The Tin Age of Energy Storage

Prototype Fourth Power thermal storage battery system

By EVWorld.com Si Editorial Team

In the race to decarbonize the grid, lithium-ion batteries have become the default solution - ubiquitous, scalable, and increasingly affordable. But as renewables surge and demand for long-duration storage grows, a new contender is heating up. Literally. Fourth Power, a Boston-based startup backed by Breakthrough Energy Ventures, is betting on molten tin and thermophotovoltaics to reshape how we store and dispatch electricity. Their pitch: a thermal battery that operates at 2,400 C and costs one-tenth as much as lithium-ion.

The concept is elegant in its brutality. Excess electricity - say, from solar overproduction - is used to heat liquid tin to temperatures that rival the surface of Venus. That tin circulates through graphite pipes, transferring heat to carbon blocks insulated by a meter-thick shell and sealed in an argon-filled chamber. The blocks can retain heat for days, losing just 1-3% per day. When the grid needs power, the stored heat is transferred to a tungsten-gold emitter that radiates light. Thermophotovoltaic (TPV) cells absorb that light and convert it directly into DC electricity. No turbines. No steam. Just photons and electrons.

It?s a system that feels more like a physics experiment than a commercial product. But Fourth Power insists it?s ready for prime time. Their 1 MWh pilot system is under construction, and they?re targeting 100 MW deployments with 5-100 hour durations - ideal for soaking up daytime solar and releasing it during evening peaks. The company claims its approach is modular, scalable, and free of rare earth constraints. No lithium, cobalt, or nickel. Just tin, graphite, and carbon.

Still, the devil is in the emitter. Gold, currently trading at around $4,000 per ounce, is alloyed with tungsten to create the high-temperature emitter that drives the TPV conversion. Fourth Power hasn?t disclosed how much gold is used, but even 100 grams adds over $12,000 in material cost. If the emitter requires more, the economics could wobble. Thin-film deposition and recycling may mitigate the impact, but the cost curve remains speculative.

Thermophotovoltaics themselves are a marvel - solid-state devices that convert heat into electricity with no moving parts. Efficiency ranges from 20-40%, depending on spectral tuning and emitter design. That?s lower than electrochemical batteries, but potentially offset by lower capital costs and longer lifespans. The TPV cells produce DC power, ideal for grid-tied inverters and battery integration.

Fourth Power isn?t alone in the thermal battery space. Antora Energy uses solid carbon blocks and TPV cells to deliver both electricity and industrial heat. Malta Inc. blends molten salt with compressed air in a hybrid electro-thermal cycle. Brenmiller and Rondo focus on steam and brick-based systems for industrial heat reuse. Each has its niche, but Fourth Power?s ultra-high temperature approach pushes the envelope - technically and economically.

The real question is adoption. Utilities are notoriously conservative, and regulators often favor proven technologies. Fourth Power will need to demonstrate reliability, safety, and cost-effectiveness at scale. But if it succeeds, it could unlock a new class of energy storage - one that?s cheap, durable, and decoupled from the volatile metals market.

In a grid increasingly shaped by intermittent renewables, the ability to store energy for days without degradation is a game-changer. Fourth Power?s tin-based system may not be the final answer, but it?s asking the right questions. What if heat, not chemistry, is the future of storage? What if photons, not ions, power the night?

Sources:
https://www.utilitydive.com/news/fourth-power-raises-19m-thermal-battery/702220/
https://gofourth.com/technology/
https://pv-magazine-usa.com/2023/12/12/gates-backed-fourth-power-pilots-thermal-storage-based-on-tpv-cells/
https://www.kitco.com/market/price/gold/

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