System and method for high pressure, passive condensing of water from hydrogen in a reversible solid oxide fuel cell system

A method for passively removing water from a stream of hydrogen gas includes receiving a stream of hydrogen gas that is water-saturated, having an initial pressure below about 1 psig and an initial temperature above about 25° C., compressing the stream of hydrogen gas to an elevated pressure, chilling the compressed stream of hydrogen gas to a low temperature, and condensing water from the compressed and chilled stream of hydrogen gas until the water content of the stream of hydrogen gas is below about 100 ppm.

FIELD OF THE DISCLOSURE

The present disclosure is related to removal of water from gaseous hydrogen. More particularly, the present disclosure relates to a system and method for high pressure, passive condensing of water from hydrogen produced by a Reversible Solid Oxide Fuel Cell (RSOFC) energy storage system.

BACKGROUND

Fuel cells are devices that convert chemical energy from a fuel, such as hydrogen, into electricity through a chemical reaction with oxygen or another oxidizing agent. There are several different types of fuel cells. Fuel cells generally include an anode, a cathode, and an electrolyte between the two. The most well-known type of fuel cell is the Proton Exchange Membrane (PEM) fuel cell, in which the electrolyte is a proton exchange membrane that allows ions (e.g. hydrogen ions) to pass through it, while electrons cannot. At the anode a catalyst oxidizes the hydrogen fuel, turning the fuel into positively charged ions and negatively charged electrons. The freed electrons travel through electrical conductors, thus producing the electric current output of the fuel cell. The hydrogen ions, on the other hand, travel through the proton exchange membrane to the cathode, where they react with a third chemical, usually oxygen, to create water vapor, which is typically exhausted as waste.

Another type of fuel cell is the solid oxide fuel cell (SOFC). Rather than a proton exchange membrane, the SOFC has a solid oxide or ceramic electrolyte. The solid oxide electrolyte conducts negative ions from the cathode to the anode, where the electrochemical oxidation of the oxygen ions with hydrogen occurs. Compared to PEM fuel cells, SOFC's can have higher efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost, in part because they do not include expensive platinum catalyst material. At the same time, SOFC's have higher operating temperatures than PEM fuel cells (typically between 500° C. and 1,000° C.), which results in longer start-up times, and they can experience degradation with repeated thermal cycling.

Fuel cells can theoretically work forward or backward. That is, they can operate to produce electricity from a given chemical reaction, or they can consume electricity to produce that chemical reaction. However, typical fuel cells, especially PEM fuel cells, are usually optimized for operating in one mode—either electricity generation mode or electrolysis mode—and are generally not built in such a way that they can be operated in both modes. Recently, however, reversible solid oxide fuel cells (RSOFC's) have been developed that can produce electricity from hydrogen fuel, or produce hydrogen fuel from electricity.

Because of these features, RSOFC's are considered good candidates for powering and storing energy on micro-grids. Micro-grids are local power distribution systems designed to supply local energy generation for both grid and off-grid connected facilities and communities, enabling a localized energy source in cases of emergencies or unreliable traditional grid use. The high cost and energy security issues associated with importing fuel to isolated or “islanded” grids has led to a growing desire to generate power onsite with alternative and renewable energy technologies, while reducing facility costs of importing electrical power. Energy storage is desirable to balance the micro-grid and improve efficiency, reduce fuel consumption, and provide power in the event of power outages. In order to stabilize a local power grid with continuous power, an RSOFC system can operate in Fuel Cell mode when needed, using the stored hydrogen to produce energy for the grid. This can allow for grid stabilization and improvement to power plant system efficiency.

Recently, there has also been interest in the energy sector in RSOFC's for energy storage, where they can be used in conjunction with renewable energy generation sources, such as wind and solar generation. In power generation systems, such as wind and solar energy systems, excess power must be stored or it is lost. Current systems available for storing energy present a variety of drawbacks, but RSOFC systems present a potential improvement in this area. Theoretically, excess power generated in off-peak hours can be sent to an RSOFC system operating in electrolysis mode to produce H2, which is compressed and stored in tanks. The H2can then be used later in the same RSOFC system operating in fuel cell mode to provide supplemental power to the grid during peak hours or when specifically needed.

Notably, full scale application of RSOFC systems as energy storage and grid-stabilization systems has not previously been done. Consequently, many of the actual features that are needed for real world application of RSOFC's for energy storage and power grid stabilization have not previously been developed.

In making the first applications of this kind, it has been found that one challenge presented by RSOFC energy storage systems relates to the removal of water from the stream of H2gas produced by an RSOFC unit operating in electrolysis mode. Hydrogen gas produced by an RSOFC unit operating in electrolysis mode will typically be saturated with water, and at a relatively low pressure (e.g. below about 1 psig). It is desirable, however, to reduce the dew point of the gas below a level which will result in condensation of water in the storage containers for the gas. This water content level of the H2may be less than about 100 ppm prior to storage, which is considered an acceptable level for storage in commercial H2tubes. Consequently, additional water removal is desired. Unfortunately, typical water removal systems that have been used in preparing hydrogen gas for fuel cell use are unsuitable for use in an RSOFC system like that disclosed herein because they generally require a significant pressure differential, and they tend to lose a significant quantity of hydrogen in the process. Therefore typical water removal systems that require a sizeable pressure drop ahead of the compression step are not practical here.

