WO2023242385A1

WO2023242385A1 - Method for producing hydrogen with adjustment of the power of a compressor

Info

Publication number: WO2023242385A1
Application number: PCT/EP2023/066203
Authority: WO
Inventors: Alain Rocheux; Ilies Driss; Serge MALVAULT
Original assignee: H2Gremm
Priority date: 2022-06-16
Filing date: 2023-06-16
Publication date: 2023-12-21
Also published as: FR3136780A1

Abstract

The invention relates to a method for producing hydrogen with adjustment of the power of a compressor according to the rate of production of an electrolyser, said method comprising the following steps: - a) electrolysing using an electrolyser producing hydrogen at a flow rate of between 0.5 and 5 standard m3/h at an outlet pressure of between 1 and 50 bar; - b) compressing the hydrogen using an electrochemical compressor. The method also comprises a step of correcting the power supply current of the electrochemical compressor with respect to a target pressure value.

Description

Hydrogen production process with adjustment of the power of a compressor

Technical field of the invention

The present invention relates to a process for producing hydrogen with adjustment of the power of a compressor. It applies, in particular, in the field of hydrogen production, compression and storage installations with a daily capacity of up to 10 kg per day.

These installations can be used in various applications such as autonomous distribution stations for mobility, or systems allowing the energy autonomy of residential homes using hydrogen as an energy vector. The innovation implemented makes it possible to greatly simplify the implementation of hydrogen compression.

Hydrogen production and compression installations for small installations use reciprocating mechanical compressors (piston or membrane) to reach pressures between 150 and 1000 bar. These generate a certain number of constraints, summarized below.

Start-up flow: when starting, an electrolyzer needs a certain time before reaching its set flow. This flow rate will therefore vary almost linearly between 0 and its set flow rate. For its part, a mechanical compressor operates at precise speeds, determined by the operating constraints of the engine. Depending on the applications, the electrolyzer can produce hydrogen at variable flow rates. It is therefore necessary to force the electrolyzer to operate at precise speeds to adapt to the speeds of the compressor.

Compression Ratio Limit: An essential characteristic of a mechanical compressor is its compression ratio. It is the ratio between the discharge pressure and the suction pressure. This ratio is determined according to the suction temperature, the nature of the gas and its ability to heat up during compression, the type of compressor, particularly whether it is cooled or not, and the mechanical state of the compressor. In the most favorable cases, with an integrated cooling system, it is possible to achieve a compression ratio of 10. This makes it possible, for example, to reach 300 bars with a suction pressure of 30 bars. A delivery pressure above 300 bar can be achieved with a second compression stage. This second stage leads to additional implementation complexities in the management of the flow of the two compressors, as well as additional costs for the purchase of a second compressor.

Synchronization of flow rates by buffer volume: standard mechanical compressors for hydrogen generally have nominal suction flow rates higher than the production flow rates of electrolysers for small installations (i.e. 0.5 to 5 Nm3/h). For this reason, a buffer volume is placed between these two pieces of equipment (electrolyzer and compressor). This makes it possible to bridge flow differences and maintain sufficient pressure to maintain an acceptable compression ratio. To maintain an acceptable compression ratio, the compressor suction pressure must be greater than 30bar in order to maintain a compression ratio below 10, to reach 300bar at discharge. The maximum pressure achievable by the electrolyzer is generally 35 bar, the difference between these operating values is close, it is therefore necessary to increase the buffer volume to limit the number of compressor starts. Generally, a buffer volume of at least 150L is necessary for a production rate of 500Nl/h.

