Charles Lee Powell Chair in Electrical and Computer Engineering and Computer Science
Professor of Electrical and Computer Engineering

Hybrid Electrical Energy Storage Systems: Design, Optimization, and Validation

—

in

Sponsor: National Science Foundation

Conventional EES systems only consist of a single type of EES element. Unfortunately, no available EES element can fulfill all the desired performance metrics of an ideal storage means, e.g., high power/energy density, low cost/weight per unit capacity, high round-trip efficiency, and long cycle life. An obvious shortcoming of a homogeneous EES system is that the key figures of merit (normalized with respect to capacity) of the system cannot be any better than those of its constituent EES element.

HEES System Design and Configuration

An HEES system is comprised of different types of EES elements (e.g., batteries and supercapacitors), where each type has its unique strengths and weaknesses. The HEES system can exploit the strength of each type of EES element and achieve a combination of performance metrics that is superior to that of any of its individual EES components.Related work:

  • M. Pedram. N. Chang, Y. Kim, and Y. Wang, “Hybrid electrical energy storage systems,” Proc. of Symposium on Low Power Electronics and Design, Aug. 2010.

HEES System Management

Based on the properties of the HEES system and characteristics of power sources (or load devices), we developed charge management policies (such as charge allocation, charge replacement, charge migration, bank re-configuration, SoH-management, and so on) to operate HEES system properly to achieve a near-optimal performance. The charge allocation is to maximize the charge allocation efficiency, defined as the ratio of energy received by EES banks and the total energy provided by power sources over a given time period, by properly distributing power of the incoming power to selected destination banks. The charge replacement problem in the HEES system is to adaptively select the EES banks and determine the discharging currents, from zero to a maximum limit, and the voltage level settings on a charge transfer interconnect (CTI) so that the given load demand is met and the charge replacement efficiency is maximized. While charge allocation and replacement deal with energy exchange with external power supply and load demand, charge migration is an internal energy transfer from one EES bank to another.

The lifetime of EES elements is one of the most important metrics that should be considered by the designers of the EES system. The EES system lifetime is usually described using the state of health (SoH), which is defined as the ratio of full charge capacity of a cycle-aged EES element to its designed capacity. State of health-aware charge management problem in HEES systems is to find charging/discharging current profiles for all EES banks and CTI voltage, aiming to improve both the cycle life of the EES arrays (mainly battery arrays) and overall cycle efficiency of the entire system.Related work:

  • Y. Wang, Y. Kim, Q. Xie, N. Chang, and M. Pedram. “Charge migration efficiency optimization in hybrid electrical energy storage (HEES) systems,” Proc. of the Int’l Symposium on Low Power Electronics and Design,Aug. 2011.
  • Q. Xie, Y. Wang, Y. Kim, N. Chang, and M. Pedram. “Charge allocation for hybrid electrical energy storage systems,” Proc. of the International Conference on Hardware/Software Codesign and System Synthesis,Oct. 2011.
  • Q. Xie, X. Lin, Y. Wang, M. Pedram. D. Shin, and N. Chang, “State of health aware charge management in hybrid electrical energy storage systems,”Proc. of Design Automation and Test in Europe,Mar. 2012.

HEES System Implementation

A HEES prototype has been built based on our proposed HEES system architecture. The hardware part of the HEES prototype is comprised of three types of module: the EES bank module, the CTI module, and the converter module. For the software part, the user interface (UI) is designed using the LabVIEW, while control policies are implemented using the Mathscript module of the LabVIEW.Related work:

  • EES bank module: The EES bank modules store the electrical energy. We install three representative EES bank modules: one supercapacitor bank, one Li-ion battery bank, and one lead-acid battery bank. We select these three types of EES elements as they have very distinct characteristics.
  • CTI module: The CTI module is the path for charge transfer between power sources, load devices and EES banks. We have used an AC-DC rectifier and unidirectional power converter in our prototype system to keep the CTI voltage stable by using the AC power source (i.e., the Grid).
  • Converter module: This module contains an AC-DC rectifier for the grid power input, which are used to convert AC power into DC power. The DC power is in turn used to charge EES banks and maintain stable CTI voltage level settings. It also contains a DC-AC inverter to support AC loads.

User interface and control unit: The user interface (UI) is designed using LabVIEW. The LabVIEW UI monitors the runtime status of the HEES prototype, including the CTI voltage, voltage and input/output current for each EES bank, and calculates the instantaneous charging or discharging efficiency using these information.

Capital Cost-Aware Design and Control of HEES Systems

The deployment of residential HEES systems has the potential to alleviate the mismatch between electric energy generation and consumption. However, its wide application in residential units are prohibited due to the lack of a convincing and complete analysis of their economic feasibility.

In this project, we provide a complete cost-aware design and control flow of residential HEES systems. Specifically, we propose a two-step design and control method: first deriving daily management policies with energy buffering strategies and then determining the global problem of HEES specification based on the daily management results. We take into consideration the real-life factors such as the battery¡¯s capacity degradation, unit capital cost of EES elements, maintenance and replacing cost of the HEES system, etc. Simulation results show that this system achieves averagely 11.10% more profits compared to the none-buffering HEES system.

In addition, we present a design flow for HEES system in electric vehicles (EVs). Different from a residential HEES system, the EV HEES system is highly restricted by the weight requirement since larger weight results in higher traction energy consumption. We propose a Li-ion battery and supercapacitor hybrid system for EVs to reduce the daily cost and achieve high efficiencies.Related work:

SIMES: A Simulator for HEES Systems

SIMES is a simulation platform targeted at fast and accurate simulation for HEES systems. SIMES models various elements in a HEES system, including different types of energy storage systems, power conversion circuitry, charge transfer interconnects, etc. Most of the models are calibrated based on measurement data of actual hardware performed in our lab.

SIMES consists of three modules: Parser, Simulator and Visualizer. Parser parses input data in the form of an XML file and constructs HEES system model. Simulator simulates the operation of the constructed system. Visualizer is a graphical user interface which can visualize both the HEES system configuration and the simulation output.

SIMES enables end users to freely explore the design space of HEES systems, as well as testing custom power management policies. In addition, SIMES provides an easy-to-use interface which allows experienced users to implement their own component models as an extension.Related work: