Steel-Based Thermal Energy Storage: A Comprehensive Overview

Technical Principles and Thermal Properties of Steel for Heat Storage

Steel and steelmaking slag can both serve as sensible heat storage media, meaning they store thermal energy by increasing in temperature without phase change. The fundamental principle is that heat Q is absorbed or released according to Q = m \, c_p \, \Delta T, where m is mass, c_p is specific heat capacity, and \Delta T is the temperature change. Steel (metal) typically has a specific heat capacity around 450–500 J/kg·K . For example, carbon steel’s specific heat is about 0.49 kJ/kg·K (490 J/kg·K) , which is lower than water’s (~4180 J/kg·K) but comparable to many solid materials. However, steel’s very high density (~7800 kg/m3) gives it a respectable volumetric heat capacity (~3.5–4.0 MJ/m3·K). Steel slag (the ceramic byproduct of steelmaking) has a specific heat in the range of ~800–940 J/kg·K at high temperature . One study reported electric-arc-furnace (EAF) slag’s $c_p$ ≈0.93 kJ/kg·K (933 J/kg·K) at 500 °C and density ~3430–3980 kg/m³ . This combination yields a volumetric heat capacity on the order of 3.2–3.7 MJ/m³·K – comparable to or greater than molten nitrate salts used in solar plants.

When steel or slag is heated, thermal energy is stored as internal energy (primarily as increased vibrational energy of the material’s lattice). Heat absorption occurs by raising the material’s temperature (sensible heat storage), and heat release occurs upon cooling, with the process being highly repeatable over cycles (assuming the material remains in solid state). Unlike phase-change materials, steel-based storage does not involve latent heat in typical operation – melting steel would require extremely high temperatures (>1300 °C) that are impractical for routine storage. Instead, steel-based systems use a wide temperature swing within the solid phase (for instance, heating from ambient up to a few hundred °C or more) to store energy. The amount of energy stored is proportional to this temperature swing; a larger $\Delta T$ yields more storage capacity. Steel’s high melting point means it can potentially be heated to very high temperatures (limited by container and system materials), allowing greater energy storage per unit mass compared to lower-temperature media.

Thermal properties: Steel in metallic form has high thermal conductivity (tens of W/m·K). For example, carbon steels have thermal conductivity on the order of 40–60 W/m·K (stainless steels are lower, ~15–20 W/m·K). This high conductivity helps steel pieces heat up and cool down relatively uniformly and facilitates heat transfer within the storage material. By contrast, steel slag (a ceramic) has a lower thermal conductivity (around 1–2 W/m·K at 500–800 °C ), meaning heat penetrates slag pebbles more slowly. Slag’s conductivity (~1.4 W/m·K at 500 °C for EAF slag ) is similar to other ceramic solids and is lower than metals, but adequate for heat storage when using appropriately sized particles. The thermal diffusivity of steel is generally higher than that of slag or concrete due to the combination of higher $k$ and moderate $c_p$. This implies steel can absorb/release heat faster internally, whereas slag or concrete may develop thermal gradients if not given sufficient time or if particle size is large.

In summary, steel and steel slag store heat as sensible heat: energy is taken up by raising their temperature and later released as they cool. Both have good thermal stability at high temperatures and do not undergo chemical or phase changes in the operating range, which contributes to their thermal cyclability. Steel slag in particular is noted to be thermally stable above 1000 °C , far beyond the limit of most heat transfer fluids. This high-temperature capability means steel-based media can store high-grade heat that can be used for efficient power generation or high-temperature processes. The high melting point also means there’s no risk of the material liquefying or decomposing under normal operation (unlike molten salts, which can decompose or solidify). Table 1 below summarizes key thermal properties of steel/slag versus other common storage materials.

Table 1: Thermal Properties and Cost of Steel/Slag vs Other Thermal Storage Media

Storage Medium	Form (Usage)	Typical Temp Range	Specific Heat ($c_p$)	Density	Thermal Conductivity	Approx. Cost	Notes
Steel (metal)	Solid blocks or scrap pieces	Up to ~800 °C1	~450–500 J/kg·K	~7800 kg/m³	15–50 W/m·K (high)	Medium (~$200/ton for scrap)	High conductivity allows fast heat transfer; may oxidize at high $T$ if in air.
Steel Slag (EAF)	Pebbles (packed bed)	Up to ~1000 °C2	~800–940 J/kg·K	~3500 kg/m³	~1–2 W/m·K at 500 °C	Very low (~$0–80/ton)	Waste byproduct; stable at high $T$; low cost but requires containment and airflow.
Molten Solar Salt (Na/K nitrates)	Liquid in 2-tank system	290–565 °C (melts at ~240 °C)	~1500 J/kg·K @500 °C (liquid)	~1800 kg/m³ (liquid)	~0.5 W/m·K (liquid)	Moderate (~$700/ton)	State-of-art in CSP; pumpable fluid with 99% thermal efficiency, but limited max temp ~565 °C and risk of freezing.
Concrete / Heat Cement	Solid monolith or modules	~120–560 °C (special HT concrete)	~880–1130 J/kg·K	~2200–2400 kg/m³	~1 W/m·K	Very low (~$10–50/ton)	Cheap and scalable; used with embedded heat exchangers; may crack under thermal cycling if not engineered.
Ceramic Bricks (Alumina, etc.)	Bricks or pellets in packed bed	Up to ~1000 °C	~800–1000 J/kg·K	~3000–3500 kg/m³	1–5 W/m·K (varies)	High if advanced (> $500/ton), but waste ceramics cheap	Durable at high $T$ (used in furnaces); can be made from recycled materials (e.g. firebrick with conductive additives for electrical heating).

1Steel can withstand higher temperatures, but in practical systems the maximum is often limited by the steel container or structure to avoid weakening (e.g. ~800 °C limit when stored in a carbon steel tank) .

2Steel slag itself is stable to ~1100 °C before melting . In practice, storage systems with slag limit operating temperature to ~800–1000 °C due to container and insulation constraints.

As shown in Table 1, steel and slag have lower specific heat than molten salts or water, but their ability to operate at much higher temperatures (well above 600 °C) is a major advantage . This allows steel-based storage to supply high-grade heat or enable higher-efficiency power cycles. For instance, molten nitrate salts in CSP are limited to ~565 °C, whereas steel slag has been proposed for use above 600 °C in next-generation CSP plants . Higher temperature storage increases the Carnot efficiency if converting heat to electricity, and provides more utility for industrial processes that require extreme heat. In terms of heat capacity per volume, steel and slag (due to their high density) compare well with alternatives – slag pebbles offer volumetric heat capacities on par with or exceeding molten salt . Additionally, steel’s high thermal conductivity aids in charging and discharging, while slag’s properties as a ceramic give it good thermal stability and resistance to thermal shock.

Design and Operation of Steel-Based Thermal Storage Systems

Steel-based thermal energy storage (TES) systems are typically designed as solid-media sensible heat storages, often in the form of packed beds or modular solid blocks. The two common design approaches are:

Packed bed regenerators using steel or slag-based filler materials.
Solid block or module systems (often steel or concrete modules) with integrated heat exchange.

