Nuclear Production of Hydrogen, Fourth Information Exchange ...

NUCLEAR HYDROGEN USING HIGH TEMPERATURE ELECTROLYSIS AND LIGHT WATER REACTORS FOR PEAK ELECTRICITY PRODUCTION
Hydrogen-oxygen storage
Unlike electricity, hydrogen can be stored inexpensively (Forsberg, 2005) for days, weeks or months in
large underground facilities with the same technology used to store natural gas. In the United States,
approximately 400 underground facilities store at high pressure one-third of a year’s production of
natural gas in the fall before the winter heating season. This is a low-cost technology, with market
prices for storage typically 10% of the value of the natural gas.
A limited number of hydrogen storage facilities now exist in Europe and the United States
to support the chemical and refining industries. Equally important, measurements of helium
concentrations in different geologies from radioactive decay and the long-term existence of natural
gas deposits provide evidence that many geologies have the low gas permeability required for
hydrogen storage. However, storage and handling economics demand large facilities (Forsberg, 2009a).
Several factors that are almost independent of facility capacity drive the facility size: siting and site
development costs (including an understanding of the local geology). In addition, gas storage requires
compression of the gases, typically to pressures of ~7 MPa (~1 000 psi). Gas equipment efficiencies and
costs are strong functions of the size of the equipment. For bulk hydrogen storage, the capital costs
are estimated to be $0.80-$1.60/kg, which is lower than the total production costs for hydrogen.
A related consideration is the cost of hydrogen transportation. Hydrogen is expensive to
transport (Summers, 2003; US DOE, 2007a), with large economics of scale. The transport costs are
several times higher than that of natural gas because of the low molecular weight, and hence low
density of hydrogen (Bossel, 2003). With hydrogen production costs between $1 to $3 per kilogram,
transportation costs can become the dominant cost of delivered hydrogen. This combination of
factors (Forsberg 2009a; Bossel, 2003) favours the centralised production, storage and use of hydrogen
for applications such as peak electricity production
Oxygen can be stored using the same technology; however, bulk high-pressure underground
storage has not been demonstrated and there remain some uncertainties. Until recently, there have
been no incentives to develop such oxygen storage technologies. Oxygen can be produced from air at
relatively low costs. While there are strong incentives to store oxygen at high pressures to provide the
high feed rates needed for peak power production, it is unclear whether the economics of oxygen
production versus storage will favour seasonal storage of oxygen or only storage for a period of weeks.
Peak electricity production
The characteristic of all methods of peak and intermediate electricity production is that the equipment
is operated for limited periods of time ranging from a few hundred hours to several thousand hours
per year. For favourable economics, the equipment to convert the fuel to electricity should have a low
capital cost. There are two methods to produce variable electricity output from this system. Both
methods are under development; thus, significant uncertainties remain.
High temperature electrolysis/high temperature fuel cell
The HTE system can also be operated as a high temperature fuel cell. This allows the operator to vary
electricity output from the nuclear station to the electrical grid from zero to ~170% of base-load
electrical production. If all of the energy from the nuclear power station is used to produce hydrogen
and oxygen, no electricity is sent to the electrical grid. If the reactor produces only electricity and the
high temperature electrolysis system is operated as a set of fuel cells, the power output will be
maximised. Use of the same equipment for both hydrogen production and peak power minimises
system capital cost – a critical requirement for peak power production where peak electricity is
produced for a limited number of hours per year. There are several special characteristics of this system:
• Oxygen. The output of the fuel cells depends upon the availability of stored oxygen because the
oxygen electrode determines fuel-cell performance. If pure oxygen is used, the performance is
maximised. If air is used, the nitrogen in the air creates a mass diffusion barrier for oxygen
reaching the surface of the oxygen electrode within the fuel cell. This effect reduces the
electricity output per unit of cell surface area by a factor of two to four, depending upon various
design details. There are large economic incentives for pressurised oxygen storage. However,
the fuel cell can be operated on air, depending upon the economics of seasonal oxygen storage.
NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010 159