The present disclosure is intended to address one or more of the above issues.

SUMMARY

It has been recognized that it would be desirable to have a method for removing water from a saturated H2product stream from an RSOFC unit operating in electrolysis mode at approximately atmospheric pressure.

It has also been recognized that it would be desirable to have a method for removing water from a saturated H2product stream from an RSOFC unit operating in electrolysis mode that is passive, and causes minimal loss of H2.

In accordance with one aspect thereof, the present disclosure provides a method for passively removing water from a stream of hydrogen gas. The method includes receiving a stream of hydrogen gas that is water-saturated, having an initial pressure below about 1 psig and an initial temperature above about 25° C., compressing the stream of hydrogen gas to an elevated pressure, chilling the compressed stream of hydrogen gas to a low temperature, and condensing water from the compressed and chilled stream of hydrogen gas until the water content of the stream of hydrogen gas is below about 100 ppm.

In accordance with another aspect thereof, the present disclosure provides a method for passively removing water from a stream of saturated hydrogen gas produced from a Reversible Solid Oxide Fuel Cell (RSOFC) unit operating in electrolysis mode. The method includes receiving a stream of saturated hydrogen gas produced from an RSOFC unit operating in electrolysis mode, the stream of hydrogen gas having an initial pressure below about 0.5 psig and an initial temperature above about 25° C., compressing the stream of hydrogen gas with a compressor to a pressure above 1000 psig, and chilling the compressed stream of hydrogen gas to a temperature below about 10° C. The method further includes passively condensing water from the compressed and chilled stream of hydrogen gas to produce a water content of the stream of hydrogen gas below about 100 ppm, separating the water condensate from the H2gas, and storing the compressed hydrogen gas in a storage container for later use as fuel for the RSOFC unit.

In accordance with yet another aspect thereof, the present disclosure provides a system for passively removing water from a stream of hydrogen gas. The system includes a conduit, coupled to deliver a stream of saturated hydrogen gas having an initial pressure below about 1 psig and an initial temperature above about 25° C., a compressor, configured to compress the stream of hydrogen gas to an elevated pressure, a chiller, configured to chill the compressed stream of hydrogen gas to a low temperature, and a water trap, configured to passively receive water condensate from the compressed and chilled stream of hydrogen gas as the stream passes therethrough, and to separate the compressed and dewatered hydrogen from the condensate, the compressed and dewatered hydrogen having a water content of less than about 100 ppm.

DETAILED DESCRIPTION

As noted above, in power generation systems, such as wind and solar energy systems, excess power must be stored or it is lost. Current systems for storing energy include flywheels, batteries, pump hydroelectric and compressed air storage, for example. Each of these methods present drawbacks that suggest the desirability of RSOFC energy storage systems. However, full scale application of RSOFC systems as energy storage and grid-stabilization devices has not previously been done.

Advantageously, a system and method have been developed for passively removing water from a compressed hydrogen gas stream that is produced in the electrolysis mode of an RSOFC system without significant loss of hydrogen, making the gas suitable for storage and later use as fuel for the RSOFC system. Shown inFIG. 1is a schematic diagram of an embodiment of a Reversible Solid Oxide Fuel Cell (RSOFC) energy storage system100, configured in accordance with the present disclosure. The RSOFC system100acts as an energy storage device to store and supply energy to an electrical grid110based on the demand requirements from the grid, or commands from a grid network manager. When excess power is generated by the grid or the grid demands the RSOFC system to store energy, the RSOFC system can operate in electrolysis or EL mode to generate, compress, and store H2from the electrolysis of water. This is accomplished by applying power from the gird110to the RSOFC system100, when operating in EL mode. When power is needed by the grid110, the stored H2is fed to the RSOFC system100operating in fuel cell or FC mode to produce power for the grid.

ViewingFIG. 1, the system100generally includes an RSOFC unit102(also referred to as a “fuel cell stack” or “fuel cell unit” or “fuel cell subsystem”), which is connected to a hydrogen compression and storage system, indicated generally at104, and a process water system, indicated generally at106. The fuel cell unit102is electrically coupled to an AC/DC converter108, which connects to the local power grid110through a power distribution box112. The fuel cell subsystem102includes sensors, controls, etc. (not shown), and can include its own subsystem controller103, which can interface with the system master controller174, described below. Alternatively, the fuel cell subsystem102and its associated sensors, controls, etc. can interface directly with the master controller174, allowing the master controller to directly control the fuel cell unit102.