In a packed bed TES, the storage material is crushed or formed into pebbles/plates and packed inside an insulated container. A heat transfer fluid (HTF), commonly air or gas, is blown through the voids in the packed bed to charge or discharge heat. For example, steelmaking slag pebbles of 1–3 cm size have been used in a packed bed configuration . During charging, hot HTF (e.g. hot air) flows through the bed from one end, transferring heat to the steel/slag media; during discharging, cooler HTF (or ambient air) is passed from the opposite end, absorbing heat from the hot media. The bed operates as a thermocline: a temperature gradient develops along its height, with the hottest material at the charging inlet and the coolest at the outlet . This allows a single tank to hold both “hot” and “cold” regions separated by a thermal gradient, eliminating the need for separate hot/cold tanks (as used in two-tank molten salt systems) . The single-tank design reduces complexity and cost, since only one vessel is needed and the filler material itself acts as both storage medium and passive heat separator .

Packed bed system design: A practical steel/slag packed bed includes an insulated storage tank (often a steel shell lined with refractory insulation) filled with the solid media typically occupying ~60–70% of volume, with the rest being void space for airflow . For instance, in a pilot at ArcelorMittal’s Sestao plant, a 5 m high, 1.5 m diameter tank was filled about 65% with slag pebbles (balance air void) . During operation, hot exhaust or a working fluid enters, and an internal distributor or plenum may be used at the inlet to ensure even flow distribution and avoid channeling. Research has shown the importance of distributor design (perforated plates, cone diffusers, etc.) to achieve uniform airflow and minimize pressure drop . Without a proper distributor, flow can short-circuit through parts of the bed, leading to poor utilization of the storage (one study found no distributor led to very high pressure drop and uneven flow, whereas a well-designed distributor kept the flow uniform across the bed) . Thus, practical systems often include engineered flow distribution devices at inlet/outlet.

Heat transfer and materials: In steel/slag packed beds, air is a common HTF because it can tolerate very high temperatures (unlike oils or water/steam which have lower limits). The storage material is often in direct contact with the air (a direct regenerator concept), meaning the air flows through the porous bed and directly exchanges heat with the solids. This direct contact design is simple and avoids intermediate heat exchangers within the tank, improving efficiency. For example, experiments in Spain demonstrated that slag-based storage can work “100%” compatibly with air at high temperatures (~1000 °C) . One challenge is that if the heat source is an industrial exhaust gas, it may contain dust or corrosive species that could foul the bed or react with the media. To address this, systems like the ArcelorMittal pilot include a heat exchanger to transfer heat from dirty exhaust to clean air before the storage tank . In that case, the exhaust gas from an electric arc furnace (laden with dust) first passes through a robust heat exchanger to heat fresh air, which then circulates through the slag bed . This prevents contamination of the storage material and corrosion of the tank by aggressive gases, albeit at the cost of an extra heat exchange step.

Capacity and scaling: Steel-based storages can be scaled by increasing tank size or using modular units. Packed beds are highly scalable – from small pilot (~MWh thermal) to grid-scale (hundreds of MWh). For example, a 1 MWht pilot with slag was built at ~5 m × 1.5 m size , and studies have designed full-scale thermocline regenerators for CSP towers requiring multiple large tanks ~15.8 m in diameter and 11.5 m high for hundreds of MWh capacity . The modular nature of packed beds (using inexpensive filler) means capacity can be added largely by adding more material and volume, without exotic engineering for larger sizes.

Another design approach is using solid blocks or plates of steel as the storage medium with heat exchange fluid passing through channels. One example is the use of steel plates or rods heated by electric resistors or by a heat transfer fluid in channels. Berlin startup Lumenion employs a steel storage core consisting of steel elements (such as plates) inside an insulated enclosure, which are heated to ~600–650 °C by resistive heating . The heat is stored in the steel’s thermal mass and can later be transferred to a secondary fluid (like water/steam or air) via heat exchangers. This design essentially turns the steel itself into a heating element and storage medium. Because steel has high conductivity, large blocks can be heated relatively evenly if designed properly. Lumenion’s 2.4 MWh pilot unit in Berlin is a steel block storage charged by electric heaters at night (using surplus wind/solar power) and discharged to provide district heating and even electricity via a steam turbine . Such a system typically consists of an insulated cylinder or vault containing stacks of steel plates or modules, electric heating coils, and plumbing to extract heat. Thermal expansion must be managed (steel expands when heated to 600+°C), so modules are often arranged with expansion gaps or flexible supports. Additionally, to prevent oxidation of steel at high temperature, some designs may use an inert cover gas or maintain a low-oxygen environment inside the storage vessel, or simply accept the formation of a surface oxide layer.

Solid media vs fluid media: Unlike liquid media (molten salt, water) that require containment but can be easily pumped, solid media storages keep the material stationary and move the heat through it via a fluid or electrical heating. This means the design often needs internal features for heat exchange – either blowing air through (packed bed) or embedding pipes (solid block). Steel-based systems can use either approach. The efficiency of heat charge/discharge in these systems depends on good thermal contact and flow distribution. In packed beds, a thermocline forms during operation: initially, the incoming hot air heats the top layers of the bed; as the bed saturates, the heat front moves downward. The goal is to achieve a steep thermocline (temperature gradient) and minimize mixing so that hot and cold zones remain distinct . This maximizes exergy and allows nearly the entire thermal capacity of the bed to be utilized (only a small buffer zone mixes at intermediate temperature). Packed bed operations often use flow reversal between charge and discharge (hot air in opposite ends) so that the hottest material is always at the hot outlet side during discharge .

Example – slag packed bed in a steel plant: In the ArcelorMittal Sestao demonstrator, the system operates as follows: flue gas at >1000 °C from an electric arc furnace first passes through a custom heat exchanger to clean the heat . The heat is transferred to air at atmospheric pressure, which is then blown through the slag pebble bed. The slag pebbles heat up to ~800 °C (the design limit set by the steel tank construction) . Later, when a new batch of scrap steel needs pre-heating, the process is reversed: cold air is blown through the hot slag bed, extracting the stored heat, and that hot air is directed to the scrap metal feed, replacing what used to be natural gas burners . In this way, the regenerator functions much like the classical Cowper stoves in blast furnaces (which preheat air using checkerbrick heat storage), but here using waste heat and slag as storage. This configuration proved that even with the harsh environment of steel off-gas (dusty, corrosive), the system can be engineered (via heat exchangers and material choices) to reliably store and deliver heat on demand .

Other design considerations: Insulation is critical, as steel-based storage runs at high temperature. Typically, a thick layer of refractory or mineral wool insulation lines the inside of the steel shell to minimize heat loss. In some designs (especially for >600 °C), a dual shell may be used: an inner refractory concrete or ceramic liner to contain the heat, and an outer steel shell for structural support at a cooler temperature. Thermal stress management (due to gradients and expansion) is addressed by using expansion joints or by limiting temperature ramp rates. Particle size for packed beds is optimized for heat transfer vs pressure drop: smaller particles give more surface area and better heat transfer with the air, but cause higher pressure drop and risk of compaction. Studies in the REslag project found an optimal slag pebble diameter around 3–4 cm for balancing these factors in a high-temperature air system .

