Abstract:
A system includes a fuel cell subsystem, switches and a controller. The fuel cell subsystem is adapted to provide power that is capable of being consumed by residential loads, and the fuel cell subsystem is sized to provide power up to a first power threshold that is less than a maximum power threshold that is capable of being consumed by the residential loads. The controller is adapted to determine the power that is consumed by the residential loads and based on the determined power, operate the switches to selectively regulate electrical connections between the residential loads and the fuel cell subsystem to keep the power approximately below the first power threshold.

Description:
BACKGROUND 
     The invention relates to residential load shedding. 
     A typically U.S. house is wired with the capacity to consume approximately 24 kilowatts (kW) of electrical power from an electrical utility company. However, the typical house consumes a much lower annual average power near approximately 1 kW. In order to consume 24 kW of power, nearly all of the electrical appliances and devices in the house would have to be turned on at the same time. 
     Conventionally, for purposes of receiving electrical power, the house is connected to a power grid that communicates electricity from one or more electrical power plants (hydroelectric or nuclear power plants, for example). However, in the near future, the house may receive partial or total power from its own fuel cell system. 
     For purposes of generating power, the fuel cell system includes fuel cells that are electrochemical devices that convert chemical energy produced by reactions directly into electrically energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), a membrane that may permit only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is oxidized to produce hydrogen protons that pass through the PEM. The electrons produced by this oxidation travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions may be described by the following equations: 
     
       
         H 2 →2H + +2 e   −   
       
     
     at the anode of the cell, and 
     
       
         O 2 +4H + +4 e   − →2H 2 O 
       
     
     at the cathode of the cell. 
     Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several serially connected fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage. The fuel cell stack may include different plates that are stacked one on top of the other in the appropriate order, and each plate may be associated with more than one fuel cell of the stack. The plates may be made from a graphite composite material and include various channels and orifices to, as examples, route the above-described reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. The anode and the cathode may each be made out of an electrically conductive gas diffusion material, such as a carbon cloth or paper material, for example. 
     A fuel cell system typically is sized to efficiently provide a predefined range of output power. In this manner, components of the fuel cell system, such as electrical devices (electrical motors for pumps and blowers, for example) and non-electrical devices (valves, for example), may be sized to produce the predefined range of output power. If the fuel cell system is sized to provide the maximum power (24 kW, for example) that may be consumed by the average house, then the fuel cell system may suffer from inefficiency at the much lower output power that is typically consumed by the house. Additionally, the base cost of such a fuel cell system may be higher due to the system components that are designed to support a larger power output. 
     SUMMARY 
     In one embodiment of the invention, a system includes a fuel cell subsystem, switches and a controller. The fuel cell subsystem is adapted to provide power that is capable of being consumed by residential loads, and the fuel cell subsystem is sized to provide power up to a first power threshold that is less than a maximum power threshold that is capable of being consumed by the residential loads. The controller is adapted to determine the power that is consumed by the residential loads and based on the determined power, operate the switches to selectively regulate electrical connections between the residential loads and the fuel cell subsystem to keep the power approximately below the first power threshold. 
     Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a residential electrical system according to an embodiment of the invention. 
     FIG. 2 is an illustration of a priority scheme for shedding loads of the electrical system of FIG. 1 according to an embodiment of the invention. 
     FIG. 3 is an illustration of a priority scheme for connecting loads to the electrical system of FIG. 1 according to an embodiment of the invention. 
     FIG. 4 is a flow diagram illustrating execution of a routine to shed loads of the electrical system of FIG. 1 according to an embodiment of the invention. 
     FIG. 5 is a flow diagram illustrating operation of a routine to connect loads to the electrical system of FIG. 1 according to an embodiment of the invention. 
     FIG. 6 is a power versus time plot. 
     FIG. 7 is a schematic diagram of a fuel cell system of the electrical system of FIG. 1 according to an embodiment of the invention. 
     FIG. 8 is a schematic diagram of a well pump system according to an embodiment of the invention. 
