Green Hydrogen Generation and Hydrogen Fuel Cells

Hydrogen is a light and storable fuel that does not generate harmful emissions upon use, but it cannot be harvested directly from nature. It is only present in water or organic compounds and a chemical reaction must be performed to extract it. The method used with water is known as electrolysis, a process that requires electricity as an input. The term ‘green hydrogen’ denotes the fact that the electricity used in this process is from a renewable source; in contrast ‘grey hydrogen’ uses fossil fuels to generate the required electricity. Currently green hydrogen accounts for roughly 0.1% of global hydrogen production due its higher costs of production, but per Goldman Sachs, it is anticipated that cost parity between grey and green hydrogen will be achieved by 2030 with green hydrogen and the renewable energy used to make it dropping in price.

Fundamentally, electrolysis is the process of using an electric current to break down a water molecule (H₂O) into its atomic constituents. There are several electrolysis methods in common use today including alkaline, proton exchange membrane, and solid oxide hydrogen electrolysis, but regardless of the method the electrolyser is fundamentally made of up the following:

• Two electrodes: an anode and cathode
• An electrolyte between the electrodes (or for the electrodes to be submerged in)
• Electrical input (direct current)
• Water as an input (ex. water as part of the electrolyte or as water vapour)

An electrolyte refers to an ion-containing medium that is electrically conductive. The general chemical reaction is the same regardless of the method:

2H₂O → 2H₂(g) + O₂(g)

where we can see that a successful electrolysis process produces both hydrogen and oxygen in their elemental state. Plate heat exchangers are used in various stages of the process including desalination of the input water, cooling the electrolyte used during the process (and re-using excess heat throughout the process), and condensing and cooling of the product gases for transport. When considering heat exchanger selection in these processes, the high pressures of the system and the corrosiveness of the electrolyte solution necessitate certain design constraints. Semi-welded and fully welded plate heat exchangers use gasket-free designs on one, or both sides, respectively to accommodate high-pressure and high-temperature processes that gaskets cannot support. By operating the electrolyser at high pressure, overall plant efficiency increases due to requiring lower loads at the compression stage.

Stainless steel plates will inevitably corrode in the presence of liquid electrolytes (ionic solutions), meaning that a plate material such as nickel or titanium must be used. These units also have the additional benefit of having a reduced risk of leakage, which is crucial for handling high-pressure gases and corrosive fluids. For example, Alfa Laval’s TK20 units feature a gasket design with optimized gasket support with minimal fluid contact, pressure ratings up to 63 bar (914 psi), temperature ratings up to 250°C (482°F), and are available titanium plates, making them an ideal solution for green hydrogen electrolysis applications. High-temperature electrolysis processes operate at up to 800°C and require gas-to-liquid heat exchangers featuring specially designed plates with asymmetric channel volumes to minimize pressure drop and gasket-free designs to handle high temperatures. Being able to recover as much heat as possible from waste streams is particularly important in such high temperature applications; however, plate exchanger units offer a higher thermal efficiency with less required floor space relative to the equivalent shell and tube heat exchanger in virtually all hydrogen electrolysis applications.

One application of generated hydrogen is to channel it into a hydrogen fuel cell where hydrogen and oxygen (from air) are used in a controlled manner to generate electricity and heat with water as the only by-product. In brief, hydrogen fuel cells generate electricity by splitting hydrogen molecules into protons and electrons at the anode. The protons pass through an electrolyte on their way to the cathode while electrons travel through an external circuit to power the load. The electrons then complete the circuit by continuing to the cathode where they react with supplied oxygen and the protons to produce water and heat as by-products.

Heat exchangers play an important role in the efficient operation of fuel cells; the following is an overview of their uses:

• Preheating of hydrogen and air before entering the fuel cell stack, optimizing the efficiency of the cell by ensuring ideal reaction conditions.
• Cooling of exhaust gases and recovery of the waste heat from the process to be used for space heating or other applications depending on heating needs in the vicinity.
• Temperature regulation of the fuel cell stack to prevent overheating and ensure efficient operation.

Since the water generated is in vapour form, a condenser is often required to make efficient use of the waste heat; fusion bonded heat exchangers
such as Alfa Laval’s AlfaNova line are a suitable option for this application.

Hydrogen is expected to see growing use in various industries through applications such as: feedstock in chemical production processes, fuel for long-haul transport (e.g. shipping and aviation), and as a longterm method of storing energy. In general, plate heat exchanger units are considered suitable for electrolysis and hydrogen fuel cell applications due to their compact design and high pressure/temperature tolerance while being cleanable, corrosion resistant, and capable of handling all of the fluids seen within these processes, including hydrogen and oxygen gas, water, air, steam, and electrolyte solutions.