Patent Description:
Electrolyzers are devices that consume electricity in order to produce hydrogen by splitting water molecules or other hydrocarbon fuel molecules. The input power source to the electrolyzer can be either a main grid (i.e., a utility grid), a microgrid, or a combination thereof. In general, the microgrid can be configured to include one or more distributed electrical resources (DERs), such as solar, wind, geothermal, hydro, storage, conventional, and the like. The main grid also can include several distributed power resources.

The electrical power system (EPS), whether it is a main grid or a microgrid, goes through various fluctuations caused by disruption in the balance between total generation power and total load power on the EPS network. Such fluctuations are increasingly present because of the intermittent nature of the power generated by renewable energy sources connected in the DERs. When the amount of power generation on the network is greater than the total load power, then the voltage and/or frequency of the power system increases. Similarly, when the total amount of load power is greater than the power generation, then voltage and/or frequency of the power system is reduced. The following prior art is relevant:.

Accordingly, the present invention is directed to electrolyzer power management and power system control that substantially obviates one or more problems due to limitations and disadvantages of the related art.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a power control system is for an electrolyzer is provided, said power control system comprising a first stage that is configured to encode power curtailment information as a DC bus voltage level using a DC droop characteristic via a rectifier configured to output an adjustable DC voltage of the DC bus voltage level; and a second stage that is configured to decode the DC bus voltage level to power information used to set an output current of a DCDC converter to the electrolyzer.

It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are intended to provide further explanation of the present disclosure, while the invention is defined by the appended claims.

The accompanying drawings, together with the present description, are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, serving to explain the principles of the invention.

The present disclosure will be explained in detail with reference to the accompanying drawings. References made to particular examples and implementations are for illustrative purposes.

Electrolyzers are devices that consume electrical energy and heat energy to produce hydrogen by splitting water molecules. The heat energy required by the electrolyzer can be produced from electrical energy internal to the electrolyzer. Alternatively, or additionally, heat energy can be supplied from an external source, typically through a byproduct of other processes, for example. In electrolyzers, where heat energy is derived from electrical energy, the total electrical energy required for electrolysis is the sum of electrical energy required for the stacks as well as the electrical energy to produce the required heat energy.

One or more power sources can be used for electrical energy. The input power source to the electrolyzer can be either a main grid (i.e., a utility grid), a microgrid, or a combination thereof. In general, the microgrid includes one or more distributed electrical resources (DERs), such as solar, wind, geothermal, hydro, storage (e.g., battery), conventional, and the like. The main grid (i.e., utility grid also can included distributed power resources (DERs) connected to it. As DER use increases, the reliability and power quality of the utility grid that is also largely dependent on DERs increases as well.

The electrical power system (EPS), whether it is a main grid or a microgrid, goes through various fluctuations caused by disruption in the balance between total generation power and total load power on the EPS network. Such fluctuations are increasingly present because of the intermittent nature of the power generated by renewable energy sources connected in the DERs. When the amount of power generation on the network is greater than the total load power, then the voltage and/or frequency of the power system increases. Similarly, when the total amount of load power is greater than the power generation, then voltage and/or frequency of the power system is reduced. In addition, the distributed power sources in the microgrid can be either AC coupled microgrid or DC coupled microgrid, or combination thereof, for example.

Whether it is main grid or microgrid, the electrical power system goes through various abnormalities caused by disturbance in balance of generation power and load power, especially due to the intermittent nature of power generation by connected DERs, such as solar and wind. The main two parameters that define grid stability, voltage and frequency of the power system network, are directly dependent on balance between the total amount of power generation to the network and the total amount of power consumption from the network. When the amount of power generation on the network is more than power consumption, then the frequency and/or voltage of the power system increases. Similarly, when then amount of power consumption is more than power generation, then frequency and/or voltage of the power system is reduced. If proper action is not taken to mitigate these voltage and frequency excursions, this imbalance of powers can lead to total collapse of grid, and this is applicable for both microgrid and other electrical grids. Ideally, one expects generation to respond immediately to meet the power demands of network but in practice that is not possible as many traditional generation systems have much slower response compared to loads so under generation is a constant real threat to the power system network stability.