Synchronization of flow rates by mechanical regulation: even if certain mechanical compressors are capable of operating at flow rates identical to the nominal flow rate of the electrolyzer, for example 500NL/h, an electrolyzer, whatever its technology, has a ramp-up time before reaching its nominal flow. This time varies from a few minutes for a PEM or AEM electrolyzer to a few hours for alkaline technology. As explained previously, the motors of mechanical compressors can only operate at precise speeds and limited by the operation of the engine, generally 4 speeds, this only allows a first rough adjustment of the flow. However, there are techniques allowing the flow rate of a mechanical compressor to be more finely adapted. These techniques induce degraded operation of the compressor and lead to modifications, either to the design or by adding regulation elements. In both cases, they result in additional hardware costs and significant development time. It is first possible to reduce the suction pressure. With each movement, the pistons compress a smaller mass of gas, resulting in a reduction in flow. This has the effect of increasing the compression ratio and leads to efficiency losses. It is also possible to recycle the compressed gas using a bypass. A bypass consists of connecting the output to the inlet of a compressor with a regulator, this allows the inlet flow to be artificially increased. Hydrogen is a gas that heats up in the compression and expansion phases, so significant overheating occurs. The need for greater cooling and the need to compress a greater quantity of gas greatly reduces efficiency. We can also cite other methods used on higher power machines because they significantly increase the complexity and cost of the machine: emptying the valves or reducing the suction capacity by delaying the closing of the valves. suction using servomotors. These techniques can be put in parallel to improve the precision of flow control but require complex study and development to limit losses in overall efficiency.

Power at start-up: limiting the number of starts of the compressor is important because each start-up requires high energy allowing the moving parts to start. Because of the power of the engine, which does not benefit from being oversized, it is generally not possible to simultaneously ensure compression and launch of the machine. For this reason, a reciprocating compressor is almost always started empty. The discharge pressure must be reduced to a value close to the suction pressure using a bypass. Decompression leads to yield losses. An increase in the number of ignitions also leads to material wear and a reduction in the life of the compressor.

Documents EP3550056, US2022025529, US2004040862 are examples of hydrogen compressors. However, the optimization of their operation has not been achieved.

Presentation of the invention

The present invention aims to remedy these drawbacks with a completely innovative approach.

Another compression technology makes it possible to overcome a large number of problems raised previously: electrochemical compression.

Electrochemical compression is a compression technology using PEM (Proton Exchange Membrane) technology, also used for fuel cells or electrolyzers. Its basic principle consists of forcing the passage of hydrogen through a membrane behind which is the high pressure storage tank. The passage of hydrogen is strictly a function of the application of an electric current. The compression ratio obtained is mainly a function of the resistance of the membranes to the upstream/downstream pressure difference.

More precisely, the invention aims to provide a compression technology which combines numerous advantages.

It operates without mechanical movement and is therefore silent and vibration-free. It does not require sound and vibration insulation like mechanical compression.

It allows fine and continuous regulation of the compression flow.

It is carried out in isothermal conditions, unlike mechanical compressors which undergo isentropic compression. For the mechanical compressor, an increase in the compression ratio leads to an increase in the heat produced. In real conditions, this effect is much less present for the electrochemical compressor, which allows compression ratios of 10 to be exceeded. This is difficult to achieve with a mechanical compressor.

The nominal flow rate is limited by the dimensions and number of membranes. Compressor sizing can be done to suit production capacity. For a mechanical compressor, the overall structure of the compressor should be reviewed (motor sizing, piston size, etc.)

These objectives, as well as others which will appear subsequently, are achieved, according to a first aspect, using a process for producing hydrogen with adjustment of the power of a compressor as a function of the production flow rate. of an electrolyzer, said process comprises the following steps:
- a) electrolyze by an electrolyzer producing hydrogen with a flow rate of between 0.5 and 5 Nm3/h at an outlet pressure of between 1 and 50 bars;
- b) compress hydrogen from step a) using an electrochemical compressor;
remarkable in that it further comprises a step of correcting the supply current of the electrochemical compressor relative to a target pressure value; and in which the correction step comprises a sub-step of modifying said target pressure value:
- as soon as the outlet pressure of the electrolyzer exceeds a target pressure threshold, the value of the supply current of the electrochemical compressor is increased;
- as soon as the outlet pressure of the electrolyzer is below an outlet pressure threshold of the electrolyzer, the value of the supply current of the electrochemical compressor is reduced.

