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Water Dissociation Technologies for Hydrogen
Solar de Almeria (PSA), which uses sunlight to get 800°C–1200°C to split water [1].
This plant has been in operation since 2008. A megawatt plant based on this concept
can be built by having several parallel reactors operated by connecting the plant
to heliostat fields (field of sun-tracking mirrors) of a suitable size [1,108,109,113].
H 2 power systems [1,110,113] have proposed a membrane system for solar dissociation of water at temperatures as high as 2200°C. The membrane separates hydrogen
as soon as it is produced in the so-called solar water cracker. Such a cracker with
100 m 2 concentrator can produce almost 1 kg of hydrogen per hour during full
sunlight conditions.
The required scale of thermal decomposition process such that it is economical remains questionable. Large volume may require exotic refractories. At present, the choice of thermal decomposition takes second place to the thermochemical
cycles described later. At a laboratory scale, thermal decomposition has also been
analyzed using solar energy as a source of heat. The overall efficiency of solar
thermal process for hydrogen generation is considerably higher than that of PV/
electrolysis [1,108–113]. As shown below, solar thermal splitting of water is aided
by multiple chemical steps, but the following three principles govern the success
of solar thermochemical reactions: (1) drive chemical reactions at the highest temperature possible, consistent with other pertinent constraints such as materials of
construction and ability to concentrate light; (2) seek simple processes with as few
steps as possible, preferably one (e.g., cracking); and (3) for multistep water splitting
thermochemical cycles, seek processes involving a highly endothermic step driven
using concentrated sunlight, followed by an exothermic step that is autothermal and
can run continuously.
11.4.1 ThermoChemiCAl deComPoSiTion oF WATer
Thermochemical cycles have been intensely investigated over the past more than four
decades [1,11,89–139] (Funk, 2011, pers. comm.; Bamberger, 2011, pers. comm.). In
this method, two-, three-, or four-step chemical reactions aided by a source of heat
such as nuclear or solar can dissociate water and separate hydrogen and oxygen at
temperatures around 800°C–900°C. The method has some inherent issues:
1. The original concept [1,11,107] was that since the method avoided the
formation of electricity by the conversion of heat to mechanical work, it
would avoid Carnot cycle, as this is the fundamental difficulty in reducing the price of hydrogen production by electrolysis method. The thermocycles were thought to produce hydrogen at a cost of about half of that
for electrolytic method. This thinking was fallacious because the methods have to have reactions carried out at different temperatures in order
that the entropic properties of the partial reactions in each cycle can be
used to maximum advantage. Furthermore, when the individual reactions
have a positive entropy change, it is desirable to carry out reactions at the
highest temperature possible to minimize the overall free-energy change.
Conversely, if the entropy changes are negative, the reactions should
be carried out at the lowest temperature. However, this requirement of