The electrolyzer is classified as a load on the power system network as it consumes active power from the power system network to produce hydrogen. The basic electrolyzer stack (i.e., the group of electrolyzer cells) requires controlled DC current through its stacks and this power is generally taken from the AC power system of connected grid/microgrid. Therefore, every electrolyzer utilizes an AC to DC conversion device also known as a rectifier, which takes power from an AC power source and converts to DC power at its output. The rectifier can be three phase or one phase depending on the power level, and it can work from a variety of voltage and frequency levels.

Some electrolyzers have a single stage power conversion in which a rectifier that converts from AC to DC and also controls the current flowing through electrolyzer stacks. Other electrolyzers have a two stage conversion in which a first stage converts AC to fixed DC voltage and a second stage converts fixed DC to variable DC current required for electrolyzer stacks. Additionally, two stage conversion may have one rectifier supplying multiple DCDCs of multiple SOEC stacks.

The present invention is applicable to systems comprising a first stage and a second stage, with single stage power conversion being presented hereinafter for illustrative purposes only. In addition, the embodiments are readily applicable to all types and classifications of electrolyzers such as a) low and high temp electrolyzers, b) splitting water or splitting hydrocarbons, and c) polymer electrolyte membrane (PEM) or alkaline or solid oxide (SOEC) type.

Current solutions provide communication from a rectifier to all connected DCDCs. This communication can be an analog communication, digital communication, or power line communication. However, having a separate dedicated communication requires additional cost and the success of this feature solely depends upon the speed and reliability of the communication channel.

The rate of hydrogen production in electrolyzers can be controlled with a single parameter (i.e., current supply to the electrolyzer stack). Several other parameters such as stack temperature, physical properties of electrolyzer stack, etc. also can influence the hydrogen production rate; however, they are usually kept constant. The power consumed by the power supply that controls the stack current is directly proportional to stack current and thus directly proportional to the hydrogen production rate.

As will now be described, the embodiments generally relate to implementing frequency and voltage droop controls in electrolyzer systems and plants. Various droop control methods are described in <CIT>, which is hereby incorporated by reference.

<FIG> illustrates a system <NUM> having a single stage power conversion of an electrolyzer.

As illustrated in <FIG>, system <NUM> includes a rectifier <NUM> that is used to convert AC voltage of input power system <NUM> (i.e., utility grid or microgrid) to DC current <NUM> used by electrolyzer stack <NUM>. AC voltage is typically three phase for these power levels, however, other numbers of phases, such as single phase, may be used.

<FIG> illustrate rectifier and controller arrangements. The rectifiers are configured to utilize hydrogen setpoints to control load and hydrogen production.

In electrolyzer operation, the operator can either program a fixed value or send a target rate of hydrogen production (referred to as the hydrogen setpoint) to configure the rectifier. Rectifier controller <NUM> receives the hydrogen setpoint command and calculates the amount of stack current required to deliver that hydrogen and communicate that current setpoint to rectifier power section <NUM>. Rectifier power section <NUM> executes the controller command and delivers needed amount of current to electrolyzer stack <NUM>.

In the various configurations, controller <NUM> cam be integral to rectifier module <NUM>, as shown in <FIG> or external to rectifier <NUM> as shown in <FIG>. Also, one controller can receive commands for multiple rectifiers/stacks from the operator, execute them, and distribute the respective current commands to those rectifiers, as shown in <FIG>.

Controller <NUM> is used to implement droop control. Controller <NUM> is physically located close to rectifier <NUM> (or integral to the rectifier module itself as shown in <FIG>) and monitors grid voltage and frequency directly at the input of rectifier <NUM>. The controller uses measured voltage and frequency at input <NUM> to determine power inputs of rectifiers based on voltage (V) and frequency (F) droop characteristics. Controller <NUM> then converts that power information into stack current information.

