DARPA aims to make renewable power practical at last

At Afghanistan combat bases, anyway

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. ®

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