Powering the Hydrogen Economy from Waste Heat: A Review of Heat-to-Hydrogen Concepts.

Ever-increasing energy demands and environmental concerns require new and clean energy supplies, many of which are intermittent and do not correlate with demand. To balance supply with demand, a universal energy vector should be employed such that intermittent renewable energy can be stored and transported and then used when needed. Hydrogen is the perfect universal energy vector and a possible solution that ensures environmental cleanliness, maximum utilization of renewable energy sources, and high efficiency, whereby the combustion of the fuel yields only water. One abundant and freely available energy source-both anthropogenic and natural-is heat. Heat can be obtained from industrial processes and is indeed often viewed as a waste product with a premium to remove but is notoriously difficult to capture, store, and transport. Capturing and storing low-grade heat therefore provides a significant opportunity and can be achieved by coupling thermoelectric generators and water electrolyzers. A thermoelectric generator is placed within a thermal energy gradient and produces a flow of current that is fed to the electrolysis unit with which it produces hydrogen and oxygen as the final products. The hydrogen can be stored for long periods and transported for "on-demand" use in fuel cells for electricity from hydrogen burners for a return to thermal energy. This Review summarizes the current state-of-the-art research into implementing thermoelectric generators and utilizing heat as a primary energy source to produce hydrogen, which could replace the need for extra electric power to run hydrogen production units. Furthermore, suitable requirements, modifications, and other related aspects associated with such a new and novel method of hydrogen generation are discussed. Hydrogen produced from otherwise-wasted energy sources can be considered to be green.