Last week, I was just warming up (pun totally intended) on the subject of energy storage as a key component of any carbon-cutting strategy for the planet; it may therefore seem like a sideways leap from there to the dinner tables at The Grizzly House in Banff, Alberta, but there’s a direct connection.

If you’re OK with having your clothes smell like smoke the next morning, The Grizzly House should be on your list of potential dining adventures, because their wild game fondue (options including wild boar and venison) can be ordered with a hot rock instead of the more conventional hot-oil apparatus. Along with your plate of meat morsels, they set in front of you a block of stone that’s been heated to 600(+)ºF – which you then use for searing the meat, one bite at a time. It’s a lower-calorie alternative to hot oil fondue, and the result tastes great – although, as already noted, you will take the aroma home with you.

What does this have to do with climate change? Storing energy as heat in a rock is more than just a foodie novelty.

Last week’s note made reference to geothermal energy, typically employed by circulating an antifreeze mixture through an underground loop that interacts with a heat pump. We can take the next step, though, beyond using the earth as a constant-temperature thermal reservoir, by constructing a “thermal battery” that lets us cycle its temperature to store and retrieve energy. At a much larger scale than a dinner table in Banff, energy exceeding the need of the moment can be used to heat “rocks in a box” – from which the energy can later be recovered. It’s hardly a new idea, dating back to an English patent issued in 1857, but centuries of improvement in high-temperature materials are now making this technology far more competitive for a range of applications.

Further, despite the “in a box” part of the nickname I just used for this concept, we need to do some out-of-the-box thinking about the ways that we convert energy from one form to another. Rather than using stored heat energy to make steam and drive a turbine, as has been prevailing practice, the temperatures involved can now be so elevated that “sun in a box” becomes a possible (and preferable, and maybe even necessary) next-generation approach.

Work on this has been described as follows:

The new system, which the team calls Thermal Energy Grid Storage-Multi-Junction Photovoltaics (TEGS-MPV)…doesn’t tap into the heat but the light – at those temperatures molten silicon shines extremely brightly. The white-hot liquid is pumped through tubes that emit the light, which is then captured by specialized solar cells known as multijunction photovoltaics and converted to electricity. The silicon, now cooling down again, is pumped back…to start the cycle over.

We need to recognize the need to consider several stages of a process, with varying costs and efficiencies, to arrive at any overall comparison of cost-effectiveness. As noted by the authors of the work above, writing in a more formal setting,

It is well known that the conversion of heat to electricity is thermodynamically limited and therefore results in a significant efficiency penalty. However, the storage of energy as heat instead of electricity can be 50–100× cheaper, and as a result the 15-40% efficiency penalty becomes a worthwhile tradeoff.

When the many moving parts of a turbine are replaced by photovoltaic cells, the researchers add, overall costs of a heat-storage system start to look quite favorable:

Currently the cheapest grid storage technology is pumped hydroelectric storage (PHS), which has a high roundtrip efficiency (RTE) ∼80–90%, as well as a low cost per unit energy (CPE) ∼$60 per kWh-e and cost per unit power (CPP) ∼$1 per W-e… Electrochemical batteries, on the other hand, have promising new chemistries, but it is unclear if any will displace Li-ion batteries whose prices continue to drop from $300–400 per kWh-e down to a predicted asymptote ∼$150 per kWh-e. There is significant concern nonetheless, that even this lower asymptote for Li-ion is still not cheap enough to enable the eventual 100% penetration of renewables…

In thinking about lower cost storage, one class of technologies that has not received much attention is thermal energy storage (TES). This is because the final form of energy needed is electricity, necessitating the conversion of heat back to electricity, which tends to occur at low efficiency (∼35–40%)…[but] storage of heat…can be 1–2 orders of magnitude cheaper than electrochemical batteries. Our own incarnation of Thermal Energy Grid Storage…may be one of the few solutions to the storage problem that is inexpensive enough to eventually enable a fully renewable grid.

If there’s a TL;DR here, it’s that no single technology is going to be optimal for every combination of duration of storage, cycle time of storage and withdrawal, facility location, or capital cost versus ongoing expense – but Asegun Henry, the Robert N. Noyce Career Development Associate Professor in the Department of Mechanical Engineering at MIT, nails it when he says that energy storage is the “most important and critical problem in energy and climate change.” More to come.