In summary, steel-based TES can be implemented either as a stationary solid bed with a moving heat transfer fluid or as a solid resistor that is directly heated and then transfers heat out. The designs emphasize simplicity (no moving storage media parts), use of inexpensive materials (scrap metal or slag), and robustness for high temperatures. The packed bed regenerator design, in particular, is a proven concept adapted from industrial regenerators and now being extended to renewable energy storage because of its one-tank design and use of low-cost filler .

Commercial Deployments and Pilot Projects Using Steel/Slag Storage

In recent years, several pilot projects and emerging commercial systems have demonstrated steel-based or slag-based thermal storage in real applications:

ArcelorMittal (Sestao, Spain) – Slag Thermal Storage Pilot: The world’s largest steel producer, ArcelorMittal, in collaboration with the European REslag project, built a 1 MWht thermal storage unit at its steelworks in Sestao. Commissioned in early 2019 , this pilot uses steelmaking slag pebbles (1–3 cm) as the storage medium in a packed bed. The system captures waste heat from an electric arc furnace (EAF) (exhaust ~1000 °C) and stores it in a 5 m tall insulated tank of slag . Later, the stored heat is used to preheat incoming scrap metal feedstock, replacing natural gas burners . The pilot had an initial scale of 400 kWht for validation and was then scaled to 1 MWht . By July 2019, performance data were being analyzed to quantify fuel savings and CO2 reduction . The motivation is both energy efficiency and CO2 reduction – initial hopes were to cut fossil fuel use by up to 80% in the scrap preheating stage . This project demonstrated the technical viability of slag as a storage material in an industrial setting, and by mid-2019 it had shown that slag’s thermal/mechanical behavior over cycles was stable and compatible with the steel plant environment . While results indicated slag particles held up well (no significant degradation over cycles) , it also highlighted that heat exchanger design is crucial to efficiently recover EAF waste heat . The ArcelorMittal pilot is a landmark in using an industrial byproduct (slag) for onsite energy storage. It has spurred interest in replicating the concept at other plants and even transferring it to other sectors (the Basque project leaders noted potential in cement or glass industries) .
REslag Project (EU) – CSP and Industrial Slag Storage: REslag (EU Horizon 2020 project, 2016–2019) built not only the steel plant pilot above (Pilot 2) but also two CSP-oriented slag storage pilots . Pilot 3a was implemented at DLR (German Aerospace Center) in Stuttgart – a 400 kWht test regenerator using EAF slag and air as HTF, mimicking a solar tower with open air receiver . Pilot 3b, at ENEA in Italy, integrated slag pebbles into a molten salt loop (slag packed-bed as a filler in a molten nitrate salt thermocline tank) . These pilots confirmed feasibility: Pilot 3a proved slag can handle cyclic charging with air up to 1000 °C and maintain mechanical integrity . Pilot 3b showed sintered slag pebbles are chemically compatible with molten solar salt at 500 °C, with no adverse reactions, enabling use of slag as filler in single-tank molten salt storage . The significance is a potential cost reduction of ~40% in filler cost for CSP storage by replacing expensive ceramic or extra salt with waste slag . Indeed, slag-based filler was estimated to bring economic savings and environmental benefits (cutting landfill waste and lowering CO2 footprint of storage) . The REslag project concluded that “slag is thermally, mechanically and chemically competitive with conventional inventory materials” for high-temperature storage , and laid groundwork for commercial adoption.
Slag2PCC / Slagstock Projects: In Europe, additional projects like SLAGSTOCK have investigated steel slag for thermosolar plants. The Slagstock project (c. 2018–2020) confirmed that EAF slag pebbles can serve as a feasible thermal storage material for concentrated solar power (CSP), validating their economic and ecological viability . Publications from these efforts note that steel slags offer thermal stability >1000 °C and cost savings up to 40% compared to traditional ceramic or salt fillers . A master’s thesis LCA of Slagstock’s system also found significant reductions in global warming potential and water use compared to molten salt tanks . While these projects were at pilot scale, they pave the way for commercial CSP plants to consider slag-based storage, especially for next-generation systems using air receivers or particle receivers (where slag particles could double as heat transfer and storage medium).
Lumenion (Germany) – Steel Heat Battery for Grids: Lumenion GmbH is a startup deploying high-temperature steel storage for renewable energy. In 2018–2020 they tested a 450 kWh prototype, and in 2020 installed a 2.4 MWh steel heat storage system integrated into a Berlin district heating grid . Lumenion’s technology, sometimes called a “steel battery”, uses resistive heating elements to heat a stack of steel modules up to ~650 °C . The system stores surplus wind or solar power as heat in the steel. When needed, the stored energy can be delivered as industrial process heat or space heating, or a portion converted back to electricity via a steam turbine (making it a type of Carnot battery) . The 2.4 MWh unit in Berlin (Tegel district) is tied into Vattenfall’s combined heat and power network, providing renewable heat to homes . Lumenion reports that their steel storage has a high round-trip efficiency (when used purely for heat, essentially 95%+ since losses are low with good insulation) and can output heat at various temperature levels (they supply 120 °C water for heating, or can go up to 450 °C steam for industry) . Crucially, the cost is competitive: steel is relatively cheap per kWh of thermal capacity, and they project larger installations (40 MWh, 100+ MWh) to be economically attractive for grid storage . This is one of the first commercial deployments of steel-based thermal storage for energy shifting, showing how “fluctuating renewable energies [can be] available 24/7 as process heat or district heating” . By 2025, Lumenion and similar systems are being considered to repurpose coal plants or provide storage in renewable microgrids.
Siemens Gamesa ETES (Electric Thermal Energy Storage): Although not using steel as the medium (Siemens used volcanic rocks), this 130 MWh pilot in Hamburg is relevant as a proof-of-concept for grid-scale sensible heat storage. It stored hot air at ~700 °C in a packed bed of rocks to later drive a steam turbine . The success of this one-week, 1000-tonne storage facility suggests that similar designs could use scrap metal or slag as the filler. In fact, a patent by German engineers proposes “scrap metal as a heat storage medium” in CSP plants as a cheaper alternative to molten salt . Using scrap iron or steel punchings could reduce costs and consume metal waste while achieving similar storage performance . To date, Siemens’ ETES and projects like EnergyNest’s ThermalBattery (using concrete) illustrate a growing commercial interest in solid media storage at scale. They validate the practicality of one-tank, cheap-material storage, into which steel/slag fits directly.
Electrified Thermal Solutions (Joule Hive Thermal Battery): In 2025, ArcelorMittal’s venture arm invested in Electrified Thermal Solutions, a U.S. company making Joule Hive Thermal Batteries for industry . The Joule Hive system uses a stack of specially designed firebricks inside an insulated steel container, which are electrically heated and can reach extremely high temperatures. While the medium here is a type of ceramic brick (with enhanced electrical conductivity), not steel, it targets the same market: providing high-temperature heat storage for industrial use (including steelmaking) to displace fossil fuels . The relevance is that it underscores a trend of commercial solutions aiming to store renewable electricity as high-grade heat using solid materials. Steel-based media could achieve similar outcomes – indeed, the choice of conductive bricks is a parallel approach to using steel elements, both allowing direct joule heating. This technology is being scaled to a 1 MW pilot and eyed for integration at steel plants (ArcelorMittal’s GasLab in Asturias) . It exemplifies the industrial acceptance of thermal storage as a decarbonization tool.

