COMPRESSOR AND APPARATUS FOR COMPRESSING TWO GASES AT HIGH PRESSURE

FIELD OF THE INVENTION

The invention relates to the field of reciprocating piston gas compressors. It relates to a compressor and an apparatus for providing two gases at high pressure, and a corresponding method, as described in the preamble of the corresponding independent claims.

BACKGROUND OF THE INVENTION

US 4,368,008 discloses a double acting hydraulically driven gas compressor using multi-stage compression. The description includes a four-way two-position valve controlling the reciprocal motion of the hydraulic stage.

US 5,238,372 shows a piston assembly being cooled by hydraulic fluid flowing through dedicated passageways in the piston.

WO 2005/019721 describes a hydrogen generator, such as an electrolytic cell, supplying hydrogen to a hydrogen compressor which in turn pressurises the

hydrogen for storage. The oxygen produced by electrolysis may also be pressurised by means of a further compressor, regarding which no details are given.

US 4,368,008 discloses a dual stage compressor for two different gases, to be used in cooperation with other elements in, for example, a refrigeration cycle. The two gases are nitrogen and hydrogen flowing in a closed circuit through the compressor and a cryostat. In order to prevent leakage, the compressor elements are of the diaphragm type. The diaphragm elements are actuated by means of a actuating fluid which in turn is driven by a common piston.

DESCRIPTION OF THE INVENTION

It is an object of the invention to create a compressor and an apparatus for providing two gases at high pressure of the type mentioned initially, which are of a simple construction and allow for efficient operation.

These objects are achieved by a compressor and an apparatus for providing two gases at high pressure according to the independent claims.

The compressor is a piston compressor comprising at least one first compression stage for compressing a first gas, the first compression stage comprising at least a first piston sliding in a first compression chamber. The compressor further comprises at least one second compression stage for compressing a second gas, the second compression stage comprising at least a second piston sliding in a second compression chamber. The second and the first piston are driven by the same mechanical actuator. The first and second gas are, within the compressor, kept separate from one another.

More specifically, the second and the first piston are mechanically coupled to the actuator to transmit motion and forces from the mechanical actuator to the pistons. In operation of the compressor, the second gas is preferably of a different composition than the first gas.

This allows to compress two gases simultaneously in a single, compact compressor having a single mechanical power source. This is in particular advantageous when the compressor is coupled to a unit generating different gases from a chemical or electrochemical reaction such as an electrolysis cell generating hydrogen and oxygen from water: the two gases are compressed in separate compression stages of a single compressor. Having only one mechanical actuator and a linear movement of co-axial components along a single axis allows for efficient operation. Only one control unit is required. The simple construction and the resulting reduction in size allows for the use in the domestic area and for example for mobility applications.

Thus, the apparatus for providing two gases at high pressure comprises a gas source unit, such as one or more electrolysis cells, for generating the two gases at a first pressure. This gas source unit thus constitutes both a first gas source and a second gas source. The compressor is arranged to compress the two gases, elevating the pressure from the first pressure to the high pressure. The invention is applicable to other gas sources which generate a pair of gases as well.

In a preferred embodiment of the invention, the first pressure, e.g. as generated in the electrolysis cell itself, is in the range of 10 or 20 bar to 40 or 50 bar, preferably around 30 bar. The compression ratio of the compressor preferably is in the range of 6 or 8 to 12, preferably 10 - 12 to 15 in claim 20. The compression ratio of the two gases is preferably at least approximately the same. The resulting high pressure thus lies around 300 bar, at which the two gases can be stored in separate pressure vessels for later use. For other uses of the invention it is possible to increase the outlet pressure for one or both gases up to 700 bar or 1000 bar.

In a further preferred embodiment of the invention, the ratio of volumes of the first and second compression stages is the same as the stoichiometric volume ratio of the gases produced by a separation process from which the gases originate. In other words, if the separation process produces the gases at a volume ratio of 1 :n (the gases being at the same temperature and pressure), then the volume ratio of the corresponding separate compression stages for the two gases is also 1 :n. It follows, when the operative connection, in particular a mechanical linkage, between the first and second compression stage is such that the stroke of the pistons in the two compression stages is the same, that the cross section areas of the pistons for the two compression stages also have the ratio 1 :n. For example, electrolysis of water gives hydrogen and oxygen (2H ₂ O + 2H ₂ + O ₂ ). For the combined pressurization of hydrogen and oxygen generated by electrolysis, the volume ratio of the hydrogen compression chamber to the oxygen compression chamber therefore is preferably two to one.

