- Why is compressed hydrogen preferred for storage compared to uncompressed hydrogen?
- Which type of compressor is suitable for hydrogen compression?
- What is the importance of H2 compression in hydrogen fueling stations?
- What are the challenges and precautions that should be considered when compressing gaseous hydrogen?
- What are the common issues associated with the operation of hydrogen compressors?
- What are the recent innovations in hydrogen compression technology?
- Which composition of material are preferable for H2 compression system
- What factors influence the efficiency of a hydrogen compression system?
- What are the material compatibility considerations for selecting materials in high-pressure hydrogen environments?
- Which type of filter suitable before H2 compression and what is the porous size of the filter?
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Why is compressed hydrogen preferred for storage compared to uncompressed hydrogen?
The hydrogen economy is a critical part of the trend toward the decarbonization of the energy sector. Major energy infrastructure changes are required to meet the production, transportation, storage, and usage needs for a functioning hydrogen economy. This includes the need to develop compressors that are different from the types currently used. The hydrogen to be a viable energy carrier, compressors must operate efficiently and reliably. Most importantly, they must be economically viable.
As hydrogen has a low energy density (0.089 kg/m3) 13 times less than that of air and is difficult to store in large quantities. Hydrogen needs to be compressed and to be stored in the most economical way. To densify this gas is to compress it by applying high levels of pressure (between 350 - 900 bar) for storage. The volume of hydrogen is much larger than that of other hydrocarbons - nearly four times as much as natural gas. For practical handling purposes, hydrogen needs to be compressed before storing it in the tanks.
In the current search for sources of alternative sources of fuel for use in mobility, instead of the traditionally used fossil fuels, hydrogen has positioned itself as an energy vector to help slow down climate change. Its use, however, poses some challenges in terms of storage. Hydrogen has a very low density (13 times less than that of air) and it occupies a great volume when at room temperature, making storing it somewhat complicated. The most economical way of densifying and storing it is by compressing it at a very high pressure (between 350 - 900 bar) before using it.
Which type of compressor is suitable for hydrogen compression?
Reciprocating and Diaphragm compressors (positive displacement type) are mostly used to compress hydrogen in industry and for transport purposes. Reciprocating and diaphragm compressors are currently used at hydrogen refueling mother stations to compress hydrogen from electrolysers (10s of bar) or tube trailers (250 – 500 bar), to refueling pressures up to 700 bar. The Ionic Compressor (IC90) uses a five-stage compression concept which meets the latest SAE J2601 fueling standard and allows for continuous, fast, and high- performance fueling of hydrogen vehicles at considerably reduced operating costs. Centrifugal compressors often have mechanical design and efficiency advantages over positive displacement compressors in high flow rate, <100 bar outlet pressure applications found in gas transmission networks. Centrifugal hydrogen compressors are comparatively less mature, as work is currently ongoing to design and build prototypes compatible with high flow rate applications.
Selection of compressors is based on the: -
- Volume
- Mass flow rate
- Inlet or suction pressure and temperature
- Outlet or discharge pressure and temperature
- Specific gravity of the gas to be compressed.
Reciprocating compressors: Low to moderate flow rates and high-pressure ratios. Used in Petro-chemical plants and oil refineries to compress the H2.
Diaphragm compressor: Low flow rates and pressure ratios and high purity and single stage compression system. Used at fueling stations.
Centrifugal compressors: High flow rates and moderate compression, centrifugal compressors will require impeller tip speeds around 3x speed higher for H2.
Ionic compressor: It is a high-performance solution that has an approved design and is easy to operate. It allows for quick, safe, highly efficient, and economical fueling of hydrogen vehicles at 35 or 70MPa.
What is the importance of H2 compression in hydrogen fueling stations?
Now, and for the foreseeable future, there are two main pressures that vehicles use for storage:350 bar and 700 bar. The 700 bar refueling applications require higher-pressure dispensing, because these vehicles only feature a finite space for fuel storage; the more gas you can put into the vehicles’ onboard storage system, the greater range the vehicle will have. 350 bar pressure is typically used for heavy duty vehicles like trucks and buses, and 700 bar for lighter vehicles.
The gas compression element of a refueling station is integral to allowing you to achieve the refueling pressures at those higher levels. The most efficient way to dispense hydrogen into a vehicle is by a method called cascading, which allows the gas to flow from a higher pressure to a lower pressure until it gets to equalization. The best way to achieve this is by taking the inlet gas pressure and compressing it to a high pressure to be held in the storage system, which ultimately feeds the dispenser to the vehicle. Refueling stations can store gas at a variety of pressures to allow for the most efficient use of this gas.
