Flow of Electricity in the System
Seems simple enough, right? Panels make electricity, which charges batteries, which runs your home. This is true, but there are a few other things you should know.
First, the PV modules (as they are known in the industry) create direct current, or DC power. This power must be managed in a way that the rest of the equipment downstream can handle. To accomplish this, the modules are wired in series (positive to negative) strings to create the correct voltage and amperage to connect to the rest of the system. Each series string is through a disconnect, typically a breaker, most likely found in a DC Combiner Box. This disconnect offers a readily accessible place for you or a technician to “turn off” one or more strings in the event that service is needed at the array. The disconnect also protects the system in case there is ever damage to the modules which could cause dangerous electrical currents to flow within the equipment.
Second, the power flowing out of the DC Combiner reaches a Charge Controller, which regulates the amount of power that enters the storage batteries, and prevents them from being overcharged. The Charge Controller does much more than this, however. This sophisticated device actually manages the overall energy harvest from the array of modules through variable charging algorithms. Another way of saying this: the Charge Controller can actually change the way that it charges the batteries depending on conditions affecting the modules. So when a cloud passes over the array, it can alter it’s charging strategy to yield the highest amount of power from the modules. It also is responsible in some ways for the overall health of the battery bank. The Charge Controller can not only limit power into the batteries, it can provide for timed overcharging of them. This is known as equalization, and is important for battery maintenance. More on that later. The Charge Controller, as you can see, is very much the brain of an off-grid system.
Third, power from the Charge Controller moves into the battery bank, the storehouse of energy. As important as they are, batteries are essentially quite dumb. They react chemically to an electric charge, and are able to reverse that process to release this charge. Each battery within the bank produces roughly 6 volts, and are wired in series to reach higher voltages to connect to the rest of the system, in much the same way as the array of modules. For a 24V nominal system, a minimum of 4 batteries must be wired in series to operate the rest of the equipment. To achieve greater depth of storage, more groups of 4 are added with parallel connections (negative to negative, positive to positive).
Fourth, power is removed from the batteries by the Inverter whenever you turn something on in your house. If a Charge Controller is the brain within the system, the Inverter is most like the heart. It is designed to do the hard work of converting DC electricity from the array and batteries into AC electricity to power lights and appliances, and do this 24 hours a day for life. The inverter also performs double-duty as a battery charger whenever you feed power back through it with your generator. Although not as smart as the Charge Controller, Inverters can sense when this power is available, and will connect to it automatically with an internal transfer switch.
Finally, power from the Inverter makes it’s way into your house wiring through it’s main service panel, where it is distributed among all of the lights, computers and other toys connected through outlets.
Array Maintenance
One of the beautiful things about photovoltaic power is that there isn’t much to do to keep modules running. They will sit on your roof or in your yard and continuously provide electricity day after day without complaint. Typical productive lifespan for PV modules is 30 years, although older panels have been found to be producing viable power for much longer. In fact, most manufacturers of PV modules warranty their power output at 80% nameplate rating over a period of 25 years. That means that a 200 watt module with this warranty would still be making 160 watts after a quarter century! This is not to say there aren’t a few things that you can do to help them make a little extra power here and there.
Rinsing off the dust and dirt that accumulates on the faces of modules does help keep them performing at their best. Heavy soiling can cause as much as a 10% reduction in module efficiency. Choosing a time to wash the array that is neither to hot or too cold is important, so that there is no risk of shattering the glass through heat-shock.
Seasonally adjusting the array can boost power output by as much as 15% if performed religiously throughout the year. Consult a sun-path chart for your latitude to determine the optimum seasonal angles for your location. In the desert southwest, summer angles are usually no more than 25 degrees from flat, and winter angles no more than 45 degrees. Adjusting array angle can be a difficult task, best accomplished with more than one person at hand. Take caution to be careful of wiring between modules, and loosen module wiring clips when necessary. It is very important to realize that the array can never truly be “turned off,” as the panels will continue to output power unless completely shaded.
DC Combiner Box
Be especially mindful of the dangerous voltages and currents within this box. Do not allow children or animals to play around this area. Do not remove the box cover unless there is significant reason to do so.
Battery Maintenance
Batteries are the least glamorous and most time consuming part of your new solar system. They are the “necessary evil” that all off-gridders must endure. They’re big, stinky (just wait!), heavy and generally cantankerous. Think in-laws wired in series and you’ve got it. For all of their faults, there just isn’t anything else quite as good or efficient at delivering back what you put into them. Did I mention expensive? So, to protect our investment, we must take good care of them while they are with us. From Alan Sindelar, President, Positive Energy in Sante Fe, New Mexico: “Respect your batteries, for they are thy storehouse, and thy gold shall be quickly turned to lead.”