Overall, commercialization of steel/slag heat storage is in its early stages but rapidly progressing. The steel industry itself is piloting these systems for waste heat recovery (e.g. ArcelorMittal, Cleveland-Cliffs in NA has also shown interest in waste heat storage). Renewable energy projects are adopting solid-state heat batteries (Lumenion, EnergyNest, ETS, etc.), with steel and slag poised as abundant, low-cost media. The pilots have demonstrated key performance aspects: high-temperature durability, environmental safety (no toxic materials, slag leaching is minimal once sintered/inerted), and significant energy density. As renewable penetration grows and industries seek to cut carbon emissions, steel-based thermal storage is moving from pilot to practical deployment, especially wherever cheap surplus heat or power can be stored and later used in place of fossil fuel.

Comparison with Other Thermal Storage Materials

Steel-based thermal storage has unique characteristics compared to other common thermal energy storage (TES) materials like molten salts, concrete, ceramic media, or phase-change materials. Below we compare them on key factors:

Operating Temperature Range: Steel and slag can handle very high temperatures. Steel slag is stable for use well above 800 °C (demonstrated up to 1000 °C with air) , and even up to ~1100 °C before beginning to soften . Steel metal likewise can be used at high temperatures limited mainly by oxidation and container strength (carbon steel loses strength above ~800 °C, but high-alloy steels could go higher). In contrast, the standard molten salt (solar salt: 60% NaNO₃/40% KNO₃) is limited to ~565 °C before it starts decomposing . Phase-change salts or alloys often melt below 400 °C for practical reasons. Concrete storage is usually limited to ~400 °C (some advanced concretes up to ~550 °C) to avoid cracking and dehydration of binders. Ceramics (alumina bricks, etc.) can also handle >1000 °C similar to slag, but are more expensive. Thus, for high-temperature (>600 °C) applications, steel/slag has a clear advantage, enabling higher efficiency power cycles (e.g. supercritical steam or sCO₂ turbines) and serving processes (like metals, cement) that require extremely hot heat.
Energy Density: By mass, steel/slag have moderate specific heat (as noted, ~0.5 kJ/kg·K for steel, ~0.9 kJ/kg·K for slag at high T). Molten nitrate salt has a higher specific heat (~1.5 kJ/kg·K), but much lower density (~1800 kg/m³ liquid vs 3500+ for slag, 7800 for steel). Consequently, by volume, steel and slag can store comparable or more energy than nitrate salts per unit volume. For example, slag pebbles (density ~3.8 g/cc, $c_p≈0.8$ J/g·K) have volumetric heat capacity ~3.0 J/cc·K, whereas molten salt (1.8 g/cc, $c_p≈1.5$ J/g·K) is ~2.7 J/cc·K. Concrete is less dense (~2.3 g/cc) and has $c_p≈0.88$ J/g·K, giving ~2.0 J/cc·K. Thus, slag and steel provide high energy density in solid form – an important factor when space is a premium (industrial retrofits or urban installations). Furthermore, steel/slag systems can often utilize a larger $\Delta T$ range. For instance, a slag storage might operate from 200 °C up to 800 °C (ΔT = 600 K), whereas solar salt might only use 290–550 °C (ΔT = 260 K in practice). The larger temperature swing effectively increases the storage capacity (in J/kg) of the solid media, partially offsetting the lower specific heat. In terms of total storage capacity per volume, properly designed steel/slag packed beds can rival two-tank salt systems .
Efficiency and Heat Losses: If we consider thermal efficiency (fraction of stored heat that can be retrieved), steel/slag systems can be very efficient, typically >95% for sensible heat storage over the charge-discharge cycle. There are two aspects: heat transfer efficiency and stand-by heat losses. On heat transfer, well-designed packed beds or heat exchangers can achieve near equilibrium heat exchange; for example, regenerative packed beds can recover a high fraction of heat with only a small “thermal deadband” (the thermocline region that remains in the tank). Studies have reported thermal recovery efficiencies around 90–99% for packed beds, similar to molten salt tanks which often exceed 90% thermal efficiency . The main losses are convective/radiative losses through insulation. Steel/slag storages, being high-temperature, do have significant driving gradients, so thick insulation is required to keep losses low (often a few percent per day). In practice, a large steel slab storage (like Lumenion’s) is claimed to lose only ~1% of heat per day with proper insulation . Molten salt tanks also lose heat and require heaters to prevent freezing overnight, incurring standby losses. Thus, both systems are comparable in efficiency if well insulated. A minor difference: parasitic energy – molten salt uses pumps to circulate fluid (parasitic electrical load), whereas an air-based steel/slag storage uses fans or blowers. Both consume some electricity, but are relatively small (on the order of a few percent of thermal power). For round-trip electricity storage (Carnot battery mode), the overall efficiency will depend on the power cycle. A high-temperature steel-based system (800+°C) driving a modern turbine could achieve ~40–50% electric round-trip efficiency (similar to a molten salt CSP plant’s turbine efficiency), whereas a lower temperature system (e.g. <400°C steam from concrete storage) would have lower efficiency. So in summary, for direct heat use, steel/slag TES has near-equivalent efficiency to other sensible heat storages; for power generation, it holds an advantage in potentially higher temperature (thus higher Carnot efficiency).
Cost: One of the strongest points of steel slag (and to a lesser extent scrap steel) is low cost. Steel slag is essentially a free byproduct – worldwide ~20 million tons of steel slag are produced annually in the steel industry , and a significant portion is landfilled as waste . In the slag storage pilot, it was noted “the cost [of slag] is almost zero – only transport” . Estimates put prepared EAF slag pebbles at about €80 per ton in some cases (still only ~$0.008 per kg, an order of magnitude cheaper than salts). Even including processing (sieving, sintering into pebbles), slag is dramatically cheaper per kWh stored. Scrap steel is a traded commodity (its value fluctuates, e.g. $150–300/ton depending on market), but using low-grade scrap or pig iron specifically for storage can be cost-effective. A patent claims that using scrap metal as TES medium would “reduce storage media costs compared to salt mixtures” and also consume surplus scrap in an eco-friendly way . By comparison, molten salts are relatively expensive (solar salt might be $700–800/ton plus the cost of heat tracing and pumps), and high-purity ceramic media (like alumina balls) can cost hundreds to thousands of dollars per ton. Concrete is extremely cheap (cement, sand, etc. for <$100/ton of mix), but requires formwork and piping which add cost at system level. On pure media cost per kWh_th: slag can be <$1 per kWh_th (one analysis found slag storage material cost yields ~0.64 €/kWh_th vs ~5–10 €/kWh_th for molten salt) due to slag’s low price and large ΔT . Overall, steel slag offers one of the lowest-cost thermal storage materials on a $/MJ basis, and scrap steel is also competitive especially if one uses otherwise low-value metal (like pig iron cast-offs).
