[ris]

I implore people to support something far more practical like http://www.solar-aid.org/ than this.

Solar lights are many times more thermodynamically efficient, cheaper to make, smaller and much more robust (no moving parts!).

This "lifting weights" thing is a silly gimmick. A lifted weight stores a miniscule amount of energy compared to a battery that's been charged by the sun.

[kbenson]

Don't both have their places? There are benefits to having a power source that is a) not dependent on a resource that is not available at all times and b) does not require a charging period.

It's sort of like people in the late 19th century saying you shouldn't support the development of automobiles, because boats are much more efficient way to move cargo. While true, it does nothing to address the fact that in some cases the use of a boat makes no sense, such as transporting to a land-locked location.

[ris]

"Don't both have their places?"

It's more that this "invention" has been bouncing around for a while and gets an unwarranted amount of attention for the amount of energy it can provide. Weird to think, but solar isn't "sexy" in the same way.

[scott_s]

"D'oh, we forgot to charge our solar light earlier today."

"No worries, we'll just use the gravity light for now."

That scenario seems reasonable to me. The point is not the amount of energy the technique provides. The point is the utility people get out of it. And I see a place for both; GravityLight and Solar Aid seem like great projects.

[aplummer]

I think they would be good in a mine safe room or something, no need to ever replace batteries right. There are a bunch of cases you might not have a charged light.

[kbenson]

Maybe that "unwarranted amount of attention" is because it has a different set of requirement for use than solar which makes it particularly attractive. Considering only the energy efficiency and not how it could or would actually be used by the target consumers seems shortsighted to me. It may yield a the most efficient product, but it may not yield the most effective product.

[ris]

Yeah, sure, but that set of requirements is, I'd argue anyway, probably more appropriately served by something clockwork or something crank-based that charges a battery.

[shaneofalltrad]

exactly boats are still used when an automobile can't. Just the same as in low light areas, gravity is your only option. Plus I think gravity is probably a cheaper solution to offer, as the weight can be used from local resources while a solar panel is an added cost.

[DanBC]

Gravity is not the only option.

This isn't a boat with oars. This is a boat where you're given some wooden spoons.

[aaron695]

Coal and kerosene are cheap.

We need to stop dicking around with idealistic ideas involving electronics and design more efficient and safer uses of these existing cheap fuels for lighting and more importantly cooking and phone charging.

They are being used because they work for people in these situations.

http://www.gatesnotes.com/Energy/Two-Videos-Illuminate-Energ...

[raverbashing]

> Solar lights are many times more thermodynamically efficient, cheaper to make, smaller and much more robust (no moving parts!).

Beyond the "no moving parts" (and maybe cheaper to make), I think you're just spilling a marketing blurb.

> This "lifting weights" thing is a silly gimmick. A lifted weight stores a miniscule amount of energy compared to a battery that's been charged by the sun.

Potential energy = m.g.h So, lifting a 10kg weight at the height of one meter gives approx. 98J

Incident solar power is 100W/m2 (best case). A square solar cell 10cm of side hence gets 1W (or 1J/s), after efficiency losses this is even less (which are present on the GravityLight as well). Between 0.5W and 5W seems common on sale websites

So, lifting the weight above (which takes 1s approximately) gets you as much energy as 36s of solar energy (of course, you can't recharge it at night)

Also consider the cost of the battery

Edited for wrong math

[meric]

There's a running joke among cantonese who've watched the comedy 國產凌凌漆[1] (= Domestic-produced 007), the character who acted as Agent 007's inventor, built for the agent a solar-powered torch that only works in the sun. Thanks for the chuckle.

[1] https://en.wikipedia.org/wiki/From_Beijing_with_Love

[CodeWriter23]

The weight option has the benefit of durability. A battery needs to be replaced after x charge/discharge cycles. The goal of GL is to reduce poverty cycles by making a one-time investment that pays for itself in a month. Adding the recurring cost of a battery, even if still cost effective, runs counter to that goal.

[makomk]

The weight option has lots of moving parts under tension from about 10kg of weight hanging off the mechanism. That's going to present its own durability problems, and the parts are going to be a lot harder to obtain and fit than a new battery.

[zebra]

The inuit people cannot use solar. I think that it is better to have both solutions in the field and the time will tell which solution is better for a given region.

[kragen]

Your comment is a mixture of the irrelevant and the incorrect.

First, you’re mistaken about the robustness of batteries. Batteries tend to have a very short lifetime, measured in months to years, and they can typically only be recharged a few hundred times. Generators have longer lifetimes, measured in generations to centuries. Batteries are usually a lot cheaper than generators, so it might make sense, as you say, to just replace the batteries a few times. But how much does a 100mW generator cost? Amazon is selling an “X Factor” brand 3000mW generator along with accessories for US$17.56, so I suspect that the retail cost of a 100mW generator would be about US$1. (Little 1-watt brushless DC motors do cost about US$1 to US$3. I can’t find any 0.1-watt DC motors.) By comparison, the retail cost of an 800mAh AAA NiMH battery (3700 J, 1000 charges, thus lifetime about 3–5 years, Tenergy Centura brand) is about US$1.50 to US$2.

Second, it is incorrect to say, “A lifted weight stores a miniscule amount of energy compared to a battery,” except according to irrelevant figures of merit. Batteries do indeed have orders of magnitude higher energy density per kilogram and per liter, about 400kJ/kg and 1MJ/ℓ, while a bag of sand that has been lifted two meters only stores 20 J/kg and something like 20 J/ℓ. But those are not relevant figures of merit here. The target audience has tens of thousands of liters of available storage space, even in the worst slums, and can store tons of material, and the target application only needs to store 120 J. Reducing the weight of the stored energy from 6 kg and 6 ℓ down to 300mg and 0.12 mℓ (or, more realistically, 4g and 2mℓ, since you need to store more than 20' of energy) is not needed to replace a kerosene lamp; it provides you with a flashlight. Not to say that flashlights and solar lights aren’t useful, but if this is cheaper than a flashlight, maybe it’s useful anyway. Solar lights and rechargeable flashlights cost about US$6 to US$10 at retail today, which is a significant amount of money to a lot of people.

It’s true but irrelevant to say that lifting a weight by hand is not particularly thermodynamically efficient. It’s incorrect (but still irrelevant!) to say that solar lights are more thermodynamically efficient. Lifting weight is about 25% efficient, due to the inefficiency of your own muscles. (Generators are typically better than 90% thermodynamically efficient.) This is substantially more thermodynamically efficient than solar lights, which lose 85% of the sunlight on input and another 50% or so in the battery. If you can get the energy from somewhere else, rather than muscle power, then the weight is many times more thermodynamically efficient.

The efficiency question is irrelevant because we’re talking about 120 joules of energy for 20 minutes of light, which is an amount of energy that costs US$0.000003 if you have an actual power grid, or 0.03 grams of carbohydrate if you’re powering your 25%-efficient muscles (between one and two grains of rice). The only people who worry about conserving the energy in individual grains of rice are people currently experiencing a famine, mentally insane people, and commenters on Hacker News.

The one thing in your comment that might be true and relevant is the statement that solar lights are cheaper to make, because we don’t know how much the final retail cost of these Gravitylight gizmos will be. However, it seems implausible that they are many times cheaper to make. It seems likely that the vast majority of the cost in either case will be non-electromechanical components like the plastic housing, the gear train, glass or plastic to protect the solar cells, marketing, and so on, so the two solutions are likely to be almost exactly equal in cost. One will be portable but need to be left out in the yard to charge; the other will be heavy and nonportable, but you’ll be able to charge it in the middle of the night.