The energy transition involves numerous challenges that are also affecting the area of mobility. For NEUMAN & ESSER, this means that one application is moving even more into focus: hydrogen refueling stations. Hydrogen refueling stations are used to refuel vehicles that are powered by a fuel cell. The fuel cell requires compressed hydrogen, which is free of any oil in the gas. Thus, any kind of lubricated piston compressor application is not an option.
A diaphragm compressor, in contrast, is perfectly suited for compressing hydrogen for refueling stations. As they both are recips, the principle of the diaphragm compressor is very similar to that of the classic piston compressor. Unlike the piston compressor, however, the gas is not compressed and conveyed directly by the piston. In the diaphragm compressor, the piston moves oil, which then separately pushes out the gas through a steel diaphragm. This construction principle ensures that the gas cannot be contaminated with oil under any circumstances whatsoever. Uncontaminated compression is a prerequisite for the hydrogen filling stations where the compressors are used to fuel cars and trucks with hydrogen. Unlike, for example, a piston compressor, only static and no dynamic seals are required via piston rings and packings. Therefore, this design principle is very well suited to meet these requirements of an HRS.
At the present frequency of refueling, however, a disadvantage becomes apparent when using the diaphragm compressor. In intermittent operation, the diaphragm of the diaphragm compressor has a limited working life before it has to be replaced at relatively high cost.
The compression principle, that is also used on account of the time-consuming replacement of the diaphragm, is that of the hydraulically driven piston compressor. The compression principles for hydraulically driven piston compressors and classical recips are similar. However, the piston drive is not made via a crank drive for the hydraulically driven compressor, but via hydraulic oil which drives a hydraulic piston. Therefore, the power is actually transferred hydraulically as opposed to mechanically. The very low hydraulically driven piston compressor’s double stroke rate keeps the average piston speeds very low. Thus, the 1000 bar required for hydrogen compression can be achieved with reasonable wear time for the piston sealing elements.
Normally, immobilization of up to six months is possible if the refueling station compressor is used accordingly little. The hydraulically driven piston compressor’s sealing elements are extremely easy to replace. Only four lid screws need to be loosened, the entire piston removed, the piston sealing elements replaced and the screws fixed back in place. The same is true for replacing the valves. Full inspection of a hydraulically driven piston compressor takes a maximum of 30 minutes until all of its wear parts have been replaced.
The very low number of double strokes or rotary frequency of the hydraulically driven piston compressor means that no very extreme quantities can be conveyed. Due to the small design quantity and the so far low number of vehicles with this drive, this poses no problems with the current global dimensioning of the compressors. According to the opinion of NEUMAN & ESSER, there will be more similar fuel cell-driven vehicles in the future, which is why the diaphragm compressor will again enjoy stronger demand. As soon as the diaphragm compressor can be operated continuously as a result of more fuel cell-driven cars, the diaphragm compressor is extremely suitable for this application. The diaphragm’s very long service life limits servicing work merely to replacing the valves, which can be just as easily replaced as for the hydraulically driven.
Considerably increasing numbers of fuel cell-driven vehicles mean, however, that a larger quantity of hydrogen has to be compressed and this cannot be achieved by the diaphragm compressor alone. Therefore, so-called hybrid solutions will be drawn on more frequently. Here, the low-pressure stage, a classical piston compressor stage with which a larger amount can be compressed because the final compression pressure will still be within the range which also poses the dry running piston compressor no problems. The combination of a classical piston compressor stage and the final diaphragm compressor stage is ideal for the oil-free conveyance of the gas whilst avoiding having to change to two completely different compressors.