     FIG. 9 is a schematic diagram of a load sense and switch circuit of the electrical system of FIG. 1 according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an embodiment  10  of a residential electrical system in accordance with the invention includes controlled residential loads  16  (controlled residential loads  16   1 ,  16   2  . . .  16   N , as examples) and uncontrolled residential loads  18  that receive power from a fuel cell system  12  (of the residential electrical system  10 ). The controlled residential loads  16  may be, as examples, appliances (a refrigerator and/or an electric oven, as examples) and/or other electricity consuming devices. The components of the fuel cell system  12 , such as flow control motors  14 , valves, etc. are sized to produce a maximum output power that is below the maximum power that may be collectively consumed by the residential loads  16  and  18 . To ensure that the fuel cell system  12  is not overloaded, a fuel cell control circuit  15  (of the fuel cell system  12 ) is designed to monitor the output power of the fuel cell system  12  and shed the appropriate controlled loads  16  to regulate the output power. 
     Unlike the controlled residential loads  16 , the uncontrolled residential loads  18  may not be shed from the electrical system  10  for purposes of reducing the power output of the fuel cell system  12 . Instead, each uncontrolled residential load  18  is coupled to the fuel cell system  12  by wiring of the house that may be disconnected from the fuel cell system  12  only by a standard circuit breaker panel switch (not shown). In this manner, the circuit breaker panel switch may be manually turned off or may automatically turn off if the current through its associated house circuit exceeds a predefined level. Similar to the controlled residential loads  16 , the uncontrolled residential loads  18  may be, as examples, appliances and/or other electricity consuming devices. 
     A technique of storing electricity, such storing energy in a storage battery  85  (a lead acid battery, for example), may be utilized for extending the peak power output capability of the fuel cell system  12 . This way the maximum peak power output of the fuel cell system  12  is equal to the combined peak outputs of both the battery  85  and the fuel cell system  12 . The battery  85  provides extra power capability for limited periods of time based on its capacity and state of charge. Load shedding may be used to prevent completely discharging the battery  85  by limiting peak load based not only on the peak output of the fuel cell system  12 , but also taking into consideration the battery&#39;s state of charge. During periods of low load (i.e., low output power), the battery is recharged using the fuel cell system&#39;s excess capacity. 
     The fuel cell control circuit  15  may shed the controlled residential loads  16  from the electrical system  10  based on the output power (of the fuel cell system  12 ) that is depicted by an exemplary waveform  79  in FIG.  6 . In some embodiments, the fuel cell control circuit  15  regulates the total load on the fuel cell system  12  to satisfy the following criteria: 1. The output power does not exceed a maximum power threshold level (called TH U  (see FIG.  6 )); 2. The output power does not exceed a middle power threshold level (called TH M ) for a time interval that is longer than a predefined window  100  of time; and 3. The output power does not exceed a lower power threshold level (called TH L ) for a time interval that is longer than a predefined window  105  of time. The window  105  is longer than the window  100 , as depicted in FIG.  6 . 
     Although the fuel cell system  12  may be sized to provide an output power up to the TH L  lower power threshold level, the ability of the fuel cell system  12  to temporarily furnish power above this level due to the battery  85 . In this manner, as further described below, the fuel cell system  12  may use the battery  85  to temporarily boost the output power when the output power exceeds the TH L  lower power threshold level. However, because the energy that is stored by the battery  85  is limited, the fuel cell system  12  may monitor the remaining energy that is stored in the battery  85  and set the durations of the windows  100  and  105  accordingly. Thus, the durations of the windows  100  and  105  are dynamic and are a function of how long the battery  85  has recharged between successive power surges. As a more specific example, at a particular moment, the duration of the window  100  may be approximately one half of a second (as an example), and the duration of the window  105  may be approximately 30 seconds (as an example). However, the fuel cell system  12  may change the window durations as needed in accordance with the available energy that is stored in the battery  85 . 
     In some embodiments, to accomplish the above-described regulation criteria, the fuel cell control circuit  15  may operate switches  29  (see FIG. 1) in the following manner to shed the controlled residential loads  16 . If the fuel cell control circuit  15  determines that the output power of the fuel cell system  12  is above the TH U  upper power threshold level, then the fuel cell control circuit  15  immediately switches off one or more of the controlled residential loads  16  to return the output power below the TH U  upper power threshold level. If the output power rises above the TH M  middle power threshold level, then the fuel cell control circuit  15  sheds one or more of the controlled residential loads  16  to bring the power level under the TH M  middle power threshold level within the window  100 . If the output power is above the TH L  lower power threshold level, then the fuel cell control circuit  15  sheds one or more of the controlled residential loads  16  to bring the output power below the TH L  lower power threshold level within the window  105 . 
