OK, so energy storage isn’t the most exciting topic in the world, but it really matters. The problem of energy storage (or changes in energy use that make it unnecessary) is the biggest remaining obstacle to a transition to renewable energy. So, here are some observations, labelled for convenience and partly derived from this study by the US Department of Energy
(a) Any reversible energetic process represents a potential storage technology. Reversibility entails that some energy is stored (as potential or chemical energy) when the process goes one way, and released when it goes the other. Of course, the Second Law of Thermodynamics implies that we will always add entropy (that is, lose useful energy) in this process
(b) Any technical or social change that shifts the time at which energy is finally used replicates the effects of storage
(c)Energy storage is in much the same position as renewable electricity generation was, say, 15 years ago.
(d) There are a lot of potential approaches, most of which have been developed in niches where particular characteristics are required. For example, car batteries need to store a lot of energy for given weight, household batteries need to store energy for a long time and so on. The needs of a renewable-dominated electricity system are very different and will require substantial modifications of these technologies
(e) With one big exception, there is currently no price incentive, in most jurisdictions to use storage technologies and therefore none are used
(f) The big exception is off-peak hot water. Coal and nuclear systems generate baseload supply when it is not needed for consumption. Price incentives are used to encourage people to store the resulting excess energy in the form of hot water
(g) There’s no technological obstacle, given the availability of smart meters, to changing the timing of hot water systems to reflect actual availability of excess electricity rather than reflecting the assumptions of a coal-based system
(h) All of this applies to electric cars. Even ignoring the possibility of feeding power back into the grid, the economics of electric cars would be drastically improved if they could be charged using low-cost power in times of excess supply (in the case of solar PV, around midday when lots of cars are sitting in parking lots)
(i) Something I just found out from the DoE study: Electric car batteries are considered unfit for services when they fall to 80 per cent of their original charge capacity (recall that energy density is critical for car batteries). But they still have a long potential life as static storage devices. This enhances both the economics of electric cars (since the battery has resale value) and of storage (since the opportunity cost is zero)
Here’s an older post, with a really simple example of how the argument works, once you get away from the fixation on replicating the characteristics of a coal-fired system.