Transitioning to 100% renewable energy globally would be cheaper and simpler using firebricks, a form of thermal energy storage with roots in the Bronze Age, to produce most of the heat needed for industrial processes, according to a new Stanford study.

Today’s industries require high temperatures for manufacturing, which are achieved largely by continuously burning coal, oil, fossil gas, or biomass. With much of the world focused on reducing emissions by transitioning away from fossil fuels to renewable sources like wind, solar, and hydro, the question is how to provide industries with on-demand continuous heat in a 100% renewable world.

In a recently published study, researchers from the Department of Civil and Environmental Engineering at Stanford University proposed that an ancient solution, firebricks, could be the answer.

“By storing energy in the form closest to its end use, you reduce inefficiencies in energy conversion,” said Daniel Sambor, a postdoctoral scholar in civil and environmental engineering and a study co-author. “It’s often said in our field that ‘if you want hot showers, store hot water, and if you want cold drink, store ice’; so, this study can be summarized as ‘if you need heat for industry, store it in firebricks.'”

As energy from wind and solar fluctuates, it’s important that sources replacing combustion fuels are capable of electricity or heat storage. Refractory bricks, which can withstand high temperatures without damage, have been used for thousands of years – likely since the early Bronze Age – to line furnaces, kilns, fireplaces, and ovens.

Similar to refractory bricks, firebricks can store heat or insulate, depending on what they’re made from. Firebricks used for heat storage should have a high specific heat – the amount of heat 1 g of a substance must absorb or lose to change its temperature by 1 °C (1.8 °F) – and a high melting point. Ideal low-cost firebrick materials with these properties include alumina and magnesia or low-grade graphite. Insulating firebricks must withstand high temperatures but have low thermal conductivity to resist heat flow and obtain heat slowly from their surroundings. Silica has a low thermal conductivity, so is regularly used in these types of firebricks.