     As depicted in FIG. 1, each switch  29  is located between the power lines  21  and a different set of power lines  27  that extend to an associated controlled residential load  16 . Therefore, if the fuel cell control circuit  15  determines that the output power needs to be reduced, then the fuel cell system  12  sheds the appropriate controlled residential load(s)  16  by opening one or more of the switches  29  to disconnect the controlled residential load(s)  16  from the electrical system  10  (and from the fuel cell system  12 ). The uncontrolled residential loads  18  may be directly connected to the power lines  21 . 
     Each switch  29  may be part of a load sense and switch circuit  24  (circuits  24   1 ,  24   2 ,  24   3 , . . .  24   N , as examples) that is associated with the same controlled residential load  16  as the switch circuit  29 . In some embodiments, each switch circuit  24  may provide various indications to the fuel cell control circuit  15 . For example, the circuit  24  may communicate an indication of whether its associated controlled residential load  16  is turned on or off and may communicate, for example, an indication of the power that is currently being consumed by its associated controlled residential load  16 . Therefore, for example, if the fuel cell control circuit  15  needs to connect one of the controlled residential loads  16 , the fuel cell control circuit  15  may communicate with the associated circuit  24  to determine the most recent historical power consumption profile of the associated controlled residential load  16  for purposes of ensuring that turning on this controlled residential load  16  does not exceed the TH L  lower power threshold level. For example, the circuit  24  may track the maximum power level that has been consumed by the associated controlled residential load  16  during the last hour (for example) and communicate this maximum power level to the fuel cell control circuit  15 . Other arrangements are possible. 
     The fuel cell control circuit  15  may, in some embodiments, use the indications from the circuits  24  to identify the connected controlled residential loads  16  that are connected and are not currently consuming power, i.e., to identify which controlled residential loads  16  are turned off. In this manner, the fuel cell control circuit  15  may shed these identified controlled residential loads  16  to prevent a momentary overload that may occur if the identified controlled loads  16  are turned on. 
     Referring to FIG. 2, the fuel cell control circuit  15  may follow a predefined priority scheme when shedding, or disconnecting, the controlled residential loads  16  to bring the output power within the specified range. For example, in some embodiments, the fuel cell control circuit  15  assigns a different priority level to each controlled residential load  16  and may use a round robin disconnection priority scheme  40  to select the next controlled residential load  16  that is to be disconnected from the electrical system  10  (and from the fuel cell system  12 ). As an example, in the disconnection priority scheme  40 , the controlled residential loads  16  may have the following disconnection priority (according to the reference numbers of the controlled residential loads  16 , as listed from the top to the bottom disconnection priority level in order):  16   1 ,  16   2 ,  16   3 , . . .  16   N . Thus, the fuel cell control circuit  15  may disconnect the controlled residential load  16   1  (assuming that the controlled residential load  16   1  is connected) before disconnecting the controlled residential load  16   2  (assuming that the controlled residential load  16   2  is connected), as depicted in FIG.  2 . 
     In some embodiments, the controlled residential loads  16  that consume more power may have a higher disconnection priority and therefore, may be disconnected before the other controlled residential loads  16 . In some embodiments, if a particular controlled residential load  16  is associated with an electrical device/appliance that has a substantial potential energy, then the controlled residential load  16  is assigned a higher priority for shedding purposes. 
     For example, a high thermal mass is one type of potential energy that permits a particular electrical device/appliance to function after being disconnected and thus, the disconnection may go unnoticed inside the house. For example, one such device that has a high thermal mass (and substantial potential energy) is a hot water heater, as the hot water inside a tank of the hot water heater may remain hot for a substantial time after electricity to the water heater has been disconnected. 
     Similarly, an air conditioner may have a high thermal mass (and substantial potential energy) in that evaporation coils of the air conditioner may remain cold when a compressor (that is part of a controlled residential load  16 ) is turned off. The blower of the air conditioner may remain electrically connected to the system  10  (i.e., the blower is not part of the circuit that forms the controlled residential load  16  that is disconnected) to continue to blow air over the evaporation coils to produce cold air in the house. Therefore, disconnection of the compressor may go unnoticed for a substantial time. Other controlled residential loads  16  that may have a high thermal mass (and substantial potential energy) may be a heat pump, an oven, an electric dryer and a pool heater, as just a few examples. 