Within a standard hydrogen refueling station, there are three key elements: compression, followed by storage, and finally dispensing. Having the right combination of these three elements is critical to the efficiency of the station. Currently, the average hydrogen refueling station is designed to dispense around 400-500 kg a day. As hydrogen HGV trucks come online, that figure will increase quite significantly. We can see a trend in motion for stations capable of dispensing 1-2 tons a day. By 2030, many believe stations placed on the main transport corridors fueling the HGV (heavy goods vehicle) sector will be dispensing up to 6 tons a day. The challenge for industry will be to develop compression technology to meet the requirements of the larger refueling stations of the future.
What are the challenges and precautions that should be considered when compressing gaseous hydrogen?
Hydrogen compression has specific challenges.
- One of the problems is hydrogen embrittlement, where hydrogen can enter and weaken the compressor's material, making it more prone to cracking and failures. To address this, materials with lower susceptibility to embrittlement and adding stainless steel cladding or coatings can help.
- Another issue is sealing the compressor due to the small size of hydrogen molecules. They can easily escape through tiny gaps. Designing effective seals with special materials and techniques is necessary to prevent leakage. Managing hydrogen embrittlement and sealing challenges is important for safe and efficient hydrogen compression.
What are the common issues associated with the operation of hydrogen compressors?
One of the main issues with compressing hydrogen is the significant energy penalty associated with it. Compressing hydrogen to 35 MPa requires 14.5 MJ per kg, while compressing it to 70 MPa requires 18 MJ per kg. This means that about 15% of the energy in the hydrogen is spent on compression when stored at 70 MPa. Despite this, compressed hydrogen remains the preferred technology for the chemical industry and most commercial hydrogen fuel cell vehicles due to its good energy densities compared to other storage methods. Hydrogen is compressed to higher pressures (450 or 900 bar) than any other gas to achieve suitable density for storage. Additionally, because hydrogen is a very light molecule, it requires more energy per kilogram of gas to compress compared to other gases. Compressing hydrogen up to 1000 bar typically
requires around 2.6 kWh/kg for an ideal isothermal compress.
Compressors face various problems, including noise and vibration, pressure fluctuation, starting and stopping difficulties due to liquid presence, and accumulation of foreign substances causing biting and abrasion. These issues impact compressor efficiency, reliability, and performance. The hydrogen compressor requires high manufacturing accuracy and maintenance. It operates by the rapid rotation and reciprocation of the crankshaft, which drives the crosshead parts and ultimately compresses hydrogen. However, these moving parts are prone to wear over time, which can affect performance. Regular maintenance is necessary to ensure the safety and stability of the compressor.
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The drawback of current mechanical compressors, which use a variety of physical techniques, including the piston method, is their unpredictable durability and poor efficiency. Improvement is required since noise is produced and hydrogen can be contaminated, for example by mixing the lubricants used in the compression process.
What are the recent innovations in hydrogen compression technology?
A new hydrogen compressor developed at Southwest Research Institute (SwRI) can improve the efficiency and reliability of hydrogen compression used in the refueling of fuel cell electric vehicles (FCEVs). The SwRI-developed linear motor-driven reciprocating compressor (LMRC) is designed to compress hydrogen as a fuel source for FCEVs and other hydrogen-powered vehicles. Unlike most hydrogen compressors, SwRI’s LMRC is hermetically sealed and has a linear motor design that increases its efficiency and reliability. “The LMRC was built and designed to compress hydrogen for refueling vehicles with hydrogen fuel cells”. “To refuel hydrogen vehicles, the gas must be compressed to high pressures first. So, we set out to design a more efficient, leak-proof compressor. “A key challenge for hydrogen compression is hydrogen’s is - small particle size, which increases the potential for leaks as the gas flows through equipment. The LMRC is an improvement over conventional reciprocating compressors as it minimizes the mechanical part count, reduces leakage paths, and is easily modularized for simple field installation. As mentioned previously, the isentropic efficiency is improved with the LMRC design and mechanical losses are reduced by reducing secondary systems. This results in an increase in overall efficiency for the system. It can compress the gas from 20 bar to 875 bar.
