[Image above] Firebricks can not only provide thermal insulation but also store heat energy for high-temperature materials processing operations. Credit: yasin hemmati, Unsplash
Since at least the early Bronze Age (4,000 to 3,000 BCE), humans have used refractory bricks to line the inside of kilns and furnaces. These ceramics can withstand high temperatures without damage to their structure, and so they can provide thermal insulation during heat-intensive materials processing operations.
There are different types of refractory bricks, and each type can provide additional benefits besides thermal insulation. For example, firebricks can be used for heat storage as well as insulation depending on their specific composition. This ability is starting to attract attention in the renewable energy community, as explained in a recent open-access paper by Stanford University researchers.
About 17% of global emissions are attributed to burning fossil fuels to power low-temperature (<100°C for paper manufacturing) and high-temperature (1,800°C for Portland cement) industrial processes, according to the researchers. Some of these processes could potentially be powered by renewable electricity instead. But the variability of wind and solar resources would necessitate the construction of large-scale energy storage systems to ensure consistent supply.
Currently, renewable energy is often stored as electricity in batteries. But storing renewable energy as heat could be more cost effective from both a capital cost and lifetime perspective.
To store renewable energy as heat, electricity can be converted to heat using either electric resistance heaters connected to a heat storage material, such as soapstone, or through direct resistance heating of the storage material itself. The storage material is carefully insulated to keep the heat from escaping, and then channels of fluid or air are used to transfer the thermal energy so it can be used either as heat or converted back to electricity.
Firebricks have been used as heat storage materials in regenerators for glass and steel manufacturing. Regenerators obtain heat from a high-temperature flue gas, store the heat for about half an hour, and then use the heat to preheat air for combustion.
In the open-access paper, the Stanford researchers examined the potential impact of widespread use of firebrick thermal energy storage systems on global energy costs. They investigated this impact in a hypothetical future where each country has transitioned to wind, geothermal, hydropower, and solar for all energy purposes.
To investigate this impact, they ran simulations of matching electricity and heat demand with supply, storage, and demand response in 149 countries across 29 world regions. Three types of models were used:
- A spreadsheet model to estimate energy demand in 2050 from a business-as-usual and renewable energy (wind, water, and solar) perspective.
- A global weather–climate–air pollution model, which takes results from the spreadsheet model to predict demands for different types of energy supply worldwide.
- A model that uses data from the second model to match demand with supply, storage, and demand response every 30 seconds for three years.
The simulations showed that, relative to a base scenario with no firebrick thermal energy storage systems, the use of these systems will result in a 14.5% reduction (32.2 TWh vs. 32.7 TWh) in battery capacity by 2050. Additionally, reductions in other areas will be seen, specifically
- Annual hydrogen production for grid electricity by 31%
- Underground low-temperature heat storage capacity by 27.3%
- Onshore wind capacity by 1.2%
- Land needs by 0.4% (2,700 km2)
- Overall annual energy cost by 1.8%
Furthermore, the cost per kilowatt-hour of electricity for a firebrick storage system was estimated to be less than one-tenth that of a battery storage system. Using firebricks reduced the capital cost of transiting to renewable energy by $1.27 trillion, or 2.2%. The levelized cost of energy and the annual energy cost were also reduced slightly. The energy cost payback time decreased by 3.2%.
Overall, the all-storage maximum discharge rate was increased, though there was a decrease in the all-storage maximum capacity. Although the daily loss rate of heat may be an issue, the researchers cite the work of energy equipment and solutions provider Rondo Energy to demonstrate that the impact on energy cost will likely be minimal. For processes not applicable to firebricks, other electric heating technologies could be used.
The researchers conclude that firebricks used as heat storage could be “a large-scale solution to addressing industrial process heat emissions” and “a remarkable tool in reducing the cost of transitioning the world to clean, renewable energy.”
However, they caution that incentives and policies are required to transition to firebricks in time to address the climate and energy security problems. They previously estimated that an 80% transition of all energy by 2030 (100% by 2035) is needed to avoid sustained 1.5°C global warming.
The open-access paper, published in PNAS Nexus, is “Effects of firebricks for industrial process heat on the cost of matching all-sector energy demand with 100% wind-water-solar supply in 149 countries” (DOI: 10.1093/pnasnexus/pgae274).
