The onset of the green energy revolution has led to a burst of technologies around the generation and storage of clean electricity. A key advantage of erstwhile coal and gas-powered plants over their renewable energy counterparts is that power generation could be started and stopped with a flip of the switch. This also ensured that storage was not a key concern since electricity is available ‘on tap’ as it were.
With both solar and wind power, in contrast, power generation depends on the elements and the intensity with which they choose to act. While wind energy can vary erratically depending on the force and direction of the winds, solar power is available mainly during certain peak hours of the day, although devices like solar trackers allow us to maximise the energy we harvest during these hours.
The other issue, of course, is storage. With renewable energy, the ability to store becomes critical to ensuring that the supply to the grid does not suffer the same vagaries as the energies received.
One of the main methods to store energy uses green hydrogen.
Green hydrogen involves using the harvested energy to power an electrolyser, which in turn converts fresh water into hydrogen and oxygen. This hydrogen is then stored in tanks and later burned (the by product being water) to create power for the grid. Across the world, companies are scrambling to set up green hydrogen plants, as they form a critical link to allow renewable energy to become the mainstay for future power needs.
In this endeavour, the efficiency of the electrolyser becomes paramount in ensuring that minimal energy is lost in the overall process.
An electrolyser is a device capable of splitting water molecules into their constituent oxygen and hydrogen atoms. It consists of a conductive electrode stack separated by a membrane to which a high voltage and current is applied. This causes an electric current in the water which causes it to break down into hydrogen and oxygen.
At present, there are different types of electrolysers depending on their size and function. The most commonly used are:
They use a liquid electrolyte solution, such as potassium hydroxide or sodium hydroxide, and water. Hydrogen is produced in a cell consisting of an anode, a cathode and a membrane. The cells are usually assembled in series to produce more hydrogen and oxygen at the same time. When current is applied to the electrolysis cell stack, hydroxide ions move through the electrolyte from the cathode to the anode of each cell, generating bubbles of hydrogen gas on the cathode side of the electrolyser and oxygen gas at the anode.
Proton exchange membrane (PEM) electrolyser
PEM electrolysers use a proton exchange membrane and a solid polymer electrolyte. When current is applied to the battery, water splits into hydrogen and oxygen and the hydrogen protons pass through the membrane to form hydrogen gas on the cathode side. They are the most popular because they produce high-purity hydrogen and are easy to cool. They are best suited to match the variability of renewable energies, are compact and produce high-purity hydrogen. On the other hand, they are somewhat more expensive because they use precious metals as catalysts.
Expanded PTFE (ePTFE) in electrolysers
Given the presence of liquids and chemicals, it is imperative that proper sealing exists between the stacks of the electrolysers. In this regard ePTFE tapes are used between the stacks to provide superior sealing. Expanded PTFE not only has a compressibility of up to 60% - allowing it to make a very robust seal even at low torques – but is also weatherable, resistant to chemicals, and highly effective even in extreme pressures. The exact dimensions of the ePTFE tape can vary from project to project, depending on the construction of the electrolyser. However, a thickness of 1.5-2.5mm is typically used with a width of 25-50mm. The tape is easily applied and can even be layered on to itself, eliminating the use of a standard cut gasket. This is relevant because the diameters of the electrolysers can be as high as 2 meters, meaning that a standard cut gasket would be very wasteful. Considering that a 5 MW alkaline electrolyser requires around 500 seals, this saving is particularly vital.
Over the past few years, a number of green hydrogen projects have shifted to ePTFE for the sealing of the electrolysers. While other materials such as EPDM, Low-hardness FKM, and butyl have all been tested and found reasonably effective, the efficacy and ease of use of ePTFE has proven unparalleled. It is likely that in the coming years, ePTFE gasket tapes will be a mainstay of any electrolyser plant.