US Government funds pilot project for heated sand energy storage
A U.S.-funded pilot project is exploring “sand batteries” that store renewable energy as high‑temperature heat in sand, then recover it via gas turbines to generate electricity. Commenters weigh the appeal of sand as a cheap, abundant, and environmentally benign storage medium against its relatively low round‑trip efficiency compared with batteries or pumped hydro, noting it may be best suited for district heating or multi‑day grid backup rather than routine power storage. The thread also highlights key engineering tradeoffs—such as moving sand versus piping a working fluid, temperature limits, insulation and scale effects—and skepticism about whether long‑duration thermal storage can compete with simply overbuilding renewables or producing fuels like green hydrogen.
Technology and Design
- System heats sand to ~1,100–1,200 °C with cheap renewables, then gravity-feeds it into a heat exchanger to heat a working fluid that drives a combined-cycle turbine.
- This is explicitly not “heating coils in a sand pile”; it uses engineered silos and Babcock & Wilcox–style fluidized bed heat exchangers for rapid, compact heat transfer.
- Sand is thermally stable up to ~1,200 °C; higher temps would raise efficiency but cause significant materials problems.
Why Move Sand Instead of Fluid?
- Moving sand avoids poor thermal conductivity and air gaps limiting heat transfer when piping fluid through a static sand bed.
- A central heat exchanger can stay the same size as storage scales, unlike embedded piping which must grow with volume.
- Cycling only the sand being used lets the bulk remain at near-optimal temperature instead of cooling the whole mass.
Energy Density, Scale, and Use Cases
- Sand is environmentally benign, cheap, and uses no rare materials. Worst failure mode is a pile of very hot sand.
- Volumetric energy density is low versus fuels:
1000 gallons of sand heated +1000 K stores ~6.6 GJ vs ~146 GJ for 1000 gallons of oil (22× more volume needed). - Large-scale installations benefit from better volume-to-surface-area ratio, so long-duration storage losses can be just a few percent per month at town scale.
- Finnish “sand batteries” discussed as heat-only storage; the US system targets electricity generation, which is notably more complex.
Efficiency and Comparisons
- Government report estimates ~50% round-trip electricity efficiency, maybe 55% with more complex turbines.
- Combined cycles at these temperatures can theoretically approach ~65% heat-to-electric efficiency, but falling sand temperature drags the average down.
- Pumped hydro is cited at ~80% round-trip, with geographical constraints. Pumped thermal storage and other LDES concepts (e.g., hydrogen, compressed air) are mentioned as alternatives.
Residential and Seasonal Heating
- Thread consensus: seasonal storage for a single house with sand is impractical; too much heat loss and low energy density.
- Safer than oil tanks and non-flammable, but 1000 °C+ temperatures are seen as dangerous for homes and better suited to district or industrial systems.
- Some DIY and small “sand battery” experiments exist, but they target daily buffering, not whole-winter storage.
Skepticism and Market Fit
- Supporters emphasize ultra-low material cost, safety, and suitability for using otherwise-curtailed renewables.
- Skeptics argue long-duration thermal-to-electric storage will struggle against ever-cheaper batteries, more renewables, and power-to-fuels (e.g., hydrogen-derived fuels).
- Several comments frame heat storage as excellent for heat, but only marginal for electricity where conversion losses and capital costs matter.