Abstract:
A method for operating a fuel cell system connected to a power grid includes determining a frequency of the power grid, and adjusting the operation of the fuel cell system based on the determined frequency.

Description:
BACKGROUND 
       [0001]    The following description is provided simply as an aid in understanding the disclosure and is not admitted to describe or constitute prior art. 
         [0002]    The present invention relates generally to optimally operating electric power grids, and more particularly, to methods and systems for operating solid oxide fuel cell systems (SOFCS) in concert with electric power grids. 
         [0003]    Electric power grids typically include a number of power generating systems, such as SOFCS, that supply electricity to the grid and a number of consumers that draw electricity from the grid. When the generation and consumption of electricity are substantially equal, the grid frequency is substantially constant at a particular nominal value. This is the preferred state for optimal efficiency and functionality. The nominal grid frequency is a parameter established by the governing power distribution entity. Examples of nominal standard grid frequencies for the European and North American systems are 50 Hz and 60 Hz respectively. 
         [0004]    Transient frequency deviations result from changes in energy consumption and/or the removal or addition of power generation systems. Decreased consumption or increased generation tends to cause an increase in the grid frequency, and vice versa. Power consumption and generation are time-dependent variables which may cause short—i.e., measured in second or minutes—deviations of small magnitude. Larger frequency transients, such as those having a magnitude of greater than 0.3 Hz, may be due to the sudden loss of a significant power generator. 
         [0005]    One known way to mitigate the frequency transient magnitude and duration is to have some amount of standby power generation capacity, sometimes referred to as a system reserve. A spinning reserve is an aspect of the system reserve that is derived from already operating generators and is readily deployable. For example, a spinning reserve can be defined as any back-up energy production capacity which can be made available to a transmission system with short notice and can operate continuously for several hours once it is brought online. Frequency generation is often managed by varying the output of fossil fuel-fired generators connected to the electric grid. Not all generators can be effectively operated with constantly varying output, and those that are thus operated incur costs from increased fuel consumption and maintenance. Additional conventional solutions include rapid deployment or removal of load, or added transmission power from other grids. Accordingly, there is a need for both rapidly responsive reserve capacity, and methods and means for utilizing excess power during periods of low consumption by end users. 
         [0006]    Solid oxide fuel cells are electrochemical devices that convert chemical energy produced by a reaction directly into electrical energy. Reversible or regenerative cells may also do the reverse, consuming electricity and converting chemicals in a reaction to produce hydrogen gas—i.e., electrolyzing steam. Multiple fuel cells may be assembled together to form an arrangement called a fuel stack. The fuel cell stack may include interconnects/gas separator plates for routing reactants and products through the stack. An example of a fuel cell system is described in U.S. Pat. No. 7,422,810, which is incorporated herein by reference in its entirety. Parameters such as temperature, reactant type and flow rate are determined by a controller, which thereby regulates the type and magnitude of the fuel cell stack output. 
       SUMMARY OF THE INVENTION 
       [0007]    One embodiment of the invention relates to electrically coupling a SOFCS to a power grid and operating the SOFCS responsive to grid frequency. A reversible SOFCS (“SORFCS”) may be operated in either fuel cell mode or electrolyzer mode responsive to grid frequency. The grid frequency is provided either by a sensor and/or is communicated by a grid control center. A deviation in grid frequency below a predetermined lower threshold initiates an output or an increase in the existing output level of the SOFCS power to the grid. Conversely, a deviation above a predetermined upper threshold leads to a stoppage or a decrease in the existing output level of the SOFCS power to the grid. For a SORFCS, the output state of the SORFCS may be switched from electricity output in the fuel cell mode to hydrogen generation in electrolyzer mode. 
         [0008]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Features, aspects and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below. 
           [0010]      FIG. 1  is a schematic diagram of a power distribution system according to an embodiment of the invention. 
           [0011]      FIG. 2  is a flow diagram depicting an operation scheme for the SOFCS according to an embodiment of the invention. 
           [0012]      FIG. 3  is a graphical representation of the SOFCS output according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Embodiments of the disclosure will be described below with reference to the accompanying drawings. It should be understood that the following description is intended to describe exemplary embodiments, and not to limit the claimed subject matter. 