Since the two gases may be generated by separating the constituents of one molecule which may recombine in an exothermic reaction, they should remain separated throughout the compressor. In particular, even the mixture of minuscule quantities should be avoided. This is particularly relevant since the two compression stages share the same mechanical actuator, and gases escaping trough the piston seals might mix in or around the mechanical actuator, forming an explosive mixture. Also, one of the gases may form an explosive mixture with the ambient air. For example, H ₂ and O ₂ issued from water electrolysis form an explosive mixture but H ₂ alone will also form an explosive mixture with the ambient air, and so the containment of H ₂ alone is a safety concern as well.

Therefore, in order to detect the leakage of one gas or the other towards the common actuator, the following arrangement is preferably implemented: For at least one of the compression chambers, an inner seal is arranged in the annular gap between said

compression chamber and the piston operating in said compression chamber, in order to prevent gas from escaping from said compression chamber. An outer seal is arranged in a further location along said gap. The inner seal and the outer seal define a security chamber, said security chamber being an enclosed section of said gap between the inner and the outer seal. The security chamber thus separates the respective compression chamber from the ambient air or from the hydraulic medium or from the other compression chamber.

In a preferred embodiment of the invention, a pressure sensor is arranged to provide a signal indicative of the pressure in the security chamber. This pressure sensor and the subsequent processing of the signal serve to provide an alarm and/or to shut down operation of the compressor in case of an excessive pressure or pressure gradient in the security chamber is detected. The magnitude of the pressure gradients that can be detected in this fashion is correlated with the rate at which the gas may escape from the chamber, e.g. by means of a vent.

With a sufficiently low escape rate, the pressure sensor can detect a deterioration of the seal quality, that is, its tightness. Such a security chamber may be implemented in other compressor types as well, regardless of whether two gases or only one gas is compressed, and how many stages the compressor has.

In addition to the pressure sensor, or alternatively, the security chamber may also be provided with one or more gas sensors that are sensitive to the presence of the gas that may leak in from the adjacent pressure chamber. This provides a further means for detecting leakage of the seals, and even a gradual deterioration of seal quality.

Thus, both the pressure sensor and the gas sensors serve as sensors that are indicative of the presence, in the security chamber, of the gas being compressed.

The inner seal and the outer seal may themselves comprise one or more seals with intermediate chambers. For example, the complete sequence of seals and chambers may be: a first seal, a first chamber, a second seal, the security chamber, a third seal, a second chamber, and a fourth seal. In this example, the first seal, first chamber, and second seal are considered to be the inner seal. The third seal, second chamber and fourth seal are considered to be the outer seal. The first and second chamber are called "sealing chambers".

Alternatively, the first and/or second chamber may comprise a sensor (gas and/or pressure), and will then be called "security chamber", whereas the other chambers may just serve as sealing chambers. In further preferred embodiments, more than one of the chambers comprises a gas and/or a pressure sensor.

In a further preferred embodiment of the invention, a vent connects the security chamber to a further volume, in particular the ambient air or a ventilation system, and allows a predefined outflow of gas from the security chamber. For example, the vent is a small diameter pinhole or may be adjustable, e.g. by a needle valve.

Alternatively, the vent may be a permeable membrane. In both cases, the amount of gas vented by the vent is minuscule but serves to prevent a pressure build up by the allowable leakage of the compression chamber through the inner seal due to normal diffusion. Larger leakage - more than can be vented - leads to a pressure buildup that is detected.

In another preferred embodiment of the invention, the security chamber is filled with a further, inert fluid such as nitrogen gas or purified water. The pressure of this further fluid is kept slightly higher than the compressor's outlet pressure. This serves to prevent any leakage of the gases under compression into the security chamber and to other places where they could potentially form an explosive mixture. The source of this higher-pressure fluid can be, for example, a small tank of commercially- available highly compressed gas. In this embodiment, the security chamber may also

comprise a pressure sensor, for detecting a drop in pressure in the security chamber, which indicates a deterioration of at least one of the seals.

