Reader Chris B. had seen someone promoting a product  that could be retrofitted to passenger lifts to recover kinetic energy rather than dissipating it in friction brakes. He wrote to ask if it was a plausible offering. 

Certainly there are lift systems that use regenerative braking, that is, motors which turn into generators when switched into reverse to decelerate the descending car. It is a legitimate idea, but usually a feature designed into the installation at the outset. As such it offers other advantages such as reduced heat dissipation in machine rooms. But it’s hard to believe that it would be viable to retrofit an existing installation because the available energy is not as much as you might think, as you can see from this rough calculation making some very optimistic assumptions.

Suppose a lift drops 36 m vertically carrying an excess mass equivalent to ten 80-kg passengers. As gravitational field strength is about 9.8 N/kg its change in potential energy would be 36 x 10 x 80 x 9.8 = 282,240 joule. That’s 282,240 / (3,600 x 1,000) = 0.0784 kWh. If you could harvest all that energy it would be worth approximately one penny. Factor in some more conservative assumptions and realistic conversion efficiency, and the value of the recovered energy is totally negligible.

Why are worthless bolt-on products like this promoted? In the case of products supposedly under development, it is usually to lure naive investors. For readily-available and easily-deployed products like bogus boiler additives and fuel-line magnets, it is to lure naive franchisees.

Learn about energy-saving techniques that actually work at this tutorial: “Energy efficiency A to Z” on 26-27 May