Thermal Energy Storage for Industry: How Molten Salt and Resistance Heating Decarbonize

Industrial heat is an inconveniently large problem. Manufacturing processes that require high temperatures — steelmaking, cement production, glass manufacturing, chemical processing, food production — account for roughly 20 percent of global energy consumption and approximately one-third of industrial CO₂ emissions. For context, this is more energy than the entire global commercial aviation sector, and it has proven substantially harder to decarbonize.

The difficulty is not purely technical. Natural gas and fuel oil are currently the cheapest way to deliver thermal energy at the temperatures industrial processes require — sometimes exceeding 1,500°C — and replacing them requires either a source of high-temperature electricity, a different fuel with similar energy density and cost, or a storage system that can absorb intermittent renewable electricity and release it as heat on demand. Thermal energy storage (TES) occupies that third niche, and two technologies — molten salt systems and resistance heating with solid materials — are emerging as the most commercially viable approaches.

## Why Industrial Heat Is Hard to Electrify Directly

Before examining TES technologies, it's worth understanding why direct electrification of industrial heat is more complicated than it appears.

Electric arc furnaces — already widely used for steel recycling — are a success story of industrial electrification. But primary steel production from iron ore requires reducing iron oxides at temperatures above 1,500°C, and electric arc furnaces need metallic feed stock (scrap) that is in limited supply. Cement production requires kiln temperatures around 1,450°C for clinker formation, and direct electric heating of rotary kilns has been demonstrated but introduces materials and process control challenges at scale. Ceramics, glass, and specialty chemicals each have specific temperature profiles, atmospheric requirements, and product contact constraints that complicate direct electric heating.

The intermittency problem compounds these difficulties. Even where direct electric heating is technically feasible, connecting a manufacturing process directly to grid electricity creates vulnerability to price volatility and supply reliability issues. A cement kiln cannot simply pause for four hours when grid prices spike; the thermal inertia of the process means that interruptions cause quality defects, equipment damage, or both.

Thermal energy storage addresses both problems simultaneously: it decouples the electricity consumption from the heat delivery, and it allows industrial facilities to charge storage during low-price or high-renewable-generation periods and discharge during production hours regardless of the grid state.

## Molten Salt: The Concentrated Solar Spinoff

Molten salt TES originated in the concentrated solar power (CSP) industry, where it has been used since the 1980s to store daytime solar thermal energy for nighttime electricity generation. The Solana and Crescent Dunes plants in the American Southwest demonstrated that molten salt could store gigawatt-hours of thermal energy reliably over decades of operation.

The industrial adaptation of molten salt TES uses the same fundamental concept: two insulated tanks at different temperatures, with fluid pumped between them depending on whether the system is charging or discharging. During charging, resistance heaters — powered by grid electricity or directly connected to renewable generation — raise the salt temperature in the hot tank to 560°C or higher. During discharge, the hot salt circulates through heat exchangers that deliver steam, hot air, or process heat to the industrial load.

**Practical properties of commercial molten salt mixtures:**
- Operating temperature range: typically 290°C (freeze point) to 565°C (thermal stability limit) for standard solar salts (60% NaNO₃, 40% KNO₃)
- Energy density: approximately 250 kWh per cubic meter of salt volume — sufficient for practical industrial storage at manageable tank sizes
- Round-trip efficiency: 85 to 92 percent for the thermal storage cycle, though the efficiency of subsequent heat-to-electricity conversion (if applicable) reduces overall round-trip efficiency further
- Freeze protection: the 290°C freeze point requires heat trace systems on all piping and a minimum operating temperature maintained by parasitic heaters during standby

The freeze protection requirement is one of the main operational challenges in temperate industrial environments. A cold snap that causes a power outage at a facility using molten salt storage creates a potential scenario where the freeze protection system itself loses power, risking solidification of salt in piping — an expensive and time-consuming recovery. Industrial installations address this through backup power connections and piping designs that allow gravity drainage, but it remains a maintenance and reliability consideration that operators take seriously.

**Industrial temperature limitations** are the second challenge. Standard solar salt mixtures begin degrading above 600°C, which excludes them from many high-temperature industrial processes. Research programs are developing alternative salt chemistries — including chloride salts and carbonate mixtures — capable of operating to 750°C or above, but these materials introduce more aggressive corrosion environments that require more expensive alloy piping and heat exchanger materials.

