you are looking for ways to make it seem as though this can't work vs collaborating on ways to make it seem that it can work.
Not at all.
So, again, the question was: Why can't it work" and the answer is "It can work, but it may be inconvenient".
I would put it another way. You can use gravity to store energy but it is useful in only a very small number of very specialized cases. Man moveable masses are only practical for powering things of very low wattage, like a few LEDs. As others pointed out, extremely large masses do have use for storing excess power plant energy. Examples would be water pumped storage or the new ARES rail system which uses train cars and regenerative braking technology.
Still not sure about your numbers, but let's assume that they are correct...Still, something seems a little off in all this and my hunch is that there is a mismatch in units somewhere
No need to assume as it is easy enough to verify. To keep it simple we can use the SI system units and just convert Joules to Watt Hours at the end.
Potential Energy of hanging weight = Mass X Height X gravitational constant
Mass = 1 pound = 0.4536 kilograms
Height = 1 foot = 0.3048 meters
Gravitation constant = g = 9.8 meter / seconds
2Therefore potential energy of 1 lb mass at 1 foot is:
PE = 0.4536 kg * 0.3048 m * 9.8 m/s
2 = 1.357 kg m
2/s
2 = 1.3549 Joules
1 Watt-Hour = 3600 Joules so:
1.3549 Joules/(3600 J/W-H) = 0.000376 W-H
This is very close to what I calculated before. Originally I used the imperial system and used 32 f/s
2 for g where I would have needed use 32.152 f/s
2 to correspond more precisely to the 9.8 m/s
2 used here.
Net, I see no error in my calculation.
You suggest lifting a 27# weight 7' in the air? Why? Why not lift our 27# weight 70' in the air?
kckndrgn asked the specific question of how many times the deciwatt gravitylight would need to be reset. If you look at the specs, you will see that there are maximum design limits set by the manufacturer. One is a maximum weight of 27 lbs. Another is a maximum height of about 7 feet. Why does it have these limits? There are several probable reasons on which we can speculate:
One is that the device uses an electrical vs. mechanical escapement. Specifically, it is the LED circuit itself which slows the drop of the weight. This is actually quite clever as it keeps the mechanism very simple. Despite how much weight you put in the device, it falls at about the same rate and maintains about the same voltage. What changes is the current flowing through it. Eventually this current will rise to the point where the circuit can't handle it. If it is fused, it will blow. Otherwise it goes *poof*.
Another issue with the weight is the gears. The gears are plastic. This has manufacturing and performance advantages (esp. not needing lubrication). But it also means the teeth will break if over stressed. The manufacturer made use of a planetary gear system to push the limit of this. This setup allows multiple teeth to be engaged with the main gear splitting the force among them. But again you can only go so far with such a setup. Similarly, the plastic chain also causes a weight limit as it will stretch or snap if the weight is too great.
Regarding height this is limited in any hanging weight clockwork system like this. Chains have mass. So as the clock runs, the chain moves from one side of the clockwork to the other. This changes the ratio of mass disrupting the movement.
The manufacturer of the gravitylight spent nearly a million dollars optimizing these pieces to come up with the best device they could. It is actually quite impressive piece of engineering. But the real brilliance (Ah, a pun worthy of Carl!) is the LED. It is amazing just how efficient LEDs are at creating light. We have to remember that their creation was worthy of a noble prize in physics.
I've made batteries out of pint mason jars using cheap magnesium fire starter bars and flattened copper pipe as the cathode and anode They use plain old water as the electrolyte. Four of them in series will light a bright LED...practically forever, or at least until the metal is used up.
...
I don't see why creative thinking couldn't use gravity to power electrical devices.
The natural forces within chemical storage are much, much stronger than gravity. Gravity is very weak. Think of it this way. We live on a big rock with a mass of about 6,000,000,000,000,000,000,000,000 Kg. Yet, with just a little thought we can send a tiny current into our muscles and completely overcome the gravity to propel ourselves. Or how easy it is to lift a small magnet off a wood table (gravity only) vs. a metal table (+ magnetism). The number calculated above shows the potential energy per pound per foot. That is what we have to work with in gravity systems.
