The Gravity Light has been designed to bring safe, cheap light to the developing world using the force that binds the universe together. Andrew Wade reports
When Jim Reeves was approached in 2009 to design a solar lantern for the developing world, he soon clocked that the concept was flawed.
“We quickly realised that the batteries and the solar panel contributed a fixed 60-70 per cent of the cost of micro-solar products,” he told The Engineer. “So we started to question the premise of using batteries to store power.”

Not only was the battery the first thing to fail, it was also the most expensive component, and not easily replaceable. On top of this, at the end of its life it would generally end up in landfill, adding to an already substantial environmental footprint. Reeves and design partner Martin Riddiford knew that to hit the solar brief they would need to use lower-quality components, compromising the integrity of the product. Instead, they explored other ways to provide off-grid light that could replace the expensive and dangerous kerosene lamps still used by billions of people around the world. As with Newton’s apple, a simple solution eventually fell to them.
Harnessing one of the universe’s primary forces, Gravity Light uses the kinetic energy from a 12kg bag of rocks or sand, which descends at about 1mm per second. This powers a drive sprocket, which rotates very slowly with high torque. A polymer gear train turns this input into a high speed, low torque output that drives a DC generator at thousands of rotations per minute, powering an LED bulb. The brightness is six times that of a kerosene lamp, and each time the bag is raised, 20 minutes of light is produced.
“The progress that’s been made over recent years in the efficiency of LED lighting technology started to bring the power required into the realm where simply raising a bag of rocks or locally found material could provide you with an energy reservoir that was a viable source of power,” Reeves explained. “The sort of elegance and simplicity of raising a bag of locally found weight had such appeal, because you’re providing a fabric bag as the means to enable that mass to be coupled with the product.”

The Gravity Light Foundation has already received funding of £200,000 from Innovate UK, with additional support coming via the Shell Springboard programme. In October it was backed by Siemens Stiftung, the charitable arm of the German multinational that promotes sustainable social development. Perhaps most impressively, £400,000 of early funding was raised through crowd-funding site Indiegogo, with Gravity Light hitting its initial target in just four days. This helped finance a global field trial, with a first run of 7,500 units distributed across 26 countries, as well as among Indiegogo investors.
“We learnt vast amounts from that trial,” said Reeves. “It showed up some incredibly surprising aspects and issues.”
The team had assumed people would want to adjust the brightness in exchange for duration, and provided three settings as a result. But it found users had little interest in this, with lights invariably remaining on the same setting as when delivered.
“Learning that it wasn’t a required function, we eliminated the feature,” Reeves explained. “It was several parts, it was potential points of failure, it was complexity, and was simply not needed as a feature set.”
Unsurprisingly, it was also found that children enjoyed swinging from the bag, so durability had to be improved. But overall the trial was a major success, with 90 per cent of users saying they would replace kerosene lamps with Gravity Light if they could. That first iteration required the user to pick the bag off the ground and physically lift it with their hands. This led to an unorthodox use pattern, with periodic darkness when the bag was being raised, and the height and strength required also creating a barrier for young, elderly, or disabled users. Gravity Light 2.0 addressed this by introducing a pulley mechanism, which delivers persistent light.

“It does three things really,” said Reeves. “It means the strength of the user is no longer coupled to the amount of energy they can deliver. We’ve given a mechanical advantage, so you pull down with a couple of kilos and you raise 12. We’ve decoupled the height issue, because it means if you’re of lesser height you can pull on the cord and raise the weight far above your head. And, as a result, we’re able to keep the light on continuously. So as long as you interact with Gravity Light for about 30 seconds an hour, you’ll keep that bag off the ground and have uninterrupted light for as many hours as you want into the evening.”
Being aware that most off-grid households don’t have access to scales, Gravity Light essentially measures the weight for you. Small packets are provided with the kit, then filled with material and placed in the bag. When the maximum weight is exceeded, the light glows bright red. Removing one packet leaves the bag at optimum weight, with the LED operating at peak efficiency.