<FIG> illustrates a system <NUM> for droop control implementation in an electrolyzer with single stage conversion.

The implementation logic of frequency control is described. Controller <NUM> measures the frequency of AC at input <NUM> and derives the new power consumption setpoint for each rectifier based on the frequency droop characteristic. The rectifier then converts that input power information into current that is supplied to respective stacks. Similarly, controller <NUM> also receives the hydrogen production rate setpoint from the operator through another site level controller, cloud-based controller, or a fixed number stored inside controller <NUM> itself. The controller then converts that hydrogen production rate setpoint to an equivalent stack current. Controller <NUM> compares stack currents derived from droop characteristics and operator commands and sends the lower of these two current values to its corresponding rectifier for execution. Controller <NUM> executes these power and current calculations periodically and updates the stack currents. In other words, rectifier output currents execute the operator hydrogen command in normal operating conditions, but rectifier output is limited to current derived from droop characteristic if current derived from droop control is less than the operator hydrogen command during abnormal conditions of the grid.

The electrolyzer may be configured to overproduce hydrogen (i.e., go beyond its nominal rating) for a short period of time. This type of electrolyzer also can be used to increase load on the power system network during over frequency (i.e., generation is greater than consumption). Example selection logic in electrolyzers with temporary overload capability is also shown in <FIG>. When there is an over frequency scenario, controller <NUM> chooses the minimum of 'maximum short time overload setpoint' and 'current value derived from droop characteristic' and sends the result to the rectifier. If the grid frequency is not determined as being in an over frequency scenario, then the rectifier chooses minimum of 'current value derived from operator hydrogen setpoint' and 'current value determined from droop characteristic' and passes that value to rectifier.

As will be readily understood by one of ordinary skill in the art, the implementation of voltage droop control and current selection criterion are performed in the same way as above-explained for frequency droop implementation, with voltage being a forcing function rather than frequency. The examples are not limited to use of either frequency or voltage droop control, but rather may be applied to both.

In some instances, a grid event with different combinations of over/nominal/under scenarios of frequencies and voltages can occur. Therefore, voltage and frequency droop control loops can output different power setpoints.

<FIG> illustrate a selection criterion of stack current in a single stage conversion electrolyzer. The selection criterion of power setpoint follows the logic in <FIG>.

As illustrated in <FIG>, the rectifier controller (e.g., <NUM>) derives two current levels from voltage and frequency droop characteristics, compares the two levels in block <NUM>, and sends the lower of the two values to block <NUM> to compare with the current setpoint value derived from the operator hydrogen setpoint. Block <NUM> then communicates the lower of the two values to the rectifier to control the stack current.

The selection criterion logic for electrolyzers with temporary overload capability is illustrated in <FIG>. The main current selection logic is same as the above scenario with an addition of block <NUM> to cover the over frequency scenario. The controller (e.g., <NUM>) compares 'maximum short time overload setpoint' with 'current derived from frequency droop' in block <NUM> and outputs the lower of the two at its output. The software logic switch chooses the current setpoint to rectifier based on the frequency status. If the frequency is determined as an over frequency condition, then output of block <NUM> is sent to the rectifier. If frequency is not determined as an over frequency (i.e., normal frequency or under frequency), then output of block <NUM> is sent to the rectifier. When it comes to active power control, frequency droop takes higher priority compared to voltage droop.

<FIG> illustrate an electrolyzer system <NUM> with two stages of power conversion according to example embodiments of the present invention.

As illustrated in <FIG>, the first stage contains a rectifier <NUM> that converts AC input power from utility or microgrid <NUM> into a constant DC voltage <NUM> and in second stage, this constant DC voltage <NUM> is converted to variable DC current <NUM> for the electrolyzer stack <NUM> using a DCDC converter <NUM>. The rectifier <NUM> is generally designed to output constant DC voltage <NUM> which supplies power to DCDC converter <NUM> as needed.

In some configurations, multiple stacks and their respective DCDCs may be used in one electrolyzer. In addition, multiple electrolyzers can be connected to a common rectifier as shown in <FIG>. In such configurations, second stage DCDCs can be located physically far from the rectifiers or grid.

