Electrolyser
Electrolysers are a key component of renewable-hydrogen systems and they account for a substantial portion of the cost of such systems. The choice and size of the electrolyser to use in a hydrogen buffering system impacts, to a great extent, on the unit cost of energy produced from such systems. Alkaline electrolyser technology is very well proven and is the electrolyser type used in this project. The information considered in making this choice as well as typical characteristics of an alkaline electrolyser is presented in this section.
Types of Electrolysers
Three types of electrolysers for hydrogen production exist. These are the alkaline electrolyser, the proton exchange membrane (PEM) electrolyser, and solid oxide electrolyser (SOE). The alkaline electrolyser and the proton exchange electrolyser are well proven technologies and have been used for commercial electrolytic hydrogen production. Solid oxide electrolysers are still in the developmental stage and questions as to its viability remain unanswered [1]. Table 1 provides a summary of the key features of the main types of electrolysers.
Alkaline Electrolyser
The principle of operation of the alkaline electrolyser is shown in Fig 1. Water molecules are reduced at the cathode to atoms of hydrogen and hydroxyl ions as direct current is passed through the potassium hydroxide electrolyte. The hydrogen atoms combine to form gaseous hydrogen which escapes from the cathode. The hydroxyl (OH-) ions migrate from the cathode under the influence of the applied electrical field through a porous diaphragm to the anode where the OH- ions give up electrons releasing oxygen atoms which combine to form gaseous oxygen released at the anode. Alkaline electrolyser conversion efficiencies of over 90% and gas purities of about 99.8 are common place. The hydrogen produced often has to be further purified to about 99.998%.
Alkaline electrolysers are the most widely deployed electrolysers for commercial production of hydrogen. Alkaline electrolysers are very popular because they are very robust and have relatively long operational lives usually measured in decades rather than years. Alkaline electrolysers employ a 25 – 30% (by weight) caustic solution (usually potassium hydroxide) as electrolyte, with typical current densities around 0.2 – 0.4A/cm2. Alkaline electrolysers use relatively cheap metals such as Nickel as catalyst unlike other electrolysers which use precious-metal catalysts. Alkaline electrolysers have been designed for operation at atmospheric pressure and at pressures up to 30bar [1]. Alkaline electrolysers are capable of producing hydrogen with purity of 99.8%, and 99.997% after further purification.
Advantages
The alkaline electrolyser is currently the commercial electrolyser of least unit cost and has the greatest conversion efficiency (up to 95%). The alkali electrochemistry employed obviates the need for precious metal catalysts, so that relatively cheap catalysts (such as Nickel) can be used. Alkaline electrolysers are very well proven and are the earliest known commercial electrolyser technology. As alkaline electrolysers with relatively high capacities (up to 200Nm3/h of H2) can be manufactured, they currently provide the principal route for producing electrolytic hydrogen and oxygen. The relatively low operational temperature range (50 – 100oC) makes for an easy start up. Alkaline electrolysers have the reputation for being robust and can be operated reliably for decades.
Drawbacks
Alkaline electrolysers use liquid potassium hydroxide (KOH) as electrolyte. This liquid electrolyte limits their response to fluctuating electrical input characteristic of renewables and this has the effect of increasing energy wastage. The liquid electrolyte also makes them more susceptible to leakages thus increasing maintenance. Alkaline electrolysers typically operate at low current densities so that operational pressures are limited to about 30bars. This low pressure gas output often necessitates auxiliary gas compression equipment which adds to unit cost. Furthermore, the product gases from an alkaline electrolyser often contain traces of electrolyte (KOH) which has to be removed by scrubbing in order to increase gas purity. This further adds to the auxiliary equipment required and to unit cost.
References:
[1] Smith, A. F. G. and Newborough, M., Low-Cost Polymer electrolysers and electrolyser implementation scenarios for carbon abatement, Report to the carbon trust and ITM-Power PLC.
[2] http://www.itpower.co.uk/investire/pdfs/electrolyser.pdf
[3] Irvine, J., Solid oxide electrolysers, Electrolysis seminar, Herriot-Watt University, September 8 2004.
[4] http://www.spaceflight.esa.int/users/images/foton/highres/illustr_electro.gif
