FIGURE G-6 The energy needs for hydrogen production by the gas turbine modular helium reactor (He cycle) high-temperature electrolysis of steam (HTES) and the supercritical CO2 (S-CO2 cycle) advanced gas-cooled reactor HTES technologies. NOTE: HHV = higher heating value.

sulfur cycles are depicted schematically in Figure G-7.) The most promising form consists of the following three chemical reactions, which yield the dissociation of water (Brown et al., 2003):

A hybrid sulfur-based process does not require iodine and has the same high-temperature step as sulfur iodine but a single electrochemical low-temperature step that forms sulfuric acid. That electrolysis step makes sulfuric acid at very low voltage (power). The low-voltage electrolysis step (low power compared with electrolysis of water) may allow much larger scale-up of the electrochemical cells. (High-voltage systems have high internal heat generation rates that often limit the scale-up of a single cell.) The efficiency of this process is about the same as that of the SI process, but is influenced by the efficiency of the electrical power cycle. It is one of only four processes for which a fully integrated process has been demonstrated in a hood. It is the only process for which a full conceptual design report for a full-scale facility has been developed. Lastly, like the SI process, it has the potential for major improvements.

The SI cycle requires high operating temperatures but offers the opportunity for high-efficiency conversion of heat to hydrogen energy, ηH, as shown in Figure G-8. The SI cycle can be coupled to the modular high-temperature reactor (MHR) (a version of the HTGR) (LaBar, 2002). This reactor consists of 600 megawatt-thermal (MWth) modules, which are cooled by helium gas, with high coolant exit temperatures that can provide the necessary heat to the SI reactions. The coupling of the MHR and SI cycle, MHR-SI, provides a large-scale, centralized production of hydrogen.

The MHR-SI is a capital-intensive technology. Future cost reduction can be achieved from high efficiency by devising materials that can withstand higher temperatures. Reactor materials that are temperature-, irradiation- and corrosion-resistant would be needed. Also, possible reduc-



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