GravityLight: The Light Powered by Sand and Gravity

Such a simple idea that could make a big difference for many in developing nations:

The GravityLight is simply charged by a bag that is filled with around 9kg of material and hung from a cord below the light. As the bag descends, a series of gears inside the device translates this weight into energy, providing 30 minutes of light. The light strength can be adjusted, from strong task lighting to a longer-lasting low-level glow, and two terminals on the front allow it to be used as a generator so it can recharge other devices including radios and batteries.

The project originally emerged from a brief by charity Solar Aid to come up with a low-cost light source as an alternative to the ubiquitous kerosene lamps that provide the main source of light across the developing world – but which come with their own set of health problems.

The World Bank estimates that 780 million women and children around the world inhale a volume of smoke equivalent to smoking two packets of cigarettes a day – leading to the statistic that 60% of female lung-cancer victims in developing nations are non-smokers. The fumes also cause eye infections and cataracts, while 2.5 million people per year suffer severe burns from kerosene lamps in India alone. It also comes with a huge financial burden: the cost of kerosene for lighting alone can account for 20% of household income.

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  • Jer

    My first thought was this isn’t all that cool. I have LED flashlights that you can twist or squeeze a handle a few times and get 5 minutes or so of good light. What are the limitations of scaling this up?

    Batteries. This is storing potential energy in a much cheaper efficient way.
    I do wonder how quickly the gears wear out. Would they be under a lot of strain from a 20 pound sand bag?

    • The Vicar

      It’s not the strain of cranking, it’s the relative frequency and length of time you have to spend. Next time there’s a substantial power outage at night, try using only crank- or twist-powered lights and see how long it is before you say “the heck with it, I’m just going to go to sleep until the sun rises”. This thing, on the other hand, would actually make it possible to read or write at night — a few seconds every 25 minutes or so is actually very, very little, comparable with candles or oil lamps. (As is the amount of light this device is supposed to give off. We forget how crummy most flame-based illumination is.)

      As for the weight wearing the gears out: depends on what they’re made from. Old long-case clocks (including grandfather clocks) used weights to drive the clock (usually a separate set for the chimes) and those could last for centuries of more or less continuous use, with care — although they were probably better-made than most stuff manufactured these days. Clock weights tend to run from 5 to 20 pounds, so presumably a 20-pound weight COULD work without wearing the works out.

      A better question is: when will we see a mechanical battery, which uses a small solar panel or windmill to wind a series of weights on cables up, combined with a mechanism for releasing said weights one after another after the device is set to discharge, to produce automatic lighting? (Or which could be hooked up in parallel to produce a larger amount of current?)

      • Andrew G.

        You won’t see a mechanical battery for the simple reason that the idea doesn’t scale well. A single AA-size high-capacity NiMH cell stores enough power to lift more than a tonne weight through one metre of height. A square metre of solar panel can generate enough power in a day to lift 100 to 500 tonnes by a metre. A litre of kerosene, converted to mechanical energy at 10% efficiency, will lift over 360 tonnes by a metre. So to store any useful amount of power you need either a long drop or a really heavy weight or both, and either way you have significant construction problems.

        The only practical large-scale gravity power storage is pumped hydro, which requires a lot of water and suitable terrain.

  • Andrew G.

    We went through this on Slacktivist – once you do the sums, the limited energy storage is a problem.

    10 kilos raised 2 metres stores only 196 J, which is enough to power a 0.14 watt light for 23 minutes at perfect efficiency. The most light output you can get from 0.14 watts using white light and current production technology is about 22 lumens (equivalent to a quite dim kerosene light – or an incandescent bulb running at about 2 watts), and using R&D-stage technology maybe 35 lumens. (That upper figure requires 250 lumens/watt, which has been claimed in the lab, but which is pushing quite close to the theoretical limit of 300 lumens/watt for white LEDs.)

    By comparison a solar panel of 0.01 square metres will develop 10 kJ to 50+ kJ per day, enough to run a 0.7 watt lamp for at least 4 hours, with the advantage of no moving parts or mechanical wear.

