Green Technology Good. Actually Works Optional.

I think most people underestimate how much energy it really takes to power things. A quick trip to your local science and technology museum / centre should quickly dispel any misconceptions. These places usually have a stationary bicycle connected to a generator and you are encouraged to peddle to try and light a lamp or two. It doesn't take much peddling to realize that it takes a whole lot of work to light a lamp.

This realization did not occur to the inventor of an environmentally friendly lamp dubbed the Gravia or to the judges judging it.

The idea is that the energy from a slowly descending weight is converted to electricity and used to power high efficiency LEDs, providing approximately 600-800 lumens of light over a 4 hour period. At the end of 4 hours, you just flip the lamp over and the whole process begins anew.

It sounds great ... except the energy output seems a little out of whack - especially if you have ever energetically peddled one of those stationary bikes in a science centre. (I also have a dynamo powered flashlight and have a feel for how much effort is needed to keep the light reasonably bright - though, nowhere near bright enough to light a room.)

A 40W light bulb gives off between 400-500 lumens of light (the amount depends on the bulb - long life bulbs give fewer lumens because they are not run as brightly) and a 60W light bulb between 600 and 800 lumens.

The total power available from the system (assuming a perfect system with no losses) is 270 Watts. Period.

This is enough to light a 40W bulb for 6-3/4 seconds or a 60W bulb for 4-1/2 seconds.

A high powered LED (420 lumens @ 8W) would burn through the power in 33-3/4 seconds.

Assuming we had device that converted 100% of the energy into light, at 600 lumens output, we would still burn through the energy in 5 minutes and 7 seconds. At 800 lumens, we would use up the energy in 3 minutes and 51 seconds

None of those remotely approach the promised 4 hour running time. If the energy were delivered over a 4 hour period, the device would be capable of a continuous output of 18.75 mW. Good enough to run an LCD watch or clock or calculator, but not much else.

Using our mythical 100% energy to light converter, we get an output of 12.8 lumens at that power level. Which is pretty dim. Dimmer than a nightlight.

As the website says for its judging criteria: Entries were evaluated on the following criteria: innovation, clarity of design, originality, form and presentation. Being even remotely plausible was not a criteria.

Image nabbed from here.

We can calculate the energy available in the system (assuming a perfect system with no losses): it is a 50lb weight falling over 48 inches. Converted to metric that is 22.73Kg over 1.22m.

Force (F = ma) available from the free falling weight is 22.73Kg x 9.8m/s2 = 222 Newtons (Kg-m/s2).

The entire energy (E = Fd) available in the system is 222N x 1.22m = 270 Joules (Kg-m2/s2).

If you use up 1 Joule per second, you use 1 Watt (N-m/s2) of energy. So the whole system has a grand total of 270 Watts of power available for use.

The continuous power the device is capable of outputting per second if run over a 4 hour period: 270W / (4 x 60 * 60) = 0.01875 W/s or 18-3/4 mW per second.

The theoretical maximum lumens per watt of power is 683 lm/W.


b said…
All this science speak makes my head spin a bit!! :) But, I do catch your drift and the example of the stationary bicycle particularly sheds light (hehe) on your point. It has been awhile since I considered how much it truly takes to power things like a lamp. It's amazing how quickly we "forget" such information and continue taking it for granted.
Richard said…
breal: I moved the calculqtions to the end of the post because i fiured they would just confuse people.

I am sure someone is going to come along and grumble about the fact that I loosely talk about power in the main part when I should really be talking energy (Joules), but I think for the pueposes of the post being a little loose was ok.
Barbara said…
Anything that requires one to flip it over every 4 hours is not going to fly in the real world.
Richard said…
barbara: really? I would go for it. I don't think the inconvenience of having to turn over a reading lamp every 4 hours is really troublesome. Of course, his concept does weigh over 50lbs - which could be a bit of a damper for some frail, petite person. And further, it does not work, it cannot work. The total available energy in his concept is at least 5 orders of magnitude too little in order to work as he wants. Even in a theoretically perfect universe, the difference between delivered energy and desired energy is still 4 orders of magnitude, so even improved technology won't help rescue this idea.
carra said…
Well unfortunately science theory and science practice are two completely different things. The concept is beautiful, now they need somebody really clever, who could challenge the laws of physics and make it work. I'd buy it if it worked.
Richard said…
carra: I would even go for a wind up spot lamp that could be used for reading if it lasted at least an hour or so. Of course, price would be a consideration as well. I might be willing to pay $20 for such a lamp, but not $700.

Something can be done, but it takes a little more than just challenging the laws of physics, it would require rewriting them.

However, there is some good research occurring now on tapping into waste energy. I plan to blog on it one day. I had blogged about it earlier, but since that post, I have learned of projects actually looking into some of the ideas I had written on (I wish I could claim to have been the inspiration, but I know they were independently conceived). There is research into piezoelectric mats, building materials and clothing, I discovered a patent for extracting energy from revolving doors, and numerous other waste energy reclamation projects.
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