over paneled

Well its been a lot in planning and significant preparation and then construction but my "shed project" has finally made it to "solar shed project" status with the mounting of my 250W panel... 

...but lets perhaps go back to the start and explain the title...

In the foreground there you can see a small 10W panel which I mounted some time ago which I discussed in this blog post, and has powered my simple lighting system from a humble Gel Cell 12V battery for the last few years.

The whole idea was naturally an extension of wanting a shed for tool storage and workshop (so that I'm not working on things in the house), the construction of the shed (and a bit of history) is discussed here, but in this blog post I'll focus only on the solar aspect of it, what I did and why.

Needs Assessment

My first step was an assessment of power needs which I did with one of these plug in power meters (easily obtainable from eBay or your local hardware shop).

I have been working outside on "flatish" areas when doing things such as building, and while I make pretty extensive use of 18V lithium battery tools I do still need 240V for things, a more powerful orbital sander, drop saw, circular saw, workshop vacuum cleaner and the like.

My normal process has been to run an extension lead out the window to where I'm working, which provides an ideal point to measure exactly what I use.

So doing some work on a few occasions I made some measurements and found that the number was about 200Wh for the days work (I took a few measurements over different days). Which isn't a lot really.

Basically if I got two 50W 22Voc panels that would give me entirely sufficient power to replenish my needs. Even if I used 1000Wh on a particular day that's only 10 hours charging, meaning that the battery bank would be back up to "float charge" after 2 days ... certainly no time for sulfation to occur there. However typically such a panel would cost about $60 which is about twice what I paid for this panel and I'd need two of those! Something like this was in my view ideal and was $160 per panel.

I'd need two because I will be charging 24V (2 FLA batteries) ... however I chose to go with the one higher Voltage panel because it was so darn cheap.

Storage

As I wanted to use 1200W tools in the shed (vacuum cleaner and compressor) I didn't want high amp loads coming from the battery to the inverter (not least because high amp loads distress the cables and distort the Amp hour rating of the battery) I immediately turned to 24V as the low hanging fruit (halving the Amp draw right there).

This has a knock on effect that you don't need as many Ah out of each 12V battery and so I picked a pair of batteries which were rated at RC95 (RC is Reserve Capacity), I'll spare you the cookie fest and paste below a definition (or you can seek your own)

Reserve minutes, also called reserve capacity, is the number of minutes a fully charged battery can sustain a designated constant load (usually 25 amps) before it is fully discharged. For a 12-volt battery, that means battery voltage will have fallen to 10.5 volts.

So I reckoned that I was not going to go close to needing 90 continuous minutes of 600W (by any stretch of the imagination) and so it was settled quickly there:

• 24V 2000W (4000W peak) high frequency inverter (AU$240)
• two 12V batteries

So we have energy captured by the solar, stored in the battery charge and delivered by inverter to the power points on the bench. Conveniently the solar charge controller has USB output which allows me to charge my phone and my Bluetooth speaker there. (*point, notice the multi meter on the bench? If you're even thinking about  this topic you must have something like that or frankly your're working blind.)

I recently did a draw down test on the system (panel, battery and inverter) running my fridge during an 8 hour power outage (IE most of the day) which was enough to pull the battery bank down to 24.5V. Now while this isn't ideal (and indeed is disappointing in some ways) it shows two things

1. the system is entirely adequate for my needs
2. the use of inverters off the battery does not allow the controller system to "see" what is going on and thus not divert sufficient (actually available) power to the "load" on the system (that's a long story right there) if one was to be going into the night and needing to keep sustaining the fridge (which is about 190W)

The Panel

One of the great things (for me at least) is the market distortions occurring due to government subsidies of solar. Pretty stupidly (but sadly not atypically)  what is happening is that are getting the subsidies to put new  systems on their roof and this includes people with existing (say 10 year old) systems and the older panels are removed and sold as surplus. 

This means I got my 240W 60 cell panel for just $20 ... fuck me dead, I can't even buy a (so called) 12V 100w panel for that (and BTW they aren't actually 12V), more like $120 each, and so getting a panel which is (more or less 32V (at maximum power) is a bargain.

The specs on the panel are something you should learn well, and you need to understand Voc, Isc, Vmpp and Impp.

Anyway to get that for a few bucks is just fantastic even if in excess of my needs. Basically this means that even if on a big day I run the workshop battery down to nearly "inverter power shut off" voltages, by the end of the next day it will be fully recharged and cell plate sulfation avoided (and my battery investment maximised). This is unlikely to ever happen as I simply don't need that much power in a day and I don't use the shed power even remotely hard most days.

So, next, the solar panel needs to be on the roof, and needs to be secure there.

Mounting

The shed I have (as you may have seen) is a basic 3 x 3 meter garden shed which I've used as the basic starter (for materials) and added a steel frame and other enhancements, accordingly the roof is not a trafficable roof and I'm against drilling holes (to later leak in the rain), and further because there is nothing underneath the "Trimdek" (like any battens) to screw into I decided to flush mount the brackets and to use a modern adhesive like Sikaflex 291 (rated to 1.8MPa tensile strength or about 140PSI) to secure the brackets to the "Trimdek" which the shed is made of.

The brackets (as you can probably make out in the first picture) are 25mm square section tube (so more or less an inch by an inch foot print on the Trimdek) and are 1200mm long (covering 7 high points). I added extra "feet" to extend out either side of the square section tube to prevent any rolling when I slid the panel up and across them, as well as add more grip surface for the adhesive.

To prevent any "movement" (slippage) while the adhesive was "skinning" I used two wooden "supports"

and again (with tape) on the upper side, giving me two nice mounts for the panel you saw back at the start of this blog post)

note that I've added some vertical mounts to secure the panel to the square tube too. These are screwed (and a small blob of sikaflex applied) into the aluminium frame of the panel. The whole thing then has overall 7 good joints like this...

... at each end to hold the panel down.

Now its important to note, that Sikalfex is humidity cured, and due to the thickness of the apparent bead (being squeezed in between the metals) you need to leave it for at least a few days to cure ... which I did.

A friend came over and helped me slide it up and screw the panel onto the side mounts.

So there we have it ... job done! (I might put some covers over the end ... I might not).

what's left?

basically the final sections are to

• neatly mount the Solar Charge Controller to the wall beside the door inside the shed
• add in some fuses / circuit breakers
• add in a neat isolator for the panel (I'm intending to insert a power monitor in there now and then) which I'll do with XT-60 connectors

However right now its fully functional, and so the components for this are:

• 250W panel (32v) = $20
• MPPT solar charge controller = $40
• 2x RC95 lead acid car batteries @$90 = $180
• 2000W (4000peak) inverter = $240
• sundry wiring

Now some may be asking "why didn't you just get an electrician to wire it to the house?", well the answer is that because of the distance down to the shed (about 30 meters) it would cost something over $1200, instead of $424.38 and I'd have learned nothing new.

Win Win

PS: I've now mounted the solar charge controller, run the wires neatly, and added a circuit breaker / fuse system which of course acts as an isolator.

The top one is for the panel (I sincerely doubt I need a circuit breaker, but as it has a manual operation its a handy isolator) and the bottom one to the battery is really for (hopefully) protecting the internal fuse that's built into the SCC.