The last time you’ve thought about clay was probably, well, never. Its existence eludes our thoughts despite being so prevalent in the composition of many objects in our everyday lives, from the ceramic cups we drink from at home, to the decorative vases on our shelves and the tiles on our roofs — all of which contain clay as the key ingredient. But what if clay ceramics could be used for much more than what you initially perceived they could do? What if they could also store thermal energy for later extraction and use? For a material with such a seemingly straightforward purpose, it’s difficult to imagine that it would serve any other function aside from holding your morning cup of coffee or a couple of flowers. Dr. Claire White, Assistant Professor of Civil and Environmental Engineering at the Andlinger Center for Energy and the Environment, however, would tell you otherwise.
Energy storage, which refers to any method that can absorb energy and store it for use at another time, is essential in a world so dependent on electricity, fossil fuels, heat, and many other forms of energy. Multiple types of energy storage exist: electrochemical energy storage such as rechargeable batteries, hydroelectric energy storage via dams, and thermal energy storage, which commonly relies on materials of high specific heat capacity and durability to capture and release heat. Molten salts, which are managed at very high temperatures, and concrete are popular materials for this purpose. Salts tend to have very high specific heat capacities, allowing for more heat energy to be stored per degree raised during the heating process, and concrete has a decent specific heat capacity while also being quite inexpensive. However, the former generally require complicated systems to manage, and the latter is typically suitable only for energy storage under 500 degrees Celsius, since it begins to deteriorate at temperatures any higher.
In a search to utilize thermal energy storage while ameliorating these problems, White, in collaboration with visiting professor Dr. Pierre-Marie Nigay and former Professor of Mechanical and Aerospace Engineering Winston Wole Soboyejo, turned to clay ceramic materials, which can withstand temperatures up to 1000 degrees Celsius and are therefore capable of high-temperature thermal energy storage. Furthermore, ceramic can be manufactured in the form of blocks, making the stored thermal energy easy to transport. Despite resolving the issues presented by molten salts and concrete in thermal energy storage, ceramic materials have their own drawback: they have low specific heat capacities compared to those of concrete and molten salts. Thus, ceramics store less heat energy per degree and increase in temperature at a faster rate.
In a search to utilize thermal energy storage, White, Nigay, and Soboyejo turned to clay ceramic materials.
To address this flaw which made ceramics inefficient for thermal energy storage, the research team decided to try mixing clay with organic additives to cause an overall increase in the specific heat capacity of the new material. The additive used was biochar, which is a product of subjecting biomass, obtained from grinding hard wood, to pyrolysis, a decomposition process involving high temperatures and the absence of oxygen. The biochar was then mixed into the clay and baked at 950 degrees Celsius. By doing this, White, Nigay, and Soboyejo hoped to increase the specific heat capacity. The biochar functioned as impurities that, at a tiny average size of approximately 20 micrometers per particle, did not compromise the strength of the clay ceramic. This process is analogous to the properties of concrete, which becomes stronger with smaller impurities and weaker with larger ones.
“The biochar can be thought of more as a filler material, increasing the specific heat capacity without adversely changing the chemical or mechanical properties of the fired clay,” White explained. “It’s the size of the filler particles that controls the strength of the composite.”
“The biochar can be thought of more as a filler material, increasing the specific heat capacity without adversely changing the chemical or mechanical properties of the fired clay,” White explained.
The resulting specific heat capacity of the biochar-infused ceramic was nothing short of astonishing. Not only was the measured specific heat of the ceramic with 15 percent by weight added biochar approximately 55% higher than the average specific heat capacities of both ceramic without biochar and concrete, but the predicted specific heat of ceramic with 30 percent by weight added biochar — according to an extrapolation of a model created with data obtained from the research — should be approximately 71% higher, almost matching the specific heat of most molten salts. Hence, biochar-infused ceramics can potentially function just as well — or better — than molten salts and concrete can for thermal energy storage, but with the added advantage of being easily moldable into bricks for effortless transport, as well as being able to withstand high temperatures.
Our society will only become more dependent on reliable energy sources as time progresses. While discovering new methods of clean, sustainable energy production is essential to helping both our and future generations thrive, it is equally important to search for energy storage techniques that can be employed to conserve energy. As clay ceramics are prominent in more ways than we may realize. They are the perfect candidate for such a role when modified with organic additives like biochar, potentially bringing us closer to a society that is more environmentally-aware and oriented around sustainable energy. The next time you reach for your coffee mug, remember that it’s made of the same material that may eventually be used to store thermal energy and power our lives.