Water electrolysers

Electrolysers can be classified into three basic kinds, based on the type of electrolyte used alkaline water electrolysers, membrane electrolysers (PEM electrolysers) and high-temperature electrolysers. Alkaline water electrolysis, the oldest and, therefore, most widely used technology, is described in more detail below. Figure 10.6 shows a diagram of an alkaline water electrolyser. 

Figure 10.6. Block diagram of alkaline water electrolyser.

Canada s hydrogen transport and distribution infrastructure program comprises refuelling stations (various locations and various companies), and includes work on large-scale water electrolysers, hydrogen compressors up to 700 bar, hydrogen dispensers, controls, and codes and standards. 

This process, illustrated in Figure 1, has been carried out by the team of Doenitz in the 80s (Doenitz, 1982, 1985, 1988a, 1988b). They showed that the main drawback of water electrolysers is their high electricity consumption. Electricity is known to be an expensive form of energy. As a result, electrolytic hydrogen is currently more expensive than steam-reformed hydrogen by a factor of at least two. 

The sulphuric acid decomposition reaction to regenerate S02 and to produce oxygen, (2), is common to all sulphur cycles, including the sulphur-iodine (S-I) cycle. What distinguishes the HyS process from the other sulphur cycles is the use of sulphur dioxide to depolarise the anode of a water electrolyser, as shown in reaction (1). 

Another interesting work is the recent report by Licht et al [72, 75, 94]. Although the system they studied was not a strict photoelectrochemical one, since the photovoltaic system was separated from the water electrolyser, their study is of general interest for the water oxidation field. The photovoltaic cell was connected to a water splitter catalyst system of considerably larger area than the solar cell. With this design, it was possible to combine a high solar cell efficiently with a low photocurrent density over the electrolyzer (jph = 0.44 mA/cm2), which minimized the overpotential needed for water oxidation. An overall efficiency as high as 18.3% was obtained. 

According to Zoulias el al., 2006, water electrolysers have found to comprise a major cost factor in a complete hydrogen-based autonomous power system. In this case a cost of 8150 per N m3/h of hydrogen produced was used in our calculations. The high cost of commercial electrolysis units is attributed to the lack of mass production from all manufacturers. Mass production of electrolysis units is expected to result in a 50% reduction on the capital cost. Two sizes for electrolysis units have been considered in the analysis 3.2 N m3/h of hydrogen produced (16 kW) and 4.2 N m3/h of hydrogen produced (21 kW). The lifetime of the electrolysers was 20 years. 

The optimisation results revealed that the most economically favourable system configuration for Rauhelleren comprises a wind turbine with a capacity of 140 kW, a 30-kW PEM fuel cell, water electrolyser capable of producing 8 N m3/h of hydrogen and a 400-kg hydrogen storage tank. 

Low availabilty and high cost of small water electrolysers Another important problem for the commercialisation of hydrogen-based autonomous power systems, since there are not many small water electrolyser manufacturers all over the world and the cost per N m3 of hydrogen produced is extremely high. These problems can be tackled as soon as a mass production of respective small electrolysers is available and technology breakthroughs are made in order to use inexpensive materials used in the construction of electrolysers. 

Stucki, S. et al., PEM water electrolysers evidence for membrane failure in 100 kW demonstration plants, J. Appl. Electrochem., 28, 1041, 1998. 

Agranat V., Zhubrin S., Maria A., Hinatsu J., Stemp M. Kawaji M., CFD modelling of gas-liquid flow and heat transfer in a high pressure water electrolyser system. Proc. of FEDSM, July 17-20, Miami (Florida) 2006. 

Marshall A, Borresen B, Hagen G, Tsypkin M, Tunold R (2007) Hydrogen production by advanced proton exchange membrane (PEM) water electrolysers—reduced energy consumption by improved electrocatalysis. Energy J 32 43 I —436... 

Marangio F, Santarelli M, Cali M (2009) Theoretical model and experimental analysis of a high pressure PEM water electrolyser for hydrogen production. Int J Hydrogen Energ 34 1143-1158... 

Grigoriev SA, Millet P, Fateev VN (2008) Evaluation of carbon-supported Pt and Pd nanoparticles for the hydrogen evolution reaction in PEM water electrolysers. J Power Sources 177 281-285... 

A small percentile of world production has been based on water electrolysis from cheap hydroelectric eneigy in Canada and Scandinavia H O = H2+(1/2)02, with h = 0.65. It is the purest H. As fossil fuels are being exhaust, water electrolysers seem to be the most popular hydrogen sources in the future. Electrolysers with liquid... 

The hydrogen evolution reaction is historically very important since its study has contributed much towards our understanding of electrode reactions. It is also met in corrosion and as a cathode reaction in water electrolysers, some chlorine cells and other oxidation processes. 

Figure 2.12 Schematic diagram of a monopolar, tank water electrolyser.

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