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Monday, September 6, 2010

Solar Cars

Are solar car possible?
Forget about the cost of solar panels, the cost of batteries. Let's focus on how much power solar panels can produce and how much power solar cars need.

This is the very first step we must do, to understand if a dream like solar cars are something that we are going to see in the near future of if they are going to stay in our dreams.

Let's start from some facts: in the US there are 250 million cars, and they use about 9 million barrels of gasoline every day. Quite impressive: more than 3000 gallons per second!

Let's go ahead with the calculations.

As a liter of gasoline "contains" 34.8 MJ (mega joule), that means that US cars need about 15 Exa-Joules per year. Exajoule is a huge number. It's 1 000 000 000 000 000 000 joules.

Averaged over the year and divided by the number of cars, it means that every US car use 2 KW (kilo watts) of power. Of course a car uses gasoline only when moving around, but if we average its annual consumption over the year, we get 2KW.

Now, internal combustion engines (the one we have in every car) have a low efficiency. Only 20%, one fifth, of the energy contained in gasoline is converted into mechanical power, the rest is wasted into heat.

Electrical engines, or drives, on the other hand, have a much higher efficiency, even above 90%.
Then to make calculation easier, suppose that our solar cars only need one fifth of the power of the regular car, to do the same trips. It then requires 400 W (watts).

Now the crucial question: how large is a solar panel that gives you 400W in average?

The true output of a solar panel depends on where the panel is located. For example, a solar panel of 1 square meter in London gives you an average of 21 watts. Not constantly during the year, though. In July the sun is higher in the sky, so it may reach 38 watts. In December, on the other hand, it falls to about 4 watts.

Therefore, to power an electric car in London, you need a solar panel of 20 square meters: 4 by 5 meters, for example, or 14 by 16 feet.

If you want to use the car in December, though, this panel has to be 5 times larger, as you cannot store energy from the summer to the winter.  10 by 10 meters, or 33 by 33 feet, would suffice.

Quite a big panel, unfortunately. For sure, you cannot have a solar car with solar panels on the car roof, there is no way to achieve that with today's technology. And there is no hope that this will be possible in the very near future. 

This doesn't mean that solar cars are of no interest, of course. It's just something that we should keep in mind when talking about possible solutions for the problem of scarcity of energy resources and for the switch to renewable energy sources.