Grid batteries usually live in shipping containers on the outskirts of town. But molten salt — the same nitrate mix (60% NaNO₃, 40% KNO₃) that runs concentrated solar plants like Crescent Dunes — stores heat cheaply at ~$25/kWh-thermal. What if we made the building itself the tank?

Imagine a 60-story tower with a central core that isn't elevators — it's a stack of insulated tanks holding 10,000 tonnes of "solar salt" cycling between 290°C (cold) and 565°C (hot). The salt is heated by cheap off-peak grid electricity through resistive immersion heaters (round-trip electric efficiency ~40% via steam turbine on discharge), and the "waste" heat leaking through the tank walls is exactly the heating load for the offices wrapped around it.

The energy math. Solar salt has a specific heat of ~1.5 kJ/kg·K. Cycling 10,000 tonnes through ΔT = 275 K stores:

Q = m·c·ΔT
  = 10⁷ kg × 1500 J/kg·K × 275 K
  ≈ 4.1 × 10¹² J
  ≈ 1,150 MWh thermal
  ≈ 460 MWh electric (at 40% conversion)

That's roughly enough to run a mid-sized US city (say, 15,000 homes at ~30 kWh/day) for one evening. For comparison, the Moss Landing lithium plant stores 3,000 MWh at ~$400/kWh; our salt core stores 460 MWh electric at maybe $80/kWh installed. The catch is you need real estate for a steam turbine and cooling loop in the basement.

The heating bonus. A 60-story office needs ~5 MW of heat in a cold-climate winter. Our tank, at 565°C surface temp behind (say) 40 cm of microporous insulation with k ≈ 0.025 W/m·K, leaks:

Q_loss = k·A·ΔT/L
       ≈ 0.025 × (π·8m × 240m) × 545 / 0.4
       ≈ 205 kW

That's less than the heating demand — so we tap the tank on purpose through a controllable heat exchanger. Round-trip combined heat and electricity efficiency climbs to ~85% because the "waste" is the product.

The structural nightmare. 10,000 tonnes concentrated in a 200-m column is a compressive load of ~1 MPa at the base plus a huge lateral hazard: solar salt freezes at 220°C. Lose heat trace for 48 hours and the entire core solidifies into an unpumpable ingot. Crescent Dunes learned this in 2016 when a cold-salt tank leaked and cost them eight months. In a skyscraper, a leak of 565°C salt into an elevator shaft would be catastrophic — salt at that temperature ignites organics on contact and eats stainless 304 at ~0.3 mm/year (you need Inconel 625 for the tank, ~$40/kg, adding maybe $400M to construction).

Seismic problem. A slug of liquid at 200 m height has a sloshing resonant period of T ≈ 2π√(H/g·π) ≈ 15 seconds — dangerously close to the fundamental sway period of a tall building. You'd need internal baffles and probably a tuned mass damper synced against the salt itself. Nature would prefer these two systems didn't share a resonance.

The verdict. As pure grid storage, hopelessly worse than a battery farm. But as combined storage + heating for a district energy zone (think Helsinki or Toronto), the numbers pencil. Vantaa, Finland is literally building a 90 GWh underground sand version of this — same physics, safer medium.