It’s getting hot in here: A tale of temperature and energy

We often assume that making things hotter means that you use more energy. In many cases, this is true. When your home is cold, your energy use spikes as you increase the temperature on your heater.

But temperature and energy are actually unique properties, and they are not always so nicely correlated. Think for a moment about trying to pull apart two pieces of wood that are hot glued together. You’re going to need energy to pry the boards apart. The question is: how much and how do you use that energy smartly? If you go the brute force route, you’ll need a lot of energy. But if you use energy to increase the temperature of the glue so that it melts – the boards will come apart with relative ease. In this example, temperature and energy don’t correlate. In fact, by increasing the temperature, you may use less energy. 

The same can be true when it comes to CO₂ removal from the atmosphere. CO₂ is naturally absorbed from the atmosphere and bound to certain materials – calcium hydroxide rocks in our case, which forms limestone. The process for removing CO₂ from any material – including limestone – is not so dissimilar from pulling apart two boards glued together. The question is the same: how do you remove CO₂ efficiently?

There are different ways we can capture and release CO₂. Some require lower temperatures and some higher, but it really comes down to two things: how strongly the CO₂ is ‘stuck’ on the capture material, and how much the material favors CO₂ relative to other gasses (for example - if your material favors water and CO₂ equally, you will have to use energy to remove the water to also remove the CO₂ – which drives up overall energy requirements per ton of CO₂ removed from the air).  With some sorbents, just as with hot glued boards, it takes more energy to remove CO₂ at a lower temperature than it does to remove it by heating the material to a higher temperature. 

Heirloom uses a thermal regeneration process, where the limestone is heated to around 900℃ in a kiln using renewable energy to produce a pure stream of CO₂. The energy requirements of our system are currently 2,500 kWh/tCO₂ with a clear pathway of getting to 1,500 kWh/tCO₂  – which we’ll get to by implementing efficiency improvements to our current process (through things like heat integration). And while 900℃ sounds hot (and it is hot), it’s actually the sweet spot when it comes to energy use for our process. 

Why does this all matter? Because energy use is one of the biggest cost drivers of most DAC technologies, including ours. Pulling CO₂ from the atmosphere has a minimum energy requirement dictated by the second law of thermodynamics - and that energy requirement is high because CO₂ is such a diffuse gas in our atmosphere (just 0.04%).

Finding the sweet spot where we can use energy smartly and efficiently – like cranking up the temperature on our kiln – will be key in making sure we keep costs as low as possible, so that we’re on a path to remove CO₂ from the atmosphere for less than $100 per ton - the holy grail of the carbon removal industry. Some like it hot – but we like it hotter. 🔥 🔥