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 | | Posted by: Tom Farin | 4/11/2008 10:43 AM | This post goes through the various components that might be found in an off-grid solar system
Components of an Off-Grid PV System
The simple off-grid PV system discussed a few posts back (Harbor Freight Solar Powered Yard Light) has the most important components of an off-grid photovoltaic system. But as an off-grid PV system gets upsized to deal with more and more power requirements, additional components may be necessary. So this is a good time to review the potential compenents of an off-grid PV system.
Photovoltaic Array
The system begins with the photovoltaic array that turns the sun's energy into direct current electricity. As our electrical needs grow, we will need to upgrade from the small (2" x 2") array in our yard light to a much larger array as we add power demanding components to our system. In a whole house system, an individual PV panel might provide 10 amps of DC at 12 volts, 120 watts of electrical power. Need more watts of power? Add more panels. Panels can be wired in series (increasing voltage) or in parallel (increasing amperage).
PV Array Disconnect
This allows the array to be disconnected from the rest of the system for maintenance. That's important with a big PV array as large DC loads can be as fatal as large AC loads. It also protects components down stream from the PV array from overload. For example, should an incoming house 240 V AC line be knocked down by a wind storm and make contact with the right spot on the array, the off grid system is suddenly an on grid system and the load might be way beyond the capacity of the wires and other down stream components. The PV Array Disconnect's main breaker would trip, protecting the components.
Charge Controller
The primary job of this component is to protect the battery bank from being overcharged. It does so by monitoring the charge levels of the batteries cutting of the current from the PV Array when the batteries are fully charged. It might provide a trickle charge to maintain the battery charge level once the battery is fully charged. It might also monitor battery voltage levels to make sure the charge doesn't get too low. It can be very hard on a battery to go through multiple cycles where the battery is full discharged then fully charged. Full discharge cycles can significantly shorten the life of the batteries (often the second most expensive component of an off-grid system.
It is not uncommon for charge controllers to cut off output from the battery bank to power users when the batteries are 50% discharged. That is an important consideration in designing battery capacity of an off grid system. You will need batteries totalling twice the amp hour capacity you will need to power your devices if battery discharge is limited to 50% of capacity.
Battery Bank
This is the bank of batteries used to store the current generated by the solar panel until needed by devices using the power. Keep in mind a solar panel will generate electricity during daytime hours only. The generation will be greater in full sun than when the PV array is shaded or there is cloud cover. In Wisconsin, we average 4 1/2 hours of full sun per day over the course of a year. The batteries are needed to deal with power needs at times of day when power is not being generated and during stretches of overcast days.
It is common for small off-grid systems to be 12 volt systems. That's because 12V is a common battery size and many electrical devices are designed to operate at 12 volts DC. We could store and deliver 12 volts with a single 12 volt battery, multiple 12 volt batteries wired in parallel, two six volt batteries wired in series, or some combination of the above.
But larger off-grid systems are often 24 or 48 volt systems, created using 6 or 12 volt batteries wired in some combination of series and parallel. Why the higher voltages? Because for a given number of watts of power, the higher the voltage, (and the smaller the amperage) the smaller the wires needed to distribute electricity in the system. Because PV arrays can be wired in any combination of series and parallel, just like the batteries, you could construct a 48 volt array to charge a 48 volt battery bank. The electrical wires wouldn't need to be anywhere as large for a 48 volt system as those needed for a 12 volt system producing the same amount of current. Anyone who has tried to buy brass track lately knows that copper is the new gold. Smaller wires on long runs between PV arrays and battery banks can significantly reduce cost.
The down side of a 48 volt system is transformers are needed to step the voltage down to 12 volts if 12 volt DC devices are to be powered.
System Meter
Larger systems often have system meters that monitor things like the state of the battery charge, and current utilization by the system. They provide much the same function as the gauges (or idiot lights) on your auto that monitor battery voltage, alternator charge rates and the like.
Main CD Disconnect
This component does much the same thing as the AC panel found on your home. There is a main breaker that protects the electrical lines going to the devices and the battery bank from current loads in circuits that are abnormal (short) or beyond the capacity of the wiring. In addiiton there may be breakers controlling current to individual DC circuits for lighting, pumps, etc. This panel may also include the transfomers need to step voltage down from the 24 or 48 volts provided by the batteries to serve 12 volt circuits powered by the system.
Inverter
If you are going to power 120 V AC devices with your off-grid solar system, you need to convert 12, 24 or 48 volts DC to 120V AC. In addition the inverter may monitor incoming power vs. outgoing demand for power and tap into a supplemental power source when demand for power exceeds supply.
Backup Generator
Some inverters are sophisticated enough to turn on a backup generator to increase the supply of power when the demands for power exceed supply as a result of a load beyond the PV system capacity or when the battery bank source is turned off by the Charge Controller because of the level of discharge of the battery bank.
AC Panel and Disconnect
Many smaller inverters allow you to plug AC devices directly into the inverter. But in larger systems there may be multiple AC circuits, each needing an appropriate capacity breaker. In addition the main breaker will allow the AC lines to be disconnected from the inverter and trip when the AC load exceeds the capacity of the inverter and DC system components.
Not all of these components will be needed to power a model railroad. But this is a pretty good checklist to go through in deciding whether the right components are in place to meet your needs. And each of these components need to be appropriately sized based on the electrical demands of the system. What i've done in a few posts is take you from one of the simplest off-grid PV system (solar powerede landscape lights) to a sybolic diagram of a system that could be used to take an entire home or cabin or workshop off grid.
In designing my GRR solar PV system I will cut down on the number of components to those that are actually needed for this application. But because your needs and application may be different than mine, I thought I'd start big and scale down.
As we move to greater capacity components, total cost of the system can rise significantly. This is particularly true of the investment in the PV Arrays and battery banks. That's why it is a really good idea to first do everything you can to reduce system power use (substitute LEDs for incandescent lights) before sizing the system.
A few posts back I dealt with reducing the power demands of my yard lights. My solution was to convert from 18v incandescent lights to LEDs then power them with individual solar units. As a result, my main off-grid PV system won't need to deal with powering yard lights. In my next few posts, I'll deal with the power needs of the pump powering my water feature and the circuits needed to charge the bateries powereing my radio controlled battery operated railroad.
Once the power needs are defined, I'll move on to selecting and sizing the components.
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