Sometimes system expansion is planned from the start, and sometimes it is done out of necessity. So how can you best anticipate changes, and meet those future needs?
Can I add more modules to my system next year? Should I oversize my inverter from the get-go? PV system designers and installers are frequently asked these very questions. Sometimes system expansion is planned from the start, and sometimes it is done out of necessity. The reasons are many, ranging from a limited budget, to increased loads putting additional strain on an off-grid system, to changes in module prices and rebate structures. So how can you best anticipate changes, and meet those future needs?
Here are the component considerations (and challenges) for expanding your PV system.
Increasing the length of existing PV source circuits by putting more modules in series may be one option for expanding the system. However, sourcing modules of the same make and model may be difficult. Models change and manufacturers come and go. Some larger integrators may keep extra modules on hand, often as replacements for damage that may occur—but don’t count on it. Besides aesthetics, mixing new and old modules means paying attention to size constraints and additional wind and weight loading on the roof or mounting structure.
Module size has increased dramatically over the years. Not that long ago, 80-watt, 12-volt (~18 Vmp) nominal modules were common; today, 200 W (or larger) modules, ranging from about 30 V to 60 V, are more typical. If different modules are going to be used, the new ones should have voltage and current characteristics as close to the originals as possible. Note that wiring additional modules in an existing series string doesn’t allow much expansion, as maximum circuit voltage will typically limit the number of modules that can be installed. For example, if a module’s maximum voltage is 44.1 V (based on low temperature data and the temperature coefficient of open-circuit voltage), and there are already 12 modules in series (12 × 44.1 V = 529.2 V), then only one more module could be wired in series without exceeding the 600 V maximum limit for dwellings (14 modules in series would equal 617.4 V—too high—while 13 in series would be 573.3 V).
Grid-tied inverters do their best to deal with array current-voltage curves affected by the inherent, slight differences between all modules. The greater the dissimilarity—for instance, adding 200 W modules to an array of 85 W modules—the more difficult it is to extract maximum power.
When modules with different currents and voltages are connected in series, the current of the new string will more closely reflect the module(s) with the lower currents. For example, if a 7-amp module is wired in series with a 4 A module, the higher- current module will be current-limited to about 4 A. The voltage will be additive, as expected with series wiring, but the overall gain in power will be limited by the lowest-current modules.
Central String-Inverters. For grid-tied systems, the size of the string-inverter can limit expansion. It is common to design systems with an array-to-inverter power ratio of 80% to 120%, depending on the location of the system and where the inverter is installed (see the “2012 Grid-Tied Inverter Buyer’s Guide” in this issue). For example, a 4,000-watt inverter may be coupled with a PV array ranging from 3,200 to 4,800 W STC. However, a design in this range will not allow for significant future expansion. For expansion purposes, consider installing a 7,000-watt inverter in place of the 4,000. Because inverter efficiency curves are relatively flat once the array size exceeds 30% of the inverter rating, loading an inverter “halfway” to allow for a future doubling of the array size does not greatly reduce the inverter’s operational efficiency.
A larger inverter can allow for additional PV source circuits, or “strings”—for example, adding eight 200 W modules to an existing 3,200 W array of 16 modules. Ideally, the original system was installed with a 5,000-watt inverter with future expansion in mind. Because installers must size the AC output wiring based on the full inverter capacity, the required AC output wiring and overcurrent protection for the expanded array is already in place. Sizing additional components, such as transition boxes, conduit, and disconnects on the DC side for the future full array, is a big help when it is time to add the new string.
As systems age, expansion and maintenance are likely to go hand in hand. Inverter replacement is a known maintenance cost—most inverters are only warranted for 10 to 15 years, while most PV modules carry a 20- to 25-year warranty. When the time comes to replace an inverter, a concurrent expansion in system size is a possibility. If expansion is likely to occur, then designing for it can make adding more power much easier.
Issues with mixing and matching modules still pertain, but—especially with an older array—it may also be helpful to recalculate the operating voltage of the older modules. As they age, modules lose some voltage; additional resistance in the connections and terminations in the circuit result in further reduction of operating voltage. If a new inverter is being installed, make sure to test voltage (or measure the IV curve) of the existing modules to verify that the new inverter’s DC input voltage window matches.
Multiple MPPT. String inverters that have multiple MPPT inputs make it easy to add strings of different modules because the inverter is able to track two separate subarrays. Multiple MPPTs minimize losses when source circuits are on different roof orientations or a string of new modules is added to an old array.
Microinverters & AC modules. Using microinverters or AC modules can avoid future inverter-size limitations—each module is connected to its own inverter. While wiring and rack considerations must be taken into account (as there is a limit to how many can be connected to a single AC branch circuit), using microinverters or AC modules can allow system expansion one module at a time.
If an existing string inverter has no room for adding more modules, microinverter/module pairs can be added to increase the system’s size. Depending on their definition of “photovoltaic system,” local inspectors may require a separate conduit from the DC circuits for the microinverters’ AC output. Most microinverter systems require a 15 A or 20 A interconnection breaker, which can also limit the ability to add on (see “Adding a Second System” sidebar).
DC–DC converters. Similar to microinverters and AC modules, these are connected to each module individually and offer module-level MPPT. Thus DC optimizers can allow different modules to be added without module-matching concerns, and allow expansion by one or more modules at a time, as long as the existing string inverter can handle the additional power.
Installing larger wires than required adds to the initial system expense, but will pay for itself in the long run if expansion occurs. Match the conductor with an appropriately larger overcurrent device so that it doesn’t need to be replaced, either. As long as the overcurrent device protects the conductor, loading it lightly simply means less voltage drop. Just be sure to consider voltage drop when the current level increases from expansion. Combiner boxes also can be oversized so that they will hold the additional conductors and fuse-holders or breakers that will be required when additional PV source circuits are added to the array.
Most existing systems weren’t planned for expansion, so don’t underestimate the costs to add to a system. Expansion may require upgrading conductors so that they are code-compliant, adding overcurrent protection and conduit, and dealing with other “issues” from the original system. This can make the cost of an addition significantly more than that of a new installation. Depending on the demands of the local inspector, adding to an existing system may also mean retrofitting newly required components—such as the arc-fault protection now mandated by the 2011 NEC.
There is a range of design and installation considerations for systems that will be expanded. Installing additional racking in advance is only cost-effective when the expansion will absolutely happen, or to allow for the addition of one or two modules. This is particularly effective for pole- and ground-mounted systems—for example, installing a 10-module rack but only including eight modules initially.
For roof-mounted systems, carefully utilizing the available space is important. Rather than centering an array in the middle of a large roof, putting it to the side or higher or lower will make it easier to add modules.
If expansion will be additional source circuits on a roof with a different orientation, then using distributed MPPT equipment (i.e., microinverters, AC modules, DC–DC converters, or a string inverter with more than one MPPT) can alleviate issues due to differing voltages and currents.
While upsizing conductors in advance is a good strategy, a less expensive alternative is to install larger-than-necessary conduit that can hold additional conductors. For ground- and pole-mounts, an easy solution is to put an extra conduit in the ground and cap it on both ends, leaving a cord inside to pull conductors. This makes adding extra wires a snap.
Brian Mehalic is a NABCEP-certified PV installer and ISPQ-certified PV instructor. He has experience designing and installing both PV and solar thermal systems, and is a curriculum developer and instructor for Solar Energy International.