Thanks to these arrangements, the present invention is technically simpler and less expensive to purchase and use for the compression of hydrogen at rates greater than 10 and for flow rates less than 5Nm3/h. It makes it possible to significantly reduce the buffer between the hydrogen production and compression stage. There is therefore also a significant saving in space occupied by the system. This is made possible thanks to dynamic flow management of an electrochemical compressor and gas humidity management perfectly integrated with the needs of the compressor and the application for small hydrogen production and compression installations.

The invention makes it possible to adapt the power of the compressor according to the production flow of the electrolyzer. It is therefore possible to operate the electrolyzer and the compressor in parallel in an automated manner at an identical flow rate. This allows the buffer volume to be reduced by a factor of 500. For this a program is implemented in a control card. This control card varies the current setpoint according to the pressure between the electrolyzer and the compressor. The compressor is capable of reaching its nominal flow rate in a few seconds, so it will be able to follow the production of the electrolyzer in these start-up and shutdown phases.

The value of the electrochemical compressor supply current changes proportionally to the difference between the measured pressure and the target pressure. The greater the pressure difference, the more the current setpoint increases to reach the target pressure.

The value of the electrochemical compressor supply current changes proportionally to the pressure difference between it and the target value. The greater the pressure difference, the more the current setpoint will decrease until the compressor completely stops.

The invention is advantageously implemented according to the embodiments and the variants set out below, which are to be considered individually or according to any technically effective combination.

In one embodiment, it further comprises a step of humidifying the hydrogen by a humidifier so that the hydrogen produced in step a) has a relative humidity of between 80% and 99%.

Using an electrochemical compressor means that the incoming hydrogen must be almost saturated with water (80-99% RH) to ensure the compressor operates properly. This condition is not met if the hydrogen comes directly from an electrolyzer, nor from a standard storage tank.

Relative humidity gives the percentage of saturation of the gas with water. This humidity varies depending on pressure and temperature.

In one embodiment, during the correction step the target pressure value is regulated by a proportional integral or proportional integral derivative type regulator.

In one embodiment, during the correction step the target pressure value uses data from at least one of the following sensors: a pressure sensor and a temperature sensor.

In one embodiment, during the correction step the pressure sensor is positioned between the electrolyzer and the electrochemical compressor on the humidifier. Thus, the sensor can be positioned before inside or after the humidifier.

In one embodiment, during the humidification step, the humidifier has a buffer volume of less than 500ml.

The buffer volume is in the humidifier. It is located between the electrolyzer and the compressor. The buffer volume therefore has the dual function of humidification and buffer volume.

This is an advantage that allows us not to install an additional tank, using the humidifier tank.

Brief description of the figures

Other advantages, aims and characteristics of the present invention emerge from the description which follows, given for explanatory and in no way limiting purposes, with reference to the appended drawings, in which:

There represents in flowchart form the steps implemented in a particular embodiment of the process which is the subject of the present invention.

There represents in flowchart form the steps implemented in another particular embodiment of the process which is the subject of the present invention.

The present invention relates to installations allowing gas humidity control operating at pressures greater than 10 bar.

Previous systems allow precise control of the humidity level in the gas, they are very expensive and are not adapted to the needs of small installations.

It is possible to use a humidity regulation system perfectly suited to operation in this type of application and made up of standard elements.

Incoming gas:

To achieve a relative humidity of the incoming gas of between 80 and 100% at the inlet, the humidifier is maintained at a temperature close to the temperature of the incoming gas.

An identical temperature makes it possible to reach 100% relative humidity.

The humidifier must therefore be maintained at a temperature a few degrees lower than the temperature of the incoming gas to keep the relative humidity level within the required range (80% ≤ RH ≤ 99%).

If the temperature of the humidifier is higher than the temperature of the incoming gas, condensation will occur, which can adversely affect overall operation.

To do this, the incoming gas is temperature regulated by means of a thermoregulator. Thermoregulation is carried out at the humidifier using a finned tube and a fan.