While a single fuel cell unit102is shown inFIG. 1, this is for illustrative purposes only. Those of skill in the art will recognize that multiple fuel cell units or stacks102can be electrically coupled together in parallel and/or series to provide the desired output voltage and current from the fuel cell system100. The RSOFC unit102produces power from oxidation of stored hydrogen (H2) from the hydrogen compression and storage system104when in fuel cell or FC mode, and generates hydrogen (H2) through electrolysis of water from the process water system106when in electrolysis or EL mode. The reversible solid oxide fuel cell unit102includes various features that are not specifically shown, such as an anode vent and anode vent valve, a stack preheater, condenser, etc. These types of components are generally known, and will be appreciated by those of skill in the art.

The hydrogen compression and storage system104includes a group or array of hydrogen fuel storage tubes or cylinders118and a hydrogen compressor120. The hydrogen fuel storage tanks118are configured to store hydrogen gas at high pressure, and are coupled to the fuel cell unit102via a hydrogen fuel line122. As used herein, the terms “high pressure” or “elevated pressure” in reference to the hydrogen storage, means any pressure above about 250 psi. In one embodiment, the hydrogen fuel storage tanks118and the system100as a whole are designed to store hydrogen at a pressure above 1000 psi and more particularly up to about 2500 psi, but much higher pressures can also be used. For example, some hydrogen fuel cell systems store hydrogen at pressures of 10,000 psi and higher. The hydrogen fuel line122includes a pressure reducing valve124and a main hydrogen control valve126. A high pressure hydrogen vent128and its associated valve130are also coupled to the hydrogen fuel line122to vent hydrogen gas in case of overpressure in the hydrogen fuel storage array118. The pressure reducing valve124allows hydrogen gas to be provided from the hydrogen fuel storage array118, which is at high pressure (e.g. 2500 psi), to the RSOFC unit102at lower pressure (e.g. 75 psi) when the RSOFC unit102is operating in fuel cell mode.

The compressor120is coupled to the RSOFC unit102via the fuel cell output line132, which directs low pressure output from the fuel cell102through a condenser168, which initially separates a significant quantity of water from the fuel cell output, and thence into a low pressure hydrogen line133. The compressor120receives low pressure hydrogen gas as output from the RSOFC unit102via the low pressure hydrogen line133when the RSOFC unit102is operating in electrolysis mode. A hydrogen recycle line182connects the low pressure hydrogen line133to the hydrogen fuel line122, and includes a hydrogen recycle blower184. This hydrogen recycle line182is useful in fuel cell mode for pumping residual hydrogen back for consumption in the fuel cell unit102. When operating in fuel cell mode, some residual amount of hydrogen gas may pass through the fuel cell102without reacting and producing electricity. This residual hydrogen will be separated from the water vapor in the fuel cell output line132by the condenser168, and is returned to the hydrogen fuel line122by the hydrogen recycle blower184. Disposed in the low pressure line133are a compressor suction valve134and a fuel cell exhaust vent136and fuel cell exhaust vent valve138. A buffer tank140is also coupled in line with the low pressure line133near the intake of the compressor120. The buffer tank140is fed H2generated by the fuel cell subsystem102. The buffer tank140is coupled to the compressor suction vent142via a buffer tank vent line141and buffer tank vent valve143. Pressure is monitored in the buffer tank140to ensure that pressure does not get too high or too low, since this could cause elevated operating pressure in the fuel cell subsystem102, or, in the case of low pressure, create a vacuum that could draw air in if there is a leak.

The compressor120includes a compressor suction vent142, along with a drain146for allowing drainage of water from the compressor condenser (not shown). The compressor120is configured to receive low pressure hydrogen gas produced from the electrolysis of water in the fuel cell102, and compress this gas and provide it to the hydrogen fuel storage array118via the compressed hydrogen supply line148. A compressor recycle line150is also connected between the hydrogen fuel line122and the low pressure line132, with a compressor recycle line valve152therein. The compressor recycle line valve152can be a pressure reducing valve, since the pressure in the low pressure line133is generally below the pressure in the hydrogen fuel line122. Alternatively, a separate pressure reducer (not shown) can be included in the compressor recycle line150. This compressor recycle line150is coupled to the hydrogen fuel line122downstream of the pressure reducing valve124, and allows low pressure hydrogen to be recycled through the compressor120if desired, rather than being fed to the fuel cell unit102, as discussed in more detail below. Other features of the compressor120, such as a condenser, cooler, etc., are not shown inFIG. 1, but will be appreciated by those of skill in the art.

The process water system106includes a water storage tank or reservoir154and a desalinator/deionizer unit156, along with a water supply pump158and a process water pump160. The water storage tank154can include a drain155for allowing the process water to be drained from the tank. The process water system106provides deionized water to the RSOFC unit102, whether from the water storage tank154or the deionizer unit156or both, when the RSOFC unit102is operating in electrolysis mode, and can receive exhaust water from the RSOFC unit102when the RSOFC unit102is operating in fuel cell mode (water output from a fuel cell operating in fuel cell mode is naturally deionized). Deionized water is desired for electrolysis in order to avoid the introduction of minerals and chemical species that can interfere with the electrolytic reactions of the fuel cell unit102or degrade its condition.

