Reviewing the numbers from one month of solar harvesting

[Mathieu] just finished analyzing the numbers from a month of solar energy harvesting. You may remember that he was curious to see what kind of energy can be collected from small solar cells used indoors. He built several copies of a test platform which collected data between December 16th and January 16th.

First of all, it’s not shocking to find out that rooms with no sunlight produced negligible energy during that time. When you think about it, if they had been gathering a statistically significant amount wouldn’t that mean the lighting used in those rooms was incredibly inefficient? In other words, there’s no way you need to be making that much light.

But he did find that proper positioning in rooms that catch sunlight during the day can result in usable energy for small loads. He established that a 0.5 Watt panel harvested just a bit more than half of what a 1 Watt panel did. But perhaps the most useful discovery was that it’s quite a bit more efficient to have a charging circuit store energy in a battery rather than directly powering a fixed load.

It will take us a few more viewings to really decide what we can take away from the experiment for our own projects. But we appreciate [Mathieu’s] quest for knowledge and his decision to put this information out there so that others can learn from it.

19 thoughts on “Reviewing the numbers from one month of solar harvesting”

1. ” it’s quite a bit more efficient to have a charging circuit store energy in a battery rather than directly powering a fixed load.”
I didn’t see that in the article. He did suggest using a big battery to store energy from the better months.
However, solar cell efficiency is partly dependent on the voltage maintained across the cell. An optimal load maintains an ideal voltage at the cell. So, a good solar-optimised regulator/charger would indeed yield more energy than most loads directly connected to a cell/panel.

1. Dax says:

The solar cell acts like a diode with the solar radiation pushing current through it, so the voltage you see behaves exactly the same as the forward drop in a forward biased diode.

Series resistance and temperature tends to mess things up though.

2. Dax says:

It costs between 80-300 kg of CO2 to produce a square meter of solar panel, so it makes very little sense to put any on small gadgets and especially indoors.

It would be polluting more than using a tiny gasoline powered generator to charge up your cellphone.

1. MisterX says:

That’s funny as hell Dax, Great Troll!

Of course, if one takes onto account all the embodied energy that went into the extraction of the metals for your generator, and extracting the petroleum/transporting/refining/transporting the refined gasoline to power the generator, your premise FAILS miserably. And lets not even mention the carbon footprint of your faulty comparison.

Ha, ha, ha, you funny guy.

Dax sez:
>
> It costs between 80-300 kg of CO2 to produce a square meter of solar panel, so it makes very little sense to put any on small gadgets and especially indoors.
>
> It would be polluting more than using a tiny gasoline powered generator to charge up your cellphone.

2. Dax says:

No, I’m not being funny.

Gasoline has an embodied energy cost of roughly 1.25 times its energy contents. To turn it into electricity you need four times as much gasoline, which means 5 times the energy.

A kilogram of gasoline produces roughly 3.2 kg of CO2 and contains 8.76 kWh of energy, therefore you produce 1800 grams of CO2 for every kWh of electricity produced.

Then let’s take a solar panel. Let’s take the best case scenario and say that it only produces 80 kilograms to manufacture it. We take a 10×10 cm square, or 1/100 of it, which is worth 800 grams of CO2. Then we use it indoors to produce roughly 20 mWh per year for 20 years. That’s a total of 0.4 Watt-hours, or 0.0004 kWh, or adjusted to the same units: 2 million grams of CO2 per kWh.

1800 g/kWh is so much less than 2,000,000 that we don’t even have to take into account how much energy went into producing the tiny generator. Any way you make it, it’s going to be more ecological to simply not manufacture the solar panel by a factor of a thousand.

3. Dax says:

Or to put it in other words, there should be at least a thousand times more light indoors before using solar panels on gadgets would be on par with running a diesel generator in your back yard.

2. Guest says:

“He established that a 0.5 Watt panel harvested just a bit more than half of what a 1 Watt panel did.”

Well, knock me over with a feather.

3. He established that a 0.5 Watt panel harvested just a bit more than half of what a 1 Watt panel did.

I’ve read this over a few times, and I can’t seem to decipher what was meant here (beyond the painfully obvious).

1. I think he was pointing out that the TI device scales with input power which is a good thing. He noted that his two solar panels had slightly different efficiencies.

4. This article kind of proves the obvious: there’s no place for solar cells inside.

I don’t really see why it could be feasible to charge the battery more in sunnier months and use the energy in darker ones because: batteries have discharge rate that might be comparable to the extra charge in summer and if you are getting so little energy(at this scale) isn’t it cheaper to use alkaline batteries instead of solar panel+rechargeable? the cost of the solar version might be equal to enough alkaline batteries to power the application throughout it’s life.

Example: the 1W panel extracts 50uWh per day. That’s 18mWh per year. An AA cell has about 3Wh(1.5V * 2Ah). Even if you get 10X more in summer, it is still more than 10 times less than an AA battery.

1. Hi Bogdan,

Yep, but it had to “proven”. The thing is that you don’t find people doing the same kind of experiments on the internet, so you can’t really know for sure that small solar panels are completely useless.
However, for a small sensor network, that might just do the trick. In my opinion, li-ion discharge rate is not that big.

But still, AA cells are still a better way to go.

1. Greenaum says:

Of course indoors we have mains electricity. Horses for courses and all that.

5. Mike says:

Having a quick look at http://www.physibel.be/voltra_sol2.htm gave some interesting ideas. An average sized room with an average sized window looks to give approx 5W solar energy per square meter on a sunny day.

Given that a 0.5W panel is only 55m x 70mm, and efficiencies are around 15% it is not unreasonable to expect that at most 3mW that can be generated during sunny times, and looking at the graphs that was only on day in three!

When you allow for the aspect of the window (e.g. south facing) you might only get that level for a couple of hours a day, making your 1W solar cell average under a mW per day….

1. Dax says:

Here’s the thing though, solar panels are awful at collecting light from off angles. They only “see” the light that’s coming more or less straight at them, which is a tiny portion of the available ambient light.

There are alternatives, like: http://www.physorg.com/news/2012-01-boost-solar-efficiency.html

They basically use a material that collects all incident light, and then re-radiate it at a different wavelenght that the actual solar panel can pick up.

1. David says:

i’ve often wondered if it would be feasible to use the magnifying glass effect on a solar panel. basically using a few lenses and a prism to collect and then scatter the light.. assuming of course just focusing the ant-ray of doom on one directly would burn out the collector

2. Dax says:

They are doing that as well.

Main problem being the cost of high quality lenses that don’t absorb too much of the light, and making so many of them.

6. Chris G says:

Probably easier and cheaper to set up an array of small mirrors to focus sunlight on the panel. Mirrors can be had at Goodwill.

7. I agree, but we all need to understand that adding Solar to their home is an asset that should boost the actual valuation of their property if / when they make a choice to sell. With the environment the way it is going we are unable to overlook any product or service that provides zero cost power at no cost to both the consumer and more significantly the earth!

8. mike says:

I think it’s better to use a eneloop AA battery (keep its charge for a longer time) and recharge the battery when needed (with solar panels placed outside).