     Referring to FIG. 8, as another example of a load that has a substantial potential energy may be a system that includes a well  202  and a well pump  200 . The well pump  200  pumps water into a pressurized reservoir tank  204 . In this manner, the pressurized reservoir tank  204  may house an air bladder  206 , for example, that is compressed when water  208  is stored in the reservoir tank  204 . Therefore, by temporarily disconnecting the water pump  200 , water may still be supplied to a water outlet line  210  (of the reservoir tank  204 ) that furnishes water to the house, and residents inside the house may not notice a temporary disconnection of the water pump  200 . 
     Referring to FIG. 3, for purposes of connecting the controlled residential loads  16  to the electrical system  10  when the fuel cell system  12  is producing output power that is less the TH L  lower power threshold level (see FIG.  6 ), the fuel cell control circuit  15  may use a connection priority scheme  42 . In particular, the connection priority scheme  42  may include connecting the controlled residential loads  16  in an order that is opposite to the disconnection order of the disconnection priority scheme  40 , described above. In this manner, in the connection priority scheme  42 , the controlled residential loads  16  may have the following connection priority (according to the reference numbers of the controlled residential loads  16 , as listed from the top to the bottom connection priority in order):  16   N ,  16   3 ,  16   2 , . . .  16   1 . Therefore, as an example, the fuel cell control circuit  15  may connect the load  16   3  before the fuel cell control circuit  15  connects the load  16   2 . 
     In some embodiments, the fuel cell control circuit  15  may be a processor-based (a microcontroller-based, for example) circuit that stores a program  19  that the fuel cell control circuit  15  executes to perform the above-described disconnection and connection of the controlled residential loads  16 . For example, referring to FIG. 4, the program  19  may include a routine  23  that the fuel cell control circuit  15  executes to perform the shedding, or disconnection, of the controlled residential loads  16 . In particular, the fuel cell control circuit  15  may determine (diamond  62 ) whether the output power is below the TH U  upper power threshold level. If so, then the fuel cell control circuit  15  selectively disconnects (block  64 ) one of more of the controlled residential loads  16  (pursuant to the disconnection priority scheme  40  described above) until the power output of the fuel cell system  12  is below the upper power threshold level TH U . 
     Next, the fuel cell control circuit  15  determines (diamond  66 ) whether the power output of the fuel cell system  12  is below the TH M  middle power threshold level during a window  100  from a time when the output power exceeded the TH M  middle power threshold level. If not, then the fuel cell control circuit  15  selectively disconnects (block  68 ) the controlled residential load(s)  16  until the power output of the fuel cell system  12  is below the TH M  middle power threshold level. Subsequently, the fuel cell control circuit  15  determines (diamond  70 ) whether the power output is below the TH L  lower power threshold level for a window  105  from a time when the output power exceeded the TH L  lower power threshold level. If not, then the fuel cell control circuit  15  selectively disconnects (block  72 ) the controlled residential load(s)  16  until the power output of the fuel cell system  12  is below the TH L  lower power threshold level. The routine  23  may cause the fuel cell control circuit  15  to return to the diamond  62  as long as the power is above the TH L  lower power threshold level. 
     Referring to FIG. 5, for purposes of connecting the controlled residential loads  16  to the electrical system  10 , the program  19  may include a routine  31 . In particular, the routine  31 , when executed, may cause the fuel cell control circuit  15  to determine (diamond  74 ), in accordance with the connection priority scheme  41 , whether a particular controlled residential load  16  is connected. If so, the fuel cell control circuit  15  returns to diamond  74 . Otherwise, the fuel cell control circuit  15  determines (diamond  76 ) whether the total output power of the fuel cell system  12  will remain below the TH L  lower power threshold level when the targeted controlled residential load  16  is connected. The fuel cell control circuit  15  may make this determination by, for example, interacting with the associated circuit  24  to retrieve the most recent history of the controlled residential load  16 . If the fuel cell control circuit  15  (based on the power history that is provided by the circuit  24 ) determines that the power output after connection of the targeted controlled residential load  16  will remain below the TH L  lower power threshold level, then the fuel cell control circuit  15  connects (block  78 ) the targeted controlled residential load  16 . 