The solution consists of encapsulating the humidifier and gas assembly in the same thermo-regulated circuit. This avoids electricity consumption for maintaining the temperature (resistors and heating cables). The humidifier and the coaxial tube containing the incoming gas help dissipate heat in the event that the system produces an exothermic reaction requiring cooling.

Consequently, the dimensioning of the thermoregulator is slightly lower than the initial configuration necessary for the same quantity of incoming gas.

The present invention aims to guarantee the saturation of the gas with humidity whatever the operating temperature.

In an exemplary embodiment, the present invention is integrated into a process for regulating the humidity of a gas

The process for regulating the humidity of a gas includes the following steps:
- a first step of regulating the humidity of a gas entering a humidifier at a flow rate of between 500 NL/h and 50,000 NL/h, said humidifier comprises the following elements:
- a humidifier tank comprising an inlet and an outlet,
- a thermoregulation tank which surrounds said humidifier tank,
- a volume of thermoregulation water contained in said thermoregulation tank,
- a thermoregulation water circulation pump;
- said humidifier tank comprises a first porous matrix located at the bottom of the humidifier tank near the inlet;
the gas entering the humidifier through the inlet of the humidifier tank passes through the first porous matrix, this creates gas microbubbles allowing better absorption of humidity. This matrix is positioned in a volume of water contained in the humidifier tank, the humidified gas leaves through the outlet of the humidifier tank and circulates towards a compressor; the volume of thermoregulation water circulates in a closed circuit passing through the thermoregulation tank, the circulation pump and the compressor;
- a second stage of drying the gas from the first stage, the gas leaving the compressor passes through a T-shaped tube comprising an inlet and two outlets, one of the outlets is vertical and by the effect of gravity leaves s flow the liquid phase of the gas creating a condensate, the other outlet is connected to the bottom of a desiccant tank; at the outlet of the desiccant tank, the dried gas is stored in a tank and has a humidity lower than a predetermined threshold.

Thanks to these arrangements, the gas entering the compressor is saturated with humidity, i.e. 80%≤RH≤99% and optimizes its operation; and after the dryer stage, the outgoing gas carries a water concentration of less than 5ppm which allows hydrogen to be used in various applications.

There are other advantages such as the need for less cooling compared to other processes, a process whose exiting gas conditions are ensured and respected.

The internal volume of the humidifier is also used as a shock absorber for variations in incoming gas pressure, thus allowing simplified management of the compressor.

The humidifier maintains an optimal water level using an automated purge and fill system.

A single temperature regulator installed around the humidifier allows the temperatures of the humidifier and the associated compressor to be adjusted.

By separating the condensate upstream of the gas dryer, we minimize gas losses during purges of this condensate.

In one embodiment, during the first step, the temperature of the thermoregulation water is ensured by a thermoregulation element.

In one embodiment, the thermoregulation element of the first stage comprises fins placed around the thermoregulation tank.

In one embodiment, the thermoregulation element of the first stage comprises a fan configured to ensure the circulation of air in contact with the fins.

In one embodiment, during the first step, a second porous matrix is located in the upper part of the humidifier tank near the outlet of the humidifier tank, the humidified gas passes through the second porous matrix before leaving the humidifier tank . It is used as a safety element preventing the passage of liquid water.

In one embodiment, during the first stage the relative humidity of the gas entering the compressor is between 80% and 99%, preferably between 95% and 99%.

In one embodiment, said method comprises a third shutdown step to purge the volume of water contained in the humidifier tank then fills it with a new volume of water.

The humidifier tank is used as a buffer volume. This makes it possible to do without an additional tank, which saves space and reduces costs.

In one embodiment, during the third shutdown step, the condensate contained in one of the outlets of the T-shaped tube is purged.

In one embodiment, when the humidity level of the gas is greater than the predetermined threshold, preferably when it exceeds the threshold of 5 ppm, said method comprises a visual, audible or terminal alert step indicating the need replacement of the desiccant.

This humidity control process ensures that humidity is close to 99% regardless of the pressure and temperature conditions in the compressor.