The deionizer unit156can be connected, as indicated at162, to any suitable water supply, such as a local municipal potable water distribution system, or even to a sea water intake. The deionizer unit156can be configured to remove salt from the water through a multistep process of reverse osmosis, followed by a deionization step to further purify the water. Where the water supply is a potable water source, the process can involve only the deionization step. A sensor (not shown) can be placed on the exit of the deionizer unit156to measure the conductivity of the water, to determine the completeness of deionization and its suitability for use in the RSOFC unit102. A drain157can be provided for drainage of brackish water from the deionizer unit156.

The process water system106is coupled to the fuel cell unit102through the process water pump160via a water supply line164and a water return line166, which can return water from the condenser168to the water storage tank154. With this configuration, the RSOFC system100can be a closed-loop system, retaining and recycling process water whether operating in electrolysis or fuel cell mode. In fuel cell mode, water exhaust produced in the fuel cell unit102can be condensed by the condenser168and returned to the water storage tank154via the water return line166. In electrolysis mode, process water can be drawn from the water storage tank154and/or the deionizer unit156and pumped via the process water pump160to the fuel cell unit102, where the water is split into hydrogen, which is compressed and stored, and oxygen, which is exhausted to the atmosphere. Alternatively, the RSOFC system100can be an open-loop system, exhausting water vapor to the atmosphere when operating in fuel cell mode, and drawing water from the process water system106as needed when operating in electrolysis mode. In electrolysis mode, the condenser168acts as a heat exchanger, reducing the fuel cell exhaust temperature (e.g. from about 250° C. to about 40° C.) so that inlet temperatures for the anode recycle blower and the compressor are in a desired range. In this process, water is condensed out of the output stream, and this water is returned to the water storage reservoir154via the water return line166.

The RSOFC100also includes an electrical subsystem, indicated generally at170. The electrical subsystem includes the AC/DC converter108, the power distribution box112, as well as a connection from the power distribution box112to a 24 volt DC power supply172for powering electrical subsystems of the RSOFC system100, including a master controller174, and a connection to supply power to the plant electrical subsystems180, sometimes also referred to as the “balance of plant” electrical. As noted above, the fuel cell unit102is electrically coupled to the local power grid110through the power distribution box112. The AC/DC converter108is a bi-directional converter that converts DC output from the fuel cell unit102into grid power (e.g. 3 phase-4 wire, 480 VAC 60 Hz) for transmission into the power grid110when the fuel cell unit102is operating in fuel cell mode. Conversely, the AC/DC converter108also converts AC input from the power grid110into DC input for the fuel cell unit102when it is operating in electrolysis mode. The electrical subsystem170can also include other elements, such as a transformer (not shown) to convert grid power to 110V AC for utilities use, a ground fault detector (not shown) to measure any leakage current for the entire power distribution box112, current sensors (not shown) for reading the current being drawn by each individual load, including total parallel current for all AC/DC loads, and a line monitor (not shown), which measures voltage on the electrical line connecting the RSOFC system100to the power grid110to read both grid voltage and current. The power grid110can include a grid controller186, which is coupled (e.g. via Ethernet, Internet, wireless connection, etc.) to the master controller174and provides signals indicating a power demand or power surplus condition of the grid110. When the grid110demands power, the master controller174can cause the system100to enter fuel cell mode, and produce power for the grid110, so long as it has a suitable fuel supply. Conversely, when the grid110has a surplus of power, the master controller174receives a signal from the grid controller186indicating this, and causes the system100to enter electrolysis mode to produce and store hydrogen gas.

The power grid110can also include grid-coupled solar, wind or other renewable energy generation systems. The RSOFC unit102can thus receive electricity from these renewable energy generation systems when operating in electrolysis mode, thus allowing the system100to store excess energy that is generated from these variable and intermittent sources. It is to be understood that wind and solar generation systems are only two examples of many types of energy input sources that could be coupled to the RSOFC system100through the grid110. Those of skill in the art will recognize that there are other energy input sources that could be associated with the RSOFC system100.

The master controller174is a microprocessor device, having a processor and system memory, and provided with suitable software for monitoring and controlling all of the systems and connections of the RSOFC system100. The master controller can include or be coupled to a computer terminal176and/or a control panel178for allowing user input and monitoring.

The RSOFC system100also includes a variety of sensors (not shown) that are either associated directly with various components of the RSOFC system100, or are associated with fluid conduits, valves, electrical connections, etc. These sensors are coupled to provide sensor data (e.g. via wired electrical connections) to the master controller174. For example, the hydrogen fuel line122, the low pressure hydrogen line133, the compressed hydrogen supply line148and any other components that handle the storage or transmission of hydrogen can include sensors for pressure, temperature, flow rate, hydrogen presence, water content, etc. Pressure in the hydrogen storage tanks118and downstream regulated pressure can be monitored, and values can be sent to the master controller174. Other sensors can be associated with the various valves to provide indications of valve state (e.g. open or closed). Similarly, pressure, temperature and flow sensors, as well as pump operating sensors can also be associated with each component of the process water system106.

