June 23, 2010

Could we use solar power for transportation?

I often hear that installing wind and solar power doesn't help with our oil import problems, because we don't use oil for electrical generation anymore. I think that's not thinking far enough "outside the box".

So, do renewables help with our problems with oil?

Yes, solar power should be an important part of our transportation solution portfolio.

Transportation needs to be electrified, and all forms of existing transportation have power-hungry electrical systems, which draw power from the engines. PV can provide electricity more cheaply than can gasoline/diesel engines functioning as generators1.

We could reduce transportation fuel consumption by aggressively deploying PV on planes, trains, ships and automobiles. Let's discuss the hardest case, flying.

1st, Manned non-commercial planes that run on PV2 exist right now, and PV can certainly provide "hotel" electrical consumption (lighting, instruments, etc) on commercial aircraft. Planes travel above the clouds, and mostly during the day, which raises the "capacity factor" - the % of time the PV would generate power. The surface area of a plane could be maximized with trailing surfaces and longer wingspans. Taking the surface area of a large existing plane, one might generate 5% of overall energy needs using 20% efficient current commercial technology.

PV efficiency is likely to rise to something close to it's theoretical 66%, tripling the % that it can provide, while energy requirements are likely to fall: In the long term, design changes can reduce fuel consumption by 70%: "CAMBRIDGE, Mass. — In what could set the stage for a fundamental shift in commercial aviation, an MIT-led team has designed a green airplane that is estimated to use 70 percent less fuel than current planes while also reducing noise and emission of nitrogen oxides (NOx). source

The combination of 3.3x the PV output and 1/3 the energy requirement brings the PV % up to perhaps 50% of energy needs.

Finally, the remainder of the power could come from fuel (SOFC, hydrogen, etc) cells, which make much more sense for aviation than for personal transportation because infrastructure requirements are much easier to deal with (there are a relatively small number of airports). It's perfectly possible aviation will go to fully electric drivetrains.

Lets look at shipping, starting with the Emma Mærsk . With a length of 397 metres, and beam of 56 metres, it has a surface area of 22,400 sq m. At 20% efficiency we get about 4.5MW on the ship's deck at peak power. Now, as best I can tell it probably uses about 10MW at 12 knots (very roughly a minimum speed), 20MW at 15 knots, and 65MW (80% of engine rated power) at 25.5 knots (roughly a maximum). So, at minimum speed it could get about 45% of it's power for something close to 20% of the time, for a net of 9%. Now, if we want to increase that we'll need either higher efficiency PV, or more surface area from outriggers or something towed, perhaps using flexible PV. You could add a roof, or you could incentivize 10% of the containers to be roofed with PV - they could power ships, inter-modal rail, inter-modal trucks...

Here' a fun example of a boat that's 100% PV powered, and here's another.

Rail would be relatively easy, as most trains are driven by electric motors. A straightforward solution would be to build PV into the roofs of shipping containers, which could be plugged into ports on both trains and container ships.

1 $3 gasoline in a car translates to $.30/kWh (there are about 35kWh in a gallon, of which the most efficient generators can extract about 10), which PV can beat handily. $2 diesel or jet fuel translates to $.10/kWh (there are about 40kWh in a gallon, of which the most efficient diesel engines can extract about 20). That would require PV that cost $2 per peak watt, which the best existing PV modules can provide. Balance of System costs (structures, wiring, inverters, installation) would be greatly reduced by building panels into rolling stock as part of the manufacturing process. Inverters wouldn't be needed, as power would feed directly into vehicle electrical systems.

We can expect diesel to rise in price, while PV will continue to fall.

2 This may look very far from a practical solution, with it's small single-person manned capacity and it's very wide wingspan, but these guys are trying something very hard: continous powered flight during the night. They're trying to solve a problem that's much harder than commercial daytime aviation from Chicago to New York.


Adam said...

Hey Nick,
I really like your boating example, and for sure you are right on with the math and general idea. I am not sure how different the equation would be for boats with regards to where exactly they are located, but my hunch would be they surely would not have many issues with shading(although there may be a ton of trees out on the open seas that cast massive shadows on the decks of boats....
great posting

Nick G said...

Thanks, Adam!