There are a wide variety of hydrogen compressor and pump technologies in use today. Types of compressors include diaphragm, reciprocating piston, and centrifugal. Pumps for hydrogen applications consist of various kinds of positive displacement systems. Each compressor and pump system operates using different methods, and each is used for specific reasons and for specific markets.
To introduce readers to this topic, we asked several hydrogen compressor and pump manufacturers to describe the systems used in hydrogen applications today.
Hydrogen is the simplest and most abundant element in the universe. (See CGI, Feb. 2008, p. 52.) It is an efficient, non-polluting, renewable fuel. Emerging hydrogen technologies require the storage and the use of hydrogen at high pressures.
Compressors are used to increase the pressure of gaseous hydrogen (H2). In general, however, liquids are not deemed compressible. Pumps therefore are used to increase the pressure of liquid hydrogen (LH2) at the use point by providing a constant flow. Restrictions and final use back pressure cause the pressure increase. LH2 at high pressure is then converted to vapor as it passes through a vaporizer, and is used or stored at the elevated pressure. Gaseous compressors and liquid pumps are both used in hydrogen applications.
Although the basic principles of compressing and pumping are generic to most gases and liquids, there are unique differences and requirements, including safety, when dealing with hydrogen. One of the biggest challenges to using hyrogen is its safe containment, due to its low molecular mass.
The three basic types of compressors — diaphragm, reciprocating piston, and centrifugal (also referred to as radial) — have different characteristics that make them appropriate for use in different settings.
“Diaphragm compressors,” advises Osama Al-Qasem with Pdc Machines Inc., “are a good choice for compressing gases without incurring contamination of the process media or leakage of gas to ambient air.” H2 is isolated from the mechanical parts of the compressor and from the environment by a set of three metallic diaphragms. These are clamped between two precisely contoured concave cavities in upper and lower plates. The three diaphragms are nested and act together as one. The top diaphragm is in contact with the H2 and the bottom is in contact with the hydraulic oil. A three-diaphragm set is used to ensure there is no cross-contamination between the hydraulic oil and the H2 being compressed. The center diaphragm, used for leak detection, has lines scribed on both sides. If a leak develops in the upper or lower diaphragm, or if the O-rings wear, the media will seep along the scribe lines into an accumulator. When an accumulator pressure reaches a set limit, the compressor will automatically stop. “As static seals are used,” advises Al-Qasem, “there is no leakage of gases to the environment, and no need to purge or vent the crankcase.”
A motor-driven crankshaft connected to a piston moves a column of hydraulic fluid up and down. Compression occurs as the hydraulic fluid is pushed upward to fill the lower oil-plate cavity, exerting a uniform force against the bottom of the diaphragm set, deflecting it into the H2-filled gas-plate cavity above. The displacement of the diaphragm against the gas-plate cavity compresses the H2, pushing it out the discharge check valve. As the piston, which moves the hydraulic fluid, strokes downward, the diaphragm is drawn back down into the lower cavity, the inlet check valve opens, and the upper cavity fills with H2. The cycle is repeated.
The main advantage to diaphragm compressors is there is not the concern for leakage as with other compressors or pumps.
Stephen St. Martin of Gas & Air Systems, Inc. reports that “Diaphragm compressors are used to compress H2 in cylinder trans-filling and tube trailer offloading operations, and for gas recovery from the vapor space of cryogenic storage vessels. Due to its high pressure capability, and inherently oil-free compression, the diaphragm compressor is also commonly used in vehicle hydrogen fueling stations, where pressures of 10,000 psi and above are currently used.”
The hydrogen fuel cell requires ultra-purity H2 to function properly. “The diaphragm compressors,” according to Osama Al-Qasem, “are designed to provide exactly this feature. Hundreds of diaphragm compressors have been installed globally as part of the renewable energy program to find alternative sources for oil.” Al-Qasem states that 85 to 90 percent of this market needing diaphragm compressors were supplied by Pdc.
