History as a Guide to Understanding the Future of Storage

Human energy use is derived from sources that can be characterized as either stocks or flows. In this view, the solar energy reaching the earth is an energy flow, but the energy embodied in wood that was derived from solar energy, via photosynthesis, is an energy stock. Energy storage deals with the relationship between stocks and flows: storing energy, whether by natural or anthropic processes, involves the accumulation of flows as stocks; exploiting stored energy involves the conversion of stocks to flows.