Some engineers have developed a device that provides lighting to rural areas with no access to grid
Question:
Some engineers have developed a device that provides lighting to rural areas with no access to grid electricity. The device is intended for indoor use. It is driven by gravity, and it works as follows: A bag of rock or sand is raised by human power to a higher location. As the bag descends very slowly, it powers a sprocket-wheel which also rotates slowly. A gear train mechanism converts this slow motion to high speed, which drives a DC generator. The electric output from the generator is used to power an LED bulb.
Consider a gravity-driven LED bulb that provides 16 lumens of lighting. The device uses a 10-kg sandbag that is raised by human power to a 2-m height. For continuous lighting, the bag needs to be raised every 20 minutes. Using an efficacy of 150 lumens per watt for the LED bulb, determine
(a) The velocity of the sandbag as it descends
(b) The overall efficiency of the device.
Step by Step Answer:
Thermodynamics An Engineering Approach
ISBN: 9781259822674
9th Edition
Authors: Yunus Cengel, Michael Boles, Mehmet Kanoglu