Electrical sensors and switches (not shown) can also be associated with the AC/DC converter108and the power distribution box112and their related electrical connections, both internally and with the power grid110, to allow the master controller to receive input signals related to the conditions of the electrical subsystem170and the control system of the power grid110. The master controller174is thus coupled to all components of the RSOFC system100and can control the modes of the RSFOC unit102and the related devices (e.g. the hydrogen compression and storage system104and the process water system106) based on sensor data.

The RSOFC system100can also include a variety of other subsystems that are not specifically shown inFIG. 1, but can be included for control and operation, and will be familiar to those of skill in the art. For example, the various valves of the RSOFC system100can be power actuated valves, allowing remote control and monitoring of their state. For example, these can be pneumatically actuated valves, and the system100can include a compressed air subsystem (not shown) for providing power for actuating all of these valves in response to electrical control signals from the master controller174. The compressed air pressure can be measured and monitored at the output of the air compressor (not shown) in order to be maintained higher than some desired working pressure, such as 80 psi.

The RSOFC system100can also include a flammable gas detection system (not shown) that includes multiple sensors to monitor for a combustible environment that could occur in the event of a leak of hydrogen. Sensor signals from the flammable gas detection system can be transmitted to the master controller174, which analyzes these signals for any indication of a combustible environment.

The RSOFC system100can also include a thermal management system (not shown) that is configured to supply cooling water to the H2compressor and to the fuel cell unit102for its condenser. This cooling water system can be designed to supply an inlet temperature of about 25° C. and about 30 psi pressure to each system, for example.

Advantageously, the RSOFC system100shown inFIG. 1can be scaled up or down as needed for a given location and application. It has been suggested, for example, that this type of system can be provided in a modular form in a standard size shipping container or the like, so that it can be easily transported to any location where it is to be used.

The various sensors of the RSOFC system100indicate the status of the various components of the system, and, based on the sensor input, the master controller174can determine a state of the RSOFC system100by applying a conditional logic algorithm. The master controller174can thus transition the RSOFC system between the fuel cell mode and the electrolysis mode based upon the sensor data and the system state. The result is an energy storage system that converts excess energy into hydrogen when in electrolysis mode, and compresses that gas for later use. In order to stabilize the power grid110with continuous power, the RSFOC system100can then switch to fuel cell mode and use the stored hydrogen to produce energy for the grid110. The master controller174can thus control and orchestrate the various modes of the RSOFC system100to help ensure that power output meets the demand for power, and that excess power is not lost.

As noted above, the master controller174interfaces with the control system of the power grid110and the various subsystems of the RSOFC system100to determine mode transition and status of the overall system. Shown inFIG. 2is a mode transition diagram for an RSOFC system like that ofFIG. 1. The RSOFC system100has four general operational modes, which are idle mode200, heat up mode202, online FC mode204and online EL mode206. Generally, the master controller (174inFIG. 1) can place the system in FC (fuel cell) mode204or EL (electrolysis) mode206based on the commands or power requirements from the grid110. Between the power generation mode204and hydrogen generation mode206are two changeover modes—a changeover from fuel cell (FC) to electrolysis (EL) mode208, and a changeover from electrolysis (EL) mode to fuel cell (FC) mode210.

At any time during operation in heat up mode202, online FC mode204, online EL mode206, or during the changeover modes208and210, the system can be prompted (e.g. by an operator or through programmed operation by the master controller174) into a controlled shutdown mode212, in which the system will return to idle mode200or await an operator reset signal216. In controlled shutdown mode212the master controller leaves the coolant pump ON in order to provide the necessary cooling to the system. A list of measurements are recorded and monitored. All subsystems follow their own controlled shutdown protocol and send confirmation to the master controller that controlled shutdown mode212has been successfully entered. The system can enter idle mode200when the fuel cell subsystem has indicated it has completed its controlled shutdown. When returned to Idle, the pump is turned off and all valves are returned to their fail safe mode.

Alternatively, at any time during operation in the above-mentioned modes, the system can be prompted by the master controller174into an emergency shutdown mode214. An emergency shutdown is an immediate shutdown of the system due to an operational failure. Emergency shutdown214can be initiated in response to a variety of situations, such as a signal indicating a combustible environment (i.e. a hydrogen leak), a failure of a subsystem of the RSOFC system100, etc. In one embodiment, emergency shutdown only occurs if there is an electrical ground fault detected or a combustible gas sensor (not shown) detects gas concentrations at some level relative to a combustibility limit (e.g. sensing combustible gasses at 50% of a lower explosion limit). After the system enters emergency shutdown mode214, the system remains in this mode until the fuel cell indicates that it is “shutdown,” and until the operator manually resets the system. The operator cannot reset the system until the fuel cell is “shutdown.” At this point the system can go into idle mode200. The Emergency Shutdown mode216occurs when there is a catastrophic operational failure to one of the RSOFC subsystems. In such a situation, all equipment is turned off and placed into fail safe mode. All valves are de-energized, and pumps are turned off. Advantageously, the system is designed to fail safe. However, in emergency shutdown mode214, selected sensors will remain online to allow monitoring, while the other subsystems of the RSOFC system100are shut down. The sensor systems remain online to monitor components of the system.

