Original URL: http://www.theregister.co.uk/2011/04/13/darpa_des/

DARPA aims to make renewable power practical at last

At Afghanistan combat bases, anyway

By Lewis Page

Posted in Science, 13th April 2011 10:09 GMT

Pentagon boffinry bureau DARPA, which deals with established technology paradigms in much the same way as Patrick Swayze and Demi Moore dealt with clay in their 1990 supernatural romanta-flick outing - that is as material for squidging into improbable shapes so as to satisfy the squidger's raging occult lusts - has done it again.

In this case the deep-seated desire which DARPA is trying to satisfy is the need to remove a major obstacle facing renewable power sources. As the war-boffins put it in a request for proposals issued yesterday:

For example, using photovoltaics for power generation, a short-term interruption could be caused by a cloud passing in front of the sun, a medium term interruption would be caused by the sun setting, and longer term power interruption could be caused by a multiday storm or seasonal event. Available wind power manifests power fluctuations of similar timescales.

This means that something else must be ready to step in when renewable power gets interrupted. This could be conventional fossil-fuelled (or nuclear) "thermal" generation. This is mostly what happens when major grid-scale renewables kit suffers interruptions.

But having full thermal backup for renewables is expensive: it makes the idea of renewables much less attractive once you are using renewables for a large amount of your load, both on a grid - or at a US military forward operating base in a warzone. At the moment such US bases are powered largely by diesel generators, meaning that fuel has to be trucked in along roads plagued by ambushes, mines, bombs - and occasionally by recalcitrant neighbouring governments.

This has previously led DARPA to look at powering such bases with small nuclear reactors, which could have the added benefit of perhaps allowing vehicle and aircraft fuel (another big logistics burden) to be produced locally from soldiers' poo using surplus reactor power.

However, another plan which has lately been gaining much currency across the US services is the installation of solar or wind power at FOBs.

Thus it is that DARPA plans a project called Deployed Energy Storage (DES) to store surplus power from the renewable generators when output is high, and use this stored juice to make up the shortfall when the sun goes in or the wind drops.

This will be amazing stuff indeed:

The system at full charge will be capable of providing uninterrupted power to a 150 kW average load for 9 days with 90% reduction and 30 days with a 30% reduction in available generated power from an appropriately sized renewable power generation plant.

Or in other words the kit will be capable of holding and releasing no less than 32.4 megawatt-hours, and will laugh at fast charge/discharge rates and rapid charge cycles. And it will be deployable to a military FOB, so it has to be something that fits on a reasonable number of trucks.

Some power utilities have tried using large, specialist li-ion battery installations in such tasks. Typical installations can hold 500 kilowatt-hours and fit into a truck trailer: to meet DARPA's requirements one would need a fleet of 60 or 70 trucks to deliver the DES kit.

By contrast a single truck can carry a 150 kilowatt diesel generator. Another single military supply truck could also deliver 20,000 litres of diesel, which would suffice to run that genny at full load for more than 20 days - or let it replace a DES system for at least 60 days; probably a lot more.

Basically for a li-ion DES to involve fewer truck deliveries than normal diesel generation, your FOB would need to be in place for decades. It would seem that this isn't a viable way forward.

There are other ways to store power and release it using limited amounts of equipment. Some long-endurance solar powered aircraft now in development are intended to store energy by using surplus power to crack hydrogen out of water. The hydrogen would then be used in fuel cells to generate power at night (and the resulting water exhaust, in that case, saved up to be cracked again next day).

That's new stuff, but the fact that designers think it could go on aircraft might make it viable at least in a military context. Storing the hydrogen might be a bit of an issue, though: assuming it could be kept as a gas under say 200-250 bar pressure and allowing for inefficiency in turning the stuff back into 'leccy one would be looking at a high-pressure tank farm with well over 100 cubic metres of volume: a pretty large, dangerous and vulnerable thing to have around a combat FOB - or indeed anywhere.

The tank farm on its own would represent many truckloads of equipment. Liquefying the hydrogen so as to cut down on volume probably wouldn't be practical. It would be better to store the energy as some kind of hydrocarbon - for instance methane, as some German engineers are trying to do. But that would still involve a biggish tank farm, and would require a source of carbon as well.

All in all it's difficult to see how DARPA think they're going to pull this one off, even in a military context where cost is less important than size and weight.

On the other hand, if they did achieve the goal at any kind of reasonable cost, the implications would be huge for renewable power - and for nuclear, too. Nuclear powerplants, contrary to the assertions of many greens, can and do operate in load-following mode - but it would improve their economics if they could run all the time at a high load factor, which effective and easily scaled-up energy storage kit would allow them to do.

DARPA promises that "additional parameters" on DES will be issued soon. The agency anticipates that the trial system "will be continuously demonstrated for 30 days at a government facility using renewable energy sources".

We'll let you know more as we get it. ®