RU2733200C1 - Compact system for stabilizing hydrogen pressure in portable power source based on chemical reactor - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to electrical engineering and can be used to stabilize hydrogen pressure in portable power sources including a chemical hydrogen source and an electrochemical generator. Feed buffer is connected in series with a chemical hydrogen source in a portable source of ethane to the hydrogen gas line. Storage buffer used is a thin-walled polymer vessel with a screwed cover, which is equipped with a sealed nozzle, outside the cover the union ends with a threaded connection for connection of a gas threaded fitting, and opposite end of nozzle has cylindrical place with groove for fixation and sealing of elastic membrane. Elastic membrane divides the total volume of the vessel into two compartments of variable volumes. On the shell of the vessel there are outlet and inlet valves, pressure in compartment with hydrogen is determined by pressure in gas main line of hydrogen. To adjust the pressure inside the polymer vessel there are inlet and outlet valves, or holes with diameter from 1 mm to 3 mm with smooth edges. At abrupt jumps of hydrogen pressure in the gas main the elastic membrane in the buffer-accumulator provides the required inertia of change of volumes of compartments and limits pressure drops of hydrogen in gas main line of the power supply source in safe limits.

EFFECT: technical result of invention is smoothing of non-uniformity in generation of hydrogen and maintenance of constant level of power of portable power supply.

6 cl, 11 dwg, 1 ex

Description

The technical field to which the invention relates:

The present invention relates to the field of hydrogen energy, more precisely to methods for obtaining a stable hydrogen pressure in mobile energy sources based on chemical hydrogen sources and electrochemical generators.

Hydrogen fuel cell (FC) power systems are used in unmanned aerial vehicles, robotic systems, mobile and portable energy sources. They provide a high specific energy reserve that is 2-3 times higher than lithium-ion storage batteries, are characterized by a high (up to 60%) efficiency of the process of converting chemical energy into electrical energy and a high specific energy content of hydrogen used as a fuel. Despite the indisputable advantages, there are a number of problems, without the solution of which there is no rapid spread of power systems to thermal power plants.

One of these problems is the problem of efficient and safe storage and transportation of hydrogen. There are several ways to store hydrogen, namely: storing gaseous hydrogen in a compressed form, storing hydrogen in a liquefied state, storing hydrogen in metal hydride compounds, storing hydrogen in mesoporous sorbents. Physical methods of hydrogen storage (compressed or liquefied hydrogen) are characterized by a high hazard when handling a hydrogen source, which imposes severe restrictions on the conditions of storage, transportation and refueling of the hydrogen source. Methods for storing hydrogen in metal hydride compounds or mesoporous sorbents provide increased safety when working with a hydrogen source, since with these methods there is no danger of the formation of large volumes of an explosive gas mixture of hydrogen with air if the vessels are damaged or depressurized. At the same time, the gravimetric storage capacity of hydrogen in mesoporous sorbents is less than 2% by weight, which severely limits the use of these hydrogen sources in mobile and portable systems. Metal hydride hydrogen storage systems appear to be the optimal solution for mobile and portable applications due to the combination of high gravimetric hydrogen storage capacity (from 2 wt% to 12 wt%), absence of overpressure and cryogenic temperatures. Classic examples of metal hydride compounds used as hydrogen sources are sodium boron hydride (NaBH ₄ ), magnesium hydride (MgH ₂ ), aluminum hydride (AlH ₃ ), intermetallic hydrides (LaNi ₅ H ₆ and FeTiH ₂ ).

Extraction of hydrogen from metal hydrides is possible with reversible desorption of hydrogen from compounds, or with an irreversible chemical reaction. The first method is used in stationary energy systems and hydrogen energy storage systems. The latter method does not require energy costs and allows for greater efficiency from a hydrogen source, which is most important for mobile and portable applications.

