Denmark's Molten Salt Battery: A Detailed Analysis of its Functionality and Potential

1. Introduction

The increasing global demand for sustainable energy solutions has placed significant emphasis on the development of efficient and reliable grid-scale energy storage technologies. The intermittent nature of renewable energy sources, such as solar and wind power, necessitates robust storage solutions to ensure a consistent and dependable energy supply. While existing energy storage technologies have made considerable progress, limitations in energy density, storage duration, cost-effectiveness, and safety continue to drive the search for innovative alternatives. In this context, Hyme Energy's molten hydroxide salt battery technology, developed in collaboration with Sulzer, represents a notable advancement, offering the potential for long-duration energy storage and a sustainable approach to industrial heat supply. This report provides a detailed analysis of the operational principles, key features, and potential impact of this groundbreaking technology, highlighting the synergy between Hyme Energy's innovation and Sulzer's expertise in fluid engineering.

The challenge of matching the supply of intermittent renewable energy with fluctuating demand underscores the critical need for effective energy storage. Technologies capable of storing large amounts of energy for extended periods are essential for grid stability and for enabling a greater penetration of renewables into the energy mix. Hyme Energy's approach directly addresses this challenge by offering a system capable of storing energy for over ten hours. Furthermore, the technology's ability to achieve efficiencies of up to 90% for industrial heat applications suggests a dual benefit, catering not only to electricity grid needs but also to the significant energy consumption of industrial processes. This dual-market potential could enhance the economic viability and overall impact of the technology.

2. Core Technology and Operational Principles

2.1. Charging Phase: Converting Renewable Electricity to Thermal Energy

The operational cycle of Hyme Energy's molten salt battery begins with the charging phase, during which surplus renewable electricity is converted into thermal energy and stored within the system. This process typically occurs during periods when renewable energy generation exceeds demand, such as peak sunlight hours for solar power or periods of high wind speeds for wind energy. Instead of complex mirror installations traditionally associated with concentrated solar power, Hyme Energy proposes utilizing this excess renewable electricity to heat molten salt. The conversion of electrical energy to thermal energy is achieved through the use of electrical resistance heaters immersed within the molten salt. As electricity flows through these heaters, it encounters resistance, causing the heaters to generate heat, which is then transferred to the surrounding molten salt, raising its temperature significantly. The system is designed to heat the molten hydroxide salt to a temperature of up to 600°C (1,112°F). The successful validation of this fundamental concept was demonstrated by the Molten Salts Storage (MOSS) project in Esbjerg, Denmark, a collaborative effort between Hyme Energy and Sulzer. This pilot facility served as a crucial step in proving the feasibility of using excess renewable energy to heat molten salt for later use.

The core principle of using excess grid electricity to heat molten salts draws a parallel with Concentrated Solar Power (CSP) technologies, where sunlight is focused to achieve high temperatures in molten salts. However, Hyme Energy's innovation lies in its ability to decouple the energy input from direct solar radiation, allowing the system to be powered by any source of renewable electricity, regardless of location or time of day. This broadens the applicability of the technology, making it potentially viable in regions with less intense sunlight or during nighttime hours when wind generation might be high.

2.2. The Role of Molten Hydroxide Salt: A Cost-Effective and Efficient Storage Medium

The heart of Hyme Energy's technology lies in its choice of energy storage medium: molten hydroxide salt, specifically sodium hydroxide (NaOH), commonly known as caustic soda or even drain cleaner. This particular salt offers several key advantages that contribute to the overall efficiency and economic viability of the system. One of the most significant benefits is its low cost, as sodium hydroxide is an industrial byproduct of chlorine production from seawater, making it a readily available and inexpensive resource. This contrasts with other battery technologies that rely on more expensive and potentially supply-constrained materials. Furthermore, hydroxide salts possess a high energy density, enabling the storage of a substantial amount of thermal energy within a relatively compact system.

Operating with molten salts at high temperatures presents engineering challenges, particularly concerning corrosion. Hyme Energy has addressed this through its proprietary hydroxide salt corrosion control technology. This innovation is crucial for ensuring the long-term operational stability and lifespan of the system, preventing degradation of the storage tanks and other components due to the corrosive nature of molten salts at 600°C. The company emphasizes that their unique family of hydroxide salts enables greater efficiency and cost savings compared to other salts used in the thermal energy storage market. These salts are not only sustainable and abundant, derived from seawater, but also offer high thermal conductivity, which boosts the efficiency of the heat transfer processes within the system.

