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.
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:
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.
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.
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.
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.
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 firstname.lastname@example.org. This is the last part in the electrolyser series. Read the previous parts here: part 1 and part 2.