“The LED dictates the duration in combination with the gear train and generator, and the brightness is driven higher and higher the greater the weight, which is the other reason why you need to indicate when you’ve overloaded,” said Reeves.
As well as providing light, the system can be used to charge AA or AAA batteries. The foundation is exploring the possibility of phone charging too, but the power requirements of smartphones mean this may not be practical using Gravity Light in its current format. However, power from the central dynamo can be shared among up to four SatLights, auxiliary lamps that can be linked to form a mini home-lighting system.

“You can daisy chain these SatLights, and you share the brightness across the number of lights you have switched on at any time,” Reeves said. “And there are other appropriate applications that are within that sort of power envelope. One of them is FM radio, which is used extensively across rural Asia and the developing world for delivering education. Coupled with the ability to provide lighting, you’ve really taken a big step towards enabling education.”
Gravity Light’s power mechanism makes it suitable for emergency, as well as off-grid lighting, according to Reeves.“One of the things we’ve come to learn over this journey is just how broad the application for Gravity Light is… after Hurricane Sandy I think there were over eight million people in [the US] without power for a fortnight.”
For now, however, the developing world remains the top priority. Having sized up the possibility of manufacturing in Africa, the team realised the infrastructure and supply chain were not quite mature enough. Consequently, production is taking place in China, with assembly located just outside Nairobi, Kenya.
“With Gravity Light, assembly is the bulk of the creation of employment, and has relatively low investment and infrastructure required for an assembly line,” said Reeves. “We can control quality of materials and components at source, and explore the viability of assembling products locally, and if that can be made to work it’s a model that could be replicated and rolled out in other markets where there’s a need for Gravity Light.”
The product is currently being introduced across Kenya as part of a 50-stop roadshow. It’s a country where 80 per cent of people don’t have access to electricity, and where kerosene lamps are adversely impacting the health and finances of millions of people. By 2018 it’s hoped that around 100,000 people will be using the technology, with gravity bringing light to virtually every corner of the globe.
That’s fabulous. What a great product!
The name “Gravity Light” is a bit misleading. This is a device fuelled by the human body. It runs off food, not gravity.
Good on them though, it’s a neat idea.
That’s fantastic. Also, where can i get one?
This is fantastic, I love the idea that the power source is a falling weight. It is just a shame that we use so much more power in UK home otherwise you could scale it up and have a 20Kg Weight descending a lift shaft in a UK home, which would neatly solve our pressing domestic power problems and adult obesity at the same time.
Major Kudos to the developers of gravity light. I love this concept. I think this could also be a viable way to safely store home generation for some off-grid activities. Just make a large one capable of running some household appliances one per day, and the refrigerator, and HVAC in winter (gas fired heating with just the fan motor needing power). I suppose there would have to be a vertical shaft in the ground with a very large ballast weight?
Wonderful invention, medals for Jim Reeves and all the team. Hope the manufacture, assembly, distribution, eventual use and adaptations/improvements are all successful.
I think it’s rather petty to argue over the name-it runs due to the pull of gravity, surely it’s as simple as that, which law of nature is the force that gave them the inspiration-beautifully simple and novel. No gravity-no lamp
Having heard Trevor Bayliss speak: primarily about the issues he faced from the ‘opinions’ of experts – commissioned by ‘the banks’ who initially refused him funding- before his amazing invention was finally accepted….for this group to have got so far so quickly is an example to us all.
The ‘bag’ of earth or rocks: must it be fabric? Could it be an animal ‘bladder’ or similar. If it has to be fabric, might I suggest that the work done 30 years ago to enhance the strength of ‘bags’ (used to relief ‘drop’ food grains directly from low-flying aircraft) which initially burst on impact, spilling out all the food might be of value. We came-up with a design, using a ‘partially’ drawn fibre to weave the fabric: such that on impact the bag ‘extended’ (literally ‘up the stress/strain curve’ as in a tensile test…) but still retained the integrity of the majority of the fabric such that only a small percentage of the contents were lost. Oxfam and UNICEF know more.