In further preferred embodiments of the invention, the compressor comprises a hydraulic stage with a hydraulic piston arranged to drive the first piston and the second piston. The hydraulic medium powering the hydraulic piston is preferably water. This is advantageous for small-scale domestic applications of the compressor, and/or when the hydraulic piston is powered by water provided from a water mains line, the pressure of the mains line being maintained by a remote pump or elevated water storage operated by the water utility. The hydraulic medium (water) also serves to cool the compressor, which keeps the gas temperature increase small and thereby allows the high compression rates mentioned.

Alternatively, hydraulic oil may be used, with power provided by e.g. an electrical pump. In other embodiments of the invention, the two compressing pistons are actuated directly by a single electrical motor, e.g. with a mechanical gearing for converting the motor's rotation into a reciprocating movement.

In a further preferred embodiment of the invention, at least one of the first compression stage and the second compression stage is part of a multi-stage compression arrangement, wherein such a multi-stage compression arrangement comprises at least two serially connected compression stages, optionally with intercooling. According to the present invention, a central hydraulic stage can have, on each side and coaxially arranged, a group of two or more coaxial and serially connected compression stages each. Again, these coaxial stages are preferably driven by the same actuator. A first group of stages compresses the first gas, a second group of stages compresses the second gas.

For example, a sequence of serially connected compression chambers in subsequent compression stages is known for single-gas compressors, described, e.g. in US

4,368,008. Therein (in Fig. 12), a three-stage compressor is shown, the three stages being arranged to one side of a hydraulic stage. Thus, according to the present invention, a second three-stage compressor is arranged on the opposite side of said hydraulic stage and is also driven by the hydraulic stage.

In a further preferred embodiment of the invention, the compressor comprises a control unit configured to, for each stroke of the pistons, drive the pump to operate, for a first time interval, at a substantially constant speed and, for a second time interval, at a progressively increasing speed as the pressure in the pressure chamber or chambers being pressurized increases. The first time interval in which the pump operates at constant speed may also be zero, that is, the pump speed increases monotonously, from the very beginning of the compression stroke.

This control scheme is based on the realisation that, during a first phase of the pressure buildup, the counterpressure in the compression chamber is low, and therefore it is sufficient to drive the hydraulic pump at a lower speed. The pressure generated at this lower speed is sufficient to overcome (via the hydraulic piston and the gas compressing piston) the counterpressure in the chamber. Running the pump at lower speed reduces its energy consumption. Only when, in a later phase of the piston stroke, the counterpressure in the compression chamber rises, is the speed or the pressure of the pump increased, in order to overcome the counterpressure. The acceleration and therefore the speed of the piston is given by the balance of forces from the gas and hydraulic fluid acting on the pistons. The flow of the hydraulic fluid is proportional to the speed of the piston. The movement of the piston may decelerate due to the rising counterpressure of the gas, while the speed of the pump has to be increased to drive the hydraulic pressure further up. The increase of the pump speed may be stepwise or linear or progressively increasing, e.g. according to a predetermined curve. The increase may be controlled by feedback from pressure or stroke measurements from the compression chamber. Optionally, the source pressure and the storage pressure are used for controlling the (increase of) pump speed.

Alternatively, the pump speed may be open-loop controlled, that is, following a predetermined trajectory over time. The gradual increase of (nominal) pump pressure or pump speed during each stroke reduces overall power consumption of the pump and therefore increases the efficiency of the compressor as a whole.

Under conditions in which the counterpressure is low, that is, if the current storage pressure still is low, the decelerating force on the piston is low as well. Then, the controller may decrease the pump speed towards the end of the stroke of the piston, allowing the piston to decelerate towards the point of lowest gas volume (instead of slamming into the end position). This also reduces energy consumption, mechanical stress and gas outflow speed.

The above description regarding speed control is based on the assumption that a pump is used which allows to generate higher pressures by increasing the speed of the pump. The pressure that is ultimately generated by the pump is a function of the pump parameters such as pump speed (for a particular type of pump), and the hydraulic properties of the system driven by the pump, e.g. the hydraulic capacity and resistance of the system. Therefore, the actual pressure generated is to be distinguished from the nominal pressure. The nominal pressure corresponds to the maximum pressure that can be reached at the current operating condition (such as speed) of the pump. Thus, in more general terms, the above control scheme comprises driving the pump, in the first phase, in a first operating condition to generate a lower nominal pressure, and in the second phase, in a second operating condition to generate a higher and preferably increasing nominal pressure.

