Off-grid solar PV upgrade: 750W to 1,250W

Yesterday we upgraded our solar PV - again! I'll go through some of the pictures first, then get into technical stuff later for those that are interested... Here's the finished system, installed next to our existing solar thermal (hot water) installation - as we were working above the flat roof, it was a lot easier than last time, no scaffolding required!

As before, my friend Rich gave me a hand with it, here he's fitting in one of the roof hooks that hold the rails on which the panels are mounted. Thanks again to Midsummer Energy for being very helpful in the supply of Grace Solar mounting equipment for the panels.

Once the hooks are in, they look quite neat. For more detail on the fitting process, see the blog post on the work we did last July.

The only tricky bit was the route up, on a ladder past the existing solar panels. Easy enough to climb, but not so easy to get the solar panels up there!

I chose to mount the panels one above the other, as this will minimise afternoon shading from the chimney.

The fiddly bit was actually doing the wiring - there's now five pairs of wires coming in, but I've also added blocking diodes - the row of components bolted to the top bus-bar here:

Anyway, that's it for the summary, now for the detail...

Why the upgrade?

Three reasons:

1. I had to go up on the roof to fix the chimney cowl after the wind in December, and while I was up there I realised there was space for one or two more panels...
2. When I set up the original system, I knew there would be a bit of spare capacity in the Morningstar Tristar MPPT controller, as it can handle nearly 900W on a 12V system, and I only had 750W installed. However, what I hadn't thought about enough was that while the Suntech panels I've used output 250W each under 'standard test conditions', under what they call 'nominal operating cell temperature' (NOCT) conditions it's only 183W each. This is because the panels are less efficient when hot, and also allows for not having the sun directly overhead. This meant there was a lot more spare capacity than I originally thought.
3. Finally, I saw a couple of identical panels to the ones I already have on ebay for a good price, so that was the final prompt to get installing!

Power advantages
Our Tristar MPPT controller will handle up to 882W on our system with the way I have it set up, though this falls as the battery temperature rises. So, when the panels do output at their rated power, the controller will be shifting the voltage to dump the excess power. However, this only occurs in rare conditions, usually on a cool day with patchy cloud - as the sun appears between clouds you get 'cloud edge effect', where in addition to the direct sun you also have reflected light from the edges of the clouds round the sun, with the result that the panels sometimes even generate more than their rated output for a short time! In practice though, our five panels can output 915W under NOCT conditions, so there's only a bit of power going to waste.

Where the advantage really comes in is on the duller days and during the winter - at these times all the extra power produced will be harvested, allowing us to run more of the house off-grid for more of the year. At the sunnier times of year, the boosted capacity will also enable us to run the washing machine off grid more often, and also power a slow cooker, thus saving us some gas (our normal cooking energy source). I'm even considering an electric chainsaw for solar-powered firewood production!

The need for blocking diodes
One downside of the location of the additional solar panels is that one of them will get shaded by the chimney from mid afternoon, and the other from late afternoon. Because of the way solar panels work, if you shade even a small part of them, the power generation drops dramatically (all 60 cells on each of our panels are in series - so shade one, and the current is limited for all of them). When you have panels in parallel, this can even mean that some of the power from the unshaded panels could feed backwards through the shaded panel, thus losing some power and potentially even damaging the panel.

To avoid this problem, I added some blocking diodes. A diode acts like a one-way valve, allowing electricity to flow in only one direction. I used Schottky barrier diodes, as these drop a smaller amount of voltage than normal diodes, so reducing the losses. The model I picked is the VT4045BP from Vishay Semiconductor, which can handle 40A and a reverse voltage of 45V - a lot more than our panels will ever generate. I reckon that typically about 1% of the system power will be lost in the diodes, but the avoided shading losses should more than make up for this. You can see the diodes bolted to the upper busbar in this picture:


So, it's all happily running now, I just need to sit back and watch the kWh flow in! :-)

Mike

Related posts:

• Insulating the cavity walls
• Insulating the loft
• Flat roof insulation
• Insulating the dormer roof
• Sweeping the chimney
• Installing the wood stove
• Plumbing and testing the wood stove
• Installing solar water heating
• Installing off-grid solar PV (part 1)
• Installing off-grid solar PV (part 2)
• Off-grid solar PV upgrade to 750W
• Off-grid solar PV upgrade to 1250W

Click here to read the rest of this post.

Off-grid solar PV upgrade

Sorry there's been no posts for a while, but as you can see I've had a lot going on!

