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Technology: Electrolysers – part 3

Electrolyser technology

The last part of the electrolyser series is a comparison between Alkaline (ALK), Polymer Electrolyte Membrane (PEM) and Solid Oxide electrolysers (SOEL),  explaining the advantages and disadvantages with the different technologies.

Did you miss the previous parts in the electrolyser series? Read them here: part 1 and part 2.

Maturity and costs

The alkaline electrolysis stacks have been available on a MW-scale for a long time, and a scale-up of PEM has been realized the last few years, largely driven by the drive to run electrolysis from Variable Renewable Energy (VRE) and to reduce plant footprint. Alkaline stacks are available up to 6 MW and PEM stacks up to 2MW. The SOEL is still in laboratory scale with up to 10kW. 

Alkaline electrolysers have the lowest cost per kW. In commercial scale plants (2 MW +), Alkaline electrolyser plants have a capital cost of $ 800 – 1 000 /kW. PEM electrolysers come in at an overall higher capital cost at 1 400 – 1 700/kW. The price difference between Alkaline and PEM electrolysers is largely explained by the maturity of the technology and the use of precious metals in PEM electrolysers. There is an uncertainty regarding the investments cost due to the pre-commercial status for SOEL.

Renewable energy applications

Usage of intermittent power sources requires flexibility. A State-of-the-art PEM electrolyser can operate more flexibly than current Alkaline technology, and these characteristics are suited for variable renewable energy. Historically, the alkaline electrolyser was designed for stationary applications with grid connections and must be adapted to the new flexibility requirements.

Advancements in Alkaline technology, specifically ‘Pressurized’ Alkaline electroysers have made them once again suitable for variable renewable energy if the system components are engineered to operate with an intermittent power supply. 

While PEM electrolyers offer best in class use of intermittent power sources, both PEM and (pressurized) alkaline electrolysers can offer fast load dynamics when they are in operating temperature and is suited for grid stabilizing.

Figure 1 compares parameters regarding the different electrolyser technologies:

Start-ups

The development of PEM has been driven by energy storage application. PEM has short startups, especially from cold. Alkaline electrolysers have a slower start-up taking up to an hour. The SOEL has a cold start-up time up to several hours and needs a high energy consumption (in the form of heat) to maintain a temperature that allows a short start-up time. 

One of the most significant issues with start-ups is when shut downs extend for a extended period of time resulting in the need to purge the equipment using nitrogen. This can increase the start-up time signficantly beyond normal start-up periods. Nitrogen purge requirements however largely differ from manufacturer to manufacturer.

Flexibility

The production rate for PEM can be varied over the full load at 0-100%, but the alkaline electrolyser typically has a load limit of 20% meaning it cannot operationally drop below 20% of the nominal load. It is worth noting that some alkaline electrolysers can offer higher flexibility such as pressurized alkaline. The SOEL has the ability of co-electrolysis of CO2 and steam to produce syngas containing H2 and CO2 for synthesis of fuels. It is also possible to have the reversible operation. This allows operating range from -100 to 100%. However, SOEL is still at the research stage based on single-cell or short-stack tests and any capabilities such as this may not be realized on a commercial stage.

A lower operating limit and the number of stops permitted by the manufacturers are limitations for the commercial Alkaline electrolysers. Reducing the electrolysers’ lower operation limit and improving the response times are key aspects in development.

Pressure

The outlet pressure of each of an electrolyser can influence the overall production facility and maintenance requirements. While the alkaline typically offers low outlet pressure of only 2 – 3 bar, SOEL has a slightly higher output pressure around 5 bar, and the PEM offers the highest outlet pressure in the range of 20 – 30 bar.

Lifetime

Alkaline electrolysers offer the longest lifetime, some have been in regular operation for 30+ years. Typically, cell stacks need replacement after 80 000 operational hours. PEM electrolysers also offer long times, however, degrade faster than their Alkaline counterparts requiring stack replacement after 40 – 50 000 hours. Finally, SOEL offers little to no lifetime with a ~8 000 hours lifetime – this is due to very high temperature used in the process causing material breakdown.