Scalability and Footprint: All sensible heat systems (steel, salt, concrete, etc.) are quite scalable, but there are practical differences. Molten salt two-tank systems have been built at GWh-scale (e.g. 1000+ MWh_th in large CSP plants). Steel/slag storage is modular and can similarly be scaled by adding tanks or increasing tank size. The footprint for a given energy may be somewhat larger for a solid media bed than a salt tank, due to void spaces and lower operating ΔT in some cases, but clever design (tall tanks with thermocline) mitigates this. For example, a study for a full-scale slag-based CSP storage envisaged multiple large silos (~15 m diameter, 12 m tall each) to store many hours of heat . This is comparable in land use to salt tank farms. One advantage is vertical scaling: packed beds can be made tall to maximize ΔT stratification, using land area efficiently (one tall silo vs two wide salt tanks). Concrete or steel module systems can be stacked vertically as well. Manufacturability also favors steel and concrete: these storages can leverage existing silo tank fabrication and concrete casting techniques, whereas molten salt systems involve more specialized pumps, valves, and heat tracing infrastructure. Therefore, for rapid deployment at scale (e.g. repurposing a decommissioned tank or building a storage farm), using steel or slag might simplify construction (less welded pipe, more bulk material handling). On the other hand, molten salt’s advantage is fluid transferability – energy can be moved via pipes to a separate location (e.g. tower receiver to ground storage), whereas a solid storage is usually co-located with the heat source/use or requires a secondary heat transfer loop to move energy in/out.
Durability and Lifetime: A well-known issue for thermal storage media is how they hold up over many heating-cooling cycles. Steel slag has been tested for thermal cycling and found to have good mechanical stability with minimal degradation. In tests of sintered slag pebbles cycled between ambient and 1000 °C in air, the weight loss or change was very small after many cycles . Some initial conditioning occurs – slag pebbles might undergo minor structural transformations or surface oxidation in the first few cycles (one study observed a slight mass change in the first cycle due to oxidation of FeO to Fe2O3, but then stabilized) . After that, slag is essentially inert: it doesn’t significantly corrode with dry air, and compatibility tests showed “no chemical corrosion” between EAF slag and air at 1000 °C . Slag also proved compatible with molten nitrate salt and synthetic oil (no appreciable reaction at 500 °C and 400 °C respectively) , highlighting its versatility for different system types. Steel metal in a hot air environment will oxidize over time (forming iron oxide scale). This is a limitation – a steel filler may gradually flake off oxide, losing some mass/heat capacity and potentially fouling airflow. Historically, attempts to use metal (iron) in regenerators were abandoned due to oxidation; however, in a sealed or inert atmosphere, steel would not oxidize appreciably and could have a very long life. Some newer concepts propose coating metal particles or using low-oxygen purge gas to mitigate oxidation if steel media are used openly. In general, ceramics (slag, alumina, brick) have an edge in inherent durability (no oxidation, very high melting point).
The tank and structure also limit lifetime: salt tanks suffer corrosion of tank walls and need careful materials (often 304/316 stainless steel or A516 carbon steel with nitrite inhibitors). Slag/steel storage tanks operate either in air (corrosion at high temp is a factor for the steel shell interior) or with refractory linings. A well-designed slag storage tank will have an inner refractory, protecting the outer steel shell from extreme heat and corrosion, thereby extending life (many furnace regenerators operate for decades with brick and steel shells). Concrete storage durability depends on preventing crack propagation – thermal stress can cause micro-cracks, but modern heat concrete formulations and rebar can give many thousands of cycles of life.
Environmental and Safety: Steel and slag are generally benign materials. Slag is a solid waste, often used in road aggregate; encapsulating it in a TES gives it a second life and prevents landfill. It’s chemically stable in solid form, especially if from EAF (mostly oxides of Ca, Si, Fe, etc.). There is little environmental risk – unlike molten salts, there’s no risk of spills of hot liquid or salt contamination of soil. Also, no risk of fire or toxicity (some PCMs like organic phase-change materials are flammable; molten salts can release toxic fumes if overheated). Steel media likewise pose no chemical hazard. One safety consideration is that steel/slag storage operates at high temperature and often at atmospheric pressure (if using air). Atmospheric pressure operation is intrinsically safer than pressurized steam accumulators or tanks. In case of a breach, hot solid material will cool in place rather than flow. Molten salt, by contrast, is liquid and can cause severe thermal incidents if leaks occur. Freeze protection is a unique concern for molten salts – heat tracing and careful procedures are needed to avoid solidification in pipes, whereas solid media storages don’t have a freeze issue (they’re always solid). This reduces operational complexity and risk of catastrophic freezing-induced downtime.
Performance (Charge/Discharge Rate): Steel-based storage can have high power capacity if designed for it – e.g. blowing a large mass flow of air can extract heat quickly from a packed bed. The rate is primarily limited by heat transfer coefficients and the material’s thermal diffusivity. Metals like steel can absorb heat very rapidly (especially if directly resistively heated or via induction) due to high thermal conductivity, allowing fast charging if enough power is applied. Ceramic slag, with lower conductivity, charges a bit slower; but by using small particles, one can achieve high surface area and good convection, mitigating this. Molten salt can typically charge/discharge at a rate limited by heat exchanger and pump capacities, often sized for 6-8 hours of charge in CSP. Packed beds can be designed for faster turnaround if needed (with higher flow rates and some efficiency trade-off). There is also the concept of stratification: molten salt storage can provide constant temperature output (from the hot tank) until switching to cold tank, whereas a thermocline will see a temperature drop-off as the thermocline passes. For processes requiring stable temperature output, control strategies (mixing, or oversizing the storage to avoid delivering the bottom of the thermocline) are needed. This is a known issue with single-tank systems, but one that can be managed by design (e.g. using a filler that maintains a sharper thermocline, layering different materials, or recirculating to homogenize output).

