California Storage Mandate

A transition to a low carbon energy system will require a large increase in energy storage, and policies to promote storage technology are necessary. This applies more obviously to renewable energy - we are all aware that the sun goes down - than nuclear energy. Nuclear power plants however run must economically at full tilt all the time, therefore there is a preference to be able to store that electricity when people don't need it for when people do not need it. Therefore recent moves by the state of California to promote energy storage should be welcomed. However a closer look at the details raises some serious questions.

California's policy is as follows. By 2024 California's three investor owned utilies must invest in 1.325 billion watts (1.325 GW) of energy storage capacity. The problems with this mandate are multiple. Let's begin with the most obvious and fundamental. Energy is not measured in watts. By convention we measure it in joules, or the more mediaeval unit called the British Thermal Unit. A watt is instead a unit of the rate of energy use. If I was to use an anology then I would say this mandate is rather like a fitness instructor telling me to get fit by running at 15 kilometres per hour each day. To see this consider the most commonly used form of energy storage: pumped storage.

Pumped storage is a rather straightforward, if expensive, way to store electricity. You use electricity to pump water up a hill, this is then stored in a reservoir, and when you want electricity you let the water flow back down the hill and electricity is generated by that water turning turbines. Typically this stores electricity with an efficiency of around 75%.

However meeting this 1.325 GW energy storage mandate entirely with pumped storage would give very uncertain results. Consider the pumped storage stations in the United Kingdom. Not blessed with easily pronounced names their capacities and storage capabilities are: Ffestiniog (capacity: 0.36 GW; storage: 1.3 GWh), Cruachan (capacity: 0.4 GW; storage: 10 GWh), Foyers (capacity: 0.3 GW; storage 6.3: GWh), Dinorwig (capacity: 1.8 GW; storage: 9.1 GWh). These numbers make the problem clear, there is a very unclear relationship between capacity, which is what California is mandating, and actual energy the station can store, which is what the mandate is supposed to deliver. Cruachan can store slightly more energy than Dinorwig, however Dinorwig can supply energy five times faster than Cruachan. In other words if California was to meet its mandated energy storage capacity with systems similar to Dinorwig it would actually be able to store five times less energy than if it used systems similar to Cruachan.  Quite clearly a quantiative target that can result in energy storage levels that vary by a factor of five has some serious problems.

The mandate also veers into near meaninglessness when some other forms of energy storage are concerned. Power to gas is an energy storage system with some benefits over existing energy storage methods, and is currently being tested in Germany and elsewhere. In pumped storage you convert electrical energy into potential energy, then into kinetic energy and then back into electrical energy. In power to gas you convert electrical energy into chemical energy, in the form of methane. This process is about 60% efficient, but burning it for heating or electricity in power plants will result in further losses. The fact that you can do whatever you want with this methane raises an obvious problem. How would you decide how this contributes towards the energy storage mandate? There appears to be no sensible answer, and I can only conclude that for this energy storage technique the mandate literally makes no sense.

Energy storage in most cases - though not for power to gas - is rather like a battery. As everyone knows some batteries last a lot longer than others, but can still power the same devices (until they run out of energy of course). Few people would buy batteries in a shop without thinking about how long those batteries would last. The same basic considerations should also apply to energy storage mandates, but apparently they do not.