Energy and exergy analyses of electrolytic hydrogen production

Abstract

The thermodynamic performance is investigated of a water-electrolysis process for producing hydrogen, based on current-technology equipment. Both energy and exergy analyses are used. Three cases are considered in which the principal driving energy inputs are (i) electricity, (ii) the high-temperature heat used to generate the electricity, and (iii) the heat source used to produce the high-temperature heat. The nature of the heat source (e.g. fossil fuel, nuclear fuel, solar energy, etc.) is left as general as possible. The analyses indicate that, when the main driving input is the hypothetical heat source, the principal thermodynamic losses are associated with water splitting, electricity generation and heat production; the losses are mainly due to the irreversibilities associated with converting a heat source to heat, and heat transfer across large temperature differences. The losses associated with the waste heat in used cooling water, because of its low quality, are not as significant as energy analysis indicates.