In one preferred embodiment of the invention, the control means is configured to determine the increase of the pump speed as a function of a measured value of the storage pressure; this is done e.g. by selecting, according to the measured storage pressure, a trajectory from a predetermined set of speed trajectories.

A method for simultaneously compressing gas according to the invention comprises the steps of

• generating, in a gas source unit, a first gas and a second gas by a chemical or electrochemical separation process, • compressing, in a first compression stage and by means of a first piston, the first gas,

• driving the first piston by a mechanical actuator,

• compressing, in a second compression stage and by means of a second piston, the second gas, • driving the second piston by the same mechanical actuator,

• keeping the first and second gas, within the compressor, separate from one another.

In a preferred embodiment of the invention, the method further comprises the steps of:

• pressurising the hydraulic fluid by means of a pump;

• switching, by means of one or more valves, the hydraulic fluid to flow, alternately, into opposing chambers of the hydraulic stage, thus giving rise to a reciprocating movement of the hydraulic piston and the first and second piston;

• by said reciprocating movement of the pistons, alternately compressing the first gas and the second gas in the respective compression stages;

• controlling, during each compressing movement of a piston, a pressure generated by the pump to rise from a first pressure level to a second, higher pressure level, such as to compensate for the increasing counterpressure exerted by the gas being compressed.

Further preferred embodiments are evident from the dependent patent claims. Features of the method claims may be combined with features of the device claims and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:

Figure 1 shows a compressor in a perspective view; Figure 2 shows a compressor with part of a outer housing cut away;

Figure 3 schematically shows a compressor with additional equipment;

Figure 4 shows a detail view of a security chamber;

Figure 5 shows alternative embodiments with regard to certain constructional details; Figure 6 show trajectories of gas pressure P, pump speed setpoint Fs and pump speed Fa over time t; and Figures 7 and 8 show pressure trajectories over time.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The structure of a preferred embodiment of a compressor according to the invention is explained with reference to Figures 1 through 3: Figure 1 shows a compressor 1 in a perspective view, and Figure 2 in a similar view, but with part of an outer housing cut away. Figure 3 schematically shows a compressor 1 with additional

equipment forming the context in which the compressor 1 operates, in particular a high pressure gas providing apparatus 2.

The compressor 1 comprises a first compression stage 11 for a first gas, a first compression chamber 12 for a second gas, and a hydraulic stage 31 for driving the two compression stages. The first compression stage 11 comprises a first piston 13 operating in a first compression chamber 12, the second compression stage 21 comprises a second piston 23 operating in a second compression chamber 22, and the hydraulic stage 31 comprises a hydraulic piston 33 separating a first hydraulic chamber 32a from a second hydraulic chamber 32b. Guiding pins extending from the housing into corresponding bores in the hydraulic piston 33 prevent the hydraulic piston 33 from rotating around its axis. The three stages 1 1, 21 and 31 and the three pistons 13, 23 and 33 are cylindrical and are arranged coaxially, i.e., with common axis, to one another. The three pistons 13, 23 and 33 are made of separate pieces that are attached to one another in a fixed relationship, or of a single piece of material.

Further elements that may be required for the safe operation of the system, such as shut-off valves, control valves, safety or pressure relief valves, pressure regulators, flow control valves, throttles etc, are known to one skilled in the art and are not shown in further detail.

The hydraulic piston 33 is driven by hydraulic fluid, preferably water, alternately being forced into the first hydraulic chamber 32a and the second hydraulic chamber 32b through corresponding inflow/outflow openings 34. The inflow/outflow openings 34 for each of the hydraulic chambers are preferably arranged symmetrically with respect to the common axis of the pistons. That is, each of the chambers comprises at least two associated inflow/outflow openings for the hydraulic fluid, with the at least two openings being arranged symmetrically with respect to the longitudinal axis (or roll axis) of symmetry of the hydraulic piston. This gives rise to a symmetrical flow in the hydraulic stage, and thus also to

symmetric forces on the piston. Preferably the forces generated by the fluid flowing against the hydraulic piston 33 are at least approximately co-axial with the longitudinal axis of the hydraulic piston 33. This reduces friction and wear. In the view of Figure 1 only one inflow/outflow opening 34 is shown per chamber, an opposing inflow/outflow opening 34 being located on the invisible side of the compressor 1.