One of the most difficult problems arising during the operation of portable power sources is the problem of maintaining the generation of hydrogen flow at a constant level, which corresponds to the nominal power of the electrochemical generator. The amount of hydrogen released during an irreversible chemical reaction depends on many process parameters. In the case of using magnesium hydride powder and water / water vapor as reagents, the amount of released hydrogen will depend on the temperature in the reaction zone and the concentration of the initial components, the permeability of the layer of reaction products for steam and hydrogen, and, on the design features of the reactor - a chemical source of hydrogen. The reaction product in the described case is a mixture of magnesium oxide and hydroxide, which can also absorb and release water vapor depending on temperature, pressure, and specific surface area of the powder. During the reaction at elevated temperatures, magnesium hydride can decompose into metallic magnesium and hydrogen. In this case, over time, a mixture of magnesium and magnesium hydride powders appears in the reaction zone, the interaction parameters of this mixture with water turn out to be completely different in comparison with pure magnesium hydride powder. With an increase in the pressure gradient in the reactor, the powder mass becomes denser, while the time of diffusion of water vapor to the reaction zone increases and the controllability of the reactor worsens. The ratio of the hydrogen outlet volume to the water / steam supply volume is highly non-linear, pulsating. A similar behavior of chemical sources of hydrogen, to one degree or another, is observed when using other types of hydrides: hydride of calcium, sodium, lithium and their mixtures with other chemical components, for example, silicides. Systems using solutions of boron metal hydrides, such as sodium boron hydride (NaBH ₄ ), also have problems with maintaining a constant level of hydrogen generation. As a rule, such chemical sources of hydrogen are built according to the same principle: a solution of sodium boron hydride in water is pumped through a mesh / filter with a supported catalyst, which stimulates the reaction of interaction with water with the release of hydrogen. Over time, the temperature of the solution changes, the concentration of sodium boron hydride in water decreases, the activity of the catalyst deteriorates (for many reasons), which changes the hydrogen yield per unit of time and requires adjustments to the operation and controllability of the chemical hydrogen source.

Thus, there is a need to develop a device that would allow smoothing irregularities in hydrogen generation for short periods of time and provide a constant power level of a portable power source.

Vehicle tier:

US Pat. No. 4,155,712 describes a method for controlling the rate of hydrogen generation in a chemical reactor based on metal hydride compounds CaH ₂ or LiAlH ₄ by metering the supply of water vapor. The section with water in the reactor is separated from the section containing the metal hydride powder by a hydrophobic porous Teflon membrane that allows water vapor to pass through and does not allow water to pass through in liquid form at low pressure drops. With an increase in the hydrogen pressure in the compartment with metal hydride, the pressure drop across the porous membrane decreases and the flow of new portions of water vapor through the membrane into the compartment with metal hydride stops. The authors of the patent believe that such a solution can provide an automatic supply of water vapor while maintaining a predetermined excess pressure in the compartment with water in the reactor. But the description does not take into account that during the operation of the reactor there is an increase in temperature, a change in the concentration of metal hydride, an increase in the layer of reaction products on the surface of particles, etc. All of the above factors have a significant effect on the rate of interaction of water vapor with metal hydride particles and cause nonlinearity in the dependence of the hydrogen flux on the amount of supplied water vapor. In addition, the patent does not contain graphs of the experimental dependence of the flow and pressure of hydrogen during the operation of the proposed reactor with a hydrogen consumer providing constant output power.

The authors of US Pat. No. 5,593,640 propose to solve the problem of uneven hydrogen flow from a chemical hydrogen source based on high-temperature hydrolysis of LiAlH ₄ by using an alloy buffer capable of reversibly absorbing and releasing hydrogen upon changes in pressure and temperature. Compounds of rare earth metals with a high nickel content are efficiently absorbed and evolved at temperatures close to normal and pressures of the order of ten bar. The patent discusses Alloy M manufactured by Hydrogen Consultants Inc. USA. The proposed method is designed to operate under pressure significantly exceeding atmospheric pressure, which necessitates the use of containers, pipelines and connections of appropriate strength. Therefore, due to its heavy weight, it is not suitable for mobile applications. In addition, the graph of hydrogen evolution given in the text of the patent demonstrates that the dependence of the hydrogen flux on time has a non-linear character with small local maxima and minima, which indicates the imperfection and high inertia of the proposed system.

