Sun strength for the last 2.5 days.

I’m not planning a solar-umpc tour this year but I am planning to get some use out of the panel. At the moment, the plan is to run a UMPC from the solar panel that could serve this blog. I doubt i’ll be able to do it 24/7 for the whole of summer but I’m running some tests on the Raon Digital Everun S6S at the moment to see exactly how much I could get out of it with the 24W panel.

The Everun, when configured for 400Mhz with WIfi and screen off, will consume a tiny 4W at idle and about 6W at full power which is about as low as you can go with a PC without having to switch to an ARM architecture. I don’t want to do that because the plan is just to install a basic Ubuntu image, fire up Apache and SSH and serve this blog as a set of static web pages. WIthout Mysql or PHP running it should keep the processor load down to a minimum although i’m not sure that Ubuntu will be able to switch the CPU into 400Mhz mode. It might have to stay locked at 600Mhz.

At 5W drain, the server would need 120Wh of energy to power it for a full day. With a 24Wh panel I’m only going to get about 80-100Wh per day on average (using the 4-hours sun/day  rule that applies to this part of the world) so at some point, back-up power is going to be needed. The problem is, how can I switch-over to back-up power (or gracefuly switch over to another server.) Switching the servers is OK but getting the Everun to shutdown when it reaches 5% power is impossible under Linux right now.

The Alternative is to shut-down the server between 0300 and 0900 every day and to try and regulate it that way or just keep topping up the SLA battery from other sources when needed.

I’m going to need a couple of new items for this project though.

1) Power usage measurement tools. I want to feed the data back into the website. Power used, power given by panel, current battery level. Any tips there would be much appreciated. Should I buy a dedicated data logging system with software or are there simpler ways to do this.

2) An ethernet port. The Everun only has a WIfi connection and that would take too much power so I’m looking at a USB to Ethernet adaptor. This one from SMC takes about 150mw which will be fine.

3) Bigger SLA battery. Rather than using the 56whr battery that I have, I should get one that takes about 200wh so that if we have two or three good days of sunshine, I can store the engergy and use it on days where the sun is weaker.

Solar panel positioning is going to be a major problem at my house which is badly oriented for a solar panel. I’m also a bit worried about loss along the length of cable I will need in order to position the solar panel correctly.

Over the next few weeks I hope to at least have a partial solar-powered web server running but if this blog disappears, you’ll know what’s happened!