Finland based PV technology testing company Endeas Oy says using a flash tester, it has developed an innovative method that equates the accuracy of steady state IV measurements of PV cells and modules. It calls this method 'Capacitance Compensation (CAC)'. A short flash pulse helps accurately measure the maximum power and other IV parameters, also in the highest efficiency PV technologies, it says.
Jaakko Hyvärinen, managing director of Endeas, says that manufacturers are not satisfied with current solar simulators that may be underestimating the maximum power of their high-efficiency products, forcing them to sell their products at a lower price.
"The new CAC method is perfectly suited for power measurements in PV manufacturing, as measurement results comparable to steady-state solar simulators can be provided for any PV technology with compact and proven flash testers that are straightforward to integrate into a manufacturing line," said Hyvärinen.
Typically, flashers with short pulse duration are employed to estimate the IV characteristics of PV devices. However, with high efficiency cells increasingly becoming available, the accuracy of the measurement is at stake. This is mainly due to the fact that these high efficiency technologies exhibit high capacitance – the ability to store electric charge –, which varies depending on cell technology. Due to well-passivated surfaces and high effective minority carrier lifetimes, advanced cell architectures such as PERC, HJT and IBC, store a considerable amount of electric charge within the devices. Consequently, charging and discharging of the device during flash testing distorts the measured I-V characteristics when short flash pulses are applied.
An alternative would be a steady state measurement that allows the sample under testing to soak in the simulated sunlight for sufficient time to generate more accurate IV curves. However, steady state simulators not only have higher operational costs, they also increase the temperature of the sample. That means, a precise active cooling is required, which complicates the entire IV testing setup.
Endeas says its (CAC) can bring in the similar measurement accuracy levels to flasher in comparison to steady state sun simulators. The method actually measures the capacitance of the cell or module based on current and voltage recorded during a normal flash pulse. By observing how the measured current behaves while a forward sweep (from short to open circuit) charges and a successive reverse sweep discharges the device, the method accurately measures the device's capacitance. Once the capacitance is known, the influence of capacitive components is eliminated, thus the results are equivalent to steady-state IV curves and the maximum power are formulated.
Usually the accuracy level isn't that perfect when measuring the maximum power of PERC, or even higher efficient HJT or IBC cells, owing to charging of the cells. According to Endeas, CAC is reliable in its measurement using a single flash pulse of only 40 ms, as it is based on measuring the capacitance of the tested device during the flash pulse.
It claims these results are comparable to steady-state solar simulators for any PV technology with compact and proven flash testers that are 'straightforward to integrate into a manufacturing line'. The new method from Endeas addresses the need of PV manufacturers for longer and longer flash pulses.
However, with cell and module manufacturers moving to higher efficient device technologies there is a trend to longer pulse flashers. H.A.L.M. from Germany, for example, says its latest heavy duty system for cell lines is able to run 60ms flashes in 1 second cycles, while for module systems, repetition rates of 10 seconds with 60 ms flashes can be achieved.
Endeas' method is included in its QuickSun 600 all-in-one module testing stations and other current solar simulator models. It will present this method at the upcoming EU PVSEC conference in Brussels in September 2018.