US Pat. No. 7,951,349 proposes a method for controlling the amount of evolved hydrogen in the reactions of high-temperature hydrolysis of magnesium and magnesium hydride powders, as well as in the reaction of thermolysis of magnesium hydride powder supplied to the reactor in the form of pellets or disks, by dosing or by adjusting the flow of heat removed from the reactor. But even in this method, judging by the given graphical dependencies, it is not possible to achieve a uniform and constant flow of hydrogen in time.

US 2010/0064584 describes a method for maintaining a constant level of hydrogen generation using a dispenser that feeds briquettes or pellets of hydrides or metal hydride complexes into solution in response to a decrease in pressure at the outlet of hydrogen from the system. With such a discrete method of controlling the concentration of the solution, pulsations in the pressure and the amount of generated hydrogen at the outlet from the system will be observed.

Thus, the indicated disadvantages of the above methods for controlling the amount of generated hydrogen cause fluctuations in the hydrogen pressure in the system, which negatively affects the operation of fuel cell batteries in mobile energy sources.

Disclosure of the invention:

The objective of the invention is to propose a solution to the problem of compensating for pressure surges of hydrogen in the system of a portable source, which has two main components: a chemical hydrogen source and an electrochemical generator. Hydrogen pressure surges at a constant payload on a fuel cell arise for two main reasons - the difficulty of providing a constant flow of hydrogen from a chemical hydrogen source and because of the need to periodically purge the electrochemical generator with hydrogen to discharge ballast gases.

The most popular on the market are portable power sources with power up to 300 W. Depending on the efficiency of electrochemical generators, which usually ranges from 35% to 40%, the required hydrogen flow is 0.24-0.27 m ³ / h. Purge of electrochemical generators is carried out at intervals of once every 2-3 minutes, depending on the generator model and its manufacturer. The volume of hydrogen required for flushing (at normal hydrogen pressure) is from 40 cm ³ to 70 cm ³ . The inertia of the reaction of a chemical source of hydrogen (change in the rate of hydrogen generation) to a change in the rate of supply of reagents (water or a metal hydride compound in the form of powder or pellets) ranges from several seconds to one to two tens of seconds. In addition, in chemical sources of hydrogen of the hydrolysis type, conditions periodically arise for a spontaneous, short-term and very sharp acceleration of the reaction caused by local overheating and release of adsorbed moisture from the reaction products or mechanical destruction of the layer passing through the catalyst. Such processes of pulsation of thermokinetic chemical reactions were investigated by Professor Frank-Kamenetsky in the 30s of the last century. If the volume of gas in the system, including the volume of gas in the chemical source of hydrogen, the volume of pipelines and the electrochemical generator does not exceed 100 cm ³ , then during the purging of the electrochemical generator and when the rate of hydrogen generation by the chemical source of hydrogen changes in the system, large pressure jumps are observed from values less than 0, 01 MPa to a value greater than 0.1 MPa. Most of the widespread PEMFC fuel cell batteries with a polymer membrane in a membrane-electrode assembly are designed to operate in the hydrogen pressure range of 0.03-0.08 MPa. At a pressure of less than 0.03 MPa, hydrogen starvation begins at the electrochemical generator, which negatively affects the long-term electrophysical characteristics of the device. Pressures above 0.08 MPa are dangerous for the membranes of the fuel cells of the electrochemical generator due to the possibility of membrane rupture. Thus, it becomes necessary to use in the gas system a mobile power source some kind of gas buffer storage, which has the ability to very quickly consume and release hydrogen in volumes not exceeding 1 dm ³ and operating in the hydrogen pressure range from 0 MPa to 0.15 MPa. In addition, the storage buffer should be lightweight. The proposed solution is a thin-walled container made of polyethylene terephthalate, lavsan or polyester closed with a screw cap made of similar materials. The screw plug has a sealed fitting - a gas inlet, which ends with a threaded connection on the outside for connecting a gas threaded fitting, and inside it has a cylindrical seat with a groove for a rubber sealing ring for fixing and sealing an elastic inflatable membrane. There are two additional valves on the shell of a thin-walled polymer container: an inlet valve that opens when the pressure inside the container drops below atmospheric and an outlet (safety) valve that opens when the pressure inside the container is above 0.04 MPa.

