Strategies for the reduction of energy storage capacity for high penetration of wind and solar power

Wind and solar energy are currently the most promising carbon free sources of electrical energy. However, high penetration of solar and wind into the electrical grid will require energy storage due to the intermittent and unpredictable nature of these sources. Using the Midcontinent Independent System Operator (MISO) energy market in the central region of the United States for the years 2007, 2008, 2009, and 2010 as a case study, we show that the amount of storage required for a 100% wind and solar energy portfolio would correspond on average to approximately 4.5% of the load energy for the year. Allowing for 10% of this energy to be produced by other (necessarily dispatchable) sources, and using 20% over-generation of solar and wind, reduces this fraction to approximately 0.03%. Assuming optimistic capital costs for wind and solar capacity as well as storage capacity, a simple model shows that the corresponding ratio of storage to generation capital costs is reduced from 10 to 0.06.