The pressurized hydraulic fluid is provided through hydraulic supply lines 35, the flow of the fluid being controlled by one or more valves 36, e.g. a four way valve as known from the prior art, e.g. US 4,368,008 or US 5,238,372. Alternatively, a set of four separate valves can be used to control the flow of pressurized water: The valve or valves 36 are configured to alternately provide the pressurized fluid to one of the hydraulic chambers 32a, 32b while allowing the fluid to exit from the opposing chamber. The fluid exiting in this manner leaves the valve 36 through a hydraulic discharge line 38.

In one preferred embodiment of the invention, the hydraulic fluid is water. The pressure required to power the hydraulic stage 31 is either already present in a hydraulic supply line 39 such as the water mains, that is, the pressure and the power for driving the compressor 1 is provided by the water utility, or the pressure is provided by a pump 37 driven by, e.g. an electrical motor. The water exiting the hydraulic discharge line 38 may be recycled through a reflow line 39a to the pump 37 or may be used (for example, when no pump 37 is present) for domestic purposes.

The compressor 1 may thus be used in a domestic setting, thanks to its simple and compact construction, and the working fluid (water) can be re-used.

In a preferred embodiment of the invention, the compressor 1 further comprises • a first supply line 18 for connecting the at least one first compression stage 11 to a first gas source generating the first gas by a chemical or electrochemical

reaction, and a first discharge line 16 for discharging the pressurized first gas; and • a second supply line 28 for connecting the at least one second compression stage

21 to a second gas source generating the second gas by the same chemical or electrochemical reaction as the first gas, and a second discharge line 26 for discharging the pressurized second gas.

The volumes, that is, the lines and compression stages for containing the first gas are distinct from the lines and compression stages for containing the second gas. That is, there are separate, nonidentical pathways for the two gases through the compressor 1.

In the preferred embodiment of the invention shown in Figure 3, the first and second gas sources are realized by a electrolysis unit 40. The first gas source corresponds to a first section 40a of the electrolysis unit 40 in which e.g. oxygen is generated, and the second gas source corresponds to a second section 40b in which e.g. hydrogen is generated. The electrolysis unit 40 comprises one or more electrolysis cells, electrical leads 41 powering the cell(s), and a water supply 42 for replenishing the water which is converted to oxygen and hydrogen.

The first supply line 18 leads to the first compression chamber 12 through a first inlet check valve 14, the second supply line 28 leads to the second compression chamber 22 through a second inlet check valve 24. The first compression stage 1 1 may comprise a first sensor 19 for measuring the pressure and/or the temperature in the first compression chamber 12, or for detecting an overpressure. Likewise, the second compression stage 21 may comprise a second sensor 29. The compressed first gas exits the first compression chamber 12 through a first outlet check valve 15 and a first discharge line 16 leading the first gas to a first pressure vessel 17. The compressed second gas exits the second compression chamber 22 through a second outlet check valve 25 and a second discharge line 26 leading the second gas to a second pressure vessel 27. The first and second pressure vessel 17, 27 can be

disconnected (by a disconnecting means not shown in the figure) and the hydrogen/oxygen used at another location or in a vehicle.

Further valves, check valves, pressure sensors and safety equipment commonly known to one skilled in the art may be arranged at appropriate places in the overall arrangement. The compressor and auxiliary equipment are controlled by means of a control unit 45 connected to the process by signal lines from sensors and control lines (not shown for clarity) to actuators, in particular valves 36.

Figure 4 shows a detail view of a security chamber 50 formed between two seals, that is, an inner seal 51 and an outer seal 52 (in Figure 4, the seals are shown for the second compression chamber 22 only). The inner seal 51, here shown as comprising two sealing rings, seals the gap between the compression chambers 12, 22 and the corresponding pistons 13, 23. The outer seal 52 seals the gap between a hydraulic chamber 32a, 32b and the corresponding piston. The two seals arranged along the same piston define an annular volume 50 between the seals. This volume serves as an intermediate security chamber 50 and provides an additional separation between the hydraulic chambers and the respective gas compression chambers. If only one seal were arranged between the gas and the hydraulic fluid, the two gases could dissipate into the hydraulic fluid and might accumulate in the fluid to form an explosive mixture. No indicator of the wear rate of the seal would be available.

