23-06-2012, 04:55 PM
Hydrogen as a Storage Medium for Renewable Energy
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Introduction
Today the energy consumers on earth are heavily dependent on air polluting fossil
fuels, and with the present production rate, the oil reservoirs could be emptied in less
than a hundred years. The coal reservoirs could probably cover the world’s energy
demand for several hundred years more, but this development is not compatible with
the vision of a sustainable global energy supply. The environmental situation looks
even more critical if we consider a scenario where coal becomes the main resource and
the energy consumption per capita in heavily populated developing countries continues
to increase with the same rate as today. So far around 2 billion people in the world do
not have access to electricity (Friedland and Speranzs, 1999). Providing such a number
of people with electricity will have a major effect on the environment if fossil fuels are
used.
Hydrogen production from renewables
A possible storage system for the conservation of electrical energy consists of a hydrogen
production unit, a storage medium and a unit which converts the chemical energy
stored in the hydrogen back to electricity. An example of such a system is shown in figure
1, which is taken from Barbir (1999). The hydrogen system is connected to a
renewable power source, for instance a photovoltaic array (solar cells) or a wind turbine
generator which provides a local load with electricity.
Experimental studies
There have so far been fewer studies on genuine hydrogen storage systems than on
simulated systems, mainly because of material problems, high costs and uncertainty
about the technology. One of the most successful demonstration projects so far has
been reported by the University of Helsinki, where an experimental 1 kW system consisting
of a polymer membrane electrolyser and a polymer membrane fuel cell was
tested (Vanhanen et al.,1998). In an earlier project, the research group had a system
consisting of an alkaline electrolyser, pressure vessel for hydrogen storage and acid
phosphoric fuel cell (Kauranen et al., 1994), but this system had a poor operating efficiency,
mainly because of problems with the fuel cell. An interesting detail of the new
system is that the hydrogen is bound in so-called metal hydrides instead of stored in a
pressure tank.