The weather has been kind this week so I began re-wiring the amorphous array for the triplets running at 51.7V configuration.
The frames for the 15W panels are somewhat modular and I had a spare T bar (the broken one that I mended with super-glue) to make up a pair of two panel expansion units. All I had to do to the existing four panel unit was unbolt the end piece and replace it with the two panel expansion.
You can see the new six panel array here along with a new steel cored plastic rope as an added safety measure to stop the Sharp array taking off in any storms. It is secured to the roof support girder underneath.
I lengthened all the wires so they could comfortably reach the waterproof junction box and used two independent diodes for the two triplets of panels connected in parallel. This replaces the single diode at the main junction box at the end of the long feed wire. This should lower the diode forward voltage drop, as only half the current is now passing through each diode. It also means that part shading of this array will not cause local back-feeding through the other triplet.
Here's how it's wired up now:
You'll notice that some of the 12W panels have built-in blocking diodes. Despite this, they still put out 17.5V as their maximum power point as they have more cells than the 15W modules. I decided to still use external diodes as I don't know the breakdown voltage of the internal ones. You can bet they will be cheap diodes with less than 50V reverse breakdown rating (as they were only intended to work at 25Voc) so I decided to shore them up with the 1000V 1N5408 diodes I had already installed on the other pairs of panels.
The string of zener diodes is working well, capping the open circuit voltage at about 68V. The diodes have a voltage tolerance of 5%.
With the sunny weather lately, I'd been watching the battery get full at lunchtime and wondering what to do with all the power being wasted... Most of the kitchen is a non-starter as things in there consume way too much power. The fridge might be possible but there isn't enough solar power to run the thing 24x7 and if I forget to switch it back to grid power, the freezer would melt.
Then I realised that the ideal candidate would be the computer room upstairs. The comms kit and server are on 24x7 and chomp their way though about 3kWh per day and are fed by a UPS. That means I can swap the plug over from grid to solar and back again without interrupting the computer or network, as they're happy to run from the UPS for a few seconds.
So I ran a new extension of the solar power up to the computer room and put a trailing socket near the UPS under my desk. Now when it's sunny enough and there's spare power about, I can switch the computer room over to solar power. I'm toying with the idea of using a 240V coil change-over relay to do the switching automatically (or at least with a wall switch) so I don't have to crawl under the desk to swap the power lines over.
In the first two days, I increased my daily solar power utilisation from an average of 0.7kWh to 1.5kWh, using solar power for the computer room from about lunchtime (when the battery reached absorption mode) until about 9pm, when the computer room started to eat into the battery too much (what with all the lights, TV and kettle being used in the evening as well).
If I was
really in the mood for a project, I could have the server monitor the solar power level and battery state by the serial port on the Morningstar controller. The server could then choose its own power source by activating the mains transfer relay automatically!
Since the start of December, I've been keeping detailed statistics on total energy used in my home (Gas, Electricity and Solar) and you can see the results below.
December and January are pretty much a dead loss for the solar system, with big negative spikes where I had to use grid power to charge the batteries to keep them from rotting. However, from mid-February, things pick up and I've not had to use any grid power to charge the batteries and have been able to extract progressively more power from the solar system.
But as the total energy use graph shows, solar power is only accounting for a tiny fraction of my total energy use (dominated by gas central heating). The peak daily offset for electricity has been that really sunny day a couple of days ago when I ran the computer room from the solar power (making just over 12% of my total electricity consumed that day). The best weekly total offset is only about 5.2% though.
The last graph shows the daily solar utilisation and a lifetime electricity offset trend line (total solar kWh as a proportion of total electricity consumed since December).
Of course, the figures are somewhat skewed as over the last couple of months I've installed a load more solar panels and changed their configuration and so on. But it's still an interesting record.
You can see some changes I made to energy use as a result of keeping records. Around mid-February, I decided to switch from gas water heating to electric water heating. The gas boiler circulates hot water to a tank upstairs and heats the water in the tank by a heat exchanger coil. Not very efficient. By switching to direct electric water heating in the tank, I increased my daily electricity consumption by about 5kWh but reduced the gas consumption by some 20kWh. Even though electricity is three times more expensive than gas, it saved me money and reduced my total energy consumption.
. Now there's lots of cheap stuff being sold off from companies going bust - hence the van load of cheap computers and LCD screens at the weekend.