         [0014]      FIG. 1  is a schematic illustration of an exemplary power distribution system  100 . The power distribution system  100  includes a power grid  102  that communicates power to grid loads  104 , which may include various industrial and residential power consuming devices. A grid utility  106  and an auxiliary power generator  108 , such as a SOFC power generation system, provide power to the power grid. While a solid oxide fuel cell system is described herein as the power generator  108 , other fuel cell systems, such as PEM, phosphoric acid, molten carbonate, etc., system may also be used. Additionally, other renewable electricity generation systems, such as photovoltaic (solar), wind turbine, geothermal, etc., may be used together with or instead of the SOFCS. The grid utility  106  is controlled by a grid control center  114  such as an Independent Systems Operator or Regional Transmission Organization, or the like. While one power generator, such as SOFCS  108  is shown in  FIG. 1 , it should be noted that there may be plural systems  108 . The SOFCS  108  may also optionally provide power directly to a local load  110 , such as a building, vehicle, data center, storage battery, etc. 
         [0015]    The operation of the SOFCS  108  is governed by a SOFCS controller  116 , such as a general or specific purpose computer or a dedicated logic device. Controller  116  may control one or more SOFCS  108 . 
         [0016]    The SOFCS  108  is in fluid communication with at least one of a fuel source  122  and a fuel pipeline  124 . The fuel source  122  may comprise a fuel storage vessel, such as a hydrogen or hydrocarbon fuel vessel, such as a natural gas tank. Source  122  provides suitable hydrocarbon or hydrogen fuel that the SOFCS  108  consumes during operation in fuel cell mode. The pipeline  124  may comprise a natural gas, hydrogen or other fuel pipeline which provides fuel to the SOFCS  108  instead of or in addition to the source  122 . 
         [0017]    If the SOFCS  108  is a SORFCS, then the SORFCS may provide a hydrogen outlet stream to the source  122  and/or to the pipeline  124  while operating in electrolyzer mode, as described for example in U.S. patent application Ser. No. 10/446,704, filed on May 29, 2003 and incorporated herein by reference in its entirety. 
         [0018]    The grid control center  114  and optionally the SOFCS controller  116  are designed to monitor grid frequency by virtue of one or more frequency transducers  112 A,  112 B. Additionally, the grid control center  114  may be in communication with the SOFCS controller  116  via a signal transmitter  118  so as to provide the SOFCS controller  116  with grid frequency information in addition to or instead of grid frequency provided by a transducer  112 B directly to SOFCS controller. This signal transmitter  118  may be embodied by a telephone or internet line, or wireless link, for example. In the alternative embodiment, the SOFCS  108  is enabled to support the operational stability of the power grid  102  by modulating its power (i.e., electricity) output to the grid in response to changes in grid frequency without requiring any communication with the grid control center  114  by using the dedicated transducer  112 B. The SOFCS controller  116  additionally comprises a computing unit that executes the decision logic required to advantageously modulate SOFCS operation. 
         [0019]      FIG. 2  is a flow diagram depicting an operation scheme for the SOFCS  108  according to one embodiment. At the start point  200 , the SOFCS  108  is operating in fuel cell mode at nominal output state that maximizes fuel efficiency. The output power of SOFCS  108  is provided either to grid  102  and/or to the local load  110 . As a first step, the actual grid frequency is provided to the SOFCS controller  116  (see step  202 ). In the event that this measured frequency is at or below a predetermined threshold value below the nominal frequency (rectangle  204 ), the SOFCS controller  116  commands the SOFCS  108  to either initiate providing power to the grid  102  (if the SOFCS  108  was providing power only to the local load  110 ) or to increase the amount of power provided to the grid  102  if the SOFCS  108  was already providing power to the grid. For example, the reversible or non-reversible SOFCS may be set to operate in a fuel cell mode at maximum electricity output level designed for the SOFCS (see step  206 ). 
         [0020]    According to another embodiment, when the grid frequency becomes low, the grid control center  114  causes the SOFCS  108  to switch to grid independent operation of a critical load—in order to avoid a possible interruption of the critical load—particularly if the frequency is below a threshold such that a blackout is imminent. (That is, when predetermined criteria for an imminent blackout is met, then forced supporting of local load  110  is performed and the SOFCS  108  is divorced from the power grid  102  intentionally before the interruption occurs) (see step  205 ). According to one embodiment, it is unlikely that this would ever be done unless a blackout were truly eminent because the critical load (or local load  110 ) is by plan going to be less than the output capacity of the SOFCS  108 . So, forcing this divorce from power grid  102  connect would result in a net loss of output to the power grid  102  at the worst possible time (when there is already not enough power). However, if a utility has a contract to support a critical load with high reliability, this might be the right financial choice. According to another embodiment, this operation may be performed if the critical or local load is for example, a hospital or critical data center. 