<FIG> illustrate an electrolyzer system <NUM> with two stages of power conversion according to an example embodiment of the present invention. This is an architecture for droop control implementation in two stage conversion.

Here, the rectifier controller <NUM> can be separate (as shown in Fig. <NUM>) or can be an integral part of the rectifier module (as shown in <FIG>). Also, a single rectifier controller <NUM> can be configured to control multiple rectifiers <NUM>, as shown in Fig. 6c.

Rectifier controller <NUM> continuously monitors the grid voltage and frequency at rectifier input <NUM> and determines the required power curtailments during grid abnormalities. However, as mentioned in the above, the rectifiers are generally designed to produce fixed DC output voltage while supplying the power that connected DCDCs <NUM> utilize. In other words, it is not possible to control electrolyzer current directly from the rectifier or rectifier controller without active communication between them.

The embodiments of the invention provide more robust solutions without a need for active communication between the rectifier controller and DCDCs. It is proposed to use 'coded' DC bus voltage to convey the power curtailment information from rectifier controller <NUM> to DCDC controllers <NUM>. Since rectifier <NUM>, DCDC converters <NUM>, and DCDC controllers <NUM> are electrically connected to same DC bus <NUM>, they are able to monitor the voltage on DC bus <NUM> simultaneously, independently, and instantaneously. There is no need for separate highspeed communication lines between rectifier controller <NUM> and centralized DCDC controllers <NUM>.

<FIG> illustrate a frequency droop control implementation in an electrolyzer with two stage power conversion using DC droop according to example embodiments of the present invention.

The embodiments encode power curtailment information as DC bus voltage level using a DC droop characteristic in first stage and decode that DC bus voltage to power information in second stage. The desired frequency-power characteristic is shown in <FIG>. When frequency drops below f<NUM>, the power draw by electrolyzer is expected to reduce as dictated by the slope of the droop curve between f<NUM> and f<NUM>. The droop characteristic may contain multiple slopes or nonlinear slopes. The desired frequency-power droop curve can be implemented through a series of two droop curves in different controllers (i.e., frequency - DC bus voltage droop curve in rectifier controller <NUM>, and DC bus voltage - power curve in DCDC controller <NUM>).

The rectifier <NUM> is designed to provide adjustable output DC voltage within a pre-determined range. The relation between electrolyzer power consumption and DC bus voltage is determined by a pre-set droop characteristic, as shown in <FIG>. Similarly, the relation between DC bus voltage and frequency is determined by a pre-set droop characteristic, as shown in <FIG>. Slopes and dead bands of droop curves in 7a and 7b can be designed specifically to achieve the desired droop curve in 7c.

Rectifier controller <NUM> continuously monitors frequency of the connected AC input <NUM>. When frequency is within limits (f > f<NUM>), controller <NUM> commands rectifier <NUM> to produce nominal DC output voltage, Vdc<NUM> at DC bus <NUM>. When the input <NUM> frequency drops below f<NUM> the controller commands rectifier to reduce its output voltage level to a value that is less than nominal voltage level Vdc<NUM> based on the slope of DC droop and measured frequency, as shown in <FIG>. The governing equations for this frequency-droop control characteristics are provided below: <MAT> <MAT> <MAT> where fgrid is measured frequency at input <NUM>; Vdc is measured DC voltage on DC bus <NUM>.

The slope of DC droop is designed based on nominal DC voltage Vdc<NUM> minimum voltage required for DCDCs and rectifiers to operate, Vdc<NUM>, and frequency values f<NUM> and f<NUM>. The typical value of Vdc<NUM> is <NUM> - <NUM>% of nominal DC bus voltage, Vdc<NUM>.

DCDC controller <NUM> continuously monitors DC bus voltage <NUM>, Vdc and calculates the equivalent power based on the characteristic described in <FIG>. Controller <NUM> then derives the stack current level from this power information. The governing equations of 'DC bus voltage - power' droop curve are provided below: <MAT> <MAT> <MAT> where Vdc is measured DC voltage on DC bus <NUM>; P is target power of electrolyzer.

