Solar Pi Thoughts

The Solar Pi is an idea I had for a project/concept art project: a Raspberry Pi powered directly from a solar panel, hosting a website. When the sun is down, the site isn’t available. There are other solar powered websites out there, but forgoing storage is an interesting idea.

It wouldn’t have to be a Pi, other single board computers could work. The key is that the load is light enough that a solar panel itself can power it without much reserve, over a reasonable period during the day. With a larger panel, even during overcast days you might be able to keep it running somewhat. The idea is to see how useful you can make this without introducing a battery, although I’m considering a supercapacitor to provide enough storage to shutdown gracefully, and handle a single cloud going overhead.

Spiritually, this is similar to things like solar water pumping off-grid: the sun powers the pump whenever it’s out, and you pump water into a tank for use. That’s your energy (and water) storage. Or, a solar vent fan. Conventional grid-tied solar is like using the grid as a battery, although if you time your loads right you can end up using it this way too.

This has gotten me thinking about a couple things. For one, maybe running a mail server too? Email should, in theory, work if a receiving server becomes unavailable. It would be neat to include this as well.

Another idea: what about leveraging something like syncthing? The website could be shared on the Pi, and then others would just download it when it’s online. Not only that, but users could help distribute it as well.

Amusingly, given my home network, the site would probably sit behind a proxy. In other words, another web server and mail server that would relay to the Pi. These would almost certainly be powered from the grid, although I may tackle that as an other project. It kind of takes the starch out of the initial proposal, but from my perspective at least it’s kind of fun to set this all up.

A Solar Website

As if I needed another project, I had been tossing something around lately regarding renewable energy and the web. Low Tech Magazine, which showcases what it considers old and forgotten technology, has an edition of their website available served from a Raspberry Pi run entirely off-grid. Their setup consists of a Pi powered from a solar-charged battery, and they make it a point that depending on weather, that site may not actually be available at all.

I think that this is an interesting concept, although I’ve been tossing around the idea of taking this further. Rather than try to keep a small web server running all the time completely on solar power, what about running it only when the sun is out? In other words, whenever enough sun hits a panel to produce enough power to keep a Raspberry Pi going (about 3 watts, say). With a large panel, this means it could stay up even when it’s cloudy.

I’ve been playing with off-grid solar for years in a hobbyist capacity, and first wondered about this when seeing the green ‘charging’ light on my charge controller turn on even on a clear night with a bright moon. That is, with enough panelage, can a useful amount of power be extracted even in low light? And what ways are there to utilize solar power without trying to store it?

The system, in my mind, would be relatively simple: A photovoltaic panel, one meant for charging a 12 volt battery, would feed a Raspberry Pi via a buck converter. When there’s enough sun to keep the Pi running, it runs. When there isn’t, it shuts down. The converter would help to match the panel’s output to the Pi, supplying it with a constant 5 VDC. With a large enough panel (>20 watts?), it should be able to put out the required 3 or so watts over a range of weather conditions.

I’m wondering about some other bells and whistles, like somehow predicting when night is about to fall, and gracefully shutting down the Pi. And monitoring voltage and current as well. I may take this as an opportunity to design my own buck with some of this built in, but I think an off-the-shelf converter would work as well.

Anyways, I’m not sure when I’ll get to this, but it’s just a thought. I’m not sure how useful it will be, but it could be kind of fun.