Architecture Development and Design Optimization of Hybrid Electrical Energy Storage Systems

There are examples of actual deployment of grid-scale EES systems to mitigate the gap between the supply and demand. In addition, most stand-alone renewable energy sources, such as solar energy, wind power, and hydropower require an EES system. However, current EES systems are mainly homogeneous, that is, they consist of a single type of EES element, and therefore, suffer from a fundamental shortcoming that will plague every homogeneous EES: key metrics (normalized with respect to capacity) of any homogeneous EES cannot be better than those of its individual storage elements. Consequently, a homogeneous approach is not viable for any system where none of the existing types of EES elements can fulfill all the required performance metrics – such as power density, energy density, cost per unit capacity, weight per unit capacity, round-trip efficiency, cycle life, and environmental effects. This limitation is preventing the adoption of a wide range of socially and economically useful technologies, such as wide adoption of grid-scale EES and electric vehicles (EVs), and causing significant inefficiencies for many others. Hence, elimination of this limitation of homogeneous EES systems is the primary motivation for our research.

Our approach for improving performance of EES systems is to exploit different types of EES elements, where each type has its unique strengths and weaknesses, to design hybrid EES system architecture and control policies that dramatically improve the key performance characteristics of the storage system. This approach will exploit fundamental properties that provide a heterogeneous energy storage system (HEES) with the potential to achieve a combination of performance metrics that are superior to that for any of its individual EES components. In fact, in some cases, it is possible for a HEES system to attain values of individual metrics that are close to their respective best values across its constituent EES elements. For example, it is possible that a HEES system can achieve the power density of its constituent EES component that has the highest power density (which is likely to have the highest cost) and, at the same time, achieve a cost close to that of its cheapest component (which is likely to have low density). Simply speaking, we pursue HEES since it holds the promise of providing us with the best of all worlds. Such dramatic improvements can be provided only by a HEES system that is well-designed and well-controlled. Hence, development of design and control techniques for HEES is our goal.

Tutorial given at the 2011 International Symposium on Quality Electronic Design, Santa Clara, CA — Hybrid Electrical Energy Storage Systems

As of today, no single type of electrical energy storage (EES) element fulfills high energy density, high power delivery capacity, low cost per unit of storage, long cycle life, low leakage, and so on, at the same time. Following a review of conventional EES, we introduce a HEES (hybrid EES) system comprising heterogeneous EES elements based on the concepts of computer memory hierarchy. We introduce HEES design considerations aiming at the optimal charge management for various cost metrics.