After a little delay our photovoltaic generator went online – we had been waiting for the delivery of this sophisticated addition to our office decoration:
People on G+ had very cool suggestions, such as a rotating alien-fighting device throwing darts. Closest to the truth were: fuse box and fire alarm.
The box containing two knobs (actually this large box does not contain a lot):
Two switches that are connected to that big red button downstairs, positioned next to the inverter for our PV panels:
We have two strings of modules, oriented perpendicular to each other; so irradiation on these is different. I add an overlay to a screenshot from Google Maps:
Solar panels subject to different irradiance are connected in different strings – serial connections of modules; otherwise output power would suffer. The inverter has two inputs for two such strings and two MPP trackers that try to find the Maximum Power Point for each generator, by constantly probing each string’s current versus voltage curve.
Each string is connected to one of the little red knobs, which are part of yet another safety mechanism. The inverter converts DC current from the panels to standard 3-phase AC output voltage (230 V each phase). It has surge protection (another grey boy, but downstairs) and can shut off power at its DC and AC connectors – but then there is still a voltage drop across the DC cable from the roof to the inverter.
DC voltages supplied by our PV generators are about 400V, but generally they can be close to 1000V. This is a risk for firefighters connecting themselves to the circuit via a jet of water. You ‘cannot turn the panels off’ as long as there is sunlight! In order to make sure that the voltage drops to zero as close as possible to the panels, those switches are installed.
That ‘firefighters’ switch is semi-mandatory here. Lightning protection is not mandatory too, but we decided we should finally have one. Since safety standards and costs of such protection have grown exponentially in recent years, we can brag with a Faraday cage with tighter meshes and taller antenna-style tips than all our neighbours.
I am sure it protects us not only from lightning but also from alien attacks (see image below) and EMP guns – and the wiring goes well with the surface-mounted aluminium tube for the DC and AC cables for the PV generator.
The big red button is in the tech gadget closet on the left side of the driveway.
Firefighters will pull or push the red button in case of a fire. We decided for the pull option as you are less likely to pull than push something accidentally.
What we did not know before installation: The switch will also be activated automatically in case of a power outage – this means: about every 2 years for a few minutes. But when the big red button has been activated you need to switch power on again upstairs in the roof, too!
Normally, the switch box would be tucked away in an attic, above a dropped ceiling. We have no attic anymore – this is all office space, 3,5 high in the center. We could have squeezed the box into the insulation. But then after every power outage we would have needed to climb up there, remove roof tiles and switch on power again. So we spontaneously decided to have it installed on the ceiling, above the Chief Engineer’s desktop:
Last Monday The Metering Guy from the utility finally installed a smart meter, capable of metering both consumption and feed-in to the grid. He had to disconnect from the grid to do so. We switched on the inverter in bright daylight – and there was no power! Panic – what happened? I fetched the laptop and the inverter’s manual, ready for troubleshooting – until The Chief Engineer walked by, carrying a ladder, and grinning mischievously:
Have you perhaps triggered the firefighters’ switch when disconnecting from the grid?
I had forgotten about the switch only about 15 minutes after I putting big signs for firemen! But at least we knew it worked!
After one more controlled test of a power outage we were finally online. This is what power generation looks like on a nearly perfect sunny day now (2015-05-11).
Since May 5 we have consumed 11kWh / day on average; about 55% of this have been provided directly by the solar panels. Daily energy generation was about 23kWh; we used 27% of the power generated.