Background
Grid modernization has become a new refrain in the electric utility industry. The electric grid is ageing, and modernization is imperative. The many facets of grid modernization include – (a) changing mix of generation including distributed and clean energy, (b) reliability and resiliency of grid in the face of climate change and cyber/physical threats, (c) electrification of transportation, (d) interconnectivity, interoperability (information exchange), and distributed/edge control, (e) market-driven management of supply and demand, and (f) physical infrastructure for energy transport.
Many of these aspects of a flexible, sustainable, and affordable modern grid present new opportunities for Southwest Research Institute (SwRI). Extending SwRI’s on-going involvement in electrified transportation to the electric grid is a natural next step. SwRI’s access to a small, grid-connected battery storage and solar array on its campus through an arrangement with the local electric utility company facilitates this step.
One of the unique challenges of the electric grid is the need to constantly match supply to demand lest the grid become unstable. Large portion of the electric supply is relatively fixed over a short duration. The demand, however, can fluctuate unpredictably. Battery storage, as a multi-time-scale source and sink of energy, is beginning to revolutionize electric distribution. Value-stacking (that is, using battery energy storage for a variety of grid services) is becoming popular. Services such as solar shifting, arbitrage, frequency regulations, voltage support etc. subject the battery to large swings in state-of-charge (nearly full to nearly empty), power levels, and event duration (seconds to hours). However, longevity and safety of battery under such mixed operation is unclear – and that is the subject of this internal research project. Performance degradation and safety of batteries under mixed grid duty are important considerations from life-cycle cost and societal perspectives.
The main objectives of the project are: (a) to develop a unified model-based method to estimate performance degradation and likelihood of fire (due to plating/dendrite formation) in a lithium-ion cell under mixed grid duty, and (b) to implement and demonstrate the unified method/algorithm in real-time for bid/offer decision making and for predictive preventive maintenance.
Approach
Given the potential for long-term opportunities in a restructured electric grid, we initiated a Joint Industry Program (JIP) titled ‘Energy Storage for Electric Grid’ (ESEG) in collaboration with the IERE of Japan. Members participate in the JIP – an adjunct to the internal research project – by contributing operational field-data and by providing guidance to keep the program true to the needs of the industry. We use the member-supplied field data of battery dispatch to characterize daily, weekly, and seasonal trends and variances.
There are five major tasks: (1) Laboratory testing: The operational data is used to design and conduct cell-level experiments in the lab, covering the range of operation seen in the field. The laboratory experiments are designed to promote and measure performance degradation and dendrite formation. (2) Model development and calibration. The laboratory data is used to calibrate the cell-level, physics-based model. (3) Model validation: The operational data is used to construct “reference” duty cycles seen by a value-stacked battery. The calibrated model is validated against these reference cycles to verify its predictive ability. (4) Charge/discharge shaping: Charge/discharge profiles are adjusted to reduce degradation and dendrite formation. This is a trade-off with the intensity of value-stacked services. (5) Demonstration: Exhibit the model-based degradation and safety system on the small, grid-connected battery on SwRI campus.
Accomplishments
The JIP has five members as of this writing. We procured battery modules from the BMW i3 REx and extracted the 94 Ah prismatic cells. It is our understanding that these cells are also used in grid applications. The cells are presently undergoing the specifically designed tests in SwRI ESTC laboratory. We have extended the standard single particle model (SPMET) of a lithium-ion cell to account for degradation and plating/dendrite growth. We are refining the framework to identify parameters of the extended SPMET model from the laboratory data. In the coming quarter we plan to construct and test the reference duty cycles in the laboratory, and to implement the extended model for real-time operation.