With the help of my friend Rich, I've upgraded our off-grid solar PV system, removing some of the panels we installed a couple of years ago and replacing them with some much larger models. Here's a video summarising what's been done:

And if you want more detail, here's a video of the installation of the new controller that's needed to handle the increased power - it's a Morningstar TriStar MPPT 60A.

And for those who are into technical stuff and software, here's a video showing what the software Morningstar provide can do to gather and display data on the controller's operation (best watched full screen in HD so you can see everything):

Here's the photos (some of which are in the first video above).

First, here's the original system. The reason for the upgrade is simply that prices of solar panels have dropped a lot, and doing an upgrade now meant that the same roof-mounting kit was still available to buy (just!) so I could extend the rails easily. I'm finding good homes for the old panels...

First we removed the old panels, which was easily done from a ladder:

After that we set up the scaffolding that Rich had brought:

Then we removed the top mounting rail, shifting it up a bit, and extended both rails to the right, using the extra kit I managed to source from Midsummer Energy - thanks for the help guys! We'd also got some advice from Midsummer Energy on installation, so we removed the tiles we'd lightly ground using an angle grinder last time, and made some more aggressive cuts into them to fit round the mounting brackets:

This enabled the tiles to fit much more neatly - some of the ones we did last time had broken since installation, probably due to the wind making the array flex and press on them.

We also put into practice some advice on adding a bit of felt just to ensure waterproofing, although in practice this area is under the panels so gets less wet anyway in the rain.

I'd also uncovered the manual for the mounting system on the internet, from Grace Solar in China. This included useful tips like how to slot in the connectors that the clamps screw into, without sliding them all the way along the rails - very useful to know! (see the video for more info)

The new panels are Suntech 250W monocrystalline, and as they weigh nearly 20kg each we had to come up with a way of keeping them in place while we got the next one ready. The solution was a piece of wood of the same thickness, which the mid-clamps could grip onto allowing the panel to be held while we moved the scaffolding and got the next one in place.

It wasn't long before all three new panels were up, and we've kept one of the old ones to run a small 12V system that's used for phone charging, running a laptop and powering the cockatiels' night light:

Wiring up the electrics was much easier than last time, partly because a lot of the wiring was already in place and could be reused, but also because the new panels came with MC4 connectors, so I was able to prepare cables and then just plug in!

So, the system has been up and running for a week now, the fridge and freezer have been off-grid continuously during that time and we've even done two loads in the washing machine off-grid! The new controller gives me loads of data, and the Mastervolt inverter we've added now is pretty neat too, so I might write another blog post later with some extra technical details...

Mike

Related posts:

• Insulating the cavity walls
• Insulating the loft
• Flat roof insulation
• Insulating the dormer roof
• Sweeping the chimney
• Installing the wood stove
• Plumbing and testing the wood stove
• Installing solar water heating
• Installing off-grid solar PV (part 1)
• Installing off-grid solar PV (part 2)
• Off-grid solar PV upgrade to 1250W

Click here to read the rest of this post.

Improving the firewood shelter and log cutting rack

Back in March 2011, some EWB students built a firewood shelter for us, and very good it is too!

But when it was built, we did take a bit of a short-cut so it could be completed during their camping trip - we spaced the slats on the roof out, alternating them between the two sides, and used a tarpaulin to cover them. While this has worked reasonably well, the plan has always been to rebuild the roof in the style of our firewood shelter at home, and in the past week we finally got round to this...

While we were doing it we also set a batch of charcoal cooking in the kiln, ready for some BBQs that are coming up:

The first step for the shelter was to remove the old roof - here it is in 60 seconds! The delay at the end is the last two slats, where we had to get 6" nails out...

So then it looked like this:

We also had a pile of new slats ready, to add to the ones we'd taken off the roof. They're all cleaved pieces of sweet chestnut:

The previous layout of slats had them alternating, but the new one was to have them butting up against each other at the apex, with only small gaps between them on each side. We got one side done first:

I did most of the drilling, while Tracy did hammering:

Then we used some surplus roofing felt to start making it waterproof:

The end of each slat was shaped using a side axe, to make sure it fitted nicely onto the roof spars, and was not too thick to drill through - here's a picture from when we built the one at home:

As a result there was a nice pile of wood shavings left, which we'll save to use for kindling this winter:

The next trip up saw the other side of slats completed and another strip of felt on the first side:

and a third day saw the job done (bar trimming some of the excess felt a bit):

We were assisted throughout by our little mobile solar PV power station, with the addition of a 12V-to-mains inverter to charge the cordless drill:

With the roof on it was then time to sort out the inside, so that one half is ready for firewood:

While the other has assorted odds and ends in it

We used some old corrugated plastic to sort that wall out - the others will get more Hazel sticks stuffed into them until there's enough to keep the rain out.

The other little job I was getting on with was rebuilding our firewood cutting rack, which had seen better days after three years of use. Having taken the stakes out I excavated the saw chips that were there, and was surprised at their depth!

I actually got four wheelbarrows full out of there...

When rebuilding it I took account of the length of the logs we cut up (the yellow log painted on the ground is measured to fit our stove), and oriented the rack so we can get our log trolley close to it for easy loading.

Not long after that it was full and ready to cut up:

If you've not seen this from a chainsaw's point of view before, check out this video I made a few months ago:

Mike

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Parallel lead acid batteries with solar PV

Just a quick update to my previous post about wiring up our off-grid solar PV system. The friend I bought the batteries off has swapped the dead one for one the others he had, so I now have a working pair:

I'd originally been planning to put them in series for a 24V system, but I've reconsidered and am currently going to keep it as 12V, partly for simplicity and partly because that gives me greater redundancy in future if parts should ever fail and prove difficult to replace.

There are several important considerations when wiring batteries in parallel:

• It's best to have them the same make and capacity.
• It's best to have them the same age and condition.
• It's important to wire them so that the load and charging currents are spread equally across them.

I'd already got these first two points sorted. The last point requires making sure that the length of wire the current must travel to a load or from the charge controller is the same for each battery. If you don't do this, the battery that is nearer to the load/charge connection will discharge and charge faster than the other one, potentially resulting in premature failure.

The solution for two batteries in parallel is simply to connect together the positive terminals with a length of wire, connect together the negative terminals with another equal length of wire, and then make sure the positive and negative connections for the load and charge controller DON'T go to the same battery. This way, the current must always flow through one length of wire to reach either battery, so keeping them equally charged. Here's a pic, with the +to+ and -to- links on the right, and load/charge connections coming in from the left:

All I need now is some sunshine, and there's certainly no shortage of that right now!

Mike

Related posts:
Sweeping the chimney
Installing the wood stove
Plumbing and testing the wood stove
Installing solar water heating
Insulating the cavity walls
Insulating the loft
Flat roof insulation
Installing off-grid solar PV (part 1)
Installing off-grid solar PV (part 2)

Click here to read the rest of this post.

Eco-refubishment - installing off-grid solar PV (part 1)

Yesterday we installed our solar PV panels on the roof!

But let's start at the beginning... Our friend Rich came to help us do the work, as he's had past experience doing roofing work. We did a work swap, I helped him in his wood back in January, in return for his help yesterday. He also brought a lot of useful kit with him, like this scaffold tower:

The process started by removing some tiles to find where the rafters were under the batons, as the brackets needed to screw onto these for strength.

Having located these, we decided to grind a slot in the tiles so that they'd fit neatly round the brackets:

Then the brackets could be screwed down (with a small wooden spacer underneath to help them fit better):

and the tiles slotted back in:

Of course, it wasn't all that simple, as soon as you put any weight on the bracket it put pressure onto the tiles at a single point, and some broke...

To resolve this we used more spacers underneath, bent the brackets slightly and also used the grinder to thin the tile a bit at that point, giving more clearance for the bracket:

Here's the detail of how the rails fix onto the brackets:

And here's an interlocking piece of aluminium to join two rails together (we had two 2m rails). There's a couple of allen key bolts to lock it in place.

Then it was time to clamp down the panels. Here's a clamp with one panel in, and space for the next:

and here's one with both panels in and tightened up:

The first three panels in place:

And here's the finished set, with the solar thermal tubes higher up the roof:

The wires from the panels are zip-tied in place under the panels:

Then they all come down in a bundle to connect into a junction box:

That'll come in Part 2 though, where I'll explain the wiring system.


Mike

Related posts:

• Insulating the cavity walls
• Insulating the loft
• Flat roof insulation
• Insulating the dormer roof
• Sweeping the chimney
• Installing the wood stove
• Plumbing and testing the wood stove
• Installing solar water heating
• Installing off-grid solar PV (part 2)
• Off-grid solar PV upgrade to 750W
• Off-grid solar PV upgrade to 1250W

Click here to read the rest of this post.