The increasing use of renewable energy resources in power systems has led to a growing focus on energy storage technologies. Energy storage can shift energy from peak-demand hours to off-peak demand hours or absorb excess renewable energy and provide it back to the grid when desired. Batteries, with their high power density, offer particular promise; and several studies have been made of battery storage in systems with renewable resources. But much of the work has been based on day-ahead models or short-term look-ahead scheduling models. Can these models, with their limited look-ahead functionality, fully exploit battery storage features in real-time operations? Moreover, batteries have high capital costs, and they deteriorate over time. Can the cost savings achieved by using battery storage offset the investment cost of the battery?

To answer these questions, a team of researchers from Argonne National Laboratory, Arizona State University, and the University of Chicago developed a two-stage framework to evaluate the benefits of battery storage in power systems with renewable resources. In the first stage, with day-ahead scheduling, wind generation tests showed that battery storage can reduce load and reserve shortfalls, decrease the number of hours that thermal units are committed, and lower the total system costs. In the second stage, the researchers used a battery schedule approximating real-time operation, that is, one that considers the uncertainties in renewable energy generation. Again, the tests showed that when battery storage is included, fewer reserve violations occur and wind curtailment is reduced, enabling more wind generation to be dispatched.

A cost-benefit analysis conducted by the research team was equally positive. The results showed that battery storage is beneficial when current capital cost estimates, the degradation effect, and its impact of the battery lifetime all are considered.

“But one issue did arise regarding real-time operation,” said Emil Constantinescu, a computational mathematician in the Mathematics and Computer Science Division at Argonne National Laboratory. ​“The tests with the second stage of our framework showed that when the wind generation deviated from the day-ahead forecast, the scheduling may not be able to fully exploit the flexibility of the battery. Our solution: an approach with a flexible operating range,” said Constantinescu.

With flexible operation, for most of the time periods the battery is operated within the operating range. At certain time periods, however, the battery is allowed to operate outside this operating range, in order to compensate for the intermittency and uncertainties in renewable resources.

Using 150 different wind scenarios for each of six days, the researchers compared their flexible operating range approach with three other methods: a fixed-schedule approach, a no-schedule approach, and a 3-hour look-ahead approach. In almost every case, the flexible approach outperformed the others, taking advantage of the flexibility of energy storage to provide more cost savings than with the other benchmark methods. The results also suggest that the flexibility of battery storage will be especially valuable when providing ancillary services such as spinning reserves – generators that are already online and can rapidly increase their power output to meet fast changes in demand

For the full paper, see N. Li, C. Uçkun, E. Constantinescu, J. R. Birge, K. W. Hedman, and A. Botterud, ​“Flexible Operation of Batteries in Power System Scheduling with Renewable Energy,” IEEE Transactions on Sustainable Energy, 7(2): 685-696, 2016.