- How does a plate heat exchanger work?
- The plate heat exchanger is a key building block in the hydrogen economy
- How is a plate heat exchanger used in the desalination of seawater?
- Why are heat exchangers essential for electrolyte cooling during hydrogen production?
- What type of heat exchanger should be used in which type of electrolyser?
- How are plate heat exchangers used in distribution applications?
- Why do heat exchangers optimize hydrogen refueling
- What is the use of plate heat exchangers in conjunction with fuel cells?
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How does a plate heat exchanger work?
Heat exchangers are devices that transfer thermal energy from one flow of medium to another. Depending on the field of application, this refers to liquids, gases or media in the transition phase between their gaseous and liquid state. In this process, heat always flows from the warmer to the colder medium without the two media coming into direct contact with each other. In this way, these are either cooled or heated along the various process stages of the value chain.
At its core, a plate heat exchanger consists of a plate pack forming channels by the inverted arrangement of two plates. The two media flow through these alternating channels without them mixing. A special plate design ensures optimum distribution and flow velocity of the respective medium, thus maximizing heat transfer.
If a flowing fluid moves in the form of ordered layers, this is known as laminar flow. However, laminar flows should be avoided in plate heat exchangers as the heat transfer is then very poor. Laminar flow adversely affects the overall heat transfer coefficient. Instead, turbulent flow is highly desirable and is achieved in heat exchanger design by a special plate arrangement in the plate and frame heat exchanger. This ensures that the fluid is swirled to a turbulent flow even at low flow velocities, resulting in improved heat transfer and increased transfer of the heat output from the plate heat exchanger.
The right choice of plate material is also very important for ensuring good thermal conductivity. For special applications such as high pressure requirements or particularly challenging media, plate heat exchangers need to be individually configured and equipped with the appropriate plate material. With a range of options, such as gasketed versions, brazed plate heat exchanger or flat plate heat exchanger designs, and welded apparatuses of different sizes, a perfect fit for the application at hand will always be available.
The plate heat exchanger is a key building block in the hydrogen economy
Plate heat exchangers of different types are used along the entire value chain of the hydrogen economy. They are used both in processes for generating electricity from renewable energy sources and in the production of hydrogen by means of electrolysis, and also in the water treatment that is sometimes required for this. The hydrogen generated by electrolysis is then transported through distribution networks and used by end users as an energy source and raw material for further production processes. For example, hydrogen is an essential component in chemical production processes and at the same time an important energy source in the industrial, building and mobility sectors.
Plate heat exchanger in the hydrogen economy
How is a plate heat exchanger used in the desalination of seawater?
The production of 1 kilogram of hydrogen requires about 15 liters of deionized water, with quality requirements comparable to drinking water. To conserve drinking water resources, the use of treated seawater is a sustainable solution in coastal regions. After filtration, the water is prepared for the electrolyzer process with the aid of a desalination plant. In vacuum evaporation, the waste heat from the electrolyzer is used to demineralize the water. Fresh water generators function as evaporators and condensers, separating the salt molecules from the water molecules and collecting the desalinated water as condensate. If necessary, this water is then deionized in a further step and can be used in the electrolyzer process.
This increased efficiency through the use of waste heat from the electrolysis process for water desalination is particularly useful in offshore applications.
Why are heat exchangers essential for electrolyte cooling during hydrogen production?
During the hydrogen electrolysis process, heat is generated through the chemical reaction at the electrodes induced by the current applied at the electrodes. A system with an output efficiency of 65% releases 35% of the applied electrical energy in the form of heat. This heat can be removed from the process by plate heat exchangers, so that the hydrogen can be produced under optimum conditions and the maximum desired temperature is not exceeded. The electrolyte, which is demineralized water in the case of polymer electrolyte membrane electrolysis (PEM) or a potassium hydroxide-water mixture in the case of alkaline electrolysis (AEL), is cooled by another cooler medium. Maximum efficiency is achieved using heat exchangers with specially designed plates which are integrated into the circuit and ensure constant cooling of the process fluid. In this case the material of the plates is crucial. Corrosion should be prevented and the hydrogen quality and longevity of the electrolyzers optimized.
Overall energy cost can be reduced if waste heat from electrolysis can be fed into district heating networks or returned to the process elsewhere, such as in the aforementioned water desalination plant.
What type of heat exchanger should be used in which type of electrolyser?
