Gravity Light Estimation

Martin Riddiford and Jim Reeves, two London-based designers, have created a neat new product, called GravityLight. It's an LED lamp that runs off of nothing but gravity - which means the only cost of the lamp is the upfront cost of buying the product, which makes this product ideal for use in underprivileged communities that rely on dangerous and expensive kerosene lamps to light their homes in the evening.

The lamp operates on a simple concept: a cable hangs down from a gear holding a plastic bag that can be filled with dirt or any other heavy material. The energy produced from the bag being pulled down is sufficient to power an LED bulb for up to 30 minutes.

To learn more about this creative device, look here.

I did some rough calculations to determine how much weight would be required to power this LED lamp for 30 minutes.

I assumed that there were 5 small LEDs in the lamp, each being 3.2 V and 20mA.

I also assumed that the lamp would be mounted about 6 ft (1.8 m) from the ground.

Here are my calculations:

Note: I made a silly unit conversion error: 33 kg actually = 72.75 lbs!

According to my calculations, approximately 33 kg or almost 73 lbs would be needed to power this LED lamp for 30 minutes, which seems like a lot of weight, but not totally unmanageable. It looks like the user of this lamp would need to be very strong to lift this weight every 30 minutes! I will be interested to see if this technology proves to be viable in the communities it is intended for.