A Walk through our PV System

Today's blog will not be featuring the usual excellent writing, great photos and interesting topics that you are used to since Rob is providing the content. Time to set the bar a little lower.  My topic is one that I know you will love - how our off-grid system works! Alison assures me that everyone  wants to know all about this... well, some people may want to know...OK... for sure her brother Mike will be vaguely interested! An off-grid system is difficult to describe without resorting to at least a few technical terms ("arc fault circuit protection" comes to mind) so don't worry if you doze off occasionally…..zzzzzz

An off-grid system is simply one that generates and stores all of it's own electricity - there is no normal electrical connection. For those who don't want to read the whole post, here is a summary of the vital information:

Do they save money? - "No"

Are they expensive? - "Yes"

Are they easy to install? - "No"

So, now that we have that cleared up, it becomes apparent that there are only two reasons that one would install an off-grid system: 

1. you live a gazillion miles from available power and have no other option 

2. you are seriously deranged and simply want to live the off-grid lifestyle. 

Let's just say that we do not live a gazillion miles from available power so draw your own conclusions.

Before starting, a brief digression on electrical inspections. Code requirements for PV systems are a fast moving target. After completely installing the system it failed inspection due to not having arc fault circuit protection (there's that term). This is actually the first inspection of any type that I have ever failed. The requirement has been in the code book since 2012 but was not enforced because at the time of purchase the necessary equipment wasn't available for off grid systems. Unfortunately, it took a year and a half for me to get the system completely installed and by then the inspector had started enforcing the rule. Timing is everything! I had to remove and reinstall a bunch of stuff and I then had to rewire the system a third time because the wiring diagram provided by the manufacturer was not the correct one for my system. Like you,  I am starting to really dislike "arc fault circuit protection".

So here we go.

The solar panels:

Our system uses 24 roof mounted solar panels (the array). In full sun, each panel produces about 255 watts of power at 35V so the array can produce a total of about 6.2 kW of power.  Panels can be wired in series to increase the voltage available or in parallel to increase the power (and usually both configurations are used in the system design). For voltage, you need to have enough to charge the batteries but not enough to damage the charge controller. Our panels are thus wired as 3 panels in series to produce 107 Volts (these are called a "string") and the resulting  8 strings are then connected together in parallel (in two sets of 4 strings, one set for each of the two charge controllers) to maximize power. One counter-intuitive complicating factor is that panels actually produce higher voltages as the temperature goes down, so the whole system has to be designed using a winter design temperature of -40 degrees (which is the lowest temperature recorded in this area. (I know, I know...why would anyone want to live here?)

An aside: Mounting the panels to a metal roof was supposed to be easy - they make clips that simply attach to the ribs on the roof, the rails then attach to the clips and the panels attach to the rails. Unfortunately, when I tested the clips they ripped the roofing apart before they were even close to the design uplift load. I then had to find a different attachment method that could handle the load and that didn't cause leaks in the roof.

Going through the roof:

Each string terminates with a single pair of wires (positive and negative)and these are routed into rooftop transition boxes at the top of the array (two boxes are required because of the number of strings installed). The only purpose of the box is to transition to less expensive wire and feed it through the roof with a waterproof seal. One of the difficulties in installing the panels was keeping track of the 18 different wires, so all wires were labelled at both ends before they were installed and colour coding was used to ensure that polarity is not accidentally switched.  I also used a voltmeter to confirm correct installation.

An aside: when I later had to rewire the system there was no longer any reason to use a transition box because the combiner was now just inside the roof and the wires were only 2 ft long. I had to rip out and replace all of the wiring that ran to the battery room and I could have spent the money much more wisely on a ton of beer!  (Alison frowns...)

Into the Combiners:

I will not bore you with the details of the combiner that I had to rip out, but let me say that it was inexpensive, easy to install and tidy. The replacement arc fault/rapid shutdown combiner boxes were expensive and a pain to install - but on the bright side, still pretty tidy. They are shown above - located just inside the roof of my shop.

As mentioned, there are 8 strings of panels used in our design. A single combiner can only handle a maximum of 6 strings thus there are 2 combiners - 4 strings are connected to each one.  As the name implies, each combiner merges the 8 wires from the panels into a single pair of much heavier wires and these then run through the gray tray underneath the combiners to the conduit on the lower left that connects to each of the two charge controllers (in other words, half of the panels will connect to one charge controller and half of the panels connect to the other). The combiners  also provide fusing and arc fault detection to protect against overload if a problem occurs with the rooftop circuits. Finally, there is also a box at ground level on the outside of the shop which allows switching the panels off in case of an emergency (it disconnects the rooftop panels and remotely trips another set of breakers to disconnect the charge controllers).

An aside: safety is a major consideration with these installations because if there is sun, there is power. You can't actually turn the panels or the batteries off; you can only disconnect them. Since the voltages are lethal, you really need to pay attention and be sure that everything is disconnected before working on the system!