In conclusion, steel-based vs other materials: Steel and slag excel in high-temperature, low-cost applications, providing durable storage without exotic maintenance (no freeze, no pump leakage). Molten salts excel in well-proven, medium-temperature storage with fluid handling (especially for direct two-tank setups in CSP plants so far). Concrete/brick solutions target cost and simplicity, albeit at moderate temperatures. Table 2 summarizes a qualitative comparison:

Table 2: Qualitative Comparison of Steel/Slag TES vs Other Materials

Aspect	Steel/Slag TES	Molten Salt TES	Concrete/Brick TES
Temp. Range	High (up to 800–1000 °C) – enables high-grade heat and efficient power cycles.	Moderate (290–565 °C typical) , limited by salt stability.	Low-Moderate (up to ~400 °C, special mixes ~550 °C).
Energy Density	High volumetric (dense media); moderate gravimetric. Large ΔT possible.	Moderate (liquid but lower density; ΔT limited by stability).	Low-moderate (needs 2× volume of steel for same energy).
Capital Cost	Low media cost (slag ~$0–10/ton , scrap steel <$300/ton). Simple tank, but blower/exchanger needed.	Medium media cost (salt $700+/ton). Needs expensive pumps, valves, two tanks.	Very low media cost (cement, brick cheap). Requires heat exchanger tubing (cost).
Thermal Efficiency	~95%+ (sensible heat recovery); well insulated for low losses. Single-tank thermocline has slight temperature glide.	~98% (two-tank keeps hot fluid separate, minimal mixing). Needs tracing to avoid freeze losses.	~90–95%; losses low if insulated. Possibly larger temperature drop during discharge.
Cycle Life	Excellent (slag inert, steel stable if protected from oxidation). No degradation observed after many cycles .	Good (salt can slightly decompose over years; pumps and valves wear). Tanks proven ~30-year life.	Good (concrete may crack over thousands of cycles; proper design can achieve long life). Firebrick proven in furnaces.
Response Time	Fast if designed for it (steel can be resistively heated quickly; air can ramp fast, limited by blower power).	Moderate (ramp limited by salt pump rate and risk of thermal stress in HX).	Moderate (thermal inertia of concrete/brick is high; typically slower charge/discharge rates).
Complexity	Medium – solid storage, but requires blower and possibly heat exchanger for integration. Mostly passive solid media.	High – liquid handling system with heat tracing, freeze management, pumps, instrumentation.	Low – very few moving parts (just fluid circulating in pipes or simple dampers). Construction similar to industrial furnace.
Use Cases	Ideal for waste heat recovery, high-temp industrial heat, Carnot batteries, and future high-temp CSP (open air towers, particle systems) . Often integrated on-site due to solid nature.	Ideal for current CSP plants (proven tech), energy storage where integration with heat exchangers is designed (e.g. PTES concepts). Best when fluid transport is needed (e.g. from receiver to ground storage in solar tower).	Ideal for distributed heating (district heat blocks), or as backup/peak shaving for buildings (night storage heaters). Also being used in electrified industrial heat (brick heaters for kilns, etc.).

Each technology has trade-offs, but steel and slag offer a compelling mix of high temperature capability and low cost, which is hard for other materials to match simultaneously. For instance, ceramics can handle high temperatures but are costly; concrete is cheap but limited in temperature; molten salt is proven but can’t go as hot. Steel/slag hits a sweet spot for sensible heat storage in the energy transition, especially for applications that demand both low cost and very high temperatures.

Key Use Cases for Steel-Based Thermal Storage

Steel and slag thermal storage systems are being explored in a variety of use cases across industrial, power generation, and heating sectors. Some of the most promising applications include:

Industrial Waste Heat Recovery

Industries such as steel, cement, glass, and chemicals often reject large amounts of high-temperature waste heat. Steel-based TES can capture this heat for later reuse, improving energy efficiency and cutting fossil fuel consumption. The steelmaking EAF example discussed earlier is emblematic: an EAF furnace’s off-gas (~1000 °C) is normally wasted, but with a slag storage tank, that heat can preheat scrap metal or combustion air, significantly reducing new energy input . By integrating a slag regenerator at steel mills, studies estimate up to ~15% of primary energy (that was lost in off-gas) can be recovered , translating to major cost and emission savings. This use case directly supports industrial decarbonization by recycling heat on-site.

Other heavy industries can benefit too. The Basque project team noted interest in applying slag storage to cement kilns (another industry with high-temperature exhaust) . A cement plant could use stored heat from kiln flue gas to preheat raw meal or to generate steam for power, displacing some fuel. Similarly, in glass manufacturing, regenerator furnaces currently use special checkerbrick regenerators to capture furnace exhaust heat – these could potentially be augmented or replaced with engineered steel/slag beds for better performance or using cheaper media. Essentially, any process with intermittent or continuous high-temp exhaust can employ a steel/slag TES to time-shift heat to when and where it’s needed.

A specific emerging opportunity is ‘batch process’ industries – e.g., forging furnaces, ceramic kilns – where heat from one batch (or one part of a cycle) can be stored and then used to warm the next batch or maintain temperatures. For instance, a forging shop could use a steel module heat battery to capture heat from quenching or furnace cooling and supply it to preheat the next load, leveling out demand.

Concentrated Solar Power (CSP) and Renewable Power Plants

CSP is a natural fit for high-temperature solid media storage. Traditional CSP plants (parabolic trough, power tower) have mostly used molten salt two-tank systems. But next-generation CSP designs are moving toward higher temperatures and alternative HTFs (like air or particles), where steel-based storage shines. In a solar tower with air as HTF, a steel slag packed bed acts as a direct thermal battery: solar-heated air (say at 700–800 °C) from the receiver passes through the slag bed and heats it during the day; at night, the flow reverses, hot air from the bed is sent to a steam generator or a Brayton cycle to produce power . This concept was successfully demonstrated in the Jülich experimental solar tower in Germany which uses a regenerator (ceramic-based) for storage . The REslag project took it further by proposing slag as a cheaper inventory for such regenerators . If widely deployed, slag-based TES could lower the cost of CSP storage (since slag is essentially free and single-tank) and allow higher operating temperatures than nitrate salts, enabling more efficient power blocks . Notably, slag and other particles can also serve as the solar absorber in so-called “particle receiver” towers – meaning the same material absorbs sunlight and then is stored in an insulated bin (this approach is being explored with sand, ceramics, etc., and slag could be a candidate).

Even in current salt-based CSP plants, slag can contribute via hybrid thermocline systems. A thermocline tank filled with slag pebbles and a smaller amount of molten salt can replace the two-tank design . The slag acts as filler and thermal flywheel, reducing the needed salt volume by ~55–75%. This was tested (Pilot 3b in REslag) and showed compatibility . The benefit is a more compact and cheaper storage for new CSP installations or as an retrofit to existing plants looking to expand capacity cheaply.

Beyond CSP, pumped thermal electricity storage (PTES) or “Carnot batteries” are emerging for grid storage. These typically use surplus electricity to heat a storage medium and later drive a turbine. Steel or slag can function in such systems: for example, a PTES design might use electric heaters to heat a bed of scrap steel or slag to ~600–800 °C, store it, then run a gas turbine or steam cycle. The Siemens Gamesa ETES pilot, though it used rocks, proved the concept of using a solid thermal store to time-shift electricity . Steel scrap or slag could be alternatives to rocks with potentially better controllability (e.g. induction heating of metal scrap is feasible for quicker charging). Research in China and elsewhere on Carnot batteries has noted that using solid media simplifies the system and that packed beds with cheap materials are attractive for medium-duration storage . As renewable penetration grows, we may see retrofitting retired coal plants with large electric heaters and slag/steel storage in the existing boiler drums or silos – effectively turning them into giant “thermal batteries” that feed steam turbines. This is an area actively being explored (e.g. by NREL and others calling them “geological batteries” when using solid material) .

Industrial Process Heat on Demand

Many industries require on-demand high-temperature heat (e.g. for drying, curing, smelting) and currently fire up gas burners or electric heaters as needed. A steel-based thermal store can act as a buffer to provide heat quickly and more efficiently. For instance, in metal casting or foundries, a steel heat reservoir could supply heat for keeping ladles warm or for providing start-up heat, reducing the need to keep burners idling. Electrified heat is often intermittent (drawing power when rates are low or when renewable surplus is available); a thermal store can take in energy during those periods and then deliver steady heat to the process when required. This improves the utilization of equipment and can stabilize operations.

