Pumped up: how 'high density hydro' could supercharge global energy storage

To hikers trekking the hills of an off-season ski resort in the Laurentian mountains of Quebec two summers ago, the small group led by Tamás Bertényi could have been mistaken for fellow ramblers.

In fact, they were engineers from U.K. start-up RheEnergise who were about to start testing an innovative energy storage concept that could radically change the way industry thinks about balancing the clean power-fed grids of the future.

Say energy storage to most people and lithium-ion (Li-ion) batteries will come to mind. Best known for powering electric vehicles, batteries can also be stacked together to create plants with almost 1-gigawatt of capacity, enough stored power to supply 750,000 North American homes.

Batteries, however, are limited by a four-hour charging cycle, unlike a growing number of so-called “long duration energy storage” (LDES) technologies that have already been shown to often be a cheaper storage solution than Li-ion, according to Bloomberg New Energy Finance (BNEF), an industry research house.

BNEF analyzed 20 LDES technologies — including thermal, chemical and gravitational designs — with energy storage lasting for weeks rather than hours. It found the least expensive design would require an average capital spend of $232 per kilowatt-hour (kWh) to build, much lower than the average of $304/kWh for the latest Li-ion facilities.

Among this wave of next-generation LDES technologies is the “high density” pumped hydro system that RheEnergise successfully piloted in Quebec in 2022. A large-scale demonstration project now under construction near Plymouth, U.K. will be brought online later this year.

RheEnergise’s technology has been in development for just over five years, but the idea behind it is almost as old as hydropower itself.

Pumped hydro was pioneered in Switzerland in the 1890s. The concept involves pumping water from a lower reservoir in a hydropower complex up to a higher reservoir, to store the energy until it is needed, then releasing the water through a turbine array to generate electricity.  

Straightforward to engineer and inexpensive to maintain, this type of industrial-scale pumped hydro energy storage has been widely adopted over the years, with a total of 140-gigawatts projects now operating in countries including China, the United States, Norway, Japan, India and Canada, where the Sir Adam Beck generating station on Ontario's Niagara River is the country's only facility. 

But conventional pumped hydro depends on three non-negotiable requirements: a deep mountain valley, a waterway to fill the reservoirs, and a 15-20 year construction timeline to complete a project — all geographic or economic limitations.

RheEnergise’s twist on this tried-and-true energy storage process is High-Density Hydro (HDH). Electric motors pump a proprietary mud-brown slurry that is two and a half times denser than regular water through a long loop of plastic pipe installed on a hillside, storing the energy until it is released back onto the grid. (See Factbox (below): Closed loop and a 'secret sauce': how High Density Hydro works.)

Unlike conventional pumped hydro, a full-scale HDH system can be up running in 2-3 years on any hillside with a slight 15-degree incline, and needs no water source.

Carbon-intensive legacy infrastructure

"The world is trying to move away from legacy infrastructure, which is all built around carbon-intensive sources, led by wind and solar, two intermittent renewables,” said Bertényi, RheEnergise's chief technology officer. “The energy transition needs to resolve this or face serious grid instability.”

LDES is expected to be a linchpin technology in the grid of the future, making it possible to balance demand with variable power supply from wind and solar, using power dispatched from utility-scale storage plants.

“We are ultimately trying to move toward having balanced, dispatchable [on-demand] renewables-powered electricity. So this needs some clear thinking," Bertényi told Canada's National Observer at RheEnergise's laboratory and test centre in Montreal.

McKinsey & Co, a management consultancy, put the challenging opportunity into hard numbers in a recent eport that forecast 100-fold growth in the demand for LDES technology through the next 15 years, with the sector expected to become a $4 trillion market by 2040. 

In the eyes of Bertényi and other industry players that kind of stratospheric expansion will not be possible with existing technologies.

"Achieving 100-times growth in deployment [of LDES] by then? The status quo is not going to cut it,” Bertényi said. “The short version: energy storage is a trillion dollar market for whoever can deliver a low-cost and scalable solution.”

The operative word for RheEnergise is “scalable.” The energy storage technology has to be a high-volume, long discharge-cycle concept that can be rapidly engineered and built at a large number of sites around the world.

The design of the 500 kWh system near Plymouth could be sized-up for huge 10-50 megawatt-hour facilities and run with the minimal material losses that are ideal for LDES technology.

As the concept needs 2.5 times less vertical elevation than conventional pumped hydro systems, there are more than a million sites globally that are "well suited" to HDH installations, according to satellite mapping conducted by the company.