The utilization of an industrial byproduct like sodium hydroxide for energy storage aligns with principles of a circular economy, potentially reducing waste and lowering the environmental footprint associated with raw material extraction. This approach offers a more sustainable alternative to technologies that depend on mined resources, contributing to a greener energy future. The development of proprietary corrosion control technology is a critical aspect of Hyme Energy's innovation, as it directly impacts the reliability and durability of the molten salt storage system over extended periods. This expertise likely represents a significant competitive advantage in the field of thermal energy storage.

2.3. Two-Tank Design for Long-Duration Energy Storage

Hyme Energy's molten salt battery employs a two-tank design, a configuration commonly used in concentrated solar power plants for thermal energy storage. This system consists of two insulated storage tanks: a cold tank, which stores the molten salt at a lower temperature, and a hot tank, which holds the salt heated to the higher operating temperature. During the charging phase, the cold salt is pumped from the cold tank through the electrical resistance heaters, where its temperature is raised to approximately 600°C, and then transferred to the hot tank for storage.

The two-tank design facilitates energy storage for extended durations, with the potential to retain thermal energy for up to two weeks with minimal heat loss. This long storage capability is a significant advantage compared to conventional grid-scale batteries, such as lithium-ion batteries, which typically provide power for around four hours. The ability to store energy for such prolonged periods makes Hyme Energy's technology well-suited for addressing the dispatchability challenges associated with renewable energy, allowing for the capture of energy during periods of high generation and its release when demand is high, even if these periods are separated by several days. The adoption of a two-tank system, a proven design in the CSP industry, suggests that Hyme Energy is leveraging established engineering principles to enhance the reliability and performance of its energy storage solution.

The capability of storing energy for up to two weeks positions this technology to play a crucial role in ensuring grid stability, particularly when faced with prolonged periods of low renewable energy generation due to weather patterns. This extended storage duration offers a significant advantage over shorter-duration storage solutions, enhancing energy security and the overall resilience of the power grid.

2.4. Discharging Phase: Generating Steam for Electricity and Industrial Heat

When energy is required, the discharging phase of the cycle commences. The hot molten salt, stored in the hot tank, is pumped through a steam generator, which acts as a heat exchanger. Inside the steam generator, the high-temperature molten salt transfers its thermal energy to water, causing the water to vaporize and produce high-temperature steam. This steam can then be utilized in two primary ways, depending on the specific application and energy needs. Firstly, the high-pressure steam can drive a conventional steam turbine that is connected to an electricity generator, thereby producing electricity that can be fed back into the grid. This process is similar to how electricity is generated in traditional thermal power plants, but with the heat source being stored renewable energy rather than fossil fuels or nuclear fission. Secondly, the high-temperature steam can be directly supplied to industrial processes that require significant amounts of heat, such as those found in food processing, chemical manufacturing, and other energy-intensive sectors. After the hot salt has passed through the steam generator and transferred its heat, the now cooled salt is pumped back to the cold tank, ready to be heated again in the next charging cycle. This closed-loop system ensures efficient utilization of the molten salt and minimizes material losses.

The ability of Hyme Energy's technology to produce either electricity or industrial heat provides a significant degree of flexibility, allowing the system to adapt to varying energy demands and market conditions. This dual functionality could enhance the economic viability of the technology by providing multiple revenue streams. Furthermore, the potential to integrate with existing steam-based industrial infrastructure could facilitate a smoother and more cost-effective transition for industries seeking to decarbonize their operations.

3. Efficiency and Performance

Hyme Energy reports impressive efficiency figures for its molten salt battery technology, particularly in applications involving heat. For industrial heat applications, the system is projected to achieve efficiencies of around 90%. This high efficiency is likely due to the direct utilization of the stored thermal energy for heating purposes, minimizing energy losses associated with conversion processes. In co-generation mode, where the system simultaneously produces both heat and power, the efficiency is reported to be between 80% and 90%. This indicates that a significant portion of the stored thermal energy can be effectively converted into usable energy outputs. However, for power generation alone, the expected efficiency is lower, around 40%. This is consistent with the fundamental thermodynamic limitations associated with converting thermal energy into mechanical energy and then into electricity. The efficiency of this conversion is governed by the Carnot cycle and is inherently less than 100%.

