Key Takeaways & Executive Findings
- •• Ferro-alloys, particularly Fe–Si–Ti compositions, achieve energy storage capacities up to 1.5 MWh·m−3, exceeding the 1 MWh·m−3 target for high-temperature LHTES. • The Fe–Si–B system shows high latent heat due to boron, but its high melting point and cost limit practical application. • Fe–Si–Cr alloys like Fe–34Si–38Cr and Fe–34Si–43Cr offer excellent energy storage density and favorable phase transition temperatures. • Thermodynamic assessment using FactSage identifies promising ferro-alloy PCMs for operation above 1000°C, outperforming some state-of-the-art metallic PCMs.
Abstract
Latent heat thermal energy storage (LHTES) is an attractive method for enhancing the functionality and availability of renewable energy sources, and it is extensively used to support concentrated solar power technologies. The main feature of every LHTES system is a phase change material (PCM), i.e., a substance used to absorb/release energy upon cyclic melting/solidification. This study investigates the potential of ferro-alloys as high-performance PCM candidates, targeting energy storage capacities exceeding 1 MWh·m−3, and operational temperatures above 1000°C. A thermodynamic assessment of binary and ternary Fe-based systems, alloyed with Si, B, Cr, V, and Ti, was conducted to identify compositions with optimal phase transition characteristics and heat storage potential. The results highlight the significant potential of the Fe–Si–B system, where boron’s exceptionally high latent heat enhances energy storage capacity despite challenges posed by its high melting point and cost. The Fe–Si–Cr system revealed promising alloys, such as Fe–34Si–38Cr and Fe–34Si–43Cr, offering excellent energy storage density and favorable phase transition temperatures. In the Fe–Si–V system, vanadium additions produced alloys like Fe–36Si–14V and Fe–34Si–10V, which meet energy storage criteria, although the high melting points of some Si–V phases may restrict their practical applicability. The Fe–Si–Ti system showed standout compositions, including Fe–38Si–20Ti and Si–48Ti, achieving energy storage capacities of approximately 1.5 MWh·m−3. This study compares ferro-alloy PCMs against state-of-the-art metallic PCMs, highlighting the performance of certain ferro-alloys.
1. Introduction
Latent heat thermal energy storage (LHTES) systems have emerged as a promising solution to address the challenges posed by the intermittency of renewable energy sources, such as solar and wind. These sources are abundant and virtually infinite, but by their very own nature, they alternate between periods of availability and scarcity. An example is the predictable, yet unavoidable, day and night cycle, which limits solar energy availability during nighttime. The primary role of LHTES systems is to store excess energy during peak production periods and release it during periods of high demand or low renewable energy generation. By effectively balancing the fluctuating nature of these sources, LHTESs enhance the stability and reliability of energy grids [1–2].
At the heart of LHTESs are phase change materials (PCMs), the substances capable of absorbing and releasing large amounts of energy during phase transition. Most commonly, PCMs transition between solid and liquid states at specific temperatures, storing or releasing latent heat during this process. Another critical component of LHTES systems is the generator used for thermal-to-electric energy conversion. Its design must be closely integrated with the PCM to ensure compatibility and optimal performance. One key parameter is the generator’s working temperature. In this study, we envision a thermophotovoltaic generator that can benefit from high temperatures above 1000°C [3]. Higher working temperatures significantly enhance generator performance, but they introduce technical challenges related to containment and cost. While materials like pure boron or silicon could theoretically achieve this, boron’s prohibitive cost and silicon’s tendency to expand during solidification pose significant challenges [4].
LHTES systems offer several advantages over conventional energy storage technologies. Firstly, they boast higher energy storage densities, allowing more energy to be stored within a given volume of material. Moreover, they exhibit excellent thermal stability and can undergo numerous charge-discharge cycles without significant degradation, ensuring long-term reliability [5]. In recent years, there has been a growing interest in advancing LHTES technology [6] to address the evolving needs of the energy sector. Research efforts have focused on developing novel PCM formulations with enhanced thermal properties [3–4,7–9], optimizing system designs for improved efficiency, and exploring innovative applications in renewable energy integration, grid stabilization, and waste heat recovery.
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Paolo Lai Zhong Lo Biundo, Wojciech Polkowski, Jianmeng Jiao, Maria Wallin, Merete Tangstad (2025). Ferro-alloys as high temperature phase change materials. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3187-6
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Frequently Asked Questions
What are ferro-alloys and why are they considered for high-temperature phase change materials?
Ferro-alloys are iron-based alloys with added elements like silicon, boron, chromium, vanadium, or titanium. They are considered for high-temperature PCMs because they can offer high energy storage densities and operate above 1000°C, making them suitable for advanced LHTES systems such as those coupled with thermophotovoltaic generators.
Which ferro-alloy systems were investigated in this study?
The study investigated binary and ternary Fe-based systems alloyed with Si, B, Cr, V, and Ti, specifically focusing on Fe–Si–B, Fe–Si–Cr, Fe–Si–V, and Fe–Si–Ti systems.
What were the key findings regarding energy storage capacity?
The Fe–Si–Ti system showed standout compositions like Fe–38Si–20Ti and Si–48Ti, achieving energy storage capacities of approximately 1.5 MWh·m−3, exceeding the target of 1 MWh·m−3. Other systems also showed promise but with limitations such as high melting points or cost.
What are the main challenges for using boron in ferro-alloy PCMs?
Boron has exceptionally high latent heat, which enhances energy storage capacity, but its high melting point and prohibitive cost pose significant challenges for practical application.
How does this study compare ferro-alloy PCMs to state-of-the-art metallic PCMs?
The study compares the performance of ferro-alloy PCMs against state-of-the-art metallic PCMs, highlighting that certain ferro-alloys, particularly in the Fe–Si–Ti system, offer superior energy storage densities and favorable phase transition temperatures, making them competitive alternatives for high-temperature LHTES applications.
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