"There are enough locations to meet the energy storage needs of the energy transition several times over,” Bertényi said. “But more importantly, these solutions have to be truly global — just as climate change is. They have to be translatable to any part of the world."

"Half the price" of Li-ion batteries

HDH could cost less than half the price of a Li-ion battery-based system (currently over $270/MWh) and a quarter of the nearly $450/MWh cost at a green hydrogen facility for eight hours of storage, U.K. consultancy Mott McDonald said in an analysis commissioned by RheEnergise.

The company said HDH is closing in on the cost of conventional pumped hydro, currently the cheapest energy storage solution, with projects operating at around $120/MWh.

Based on data crunched for the U.K. demonstration project, a 160MWh version of the HDH design — 20MW for 8 hours — could be built for under $50 million, roughly a quarter of the price of traditional pumped hydro. 

With HDH moving through its demonstration phase, RheEnergise has signed memorandum agreements (MoUs) with potential partners in target markets such as the U.S., Australia and Canada.

Last year RheEnergise signed an MoU with Chile’s biggest hydropower operator, Colbun, marking the Canadian company’s first foray into South America’s energy market. 

Since then, the company has been working to develop its HDH technology within a four-stage, 80-megawatt storage system taking shape in Chile’s Atacama desert.

Bertényi said the company’s project pipeline — a third of which is located in North America — is worth around $1 billion if the MoUs progress into built projects.  

"We are optimizing for different factors and the big one is time: if we don't roll out vast amounts of energy storage, the energy transition is going to stall," he said.

RheEnergise scopes out focus markets in terms of energy transition “pain points,”  Bertényi said.

"Prime opportunities" in Canada

In Canada, the prime opportunities are in Ontario, where next year’s round of power project leasing will include energy storage for the first time, and Alberta where the shift from hydrocarbon-fuelled to renewables-powered grids is inching forward.

"These two provinces are in the spotlight for us, but even Quebec — with its incredible big green energy source [its hydropower network] — is going to have to deploy a lot of renewables," he said.

Closed loop and a 'secret sauce': how High Density Hydro works

RheEnergise's High Density Hydro concept is a new approach to energy storage inspired by conventional pumped hydro, a technology first installed in 1907 at the Engeweiher generating station near Schaffhausen, Switzerland. Instead of constructing and filling a giant dam in the mountains outfitted with a pumping system that uses water as the working fluid, the UK start-up has created a mineral-based “secret sauce” that is 2.5-times denser than water and circulated through a large closed loop of plastic pipe to store energy.

When power demand on the grid is low, the fluid dubbed R-19, is pumped uphill - using low-cost energy sources such as wind and solar - to a storage reservoir at the top of the installation. As electricity demand rises and pushes up the power price, the stored R-19 fluid is released downhill in the closed loop piping system and passes through an array of turbines, generating electricity that flows onto the grid.

The mineral mix at the heart of R-19 is closely guarded intellectual property, according to RheEnergise chief technology officer Tamás Bertényi. However, he said it comes from "globally abundant [sources] already used in applications such as paints, papermaking and drilling mud for oil and gas."

The largest HDH project so far is a 500-kilowatt facility in Devon in southwest Britain that is due to open in late 2024. RheEnergise sees future installations of between 10-50 megawatts of storage capacity - big enough to stabilize existing grids but with a plant that can be 2.5-times smaller than a facility for conventional pumped hydro.

Hydro Quebec plans to add 10 gigawatts of wind farm electricity to the provincial grid to help meet an expected near doubling in demand in the next decade. “Storing that power will require billions of dollars in investments by Hydro Quebec,” he said.

"It is a very real chance that one of our first commercial projects, which will help define the global niche for our technology, will be in Canada," Bertényi said.

"The ingredients — the looming need for big energy storage, the industrial hydropower supply chain, the government support, and even the key minerals — are all here."

Financing to develop the company’s HDH concept has so far relied on clean-tech angel investors and government funding for new technologies. But with the 500kW UK demonstrator project set to switch on later in 2024, Bertényi said the company is in talks with a number of partners to support its plans.

Though Bertényi naturally backs RheEnergise’s HDH concept to win the LDES race, he believes there is room for a wide range of energy storage solutions given the scale and demands of a future decentralized grid.

"There isn't a one-size-fits-all. The [energy storage] sector is still really in its infancy,” Bertényi said. “Commercial energy storage from batteries to LDES will be about finding your niche. We think [HDH] will fill quite a big niche.”