Humans as generators

I was watching the show "Invention Nation" on the Discovery Science Channel. The hosts visited a company that, apparently, is working on a revolving door that, when operated by patrons, generates electricity by moving neodymium magnets across coils of copper wire. The mechanism is exposed, so that patrons of an establishment that has such doors will be able to see the means by which they are generating power.



I was skeptical as to the significance of such a device, the show hosts used a prototype to light a small bank of L.E.D.'s. So I performed a Google search on the terms "generating power with revolving doors." I found several sites that mentioned the use of various human activities to generate useful power, including revolving doors and other methods (e.g., piezoelectric crystals in floors). This led me to consider the possibilities (quoting Marcellus Wallace, "All I'm doing is contemplating the 'ifs'").



As best I can tell, the human body, when purposefully performing work (riding a bicycle, lifting, etc.) has an efficiency of somewhere between 11% and 14%. That counts only how many calories (actually kilocalories) of food it takes to do a given amount of "useful" work. It does not count the sun to plant to animal to slaughterhouse to processing plant to distributor to store to house to stove to mouth efficiency (leave out some of those if you're a vegetarian). So, unless someone is exercising to remain physically fit, utilizing the human body to convert sunlight to electricity is quite inefficient.



Let's run some "back of the envelope" calculations though. There are about 3*10^8 people in the U.S. Say 1*10^8 of them walk on office, factory, or school floors, walk through revolving doors, etc. Now, the average adult uses something like 2500 kilocalories per day, let's say 100 of those are used putting feet on floors, using doors, etc. (very generous in my opinion). At 14% efficiency by the human and 50% efficiency by the generator (piezoelectric, magnetic, etc.) we have: 100 kilocalories*0.14*0.5 kilocalories of useful work per day per person to be captured.



Work divided by time is power so the above can be converted to watts per person (I typically use Google's calculator). This yields 0.339 watts per person. This is the effective continuous power output per person on average. Multiply this by 1*10^8 to total 33,900,000 or 3.39*10^7 watts available nationwide calculated on a continuous basis. According to the CIA World Factbook, in 2005 we used electricity at the rate of 3.816 trillion kilowatt hours/year, or 4.353*10^11 watts. Hence, using these extremely optimistic assumptions, this scheme could generate 0.008%, or 8 one thousandths of 1% of our electricity.



As I said though, when we do this, we're converting solar power inefficiently into electricity. Better to invest the money into more efficient generation schemes, except at health clubs, etc., where people are working out into a load and it might just as well be an electrical load that serves a purpose.