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Cheap public transport air-conditioning

 Use surplus braking energy to air-condition buses and trains
  (+4)
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Buses and trains in London aren't air-conditioned, so why not use all that wasted heat energy generated during braking to air-condition them?
Here's how it would work: During braking, the train or bus axles would be linked via a gear to a compressor, which would compress air into a storage tank. (Note: This probably couldn't be used to dissipate *all* the energy currently lost as heat through braking). When the brakes are released and the train or bus starts going, this would release the compressed air into the train or bus. As the air expands, it will cool rapidly and provide commuters with a nice cool breeze at every bus or train stop.
—hippo, Aug 17 2000

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       While this certainly sounds like it'd be better than nothing, I think it's informed by the same thinking that calls the rush of air that comes in when a subway train leaves the station "ventilation." In other words, not quite enough to just air-condition based on relatively random events. Better to have some enviromentally terrible freon-hogging monster.
—mcfrank, Aug 18 2000
   

       As the air expands, it will cool rapidly and transfer all the heat energy generated by braking to the commuters. This would be worse than nothing: it'd heat the bus up.   

       I think you'd need a heat pump with the low--pressure part of the heat-pump circuit _outside_ the bus, where expansion will release the heat into the environment. But you could use braking energy to lower the pressure of the freon or whatever you're using for the heat-pump circuit. You wouldn't want to use the brakes to increase the pressure in the part of the heat-pump circuit _inside_ the bus: again, that would only heat the bus up. Let the heat inside the bus increase the pressure in the freon in the heat-pump circuit, and then decompress the freon outside, to release that heat.
—fc, Aug 18 2000
   

       ......Or you could open the window.....
—Haemavore, Sep 20 2000
   

       Regenerative braking (using brakes as generators) is already commonly used on trains and newer buses. Why not use some of this energy to power conventional air conditioners?
—andrewm, Feb 03 2007
   

       The regen energy is returned to the grid. Peltiers are massively inefficient. There's no saving over having conventional, grid-powered AC fitted, at least on the trains, and a loss in dragging all that extra gear around.   

       But if you do use compressed air, why not use an air-driven refrigeration compressor?
—BunsenHoneydew, Feb 06 2007
   


 

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