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Grid-scale lithium-ion batteries are our current go-to chemical energy storage solution, but they present their own challenges in safety, sustainability, cost, and longevity. However, the competition is … heating up. New forms of thermal energy storage systems built using abundant, cheap materials are on the rise. One company is aiming to sidestep the complications that come with chemical batteries…with bricks. And another company’s weapon of choice is crushed volcanic rock. Talk about going back to basics to store massive amounts of energy. But is simple really best?

Why are people looking to store energy in bricks and crushed volcanic rock? It shouldn’t be a shocking revelation to say that renewable energy like solar and wind is intermittent. The sun doesn’t always shine and the wind doesn’t always blow, so we need to have cheap, reliable energy storage to save excess renewable energy production for when we need it. How much renewable energy are we losing out on? A 2022 report by Lane Clark & Peacock for Drax estimates that enough wind energy to power 800,000 homes in the United Kingdom went to waste over two years due to lack of storage.1 2

So, to make the most efficient use of our wind and solar power, we need more reliable storage. On a grid level, lithium-ion batteries have considerable perks. They boast a high energy density, a high round-trip efficiency, and decently long cycle lives. However, Li-ion’s advantages are costly in more ways than one. Lithium is expensive, extraction is slow and not always environmentally friendly, and it has a lot of price volatility. Li-ion batteries also don’t last as long as some other forms of energy storage tech.34

Thermal energy storage (or TES) systems are heating up as an appealing alternative. For one thing, TES systems don’t necessarily require converting energy (keeping heat as heat), which means higher efficiency. In district heating systems or industrial applications, which we’ll talk about more later, the heat put into the battery is taken right back out as-is.

Another significant edge is that TES systems are based on tech that’s more down to earth. We’re talking batteries that store excess energy as heat in water, molten salts, sand, gravel, and, more recently, brick and rocks. With these materials at the core, you don’t have to worry about scarcity, environmental impact, or explosions. They don’t degrade. And they’re cheap.5

This is the foundation that the California-based company Rondo Energy and the Israeli company Brenmiller Energy built their TES systems upon. They’ve each developed heat batteries offering similar benefits in almost identical language: low-cost heat that can be stored for hours or days, decades-long lifespans, “unlimited” cycles free from performance degradation, zero emissions.678 These batteries operate using bricks and rocks, respectively.

Both companies promise similar outcomes, emphasizing reduction in the costs and the emissions associated with the industrial sector. They also both happened to make notable moves in November 2022. Early that month, Brenmiller inaugurated its latest project: the testing of a utility-scale TES at a thermal power plant in Italy as part of a collaboration with Enel, Europe’s largest energy company.9 A week and a half later, Rondo officially announced the launch of its Rondo Heat Battery or “RHB.”10

How does brick and rock translate into efficient heat storage?

Let’s start with Rondo. Its heat battery has been likened to a giant brick toaster, and the company’s CEO, John O’Donnell, has also referred to it as an “insulated shoebox full of brick.”11 I don’t want to know what kind of shoes he’s wearing. When the battery is charged, renewable energy from wind or solar, or electricity from any source, is converted into heat by its oven-like electric heating elements. This thermal radiation fires up the thousands of tons of bricks inside, which can reach temperatures up to 1,500 C. The battery can store this energy for hours or days.12

The company claims that a RHB can last over 40 years, with several of its individual components able to last even longer, as well as being recyclable.11 As O’Donnell pointed out in an interview last June, their bricks are “inert, and known to last for 100 years.” This also means that their systems don’t contain anything that, in his words, “can physically spill or release gas, or catch fire.”13

Once you want to pull the heat back out of a RHB, a blower sends air up through the bricks. The air is superheated to above 1,000 C, and the end result is heat in the form of superheated air or superheated steam released on demand.12 These high temperatures are especially useful in industrial applications, which is Rondo’s main target. RHBs are intended to directly replace fuel-fired boilers, furnaces and kilns.10

Here’s where we need to zoom out to get the full picture. Heat is central to a massive number of industrial processes, from sterilization to smelting. A 2014 study by the U.S. Department of Energy estimated that the country’s industrial sector uses about 24 quadrillion Btu, or British thermal units.14 Btu measure the amount of heat it takes to raise the temperature of one pound of liquid water by 1 degree Fahrenheit.15 24 quadrillion Btu is equivalent to roughly a third of the United States’ delivered energy supply.14 It’s a lot of energy.

