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-refurbishment - installing off-grid solar PV (part 2)

Our off-grid solar PV is now up and running! Part 1 covered installing the panels on the roof, this post is all about wiring them in. This started with screwing a waterproof junction box to the wall and then fixing a dual busbar inside it:

The next step was to wire in the PV panels. As I was setting this part of the system up at 24V, I wired the panels in pairs, as you can see below, with each PV cable having one wire screwed to the busbar and the other soldered to a wire from the opposite cable. The solder joints are then wrapped in heatshrink and PVC tape, to avoid any accidental short circuits. The wires coming out the right hand side of the junction box take the combined output of all six panels into the house.

Before you ask, yes there are seven panels on the roof, but only six wired into the junction box. That's because one of the panels feeds straight into an existing 12V system I have set up which is used for DC supplies to charge small electronic appliances and also to power a night light for our cockatiels!

Anyway, here's the junction box with all the wires neatly bundled up:

At the other end of the pair of cables heading off to the right, they disappear into an air vent...

only to emerge inside the house, under the stairs. We had this vent put in by the cavity wall insulation installer, in preparation for this use.

Once inside the cables head through a gap in the board on the left, going past the battery:

I had bought two batteries second-hand from a friend, but it turns out one of them is dead, so I'll be running the system on 12V rather than 24V. More on that later... While we're looking at the battery, here's some detail on the wiring. There's a pair of wires going to the charge controller, and another pair going straight to an inverter. The little black pods are fuse holders, the rear one's open and you can see the red fuse inside it:

Round the other side of that board is a neat little display I found, wired up to a switch so that when the switch is set to "1", it displays the battery voltage.

That photo doesn't really do it justice though, as it glows in the dark!

OK, on to the key component in the system - the charge controller, a Morningstar SunSaver MPPT. This performs several key tasks:

• It uses a Maximum Power Point Tracker (MPPT) to allow the solar PV panels to operate at the voltage at which they will deliver the most power, and then converts this to whatever voltage the battery requires. This is done dynamically, as the MPP changes according to light levels. The other advantage of this feature is that it can take an input voltage up to 70V, which is why it didn't matter that I'd wired the panels for 24V operation, but then ended up using a 12V battery. In fact, using a higher voltage reduces current losses in the cables, so this is a good thing to do anyway.
• It can auto switch to 12V or 24V. So, if I do decide to run a 24V system later on, it will work just fine (though I'd have some other bits of wiring to do on the load side of the system).
• It prevents the battery being overcharged, and actually goes through a 4-stage charging process to ensure that the battery is kept in good health.
• It prevents any loads connected to it from running the battery down so low that it is damaged.
• It includes protection against short circuit and excessive voltage on all of its connections.

With all that, you're probably not surprised to hear it cost as much as two of the 40W solar panels on the roof!

The load connections from the charge controller go to a pair of busbars. I'll make a plastic cover for one to prevent short circuits later on, for now it just has some tape over it to prevent accidents. Connected to the busbars is a 12V socket, like you'd find in a car, and plenty of room to add some more. This means anything you can get a car adaptor for, you can plug in here.

For mains appliances I've got a Victron 180W pure sine inverter. Because this can draw more power than the charge controller can handle, it's connected directly to the battery. This isn't a problem, as the inverter also can detect when the battery voltage is too low, and disconnect itself.

I've had this inverter for some time, so it was just a case of screwing it to the wall and plugging it in. I got a pure sine version so it can run all kinds of electronics with no problems. Modified sine inverters are cheap, but won't work with everything... I plan to get a much larger inverter later on, so we can run things like the fridge or freezer for a few days in the event of a power cut.

At the output of the inverter I've plugged in a monitor, to see how many kWh of electricity I've used from the system. My general plan is to use the power when the sun is out, and not when it's dark or cloudy. While this means I'm not getting the maximum amount out of it, it does mean the battery is always full, ready for any power cut that may happen.

Finally, the battery is boxed into a small compartment under the stairs, with the air vent inside it. This is essential, as charging a battery produces hydrogen, and as we saw in Fukushima, hydrogen mixed with air in confined spaces is not a good idea... Keeping it sealed away with a vent should eliminate any explosion risk.

So, what have I learned from all this? Several things:

• How to use a hydrometer to check if all the cells in a battery are working - and to check second hand batteries before buying them, even from a trusted source (as they may not know they're dead).
• How to do the basics of fitting roof tiles - after the PV was done, I went round the other side of the house and replaced a few low down cracked tiles myself.
• That the ancillary components of an off-grid PV system can cost more than the panels themselves!

It certainly would have been easier to pay someone to fit the system, but doing it myself was much more satisfying (and cheaper). It also meant that I was able to make some decisions as I went along, and as I designed and built the system, I know exactly how to fix it if it goes wrong.

So, I'm writing this blog post with my laptop and router running off grid! Very satisfying! :-)

Mike


UPDATE: I've got two batteries now, wired in parallel.

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)
• 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 - cavity wall insulation

Last week we had cavity wall insulation installed. It's well worth anyone doing, as there's grants available in the UK (some people even get it completely free!), and even if there weren't it would pay for itself in heating costs within a couple of years. Make sure you get it done by a reputable installer though - ask your local council who they use. Our installers turned up early morning, and unloaded bales of insulation material:

These are fed into a machine in the back of the installers' van, which shreds the material...

and blows it down a hose into the wall:

Before filling the walls the installers drilled a couple of holes with a core drill and put in vents. One is required to let air in for the wood stove we're having installed shortly, and the other is to provide ventilation for the place where I'm planning to install a lead-acid battery to connect to solar panels for off-grid electricity.

We've also had our new water cylinder arrive, which has two coils in it - one for the wood stove's boiler, and another for solar water heating, which will also be coming soon. For now though, it's just sitting in a bedroom waiting for the plumber...


Mike

Related posts:

• 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.

The solar-powered woodland

Last week we went up to the woods on a beautiful sunny day for dinner. The oaks are all out in leaf now and it looks fantastic!
In between them there's patches of sunlight reaching the ground, and I used one of them to test out running our woodgas stove directly from a solar panel, with no batteries involved:
The setup used a 10Wp solar panel:
A very basic charge controller (shunt mode rather than PWM, for those who are technical). This is normally used with a battery, but I left that out in this case:
A voltage regulator to give 3V for the stove:
And then the woodgas stove, which ran nicely:
Of course, all the trees in our wood are solar powered too ;-) We went for a walk to look at them, and found that the ride sides we coppiced 18 months ago are springing to life even more:
There's even grass starting to grow in the rides, now that there's some more light there:
Deeper into the wood there are still bluebells around, as they made such a late start this year. Here's a few photos of them...
It's a similar story back near our camp, though this has the benefit of being the third summer after coppicing, and oak thinning too in this case. The ground is positively green now, with new plants and more bluebells that last year:
Nice to see the Blue Bugle arriving, probably having travelled in from the wayleave:
Of course, the fruit trees needed tending to, though they're awkward to get at because of the fences we made to keep the deer off them while they're young:
I needed to get in there to remove blossoms, so the tree can focus on twigs and leaves instead of fruit this year...
There's more photos to come from the bank holiday weekend in a day or two...

Mike

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Electric fence for slugs...

My friend Alex has made a solar powered electric fence to keep the slugs and snails off his vegetable patch! Apparently the first version caused small slugs to burst(!!!), but also drained the battery too much. There's a lower power version now, which is a bit more humane I think. All in all, a neat way of protecting your food without the use of chemicals! Read more about it on his blog:
http://seacourt.blogspot.com/

Mike

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