Patent Application
20200335974
This description generally relates to energy storage using flywheels. However, the invention may be applied to other applications where frequency regulation of an electric grid is desirable.
Utilization of distributed energy storage is fundamental to modern utility grids that incorporate substantial time-variable, non-dispatchable, renewable energy generation. Distributed energy storage is also essential for stabilizing weak grid systems that do not have substantial dominant conventional rotating generation. Examples of the latter occur in relatively isolated settings with small grids, such as on islands and in the developing world. But, notably, the issue of weak regulation capacity is also present in some regions of North America, such as the ERCOT regional grid in Texas.
Distributed energy storage integrated with utility transmission and distribution systems, or on customer sites behind the meter. Based on siting, storage characteristics, and utility governance, energy storage can provide various services to utilities. These include peak shifting, load shifting, and provision of resiliency. In addition, it can provide ancillary services including provision of capacity, frequency regulation, frequency response, voltage regulation, and black start capability.
The subject invention pertains to frequency regulation. Frequency regulation concerns the regulation of utility grid frequency, with the classical underpinning based on use of rotating generation. Basically, if instantaneous real power demand exceeds generation, the overall grid frequency decreases as rotational kinetic energy is extracted from rotating generators and as well as from rotating ac motor loads. The opposite is true for the case where real power demand falls below the instantaneous generation level. Instantaneous mismatch in generation and load is unavoidable since perfectly accurate forecasting of load is not possible.
Frequency regulation has been conventionally implemented with thermal generation systems. Since a thermal generation plant can only source real power and cannot be curtailed to zero power without a complete shutdown, the dynamic range of adjustment is only a fraction of the plant's nameplate capacity rating.
In contrast to thermal plants, energy storage systems that interface with modern power electronic converters are capable of full scale up/down regulation, since full range power can be supplied or absorbed at command in such a system. Thus, whenever an energy storage system is neither fully charged nor fully discharged, it is capable of 200% dynamic range in its power capacity, accessing the full charge/discharge range.
Further, conventional thermal generation plants require a time scale of minutes to ramp from one power level to another. In contrast, an energy storage system interfaced with a modern power electronic interface can ramp in a sub-second timescale, enabling very accurate tracking of a wideband frequency regulation command signal.
Distributed energy storage is commonly realized with batteries in some form, inclusive of electrochemical, mechanical, and thermal technologies among others. However, flywheel energy storage, a type of energy storage system that stores energy as rotational kinetic energy, is emerging as an important alternative, with steel rotor flywheels exhibiting leading performance in the dollar/kwh metric due to underlying physics, steel material properties, and steel industry manufacturing experience. The features of +/−100% capacity range and essentially instantaneous signal tracking are inherent to flywheel energy storage systems.
A controllable resistive load bank can be integrated within a distributed energy storage system to increase the capacity range. Examples of resistive load banks occur in electric water heating, electric space heating, and electric heat pump systems used for water heating, refrigeration and heating-ventilating-air-conditioning (HVAC) applications. In these thermal applications, flexibility in time of use is often available since the systems under control are thermal in nature and have substantial internal thermal storage capacity. This latter utilization of controllable thermal loads is commonly accessed with the Demand Response framework where utility-controllable loads are used to augment regulation and capacity ancillary services.
The subject invention is an augmented energy system that provides frequency regulation to power grids by integrating a resistive load bank. Combining an energy storage system with a resistive load bank increases energy capacity.
Embodiments relate to a an augmented energy storage system that includes an energy storage subsystem that stores and supplies energy to an electric grid, a load bank that dissipates energy, a power control system that controls the flow of energy into and out of the energy storage subsystem and into the load bank, and an inverter that converts DC current to AC current used by the grid.
Embodiments further relate to an augmented energy storage system that includes a storage site in which a load bank is located on another section of a utility grid from the storage site.
Non limiting and non exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified.
The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.
The invention now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the invention may be embodied as methods, processes, systems, or devices. The following detailed description is, therefore, not to be taken in a limiting sense.
