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Energy storage – Part 1

Energy storage is often seen as a key to grid balancing- matching variable renewable inputs to variable energy demands.

So far pumped hydro storage is the only significant form of storage, except of course for stored fuels like coal, gas, oil or biomass. At present though, energy storage is still very expensive. So far it’s only been done- at least in the UK- when there is thought to be a serious need. For example, the giant Dinorwic underground pumped hydro project, located in a vast cavern in Wales, which at the time of its construction, was Europe’s largest infrastructure project. It was it seems initially conceived, financed and built in part to provide backup in case the Sizewell B nuclear plant went off line. It could go from zero to full generation within around 20 seconds and therefore could compensate for sudden loss of power on the grid, but also for sudden increases in demands for energy. Dinorwic could of course only hold the fort for a short while- 1-2 GW for perhaps 2 hours, while other backup came on line. And it was very expensive, so much so that when, following UK electricity privatisation, a lot of cheap and flexible Combined Cycle Gas turbines came on the grid, it became less attractive and has evidently had a harder time making money in the new market.

However, some see it as the sort of thing that will be needed in addition to more conventional pumped hydro projects, as we move to have more wind and other variable renewables on the grid. An EU wide supergrid link could also help, not least by linking into pumped hydro storage capacity around the EU – for example there is a lot in Norway. See www.theoildrum.com/node/7404

There are of course also other electricity storage options, including, for the short term, capacitors, flywheels, utility scale batteries, and flow cells; for the medium term, compressed air systems; and, for longer term storage, the conversion of electricity to hydrogen for storage and then for use in fuel cells or combustion engines to produce electricity again when needed. In parallel there’s the option of conversion to heat, which can be stored in molten salt stores.

These options all have their pros and cons- high capital costs and low energy conversion efficiency being key problems. On this basis, the hydrogen lobby has had a few years in the doldrums e.g. electrolysis is seen as inefficient and storing hydrogen as a cryogenic liquid is expensive, and there are energy losses when it’s cooled and then regassified. But recently it’s been reinvigorated by new production possibilities including a new ‘Green Hydrogen’ production cold-plasma technology, which could tilt the balance more in favour. www.gasplas.com/w3

There are also some novel storage ideas emerging, with.claims about the viability of large scale underground hydrogen gas storage in salt caverns. See: www.whec2010.com/fileadmin/html/78-4.html

On a smaller scale, Highview Power Storage has demonstrated a novel 300kW prototype cyrogenic storage system which stores excess energy at times of low demand by using it to cool air to around -190 °C. via refrigerators, with the resulting liquid air, or cryogen, then being stored in a tank at ambient pressure (1 bar). When electricity is needed, the cryogen is subjected to a pressure of 70 bars and warmed in a heat exchanger. This produces a high-pressure gas that drives a turbine to generate electricity. The cold air emerging from the turbine is captured and reused to make more cryogen. If waste heat from a nearby industrial or power plant is used to re-heat the cryogen, it’s claimed the efficiency rises to ~70%. http://www.highview-power.com

Do we really need storage?

However storage, by whatever means, is still going to be expensive. Fortunately there are alternative ways to deal with variable supplies . For example, working on the demand side of the equation, we can develop dynamic demand management techniques – delaying load peaks by a few hours or switching off some loads when wind power is low, using smart meter links. Though we don’t need it yet. At present, and for some way ahead, the variable inputs from wind generation can be easily balanced by running conventional gas fired plant up and down from full power. They already have to do this to balance the daily demand cycles, with more wind on the grid they just have to do this a few times more often, adding a small extra cost and undermining to carbon saving role of wind very slightly. As we have to accommodate more inputs from variable renewables, we may have to install more balancing capacity, but its relatively cheap and can increasingly be fuelled with biomass or geothermal heat-or perhaps even some solar heat, backed up by heat stores. As I pointed out in an earlier Blog, that could be linked to a system with Combined Heat and Power Plants feeding district heating networks and heat stores.

Moreover when wave and tidal energy are also being to fed into the grid, they can help balance varying wind (and solar) to a degree, since waves are in effect stored wind, and tidal cycles are unrelated to the weather.

Couldn’t nuclear plants also be used to balance the variable outputs from renewables like wind? The short answer is no- at least not much. Nuclear plants are usually run 24/7 to recoup their large capital costs and there are operational and safety reasons why they can’t be run up and down from high to low power regularly and rapidly. It’s not just thermal stresses in PWRs, but also the excess production of contaminating isoptopes which can interfere with proper and safe operation. Even so, it seems an EPR could ramp-up from 25% to 100% capacity, at 5% per minute, of its maximum output (i.e 80 MW per minute) e.g. from 400 to 1,600 MW in 15 minutes, but only100 times per year e.g. once every three days. Not much use for balancing regular wind variations, but maybe OK for long lulls in wind, if the nuclear operators will accept running at lower power (and loosing money) when there is wind available. Note also that nuclear plants, like all power plants, have to be backed up: for example a 300 MW light oil fired gas turbine plant is being built in Finland to back up the new, much delayed, Olkiluoto nuclear power plant.

The simple message though is that, basically inflexible nuclear is not very suited to balancing variable renewables. Moreover, if we try to have a lot of both on the grid there will be conflicts when demand for power is low- which then do you switch off? Storage might help reduce this conflict, but at extra cost. Really we don’t want to have large amounts of both nuclear and renewables on the grid system.

For more on nuclear balancing here’s an audio file : www.nuclearinst-ygn.com/ Docs/Vol5_Iss6_Track%202.mp3
And for a good overview: www.claverton -energy.com/is-nuclear-power-flexible-does-it-have-load-following-capability.html

It could be argued that, to some extent, we only need large amounts of storage if we are having problems with balancing the grid system more directly-and, if for example, we have a large inflexible nuclear component on the grid. But, even if that not an issue, it is true that if we could come up with a cheap storage system that would help grid balancing, and the hunt is on for new ideas. I will be looking at some in my next Blog.

For more ideas, there was an energy storage session at this years All-Energy Aberdeen conference: <a href="http://www.all-energy.co.uk/Power_storage_-the_holy_grail.htm”>www.all-energy.co.uk/Power_storage-_the_holy_grail.html

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