Harnessing renewable resources such as wind and solar energy is vital for the green transition. But these renewables are intermittent and unpredictable. It’s impossible to control when the wind blows and when the sun shines.

Currently, we don’t have to rely on the weather to boil a kettle. That’s largely thanks to our use of fossil fuels. To break our reliance on this finite and polluting source of power, we need to be able to store vast amounts of renewable energy cheaply and safely, but unfortunately, no technologies yet tick all the boxes.

Lithium-ion batteries are used in our phones, laptops, and electric vehicles. But lithium-ion technology depends on costly, geographically concentrated materials such as lithium and cobalt.

Cobalt mining can be a dangerous and polluting process. The organic lithium-ion-containing electrolyte (the liquid chemical between the positive and negative terminals inside a battery) is flammable. Because it poses a fire hazard, it’s risky to deploy these batteries at a large enough scale to power our homes, cities, and industries.

But there is an alternative solution: flow batteries.

In a flow battery, energy is stored in liquid electrolytes which are held in large external tanks. To charge or discharge the liquid, it is pumped through a stack of cells where electrochemical reactions generate or consume electrons. This has several advantages.

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Read more: What will batteries of the future be made of? Four scientists discuss the options – podcast

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In most batteries, two properties (energy and power) are intrinsically linked. Energy refers to how much electricity is actually stored by the battery and therefore how long that supply can run at a given rate. Power is the rate at which the energy is delivered and how much can be supplied within a given time.

Usually, it’s impossible to change one without changing the other. To store more, you add cells, and that adds power you may not need (and cost you don’t want).

However in flow batteries, this is not the case. Need to store more energy? Simply use larger tanks with more liquid. Need more power? Simply use a bigger cell stack. This ability to independently control power and energy makes the technology cheaper to scale up. Additionally, unlike lithium-ion batteries, flow batteries can’t catch fire (as they are mostly water) and are extremely durable. Some flow batteries have been in operation for more than 20 years.

Several large-scale flow battery systems have already been deployed. In China, the largest systems that connect to the grid reach gigawatt-hour scale (equivalent to powering 100,000 homes for a full day). In Switzerland, what will be the world’s largest flow battery, at 2.1 gigawatt-hours (equivalent to powering over 200,000 homes for a full day) is being constructed to power an AI data centre. Nevertheless, flow batteries are still an emerging technology, with several challenges still to solve.

Test cells

Worldwide, researchers like us are developing new electrolyte chemistries and materials to reduce the cost of flow battery systems and drive wider adoption. But getting started in flow battery research can be quite daunting. Test cells can often cost thousands of pounds and testing a new cell requires lots of ancillary equipment.

Through our research, we have developed a low-cost 3D-printed test cell. This makes flow battery research much easier and has enabled our research group to test dozens of these cells. But our team struggled to get repeatable test results. It was also difficult to replicate results from previous flow battery studies by other groups in our own labs at Queen’s University Belfast. This resulted in several months of frustration as we scrutinised every aspect of our testing protocols.

As it turned out, our research group was not alone. At a conference in early 2024, we attended a talk by Fikile Brushett, a chemical engineering professor from the Massachusetts Institute of Technology in the US. He described the lack of protocols in this field. Together, we have led a series of studies to investigate and work out how to tackle the issue of reliably replicating data from reported scientific studies.

In one study, published in April 2026, we sent our low-cost 3D-printed test cell to several leading research groups around the world. We were struck by the extent of performance variability between different groups all nominally testing the same battery. Since then, we have identified several potential causes for this variability and have suggested improvements to testing protocols.

Our latest study has involved more than 30 research groups assessing our test cells. Our testing data is now publicly available online. We are using this to pin down why performance varies across the cohort. Our findings will help the community to develop testing protocols that all researchers can use.

Eventually, this work could make it easier for newcomers to make a start in flow battery research. It could also give established groups a reliable foundation to compare results and accelerate their work. This all helps chemical engineers innovate more quickly towards storing renewable energy cheaply, safely and at scale.

Joshua James Bailey receives funding from UKRI's EPSRC.

Hugh O'Connor received funding from UKRI's EPSRC and PSDI.

William Adams was entranced by energy. As a young man, his interest was nursed by working as a clerk in a London patent office in the 1860s. This gave him an early look at some of the first British designs for exploiting solar energy using mirrors, water or both.

Adams would later recount his excitement at reading about the French mathematician Augustin Mouchot’s invention of the first machine ever to run on energy from the Sun. The device, which connected a solar boiler to a specifically designed steam engine, was warmly received by Napoleon III when it was presented to the emperor in 1866.

Inspired, Adams soon designed and patented his own rudimentary solar boiler. The only problem was, he needed more sun.

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This series is dedicated to lesser-known, highly influential scientists who have had a powerful influence on the careers and research paths of many others, including the authors of these articles.

