Monday, December 7, 2009

The Moth Light

Ever since our slightly ditsy friend Adelheidi mistook my skylight for an actual electrical fixture, she's been hearing about it. The skylight was full of dead miller moths, and Adelheidi had told me to make sure I turned off "the moth light". I looked at her and said, "You mean, the Sun"? Since then, when Kelly and I are outside and it's really hot, it's not uncommon to hear somebody complaining about The Moth Light being turned up too high. We find that we get the most bang for our buck out of references to The Moth Light when Adelheidi is within earshot. Anyway, the point here is that The Moth Light is powerful. On a clear day you have about 1000 watts per square meter of Moth Light bearing down on you. It is a testament to the efficiency of the bicycle that it is entirely possible that some day, just the sunlight hitting your body could provide the power to scoot around on two wheels. Skeptical? An in-shape cyclist might produce a few hundred watts of power for a short length of time. Let's assume that us normal folks produce about half of that, about 150 watts. If we take the area of the typical human shadow to be 0.7 square meters, then we have about 700 watts of light hitting you and your clothing on a clear day. How much of that light energy might we capture, and how? Some of my fellow geeks are actually working on integrating thin film photovoltaics with clothing and other textile products. Let's assume they are successful. The current world record efficiency for thin film photovoltaics stands at right around 20%. Putting those numbers together yields an estimate of 140 watts of harvestable photovoltaic power, a level very comparable to the energy cost of cycling. Now, I am of the opinion that most of us would benefit and even enjoy generating that power ourselves. Nevertheless, for the elderly, disabled, or dressed up among us, a sun-powered electric scooter would be useful at times. And it is sobering to contrast the idea of the sun scooter with the reality of the energy hog we know as the automobile. Even the new old Prius that Kelly and I scored a couple weeks ago needs to produce about 80,000 watts to push us around town. I find it satisfying that the humble bicycle, an invention over 100 years old, competes so well with something as modern and high tech as a hybrid automobile.


  1. I don't want be around when Adelheidi finds out that you used the moth light story for your blog post - that is for sure. But, I do like where it took you - and I love the idea of PV fabric. Go beakers!

  2. Ooh, sun scooter. I like that idea.

  3. Me thinks 80,000 watts to run your Prius is a bit high. Though it is accurate if you assume 40mpg and you are traveling around 87 mph. But this is calculated as the amount of gasoline necessary to move it so it assumes the vehicle to be 100% efficient. Which of course it isn't.

    Nonetheless, it is way more than a bicycle. I calculated the amount of energy it takes me to ride my bike in vs. drive my Prius is about 11 times less. I used data from heart rate monitor/GPS for calories burned to determine the bike energy and assumed 45mpg for the Prius. I used how much gas it takes to get me here for the Prius energy calculation.

    5.5 miles one way

    350 calories (nutritional ~ 1,000 reg calories) = 0.011 gallons gas

    Driving Prius
    5.5miles/45 mpg = 0.122 gallons gas

    0.122/0.011 ~ 11