-duration storage technologies need low energy capacity costs, which is where High-Density Hydro steps in. RheEnergise are focused on delivering a highly competitive solution on a unit cost of energy stored basis.

Like conventional pumped storage, the energy capacity cost (i.e. the storage reservoir) is independent of the power capacity cost (the turbine). This makes it relatively much cheaper and easier to increase the energy capacity by simply increasing the size of the reservoir, which increases the discharge duration. Once stored, electricity can then be discharged at peak power for more than 16 hours, thereby helping to solve the peak energy gap.

Barriers and enabling conditions

The market signals are already clear: the UK is crying out for more long duration energy storage. Last year, over 8,100 GWh of renewable electricity was wasted because it couldn’t be stored, enough to power the whole country for over 11 days. To decarbonise the electricity grid, we need a rapid build-out of clean electricity generation and storage. Lengthy delays in securing grid connections are a clear barrier preventing this from happening.

NESO’s recent reshuffle of the connections queue is a positive step, prioritising ‘shovel-ready’ projects. However, unless these projects are co-located with energy storage, we will continue to see large amounts of renewable electricity being curtailed.

Storage must be prioritised to keep pace with renewables deployment, and hybrid renewables-plus-storage projects must become the norm.

Long-duration storage projects often have high upfront capital costs. To make these projects ‘bankable’, investors need revenue certainty to ensure that capital costs can be recovered over the project lifetime. The UK government’s cap-and-floor scheme for long duration energy storage is a welcome policy support scheme as it provides this revenue certainty for investors. As we prepare for the next application window of the cap-and-floor scheme, we look forward to working with government to effect incremental improvements which will ensure deliverability of the government’s decarbonisation goals.

Going forward, it is clear that the enhanced system security benefits provided by diverse solutions and supply chains should be better accounted for in the procurement process. Targeted capital funding support could also help emerging long duration energy storage technologies reach the commercial development stage more quickly, which in turn will help achieve technological diversity and increase the tools at our disposal to bridge the peak gap, along with reducing costs for consumers.

Energy Systems Catapult’s flagship Innovating to Net Zero 2026 report and event in February will highlight the innovation priorities needed to support a flexible energy system which is affordable and fit enough to thrive in the race to Net Zero.