Storing electricity is still the expensive part of renewable energy. Lithium-ion batteries remain pricey enough that your utility keeps burning gas at night instead of banking the afternoon’s sunshine. So engineers at Sandia National Laboratories tried a different container for that energy, and it’s about as unglamorous as hardware gets: a bed of crushed gravel packed between two livestock troughs.<\/p>\n
The system’s called THERMS, short for Terrestrial Heat Repository for Months of Storage, and it does pretty much what the name promises. Electricity from a 12-kilowatt solar array runs resistance heaters, the heaters warm air, and the air carries that heat into the gravel, where it sits until someone needs it back. In this case, someone roasting coffee.<\/p>\n
According to the design slides mechanical engineer Luke McLaughlin presented at this summer’s Thermal-Mechanical-Chemical Energy Storage workshop<\/a>, the testbed’s built from farm-store parts. An eight-foot livestock trough sits upside down over a six-foot perforated one, and the 18-inch bed of gravel fills the ring between them. The blowers, valves and controls are off-the-shelf commercial gear, and Sandia’s said gravel straight from a landscaping company works fine without any special washing. There’s a patent on the arrangement, but from the looks of things there’s nothing exotic on the bill of materials.<\/p>\n If you haven’t run into packed-bed storage before, the idea’s simple. You heat a pile of cheap solid stuff with hot air when energy is plentiful, and later you blow air back through the pile to pull the heat out. Rocks don’t catch fire, and they don’t wear out either: McLaughlin’s slides<\/a> claim no cyclic degradation in the material below 932 degrees Fahrenheit (500 C).<\/p>\n They also put the storage cost at $10 to $50 per kilowatt-hour, against $200 to $300 for lithium-ion batteries. Heat batteries are a busy field right now, and a US startup is already selling 1,832-degree bricks for oil field steam<\/a>, but I haven’t seen anyone else build one out of stock tanks.<\/p>\n So why does the direction of the airflow matter at all?<\/p>\n During charging, heated air enters at the center and pushes outward through the gravel, so the hottest material ends up in the middle of the ring. To discharge, the system reverses the flow, and incoming air picks up heat on its way inward before leaving through the center at up to 1,022 degrees Fahrenheit (550 C). The hot core gets protected until late in the draw, which means your process sees usable temperature for most of the discharge instead of a long lukewarm tail.<\/p>\n Why not just let the heat find its own way out? Because rock is a lousy conductor. Sandia’s numbers put conduction at roughly 60 times too slow to move heat through the bed at a useful rate, so moving air does essentially all of the work in both directions.<\/p>\n The trouble starts when the system just sits. A charged bed holds a boundary between its hot zone and its cold zone, the same way a water heater holds hot water floating on cold, and engineers call that boundary a thermocline. Leave the bed alone long enough and conduction, buoyancy and plain old heat loss smear it out. McLaughlin’s slides name thermocline degradation during a long hold as a challenge, and they’re admirably blunt about it.<\/p>\n Sandia says the rig’s run through more than 200 days of daily charge and discharge cycles, swinging between 212 and 1,022 degrees Fahrenheit (100 to 550 C), at roughly 85% energy efficiency. That efficiency number means most of the heat you put in comes back out. The number the slides are far more careful about is the temperature it comes back at.<\/p>\n Energy that returns at 300 degrees is still energy. It just can’t roast your coffee, and it can’t make industrial steam. The slides use the word exergy for this, which is engineer-speak for how much of the stored heat is still hot enough to do the job you built the system for. That’s the whole game for industrial heat. The slides concede the exergy efficiency turns poor after a prolonged hold, because that smeared-out thermocline delivers a lower, mushier outlet temperature. How long a hold has to last before the hit gets serious is something the presentation doesn’t quantify, so I can’t tell you whether a multi-day wait comes back out at roasting grade.<\/p>\n Sandia needed a real process hanging off the discharge end, and Michael Thomas Coffee, a roaster in Albuquerque, signed on through a research agreement funded by the state of New Mexico. Sandia’s own write-up<\/a> describes owner Michael Sweeney roasting light, medium and dark batches on a fall day at the National Solar Thermal Test Facility in Albuquerque, which also runs a tower fed by desert mirrors and molten salt<\/a>, with the coffee drum drawing its heat from the gravel bed instead of natural gas. The lab published the story this July and doesn’t pin the roast itself to a year, so I won’t either.<\/p>\n The results were apparently encouraging. Systems engineer Ken Armijo said the dark roast finished a few minutes faster than it does on gas, and that an earlier experiment roasting green chile on the same kind of stored heat cut roasting time in half with more uniform results. Sweeney liked that the roaster preheated and cooled down faster, which means more batches per day out of a machine he already owns. And that’s the pitch: keep the gas roaster you’ve already got, bolt it to a gravel bed charged off your roof, and draw on stored heat that Sandia says can last for days.<\/p>\n Coffee is basically the demo. The slides list cacao and barley roasting, steam generation for food and chemical plants, and asphalt, concrete and steel production as the commercial targets, and frankly the barley one writes its own beer story. Whether a heat battery at those prices lands in any of those industries depends on someone licensing it, and Sandia’s business development people say they’re shopping the system around for exactly that.<\/p>\n McLaughlin presented the full dataset on July 29 and 30, 2026, at the workshop hosted by the Electric Power Research Institute, with more than 200 days of cycling on the books and, per his closing slide, a hops roasting demo already lined up.<\/p>\n","protected":false},"excerpt":{"rendered":" Storing electricity is still the expensive part of renewable energy. Lithium-ion batteries remain pricey enough that your utility keeps burning … <\/p>\nThe air runs one way to fill it and the other way to empty it<\/h2>\n
The 85% figure comes with a catch<\/h2>\n
So where does the coffee come in?<\/h2>\n