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Road Power

road_surface_generatorA frequent suggestion for generating power is to use vehicles passing over a device in the road. The weight of the vehicle is typically used to generate pressure or move a crank that is converted into electrical power by a conventional generator. An alternative is to use a piezo-electric material that generates an electrical current when it is compressed.

Technically all these methods will generate power with varying degrees of efficiency. The problem is will they generate enough power to justify the investment and equally important will they be acceptable to the drivers of the cars if they disrupt the driving experience. For example by subjecting the cars and passengers to constant shaking as they pass over the devices.

There is a very short answer to these questions and a very long explanation. The short answer is no, this is not an efficient way of generating power. This is because the only source of energy in this system is the fuel in the car. Using fuel in a car is already a fairly inefficient way of generating power, typically only about 30% of the available energy in the fuel is converted to useful work.

Converting the moving energy of the car into useful work is also very difficult to achieve efficiently and equally difficult to calculate. That is why it is a long explanation that is too long for this short note. It is complicated because it depends on the speed of the vehicle, the type of suspension that the vehicle has and of course the type of generator being used.

As a simple illustration consider a car with ‘soft’ suspension. If you park it on top of a generating device that depends on the weight of the vehicle then we can assume that the device is fully compressed and for a brief moment the descending weight of the vehicle generates useful power as the potential energy in the weight of the vehicle is converted into the kinetic energy of the generator. Now think of the same car travelling at high speed. Now the effect on the generator will be much reduced because most of the energy will be used in compressing the suspension of the car to absorb the shock. The proportion of energy going into the generator will depend on how compressible or compliant it is compared to the suspension. If they are of equal ‘springiness’ then they will move equal distances. The time for the car to act on the generator will be reduced as its speed increases. If the suspension is ‘hard’ then the generator will be compressed more, but the occupants of the car will experience extreme discomfort. An answer that is sometimes suggested to the shock problem is to make the compressible device continuous. The effect on the car will be rather like driving through sand with the car constantly sinking and trying to ‘climb out’.

Regardless of the actual device used the resulting power generated will be very much less than the amount of energy that was originally in the fuel of the vehicle that is being burnt to achieve all this. It would be much more efficient for the driver to ‘donate’ that proportion of their fuel at the outset of the journey and then use that fuel directly in a much more efficient generator to generate the power.

So in summary road generating devices are definitely inefficient and possibly very uncomfortable for the occupants of the cars. It would reduce the fuel economy of the car by considerably more than the useful energy extracted and therefore overall it is not the best use of scarce resources.