Industrial waste heat
Coal, natural gas, and oil are essential for the high temperatures required in the thermal processing of metals, cement, fertilizer, glass, processed foods, and so on. About 420 trillion Btus of this is wasted, and some of it could be captured without new land or water resources required. This resource is much larger than all renewable energy resources combined (Kreith).
Thermal processing is a daunting and gigantic task for renewables to perform that’s often forgotten, and essential for their being able to reproduce themselves, but an additional burden on top of their need to also build and maintain all of our infrastructure, transportation, electricity, and so on mainly with electrical power generation.
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Kreith, F, et al. 2013. Principles of sustainable energy systems. CRC Press.