### Author Topic: Everything you need to know about solar power  (Read 881 times)

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#### z.s.tar.gz

• Supreme Robot
• Posts: 540
• Linux Guru
##### Everything you need to know about solar power
« on: October 27, 2010, 10:58:30 AM »
This thread consists of twelve things that everyone who's looking into solar power for robots needs to know but that I haven't seen anywhere.

Note: this "tutorial" assumes you know what a solar panel is and the difference between a panel and a cell along with having a basic understanding of the math behind electronics.

1. The most important thing you need to know is solar panels are weak. Solar panels produce a ridiculously small amount of power (forget about fitting a 10W panel onto a robot, it's just too big.

2. Whatever the panel is rated for is about twice what you will realistically get. The most you can hope for from an 8V panel is around four to five volts under direct sunlight.

3. Solar panel power directly correlates to size. You already know this but what you may not have thought of is the fact that you need a bigger robot to hold this panel. Bigger robots are heavier. Heavier robots need more power to move and thus a cycle appears. You will always have to make the panel bigger than the chassis otherwise it just won't work.

4. Don't be cheap. Four separate panels will never produce the same power as one panel of the same surface area simply because of the wasted space on the edges. Also remember that you get what you pay for.

5. DC/DC converters are your best friend as they are the only way to reliably get a large amount of power out of a panel. Don't think you can buy a 0.5V panel and step it up to 6V though. You'll lose all of your power to inefficiency. The best thing to to is get way more panel than you need and step it down. For example say you needed 6V@1A for whatever. You would need to buy a 18-24V panel which would give you a solid [email protected] (that's the ballpark you're looking at for a large panel). Using a DC/DC converter you can turn that power into 6V@1A because your input and output needs were both 6W. So I guess number five has another message: make sure you don't try and pull 8W out of a 2W panel. Physics says it just won't work.

6. How you angle the panels at the sun makes all the difference. A few degrees off can be the difference between full power and what you get on a cloudy day. Because of this fact horizontally mounted solar panels will do best at noon and worse in the morning and evening.

7. If you go with several cells as opposed to one panel you'll notice that the power reduces drastically when even one of them gets shaded. Using a single panel and converter can help out with this [citation needed]

8. Keep the cells/panel cool either via airflow or water. The sun is hot and it'll make your solar panels hot too.

9. Mount them in such a way that vibration will not be an issue as glass can break very easily (duh  ) There is a way around this:

10. There are two kinds of panels: ones made of glass and ones made of epoxy. Always get the epoxy ones if you can because of two things: they don't shatter and you can drill them for mounting. Epoxy ones also don't seem to get as hot but that's a major [citation needed]

10 and a half: There is another school of thought where the panel is stationary and the robot simply goes over to it to get a charge every once in a while. Personally I think it's not that much harder to make it chassis mounted but you do whatever you want.

11. You can use mirrors and lenses to increase the efficiency of the panel but this will add serious weight, cost, and complexity to the project as you will need cooling if you do this.

12. There are all kinds of solar cells/panels out there so shop around. The difference in power can be tenfold from one to another. (The cost will usually be tenfold too  )

As usual, let me know if you think of something else or if something isn't correct.
[6-12 thanks to soeren]
« Last Edit: October 28, 2010, 10:21:14 AM by z.s.tar.gz »
Save yourself the typing. Just call me Zach.

#### Oller125

• Jr. Member
• Posts: 19
##### Re: Everything you need to know about solar power
« Reply #1 on: October 27, 2010, 01:35:48 PM »

4. Don't be cheap. Four separate panels will never produce the same power as one panel of the same surface area simply because of the wasted space on the edges.

Then how come lots of solar plants use lots of smaller ones?

#### rbtying

• Supreme Robot
• Posts: 452
##### Re: Everything you need to know about solar power
« Reply #2 on: October 27, 2010, 08:05:56 PM »
The "smaller ones" used in solar plants are still something like 2 feet by 8 feet... much larger than whatever you would be using.  Also, the larger the panels are, the more easy they are to break - and more expensive to replace.  This is negligible for your 1'x0.5' panel, but for a \$5000 2'x8'... there's a cost issue.

