Discussion
For nuclear heat sources, cycles with a reaction taking place at a temperature > 900\(^{\circ}\)C should not be considered because it would be higher than what Generation IV concepts are expected to provide. Also the range of 800 .. 900\(^{\circ}\)C is very questionable because of the exponential increase in cost of materials with increasing temperatures \cite{verfondern2007nuclear}.
In HTE, only a small percentage of energy is needed as heat, which is around 90 kJ/mol for an electrolyzer operating at 950°C. This splits up into ~ 60 kJ/mol of H2 around 200°C to decompose steam and only 30/40 kJ/mol (maximal realistic value) at 950°C for the electrolyzer itself, if supposed that the heat of the products is recovered to heat up the reagents. Therefore the HTGR will only provide less than 10 % of useful heat energy to the electrolyzer, resulting in an increase of efficiency from around 40 to 42% \cite{verfondern2007nuclear}.
Conclusions
\cite{verfondern2007nuclear}: The question is whether it is really worth coupling an HTE system to an HTGR and achieving such a low theoretical gain of efficiency:
- Hydrogen has to be kept separated from the nuclear plant to avoid hydrogen explosion hazard. This will be on the expense of losses in the heat exchanger, which will lower the global efficiency of the system, and of increasing investment cost for security reasons. It has been suggested to increase the number of water recirculating with hydrogen to avoid hydrogen explosion; but doing so will reduce the final efficiency of the process because the heat of vaporization of water can not fully be recovered.
- HTE systems are supposed to be very fragile, especially under thermomechanical cycling conditions. Repeated maintenance of HTGR systems will lower their lifetime and their availability.
- Allothermal instead of autothermal operation of HTE will increase the number of cells required, therefore directly impacting the already high investment cost.
Acknowledgments
The project was funded by the National Science Center based on decision number DEC-2016/23/B/ST8/03056.