Implementation of the invention:

When assembling the buffer storage, the pressure inside the thin-walled polymer container is equal to atmospheric (Fig. 4-a). The elastic inflated membrane (Fig. 4-a, pos. 6) is in a compressed state and the volume of air in it is insignificant. The buffer storage connected to the gas main works as follows. When a chemical source of hydrogen is switched on, the pressure in the gas line begins to rise (Fig. 4-b). The elastic membrane inside the polymer container begins to inflate and compress the air in it, the pressure inside the container and inside the membrane are equalized. When the pressure in the gas line rises to 0.04 MPa, the safety valve is triggered and the air in the tank is released, the inflating membrane occupies almost the entire volume of the polymer tank (Fig. 4-c). With an even greater increase in pressure, the pressure in the polymer container and inside the inflated membrane also increases linearly (Fig. 4-d).

In the case of using a storage buffer with a hole (Fig. 5), when a chemical source of hydrogen is turned on and the pressure in the system increases, the elastic membrane inflates and occupies almost the entire volume of the storage buffer (Fig. 5-a, 5-b). During normal operation of the gas system of a power source with fuel cells and a pressure in the system of more than 0.01 MPa, the entire volume of the storage buffer is occupied by hydrogen. In this case, the implementation of the pressure stabilization system, the storage buffer serves to increase the gas volume of the system and, therefore, increases its gas inertia, thus smoothing out sharp pressure drops. When the chemical source of hydrogen is turned off, the pressure in the gas system is reduced and, depending on the design of the fuel cell stack control system, can be reduced to zero by opening the bleed valve. In this case, hydrogen is removed from the storage buffer due to the tensile force of the elastic membrane (Fig. 5-c).

When a fuel cell stack with a constant rated power output is put into operation with a chemical hydrogen source, hydrogen begins to be consumed. With the coordinated operation of the chemical hydrogen source, the control algorithm of which selects the optimal water flow rate or the rate of pumping the solution of the metal hydride compound through the catalyst to obtain the nominal hydrogen flow at the outlet, the hydrogen pressure in the gas system on average remains in the range of 0.03-0.05 MPa. However, spontaneous increases and decreases in the hydrogen generation rate periodically occur, and the pressure in the gas system of the portable power source changes. In this case, the storage buffer effectively compensates for these changes in pressure surges: with a sharp drop in pressure in the gas line, the inflated membrane is compressed and within several tens of seconds is able to maintain the required hydrogen flow and the required minimum pressure; with a sharp increase in pressure, the membrane increases in size and contributes to a lower rate of pressure growth in the system to critical values, thus providing a certain margin of time to correct the water supply or the rate of pumping the metal hydride solution.

Example 1: It is proposed to use a regular PET bottle for liquids of a suitable shape and volume as a polymer container, which is designed for a maximum internal pressure of up to 1 MPa.

An illustration of the efficiency of the drive buffer is shown in Fig. 2. In this case, we investigated the operation of a portable power source with a chemical source of hydrogen based on high-temperature hydrolysis of magnesium hydride powder and a battery of 100 W fuel cells. The nominal hydrogen flow for such a nominal power and the efficiency of the used fuel cell stack 35% is 1.45 dm ³ / min. Without the use of the storage buffer, during the operation of the portable power supply for 2 hours, 15 excess hydrogen was vented when the hydrogen pressure in the system exceeded 0.055 MPa, and 21 times the emergency disconnection of the load from the fuel cell battery occurred when the pressure dropped below 0.03 MPa.

When using the storage buffer, during 2 hours of operation, there was not a single increase in pressure to 0.055 MPa, and once the payload was disconnected from the fuel cell stack when the pressure in the system dropped below 0.03 MPa. Thus, the buffer storage cardinally improves the operation of the power supply and increases the efficiency and efficiency of its use.