         [0021]    In the event that the measured grid frequency is at or above a predetermined threshold value above the nominal frequency (rectangle  208 ), the SOFCS controller  116  commands the SOFCS  108  to decrease the amount of power provided to the grid  102  or to stop providing power to the grid  102  (see step  210 ). The SOFCS  108  may instead provide power to local load  110 . 
         [0022]    According to another embodiment, when the power grid  102  frequency is high, (i.e., when there is not enough load on the grid) the grid control center  114  causes SOFCS  108 , which for reliability reasons might have been operating divorced from the grid supporting only a critical or local load  110 , to parallel with the power grid  102  and reduce output such that there is additional net load on the power grid  102  via a critical or local load  110  (See step  210 ). 
         [0023]    If the SOFCS  108  is a reversible system, then controller may command the SORFCS  108  to operate in the electrolyzer mode to draw power from the grid  102 . When SORFCS  108  operates in electrolyzer mode, it draws electric power from the grid to produce hydrogen from supplied water (see step  212 ). The produced hydrogen flows into the storage vessel (i.e., source)  122  and/or pipeline  124  (see step  214 ), and may be used during times in which energy consumption is high. Optionally, the pipeline  124  may be painted black to utilize the day-night cycle and function as a solar compressor to compress the fuel for daytime use. In another embodiment, the generator  108  may comprise a wind turbine or another renewable power generator which powers an electrolyzer, such as a solid oxide electrolyzer system and a compressor. This generator can generate hydrogen instead of electricity at night to eliminate power production at night when the grid load is low. During the day, when the grid load is high, the electrolyzer may be inactivated or operated at a lower hydrogen output while the power is provided from the power generator to the grid. 
         [0024]    For example, for a U.S. based grid, the predetermined threshold value may be 0.3 Hz and the nominal frequency may be 60 Hz. Thus, if grid frequency drops to or below 59.7 Hz, then step  206  is executed. If the grid frequency rises to 60.3 Hz or above, then step  210  or  212  is executed. These values may be different for grids in countries other than the United States. The frequency measurement and determination may be conducted at predetermined or random time intervals. 
         [0025]    In an alternative embodiment (See  FIG. 3 ), the operation state of the SOFCS  108  may be established as a continuous function of electricity output varying with grid frequency. In this embodiment, for grid frequencies lower than 59.7 Hz (region  300 ), the SOFCS  108  is operating in a fuel cell mode at a maximum designed power output level. For grid frequencies of 59.7 Hz and above, but less than 60 Hz (region  302 ), the SOFCS  108  power (electricity) output decreases gradually from the maximum value to a nominal power output (such as the power output that maximizes fuel efficiency). At a grid frequency of 60 Hz, the SOFCS  108  is operating in the fuel cell mode at the nominal power output. In region  304 , where the grid frequency is above 60 Hz and equal to or below 60.3 Hz, the power output decreases gradually from the nominal value to either zero at 60.3 Hz or to a negative value for a SORFCS indicating maximum electricity consumption and commensurate with electrolysis mode and hydrogen production. For frequency values of greater than 60.3 (region  306 ), hydrogen production remains constant at a maximum level. For decreasing grid frequencies (i.e., going right to left in  FIG. 3 ), the power output of the SOFCS  108  moves in the opposite direction (i.e., lower amount of power is drawn from grid and/or increased amount of power supplied to grid). 
         [0026]    The system  100  containing a SORFCS is capable of switching from electrolyzer mode to fuel cell mode in less than four seconds, thereby qualifying the power produced in fuel cell mode to be sold to the grid utility as spinning reserve. It advantageously provides for the recycling of excess electrical energy produced during low-consumption periods by conversion of steam to clean-burning hydrogen. 
         [0027]    The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to be limited to the precise form disclosed, and modifications and variations are possible in light of the above teaching or may be acquired from practice of the disclosure. The above-referenced embodiments were chosen and described in order to explain the principles of the disclosure and as a practical application to enable one skilled in the art to utilize the disclosure in various embodiments, and with various modifications, are suited to the particular use contemplated. It should be understood that the following description is intended to describe exemplary embodiments, and not to limit the claimed subject matter.