Seriously big batteries

Sackerson recently sent me this Telegraph piece on solar power. As you can see, it reads like science fiction, no doubt because that's what sections of the reading public enjoy. For example :-

Solar is for keeps. The more it expands, the cheaper it gets as economies of scale kick in.

Stirring stuff and maybe we need to be irrationally optimistic to push the possibilities to their limits, but unfortunately the claim is false.

Solar power is notoriously intermittent such that too much of it attached to a distribution grid causes unacceptable stability problems. Wind has a similar issue. There are other factors, but until the storage issue is resolved in a cost-effective way, stable distribution grids require predominantly fossil fuel or nuclear generation, quite apart from issues of cost.

So as we all know, a big problem with wind and solar energy is storage. How do we store it and how do we do so at a reasonable cost? Fossil fuel energy such as coal and gas are stored chemically in the fuels themselves, so might chemical storage be viable for wind and solar? From the same Telegraph piece we have:-

Cheap energy storage from flow-batteries (a Harvard research project funded by the US Advanced Research Projects Agency) will soon overcome the curse of intermittency, letting us absorb the sun’s rays by day and release them again as heating and light overnight.

Well it isn't just day and night is it? Especially here in the UK. For example, we have this thing called winter. I'm surprised a Telegraph writer hasn't heard of it.

So what is there to extract from the writer's runaway enthusiasm? Obviously we already store energy chemically in batteries and large scale battery storage seems to be the next big thing for wind and solar energy.Yet before the excitement overpowers our critical faculties, it is worth remembering that even with storage, wind and solar may not be economically viable anyway.

However, one option is flow batteries which are being pushed hard as a viable means of storing wind and solar energy. EnerVault has recently opened  an iron/chromium flow battery in California.

The EnerVault flow battery's two electrolyte tanks
store energy generated by a solar array
in an almond orchard outside Turlock (Stanislaus County). 

The EnerVault flow battery can deliver one megawatt-hour of energy from a 250 kW battery for four hours. Greater capacity could be achieved via more tanks of electrolyte. The battery is charged by a solar array.

There are complexities, but the nuts and bolts are not expensive and the technology is well understood. Iron and chromium salts are inexpensive and fairly safe to handle so a degree of optimism may not be out of place here. From spectrum.ieee.org.

By 2015, EnerVault expects to have multi-megawatt commercial systems installed. In four or five years, it hopes to have 20 megawatt or 50 megawatt-size batteries taking the place of natural gas "peaker plants," says Pape. The tanks that hold electrolyte solution can be very large but the footprint is comparable to conventional power generation equipment, he says.

The chemical storage medium is iron and chromium salts dissolved in water. Essentially all the battery does is pump them through a cell. To recharge, the process is reversed. At the heart is stable and durable technology with few moving parts, but it is still worth remembering that these are essentially political technologies. Not much seems to be novel apart from the cells.

Using iron and chromium electrolytes required developing a novel mechanical system for flowing the materials through the battery’s cells. The batteries have a series of stacked cells, with each one optimized for the state of charge of the electrolytes flowing through them, says chief technology officer Ronald Mosso.

Even so, I have a soft spot for human ingenuity and the spirit of invention and my soft spot hopes at least one or two energy projects such as this come to fruition.