One ton of CO2 is generated by making one ton of cement, which is made from limestone and a few other things heated to an extremely high temperature. But what if concrete could generate its own energy? It seems we’re getting very close to that reality after the world’s largest cement maker debuted photovoltaic concrete panels that can be used for construction while simultaneously producing energy. The company says that a typical 10-story commercial building covered in 60 percent of these panels would generate about 30 percent of its annual energy requirement.