The Ergosphere
Sunday, October 30, 2005

A reconception of marine power

From time immemorial until roughly the 19th century, boats have been powered by either muscle power or the wind.  Galleys were driven by oars and everything else raised sails to be propelled across the seas and oceans.  The advent of steam changed that.  First boilers were fired by wood and coal, and then by oil as ships got bigger and faster with greater and greater range.  Oil, as the densest form of energy available, was essential to both effective military ships and competitive commercial ships.  Some large military ships and submarines are now nuclear, and coal retains a sliver of the market in antique vessels - the SS Badger which ferries cars from Muskegon to Manitowoc is an example - but the vast majority of all shipping is still powered by bunker fuel feeding boilers or diesels.

SkySails artist conceptionAlternative energy has made some small inroads on e.g. pleasure craft for auxiliary electrical power, but major vessels are still running on oil both in and between ports.  This shows strong signs of changing; the high cost of oil is rekindling interest in sail.  Modern materials and automation have reduced the labor requirements to use it.  Roller-furling jibs are one thing, but computer-controlled parafoil kites are a whole new game.  Flying well above the waves, these kites can capture more power than even the highest topsail of a clipper ship.  With favorable winds, even large cargo ships can see substantial fuel savings, greater speeds or both.

All of this begs the questions (I'm sure you're anticipating me by now):

  1. Why should only some winds be favorable?
  2. Why should shippers be satisfied with partial fuel savings?

Solar energy isn't going to manage.  A large container ship might be over 350 meters long and 48 meters wide, for a footprint of 16800 m2.  Even if all of this area could be covered with PV cells at 20% efficiency (yielding ~200 W/m2 in full sun) the total power output would be a mere 3.36 MW.  Large container ships have engines as powerful as 80 megawatts; even if a deck-full of containers could be covered by PV, this is clearly too great a demand to be satisfied by direct sunlight.

Technology to the rescue

SkyWindPower Flying Electric Generator (FEG)Sky WindPower Corporation is attempting to commercialize the "gyromill" concept invented by the Australian professor Bryan Roberts.  The gyromill or flying electric generator (FEG) is an autogyro kite with an electric generator attached; it can be sent aloft by powering its rotors with electricity supplied through its tether, and then return power to the ground when it reaches an altitude where the wind is strong.

Sky Windpower claims a power capability of 1.5 MW from rotors totalling 24329 ft2 (2660 m2) in swept area.  The current wind-power champ is a 5 MW ground-based turbine with a single rotor of roughly 124 meters diameter (12080 m2 area); if a single-rotor FEG could return the same power per unit area, it would generate up to 6.8 MW.  One or two of these would be able to supply a substantial fraction of even a large ship's propulsive power demands.

One thing that's not used in the FEG concept is the tension in the tether.  What good is pulling the Earth around?  It won't go anywhere... but a ship does.  A wind turbine generating 6.8 MW of power from a stream of air moving at 15 m/sec requires a minimum force of 454 kilo-Newtons (~102,000 pounds) just to hold it in place; a flying generator would need even more to compensate for aerodynamic drag caused by lift (induced drag).  A force of 102,000 pounds pulling forward on a ship moving at 15 knots supplies 4700 horsepower, or 3.5 megawatts of tractive power; as the speed of the ship goes up, so does the power per unit of force.  (This force could be increased by flying back and forth across the wind, perhaps at some cost in electric generation.)  The combined electric and tractive power supplied by a 124-meter marine FEG might be 12 megawatts or more.