     Referring back to FIG. 1, in some embodiments, the fuel cell control circuit  15  may further base the disconnection/connection on other criteria than assigned priority levels. For example, the fuel cell control circuit  15  may use one or more temperature sensors  17  to sense a temperature that is associated with the controlled residential load  16  and further base the connection\disconnection on the sensed temperature. For example, the fuel cell control circuit  15  may sense an air temperature inside the house and base the disconnection of an air conditioner (i.e., a controlled residential load  16  for this example) on the sensed temperature. In this manner, if the sensed temperature is below a predetermined temperature level, the fuel cell control circuit  15  may disconnect another controlled residential load  16  (instead of the air conditioner) that has a lower priority level. As other examples, the fuel cell control circuit  15  may use the temperature sensors  17  to sense the temperatures of water in a pool and water in a hot water heater and base connection/disconnection of a pool heater and a hot water heater on these sensed temperatures. 
     Referring to FIG. 7, in some embodiments, the fuel cell system  12  may include the fuel cell control circuit  15  that stores the program  19  (that includes the above-described routines  23  and  31 ) in a memory (an electrically erasable programmable read only memory (EEPROM) or a flash memory, as just a few examples) of the fuel cell control circuit  15 . Copies of the program  19  may also be stored, as an example, on a mass storage device (a hard disk drive, for example) or on removable media (a floppy disk or an optical disk, as examples), as just a few examples. 
     The fuel cell system  12  also includes a fuel cell stack  80  that is electrically coupled to the power lines  21 . In this manner, the fuel cell stack  80  furnishes DC power to an inverter  84  that furnishes AC power to the power lines  21 . An inverter controller  82  may be coupled to the inverter  84  to monitor the power output of the fuel cell system  12  and may communicate the monitored power to the fuel cell control circuit  15  via a serial bus  81 , for example. For purposes of establishing communication between the circuits  24  and the fuel cell control circuit  15 , the fuel cell system  12  may include a power line modem  92 . In this manner, the modem  92  may use a power line transmission protocol (an X-10 bus protocol, for example) to communicate with the circuits  24  via the power lines  21 . A reformer controller  88  (that controls a reformer (not shown) via control lines  90 ) may be coupled to the serial bus  81  along with a motor driver board  86  that controls the motors  14 . 
     Among the other features of the fuel cell system  12 , the fuel cell control circuit  15  may monitor the voltages and currents of the fuel cell stack  80  via a fuel cell voltage scanner circuit  44 . The battery  85  may be coupled to the inverter  84  so that the battery  85  is discharged during power output levels that exceed the TH L  lower power threshold level (during the windows  100  and  105 ) and recharged otherwise. A supervisory controller  87  determines the energy that is currently stored in the battery  85  by monitoring the power that charges the battery  85  and the power that is depleted from the battery  85 . One way that the supervisory controller  87  may accomplish this is to use a current sensing circuit  91  (that is coupled between the battery  85  and the inverter  84 ) to monitor the current to/from the battery  85 . The supervisory controller  87  may include other control lines  89  that the supervisory controller  87  uses to coordinate the above-described activities of the fuel cell system  12 . 
     Referring to FIG. 9, in some embodiments, the circuit  24  may be packaged to be installed in a breaker panel socket and may include the following circuitry. The circuit  24  may include a reactive power measuring circuit  112  that measures the power transmitted via the power lines  21  to the associated controlled residential load  16 . A controller  114  (a microcontroller, for example) may be coupled to the circuit  112  to receive an indication of the load and communicate the results to the fuel cell control circuit  15  via a power line modem  110  that is coupled to the power lines  21 . The power line modem  110  communicates with the power line modem  92  using the power line transmission protocol. In this manner, the fuel cell control circuit  15  may communicate via the power lines  21  and the power line modem  110  to the controller  114  to instruct the controller  114  to close the switch  29 . The switch  29  may be formed from relay driver  116  and a relay  118 . The controller  114  may interact with the relay driver  116  to open or close the relay  118  to disconnect or connect, respectively, the associated controlled residential load  16 . 
     The circuit  24  may also include a current breaker switch  25  to disconnect the associated controlled residential load  16  if the current through the switch  29  exceeds a predefined current level. The current breaker switch  25  may also be opened or closed by a manual switch lever, for example. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.