Not a Watt to Be Wasted

Link: Not a Watt to Be Wasted 

The March 17/2008 TIME Magazine explored scavenging 'free energy', from sources such as:

• the leg brace invented by Max Donelan and Arthur Kuo
• vibration of bridges, dance floors and even the human heart
• energy from raindrops or motion of fabrics
• waste heat from power plants, internal combustion engines and other sources. 

With the exception of scavenging heat, most of these techniques deliver only small amounts of energy, from microwatts up to five watts for the leg brace, enough energy to power a high-efficiency LED cluster that produces roughly as much light as a 40W halogen bulb.  In comparison, model aircraft engines weighing 9 ounces (250 grams) can deliver more than 800 watts of power at maximum speed. 

Scavenging thermal energy can deliver more power, but only from high quality waste heat.  The amount of work that can be done depends on the temperature difference between the heat source and the surrounding environment.  The high temperature exhaust of internal combustion engines is a good example and can be tapped using processes such as the BMW turbosteamer concept or converted directly to electricity using the thermoelectric effect.  On the other hand, thermocouples are still relatively expensive - another approach may be to redesign the underlying processes to be more efficient and generate less waste heat. 

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