DCDC controller <NUM> derives the stack current from power P and then compares with current derived from operator hydrogen setpoint and sends the minimum of these two currents to DCDCs <NUM> (as described in Fig. 3a). DCDCs <NUM> execute that command by supplying commanded amounts of current to the stacks.

<FIG> illustrate frequency droop control of electrolyzers with temporary overload capability according to example embodiments of the invention. If electrolyzers can do temporary overload, then the droop characteristic described in <FIG> can be used to mitigate an over frequency scenario. The governing equations are given below:.

<FIG> illustrate an implementation of voltage droop control in two stage power conversion of an electrolyzer according to example embodiments of the present invention.

Voltage droop control is implemented in the same way as frequency droop control except that voltage is used as a forcing function instead of frequency. The implementation of voltage droop control in two stage power conversion is illustrated in <FIG>. The AC grid voltage - DC bus voltage droop control is implemented in rectifier controller <NUM>. The dc bus voltage - electrolyzer power droop is implemented in DCDC controller <NUM>. This configuration is readily applicable for both frequency droop and voltage droop implementations.

The governing equations for AC input voltage - DC bus voltage is given below: <MAT> <MAT> <MAT> where Vgrid is measured voltage at input <NUM>; Vdc is measured DC voltage on DC bus <NUM>.

<FIG> illustrate voltage droop control in electrolyzers with temporary overload capability according to example embodiments of the present invention. Similarly, the voltage droop implementation and governing equations for electrolyzers with the temporary overload feature is given in <FIG> and below: <MAT> <MAT> <MAT> <MAT> <MAT> where fgrid is measured frequency at input <NUM>; Vdc is measured DC voltage on DC bus <NUM>.

<FIG> illustrate a selection criterion of stack current in a two stage conversion electrolyzer according to an example embodiment of the present invention.

In some instances, a grid event with different combinations of over/nominal/under scenarios of frequencies and voltages can occur. Therefore, voltage and frequency droop control loops can be configured to output different power setpoints.

The selection criterion of power setpoint executes the logic of <FIG>. The logic in two stage power conversion follows the same logic described in single stage power conversion (as in <FIG>), but the difference is the output command from rectifier controller <NUM> to rectifier <NUM> is a DC bus command rather than a stack current command.

As illustrated in <FIG>, the rectifier controller (e.g., <NUM>) derives two voltage levels from voltage and frequency droop characteristics, compares the two levels in block <NUM>, and sends the lower of the two values to block <NUM> to compare with the voltage setpoint value derived from the operator hydrogen setpoint. Block <NUM> then communicates the lower of the two values to the rectifier.

In <FIG>, the controller (e.g., <NUM>) further compares 'maximum short time overload setpoint' with 'voltage derived from frequency droop' in block <NUM> and outputs the lower of the two at its output. The software logic switch chooses the voltage setpoint to rectifier based on the frequency status. If the frequency is determined as an over frequency condition, then output of block <NUM> is sent to the rectifier. If frequency is not determined as an over frequency (i.e., normal frequency or under frequency), then output of block <NUM> is sent to the rectifier.

The embodiments can be used in any electrolyzer technology. In the various embodiments, when there is not enough power available, priority is given to heaters over stack power and water heaters. The embodiments facilitate priority by programming droop characteristics in the connected loads. When voltage available is dropping due to load-generation unbalance, the DCDC in connected electrolyzers can determine that voltage and reduce its power to the stack. If voltage available is still dropping, then stack power can be completely reduced to zero.

Claim 1:
A power control system for an electrolyzer comprising:
a first stage that is configured to encode power curtailment information as a DC bus voltage level using a DC droop characteristic via a rectifier (<NUM>) configured to output an adjustable DC voltage of the DC bus voltage level; and
a second stage that is configured to decode the DC bus voltage level to power information used to set an output current of a DCDC converter (<NUM>) to the electrolyzer.