What Is a Hydrogen Compressor?
A hydrogen compressor is a device that compresses hydrogen gas.
Hydrogen compressors are used to compress hydrogen gas under high pressure or to liquefy hydrogen by cooling it. Compared to gas, liquefied hydrogen is much smaller in volume, which is advantageous for storage and transportation.
Hydrogen compressors come in a variety of forms, including mechanical types such as positive displacement and centrifugal types, and methods that use electrochemical reactions.
A familiar example of a hydrogen gas application is fuel cells. They are gradually becoming popular for use in automobiles, household power generation, and water heating. Hydrogen energy does not produce carbon dioxide gas and emits only water, making it an excellent energy source in this environmentally conscious age. A great leap forward is expected in the future.
Applications of Hydrogen Compressors
1. Storage and transportation of hydrogen
Hydrogen gas is an excellent method of storage and transportation because its volume is reduced to 1/800 when liquefied. Hydrogen can be liquefied by a hydrogen compressor at a hydrogen production plant and stored in a storage tank. It can also be loaded into transportation tanks on ships and tanks of trailer trucks for transportation to the destination.
2. For fuel cells
Hydrogen is supplied to automotive fuel cells by installing a hydrogen compressor at a hydrogen gas station, increasing the pressure to the required level, and filling the vehicle’s hydrogen tank. In Japan, 70 MPa is used. Fuel cells for home use are cogeneration systems and are used to generate electricity as well as to heat water. In addition, generators using fuel cells are also in practical use.
3. Other Applications
Liquefied hydrogen is used as a raw material in chemical plants, pharmaceutical plants, fertilizer production, etc., as fuel for power plants, engine compressors, etc.
Principles of Hydrogen Compressor
Hydrogen gas is the element with the lowest density, and various methods must be used to increase pressure. Mechanical compressors, such as volumetric and centrifugal types, as well as electrochemical and thermal types, are used.
Mechanical compressors have a high-pressure ratio, so most are multi-stage compressors. They also require high temperatures, so cooling is also important. The maximum discharge pressure is about 100 MPa. Electrochemical reaction type and thermal type have the features of no moving parts, no vibration, no noise, and oil-free.
Types of Hydrogen Compressors
The types of hydrogen compressors used include positive displacement type, centrifugal type, electrochemical reaction type, and thermal type.
1. Positive Displacement Compressors
The positive displacement compressor changes the volume to inhale, compress, and discharge gas. The types of hydrogen compressors in practical use include reciprocating compressors, screw compressors, and diaphragm compressors.
The reciprocating type compresses gas through the reciprocating motion of a piston. Various types are available depending on the arrangement of the cylinders. The swash plate type, in which the piston reciprocates in the axial direction, is also of this type. The screw type compresses gas by rotating a two-axis or single-axis screw to reduce the volume. Diaphragm type compressors compress by reciprocating diaphragms.
2. Centrifugal Compressor
Centrifugal compressors convert the velocity energy of the gas into pressure by rotating an impeller at high speed. Hydrogen gas is the gas with the smallest density, and increasing the pressure requires increasing the impeller speed. Centrifugal hydrogen compressors are in practical use in the U.S., and in Japan, ultra high-speed single-shaft multi-stage and multi-shaft multi-stage compressors are under development.
3. Electrochemical Reaction Type Compressor
The electrochemical reaction type is not a mechanical compressor, but a method of obtaining compressed hydrogen through a chemical reaction. When voltage is applied to the electrodes, hydrogen ions are reduced to hydrogen and hydrogen is extracted from the cathode. This method is now in practical use in Germany and the U.S., and Japan is likely to expand it in the future.
4. Thermal Compressor
This method boosts pressure by applying heat to hydrogen gas. When hydrogen gas is stored in a hydrogen storage alloy, heat is generated, and metal hydride is formed. Then, when heat energy is applied to the metal hydride, hydrogen is separated, and high-pressure gas is obtained. The output pressure is about 25 MPa. Metal hydride is a stable substance and very safe. Heating is an environmentally friendly method since waste heat can be used. In Scandinavia, it is deployed in hydrogen stations.
Other Information on Hydrogen Compressors
1. Lubrication system of Hydrogen Compressors
Three types of lubrication systems are used.
- Lubrication type
This method injects oil into the sliding and sealing parts of the cylinder and screw to lubricate, seal, and cool. There are methods such as splashing, oil pumping, and pressure lubrication. Oil mist is contained in the discharged gas.
- Oil-free type
This method does not use any oil or water in the gas passage. No oil or water is contained in the discharged gas.
- Water circulation type
Water is injected into the screw to lubricate, cool, and seal. The oil rate of the discharged gas is zero.
2. Advantages of oil-free type
Oil-free hydrogen gas has many advantages. Many industries require pure hydrogen. These include the mobility sector, power generation/storage, chemical/petrochemical, fertilizer, and refining. The advantage of oil-free is that it does not affect the chemical reactions used in its production. It is also important that it does not affect the process or equipment.