The Joule Hive (firebrick) system backed by ArcelorMittal is targeting exactly this use: it plans to supply high-temperature air or gas to industrial processes like steel furnaces, using stored electrically-charged heat . Similarly, Rondo Energy in California is deploying brick-based heat batteries to provide kiln heat for cement and food processing using renewable electricity, replacing fossil fuels. Where bricks are used in those systems, steel could also be used if appropriate (some designs might prefer ceramic due to oxidation, but conceptually similar). These systems show that steel/slag TES can integrate with industrial heat needs: e.g., supplying 24/7 hot air at 800 °C for a chemical reactor, or providing heat to an absorption chiller in off-hours, etc.

Another important industrial use case is thermal leveling in batch processes: some processes produce heat in one phase and need heat in another (e.g. exothermic reactions followed by endothermic ones). A thermal store can capture the exothermic heat and feed it back to the endothermic step. Steel or slag’s ability to handle high temperatures and rapid cycling makes them suitable for such coupling.

District Heating and Residential Heating Storage

Thermal energy storage is increasingly seen as key for decarbonizing heating in buildings. While water tanks are common for low-temperature storage (up to ~90 °C), steel-based storages can store heat at much higher temperatures, which is useful for delivering compact and high-density heat or for combined heat-and-power support. One approach is to have a central thermal battery that charges with cheap electricity or surplus heat and then discharges to a district heating network. The Lumenion 2.4 MWh unit in Berlin is a prime example: it absorbs excess wind power at night to heat its steel core to ~650 °C, and during the day it releases heat to supply an entire neighborhood’s hot water and heating needs . This helps balance the grid and ensures the residential area is using renewable heat even when the wind isn’t blowing. In Northern Europe, where district heating is common, such high-temperature storages can be placed at central plants or substations, allowing cogeneration plants to operate flexibly. Because the steel storage can also generate electricity via a turbine, it can serve dual roles (heat and power), adding value for grid services.

For individual buildings or homes, steel-based storage can appear in the form of electric thermal storage heaters. Historically, off-peak electric heaters used high-density bricks (often magnesite or ceramic) to store heat overnight and release it during the day. One could envision using cast iron or steel blocks similarly – for instance, an electrically heated steel radiator that charges when power is cheap and emits heat gradually. Steel has higher thermal conductivity than brick, so it might release heat a bit faster; advanced designs could incorporate phase-change in metals (e.g. a ferrous alloy that partially melts at ~300 °C to increase storage density, although that is speculative). At least one company, Staccato Technologies, has investigated thermal batteries for homes using metal alloys (aluminum-based, which melt at low temp). While steel (with its high melting point) isn’t used for phase-change in homes, sensible heat steel storage in the form of thermal mass under floors or in dedicated modules could be employed, especially in off-grid houses that store PV excess as heat for space heating.

In summary, steel-based TES can support residential heating by shifting electrical load (charging with renewable or off-peak power, discharging as heat during peak demand). This reduces strain on the grid and increases utilization of renewables for heating. It is essentially a thermal battery concept but geared for building heat. Compared to water tanks, steel can store heat at higher temperature, which might be leveraged in advanced high-temperature heat pump systems or in compact units (higher temperature means more kWh in a smaller volume, though one must then step down the heat via a heat exchanger to usable temperatures).

Grid Stability and Hybrid Energy Systems

Because steel/slag TES can convert electricity to heat and vice versa (through a thermal power cycle), they can act as a grid-balancing asset. For instance, a region with high solar PV might have excess midday electricity – a steel heat storage can absorb that (via resistance heating or induction) and then in the evening either feed it to a turbine to regenerate electricity or supply a local industrial load (like a food processing plant’s ovens) that would otherwise draw grid power at peak time. In effect, steel TES provides a buffer that can shift gigawatt-hours of energy with relatively low loss. This use case overlaps with Carnot batteries and district heating: it’s about integrating storage in the energy system not just for one plant but for the wider network.

One interesting concept is retrofitting old coal-fired power plants: The boiler is replaced or supplemented with a giant electric heating system and a solid media storage (rocks, slag, or iron). The existing steam turbine and generator are then driven by steam from the stored heat. This turns the coal plant into a renewable energy storage plant, dramatically reducing emissions by using clean electricity as input. Pilot projects in Germany have eyed using rock bed heaters in retired coal plants – steel or slag could also be used and might allow higher temperature steam (thus making full use of high-pressure turbines). If steel scrap is locally abundant, it could be a prime medium for such conversions.

Use Case Matrix: To consolidate, steel-based storage finds use wherever there is a mismatch in timing between energy supply and demand in the form of heat. Industrial (waste heat recovery, process continuity), renewable generation (solar/wind shifting), and heating networks (balancing and peak shaving) are key areas. Another emerging niche is integrated storage for electric ovens/furnaces: e.g., an electric arc furnace could have an integrated thermal store to buffer the rapid fluctuations in power draw, thus helping the grid (the store charges during low power periods and discharges to assist during high power demand within each melt cycle – smoothing the load). This concept is being looked at to improve grid compatibility of heavy electric processes.

Advantages and Limitations of Steel-Based Thermal Storage

Steel- and slag-based thermal energy storage systems offer a range of advantages that make them attractive for certain applications, but they also come with some limitations. Below is a summary of key pros and cons:

Advantages

High Operating Temperature: Steel media and steel slag can operate at very high temperatures (800 °C or more), far exceeding the limits of water or standard molten salts . This enables efficient power generation and supply of high-grade process heat that other TES cannot support.
Low Cost and Abundant Material: Steel slag is a cheap, abundant waste byproduct – millions of tons are available with minimal cost beyond transport . Using slag or scrap steel capitalizes on industrial waste, aligning with circular economy goals. The material cost per kWh of storage is extremely low (slag ~$1 or less per kWh_th) , helping drive down overall storage system cost.
Single-Tank Simplicity: Steel/slag packed beds allow a single-tank thermocline design, avoiding the need for separate hot/cold reservoirs . This simplification reduces construction costs (one tank instead of two, fewer pipelines) and parasitic heat losses. It also means fewer components to maintain (no hot/cold salt pumps, etc., in air-based systems).
Material Stability and Low Degradation: Sintered steel slag has shown excellent thermal stability over many cycles, with negligible chemical or mechanical breakdown . It doesn’t appreciably corrode in air up to 1000 °C , and is compatible with common HTFs (air, salts, oils) . Steel components, if protected from oxidation, also have long lifetimes. This stability translates to long service life and low replacement costs.
High Thermal Conductivity (for Metal): If using steel metal as the medium (e.g. steel plates or rods), the high thermal conductivity aids in uniform heating and discharging. It means faster response and the ability to extract heat at high power without large temperature gradients within the medium.
Mechanical Strength: Solid steel or slag pebbles are structurally robust. Unlike powders or phase-change salts, they don’t require containment to hold their shape (steel blocks are self-supporting, slag pebbles form a stable packed bed). This strength means the storage media can also serve a structural role (e.g. supporting internal fixtures in the tank).
No Freezing Issue (for Solid Media): Steel/slag are always solid in normal operation – there’s no concern about freezing or solidifying in pipes, unlike water or molten salts that require heat tracing. This makes operation and maintenance simpler, especially in cold climates or intermittent operation scenarios.
Environmental Benefits: Utilizing steel slag for energy storage has twin environmental wins: it diverts waste from landfills and reduces the need for mined storage materials . Moreover, substituting stored heat for fossil fuel cuts CO₂ emissions. A study noted that slag-based CSP storage could significantly reduce CO₂ footprint compared to molten salt production . The materials themselves are non-toxic and pose minimal environmental risk in case of leakage (slag is basically rock; steel is benign metal).
Fast Charging via Direct Electrical Heating: Steel can be directly resistively heated or induction heated (since it’s conductive and ferromagnetic). This allows very rapid charging rates and efficient conversion of electricity to heat within the medium itself, minimizing heat exchange losses. It’s an advantage over non-conductive media that require an external heater or heat exchanger.
Integration with Existing Infrastructure: Solid media storage can often be retrofitted into existing structures – e.g., an empty silo can be filled with slag and turned into a heat storage; an old boiler can be repurposed to contain a steel thermal battery. Steel scrap or slag can also often use existing handling equipment (like conveyors, hoppers) for installation.
Fire Safety: Unlike oil-based TES or chemical batteries, steel/slag storage is essentially fireproof and inert. There’s no risk of explosion or combustion of the storage material. This can simplify safety cases and reduce insurance costs.