Several factors likely contribute to the high efficiency observed in heat and co-generation modes. The excellent heat transfer properties of molten salts allow for efficient transfer of thermal energy from the heaters to the salt during charging and from the salt to the water in the steam generator during discharging. Additionally, the direct use of thermal energy for industrial heating bypasses the energy losses associated with electricity generation and transmission. The 80-90% efficiency in co-generation suggests a well-optimized system that effectively captures and utilizes the thermal energy for multiple purposes. While the efficiency for electricity generation alone is lower, it is still a valuable output, especially considering that the primary input is often surplus renewable energy that might otherwise be curtailed.

The significant difference in efficiency between direct heat provision and electricity generation highlights the potential for this technology to be particularly impactful in decarbonizing industrial heat processes, which account for a substantial portion of global energy consumption and greenhouse gas emissions. The high efficiency in this domain makes Hyme Energy's solution an attractive alternative to traditional fossil fuel-based heating systems.

4. The Esbjerg Pilot Project: Validating the Technology

The Molten Salts Storage (MOSS) demonstrator plant in Esbjerg, Denmark, represents a crucial milestone in the development and commercialization of Hyme Energy's molten salt battery technology. This pilot project, a collaboration between Hyme Energy and the Swiss fluid engineering specialist Sulzer, was inaugurated in April 2024. The MOSS plant has a storage capacity of 1 MWh and a steam discharge capacity of 1.2 MW, with a charging capacity of 1.3 MW. A key objective of the pilot project was to successfully validate the concept of storing renewable energy in molten hydroxide salt at temperatures reaching up to 600°C. Notably, the project utilizes readily available sodium hydroxide, the same chemical found in drain cleaner, as its energy storage medium.

The MOSS project aims to demonstrate the scalability of Hyme Energy's storage solution using commercially available components and to validate the patented technology in a real-world operational setting. The steam produced by the pilot plant has a temperature of 480°C. The facility is located at the premises of Semco Maritime, an engineering and construction firm with expertise in the offshore energy sector, and involves a consortium of partners from Denmark and Europe. The project has received financial support from the Danish Energy Agency's Energy Technology Development and Demonstration Program (EUDP). The successful operation of the pilot plant is intended to demonstrate that the entire supply chain required for industrial-scale deployment is ready and functional.

The successful inauguration and operation of the Esbjerg pilot plant provide critical validation for Hyme Energy's technology, demonstrating its feasibility and reliability in a practical environment. This achievement significantly reduces the technological risk associated with scaling up the technology for commercial applications. The collaborative nature of the project, involving industry leaders like Sulzer and Semco Maritime, further strengthens its credibility and potential for successful market entry.

5. The Holstebro Plant for Arla Foods: Scaling Up for Industrial Decarbonization

Building on the success of the Esbjerg pilot project, Hyme Energy is now planning to construct a significantly larger 200 MWh industrial thermal energy storage system in Holstebro, Denmark, for the global dairy giant Arla Foods. This project is touted as the world's largest industrial thermal energy storage system. The planned facility will convert renewable electricity into heat, storing it in molten salt tanks at temperatures exceeding 500°C. The stored heat will then be used to power Arla's milk powder production processes at their plant in Holstebro, with the ambitious goal of achieving a 100% reduction in CO2 emissions from process heat. Arla Foods anticipates annual savings of over €3 million in process heat costs as a result of this project.

The project is currently seeking funding from the European Union to support its development and implementation. In addition to providing clean heat for Arla's operations, the plant is also expected to generate extra revenue by offering grid stabilization services to Energinet, the Danish electricity transmission system operator. This aligns with Arla Foods' broader commitment to reducing its CO2 footprint by 63% by 2030 compared to its 2015 baseline. The project is currently in the planning stages, with the aim of having the plant operational by 2029, contingent on securing the necessary funding. The successful implementation of this large-scale project has the potential to serve as a significant blueprint for industrial decarbonization in other energy-intensive sectors globally.

The sheer scale of the Holstebro plant underscores the potential of Hyme Energy's technology to address substantial industrial energy demands and make a significant contribution to decarbonization efforts. The partnership with Arla Foods, a major player in the food industry, provides a strong endorsement of the technology's commercial viability and its appeal to industries seeking sustainable and cost-effective solutions. The combination of significant cost savings and the potential for revenue generation further enhances the economic attractiveness of this technology for industrial users.

