Renewable hydrogen from the Zn/ZnO solar thermochemical cycle: A cost and policy analysis

Flexible energy carriers are a crucial element of our energy portfolio. In a future in which a significant fraction of our energy comes from renewable sources, renewably produced fuels will be vital. The zinc/zinc-oxide thermochemical redox cycle is one approach for producing hydrogen using solar energy. This paper explores the level of carbon taxation necessary to make the cycle competitive with hydrogen production via methane reforming. In addition, the time frame for economic viability is assessed through the use of experience curves under minimal input, mid-range, and aggressive incentive policy scenarios. Prior work projects that hydrogen produced by the zinc/zinc-oxide cycle will cost between $5.02 and $14.75/kg, compared to $2.40 to $3.60/kg for steam methane reforming. Overcoming this cost difference would require a carbon tax of $119 to $987/tCO₂, which is significantly higher than is likely to be implemented in most countries. For the technology to become cost competitive, incentive policies that lead to early implementation of solar hydrogen plants will be necessary to allow the experience effect to draw down the price. Under such policies, a learning curve analysis suggests that hydrogen produced via the Zn/ZnO cycle could become economically viable between 2032 and 2069, depending on how aggressively the policies encourage the emerging technology. Thus, the Zn/ZnO cycle has the potential to be economically viable by mid-century if incentive policies - such as direct financial support, purchase guarantees, low interest rate loans, and tax breaks - are used to support initial projects.