Thermodynamic Analysis of the Use a Chemical Heat Pump to Link a Supercritical Water-Cooled Nuclear Reactor and a Thermochemical Water-Splitting Cycle for Hydrogen Production

Abstract

Increases in the power generation efficiency of nuclear power plants (NPPs) are mainly limited by the permissible temperatures in nuclear reactors and the corresponding temperatures and pressures of the coolants in reactors. Coolant parameters are limited by the corrosion rates of materials and nuclear-reactor safety constraints. The advanced construction materials for the next generation of CANDU reactors, which employ supercritical water (SCW) as a coolant and heat carrier, permit improved "steam" parameters (outlet temperatures up to 625ºC and pressures of about 25 MPa). An increase in the temperature of steam allows it to be utilized in thermochemical water splitting cycles to produce hydrogen. These methods are considered by many to be among the most efficient ways to produce hydrogen from water and to have advantages over traditional low-temperature water electrolysis. However, even lower temperature water splitting cycles (Cu-Cl, UT-3, etc.) require an intensive heat supply at temperatures higher than 550-600°C. A sufficient increase in the heat transfer from the nuclear reactor to a thermochemical water splitting cycle, without jeopardizing nuclear reactor safety, might be effectively achieved by application of a heat pump, which increases the temperature of the heat supplied by virtue of a cyclic process driven by mechanical or electrical work. Here, a high-temperature chemical heat pump, which employs the reversible catalytic methane conversion reaction, is proposed. The reaction shift from exothermic to endothermic and back is achieved by a change of the steam concentration in the reaction mixture. This heat pump, coupled with the second steam cycle of a SCW nuclear power generation plant on one side and a thermochemical water splitting cycle on the other, increases the temperature of the "nuclear" heat and, consequently, the intensity of heat transfer into the water splitting cycle. A comparative preliminary thermodynamic analysis is conducted of the combined system comprising a SCW nuclear power generation plant and a chemical heat pump, which provides high-temperature heat to a thermochemical water splitting cycle for hydrogen production. It is concluded that the proposed chemical heat pump permits the utilization efficiency of nuclear energy to be improved by at least 2% without jeopardizing nuclear reactor safety. Based on this analysis, further research appears to be merited on the proposed advanced design of a nuclear power generation plant combined with a chemical heat pump, and implementation in appropriate applications seems worthwhile.

Potential thermochemical and hybrid cycles for nuclear-based hydrogen production

“…Zamfirescu and Dincer [17] and Granowskii et al [18] have studied the use of mechanical compression heat pumps and chemical heat pumps as one of the potential options to upgrade the heat to reach higher temperatures for such thermochemical cycles.…”

Geothermal-based hydrogen production using thermochemical and hybrid cycles: A review and analysis

“…Several review articles and reports on thermochemical hydrogen production have been published [14][15][16][17][18][19][20], including a comprehensive listing of past and present activities [40]. Specific categories of thermochemical production of hydrogen have also been reviewed, including solar [23][24][25] and nuclear [41].…”

“…As mentioned previously, geothermal source temperatures are generally at 200-2501C but they can be upgraded to 550-6001C using various heat pump types [17,18].…”

“…Heat pumps could be used to upgrade the heat for higher temperature levels from existing geothermal resources. The capacity to upgrade heat can make heat pumps a better alternative than high-temperature electrical heating, since they require less electrical energy consumption for the same useful heat output [17,18]. The operating temperatures are key factors for thermochemical hydrogen production, and so optimization of heat.…”