Can steelmaking waste help solar power store energy?

UNIVERSITY PARK, Pa. — Storing energy as heat is a proven pathway to address the challenge solar power faces when it’s dark or cloudy outside by enabling the capture, storage and on-demand release of thermal energy. The process, called high-temperature thermal energy storage (TES), however, can be prohibitively expensive. Researchers at Penn State have found that steel slags — a rock-like waste byproduct of the steelmaking process — may potentially serve as a low-cost, heat storage candidate for TES applications.

They published their first-phase findings of a three-phase project today (Sept. 15) in Solar Energy. The first phase focused on identifying the morphological structure of three types of slags to evaluate their suitability for thermal energy storage. The eventual goal of the research, the team said, is to reduce steel slag waste by evaluating its potential for reuse as a TES material.

“One of the problems with many renewable energy sources, like solar, is that they are intermittent,” said Olumide Ogunmodimu, assistant professor of energy and mineral engineering at Penn State and co-author on the paper. “Solar energy is inherently intermittent and variable because sunlight is not continuously available due to the day-night cycle and changing weather conditions. This means we need to find some way to store the energy captured from the sun for later use.”

Similar to how batteries store energy, TES systems can store thermal energy for extended periods of time and release it when needed. The researchers tested the properties of three different steel slags — which comprise impurities removed from metal during steelmaking — in phase one of the project to evaluate their preliminary suitability as a candidate material for high-temperature TES applications.

“Despite clear advances, many engineered TES systems still face challenges related to material cost, corrosion and long-term degradation under repeated thermal cycling,” said Michael Enemuo, graduate student in mining and mineral processing engineering in the John and Willie Leone Family Department of Energy and Mineral Engineering and lead author of the paper. “These limitations have motivated increasing interest in low-cost, sensible heat storage media, particularly materials derived from industrial by-products.”

Enemuo recognized the challenges with TES systems and connected them with his knowledge of slags.

“For me, it started with the waste generated by the steel industry,” Enemuo said. “There are piles and piles of slags stored near the steel factories, which can be toxic to the environment, so finding a way to repurpose them was a big motivation.”

Using waste products for energy storage also contributes to a circular economy and more sustainable energy production, according to the researchers.

“Right now, it still requires fossil fuels and raw materials to create renewable energy which leaves us in a carbon debt, so we need to look both at the technology and where it comes from,” said Ogunmodimu, who is also an affiliate professor with the Alliance for Education, Science, Engineering, and Design with Africa (AESEDA). “If we repurpose waste products into raw materials, the carbon footprint can drop significantly and so will the cost of energy production. We are looking at the relationship between the raw material and what you do with it, and the motivation is really getting the raw material in a sustainable manner and at a cheaper cost.”

Slags are produced through distinct processes during steelmaking that lead to different chemical structures. The three slags the researchers tested in phase one were ladle metallurgy furnace, which is rich in magnesium oxides and calcium aluminates; basic oxygen furnace, which has high levels of calcium oxide and the presence of free lime; and electric arc furnace (EAF), which typically contains higher oxide fractions and a more stable crystalline framework.

The researchers employed a multi-technique analytical framework of tests on the slags, painting a picture of the material’s properties and how the slags might function in TES applications. They specifically looked at the thermal stability, morphological structure and assemblage and degradation of the slags.

“The different tests in combination help us understand the structure of the material, potential areas of intervention and how to classify the materials so we know how to treat them in the future,” Ogunmodimu said.

Researchers found the EAF slag was the most promising candidate for further evaluation due to its stability.

“EAF slag offers the most favorable baseline among the tested based on its low mass loss, minimal detectable free lime, comparatively stable heat-flow response and favorable phase assemblage,” Enemuo said.

Phase assemblage refers to how phases of a material — solid, liquid or gas — co-exist for optimal performance. In this case, EAF slag was solid enough to maintain a stable structure and is less prone to hydration-induced expansion.

“Based on the indirect physicochemical and thermal stability indicators evaluated in this work, the preliminary TES suitability ranking in order is electric arc furnace slag, then ladle metallurgy furnace slag followed by basic oxygen furnace slag,” Ogunmodimu said. “However, direct measurements of thermal conductivity, specific heat capacity, density and long-term cyclic stability are required before definitive conclusions can be drawn regarding practical TES performance.”

Looking ahead, the researchers said they must validate these findings and run simulations on other properties that are hard to analyze experimentally before they can begin modifying the materials for production.

The other co-author on the paper was Arash Dahi Taleghani, former Penn State professor of petroleum engineering and now at the University of Texas at Austin.

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