One of the most promising approaches for addressing the growing concerns related to the world's current fossil fuel energy system is the transition to a sustainable system based on the use of electricity and hydrogen as the primary energy carriers. Hydrogen is plentiful, sustainable, energy efficient and extremely friendly to the environment. When hydrogen is consumed in a fuel cell engine, the only exhaust is pure water. The widespread use of hydrogen as an energy carrier has often been described as the “hydrogen economy.”

However, hydrogen is a very reactive element, and it does not occur in significant quantities on earth in a pure state. Most hydrogen is combined with other elements and is found in water, fossil fuels, biomass and similar compounds. A hydrogen economy requires primary energy sources to provide the energy to release the hydrogen from these compounds. Water-splitting processes are particularly attractive, since water is abundant around the planet, and a variety of primary energy sources can be used to produce the hydrogen. Solar energy can be used to produce hydrogen by conventional water electrolysis, which first requires the production of electricity. A more efficient and potentially more cost effective approach is to use solar thermal energy to power a thermochemical process, which breaks the hydrogen-oxygen bounds through a series of heat-driven chemical reactions.

This paper will discuss research on the Hybrid Sulfur (HyS) thermochemical water splitting process. The HyS process is one of the most advanced and promising cycles for using thermal energy to conduct water-splitting. Development started in the 1970's, and extensive research and development has been restarted in the last few years. The Savannah River National Laboratory (SRNL) recently completed the conceptual design and economic analysis for a HyS process matched with a solar central receiver plant and sized for a yearly average H2 production of 100 tons/day.

The HyS plant was designed based on results obtained from a new chemical flowsheet created with an ASPEN Plus simulation. A first preliminary conceptual design of the interface system between the solar receiver plant and the HyS process is also proposed and discussed.

Capital costs for all major plant components were estimated, and the specific H2 production cost was assessed adopting the Department of Energy's H2A financing guidelines. Influence of some parameters and assumptions on the overall cost has also been investigated and discussed.

This technology could be demonstrated at an existing solar test facility, such as the Central Receiver Test Facility at Sandia National Laboratory, in the next few years. Commercialization could occur beginning around 2015. The work has been carried out as part of the DOE-EE Solar Thermochemical Hydrogen (STCH) project.