Further, the conditional logic algorithm of the master controller174or activation of an Emergency-Stop button (not shown) by an operator can shift the system into emergency stop mode218at any time. The emergency stop button opens the main circuit breaker to the RSOFC system100, thus cutting all power to the system, including power to sensors, etc. In order to restart the system after an emergency stop, the main breaker is first closed to restore power, and the system startup procedures outlined inFIG. 3are then commenced.

As indicated above, the primary function of the master controller174is to orchestrate the operation of the subsystems of the RSOFC system100to provide and store electrical power for the grid110. The master controller174receives sensor inputs from the various subsystems, and, based on these input values, the conditional logic algorithm of the master controller will instruct the RSOFC subsystems to enter the appropriate mode, and/or to transition from one mode to another. It has been found that substantial care is desirable when switching between fuel cell (SOFC) and electrolysis (SOEC) modes. That is, the FC to EL and EL to FC changeover modes208,210warrant significant control to avoid certain undesirable conditions. For example, it is desirable to remove excess water and/or hydrogen from the system before the FC to EL transition occurs, since this transition involves going from a high H2concentration to a high water concentration. Accordingly, in Changeover FC to EL mode208the master controller174reads an array of measurements from the sensor data until the system is ready to switch to EL mode206. The conditional logic control algorithm embedded in the system master controller174looks at system parameters and states along with micro-grid commands to determine when and what transition should occur.

Provided inFIG. 3is a high level flowchart showing one embodiment of an operational method, indicated generally at300, by which the master controller174controls the RSOFC system100. As shown inFIG. 2and discussed above, the RSOFC system100has four basic operational modes, which are also shown inFIG. 3. These modes include an idle mode304, a heat up mode308, online FC mode312, and online EL mode320. There are also two changeover modes: a changeover FC (fuel cell) to EL (electrolysis) mode316, and a changeover EL to FC Mode328.

After system start, indicated at block302, the next step is to enter idle mode304until the fuel cell is ready for heat up, as indicated at block306. The system remains in idle mode until the fuel cell is ready for heat up, as indicated at block306. When the system is ready, as determined by the master controller174, it enters heat up mode308, during which the fuel cell stack is heated to its desired operating temperature range. Until the desired temperature has been achieved and other sensors indicate the fuel cell is ready, as indicated at block310, the system does not move forward. Once these conditions are met, the system enters FC (fuel cell—power generation) mode312.

The system remains in FC mode312until the grid indicates that power is not needed, or the H2supply is indicated to be too low for continued fuel cell mode, as indicated at block314. When these latter indications are received, the system shifts to a first changeover mode—change over FC to EL316, which involves system adjustments to allow transition from producing power from the fuel cell to consuming power and generating hydrogen. The changeover FC to EL mode continues until the master controller determines that the RSOFC is ready for online EL (electrolysis) mode (or, where there is a separate controller103for the fuel cell subsystem102, until the controller103of the fuel cell unit102signals the master controller174that it is ready for electrolysis mode), as indicated at block318. Once these conditions are met, the system enter EL mode, block320, and generates and stores hydrogen.

When the system changes from fuel cell (FC) mode to electrolysis (EL) mode (316inFIG. 3), hydrogen gas is redirected from a condition in which it is vented to atmosphere, to a condition in which it feeds the compressor120. This is accomplished by closing the vent valve138and opening the compressor suction valve134. When the compressor120is on, a desired suction pressure is maintained in the buffer tank140(e.g. ˜0.2 psi). Pressure control via this configuration of the buffer tank140and related structures ultimately sets the fuel cell system back pressure, Which is an advantageous control aspect for the integrated RSOFC system100.

In Change over FC to EL mode316the starting pressure in the low pressure line133feeding the H2compressor120is subject to a specific control method that insures that the line pressure is prepared for transitioning the product gas from the fuel cell102and setting the fuel cell system operating pressure. In changeover FC to EL mode316, the compressor recycle line valve152is opened to set the pressure in the low pressure line133. In the event there is a higher than desired pressure, the buffer tank vent valve143can be opened to purge the gas until the desired pressure set point is reached. This prevents shocking the fuel cell unit102during transition to feeding the H2compressor, and helps ensure that air is not sucked into the compressor120due to a resulting negative pressure when the compressor is initially turned on.

Another advantageous control capability for the RSOFC system100is its ability to keep the fuel cell102“online” in the event there is a failure in the compression system120. Thermal cycling of the fuel cell102can cause accelerated degradation of its components, and it is thus desirable to keep the system “hot.” Advantageously, the present system implements a “vent” strategy in which certain alarms will shut down the compressor120, and switch the vent valve138and compressor suction valve134to positions allowing the gas to vent through the fuel cell exhaust vent136, as done in FC-EL mode316. In this condition H2is still being produced, but the fuel cell unit102will remain hot until the alarm-inducing condition can be resolved. Once any fault is remedied, an operator reset will allow the system100to resume compressing H2gas. The process to maintain proper line pressure in the low pressure line133, discussed above, is also implemented in this condition.