And then into the charge controllers:

From the combiner, the wires run to the two charge controllers and these units control how much of the power from the panels is used to charge the batteries. The charge controllers use MPPT algorithms (Maximum Power Point Tracking) to maximize the power coming from the panels. Batteries are charged using a three stage program that applies different voltages and currents according to the state of charge of the battery. Since batteries can be damaged by either overcharging or under-charging, the programming has to be set carefully and then modified according to the age of the batteries. Once the batteries are fully charged the charge controller goes into a maintenance mode and power from the panels is basically not used. If you look at the display in the photo it reads that the charge controller is "sleeping". How cute is that! 

If you use power when there is lots available (a bright sunny day), then you basically get it for free and the batteries remain charged. If you use power at night, then the batteries gradually discharge and on the next sunny day they just charge up again. Off-grid systems are normally designed to operate for about 3 days without sun - after that the available battery power is depleted to 50%. Battery life drops dramatically if they are routinely discharged to more than 50% so if the sun doesn't appear in a timely manner, a generator must be run to recharge the batteries. You quickly learn to conserve power when it is not sunny!

An aside: These are pretty cool retro looking charge controllers eh! The two main manufacturers for off grid charge controllers are Outback and Midnite, and both units look like they were removed from an old juke box. It works out that they were designed by the same person and my guess is that he (like myself) is a retired hippie. (Alison: a what hippie ????)

Batteries are next:

Batteries are the black magic of PV systems - they are expensive and depending on the technology can be finicky, require a lot of maintenance and TLC and are very easy to destroy. And did I mention expensive? The batteries are used to store the electrical energy generated by the panels so that there is power without sunlight.  Ours are a flooded lead acid type and they are contained in a battery box to make sure that nobody can play with them and, for example, accidentally short the terminals which creates a rather spectacular but short lived pyrotechnical event. They also contain acid which is best avoided and when the batteries charge they evolve hydrogen gas, which is explosive. All in all, these are well suited to our keen sense of adventure. On the plus side, the battery box also contains the hydrogen gas and it is then vented to the outside using a vent fan (the fan is programmed to switch on during the charge cycle that produces most of the hydrogen).  

Our system uses 12 batteries at 12V each and we need 48V. To accomplish this, four batteries are wired in series and as with the solar panels, these are also called strings (which can be confusing). Each of the 3 battery strings is then connected in parallel to a positive and negative bus bar (simply a point of connection that can handle a lot of power). The purpose of all of this is to increase the power available from the battery bank while maintaining a usable voltage. To ensure that all of the batteries charge at the same rate, all corresponding wires in each string have to be exactly the same length and since the wires have to handle a lot of power they end up being quite thick (the final wire is about 1/2 inch in diameter)

For safety, the circuits are fused before connecting to the bus bars and then two very heavy wires (a red and a blue wire on the bottom right) are run from the bus bars to the Load Center . This then provides the path for charging and discharging the batteries.

The load center:

The load center is where all of the components connect together and it is pretty much a rats nest of wiring due to a rather poor layout by the manufacturer. It directs the power into and out of the batteries and to and from the inverter. It also allows a generator to be hooked up and controlled and allows connection to the normal power grid (if it is available). Finally, there are circuit breakers to protect each circuit and there are connections to output the AC power to the house.  Everything is run in conduit to protect the wires from physical damage.

The Inverter

The purpose of the inverter is to convert the 48 volt DC power from the batteries into useable AC power. The system we use produces power that is exactly the same as a normal house (120/240V split phase). To do this, there are actually 2 inverters inside the box, each producing 120 V but out of phase with each other. The AC power from the inverter is fed back to the load center, then through a main circuit breaker and on to the load panel in the house (3 wires, 2 hot and one neutral).  From there it is a normal house system but it is of course limited by the available power in the batteries and the size of the inverter (we can pretty much run anything, but we can't necessarily run everything all at once).

System Control:

There are multiple systems that need to be programmed, controlled and tracked and this is accomplished using the panel shown above.  All of the parameters can also be monitored using a computer connection or they can be completely controlled and programmed over the Internet.

And here is the tidier version 

After the system is completed and the panels installed.

Does it actually work?

Happily, yes - so far anyway. The background draw of power from the system amounts to about 70 watts (which powers the system itself and the rapid shutdown). All of our lighting is LED so even with a number of lights on the draw is only an additional 35 watts. Continuous loads are the ones to watch out for and the only large, steady load in our house is the refrigerator (which has not been moved in yet). I can work in my woodworking shop all day, start any of my stationary tools (some have 5 hp motors) and run a dust collector, air cleaner and woodworking machine all at the same time.  If it is sunny, the batteries will still be at 100% at the end of the day. The real test will come during the darkest months when the amount of sunlight is limited (December-January) - so I will post in February and let you know how it went.

That's it (and I'm sure it's much more than you ever wanted to know). For me, this was really a fun project!
(Note by Editor: Fun ? Hmmmm.... well it had it's moments.  Am now throwing out electric blow dryer, and taking "clothes dryer" off of the "to buy" list.... )