Concept in Renewable Energy

Here’s something a reader sent me, a variation on the theme of capturing and reusing the energy dissipated from the activities of the human body.  In this case, it’s a door whose hinge contains a generator.  Hey, no one says you can’t do this, but let’s look at the basic physics to see how feasible such a device might be.

The first thing you need to realize is that the energy you’re recovering is some portion of the work that went into opening the door, and that this is close to zero for most doors we encounter, simply because we don’t really want to do significant amounts of work in this context–and that’s even more the case for women and kids. In addition, we generally don’t want to be slowed down as we pass from one region of space to another, but let’s forget about what we want, and focus on the matter at hand.

Let’s say we made that hinge similar to that of a rowing machine, requiring a fairly intense pull to move the door.  Work is the integral of force “dx” (over a distance).  If the force is constant, it’s force times the distance across which the force is applied.  Let’s simplify all this further and suggest that 200W, the power exerted by Olympic-trained athletes, be applied for 15 seconds, a total of 50 Watt-minutes, which we can round off to one Watt-hour.

Since the average U.S. home uses about 30 KWhs per day, we need to be prepared to ask the family who lives there to open that door 30,000 times daily.  At 15 seconds per pull, you’d need five doors all being opened 24 hours per day.

Since that seems like a stretch for the modern, on-the-go family, I’m prepared to suggest that this idea, while not physically impossible, doesn’t have much practical applicability.