The EL mode320can continue until the hydrogen storage array (118inFIG. 1) is full. Once the hydrogen storage array is full, the master controller determines whether the system is ready to transition from EL to FC mode, as indicated at block322. This determination can be based on factors such as whether the power grid is indicating a demand for power. If neither of these conditions apply, the master controller can open a system vent (e.g. the exhaust vent136inFIG. 1), and the system can continue to produce hydrogen until one of those two conditions change. If power is not needed and the hydrogen storage array118is full, the system can remain in this mode as long as desired. However, there is usually a demand for power in the grid, and thus this condition is not likely to persist for a long period of time. Additionally, the system can enter this vent state if some fault of the compressor is detected, and the system is not yet ready to changeover to fuel cell mode.

Once the power grid indicates a demand for power, the system shifts to a second changeover mode—change over EL to FC328, which involves system adjustments to allow transition from consuming power and generating hydrogen back to producing power by the fuel cell. The system remains in this changeover mode328until the master controller determines that the fuel cell is again ready to enter fuel cell (FC) mode (e.g. based on whether the fuel cell is in proper operating condition), as indicated at block330, after which the system can return to FC (fuel cell—power generation) mode312.

Throughout operation of the process shown inFIG. 3, the master controller174continually receives sensor input and evaluates the system condition. At any point, if an error or degraded condition is detected, the system can react in a variety of ways. When an error is detected, as indicated at332, the master controller determines, based upon its programming, the severity of the error, as indicated at334. If the error is a critical error, such as an electrical ground fault or detection of combustible gas above some threshold level, the system can proceed directly to the emergency shutdown stage336, which terminates operation of the system, as discussed above with respect toFIG. 2. However, if the error is not one requiring an emergency shutdown, but is more than a minor error that merely produces an alarm indication, the system can proceed to a controlled shutdown338.

Following controlled shutdown338, the system remains ready for reset, either to the idle mode304or to the heat-up mode308. Specifically, the master controller inquires at block340whether the fuel cell system is ready to re-enter heat-up mode. If so, the system proceeds to heat-up mode308. If not, the master controller next considers whether the system is ready to enter idle mode306, and transitions to that mode if the answer to the query at block342is affirmative. However, if the answers to both of these queries are negative, the system can repeat this series of inquiries until one or the other turns positive, or until some programmed limit is reached.

Shown inFIG. 4is a schematic diagram of the hydrogen compressor system120. The compressor system120is configured to receive low pressure hydrogen gas produced from the electrolysis of water in the fuel cell102, and compress this gas to high pressure (e.g. up to ˜2500 psi or more) and provide this compressed gas to the hydrogen fuel storage array (118inFIG. 1) via the compressed hydrogen supply line148. The compressor system120is both a hydrogen compression and water removal system that is suitable for use with the RSOFC system100when it is operating in electrolysis mode, or when hydrogen gas is recycled via the recycle line150.

The compressor system120generally includes a compressor unit400, a condenser402, a water trap404, and a chiller406with a coolant line407that circulates between the condenser402and the chiller406. All of the elements of the compressor system120are controlled by the master controller174. Low pressure hydrogen gas produced by the fuel cell unit102in electrolysis mode is directed toward the compressor system120via the low pressure hydrogen line133. Alternatively, low pressure hydrogen gas can be directed toward the compressor system120via the compressor recycle line150, which is connected between the hydrogen fuel line (122inFIG. 1) and the low pressure line133, via a compressor recycle line valve152. The low pressure hydrogen gas will typically have a gage pressure below some minimum threshold, such as about 0.2 psig, for example.

The low pressure hydrogen in the low pressure hydrogen line133first enters the buffer tank140, which has a volume selected to help smooth out any fluctuations in the pressure and flow rate of the incoming hydrogen. The incoming hydrogen is generally saturated, having a water vapor content of about 7%, and is likely to have a temperature of about 40° C. Any condensation of water from the hydrogen gas within the buffer tank140can be directed to the compressor drain line146via the buffer tank drain outlet416.

The low pressure hydrogen from the buffer tank140is then fed into the compressor unit400, where it is compressed. The compressor unit400includes a compressor suction vent142, and the buffer tank140is also coupled to the compressor suction vent142via a buffer tank vent line141and buffer tank vent valve143. The compressor unit400includes an air-cooled heat exchanger408, which maintains the temperature of the gas through the compression phase. This heat exchanger can be a tube-in-tube heat exchanger, such as are available from a variety of commercial sources. Those of skill in the art will be aware that the process of compressing hydrogen gas from a saturated RSOFC outlet stream will naturally cause some portion of the water in the gas stream to condense in the compressor400. This condensation can be at some pressure above atmospheric pressure, depending on the stage of compression, and is collected in the compressor400, then directed to the compressor drain line146via the compressor unit drain outlet410.