1. Heat exchangers for proton exchange membrane (PEM) electrolyzers:
Stainless steel (Alloy 316) plate heat exchangers are used in PEM electrolysis to prevent hydrogen embrittlement of the plate material. Compact fully welded heat exchangers are suitable for smaller plants up to about one megawatt, while gasketed heat exchangers are used in larger plants.
Semi-welded and welded plate heat exchangers have a high pressure resistance and offer several advantages in hydrogen production. For example, electrolysers can be operated at a pressure of more than 30 bar. This is favourable for hydrogen storage as less compression work has to be done afterwards and the overall efficiency of the plant increases.
2. Heat exchangers for AEL electrolysers:
AEL electrolysis requires heat exchangers with plates made of corrosion-resistant material such as nickel that can withstand the potassium hydroxide-water mixture. Semi-welded plate heat exchangers are often used here, welded on the potassium hydroxide-carrying side to protect against leakage. Gaskets are used on the service medium side.
3. Heat exchangers for high-temperature electrolysers:
As a comparatively new technology, high-temperature electrolysis (HTE) is still under development. The process of hydrogen production takes place at about 800 °C and this high temperature requires gas-to-liquid heat exchangers (GTL). Heat recovery reduces the cost of HTE. Gas-to-liquid heat exchangers feature specially designed plates and an asymmetric channel volume, which minimizes pressure drop. Compared to shell-and-tube heat exchangers, they offer a higher thermal efficiency and require up to 75% less floor space.
How are plate heat exchangers used in distribution applications?
Gas cooling of hydrogen and oxygen is elementary for transportation of these gases, since hydrogen for example has a very low density of about 0.09 grams per litre at normal atmospheric pressure and temperature. By compressing and cooling the gas with the use of plate heat exchangers, the density can be increased and the transported mass correspondingly increased for a given transport volume. Welded stainless steel plate heat exchangers are most suitable for this purpose due to their pressure resistance.
Why do heat exchangers optimize hydrogen refueling
During vehicle refueling, hydrogen must be cooled to a temperature of around -40 °C to prevent the gas from overheating. This requires heat exchangers able to withstand hydrogen at extreme pressures such as 350 bar or 700 bar typically found in hydrogen refueling applications.
Printed circuit heat exchangers (PCHE), which owe their robustness to a special welding technology, are ideally suited for this. Hydrogen refueling stations that use such heat exchangers enable vehicles to be refueled without waiting between refueling operations (back-to-back refueling). Thanks to their extremely compact design, these heat exchangers can be easily integrated into the dispenser devices at low installation costs.
What is the use of plate heat exchangers in conjunction with fuel cells?
In fuel cells, hydrogen reacts with oxygen to produce electricity and heat, as well as the by-product water. During the conversion process, continuous cooling of the fuel cell stacks must be provided to ensure maximum hydrogen fuel cell efficiency. The electrical energy from the fuel cell electrodes can be used directly, while the waste heat generated in the form of water vapor can be tapped through a plate heat exchanger via the condensation process, e.g. for heating of buildings. Fusion-welded gas-to-liquid plate heat exchangers are mainly suitable for this purpose.
Traditional combined heat and power systems with internal combustion engines can thus be replaced in the future by new and eco-friendly fuel cell technology.
Content contributed by Alfa Laval
Alfa Laval is an international Swedish company active in the areas of energy, marine, and food & water, offering its expertise, products, and service to a wide range of industries in some 100 countries. With our broad portfolio of thermal and separation technologies we provide solutions for enabling the transition to green hydrogen. In electrolyser production (PEM, Alkaline, or SOEC), we offer efficient plate heat exchangers for cooling electrolytes, hydrogen and oxygen. Our plate heat exchanger portfolio also includes desalination technologies that allow you to use seawater or river water in your production, which is ideal if you operate an offshore plant. And our heat transfer solutions make it possible to recover and reuse waste heat from the electrolyser to desalinate water, or for other sustainable purposes, such as district heating. We are present in both production and storage, with unique solutions for compression and cooling, as well as innovative printed circuit heat exchangers for refueling stations. Alfa Laval has spent decades developing highly efficient and robust plate heat exchanger technologies. From our fusion bonding technique that enables reliable performance at very high temperatures, to gas-to-liquid solutions that can handle extremely uneven flow requirements, we can offer unique technologies perfect for fuel cell development.
Last update: 15.01.2023