Two such units on a ship could supply 24 MW.  They would even be able to supply net power when the ship was sailing directly upwind, though the ability to fly the FEG's at different altitudes with different winds would allow the operator to pick the most favorable.  This would allow the ship to shut down its diesel engines and operate on wind power alone.  It would not run fast, but a ship capable of 27 knots on 80 MW would still manage at least 18 knots on 24 MW.  On a route of 5000 nm, the difference in trip time would be about 3 days 21 hours (7.7 days vs. 11.6 days) with the best winds.  A ship generating 80 MW by burning residual oil at 18,830 BTU/lbm1 and 50% efficiency would burn 29,000 pounds per hour; at this rate, a 5000 nm voyage at 27 knots would consume 2540 m3 (671,000 gallons) of fuel.  If this fuel costs $300/tonne, the FEG system could save $731,000 in fuel at the cost of less than 4 days at sea.  If the ship's (and cargo's) time costs less than about $180,000 per day, this looks like a profitable tradeoff.  It will only get more so as petroleum prices rise.

[1] Values for #4 diesel from CRC Handbook of tables for Applied Engineering Science, 2nd Ed. (back)

EP - The only comments I have are the practical difficulties of managing these devices on the deck of a ship. The deck would have to be uncluttered like an aircraft carrier for them to land on.

The would have to be winched down in bad storms and when docking/unloading.
They could not be on the deck of a container ship at all; it's entirely covered with containers most of the time, and it has to be accessible to cranes for loading and unloading.

Multiple-rotor FEG's would be nearly impossible to stow, which is why I did not consider them; on the other hand, a single-rotor FEG with a tail fin/rotor like a conventional helicopter could stow much more easily.  There are a few places where such a machine with a properly aligned or folded rotor could be stowed away; atop the crew space or on booms on the side away from the pier are the two I can think of.
Cool - now what do we use to waterski? Zinc? :)
EP - I hate to rain on your parade however the impracticalities of this idea would seem to outweigh the advantages.

One is that one of the largest helicopters that routinely are used on ships is the CH-53E Super Stallion Helicopter. It has a main rotor diameter of 24 meters. You are proposing what is in effect a rotor diameter of 124 meters which is 5 times larger. This could be a problem landing, stopping the rotors and stowing in large seas or really strong winds. The largest helicopter flying has a rotor diameter of about 32 meters.
@Stephen: The CH53 operates on ships (LHAs) in the 20,000 ton class as a minimum. Post-Panamax containerships, VL/ULCCs, and bulk carriers make those look dinky and even the Nimitz class carriers not so big. We're talking over half a million tons displacement, the biggest objects ever made by man.

The seamanship problems are certainly significant, but I think manageable, as there is plenty of space around a ship at sea. Give the thing floats, and it can be landed on water and derricked aboard.

Operating it at close quarters would be undesirable anyway, as the ship would probably be quite a bit less manoeuvrable whilst the kite was up. You'd undock and clear the fairway on engine power, then hoist it at sea for the passage, just as lazy yachtsmen do with their cruisers ;-)
What Alex said.

Also, these FEG's would be tethered.  Unlike a helicopter trying to land on a pitching deck, these units could use excess lift against the pull of their own lines to align themselves.  Last, it may not be necessary to land the FEG directly on the boat; some kind of moving arm (suitable for repositioning the docked FEG while in port) might also allow the flying and floating elements to split the difference.  Holding one unit above the other for takeoff might be necessary for sufficient clearance anyway.

This would add to the expense, but savings of many millions of dollars a year in fuel (and a cost, if double the typical $300/kW figure for large wind turbines, would be a mere $6 million per 124-meter unit) may make them extremely attractive.
EP and Alex - OK perhaps the thing is possible however I would not like to do it at night in a big sea in a storm. All of which, at sea, can happen quicker than you think.

I think that you would have a job convincing a skipper to try it.

BTW in really bad weather helicopters are tethered and winched down. With wind gusts etc it is still tricky but doable I guess.
@Stephen: Neither would I! One good point, though, is that since the last time these things were mooted (i.e. the 1970s, the last time anybody cared about energy efficiency) such things as weather forecasting, earth observation, and satellite comms have improved dramatically. You'd be far more likely to have early warning to get the thing down.

