A toaster and a cement kiln walk into a bar… what comes out later is a battery. Or, more specifically, a “heat battery” and one that could help revolutionise efforts to slash industrial emissions in Australia, and across the world.
On face value the duo don’t have much in common but they — or the materials and processes they represent — are a match for a new style of battery that might be simple enough and easy enough to make to bridge the technology valley of death that lies between early stage funding and serious project finance.
US company Rondo Energy is commercialising a hot brick energy storage system (ESS) that, it says, does the trick.
The idea was to put a toaster-like element inside a 500 kilo brick made of the same stuff that lines concrete kilns and steel blast furnaces, add some inventive aerodynamic design, and use it to superheat air.
That air could then be blasted back into the furnace — hot air is one of the key ingredients for steelmaking and cement — or use it to drive a steam turbine for electricity.
The trick now, as other speculative thermal ESS ventures are finding, is to bridge the gap between venture funding and the sort of finance that will allow them to deliver commercial volumes of their product.
“It’s easier to finance a $3 billion project than a $300 million project,” says Rondo Energy co-founder John O’Donnell.
“There’s been this valley of death-effect in early finance for projects that fully meet all the discipline, all the reporting, all the reliability, and build a track record.
“[They can’t] put in place the foundation for the next wave that’s 10 or 100 times larger. And that’s what we’re doing right now.”
In 2023, the company did a deal with Siam Cement Group to lift manufacturing capacity from 2.4 gigawatt hours (GWh) a year to 90 GWh.
What does that mean? An individual brick weighs 500kg and stores 100 kilowatt hours (kWh) of energy, meaning Rondo’s eventual manufacturing capacity will be 90,000 bricks a year.
The insulation problem
Working with the same stuff lining cement kilns, also known as aluminosilicate, did pose a problem however.
“[Aluminosilicate is] not very heat conductive. That’s why we make fireplaces out of it. It’ll insulate, it holds a lot of heat once you heat it up, but it doesn’t pass heat through,” says O’Donnell.
Other thermal storage proposals use materials with high thermal conductivity to transfer heat energy, as MGA Thermal does or the sodium sulphur technology CleanCo wanted to install in Queensland.
But with insulating brick, Rondo had to do something different.
Instead of relying on an outside heat source, it built toaster-like elements into the brick which heat it to 1500ºC. That heat is transferred by air, which is warmed as it’s blown through the unit and then used as process heat or to turn a steam turbine. Total energy losses can be kept to just 1 per cent.
Hot bricks in manufacturing. Image: Rondo Energy
“We [used] the physics of convection and have a huge brick surface area. We… found a way to embed the same electrical heating elements that are in your toaster,” O’Donnell told Renew Economy.
“The brick system turned out to be magnificent because it allowed us to have very carefully engineered air passages.”
“When you push cold air in at the bottom, it gets heated up by the brick it’s passing through. The brick stack cools off bottom up, and the same temperature comes out of the stack during the entire time. It’s a little bit like a home electric water heater. They call it a thermocline.”
The air is hot enough to make steam under the same conditions as coal fired boilers. It means industrial companies could swap out a coal fired furnace for a heat battery to drive the same steam turbine.
Not quite economic yet
The cost of charging this kind of thermal battery will be the make or break scenario.
Coal or gas are traditional preferred options for industrial heat in Australia and around the world.
“Direct electrification is challenging, because the 24/7 electricity many industrial facilities need is expensive,” wrote Clean Energy Review founder Sean Fleming last year.
“Assuming ~$0.07 per kWh electricity and ~$5.00 per MMBtu for natural gas, and 100% and 75% efficiency respectively, just the energy costs of outputting 1MWhth in heat would be ~$70 with electricity but only ~$23 with gas.”
In Australia, wholesale coal prices are hovering around $A100/tonne, while domestic gas has ranged between $12-$14/GJ for the 2024/25 financial year, according to the Australia Energy Regulator ($12/GJ is about the same as $7.90/MMBtu).
Wholesale electricity prices were cheapest in Victoria at $45/MWh ($0.045/kWh) by the end of 2024, but nationally they averaged $88/MWh ($0.088/kWh).
It means in many Australian states, a decision today by an industrial user to swap out a coal or gas heating system for an electric one might still be a coin toss based on energy costs.
One benefit of an electric system is capitalising on low energy costs during the day – perhaps even being paid to use electricity during midday duck curves, a factor that could shift the dial for industrial customers.
Race is on
With competitors such as Antora, another hot brick storage company, also rapidly commercialising, Rondo must now scale up fast.
Rondo is now building projects in five countries and working on winning contracts in nine others, with a focus on very large industrial users such as cement plants and aluminium refineries, O’Donnell says.
A 2MWh pilot at the CalGren ethanol plant in California. Image: Rondo Energy
Australia is one of the options to launch a project thanks to having Rio Tinto has a backer.
The current projects are a 100MWh installation at German materials company Covestro’s Brunsbüttel site and a similarly-sized GreenLab is connecting a 100-megawatt hour Rondo heat battery to its wind and solar power supply and expects it to be operational in about a year.
Those were funded by the European Commission, European Investment Bank (EIB) and the billionaire Gates’ innovation fund Breakthrough Energy, while other backers include Microsoft and Saudi Aramco, O’Donnell says.
“The largest project that’s in contract is in the 300 MWh range, and there are projects in the 10 GWh range that are in negotiation,” he says.