Limitations

Oxidation and Corrosion: A major challenge when using steel or other metals as storage media is oxidation at high temperatures. In air, steel will form scale (iron oxide) which over many cycles can degrade the material (make it flaky or reduce thermal conductivity). This can be mitigated by using an inert atmosphere or accepting a certain lifetime and overbuilding to compensate. Similarly, the steel containment of a slag bed must be protected (usually via internal insulation) from the high-temp oxidative environment to avoid corrosion and loss of strength .
Lower Specific Heat than Some Alternatives: Steel and slag have moderate specific heat capacities – for instance, slag ~0.8–0.9 J/g·K vs water 4.18 J/g·K or some salts ~1.5 J/g·K. This means for a given ΔT and mass, they store less heat. In practice the high density and allowable ΔT make up for it, but it still implies you need a fairly large mass of material, which can make the system heavy. For example, storing 1 MWh_th might require on the order of 50–100 tons of steel/slag (depending on ΔT). The weight and support structure must be accounted for in design.
Heat Extraction Complexity: Retrieving heat from a solid mass is not as straightforward as drawing from a hot fluid in a tank. You need good heat transfer between the solid and a working fluid. This often necessitates blowing large volumes of air (with blowers/fans) or installing heat exchanger tubing through the solid. Both approaches introduce complexity and potential inefficiencies (pressure drops in packed beds, or temperature gradients in a solid block). Ensuring uniform charging/discharging of a large solid storage can be tricky – e.g., avoiding cold/hot spots or ensuring the thermocline remains sharp requires careful flow distribution design .
Volume Expansion and Stress: Solids expand when heated. Steel expands about 1% in length going from room temp to 600 °C. In a constrained environment, this thermal expansion can induce significant stresses. Systems like concrete storage have to accommodate expansion joints; a steel plate module needs to avoid warping. Thermal stress can also occur within slag pebbles (though slag has decent thermal shock resistance, rapid changes could crack some pebbles). So the mechanical design must handle expansion/contraction cycles – either by allowing free expansion or by limiting temperature gradients. If not, components like the tank liner or the concrete matrix could crack.
Lower Heat Transfer Fluid Heat Capacity: Many steel/slag systems use air as the HTF, which has a low density and heat capacity. This means large airflow rates (and big ducts/fans) are needed to move a given amount of energy, compared to pumping a liquid. Air-based storage thus tends to have higher parasitic losses from pumping (though still small relative to total energy). Additionally, air at high temperatures can be corrosive (oxygen) and might require high-temperature fans or valves, which are specialty items.
Footprint for Low-Temperature Uses: For delivering low-temperature heat (e.g. 80 °C space heating), steel-based storage is probably overkill in temperature rating and might be less efficient volumetrically than water. Water is extremely effective for <100 °C storage, so steel only makes sense if you need the higher temperature or if water is impractical (weight/space constraints). In residential contexts, a water tank or phase-change material might store more heat in a smaller volume for daily cycling. Thus, steel thermal batteries are not universally the best choice for all thermal applications – they fill a niche toward the high-temp or high power end.
Initial Conditioning and Handling: Steel slag often needs processing (cooling, crushing, sieving, possibly sintering into stable pebbles) before use in TES. Different batches of slag can have varying composition, affecting melting point or heat capacity slightly . This inconsistency means a qualification step is needed (REslag, for example, characterized slag and sometimes sintered it into uniform pebbles to avoid dust and optimize size) . That adds a bit of upfront effort. Also, slag and steel are heavy materials, making transportation and on-site handling a logistical challenge (you need strong support pads, cranes to fill the tank, etc.). These are not insurmountable but do factor into project planning.
Thermocline Degradation and Mixing: In single-tank systems, repeated cycling can cause some mixing of hot/cold and thermocline degradation over time (especially if the flow rates aren’t well tuned or if the media size distribution changes due to attrition). This can reduce the effective usable heat (since a larger portion of the tank ends up at intermediate temperature that’s not fully hot or cold). Solutions exist (like periodically re-establishing the gradient or adding internal structures to maintain stratification), but it’s a complexity not present in two-tank systems where separation is physical.
Pressure Drop and Fan Power: Packed beds of fine solid material will resist flow. If the bed is large, the pressure drop can be substantial, requiring powerful fans or blowers (and thus parasitic power). For high-throughput systems, this could impact round-trip efficiency. Engineers mitigate this by optimizing pebble size and bed design (for example, using larger pebbles or segmented beds with multiple flow passes), but it remains a design limitation – you can’t pack slag too tightly or too deep without incurring high pressure losses . In contrast, pumping a liquid like molten salt can often be done with lower relative pressure drops in pipes.
Not a Drop-in Replacement (Integration Effort): While molten salt TES has a standardized design in CSP, steel/slag systems are newer and not as plug-and-play. Each project may need custom engineering (for the heat exchanger, for the blower sizing, for the control strategy). That learning curve and lack of off-the-shelf components could be seen as a disadvantage until the technology matures and standard designs/components are developed.

Despite these limitations, ongoing R&D is addressing many of them. For example, research into coatings for metal particles could reduce oxidation, and improved flow distributor designs solve a lot of the pressure drop and uniformity issues . The fact that multiple high-profile projects (ArcelorMittal’s slag pilot, Lumenion’s steel battery, etc.) have been successful suggests that the challenges are manageable with good engineering.

In conclusion, steel-based thermal storage systems offer a high-temperature, low-cost solution that is especially well-suited for industrial energy reuse and large-scale renewable energy storage. They leverage abundant materials (often waste) and proven thermodynamic principles (sensible heat in solids) to create a new class of thermal batteries. While they require careful design to address material and heat transfer considerations, their advantages — notably cost savings, high operating temperature, and durability — make them a promising component of the future sustainable energy infrastructure . The choice between steel/slag and other TES options will ultimately depend on the specific use case requirements (temperature, scale, budget, etc.), but steel-based TES clearly fill an important niche that complements existing technologies in the push for decarbonization and energy flexibility.

Sources:

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