Clearly you would have to operate it conservatively (conserve fuel conservatively, I suppose). And, in the last analysis, you might have to budget for the odd one being lost at sea. There is nothing on ships that doesn't sometimes wash overboard - including bridge superstructures, masts, ISO containers full of lead ingots. There's a reason why the shipping business invented insurance!

That is, though, a reason to go for the simpler (SkySails) option - you don't want too much sophistication in a device that may end up being jettisoned.

@EP: During the Second World War one class (the big, ocean-going Type IX) of German submarine carried a tethered rotor kite, not for propulsion but to carry a sailor with a good glass and a telephone 300 or so feet up, in order to spy out Allied shipping at more range than you get from the conning tower of a surfaced sub. It worked, but the only problem arose if the sub was spotted.

Although it could be brought in quite quickly, it was nowhere near quick enough for a crash dive, so the kite was released, and the sailor "drowned in the normal way" in the words of a Royal Navy report on the system.
What about keeping aircraft clear of such a device?
Meh. This isn't going to be more than 1000ft or so.
Sky WindPower proposes to fly their generators at well over 10,000 feet, so there would definitely be a possible threat to aircraft if ships did the same.  Whether this is desirable depends on available space for storing tethers, I suppose.

A surprise by a storm is one thing that would probably not happen to a ship flying an FEG.  An FEG would make an excellent radar platform, with the ability to see down to the horizon for many tens or even hundreds of miles.  Doppler radar on board would make it impossible for storms to catch the ship unawares.

Worse comes to worst, you cut it loose in a storm and proceed on diesel power.  If the ship saved $500,000 in fuel over the average 14-day leg, that's $13 million per year.  You'd have to lose 2 FEG's per year at $6 million each to wipe out your savings.

If something damages the landing system and makes it impossible to bring the FEG back to deck, you could sail past an airfield and cut it loose within autorotation range.
Alex:  there is one of the German submarine-borne autogyros in the Smithsonian Air and Space Museum.  I have photos of it.
Just a quick comment on solar power.
It would not be necessary to cover the ship in solar panels. A ship could easily float a huge solar array behind it correct? This of course, assumes cheap, high efficiency solar cells become available. Thin-film would be better too, not heavy or fragile, easy to stow.
While we're at this, is net wave energy unattainable for a moving ship? Does it break some kind of conservation of energy laws?
I don't know what the linear density of wave power is (watts per meter), but I recall seeing a concept for power-producing sponsons quite a few years ago.

A solar array sufficient to power the ship would be several times as big as the ship, and only produce during daylight.  How you manage it in port, how you keep it from being damaged by waves.... I'd love to see someone else's ideas for this, because I sure don't have any.
One problem I see is that even though this might not be much of a problem for aircraft, you will have problems with other ships using the same technology. Two 10,000 foot high kites will tangle with each other rather easily.
I think you overestimate the difficulty of flying a couple of kites out of each other's way.  (The kite system appears to fly at relatively low altitude; the Sky Windpower FEG system would have much longer cables but also much greater maneuverability.)
I doubt I'm exaggerating the difficulty of two 10,000 foot long systems (I refer to the Sky Windpower system here) passing one another edge on. Especially since there will be occasions when there are three or more such systems. OTOH, it's a manageable problem, and if the worst happens, then you jettison the entangled systems and keep going.
If you're so certain, you must have some calculations of horizontal separation vs. something like the azimuth angle of the cable.  Have you calculated the limits of the cable angle vs. relative wind and found that there are situations where two ships cannot pass within a certain each other without mutual interference?

Please post, they'd be quite valuable.
I live on the Kitsap Peninsula which is a 30-minute ferry ride to Seattle. Why couldn't a ferry generate enough electricity from each crossing to power the next crossing? Then only the first crossing would have to have to be supplied by outside fuel source.
I've ridden some of the Puget Sound ferries (and paddled around a couple of the ports they service in a kayak).  It's rather crowded there, and even the sailboat crowd often cruises through on their auxiliaries.

Short-haul ferries might be better suited to flywheel power, spinning up at each shore stop.
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