We’ve previously highlighted on this blog research showing microinverters deliver higher performance than predicted by industry-standard PV performance calculators, such as PVWatts and PVsyst. More recently, this topic received attention from the industry’s top laboratory.
Last month, the National Renewable Energy Laboratory (NREL) unveiled a new inverter testing standard to evaluate the annual impact of shade on PV system output. Using 78 different side-by-side shade configurations, the test revealed that Enphase Microinverters recover approximately 50 percent of the energy normally lost to shade by a string inverter (See Fig. 1 and
Fig. 2 for instantaneous and annual performance increases, respectively).
Microinverters achieved better performance because they do not face the same performance trade-offs as string inverters.
When a solar array is affected by shade, a string inverter must either (1) "bypass” (or turn off) the modules affected by shade or (2) operate the entire string of modules at the same power level as the shaded modules. See Fig. 3 and Fig. 4 for diagrams of how these scenarios would appear on a string’s I-V curve.
In both cases, the string inverter is exacerbating the impact of shade. Because shade typically only blocks a portion of the light reaching a solar module (50-60 percent on average, according to NREL), "bypassing" shaded modules increases the loss due to shade by 40-50%. Alternatively, operating the entire string of modules at a lower power level has an even more significant impact on performance.
Consequently, PV performance calculators are programmed to equate shade directly to power loss (e.g. an annual shade factor of 10% results in an annual performance loss of 10%). [Here is a more technical article on how PV performance calculators model the impact of shade.]
But NREL’s test results demonstrate the need to reprogram these calculators for microinverters. For example, if a solar array has an annual shade factor of 10%, Enphase Microinverters should reduce the performance impact of shade to just 5%.
And shade is not alone. Other loss factors, such as soiling, temperature and module mismatch, should be reexamined in light of the advantages of microinverters. For example, a long term study of module degradation by NREL shows a wide distribution of degradation rates, with many modules losing output at significantly faster rates than the average. Uneven module degradation should have a greater impact on a string inverter's output than on a microinverter's. But how can we test it?
The new shade testing standard developed by NREL could be one of many new “Testing 2.0” solutions that improve the accuracy of lifetime performance predictions of PV systems.