The role of storage has driven the design of hybrid systems. The early hybrid systems were either very small systems using lead-acid batteries or systems with limited renewable penetration. Wind diesel systems could have somewhat higher renewable penetration than solar diesel systems and sometimes used flywheels as a very short duration storage technology for stability.

The development of lithium batteries has enabled hybrid systems to have multiple hours of storage, greatly improving the cost-effective penetration of renewable energy, but still requiring fuel-fired backup generation. The goal of completely carbon-free electric power has created substantial interest in the potential of hydrogen as a long duration storage technology.

We will present analytic results demonstrating the relationship between the cost of storage and the economically optimal level of renewable penetration with a comparison of the economics of hydrogen as energy storage, a transportation fuel, or an industrial feedstock.