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.

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.

Eco-refurbishment - installing the wood stove

At the end of last week our wood stove was installed! It took a couple of days, and was done for us by The Barn Stoves and Fireplaces. It took a couple of days, as our fireplace needed enlarging first. Here's how it looked before they started:

The first job was to remove the cosmetic brick front, which pretty much came off in one piece, leaving it like this:

Then it was time to bring out a tool that looked like a big drill, but basically was just for knocking the wall apart:

The guys made quick work of opening it up, though it did create a lot of dust. They'd put dust sheets everywhere, but you can never stop all of it...

At this point I took the opportunity to get a photo up the chimney. It had just been swept, so what you see here is deposits caked onto the bricks I guess. This is why it'll be better after being lined, as the inside will be smooth and less likely to collect deposits.

Before going any further, a steel-reinforced lintel was cemented in place, and some of the spare bricks from the cosmetic front were reused to fill in the gaps.

The last job of day one was to put in a new hearth - by law this has to extend a minimum distance from the front of the stove door, in case of hot embers falling out during refuelling.

Day two arrived, and they guys slid the new flue liner down from the top of chimney:

They also fabricated a steel box to line the inside of the fireplace:

A slot was left to one side for the pipes to get out of the fireplace and connect to the rest of the central heating:

Here's the stove, a Woodwarm Wildwood 9kW. You can see the connections for the boilers on the back. It has a split-saddle boiler and also a roof boiler. The split saddle wraps round the back and sides, but has a gap for the rear flue exit, which is why there are six connections on the back:

The stove connects to a stove pipe, which was cut to the appropriate length:

After this was connected up to the flue and sealed with a register plate at the bottom of the chimney, vermiculite was poured in from the top to fill the gap between the flue liner and the chimney, providing insulation, and then a rain/bird guard was put on the top:

Then, at last, the stove could put moved into its place, and plastering done. You'll notice the plumber had added some pipes to the back of the stove, but I'll cover the plumbing in a separate post.

And here it is, all ready to go once the plumbing is complete:

Last but not least, here's the van the guys from The Barn turned up in - you may well see it around the Rye area, as they do a lot of work here. I was very pleased with their work, and recommend them to anyone.

Next up: the plumbing for the system...

Mike

Related posts:

• Insulating the cavity walls
• Insulating the loft
• Flat roof insulation
• Insulating the dormer roof
• Sweeping the chimney
• 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.