Energy storage systems that are properly placed on the transmission system can be used to relieve transmission congestion [1]. Similarly, storage on distribution can be used to reduce peak loads [2]. The reduction in currents produced by congestion relief and peak reduction may also result in a decrease in line losses. Further efficiency improvements could be realized by optimizing the placement and scheduling of energy storage for loss reduction and efficiency improvement.

The efficiency of the storage systems themselves, however, may limit their use in loss reduction or system efficiency improvement. Typical round-trip efficiencies for bulk storage systems are 70-85% for pumped hydro and 70-90% for electrochemical batteries [1, 3]. Comparing this with North American transmission and distribution efficiencies that usually exceed 90%, the case for using storage to improve overall efficiency may be difficult to make.

This presentation will examine the issues, comparing storage efficiencies with transmission and distribution efficiencies. It will examine selected cases to address the feasibility of optimizing the placement and scheduling of energy storage to improve system efficiency and reduce greenhouse gas emissions. Efficiency improvement will then be discussed in the context of the other potential benefits and costs of storage.