January 3, 2012

EV Unplugged: the TRULY zero emission electric car

There has been a lot of noise lately about plug-in electric vehicles. Of course, those are not really zero-emission cars: in the US, the typical 2011 plug-in EV runs on coal and gas, and causes CO2 emissions only slightly lower than that of, say, a Toyota Prius. But let's put this issue in a different light: to be precise, sunlight.

Putting solar cells on a car would make it truly zero emissions to operate, as well as more mobile, nice for those with range anxiety. One example are the solar race cars driven across the Australian desert: those are basically a magic carpet of PV panels with one bubble for the driver. Very cool, but it won't do for CelloMom, who needs space for the rest of her crew - not to mention the cello.

Another example is the solar car operated by the University of Central Florida: a van fitted with three PV panels on top. You can't miss it. And it would have plenty of room for crew and cello. But you wonder how far it can stray from campus.

Well, just for fun and out of curiosity, let us do the math. The total solar energy falling daily on the surface of the earth is about 25MJ/m2 = 7kWh/m2 (it is less at latitudes higher than 45degrees). A typical passenger car is 1.8m wide and 4.5m long, and let's say that you can find 5m2 of area on the roof and hood to put your PV cells: those would catch a total of 35kWh of sunlight on each sunny day.

The best currently commercial PV panels have an efficiency of about 15%, so the car's area can be good for an output of about 5kWh after a day's charging. Considering that the efficiency of a 2011 electric car like the Nissan Leaf is 0.25kWh/mile, a one-day charge could get you 20 miles of travel.

That, you might say, does not relieve range anxiety at all: considering that the average US car does 12,000 miles per year, that comes to an average of 33 miles a day, quite a bit more than the 20 miles we arrived at. CelloMom herself has been logging 8000 miles a year, but that still comes to an average of 22 miles a day - and what of non-average days?

But take heart: solar energy research is proceeding at a furious pace, and CelloMom is confident that power generation efficiencies higher than 15% will be commercially available when the price point is right. Already, the best demonstrated PV systems have close to 40% efficiency, and recent research on semiconductor quantum dots have yielded tantalising glimpses of quantum efficiencies higher than 100% at selected wavelengths - but let's have no illusions about this: if 35kWh of sunlight falls on your car, you will not get more than 35kWh out of your PV installation.

The other part of the equation is the efficiency of the car: vehicles like the Nissan Leaf and the Chevy Volt were designed to impress the zip-loving car afficionados who have to write the favourable reviews. For a mom like CelloMom there is way too much power in those cars; CelloMom would be more than happy driving in a far less powerful EV, especially if that means increased energy efficiency, say, 0.1kWh/mi instead of 0.25kWh/mi.

At a car efficiency of 0.1kWh/mi and a PV efficiency of 40%, the 35kWh of sunlight hitting the car could give it a range of 140 miles: enough to assuage the anxiety of all but the most determined of commuters.

Caveats: the numbers will work against you if you live in a place like Scandinavia (far away from the equator, it's mighty dark in winter), or you live in a place like the Netherlands (60 days of sun a year). This is a place-appropriate technology, but for the right place, the technology has the potential to work out beautifully.

To venture into the realm of the wishful physicist: quantum dot technology is particularly attractive because solar cells made that way don't have to be flat, and can in principle be applied to fit the contours of the car (in fact, a photovoltaic paint called Sunbelievable can be applied with a brush - cheap, but the efficiency is currently only 1%). And the submicron semiconductor particles, when embedded in the right matrix, effectively act as solar concentrators, which boosts the efficiency because even if a few photogenerated charge carriers are trapped at defects, there are plenty more to skate over the now-occupied trapping centers and reach the back electrode. Of course, the steel car roof itself can serve as the back electrode; moreover, it must be designed to be gloriously clean-lined, in order to optimise the capture of sunlight. CelloMom loves an elegant solution.

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