Fossil fuels continue to be the primary source of energy for factories precisely because burning them is a quick and easy source of heat. As you might expect, that has major consequences. According to the International Energy Agency, a quarter of the world’s emissions — about 9.4 Gt of CO2 — were directly produced by industrial activity in 2021. It’s important to note that this figure doesn’t include the indirect emissions from the electricity used to power them.16

Within the broad category of industry, the top three emitters of greenhouse gasses include the production of iron and steel, chemicals and plastics, and cement.17 The influence of cement in particular is pretty shocking: If it were a country (a really drab, gray country), it would be the world’s third-largest CO2 emitter behind China and the U.S. 70% of the emissions released by cement-making come from the chemical reaction that produces clinker, its main ingredient. So, the recipe shoulders most of the blame. But the remaining 30% is a result of using fossil fuels to fire up the furnaces.18

Cement is a component of concrete, and concrete is the most-consumed material on the planet, second only to water, which is kind of crazy.19 On a global level, cement and concrete production accounts for 8 to 9% of greenhouse gas emissions and 2 to 3% of energy demand.17 But while concrete isn’t going away anytime soon, maybe its reliance on fossil fuels can. This isn’t just about reducing emissions though, it’s also about saving money.

This is where Rondo is stepping up to help. Last July, the company announced its collaboration with TITAN Cement Group. The goal is to reduce emissions that would otherwise come from burning fossil fuels by using Rondo Heat Batteries to capture energy from kiln flue gasses, then taking that heat to produce clinker.20 Later, in September, Rondo announced a partnership with Siam Cement Group, which also produces chemicals and paper.21

So far, RHBs are making a big difference, according to Rondo’s claims. On the date of the battery’s commercial launch, the company’s Senior Vice President, Jeremy Keller, said that facilities outfitted with RHBs are “showing 50% to 90% reductions in emissions and reductions in operating costs of 30% or more.”10 A nice one-two punch on emissions and costs.

Rondo also claims a whopping 98% efficiency, which the company’s YouTube channel clarifies is because “the roundtrip efficiency of basically all electric thermal energy technologies will be in the 95%+ range.”22 In O’Donnell’s words, the conversion of electricity to heat happens at 100% efficiency every time you turn on a heating appliance. We’re back again to the toaster metaphor.23 Get your Pop Tarts ready.

As for Brenmiller Energy, its line of bGen batteries work like RHBs and are trying to accomplish the same thing: provide industry with a clean source of heat. The main differences are that bGen batteries stick within a lower range of 100 to 500 C for heat output, and instead of bricks, Brenmiller uses crushed rocks.7

Executive Vice President of Operations Nir Brenmiller explains that rocks store heat inside “cells that are stacked into a module, then into a pack.” Electricity or heat goes in, and hot water, hot air, or steam comes out. The energy input can be anything from exhaust heat, surplus steam, electricity straight from the grid during peak hours, wind, solar, or biomass.7 Dealers choice.

For example, Fortlev, Brazil’s largest manufacturer of water storage products, began using a bGen unit at its factory in Anápolis, Brazil, last August. The company needs hot air to run its machines that mold plastic into water tanks, and its 1MWh bGen battery allows the company to swap natural gas for biomass. Brenmiller claims that by making this switch, Fortlev lowers the fuel costs of heating air by over 75% and lowers its greenhouse gas emissions by about 800 metric tons a year.24

The bGen BS-7011 (a name that rolls off the tongue) is charged by the combustion of wood chips or pellets and boasts an 80% efficiency.25 That said, burning wood and wood pellets in the name of sustainability is a controversial subject to say the least. The potential effects depend on where exactly that wood is coming from. But the use of biomass as fuel could be a video subject all on its own. If you would be interested in that, let me know.

And if the concept of using earthy materials to store heat sounds familiar, you might be thinking back to the sand battery built last summer in a small Finnish town by the country’s own Polar Night Energy, which I covered in a previous video. Like Rondo and Brenmiller’s batteries, the sand battery is a TES system that takes advantage of excess renewable power to squirrel away heat until it’s needed. The sand battery is already warming homes and offices through the local district heating system. And like the sand battery, district heating is a potential application for both Rondo and Brenmiller’s tech.

District heating is basically an underground network of pipes that delivers heat to buildings from a central plant. It offers a great opportunity for decarbonization by using renewable energy to distribute steam or hot water instead of relying on individual boilers.26 As a concept, it’s actually been around since the use of geothermal water to warm houses in Pompeii. These days you can find this kind of infrastructure all over China and Europe. There’s also over 660 district energy systems throughout the United States, including in major cities like New York and San Francisco.27

In fact, Brenmiller has already been collaborating with the New York Power Authority since 2017.28 In 2020, the company installed a cogeneration system, which generates both heat and electricity at the same time, in the P.E. building of Purchase College at the State University of New York. This replaced its existing district heating loop. The system takes exhaust from a turbine and uses it to provide about half the building’s electricity alongside all its heating and hot water.2930 More recently, during a December 2022 call with Brenmiller Energy investors, Nir Brenmiller said that the partnership is currently “examining solutions for industrial manufacturing and for district heating in big buildings in cold locations.”7

These examples only scratch the surface of what’s possible. For a deeper dig into energy that travels underfoot, check out City Beautiful’s video on district heating. It’s a great YouTube channel and they’ve just released a video about this.