## Resistance Heating with Solid Media: The Simpler Alternative

A parallel approach to industrial TES uses resistive electric heating to raise the temperature of solid materials — typically ceramic bricks, volcanic rock (basalt), or graphite — which then release heat on demand. This concept is sometimes called "electric thermal storage" or, in its higher-temperature industrial variants, "heat batteries."

The operating principle is straightforward: high-temperature resistant electric heating elements embedded in or surrounding a mass of thermally stable solid material raise the material to the desired operating temperature. Insulated enclosures minimize heat loss. Discharge occurs by passing air, steam, or process gas through the heated material or via radiant heat exchangers in contact with the hot solid surface.

**Antora Energy**, a U.S.-based startup, has developed a graphite-based system capable of operating above 1,500°C — a temperature range that molten salt cannot reach and that opens the door to direct high-temperature industrial applications including steel reheating furnaces, glass melting, and calcination processes. Their approach stores energy in solid graphite blocks within a thermally insulated enclosure, using high-temperature heating elements to charge the system and radiative or convective discharge for industrial heat delivery.

**Rondo Energy** takes a different approach with ceramic brick stacks operating to approximately 1,500°C, focusing on steam generation for industrial processes. Their "heat battery" architecture uses a modular brick stack that can be sized to match specific industrial load profiles, from small food processing facilities requiring steam at 200°C to industrial chemical plants needing high-temperature process heat.

**Key advantages of solid media over molten salt:**
- No freeze protection requirement — solid materials simply cool down if not maintained, without phase change complications
- Higher temperature ceiling with appropriate material selection
- Simpler mechanical systems — no pumps, no fluid handling
- Lower upfront cost per unit of thermal energy capacity in many configurations

**Key disadvantages:**
- Lower energy density than molten salt by volume (though this varies substantially with material choice)
- Heat transfer out of solid media is less controllable than fluid-based systems — discharge temperature profiles are less flat
- Graphite systems require inert atmospheres to prevent oxidation above ~400°C, adding enclosure complexity

## The Economic Case: Where TES Makes Sense Industrially

The economic case for industrial TES depends on three factors: the cost of electricity, the spread between peak and off-peak electricity prices, and the cost of the alternative fuel being displaced.

In markets with high renewable penetration — Texas ERCOT, Germany, California CAISO — electricity prices now frequently go negative or near-zero during periods of peak renewable generation, particularly solar in midday and wind at night. An industrial facility with TES can charge during these low-price windows, reducing its effective electricity cost substantially. The arbitrage opportunity is largest when the spread between low and high price periods exceeds $30 to $50 per MWh, which occurs with increasing frequency as renewable capacity grows.

Natural gas price volatility adds to the case. European industrial facilities, which experienced dramatic gas price increases following the 2022 energy crisis, found that facilities with backup electric heating (not quite TES, but directionally similar) had substantially lower energy cost variance during peak crisis periods. TES with sufficient capacity to sustain production through multi-day gas price spikes offers a form of energy cost insurance in addition to the arbitrage benefit.

Current industrial TES system costs range from approximately $10 to $30 per kWh of thermal storage capacity, depending on temperature range and technology type — solid media systems tend to be at the lower end, while high-temperature molten salt and graphite systems with more complex heat exchange infrastructure sit higher. At these costs, payback periods of 5 to 10 years are achievable in markets with favorable electricity price spreads, and manufacturing learning curves are expected to bring costs down as the industry scales.

## The Integration Engineering Challenge

The engineering challenge that rarely appears in TES marketing materials is the integration problem: connecting a thermal storage system to an existing industrial process without disrupting production.

Industrial heating systems are not designed with TES retrofit in mind. Furnace control systems, heat exchanger sizing, piping layouts, and process temperature profiles were all engineered for direct-fired or direct-electric configurations. Inserting a storage buffer changes the thermal dynamics of the system: temperature profiles into the process may be less stable, heat transfer rates may differ from direct-fired equivalents, and the control logic must be rewritten to manage charging, discharging, and the transition between them.

This integration engineering is not insurmountable — it is simply not free. Industrial TES project costs routinely include substantial balance-of-plant and process integration work that the storage system cost alone does not capture. Early adopters in this space have generally been large industrial companies with in-house engineering capability, where this integration work can be performed with existing staff and amortized across large facility energy budgets.

The scaling of industrial TES to mid-market industrial customers — food processing, plastics, building materials — will likely require more standardized integration packages and engineering services capability from TES vendors. That market development is underway, but it is at an earlier stage than the large industrial deployments that have attracted the most attention.

Thermal Energy Storage for Industry: How Molten Salt and Resistance Heating Decarbonize

// COMMENTS

ON THIS PAGE