The first order of battery maintenance is ritual watering. Make sure to keep all cells topped off as often as you can. Most people realize how much water their bank will use after a year or so, but it doesn’t hurt to check. This amount will change over time. Newer batteries use less water than older ones, similar to oil used in a newer car versus an older one. Purchase a few large containers of distilled water and keep them by the batteries. Do not fill batteries with any other type of water. I have often used a small funnel and a little 6oz. Dixie Cup to carefully fill the cells. Each cell will have a “neck” that reaches down into the compartment. Your goal is to fill until the water is about ¼ inch below the bottom of this neck. This is difficult to see without a flashlight, or some strong overhead lighting. Try not to over-fill the cells, as this will reduce the concentration of the electrolyte in the batteries, reducing their efficiency. Not to mention, making a big mess that can be painful to clean up. When there is a spill, use baking soda to neutralize the acid. It is a good idea to have some on hand near the bank just in case. It can also be rubbed into skin and clothing to prevent burns. Eventually, the batteries will develop some corrosion on their terminals. A heavy wire brush will remove most of this along with a mixture of water and baking soda.
Applying an equalization charge regularly can help prolong the life of your batteries. This is a sustained high current charge designed to knock the build-up of sulfur off the lead plates inside each battery and back into the electrolyte where it can dissolve back into the solution. Equalization is activated through two sources: the Charge Controller, and the Inverter. You will need to tell each device to begin the cycle. If you are attempting to equalize with solar alone, you will want to minimize your electrical usage in the house to send the most available power to the batteries. If equalizing through the Inverter, you will need to connect and start the generator. Equalizing with both sources is recommended, as this increase in power to the batteries will guarantee the completion of the cycle within the allotted time. Battery manufacturers recommend different intervals for equalization, but a good rule of thumb is once quarterly. Newer batteries will not require equalization as often, older batteries a little more often. Before performing an equalization charge, it is good practice to check water levels in the cells. During the cycle, the batteries will use more water than normal.
A few other notes about your batteries: They like to be about the same temperature that you do. This is ideally 70 degrees or so as often as possible. You can insulate the building that the batteries are housed in, or build an insulated box around them. This second idea is often a better one since the power shed is a multi-use building for most. The box will prevent anything being dropped onto the batteries, or set upon them by accident. It’s all fun and games until a rake becomes welded to a few battery terminals, as well as energized. Also, be mindful if cables are to be disconnected for any reason. Insulate the handles of any tools you will use with electrical tape before you go to work.
Inverter
This is the real workhorse of your system. Surprisingly, there is little maintenance to be performed. Some models have a dust filter for their fan which will get dirty and impede the flow of air into the unit. Otherwise, there are no user serviceable parts within the inverter.
Most manufacturers include some kind of control device for programming and monitoring of the inverter. Consult the owner’s manual to learn how to navigate through the menus of your particular device. Do not fear accidentally changing important settings that could cause harm to your batteries or the rest of the system. If you reach a menu item that could lower performance of the system or worse, typically the parameter will require the entry of a password to make a change. Lesser functions often require you to press “OK” after making a change to be sure that you know you are making it. There is great information to be had from these devices. Lifetime kilowatt hours produced, diagnostic tools, current output stats, and more can be found with just a few pushed buttons. Another important function provided here is the ability to limit the amount of power flowing through the inverter’s AC to DC charger to the batteries. This will allow you to change the amperage input if you ever decide to connect a different generator to the system. Larger gensets will need to be current limited to around 20 amps AC, as the charger can only handle so much power. Give this some consideration before you buy that 20kW Kohler. You’ll really only be able to send about 2400W to the batteries, and then pass through a bit more to the house.
Final Thoughts
Living off the grid puts you in a much more direct relationship with energy. You must be conscious that your system was designed with a certain load (usage) estimate in mind. Adding appliances and plug-in devices in the future must be done with caution. Let your monitoring device be your guide. Seasonal changes specific to your site will affect the system’s output as well. Awareness of when storms will likely pass through can give you a heads-up for decreased power availability. Above all, you must maintain a good relationship with your installer, or develop one with an accredited local solar technician. This is the person you will want to be able to call at 10:00 PM for advice when your system crashes suddenly.
While owning and maintaining an off-grid PV system may seem challenging at first, you will be surprised how quickly all of this becomes routine. Most of my clients report that after a few years of life off the grid, they feel pride in their ability to understand how the system works, and happy that they accepted the responsibility. Good luck with solar, and welcome to the club!
I wrote this in an effort to put down the system orientation speech on paper, so I wouldn't forget it. Again. If this benefits other off-gridders out there, cheers!
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