One or both of the seals 51, 52, instead of being, as shown, fixedly located in the housing, with the corresponding piston sliding in the seals, may alternatively be located on and moving with the piston, sliding along the housing. This is shown in Figure 5 for the inner seal 51. One security chamber 50 each is preferably arranged in both the first compression stage 1 1 and the second compression stage 21.

A safety pressure sensor 53 is arranged to measure the pressure in each of the security chambers 50. A sudden increase in pressure indicates a leak in the seal to the

corresponding adjacent compression chamber. An optional gas detector may also be included, to detect the presence of the respective gases in the security chambers 50. When an increase in pressure or in gas concentration in a security chamber 50 is detected, operation can be stopped before dangerous quantities of gas accumulate. In order to prevent a pressure buildup due to the unavoidable small leakage of the respective seal, the security chamber 50 preferably comprises at least one vent 54. The vent 54 allows for a small gas flow out of the security chamber 50. It may be implemented by a permeable membrane or the like, or simply by a small hole or pinhole or a needle valve in the wall separating the security chamber 50 from the ambient air. As shown in Figure 4, the vent can also be a small channel leading from the ambient air to a volume in the safety pressure sensor 53 (at the location of pressure sensor 53, an alternative or additional gas detector 53 may also be included to detect concentrations of H ₂ or O ₂ ) which in turn is in communication with the security chamber 50. For larger than normal gas flows, the vent 54 acts as a throttle and causes a pressure variation in the security chamber 50.

Operation of the compressor: The compressor is double acting in that, in a cycle of reciprocal motion of the pistons, a gas intake phase in the first compression stage coincides with a compression phase in the second compression stage and, vice versa, a gas intake phase in the second compression stage coincides with a compression phase in the first compression stage. In each compression stage, the reciprocating movement of the respective pistons 13, 23 causes the respective gases to be drawn in through the respective inlet check valves 14, 24 and then to be discharged through the respective outlet check valves 15, 25.

Figure 5 shows alternative embodiments with regard to certain constructional details. One or more of these variations may be combined with one or more of the variations already described so far. • According to one preferred embodiment of the invention, the outlet check valves 15, 25 are arranged in parallel with the direction of movement of the

corresponding pistons 13, 23. Preferably, the outlet check valves 15, 25 are also arranged along the same axis of symmetry as the pistons 13, 23. The inlet check valves 14, 24 may also be arranged in parallel with the piston direction (not shown). • According to another preferred embodiment of the invention, the pistons 13, 23 are shaped as to correspond to the shape of inlet and/or outlet orifices. This reduces the amount of "dead volume" remaining when the pistons 13, 23 are at their extreme position.

• According to a further preferred embodiment of the invention, at least one piston 13, 23 is not attached fixedly to the hydraulic piston 33 , but remains "floating" in its stage 11, 21. This simplifies construction and assembly. The first and second pistons 13, 23 are thus operatively connected or mechanically coupled to the hydraulic piston 33 to transmit pressure forces or compressive forces from the hydraulic piston 33 to the pistons 13, 23, but not forces that pull the hydraulic piston 33 and the pistons 13, 23 apart from each other. During the respective compression phase, the piston 13, 23 is pushed against the gas by the hydraulic piston 33, and during the intake phase, the piston 13, 23 is pushed back by the pressurized gas (at the inlet pressure) entering through the corresponding inlet check valve 14, 24.

In a preferred embodiment of the invention, in order to conserve power, the speed of the pump 37 is adapted to the state of the pistons in the compression cycle. This is illustrated by means of the graphs of Figure 6 for a single compression stroke in one of the compression chambers: The graphs show exemplary trajectories of gas pressure P, pump speed setpoint Fs and pump speed Fa over time t. As the piston travels from the position of minimal compression to the position of maximal compression, the pressure P of the gas being compressed rises. The time for one such stroke (over a distance of e.g. 5 cm) is preferably in the range of one second, e.g. 1.2 seconds. The pump speed setpoint Fs starts at a minimal speed fsl , e.g. as required by the pump to generate a flow. After a certain time, a higher pump speed is required

in order to overcome the counterforce on the piston corresponding to the increased gas pressure P. The set point is increased to a higher value fs2. The actual speed Fa is delayed with respect to the set point due to the inertia of the pump and motor, and according to the parameters of the speed control loop.

