DOE’s Office of Fossil Energy is working through its new Advanced Energy Storage Program to improve and foster the widespread use of energy storage integrated with fossil energy applications leading to facility flexibility, power grid resiliency, cost savings, and reduced greenhouse gas emissions. One class of energy storage technology with potential for long durations and integrating with fossil assets is mechanical energy storage. Mechanical energy storage takes excess or low-cost energy and converts it into potential energy for subsequent discharge to the grid. As an example, Compressed Air Energy Storage (CAES) technology may offer an easy means of storage and power generation. It uses off-peak cheap electricity to compress air and store it in a pressurized storage reservoir. When electricity is needed at peak demand, the air is withdrawn, combusted, and expanded to drive an electric generator. With this system, air can be pre-heated by recovering heat from the compressor train or by burning fuel, such as natural gas, to improve efficiency.