Alkaline-type electrolyzers

For each wind energy system category, there is a particular approach of how to apply an electrolyzing hydrogen production unit. The most common application studied currently is the stand-alone wind energy system combined with an electrolyzer. The latter is connected in the place of the battery cluster. The electrolyzer unit can be either a PEM or an alkaline type where there is a constant feed with water. The produced hydrogen is usually stored in a tank at the output pressure of the electrolyzer or compressed at a higher... 

Accordingly, the operation of electrolyzers with intermittent sources of solar energy and the various possibilities for matching photovoltaic current with the characteristics of the electrolyzer was one of the recurrent design issues in such projects. Most of the electrolyzers were of the alkaline type and operated at low pressure. 

The photovoltaic power source can be any commercial BSPM (Battery Specific Photovoltaic Module), or can be an ESPM (Electrolyzer Specific Photovoltaic Module). The electrolyzer can either be a PEM (Proton Exchange Membrane) or an alkaline type. 

A concern for alkaline as well as solid polymer electrolysers should be the sensitivity of the electrolysers to contaminants in the feed water. The minimum required water quality of an industrial solid polymer electrolyzer is ASTM type II water [48], which contains at maximum 5 pg L 1 of sodium and chlorine, and 3 pg L 1 Si02 [52], It should have a maximum conductivity of 0.25 pS cm 1. Clean drinking water meeting Dutch standards contains around 60 mg L 1 of... 

For hydrogen production from water, pure water (pH=7.0) is seldom used as an electrolyte. Water is a poor ionic conductor and hence it presents a high Ohmic overpotential. For the water splitting reaction to proceed at a realistically acceptable cell voltage the conductivity of the water is necessarily increased by the addition of acids or alkalis. Aqueous acidic and alkaline media offer high ionic (hydrogen and hydroxyl) concentrations and mobilities and therefore possess low electrical resistance. Basic electrolytes are generally preferred since corrosion problems are severe with acidic electrolytes. Based on the type of electrolytes used electrolyzers are... 

Three different types of matching devices are used. Alkaline electrolyzer, operation temperature 80-100 C. Hydrogen stored in standard gas bottles at 201 bar. 

In contrast to PEM electrolysis, which has only been utilized for around 25 years, alkaline electrolysis systems of various dimensions and types with outputs of up to 750 Nm h hydrogen have been available for some decades. For alkaline electrolysis, usually a potassium hydroxide solution with a concentration of 20-40 wt% is used. This is determined by the operating temperature, which is usually at 80 °C, since the ohmic losses can be minimized by a suitable concentration of the alkaline solution and thus optimal electrical conductivity [8]. The current density ranges from 0.2 to 0.4 A cm. The state of the art of large alkaline electrolyzers has not changed much over the last 40 years [9]. This becomes apparent in the fact that since the introduction of water electrolysis more than 100 years ago, only a few thousand systems have been produced and put into operation. Some of the systems listed in Table 11.3 are no longer produced, or their manufacturers have vanished from the market. 

If an RFC system has a separate FC and electrolyzer, it is possible to choose different materials for the electrolyzer and for the FC electrodes. The materials can be adapted in terms of ideal performance and high durabihty. If a URFC system is considered, the electrodes have to be suitable for both hydrogen consumption and evolution or oxygen consumption and evolution. This bifunctional operation subjects the electrodes to considerable stress and can reduce their performance and durability. In addition to the type of operation (conventional or bifunctional), the composition of the electrodes is greatly dependent on the electrolytes used. In the following subsections, the different electrolytes (alkaline, PEM, SOE) are described and also suitable electrodes for each type of operation. 

A particular approach adopted by General Electric In U.S.A. is the solid polymer electrolyte (SPE) cell in which the porous cloth-type separator is replaced by a polymeric ion exchange membrane which is conductive to cations (Figure 5). The particular membrane employed, NAFION, is a perfluorsulphonlc acid pol3nner which is extremely stable in both acid alkaline solution. Appropriate electrocatalysts are coated on each face of the polymer sheet and these are contacted by a metal mesh current collector. Further research is aimed at reducing the cost and improving the electrical efficiency of the system to make it competitive with conventional electrolyzers. 

By way of illustration, Figure 2.14 shows an example of a technical specification sheet for the series of alkaline electrolyzers HySTAT type V (cooling and water purification included) made by the Belgian-Canadian company HYDROGENICS. 

Figure 2.14. Technical specifications for the HySTAT type V series from HYDROGENICS (alkaline electrolyzer) [HYDJ...

Electrolysis of water has been used for over a century to produce hydrogen and oxygen. The concept of a reversible fuel cell for space applications is based on a combined electrolytic-galvanic fuel cell. Like fuel cells, several different types of electrolyzers and catalysts have been developed, including atmospheric and high-pressure acid-based polymer electrolyzers, alkaline solution systems, and solid oxide-based systems. 

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