A hydrogen compressor is a device designed specifically to compress hydrogen gas for storage. Hydrogen's low density makes it a challenging medium to store in that it requires extremely high tank pressures of between 5,000 and 10,000 pounds per square inch (PSI). Commercial hydrogen compressors are generally reciprocating types with the most common being guided piston/metal diaphragm and rotor variants. Although these designs are all basically positive displacement pumps, there are several other hydrogen compressors such as the hydride and electrochemical types which have no moving parts at all. Most hydrogen compressor designs are, however, characterized by their high output pressure capabilities with average delivery pressures in excess of 15,000 PSI.
Hydrogen has long been a fuel component for rocket engines and has recently been the subject of intense scientific scrutiny as a potential large scale alternative to fossil fuels. All of these applications require that the hydrogen be stored under high pressure, typically in the range of 5,000 to 10,000 PSI. This is due, in part, to the low density of hydrogen gas at atmospheric pressure. At these low pressures, a gram of hydrogen gas has a volume of almost 3 gallons (11 liters), which requires the gas to be intensely compressed if it is to be viably stored. Hydrogen compressor units are used to compress the gas to enormous pressures, often in excess of 15,000 PSI, so that it may be stored in usable amounts.
Several different types of compressor are used to compress hydrogen; among the more common are the piston or metal diaphragm types. These compressors are reciprocating positive displacement type machines which function by trapping a small quantity of the gas in an enclosed space, lowering its volume by displacement, then releasing the compressed gas into a holding or storage tank. This cycle is repeated continuously until the tank reaches its maximum capacity. One type of piston compressor utilizes three pistons to achieve this; a motive piston driven by compressed air or hydraulic fluid and a pair of compressor pistons which compress the gas. The compressor pistons are attached to the motive piston via a pair of connecting rods which provide the necessary linked motion.
Another common hydrogen compressor type is the guided rotor compressor. This type of hydrogen compressor utilizes a lobed rotor mounted on an eccentric shaft that rotates in a specially designed chamber. Similar in principle to rotary internal combustion engines, this type of compressor relies on the displacement of gas within the chamber by the rotor lobes to achieve its compression values. There are several low volume compressor types with no moving parts such as the hydride compressor used to produce small amounts of pressurized hydrogen gas for laboratory use. These compressors use the thermal reaction properties of heated hydride to produce small amounts of pressurized hydrogen.
Most industrial hydrogen storage facilities use positive displacement pumps to pressurize their hydrogen stores. In some cases, these compressors are capable of producing outputs in excess of 15,000 PSI and with some delivering up to 30,000 PSI. These values mean that the average tank pressure of 10,000 PSI used by most green car research facilities can be quickly and efficiently achieved.