FIGURE G-5 Power cycle net efficiency (ηel) and thermal-to-hydrogen efficiency (ηH) for the gas turbine modular helium reactor (He) high-temperature electrolysis of steam (HTES) and the supercritical CO2 (S-CO2) advanced gas-cooled reactor HTES technologies. SOURCE: Yildiz and Kazimi (2003).

and 750°C, respectively. The thermal energy (MJ) needed to produce 1 kg H2 is presented in Figure G-6.18

Nuclear reactors coupled to HTES are capital-intensive technologies, due to both the nuclear plant and the electrolysis plant. The development of economical and durable HTES unit materials, which can be similar to those of the solid oxide fuel cell materials, can contribute to cost reduction. The development of improved HTES units with low electrode overvoltage at lower temperatures can enable their use with lower-temperature and thereby lower-cost nuclear plants. Improved HTES cell designs are currently being investigated at Lawrence Livermore National Laboratory (Pham, 2000) and Idaho National Engineering and Environmental Laboratory (Herring, 2002). In addition, attaining high power cycle efficiency at the nuclear plant with relatively low temperatures can contribute to cost reduction. Finally, development of economic high-temperature radiation-resistant graphite or ceramic-coated graphite materials for the nuclear plant is needed.

Thermochemical Water Splitting

A recent screening of several hundred possible reactions (Besenbruch et al., 2000) has identified two candidate thermochemical cycles that have the highest commercialization potential, with high efficiency and practical applicability to nuclear heat sources. These are the sulfur-iodine (SI) and calcium-bromine-iron (Ca-Br) cycles. The S-I cycle is being investigated by General Atomics and JAERI. The Ca-Br cycle, which is sometimes called UT-3 to honor its origin at the University of Tokyo, is being investigated by JAERI. Argonne National Laboratory (ANL) is currently working on achieving thermochemical water-splitting processes at lower temperatures than the SI and Ca-Br cycles. ANL has identified the copper-chlorine (Cu-Cl) thermochemical cycle for this purpose (Doctor et al., 2002).

Sulfur-Iodine Cycle and Other Sulfur Cycles The SI cycle has been proposed in several forms. (The SI cycle and other


ηth (power cycle thermal efficiency) is taken from Dostal et al. (2002), with the adjustment of 9 percent reduction for the He cycle and 3 percent reduction for the S-CO2 cycle in finding the ηel, to reflect the heat losses due to component cooling and leakage..

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