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When offered the chance to become deputy registrar of Bombay by the Indian city’s governor, Sir Philip Edmond Wodehouse, Adams jumped at the opportunity. There, he became the first Briton to design, build and test a fully-functioning solar steam engine fit for industrial purpose.

But he also came up against the conservatism of India’s colonial rulers, who did not see this Bombay bureaucrat for the energy visionary that he undoubtedly was.

‘The rays beat like missiles’

Adams arrived in Bombay in 1873 to find it in the middle of a cotton boom, with mills popping up like mushrooms across the city. The population was growing so quickly that firewood was depleted for miles around. The landscape grew “bald as a billiard ball”, as Adams put it.

Every morning before setting off for work near Bombay’s central fort, Adams would set up his outdoor laboratory at his home in the southernmost Colaba district, near the open sea. He instructed an Indian fundhi (skilled carpenter) to build a set of three-tiered wooden shelves to hold 18 looking glasses.

“Each glass was moveable on a swivel in the same manner as an ordinary toilet glass”, Adams explained, meaning he could pivot each glass by “the touch of the finger”.

Later, for open-air experiments, Adams used two banks of mirrors (36 in total) which made “the mercury in the thermometer boil, leaping up to over 670 degrees fahrenheit”. He then placed a copper cylinder containing three gallons of water in the focus of all 36 mirrors, making it boil in exactly 20 minutes.

But Adams’s ambition did not end there. To reach sufficient pressure in the boiler to drive a steam engine, this bureaucrat-cum-engineer built a giant concave mirror, 24 feet in diameter. He then sent for his London solar boiler, which was delivered by ship to Bombay in 1876.

One fine morning, Adams – wearing dark glasses for safety – turned his giant concave mirror on the copper cylinder filled with water. “The rays beat like missiles in a continuous and incessant storm of solar fire,” he wrote.

An hour later, the cylinder registered 55 pounds of pressure per square inch. He hired a steam engine of 3 horsepower and connected it to the boiler: the pressure moved the pistons. Adams had built the first working, British-designed solar steam engine.

For a fortnight, he kept the pump going near his bungalow in Colaba – proudly and sweatily displaying his innovation to government officials, newspaper reporters, mill owners and the local Indian communities. Members of the public were invited to witness his experiments too, via a notification in a Bombay newspaper.

‘An inexhaustible source of wealth’

In 1877, Adams wrote a letter to the editor of the Times of India arguing that the application of his solar steam engine would “make India the seat of the principal manufacturing industries of the world”.

Later, in his wildly ahead-of-its-time treatise Solar Heat: A Substitute for Fuel in Tropical Countries (1878), Adams argued that countries near the equator “possess, in their clear skies, a gratuitous and inexhaustible source of wealth, equal to that which western nations have to dig, with infinite labour and toil, from the bowels of the Earth”.

Adams sketched out plans to use solar heat for everything from cotton gins (engines to separate cotton fibres from seeds) to Hindu crematoria. He called upon the colonial British government to invest in this promising substitute for coal, which was then being imported to India at great expense.

Adams envisioned solar energy transforming the Raj. Just like the coal-combusting steam engine had replaced the waterwheel in England, he argued that thermal heat could now replace fossil fuels in India. But his colonial bosses were not persuaded.

‘Too subversive’

Adams was part of a 19th-century wave of global research into solar steam engines, as I explore in my postdoctoral project and upcoming book. But in contrast to fellow pioneers including Frenchman Mouchot, Adams built his solar steam engine to stimulate local Indian industry, not to benefit the colonial government.

The locals shared Adams’s belief in this technology. One even wrote to Scientific American magazine to express their desire for the rapid adoption of solar power:

My residence is in a tropical part of India … where fuel is scarce and dear … In this part of the country (about 300 miles north of Bombay), there is a great opening for cheap power in small units.

Bombay’s new governor Sir Richard Temple concluded, however, that solar heat “could not be used for commercial purposes on a large scale”. He argued that local factory owners would not like giving “the workmen a holiday on days when the sky is not clear”.

In truth, Adams’s invention was too subversive for Britain’s colonial officials and capitalists. In less sunny climes, solar energy – tethered to the seasonal rhythms of nature – might negate their commercial ambition for timeless industrial production. But they also saw India as an important market for British coal exports.

While a few mill owners adopted Adams’s auxiliary solar heater for their steam engines, most regarded it as a primitive contraption unfit to satisfy the demands of modern civilisation.

Increasingly frustrated that neither the industrial capitalists nor the colonial government supported his vision, Adams abandoned further experiments. His dream of India switching away from coal to solar power, from combustion to concentration, would not happen for at least another century.

Now, however, India is a world leader in the global energy transition. It heads the International Solar Alliance, and is the third largest solar power generator in the world.

Which begs the question: how much further advanced would this technology be had Adams’s 19th-century solar experiments been embraced by India’s colonial rulers at the time?

Sebastian Egholm Lund receives funding from the Carlsberg Foundation. His upcoming book, Changing the Climate at the Fin de Siècle, is published by Cambridge University Press (September 2026).