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Everything you need to know about solar power
« Reply #3 on: October 27, 2010, 10:08:12 PM »
Hi,

This thread consists of five things that everyone who's looking into solar power for robots needs to know but that I haven't seen anywhere.
[..]
As usual, let me know if you think of something else or if something isn't correct.
6. Angling them perpendicular to a straight line from the center of the cell/panel and the light source (usually the sun) is paramount. Even a few degrees off will reduce the output drastically.

7. Panels are like a chain, not stronger than the weakest link, so in a panel of 100 cells, shading even a single of them will have a quenching effect on the entire panel.

8. Keep the cells cool (fan or water works), when temp, goes up, output decreases extremely much.

9. Mount them in a way that dampens vibrations - with a few exceptions they're very fragile.

10. Best way to use them for robotics, is to let them charge an auxillary battery at a base, where the cells position can be optimized and the 'bot can go change it's battery when needed - solar is only usefull on-board if you build with carbon fiber and other light composites, so you don't need that much power.

10a. Mirrors and/or lenses can be a good way to concentrate more sun onto a cell/panel, but remember #8 - Added bonus, you learn a thing or two about compromises

11. Choose the right cells for the application. Large differences will be seen in output according to selection - Some have a higher output in natural sunlight, but have hardly any output in the light from fluorescent lamps. Some don't seem to care that much for the wavelength. Some output a few percent (of received photonic power) some output more than 20% - that's a huge difference when you consider the lack of space. Some are a very bendable film and can thus be used on odd shapes as "wallpaper" or as small ultra-lightweight panels.

12. Quality costs.

13. Never be afraid to experiment! (But do so within the borders of sanity and the unforgiving laws of nature).
« Last Edit: October 27, 2010, 10:13:35 PM by Soeren »
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

• Supreme Robot
• Posts: 11,669
##### Re: Everything you need to know about solar power
« Reply #4 on: November 11, 2010, 04:23:23 PM »
Actually, not nearly 'Everything you need to know about solar power'

I've begun working on my first space robot, and having been thinking of solar power (just because). But then I'm like . . . will it work fine at -60 C or without atmosphere to cool it down at 100C? If anyone ever runs into practical design information for space-based solar panel robots, feel free to send 'em my way

And yes, admin needs to show off with new insane robots to keep his street cred.
(don't ask for details, just stay tuned)

Quote
7. If you go with several cells as opposed to one panel you'll notice that the power reduces drastically when even one of them gets shaded. Using a single panel and converter can help out with this [citation needed]
I'll verify this for you.

The reason is that when you have cells in series, the current through each must be equal (current in = current out). If a single cell is mis-performing, it'll cause all others to mis-perform (ie weakest link in the chain). If you string a bunch of solar cells together, like a good one and a crappy/shaded one, the current will match that of the crappy one.

If you put them in parallel, the output voltage must be equal. The good one will be sinking power into the crappy one (or shaded one), wasting lots of power.

#### z.s.tar.gz

• Supreme Robot
• Posts: 540
• Linux Guru
##### Re: Everything you need to know about solar power
« Reply #5 on: November 11, 2010, 11:31:39 PM »
Ahh this is why they always put a diode when using solar to charge batteries.
And for the practical aspect of this forum, I believe it is everything.
Save yourself the typing. Just call me Zach.

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Everything you need to know about solar power
« Reply #6 on: November 12, 2010, 05:41:02 AM »
HI,

Ahh this is why they always put a diode when using solar to charge batteries.
It is to avoid the battery discharging through the solar panel when the latter doesn't generate a voltage potential that is higher than (or at least equal to) the battery voltage potential.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### Spoil9

• Robot Overlord
• Posts: 155
##### Re: Everything you need to know about solar power
« Reply #7 on: November 12, 2010, 10:38:40 AM »