Alternatively, the speed set point may be given by more or less than three discrete set points, or by a continuous trajectory. The set points and the time of their change (or the set point trajectory) may be adapted according to the actual pressure and/or the actual path traveled by the piston. They may also be predetermined, since most of the parameters and the gas properties affecting compression remain unchanged. The main parameters that can actually change during operation of the compressor are the inlet pressure and the outlet or discharge pressure, e.g. the pressure of the storage vessel being filled. If the inlet pressure is kept constant by the gas generator such as the electrolysis unit 40, then it is sufficient, for a simplified control scheme, to consider only the discharge pressure. Then the set point magnitude is adapted only according to the discharge pressure. Optimal set points can be determined for different discharge pressure values and stored in a look-up table of the (pump) controller 45. For example, for a lower discharge pressure the dotted and dashed setpoint values and further trajectories are representative. For a very low discharge pressure (when the storage vessel 17, 27 is hardly filled), the gas counterpressure is not sufficient to decelerate the piston. In that case, the pump speed setpoint (corresponding to its nominal pressure) may decrease towards the end of the stroke, i.e. fs3 then may be smaller than fs2, which may be smaller than fsl .

Figures 7 and 8 show pressure trajectories over time. Figure 7 shows the buildup of pressure in the first compression chamber 12 (lower curve, P_O2_c) and the outlet pressure of the first gas, which is substantially equal to the pressure in the first pressure vessel 17 (upper curve, P_O2_v, dotted). Figure 8 shows the same for the second gas and second compression chamber 22, with compression chamber pressure (lower curve, P_H2_c) and storage vessel pressure (upper curve, P_H2_v, dotted).

As the outlet pressure increases with each stroke, the chamber pressure is increased progressively in order to overcome the outlet pressure. Correspondingly, the pump speed trajectory is raised in line with the outlet pressure, as explained in the context of Figure 6.

Figures 7 and 8 also show a side effect of the different volumes of the compression chambers 12, 22: the compressor 1 used is designed for compressing oxygen and hydrogen simultaneously. The stoichiometric relation when generating these gases from water is two H ₂ molecules for one O ₂ molecule, and thus the first compression chamber 12 for oxygen has half the volume (e.g. ca. 20 cubic centimeters) of the second compression chamber 22 for hydrogen (e.g. ca. 40 cubic centimeters). In consequence, the initial pressure buildup for hydrogen (Figure 8) takes approximately half the number of strokes than for oxygen (Figure 7), and half the time (notice the different time scales of the two graphs). This assumes O ₂ and H ₂ storage vessels to have the same volume.

The trajectories of Figures 7 and 8 are intended for illustrative purposes: they are atypical in that they correspond to a storage vessel with a relatively small volume. In a larger storage vessel, being filled from an empty state, the pressure buildup would be much slower, i.e. require many more strokes of the compressor.

In an exemplary embodiment of the invention, the compressor 1 moves through one cycle in ca. 3 seconds (20 double strokes per minute), using, in the hydraulic stage 3.4 liters of water per double stroke (68 liters per minute), at a maximum pressure of ca. 6 bar to 8 bar. This corresponds - when driving the compressor 1 from the water mains - to a water consumption of ca. 4'080 liters per hour which allows to compress ca. 1.6 Nm3 (normal cubic meters) of Hydrogen gas and 0.8 Nm3 of Oxygen gas per hour.

While the invention has been described in present preferred embodiments of the invention, it is distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practised within the scope of the claims.

LIST OF DESIGNATIONS

1 compressor 31 hydraulic stage

2 high pressure gas providing 32a first hydraulic chamber apparatus 32b second hydraulic chamber

1 1 first compression stage 33 hydraulic piston

12 first compression chamber 34 inflow/outflow openings

13 first piston 35 hydraulic supply line

14 first inlet (suction) check valve 36 valve

15 first outlet (exhaust) check valve 37 pump

16 first discharge line 38 hydraulic discharge line

17 first pressure vessel 39 hydraulic supply line

18 first supply line 39a reflow line

19 first sensor 40 electrolysis unit

21 second compression stage 40a first section of electrolysis unit

22 second compression chamber 40b second section of electrolysis unit

23 second piston 41 electrical leads

24 second inlet (suction) check valve 42 electrolysis water supply

25 second outlet (exhaust) check 45 controller valve 50 security chamber

26 second discharge line 51 inner seal

27 second pressure vessel 52 outer seal

28 second supply line 53 pressure sensor or gas sensor

29 second sensor 54 vent