The Rondo Heat Battery Is a Big, Brave Toaster

Batteries aren't just for electricity, as they can also store heat—lots of heat. The Rondo Heat Battery is described as a brave little toaster: "In essence, a toaster heating bricks to help decarbonize heavy industries like steel, cement, aluminum and biofuels."

It is essentially a pile of bricks. You take intermittent, renewable, and surplus electricity and run it through toaster coils that heat the bricks up to 1,500 degrees Celsius (2,700 degrees Fahrenheit). You then run air through the bricks and get superheated air or steam. Rondo claims it entails "simple integration with boilers, kilns, calciners and process units." It also claims an efficiency of up to 98%.

The use of the heat battery in calciners (or cement plants) is particularly interesting; making clinker for cement is responsible for as much as 7% of the world's anthropogenic carbon dioxide. Rondo partnered with Titan cement to reduce the need for fossil fuels.

"Rondo’s thermal energy storage technology is of particular importance to cement manufacturing in the roadmap towards carbon neutrality. As energy transfer can take place using kiln flue gases, the carbon-neutral, high-temperature heat can be used to produce clinker, as well as for the thermal activation of minerals and by-products, alleviating a significant part of the emissions originating from fossil fuels burning."

Rondo claims its system is modular and scaleable but a battery, by definition, stores energy—it doesn't make it. You get out of it what you put in, minus a bit due to inefficiency. Rondo is talking about storing surplus energy from the grid. There is often nuclear power slopping around at night when demand is low, as well as renewable sources like solar and wind. This is where it gets interesting. Let's take the example of clinker production.

• It takes 3.3 gigajoules of energy to make a metric ton of clinker.
• A gigajoule is equal to 277.778 kilowatt-hours, so a metric ton of clinker would need 916.66 kWh of electricity stored as heat in the Rondo battery.
• The average cement plant might produce 5,000 metric tons of clinker per day; they can go up to 15,000 metric tons. But let's use 5,000 metric tons: The plant will require 4,580,000 kWh or 4,580 megawatt-hours of electricity to fill the toaster to make a day's worth of clinker.
• A 3-megawatt offshore wind turbine produces about 6,000,000 kWh per year. Imagine the size of the wind farm and the size of the pile of bricks in the heat battery needed to run a cement plant for a day. The math with solar panels is even more daunting.

I tried to figure out the amount of brick needed, given that brick has a heat capacity of 840 joules per kilogram per degree Celsius, but the number of zeros was scary. And it wouldn't have mattered because the battery can store up to 1,500 degrees C, while calcination happens at 1,450 degrees C, so it will poop out of process heat very quickly.

All of this is a roundabout way of saying that it takes huge amounts of energy to make cement, which is why the industry consumes huge amounts of fossil fuels and produces huge clouds of CO2. The battery and the wind farm would have to be huge. The entire problem with concrete is five big piles of huge.

The Rondo Heat Battery is a great idea. But as with any battery, the charging part is as important as the storage part. The Rondo is certainly more efficient than hydrogen as a way of providing process heat, but it is only 1,500 degrees C when it is full. After that, you just have warm toast, which could be used to heat air or water for other functions, say district heating of buildings.

In the end, I always circle around to the same conclusion: reduce demand. Or, as I now say, ephemeralize everything. If the Rondo Heat Battery could replace the process heat required to make cement, it would only reduce emissions by half, as the rest comes from the chemical reaction. How about instead, we just use half as much of the stuff?