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Report Date: December 2004
Appendices: No
Abstract
A design evaluation has been performed for an integrated system of high temperature steam electrolysis (HTSE) supported by a supercritical CO2 (SCO2) recompression Brayton cycle that is directly coupled to an advanced gas cooled reactor (AGR). The analysis includes the effects of major components performance parametrically and the component integration systematically for different plant configurations. The configuration (HTSE-SCO2-AGR) with thermal recuperation from the gas product streams and an intermediate heat exchanger (IHX) between the turbine exit and the feed water stream is found to be superior with respect to thermal efficiency, operational flexibility and cost.
HTSE for H2 production provides two major advantages: 1- It can promise higher efficiency (with the appropriate materials and power conversion system for intermediate and high temperature operation of the solid oxide electrolysis cell) compared to the leading high temperature thermo-chemical water splitting cycles and to traditional water electrolysis; 2- It has already been demonstrated experimentally by several groups, based on materials used for solid oxide fuel cells. Thus, there is a potential for improving its performance further with processing of new materials that can minimize the energy losses due to the electrocatalytic mechanisms specific to electrolysis on oxide electrodes.
HTSE energy efficiency depends significantly on the thermal to electrical energy conversion efficiency. The supercritical CO2 recompression Brayton cycle is a strong candidate to enhance the HTSE efficiency due to its high electrical efficiency, comparable with that of He cycle but at significantly lower temperatures (550oC vs 850 oC). In addition, it is expected to reduce the capital cost of the plant due to its simplicity and compactness. The advanced gas cooled reactor, AGR, which has extensive operational experience in the UK, can support a direct SCO2 cycle and provide heat for the HTSE. The SCO2 power conversion system is proposed to couple to the AGR with a direct cycle. This requires a design update of the AGR (which operates at 4MPa) to withstand high pressures (20MPa) and operation near 700oC. An intermediate heat exchanger that provides heat from CO2 at the exit of the turbine to boil feed water for electrolysis is proposed for operational flexibility of adjusting the hydrogen and electricity output. In addition, the IHX at this configuration is subject to less harsh temperature environment than that at the exit of the reactor.
The HTSE average process temperature is chosen as 900oC and the hydrogen pipeline delivery pressure is assumed to be 7MPa for the evaluation of each design configuration in this work. The reactor exit temperature and the SCO2 cycle turbine inlet temperature are chosen to be the same as those for the SCO2 recompression cycle design options for consistency and compatibility. For the base case design, the reactor exit temperature is selected as 550oC, the HTSE operates at 1bar with 80%-90% voltage efficiency, the SCO2 cycle is based on a conservative design and the produced hydrogen is compressed for pipeline delivery. For these operating conditions, the system can achieve 40.4%-43.5% (LHV) overall hydrogen production energy efficiency. When the operating pressure of the HTSE is increased to 3MPa, the system can achieve 42.4%-48.2% (LHV) of hydrogen production energy efficiency, where the range depends on the SCO2 cycle operating conditions and the HTSE materials performance. With improved recuperators and HTSE cells, 7MPa can be expected as a feasible product exit pressure from the HTSE system in the future. The design with HTSE operating at 7MPa can achieve energy efficiency of 41.6%-47.4% (LHV) and eliminate the hydrogen compressor before delivery of the product into the pipeline. The product gas streams remain at relatively high temperatures even after the thermal recuperation in this design. Further thermal recuperation for a bottoming steam cycle, for preheating the water or for providing heat for the SCO2 cycle can be energetically possible. However, none of these alternatives are found viable economically. Process heating by the final product can be possible depending on the proximity to the necessary industry, and is not pursued in this study.
As with all new technology concepts, there are major R&D needs for commercial realization of a recuperative HTSE-SCO2-AGR system. The major needs for HTSE-SCO2-AGR R&D have been identified as: materials processing for the durability and efficiency of the HTSE system at intermediate temperatures, the design update of the AGR with advanced materials to resist high pressure CO2 coolant, thermalhydraulics of CO2 at supercritical pressures, and detailed component design for system integration.
The energy efficiency of the recuperative HTSE-SCO2-AGR as analyzed in this work is more promising than the other leading nuclear hydrogen production technologies. Challenges for this system exist, but appears less difficult than those for the very high temperature thermochemical hydrogen production processes proposed for future VHTRs that have more uncertainties in their development. In the near term, the aim must be to expand the research on this promising technology, as summarized above, in order to have a system available for large scale hydrogen production with confidence and economy.
Program: NES: Nuclear Energy and Sustainability
Type: TR
RPT. No.: 2