6. Comparison with Other Energy Storage Technologies

6.1. Molten Salt Batteries vs. Lithium-Ion Batteries: Advantages of Molten Salt

When comparing Hyme Energy's molten salt thermal storage with other prominent energy storage technologies, particularly lithium-ion batteries, several key advantages of the molten salt approach become apparent. Molten salt batteries generally offer a significantly longer cycle life, with some systems capable of enduring over 10,000 charge and discharge cycles, compared to the 500 to 3,000 cycles typically seen in lithium-ion batteries. Safety is another major advantage. Molten salt batteries utilize a non-flammable electrolyte, eliminating the risk of thermal runaway and fire hazards associated with lithium-ion batteries. Cost-effectiveness is also a potential benefit, as molten salt batteries can be manufactured using abundant and inexpensive materials like sodium hydroxide, whereas lithium-ion batteries rely on more costly and potentially scarce materials such as lithium, cobalt, and nickel. Additionally, molten salt batteries tend to perform better in extreme temperatures compared to lithium-ion batteries, which can experience performance degradation in very hot or cold conditions. Hyme Energy's technology specifically boasts a long storage duration capability, retaining energy for up to two weeks, which is considerably longer than the typical storage duration of grid-scale lithium-ion batteries, around 4 hours. Finally, the use of non-toxic and easily recyclable materials in molten salt batteries contributes to a lower environmental impact compared to lithium-ion batteries, which contain toxic materials and can be challenging to recycle.

These comparisons suggest that molten salt thermal storage is particularly well-suited for large-scale, stationary applications like grid energy storage and industrial heat provision, where long duration, high safety, and cost-effectiveness are paramount.

6.2. Molten Salt Batteries vs. Lithium-Ion Batteries: Advantages of Lithium-Ion

Despite the numerous advantages of molten salt batteries for specific applications, lithium-ion technology still holds certain advantages, particularly in areas where energy density and charging speed are critical. Lithium-ion batteries typically have a higher energy density, ranging from 150 to 250 Wh/kg, compared to the 90 to 150 Wh/kg of molten salt batteries. This higher energy density makes lithium-ion batteries more suitable for applications with space and weight limitations, such as electric vehicles and portable electronic devices. Another key advantage of lithium-ion batteries is their faster charging speed, typically ranging from 30 minutes to 2 hours, whereas molten salt batteries generally require several hours to charge. Furthermore, lithium-ion battery technology is more mature and has a more established market presence, with widespread availability and a well-developed supply chain.

While Hyme Energy's molten salt technology presents a compelling solution for grid-scale and industrial heat storage, lithium-ion batteries will likely remain the preferred choice for mobile applications and situations requiring rapid charging due to their inherent characteristics. This highlights that the optimal energy storage technology depends heavily on the specific requirements and constraints of the application.

Table 1: Comparison of Molten Salt and Lithium-Ion Battery Technologies

| Feature | Molten Salt Batteries (Hyme Energy) | Lithium-Ion Batteries |

|---|---|---|

| Energy Density (Wh/kg) | 90-150 | 150-250 |

| Cycle Life | 10,000+ | 500-3,000 |

| Charging Speed | Several hours | 30 min - 2 hours |

| Safety | High (Non-flammable) | Lower (Risk of thermal runaway) |

| Operating Temp. | ~600°C | Ambient |

| Raw Material Costs | Low | High |

| Environmental Impact | Lower | Higher |

| Best Use Cases | Grid storage, Industrial heat | EVs, Portable electronics |

7. Conclusion and Future Outlook

Hyme Energy's molten hydroxide salt battery technology represents a significant advancement in the field of energy storage, offering a compelling solution for long-duration energy storage and the decarbonization of industrial heat. Its key advantages include the potential for extended storage durations of up to two weeks, high efficiency in direct heat applications and co-generation, the use of cost-effective and abundant materials like sodium hydroxide, and enhanced safety due to the non-flammable nature of the molten salt electrolyte. The successful validation of the technology through the Esbjerg pilot project and the ambitious scale of the planned Holstebro plant for Arla Foods underscore its potential to make a substantial impact on the energy landscape.

This technology holds significant implications for improving grid stability by enabling greater integration of intermittent renewable energy sources. Its ability to provide high-temperature steam for industrial processes offers a pathway for energy-intensive industries to reduce their reliance on fossil fuels and decrease their carbon footprint. The economic benefits, including potential cost savings and revenue generation from grid services, further enhance the attractiveness of Hyme Energy's solution. Looking ahead, the continued development and deployment of molten salt thermal energy storage technologies like this could play a crucial role in the global transition towards a more sustainable and resilient energy future. Further innovation and investment in this area could lead to even more efficient, cost-effective, and widely applicable thermal energy storage systems.