Now, back to the batteries and some of the gotchas, or potential cons, about the tech. Rondo and Brenmiller share another trait: Despite both companies’ heavy promotion of their products’ potential for industry, these things just don’t get hot enough for steelmaking. Getting serious about decarbonization requires that we tackle steel specifically. The industry was responsible for over 3.3 billion metric tons of greenhouse gasses in 2021.31 Meanwhile, wind turbines are, ironically, mostly made of steel.32

For now, Brenmiller is deliberately focusing on supplying medium range temperatures, citing the complexity of integration into steel factories. This caveat also means no cement.7 That’s not to say that bGen batteries can’t do much: in European countries, 30% of industrial heating applications require temperatures of less than 100 C. Another 27% do just fine with heat between about 100 and 400 C, which is well within a bGen’s maximum of 500 C.14

Rondo provides a higher temperature range, but its upper limit is 1,500 C.6 You need at least 1,600 C to produce steel.33 O’Donnell did say in an interview, however, that it’s possible for the Rondo Heat Battery to hit 1,800 C in the future.23

In other words, things are only just getting started. RHBs haven’t been commercially available for long and neither have Brenmiller’s pilot projects in Brazil and Italy. Brenmiller also has another TES in the works: a $9.2 million, 31.5 MWh bGen unit set to replace the boilers in a Philip Morris tobacco plant in Romania. For me that raises the question of, “why a tobacco plant?” Anyway … they plan to break ground in early 2023.7

So, we still have to wait to see what these batteries are really capable of. In the meantime, thermal storage is continuing to gain traction, with plenty of TES systems in development across the globe. Two of the largest upcoming projects are both 600MW molten salts plants, one in the United Arab Emirates, and another in China.5 Let’s hope they can do more than toast.


  1. Potential solution to unlock investment in climate-critical storage technologies ↩︎
  2. Renewable curtailment and the role of long duration storage ↩︎
  3. Applications of Lithium-Ion Batteries in Grid-Scale Energy Storage Systems ↩︎
  4. Batteries get most of investors’ attention, but grid scale energy storage solutions hold the key ↩︎
  5. Can thermal storage fire up the net-zero transition? ↩︎
  6. The Rondo Heat Battery ↩︎
  7. Brenmiller Energy Investors call ↩︎
  8. Utility scale storage solution: Brenmiller Energy bGen™ ↩︎
  9. Enel teams up with Brenmiller to test rock-based energy storage ↩︎
  10. Product launch: the Rondo Heat Battery ↩︎
  11. How a high-tech twist on a 19th-century process could clean up steel and cement making ↩︎
  12. The Rondo Heat Battery ↩︎
  13. Bill Gates-funded startup Rondo turns Solar or Wind into Heat ↩︎
  14. Renewable Industrial Process Heat ↩︎
  15. Units and calculators explained: British thermal units (Btu) ↩︎
  16. Industry ↩︎
  17. Technologies and policies to decarbonize global industry: Review and assessment of mitigation drivers through 2070 ↩︎
  18. Concrete: the world’s 3rd largest CO2 emitter ↩︎
  19. Explained: Cement vs. concrete — their differences, and opportunities for sustainability ↩︎
  20. TITAN Cement Group Joins Breakthrough Energy Ventures and Energy Impact Partners to Scale Rondo Energy’s Industrial Decarbonization Technology ↩︎
  21. Siam Cement Group and Rondo Energy Announce Investment and Plan Partnership to Bring Zero-Carbon Heat to New Industries and New Territories ↩︎
  22. How 3000 Degree Bricks Will End Battery Storage ↩︎
  23. “Brick toaster” aims to cut global CO2 output by 15% in 15 years. Seriously. ↩︎
  24. Fortlev and Brenmiller Energy Inaugurate the World’s First Renewable Energy-Powered Thermal Energy Storage System for Plastic Manufacturing ↩︎
  25. bGenTM – BS-7011 Storage Based Steam Generator ↩︎
  26. District Heating ↩︎
  27. District Energy Systems Overview ↩︎
  28. NYPA Announces $1 Million in Foundation Funding to Support Innovative U.S.-Israeli Thermal Energy Storage Project ↩︎
  29. Capstone Turbine Partners with NYPA and Brenmiller Energy on Thermal Energy Storage Project ↩︎
  30. Clean Energy Projects ↩︎
  31. Steel industry carbon emissions to drop nearly 1/3 by 2050 – Woodmac ↩︎
  32. What materials are used to make wind turbines? ↩︎
  33. Primary Steelmaking ↩︎
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