The Electrochemical Hydrogen Compressor (EHC) is a new system that can produce compressed purified H2 out of a complex mixture of gases, which includes no moving parts and mechanical losses, and operates isothermally. German research institute ZBT (Zentrum für BrennstoffzellenTechnik, GmbH) and gas equipment manufacturer Theisen are developing and qualifying a new electrochemical compressor system to increase efficiency and lower the costs of hydrogen storage, transport, and delivery. An Electrochemical Hydrogen Compressor (EHC) consists of an electrochemical cell with an anode of a PEM fuel cell, a Membrane Electrode Assembly (MEA) and a cathode of a PEM electrolyser. Compression is by means of isothermal compression, promising greater efficiency. With no moving parts, lower maintenance costs and higher availability as well as significantly lower noise emissions are also expected. Within the scope of this project, Theisen, with the support of ZBT, will develop, build, and commission a container-based compressor system with a delivery capacity of 10 kg/d of hydrogen and realizable output pressures of up to 400 bar based on HyET’s cell technology. Certification of the system is being prepared.
Which composition of material are preferable for H2 compression system?
For low pressures <25Mpa the material we use in metal hydride compressors are AB5 type rare earth hydrogen storage alloy materials and for high pressures <25Mpa- 80Mpa hydrogen output AB2 type titanium base hydrogen storage alloys. For hydrogen compression, the compressor material also must withstand Hydrogen Embrittlement, or the ingress of hydrogen into a component. This can reduce the ductility and load-bearing capacity of the compressor, and cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Finally, difficulties arise in static and dynamic sealing because of the size of the gas molecules. Hydrogen molecules are small and light, which allows them to escape and leak through gaps most other process gases cannot. The risk of hydrogen embrittlement should be managed by restricting the yield strength of the materials used and by adding stainless-steel cladding or coatings.
What factors influence the efficiency of a hydrogen compression system?
EFFICIENCY: The efficiency of a compressor for hydrogen refers to the amount of energy that is required to compress the hydrogen gas. It is important to select a compressor with high efficiency that minimises the energy consumption and costs. Hydrogen compressor efficiency is typically measured by dividing the output power of the compressor by the input power of the compressor. This ratio is expressed as a percentage with a higher percentage indicating the higher efficiency. There are the several factors that affect the efficiency of a compressor for hydrogen, including the design of a compressor, operating conditions, and the quality of the hydrogen gas. It is important to carefully consider these factors when selecting and operating a hydrogen compressor to maximize efficiency.
What are the material compatibility considerations for selecting materials in high-pressure hydrogen environments?
The mechanical properties of materials can be affected by hydrogen exposure. Factors like material type, environmental conditions and mechanical load determine the extent of deterioration. Material selection and design should consider the decline in mechanical properties due to hydrogen exposure. Metals exposed to hydrogen can become brittle, resulting in reduced strength, ductility, toughness, and accelerated fatigue crack growth. Fracture mechanics is commonly used to analyze pressure-containing components for acceptable mechanical loading despite hydrogen-induced deterioration. Materials commonly used in hydrogen gas service are:
- Austenitic stainless steels,
- Aluminum alloys,
- Low-Alloy ferritic steels,
- C-Mn Ferritic steels, and
- Copper alloys.
Materials commonly avoided in hydrogen service are:
- High strength ferritic steels
- High strength martensitic steels
- Cast irons (including gray, malleable, and ductile)
- Nickel alloys.
- Titanium alloys.
Which type of filter suitable before H2 compression and what is the porous size of the filter?
High-pressure hydrogen filters help protect hydrogen and the components of your system against contamination that could cause damage, extending the working life of your system. Hydrogen production requires that you separate it from the molecules where it occurs.
- The Chase Filters 51 Series Tee Type filter can be found in Aluminum, 303 Stainless Steel, and 316 Stainless Steel. It has the capability to handle flow rates of up to 50 GPM or 4,000 SCFM, with pressures that can reach up to 6,000 PSI. This filter offers a variety of filter medias, such as cellulose, stainless steel, micro glass, and porous sintered stainless steel. With a filtration range from 1 to 1,000 micron, it is an incredibly versatile filter suitable for a wide range of applications. The port sizes available for this filter range from 1/4" to 1-1/2" NPT and 1/2" to 1-1/2" FSAE.
- The filters offer the 52N Series T-Type filter, another hydrogen filtration option. It is available in 303, 316 Stainless and 17-4 PH Stainless. Flow rates up to 24 GPM or 11.5m3/hr. Like the 51 Series, you can choose from various micron ratings. Various Micron Ratings Ranging from 1 to 150.we can also select the media filter as micro-glass, porous sintered stainless steel, fiber metal felt or typical stainless steel for your elements. The 52N Series is ideal for removing contaminants from hydrogen. Port Sizes Range from 1/4” to 3/4” FNPT and 1/4” to 3/4” MP Port and HP Ports Both 51 or 52N Series filters can remove contaminants from hydrogen and protect your elements and extend the life of your system.