As used herein the following terms have the meanings given below:
Energy storage system—as used herein, refers to a system that stores and discharges energy. The energy storage system is typically coupled to an electric power grid, enabling the grid to store and withdraw energy as needed.
Flywheel unit or flywheel device—as used herein, includes a flywheel rotor that is typically a rotationally symmetric mass, such as a cylinder or disc, that spins. The rotor is physically coupled, directly or indirectly, to a motor/alternator that itself is electrically coupled to a converter, such as a back-to-back inverter system, constituting an AC-AC conversion subsystem. When power is received for storage, the rotor is driven, increasing the rotational speed of the flywheel rotor. The faster a flywheel rotor spins, the more energy it stores. When power is to be extracted, the flywheel rotor drives the motor/alternator. When coupled together, one or more flywheel units form an energy storage system.
Load bank or resistive load bank—as used herein, refers to one or more devices that develop an electrical load, apply the load to an electrical power source and convert or dissipate the resultant power output of the source. As used herein a load bank can be controlled so as to convert or dissipate a specified amount of power. Generally, commercially available load banks may be used to perform the functions ascribed to load banks as described herein.
Frequency regulation is a tool employed by power grid operators in cases when the system frequency gets too high or too low. The objective of frequency regulation is to maintain the grid at a specified frequency, typically 60 Hertz. Frequency regulation is accomplished by regulating power output; typically power generators increase or decrease power output for a period of time, referred to respectively as “regulation up” or “regulation down.”
Commands to a frequency regulation system may be supplied by a centralized regulator, eg. the independent system operator, to effect the intended regulation function. Alternatively, a frequency regulation system can operate as an autonomous regulator, by sensing and measuring instantaneous frequency at its point of common coupling to the utility. In this setting, the frequency regulation system develops its command signal locally by sensing the deviation from a nominal frequency setpoint. Both methods are encompassed in the scope of this invention.
As can be seen in the example of
The subject invention, referred to as an augmented energy storage system, combines a resistive load bank, or simply “load bank”, with a given energy storage system on a common DC bus, on a common AC bus, or in proximity within a distribution or transmission grid system.
As previously discussed, the load bank can be implemented using a commercially available product. Examples of commercially available load banks include the SIMPLEX Stationary Load Bank from Simplex Inc. headquartered in Springfield, Ill.
Further, the load bank function can also be realized with controllable internal dissipation functions within a given energy storage system. In the case of a flywheel energy storage system, such internal dissipation processes can include use of gas drag, other forms of friction applied to the rotor, as well as electrical losses realized within the power conversion subsystem. The latter include losses in the power electronic conversion stages and electromechanical conversion subsystem, typically a motor/generator.
Since resistive load is inexpensive in comparison to energy storage, this augmented system is especially effective for performing frequency regulation. It should be noted that the additional installation costs for a resistive load bank as detailed here can be very low, since the other elements of the energy system are already in place.
Introduction of energy storage to a utility system presents a new decision variable to the system, namely that of regulating the state of charge of the storage system at times when there is not a hard constraint to charge or discharge at a commanded power level. Opportunities to set storage state of charge occur when other dispatchable generation is on-line within the utility system, and not utilized at an extreme of capacity. Strategies for managing storage may utilize dynamic programming optimization over a receding finite horizon, also known as model predictive control (MPC), or other similar optimal control methodologies. Exemplary methods may break time into one hour segments, and then solve for hour-ahead and/or day-ahead policies, while keeping a horizon that scopes out for one week or more. Such an optimization needs to be informed by performance objectives, e.g. those that may occur in frequency regulation only, provision of capacity, peak/load shifting, or some combination of such ancillary and energy services.
In the case of pure frequency regulation with an augmented energy system that includes a resistive load bank, i.e. with no other service objective, an optimal strategy involves attempting to always keep the storage system fully charged. As seen in
Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs through the disclosed principles herein. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes and variations, which will be apparent to those skilled in the art, may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope defined in the appended claims.