    • The Vicar

      Ah, thank you Mr. First World Problems. No, this won’t run your laptop or a PS3, so obviously it’s useless.

      A candle or a kerosene lamp produces a vanishingly small amount of actual light. (Most of the energy in the fuel is converted to heat.) Up until the introduction of gas lighting in the mid to late 1800s, most people functioned perfectly well at night using single candles (they read, played cards, and even did sight-intensive tasks like sewing, which amazes me). People in the third world still rely on lighting of approximately equivalent brightness (kerosene, for example, mostly produces heat, not light, when burned). Well, guess what the light output of a candle is — it’s comfortably under 15 lumens. So right away, we’re talking about a light source which is potentially better than what some people are used to using.

      As for solar panels, they have their place. But suppose you’re a small and impoverished village in the third world, and you manage to get together $500 to spend (which is probably a vast overestimation). Which would you rather spend it on, one or two solar panel systems which will light up one location reasonably well, but will occasionally fail when the weather doesn’t cooperate and will probably be irreparable if anything goes wrong, or 100 small individual systems which you can deploy anywhere in the village, each of which is equivalent to a candle but without significant further cost, and which can be swapped out and sent off for repair easily in the event of problems? Solar panels have such a long way to go before they are a practical solution that you shouldn’t even bother mentioning them right now.

      • Andrew G.

        The wattage figures in my comment were not chosen at random – they were specifically aimed at generating 20 to 35 lumens, which is a reasonable working range for a small kerosene lamp without mantle. The comparison to a 2-watt incandescent bulb was just to make the point that this isn’t very bright.

        0.01 square metres of solar panel is 10cm x 10cm, with a wholesale cost of a few dollars, not $500. I have not verified it, but Wikipedia claims that solar lanterns or lighting kits costing a few tens of dollars retail are already available in some developing countries. And however you slice it, you’re not going to build one gravity powered light for $5. As for being repairable, I rather doubt that the kind of precision mechanics needed to efficiently generate electricity from a slowly falling weight will be repairable out of local resources.

  • Sunny Day

    Vicar please don’t attack people for things they obviously didn’t say.
    It makes you look dumb and I almost didn’t read the rest of your well written on point post because of it.

    • Sunny Day

      Now I look like a fundie because I screwed up the reply button.

    • vorjack

      It’s a point worth making. It’s easy to get lost in the numbers and forget that the developing world has to factor in more than pure efficiency.

      What can be maintained? What can the infrastructure support? What is available cheaply and in bulk? Often time the marginal economics produced by poverty make technologies that are considered obsolete and inefficient in the first world absolutely essential to the developing world.

      • Sunny Day

        Andrews post only dealt with numbers about energy storage. Nowhere did I see him outright disparage the whole concept or claim it was junk. I agree that small mechanical devices may be better utilized in areas where solar power wouldn’t be efficient. I’m ignoring the part where Vicar pulled 500$ and the supposed price of solar cells out of his ass.

  • vasaroti

    Hammacher Schlemmer will sell it to us for 49.95. I still want to know where I can get one of those cheap laptops they’re donating to African schools.

    The smoke and cancer thing is of interest to me because I once had an extended conversation on this topic with a guide at the Black Forest museum, where old farmsteads had been reconstructed. The kitchens were apparently very smoky, though much of the smoke was directed up a large chimney, and yet the guide claimed that there was no evidence of elevated cancer risk among the women. Yes, they had records to research – they’re Germans

  • Noelle

    I looked up Solar Aid on Charity Navigator, but it’s not listed. I know they don’t have an all-inclusive list, but before I send my monies away I like to get an idea of who I’m working with.

    Cool idea though.

  • Elemenope

    Is it just me or does it look like it was stolen from the Aperture Science Enrichment Center?

    • http://themikewrites.blogspot.com JohnMWhite

      I bet that light has a dry sense of humour.

  • JK

    The solar panels have a big disadvantage in this scenario: You have to store the energy in some kind of batteries, which cost more money, weigh a lot and need to be replaced from time to time.
    This lamps you can take with you. The only thing you need to use it is some weight.


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