Diaphragm compressors are great for high pressure applications. It is not surprising that diaphragm compressors are ideal for hydrogen applications, especially in the development of the emerging hydrogen economy. Osama Al-Qasem pointed out one unique and interesting emerging “double-green” technology. Pdc has compressors being used in conjunction with wind turbines. The electrical power from wind turbines can be used to supply a water electrolyzer to electrochemically split water into its elements, hydrogen and oxygen. One feature that makes the marriage of these two technologies such an interesting match is that the electrolyzer can operate with variable power input, as windmills turn at varying speeds, according to the wind. Hydrogen thus produced is compressed and stored for later use, either in a stationary fuel cell to produce electricity when there is no wind, or to supply a hydrogen vehicle.
Hydrogen compressor applications are many. They include utilizing solar energy to electrolyze water to produce H2, which, like the windmill application, is then compressed and stored for later use, either in a stationary fuel cell to produce electricity when there is no sunlight, or fuel a hydrogen vehicle. Compressors are used at hydrogen fuel cell stations, including those for vehicles, buses, fork-lifts, scooters, and residential re-fuelers for fuel cell (FC) cars; for filling and off-loading H2 from tube trailers, gas cylinders, and storage tanks; for the compression of syngas from renewable sources; and for wind and solar power. H2 compressors are used in such disparate applications as gas blending, recycling, and mixing, metal processing, hydrogenation of edible oils, specialty gas purification, float glass production, and power plant turbine cooling. Gases for semiconductor, electronics and fiber optics manufacturing need compressors. They are also used for feedstock for chemical, petrochemical and pharmaceutical industries, pressure boosting and storage of gases from on-site generation systems, and for power back-up using hydrogen FC for telecommunication towers, as well as research and development.
Multi-stage Reciprocating Piston Compressors
Multi-stage reciprocating piston compressors are commonly used for compression of H2 gas. Piston compressors work on a simple theory. Rick Turnquist, with RIX Industries instructs, “The piston in a large cylinder pushes a fixed amount of gas into a smaller cylinder, thereby causing a pressure increase. This is based on the ideal gas law, which in abbreviated form is: PV=nT (pressure x volume = Moles of gas x temperature). Thus as volume goes down, pressure goes up (note after the final stage the pressure increase is forced by the back pressure in the user’s tank or piping).”
Turnquist goes on to explain that “H2 compressors are similar to those used to compress other gases; however there are sometimes design differences due to the very small molecular size of the H2. These may be: a special valve design; special piston ring materials; overlapping piston ring design to minimize leakage; lower compression ratios; or cylinder and head castings may need to be impregnated to stop leaks caused by casting porosity. Additionally, the grade of steel used in the compression end components may need to be changed.”
Hydrogen, like all gases, is heated by compression. “Intercooling” of the gas is required when using multi-stage high pressure compressors.
The largest end-users of multi-stage reciprocating piston compressors are refineries and chemical plants. Customers include such companies as Air Products, Praxair, and Chevron Research. These compressors are also used for some refueling applications, for syngas, pilot plants, and laboratory R&D.
Centrifugal compressors are seldom used for hydrogen applications due to the molecule’s low molecular weight. However, centrifugal compressors are used in cryogenic H2 applications where flow is relatively high and the pressure head desired is relatively low. Barbers Nichols Inc. (BNI), designer and manufacturer of specialty turbo-machinery, has made cryogenic H2 centrifugal compressors for two applications. These two applications involve sub-cooling H2 by drawing down liquid boil-off gas pressure below atmospheric. Jeff Shull, with Barber Nichols, explains that “this creates a more dense liquid that can then be used in a rocket more efficiently (takes up less space and decreases overall weight). BNI used four separate centrifugal stages (four single stage machines each with a motor) to draw down the pressure to approximately 3 psia with an atmospheric pressure outlet and high flows for a propellant densification test at NASA. BNI’s H2 cryogenic compressors utilize a motor and bearings operating at room temperature with an overhung impeller on a hollow shaft to minimize heat input to the fluid. No dynamic seals are used so designs are hermetic. BNI has also supplied several H2 circulators in supercritical applications (supercritical H2 is more like a liquid than a gas, however) for cryogenic cooling.”