Compression of the H2gas will naturally tend to increase the temperature of the gas, causing the gas at the outlet of the compressor400to be quite hot. The heat exchanger408can cool the compressed H2gas at the outlet of the compressor400to a lower temperature, such as a temperature that is about the same as its initial temperature (e.g. about 40° C.), though it will be at a high pressure. As noted above, the term “high pressure” in reference to the compressed hydrogen is used to mean any pressure above about 250 psi. In one embodiment, the compressor120is designed to compress hydrogen to a pressure of about 2500 psi, but much higher pressures can also be used. For example, some hydrogen fuel cell systems store hydrogen at pressures of 10,000 psi and higher. After the hydrogen is compressed and discharged from the compressor400, its water content will be in the range of about 500 ppm. It is desirable, however, that the water content of the H2be less than about 100 ppm prior to storage, which is considered an acceptable level for storage in commercial H2tubes. Consequently, additional water removal is desired.

Typical water removal systems that have been used in preparing hydrogen gas for fuel cell use are unsuitable for use in the sort of system disclosed herein. Typical PEM electrolysis systems used for generating H2for fuel cell vehicle refueling stations operate at higher differential pressures that allow them to use filtration systems upstream of the compression. As noted above, the hydrogen pressure at the intake of the compressor400is below about 1 psi, such as around 5″ to 6″ H2O, or about 0.2 psi, which makes filtration systems that depend on differential pressures impractical. Additionally, such prior systems allow very high water removal, but also result in loss of H2in the process. For example, a typical pressure swing adsorption column is often used to purify H2for fuel cell vehicle applications, which results in a 10-20% loss in product H2. The H2that is used for fuel cell vehicles requires very high (e.g. 99.9%) purity. In the RSOFC system100disclosed herein, however, the solid oxide fuel cell102is run as an electrolyzer to produce H2. SOFC systems produce H2that is saturated with water vapor, but operate at relatively low differential pressure (e.g. less than about 0.3 psig). Therefore typical water removal systems that require a sizeable pressure drop ahead of the compression step are not practical.

Advantageously, the present disclosure provides a method for removing water from a saturated H2product stream from the SOFC unit102operating in electrolysis mode at approximately atmospheric pressure. The method is passive, and does not include electrical pressure switches, and causes negligible loss in H2. The compressor unit400is connected to the condenser402, which is coupled to the chiller406. The compressed H2exiting the compressor unit400will have a temperature of about 40° C. and a high pressure, such as about 2500 psi, with a water content of about 500 ppm. In the condenser this gas stream is cooled down to a low temperature, e.g. about 10° C., to reduce the moisture to less than 100 ppm, which is an acceptable level for storage in commercial H2tubes for this type of RSOFC system100.

The water that is drawn out of the hydrogen gas in the condenser402is captured by the water trap404, while the compressed hydrogen gas travels on to the hydrogen storage system via the hydrogen supply line148. The water trap404can be a coalescing filter that captures condensed water droplets and separates them from the high pressure gas, which flows one way, while the condensed water flows to the water trap drain line412. Suitable coalescing filters for high pressure operation are commercially available from a variety of sources. The condensed water then flows through a pressure regulating valve414, which reduces the pressure of the condensed water. This pressure reduction can be from about 2500 psi to about 75 psi. Pressure reducing valves that can be used for this purpose are commercially available from various sources. After pressure reduction, the condensed water passes through a ball float drain valve415that allows the condensed water to flow to the compressor drain line146. Suitable ball float drain valves that can be used for this purpose are available from a variety of commercial sources. Whenever a sufficient volume of water condensate has accumulated in the ball float valve415, the valve releases most of that water into the compressor drain line146(which is at substantially atmospheric pressure), but prevents any gas that might have passed through the regulator valve414and into the ball float valve from escaping.

Shown inFIG. 5is a flowchart outlining the steps in a method for high pressure, passive condensing of water from hydrogen in an RSOFC system in accordance with the present disclosure. As discussed above, the method includes receiving low pressure, saturated H2,500. This H2is presumably from a solid oxide fuel cell system operating in electrolysis mode, but can be from other sources. The low pressure H2is then compressed to about 2500 psi at step502, and the condensate created through this compression step is removed at step504, dropping the water content to around 500 ppm. The H2gas stream is then chilled to about 10° C. at step506, allowing the condensate to be removed to drop the water content below 100 μm at step508. Thereafter, the compressed and dewatered H2can be stored in storage tanks or the like at step510, for future use in the RSOFC system100, while the water condensate is directed to the compressor drain at step512.

The RSOFC system100and high pressure, passive water condensing and removal system120of water provides a fully integrated, grid-tied RSOFC energy storage system. It provides a passive method for removing moisture from a stream of compressed H2to a level that is suitable for H2storage, without substantial loss of product gas, and that is suitable for use with a reversible SOFC system that is configured to both store and generate energy from H2gas.

Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations are would be apparent to one skilled in the art.