One of the ideas for ‘arresting’ aircraft on the white elephants? considers using partially drawn material acting as a sacrificial energy adsorber? Some merit in this idea as well?
Superb, very well done.
It’s marvelous, the best idea’s are always the simplest. Think how many aplications this could be applied to.
Brillig!
Awesome. Be nice to see some funding to build a few plants in Africa, Asia and South America to manufacture about – 20 million units each…
And why not the UK?
Brilliant! …Just brilliant!
Didn’t grandfather clocks from the 1600s use a slow falling weight to drive the clockwork and chime mechanism for up to 7 days ? The gearbox – well just think of the slow revolving wind turbines increasing the rpm enough to drive an ac generator but scaled down. SO: Nothing new under the sun – the efficiency of LEDs has made this evolution possible. How many other old technologies are there waiting to be resurrected because of new technology and the ingenuity of engineers ?
I went to the bottom to post the same comment, that’s exactly how they worked and we still have one in the family that was my Grandfathers. However the reasons for that design and this show how we must not forget the history in our technical development.
What about two lights, one ascending and one descending. Like a counter weight of lift. When one reaches ground it flips a switch that starts the descent of second and ascent of the first. This way no need of human effort.
oh you mean pertetual motion, think your suggestion through carefully apply a bit of maths / physics and ………..
Never mind perpetual motion, if the weights are the same they will not move and if unequal it will only work in one direction and make the whole system less efficient. However, if the original poster has learnt something from this then I would suggest it was a worthwhile post.
If you had a source of flowing water you could replace the weights with buckets that filled when they were at the top and tipped and emptied when they were at the bottom. This system is in use at various places around the world, maybe it’s what the original poster was thinking of.
Precisely, perpetual motion doesn’t exist and two bodies of the same mass with no external force applied will not create any motion. Likewise a chap or chappess standing still but holding 30kg weights in each hand is not doing any work. I wonder if the author thought that the light at the top of it’s stroke was full of electrity so it would be heavier than the the light at the bottom which has used all its electricity, so the heavy light would lift the light light on the other end of the peice of string ! 8o)
This is an engineering magazine, can we get the terms right, this is potential energy not kinetic energy. Can we have some numbers? 12kg at 1mm/s gives you 120mW input power, what is the actual power output? How long does it take to charge a AA battery? I think a day or two, is that really practical?
Wonderful! Feels great to see a return to low-tech (mechanics), in most cases more efficient, less wasteful and beautiful than the much hyped high-tech (electronics). Not to mention using abundant, local natural resources makes it even more worthy of praise, as it is a sustainable solution. The only criticism I have is that they make it in China, that does not make any sense, they should develop the supply chain around the places of use. Hopefully that’s why they created a simple solution, not only as design, but also for the rest of the value chain and for a systemic sustainable solution.
Clive, it is kinetic energy since it is the moving bag doing work on the sprocket. Why does it matter how long it takes to charge the batteries? Anything that has multi-functional applications is good. Plus, anytime is better than zero and better than throwing the batteries on top of the pile of e-waste we are intoxicating our environment and ourselves with.
” I suppose there would have to be a vertical shaft in the ground with a very large ballast weight?
Or a tree? Those behind my house (when its windy!) surely ‘generate’ (or store?) sufficient kinetic energy to be used to ‘charge’ something?
Potential energy is the energy possessed by a body because of its distance from the centre of the earth. A body exerts a force on its supports due to gravity. If we lower the body the force moves through a distance which is how we get energy from it. If it moves at a constant speed we get constant power (Force x Velocity).
Kinetic energy is the energy possessed by a body because of its velocity. To get energy out of it we have to alter the velocity.
I hope this helps, the difference is important.
I have a door closer based on similar, jam jar full of nails is the “weight” – easy to adjust and several times more dense therefore compact than the bag of rocks (SG of the solid rock around 2.6, steel ~7.8). Not my idea, been around at least 50yrs.
The “grandfather clock” mentioned, in my experience, needed a lead weight about 3kg to operate for a day (chimes and all). That was, thro a height more than 1.5m
And a 4:1 ration pulley system ?