Efficiency

Solid Oxide Electrolysis is the most electrical efficient with electricity consumption around 42 kWh/kg, however this comes with the requirement that very high excess heat is available and therefore the true efficiency is significantly higher. Alkaline electrolysers have the highest overall efficiency with energy consumption around 52 kWh/kg produced. Meanwhile, PEM electrolysers have the lowest overall efficiency at 59 kWh/kg produced.

A caveat to the ‘efficiency’ discussion: PEM electrolysers are better suited than ALK electrolysers for operation under pressure due to smaller cell surfaces. It is more efficient to have a higher pressure inside the stack. The PEM electrolysers have a higher power consumption but the result is a higher output pressure. Given that the hydrogen is to be compressed later for distribution  or storage then the additional energy consumption from the PEM electrolyser is not entirely wasted.


 

 

Do you have a project you want to realise and need more information? Please contact us at greensight@greensight.no. This is the last part in the electrolyser series. Read the previous parts here: part 1 and part 2. 

Technology: Electrolysers – part 1

Electrolyser technology

Hydrogen has many colours and can be produced with a broad range of technologies. Green hydrogen is produced by water electrolysis with renewable electricity. Water electrolysis is the cleanest and most sustainable way to produce hydrogen. The next weeks we will discuss the different electrolyser technologies.

We will discuss the technologies in three parts:

Electrolysis is the electrochemical process splitting water into hydrogen and oxygen by supplying electrical (or thermal) energy given by the equation:There are currently three main technologies for electrolysis:

  • Alkaline Electrolyser
  • Proton Exchange Membrane Electrolyser
  • Solid Oxide Electrolyser
 

Electrolyser

The alkaline electrolyser (ALK) is a mature technology. In an alkaline electrolyser, the electrolyte is usually a 25-30% aqueous KOH-solution and is operated at 60-90˚C. The electrodes are immersed in the liquid electrolyte, separated by a separator that only allows transport of ionic charges. Historically, the separator was made of asbestos, but is currently made of Zirfon PERL.

When a direct current is applied to the water, the water molecule is split into oxygen and hydrogen. The electrolyte let the ions be transported between the electrodes. The purity is 99,5-99,9% for the hydrogen.

The electrolysis reactions: 

Alk redoks reaksjon

 

Alkaline cell

Conceptual set-up of an alkaline cell. Source: The International Journal of Energy

Electrolyser

Proton exchange membrane (PEM) systems are based on the solid polymer electrolyte concept for water electrolysis introduced in the 1960s.  The PEM electrolysers that are commercially available today, are more flexible and tend to have smaller footprint than the alkaline electrolysers. 

A proton exchange membrane separates the two half-cells, and the electrodes are usually directly mounted on the membrane. The membrane only allows transportation of hydrogen ions. It is necessary to use noble metal catalysts like iridium for the anode and platinum for the cathode. Water is supplied at the anode. The cell temperature of a PEM cell is 50-80˚C. 

The electrolysis reactions are as follow:

PEM redoks-reaksjon

The resulting purity is higher than for alkaline and is typically greater than 99,99% H2. PEM has a compact module design because of the solid electrolyte and has a high current density operation compared to alkaline.

pem electrolyser technology
Source: Wood Mackenzie, U.S. Department of Energy

Electrolyser

Electrolysis of water can be performed at high temperature using steam. A Solide Oxide Electrolyser (SOEL) is a high-temperature electrolyser that perform a solid oxide electrolysis and operates at temperatures of 700-900˚C. The technology is currently immature and has only been tested at laboratory scale. High temperature operation results in higher electrical efficiencies than alkaline and PEM, but it has challenges in material stability and also depend on waste heat. The high temperature steam is either supplied by an external heat source or by an electrical heater, therefore the applicability of SOEL is limited to specific instances (more in part 2)

SOEL use a solid ion-conducting ceramic as the electrolyte and comprise of three layers. Yttria-stabilized zirconia is often used as electrolyte.

The half reduction equations:

Solid oxide cell

Conceptual set-up of a solid oxide electrolyser cell. Source: International Journal of Hydrogen Energy

 

Part 2 will discuss the applications for the different technologies. Part 3 compares the different electrolyser technologies.


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