Pumps used in conjunction with LH2 are liquid positive displacement pumps. These are reciprocal pumps that have varying cylinder sizes, which are used alone or in conjunction with one another.
Liquid Piston Positive Displacement Pumps
In layman’s physics, liquids are deemed incompressible. When using a liquid pump, pressure develops not from compression of the liquid, but from restrictions in flow path of the constant volume of liquid the pump is pushing. For example, when you hold your thumb over the end of your garden hose, if the flow rate is constant, the flow is restricted, causing the same exiting volume of water to increase in pressure as it flows faster to exit the restricted end of the garden hose. Compression can develop from friction against the inside wall of the pipe, the length of the pipe, or any elbows and turns; from elevation changes to the flowpath; from constriction of the flow path (e.g. small diameter pipe); and from back pressure created by the end use application.
A reciprocating pump is one type of a positive displacement pump (PDP). A PDP draws in a fixed volume of LH2 from the inlet pressure section of the pump, and then forces that volume out the discharge port.
A reciprocating PDP employs a piston and cylinder arrangement, connected to a crankshaft, and is driven with an electric motor or a hydraulic pump and motor. Suction and discharge valves are integrated within the pump. The crankshaft end of the pump is referred to as the “warm end,” while the vacuum jacket insulated cylinder/piston end of the pump is called the “cold end.”
As the crankshaft rotates it pulls the piston downward on the intake stroke, drawing in a fixed volume of LH2 into the cylinder through the suction valve. The LH2 is then forced out of the pump through the discharge valve as the piston rises on the discharge stroke. As the diameter of the cylinder is much larger than the cross-sectional area of the discharge valve, a high pressure condition is created. The pressure is further increased by back pressure created by the end-use application. The flow rate stays the same (given sufficient horsepower).
Reciprocating H2 piston pumps may be a single cylinder or “simplex” pump, or “duplex” (two), or “triplex” (three). The multiple cylinders are not stages. They do not progressively increase the final use pressure. Rather multiple pumps work in parallel, increasing the flow rate, and all maintain approximately the same pressure.
Common uses for PDP reciprocating piston pumps include filling H2 tube trailers and ground storage vessels, food oil hydrogenation, and others. Arturo Martinez with ACD reports that “recently we have seen a growing number of requests for fueling applications that pump LH2 to high pressure, vaporize it, and then deliver it to vehicles.”
COMPRESSOR VERSUS PUMP
Given the end-use application, there are important differences to note when choosing an H2 compressor or LH2 pump (including the specific type of each). Energy efficiency, leakage, and product purity are potential concerns when selecting the means to increase gas pressure. Ed Huckestein, with Pittsburgh Cryogenics (ACD) advises that “in general the mechanical advantage of a liquid positive displacement pump (PDP) over a compressor is that the pump is more efficient, given the energy required per molecule to increase the same resulting volume to the same pressure.”
Sam Zouaghi with Cryostar France concurs with Huckestein, stating, “Compressors require more energy than pumps to bring a medium [H2] to required pressures. Pumps don’t compress, they use less energy and are more efficient when bringing a medium to pressure.”
However, depending upon such factors as leakage, safety, and product purity, a compressor technology may better fit the application. Osama Al-Qasem, with Pdc Machines Inc., cautions that “contamination of the process media or leakage of gas to ambient air can result from reciprocating and other types of compressors or pumps. Traditional reciprocating compressors are subject to leakage past the piston rings, which can result in contamination of the process gas with the hydraulic oil.”
Other important factors to consider are required performance, type of media, required duty cycle, initial cost, operational costs, reliability, maintenance frequency, and other costs.