Electrolysers generally operate at a pressure between 10 and 50bar. The use of the humidifier presented above allows operation in these pressure ranges. It is therefore not necessary to expand the gas to a lower pressure for humidification. This allows the process to have optimized efficiency because the pressure difference between the inlet and outlet of the compressor will be lower.

Another element makes electrochemical compression difficult to integrate for small installations. Compressed hydrogen should be stored in tanks with low humidity (<5ppm). It is therefore necessary to dry the compressed gas.

No product available on the market allows hydrogen to be dried at these flow rates and at a pressure between 500 and 1000 bar, at such a low humidity level. This requires the use of specific materials certified to operate at high pressure. Systems allowing this to be done at lower pressures are expensive in relation to production rates.

In addition, their use leads to significant gas losses during each operating cycle.

The use of a dryer as presented above offers a drying solution based on standard components, for example a high pressure tank allowing a significant reduction in costs, simplification of maintenance and greater modularity. Mechanical compressors require dry hydrogen to operate, so the drying step is mandatory in both cases. However, drying is carried out before compression. A simple drying solution allows a reduction of a factor greater than 10 in costs compared to a drying solution upstream of compression or existing drying solutions downstream of compression. In fact, the increase in pressure makes it possible to condense a large part of the gaseous water present in the gas. The quantity of water to be extracted from the gas is therefore less after compression, which allows the use of more affordable and simple to implement technologies.

There shows steps in flowchart form of the implemented steps of the method of the present invention.

The stages are: electrolyzer, humidifier, compressor, dryer, tanks and fuel cell (possible applications).

There shows more particularly the steps between the electrolyzer and the dryer.

The method consists of using a PID (proportional, integral, derivative) regulator. It is a resident program in the compressor control card which will modify the compressor current setpoint dynamically according to the pressure measured by the pressure sensor between the electrolyser and the compressor. The objective is to maintain a fixed pressure between the two pieces of equipment.

A pressure sensor therefore makes it possible to know the pressure between the two devices and to transmit the information to the control card. This will have the function of regulating the compressor supply current in order to keep the pressure constant at 30bar for example. The humidifier and dryer are humidity regulation elements that are less complex and less expensive than currently existing solutions. They make it possible to meet the needs for small installations and make hydrogen production and compression solutions less expensive.

Together, the new humidification solutions, dynamic compressor flow control, compression technology and drying make it possible to offer a product significantly less expensive than what currently exists on the market. The simplification of assembly and management also allows a reduction in maintenance time and costs as well as a better lifespan of the assembly.

Let's take an example: the system operates at nominal speed, the electrolyzer and the compressor therefore operate at the same flow rate, we modify the flow setpoint of the electrolyzer to reduce it from 100% to 90%. At this time, the drop in production flow of the electrolyzer will lead to a drop in pressure between the two pieces of equipment because the compressor still has a production setpoint of 100%. The PID will detect the drop in pressure and will therefore send the supply current reduction instruction to the compressor. This reduction will lead to a drop in compressor flow, causing the intermediate pressure to stabilize and reach 30 bar again. The PID reaction speed and stabilization speed will depend on 1) the compressor's ability to make speed changes and 2) the buffer volume. This reasoning can be the same for the start-up, stop and regime change phases. The compressor therefore has the sole objective of maintaining a constant pressure in an automated manner.

This solution has the advantage of being very simple to implement and does not require additional equipment or modification of the initial design of the compressor. It still requires a suitable humidification system.

Electrochemical compression is modular; increasing the number of cells makes it possible to increase the nominal flow rate of the compressor. Each cell is made up of a PEM membrane and two electrodes, the assembly is called MEA (Membrane Electrode Assembly). A standard cell with a surface area of 150cm2 generally accepts a flow rate of 20NL/h. When manufacturing the compressor, it is therefore possible to choose the nominal flow rate, and therefore the number of cells, depending on the application and the size of the electrolyzer. This flexibility is possible with a mechanical compressor, but it is necessary to completely adapt its mechanical structure (engine size, piston diameter, etc.).

