The US Department of Energy’s
(DOE) National Renewable Energy Laboratory (NREL) has released a new repeatable
test protocol that simulates real shade conditions and can predict with much
greater precision the effects of shade on a solar array.
The new test demonstrated that
under heavy shading conditions the use of microinverters instead of typical
string inverters can help mitigate the impacts of shade by improving system performance
by more than 12%.
“Photovoltaic (PV) Shading
Testbed for Module-level Power Electronics” was co-authored by NREL senior
engineers Chris Deline and Jenya Meydbray, as well as Jason Forrest and Matt
Donovan of PV Evolution Labs of Davis, Calif. The research was paid for by DOE.
Shade significantly impacts photovoltaic
(PV) performance, and is considered in PV system design. The effects of shade
can vary depending on the configuration of the PV modules, the extent of the
shade, and the use of shade mitigating power electronics in the system. The
industry currently lacks representative, repeatable test procedures for
evaluating the annual effect of shade on different PV systems equipped with
different shade mitigation devices.
The new report details a
repeatable test procedure for simulating shaded operation of a PV system and an
analysis model for converting these measurements into annual performance
forecasts.
Shade measurements from more than
60 residential installations provide the basis for the shading conditions
employed during the test, which are analyzed for three typical shade scenarios: “light”, “moderate”, and “heavy” shading. The relative performance of a system
using shade mitigation devices is compared against an identical system equipped
with a reference string inverter for these three shade scenarios, providing an
annual performance improvement score.
Combined with additional derates
like annual shade loss and inverter CEC efficiency, this annual shade
improvement score can allow performance modeling software such as PV Watts and
System Advisor Model to better predict annual performance for PV systems that
use shade mitigating power electronics. It also allows an accurate comparison
between different devices.
An initial application of the
test protocol was conducted by PV Evolution Labs, showing the shaded
performance benefit of microinverters compared with a typical string inverter on
identical 8-kW solar arrays. The microinverter was found to increase system
production by 3.7% under light shading, 7.8% under moderate shading, and 12.3%
under heavy shading, relative to the reference string inverter case. Additional
detail is provided in the report to allow duplication of the test method for
different power electronics devices and test installations.
Standard test methodologies using
applicable test conditions should provide value to the PV community, since
products can be compared by a common metric and accurate information can be
collected about devices’ annual performance benefit.
“This is a major step in
establishing new and realistic testing standards for PV power electronics,”
said David Briggs of Enphase Energy, a microinverter manufacturer.
