Continuing his discussion on how to map and achieve a successful solar PV installation, John Williams from Solar Panels Plus explains the importance of inverter specification and configuration in an overall solar PV system design.
by John Williams, Solar Panels Plus
Note: This is the second in a series of articles discussing successful solar PV installations — part one discussed creating a checklist for properly educating a solar customer during the initial selling process.
January 20, 2010 – When a solar photovoltaic system does not meet the expected amount of annual power production, there is a good chance of producing an unhappy customer instead. No amount of explanations or justifications will suffice after the fact. Unrealized power production expectations can be the result of not properly setting the expectations in advance of the sale, poor system design or faulty installation. Here will dig a bit into system design considerations as they are related to specifying and configuring the inverter that will be used.
One primary consideration is the efficiency rating of the inverter to be used. Note that all inverters have different efficiency performance at maximum capacity than they do at partial capacity, so it is important to look at the efficiency curve supplied by the manufacturer. Some inverter manufacturers explicitly state their maximum efficiency and you must dig for more complete data. The real-world efficiency of a “95% efficient inverter” may actually be lower than the real-world efficiency of a “94%” inverter under certain conditions. Don’t assume that all equally rated inverters are equal for every specific set of conditions.
Properly selecting an inverter
When choosing an inverter, efficiency rating and pricing are normally among the leading factors. (Here is a listing of inverter manufacturers with efficiency ratings.) Note that these “stated” ratings will vary based on percentage of maximum utilization and other factors. The table below shows the efficiency range of a typical 15KW inverter, ranging from a low of 88.7% to a high of 94.8%.
One of the most common mistakes made when designing a PV system is under-sizing the inverter. (Oversizing an inverter is also a serious problem.)
Proper sizing should not rely solely on the nameplate capacity of the inverter, additional calculations must also include the ambient temperature range of the location because in cold weather, a PV module will exceed its nameplate performance rating just as surely as it will under-perform its rating on a very hot day. The same formula that is used to calculate the reduction of module efficiency on a hot day can be used to calculate module efficiency gain on a cold day. The temperature coefficient data below comes from a Solar Panels Plus LLC 230W solar panel. All brands of similarly sized multi-crystalline panels will have about the same coefficient. For each degree C that the temperature rises or falls from 25°C, the efficiency rises or falls by 0.45%.
How do you size an inverter?
First, estimate the size of the inverter based on the approximate size of the solar array that is desired. Then use the location’s weather data along with the string-sizing calculator provided by the inverter manufacturer to size each string for the correct voltage, which indicates the number of panels per string. The calculators assume the strings are equally sized. Properly sized strings that fit an inverter will automatically result in a properly sized inverter.
What happens when the inverter or strings are over-sized?
If the low-end of the inverters operating range (the “start” voltage) is not met, the inverter will not come on. If the voltage on the strings falls below the minimum at any time during the day, the inverter will stop. Careful attention must be paid to the inverters start voltage — a hot day could render the inverter inoperative due to under-voltage.
What happens when the inverter or strings are not sized large enough?
If the string sizing wasn’t calculated properly, on a very cold day the inverter will receive too much voltage and be damaged. Fuses will not help, as they protect against current (amps), not voltage. None of the leading inverter brands offer DC over-voltage protection, only surge protection. An over-voltage condition will cause the capacitors to become damaged and render the inverter unusable.
Note that most inverter warranties will not cover damage due to an over-voltage situation, which will be easily detectable upon inspection when the swollen capacitor bank is examined. And no warranty will replace the power generation or customer goodwill lost during a preventable production outage.
John Williams is COO of Solar Panels Plus, 533 Byron Street, Suite E, Chesapeake, VA 23320 USA; ph.: 1-757-549-1494, e-mail: email@example.com.