The humidifier, when the system is in operation, includes 2 volumes. The first volume contains a volume of liquid water in which the gas passes and becomes humid as it passes. The second volume contains the wet gas. This volume is approximately 300ml for a system producing 0.5 to 5Nm3 of hydrogen per hour. This volume is sufficient to be used as a buffer in this solution. There is no need for an additional tank.

With PID optimization and more progressive accompaniment of the compressor with the electrolyser during start-up, the volume is reduced. However, it is useful for humidification in order to optimize the separation of the liquid phase from the gas phase. In comparison, current solutions require, in these operating ranges, a minimum buffer volume of 150L, or a volume 500 times greater.

The invention is suitable for installations producing less than 10kg/d of hydrogen. But it can also be applied to larger capacity installations.

The invention allows a significant saving of space because it allows a significant reduction in the buffer volume. It also allows a reduction in implementation complexity because it uses a very simple calculation program. In addition, it does not require any hardware modification to the compressor. This is particularly useful for manufacturers of hydrogen production and distribution stations for small mobility or for industrial uses of hydrogen because it allows a reduction in the size of installations and a reduction in costs.

The invention can also be used for any other system requiring hydrogen production and compression. For example for use in residential environments where green hydrogen is produced and used as a vector allowing energy autonomy. For this use, the invention allows a significant saving of space for homes where the available space may be limited. In addition, the integration of silent compression is suitable for residential environments.

The invention therefore makes it possible to resolve a significant number of constraints present today on hydrogen production and compression systems. These constraints are an obstacle to the deployment of these solutions for different uses.

PID control:

The compression function is provided by current control. Increasing compressor compression decreases the pressure between the compressor and the electrolyzer. Conversely, decreasing compressor compression increases the pressure between the compressor and the electrolyzer.

A pressure regulation function between the electrolyser and the compressor is therefore possible by controlling the current to the measured pressure.

The pressure sought represents the PID setpoint.

PID control is characterized by regulation of 3 orders:
- proportional regulation => speed in reaching the set pressure;
- integral regulation => static error between the set pressure and the measured pressure;
- derived regulation => reduction of pressure oscillations around the set pressure.

These 3 orders correspond respectively to the coefficients Kp, Ki, Kd.

According to an example, the method implements a PID of type:

With output = current; e = setpoint – input

This adaptation of hydrogen production according to the number of electrolysers and the compression power and therefore the pressure regulation is possible thanks to a dynamic evolution of the coefficients Kp, Ki, kd of the PID.

Claims

Hydrogen production process with adjustment of the power of a compressor as a function of the production flow of an electrolyser, said process comprises the following steps:
- a) electrolyze by an electrolyzer producing hydrogen with a flow rate of between 0.5 and 5 Nm3/h at an outlet pressure of between 1 and 50 bars;
- b) compress hydrogen from step a) using an electrochemical compressor;
characterized in that it further comprises a step of correcting the supply current of the electrochemical compressor relative to a target pressure value; and in which the correction step comprises a sub-step of modifying said target pressure value:
- as soon as the outlet pressure of the electrolyzer exceeds a target pressure threshold, the value of the supply current of the electrochemical compressor is increased;
- as soon as the outlet pressure of the electrolyzer is below an outlet pressure threshold of the electrolyzer, the value of the supply current of the electrochemical compressor is reduced.
Method according to claim 1, in which it further comprises a step of humidifying the hydrogen by a humidifier so that the hydrogen produced in step a) has a relative humidity of between 80% and 99%.
Method according to claim 1, in which during the correction step the target pressure value is regulated by a regulator of the proportional integral or proportional integral derivative type.
Method according to claim 1, wherein in the correction step the target pressure value uses data from at least one of the following sensors: a pressure sensor and a temperature sensor.
Method according to claim 4, in which during the correction step the pressure sensor is positioned between the electrolyzer and the electrochemical compressor on the humidifier.
Method according to claim 2, in which during the humidification step, the humidifier has a buffer volume of less than 500ml.