Brief description of the figures:

In fig. 1 is a diagram of a mobile power supply, showing that a chemical hydrogen source, a buffer-storage, an electrochemical generator are connected in series into a single gas system.

In fig. 2 (a, b) shows the graphs of pressure changes in the gas system of a portable power source providing an output power of 100 W, based on hydrogen fuel cells: without a storage buffer (a), with a storage buffer (b). As a source of hydrogen, a cartridge was taken, implemented on the principle of high-temperature hydrolysis of magnesium hydride according to patent application RU 2018123625 dated 06/28/2018

In fig. 3 shows a diagram of a storage buffer. The numbers in the diagram indicate:

1 - thin-walled polymer vessel;

2 - screw cap;

3 - sealed fitting;

4 - gas threaded fitting;

5 - cylindrical seat with a groove for a rubber ring;

6 - elastic membrane;

7 - outlet (safety) valve;

8 - inlet valve;

In fig. 4 (a, b, c, d) shows a sequence of images illustrating the operation of the storage buffer (implementation with valves):

a) The elastic inflation diaphragm (Fig. 4-a, pos. 6) is in a compressed state.

b) The pressure in the gas line starts to rise (Fig. 4-b). In this case, the elastic membrane inside the polymer container begins to inflate and compress the air in it so that the pressure inside the container and inside the membrane are equalized.

c) When the pressure in the gas line rises to 0.04 MPa, the safety valve is triggered and releases the air in the tank so that the inflated membrane occupies almost the entire volume of the polymer tank (Fig. 4-c).

d) With a sharp drop in pressure in the gas main, the membrane is compressed (Fig. 4-d) and within several tens of seconds is able to maintain the required hydrogen flow in the gas main.

In fig. 5 (a, b, c) shows a sequence of images illustrating the operation of the storage buffer (implementation with a hole):

a) When the pressure in the system rises, the elastic membrane inflates

b) The elastic membrane inflates and occupies almost the entire volume of the storage buffer (Fig. 5-a, 5-b).

c) Hydrogen is removed from the storage buffer due to the tensile force of the elastic membrane.

Claims (6)

1. A method for stabilizing hydrogen pressure for portable power sources, including a chemical hydrogen source and an electrochemical generator, which consists in connecting the buffer-storage to the gas line of the portable power source in series with the chemical source of hydrogen and characterized by the original design of the buffer-storage, which has two compartments with a movable elastic baffle, movable within the total volume of the buffer storage and providing, in case of sudden pressure surges in the hydrogen gas system, a change in the volumes of the compartments within the total volume of the storage buffer with inertia sufficient to maintain the hydrogen pressure in the gas pipeline within the specified limits. 2. A device for stabilizing the hydrogen pressure for portable power sources, including a chemical hydrogen source and an electrochemical generator, is a hydrogen storage buffer connected to the gas main of a portable power source in series with a chemical hydrogen source, characterized in that a light thin-walled a polymer vessel with a screw cap, equipped with a sealed fitting on which an elastic membrane, poorly permeable to hydrogen is fixed, limiting the part of the volume of the polymer vessel occupied by hydrogen, and two valves: an outlet valve that opens when the pressure inside the polymer vessel exceeds 0.4 kPa and an inlet valve, opening when the pressure in the polymer vessel drops below atmospheric. 3. A device according to claim 2, in which a polymer vessel is used with an ultimate burst pressure of at least 0.8 MPa and a capacity of 0.5 dm ³ to 2 dm ³ . 4. The device according to claim. 2, in which rubber balls with a wall thickness of 0.1 mm to 1 mm are used as the inflatable elastic membrane. 5. Device p. 2, in which the safety and inlet valves are made of aluminum-magnesium alloys or polymer materials. 6. The device according to claim. 2, in which a hole with a diameter of 1 mm to 3 mm with smooth edges serves instead of the inlet and outlet valves to regulate the pressure inside the polymer vessel.

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