GravityLight

It uses a bag filled with rocks or earth, attached to a cord, which slowly descends similar to the weight drive in a cuckoo clock.

[1][2][3][4][5][6] The design never proceeded beyond a limited number of early prototypes which did not appear to be practically usable by many consumers, and the company announced a change of direction in 2020.

The theoretical power output of the device can be computed by taking the simple potential energy generated by raising a mass to a specified height, and then dividing it by the desired time that the light is to stay lit.

Even a relatively large mass of 10 kilograms (22 lb), when raised to a height of 1 metre (3.3 ft) produces a maximum available energy of only about 98 joules; dividing by a desired illumination time of just 5 minutes would return a usable power of only 0.32 watts.

Moreover, this would be for an unrealistic 100% conversion efficiency; that of the University of Alabama prototype was closer to 50%, which in the aforementioned example would further reduce usable power to just 0.16 watts.

An early gravity light concept was developed concurrently by Clay Moulton and also by Ruphan[clarification needed] as part of his PhD in applied physics from the University of Alabama in 2017.

[8] As of 2020[update], the Deciwatt company was working on its NowLight, a manually-powered mechanical generator which stored its energy in a rechargeable battery.