Nuclear Production of Hydrogen, Fourth Information Exchange ...

NUCLEAR HYDROGEN USING HIGH TEMPERATURE ELECTROLYSIS AND LIGHT WATER REACTORS FOR PEAK ELECTRICITY PRODUCTION
• Liquid-to-gas conversion. In traditional electrolysis, water as a liquid is converted into two gases:
hydrogen and oxygen. Energy is required to convert liquids to gases. In electrolysis, this
energy is provided by electricity whereas in HTE, heat converts the water to steam. The
energy requirements depend upon the final pressures of the hydrogen and oxygen. For peak
electricity systems, the hydrogen and oxygen may be stored at pressures as high as 7 MPa;
thus, there is an incentive to produce steam at this pressure to avoid the need to compress
the resultant hydrogen and oxygen before storage. As it happens, the steam from PWR steam
generators is produced at that pressure, and it may be possible to use some of it as feed into
the electrolysers.
• Chemical bond breaking. The energy required to break the water molecule into hydrogen and
oxygen decreases with increasing temperature. The hotter the steam, the less electricity that
is required. However, this is a much smaller effect.
The HTE technology requires that the system operate at ~800°C for the rapid diffusion of oxygen
ions through a solid membrane. This requires heating the steam to ~800°C. There are two strategies to
heat steam to this temperature:
• High temperature reactor. The nuclear reactor can provide the 800°C steam.
• Lower temperature reactor with recuperation through countercurrent heat exchangers. The hydrogen
and oxygen products from the HTE system must be cooled back to room temperature. The use
of counter-current heat exchangers to transfer heat from the hydrogen and oxygen to the
steam (called recuperation of heat within the power cycle) can provide most but not all of the
heat required to raise steam temperatures. The added heat will be partly supplied by
inefficiencies in the HTE cells that generate heat. The remainder of the heat can be supplied
by electrical heating.
Hydrogen production from nuclear plants will have an energy efficiency that depends on the
efficiency of thermal to electrical energy conversion and on the efficiency of using electricity in the
electrolysis process. The thermal-to-electrical efficiency of existing light water reactor plants is about
33%. The efficiency of water splitting by conventional electrolysis can be about 75%. Thus, the
hydrogen production energy efficiency using today’s technology is about 25%. Using HTE in solid
oxide electrolysers would improve the total efficiency of hydrogen production (Yildiz, 2004, 2006a).
Most of the heat in the system is required for conversion of water to steam. This can be supplied by
any reactor that can produce high-pressure steam – including LWR and CANDU reactors. Higher
temperature steam is desirable, but not required. Existing reactor technology can be used.
One recent study (O’Brien, 2006) compared hydrogen production by electrolysis and HTE assuming
an advanced CANDU reactor with a thermal efficiency of electricity production of 36% – essentially
identical steam conditions as produced by a modern LWR. With traditional electrolysis, the overall
hydrogen-to-thermal efficiency was 25.7%. With HTE, the hydrogen to thermal efficiency was 33 to 34%.
The efficiency of hydrogen production was almost equal to the efficiency of electricity production
when the energy give back is the thermal energy derived from oxidising the hydrogen. There are
strong economic incentives to develop HTE for hydrogen production, particularly when coupled to
peak electricity.
With advanced high temperature reactors that can supply the HTE process with all the heat
needed for operating at 800°C, the efficiency of both electricity production and hydrogen production
may be as high as 50%. Advanced reactors that can attain higher temperature than today’s reactors
include the helium-cooled High Temperature Gas-cooled Reactor (HTGR), and the salt-cooled
Advanced High Temperature Reactor, which can achieve temperatures above 800°C leading to very
high conversion efficiency when coupled to a helium-based Brayton power cycle. Alternatively,
CO 2 -cooled reactors operating at supercritical pressure and coupled with appropriate Brayton cycle
can achieve thermal-to-electrical energy conversion efficiency around 50% with an outlet temperature
around 600°C (Yildiz, 2004, 2006b). An examination of the applicability of a direct cycle supercritical
CO 2 -cooled fast reactor to hydrogen production was performed by Memmott, et al. (2007). The reactor
exit temperature was 650°C. The electricity production and therefore the hydrogen production system
had overall efficiencies above 45%. The operating parameters (i.e. temperature and pressure) for the
major components play a role in the overall efficiency, with an apparent advantage for conducting the
electrolysis at slightly higher pressure than the atmospheric pressure.
158 NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010