Solar panels have an expected lifespan of 20-25 years, though the outdoor weather, shade, gravel or even dust can all impact on the performance of power generation, and the real challenge for solar panels begin after the installation.
Solar site comprises of arrays formed with several modules, which are then compiled into a solar formation. The installation scale is changeable, and may even possess the advantages of flexible design and modularization; however, as the old saying goes, “one rotten apple spoils the barrel”, a little shade on just some of the panels can affect solar production of the entire array, and thus reduce the revenue of the solar site.
There are numerous reasons that could lead to shade over modules. Apart from the on-site environment factors such as nearby trees and surrounding buildings, the accumulation of clouds and dust, as well as dirt such as bird droppings, will also negatively affect the power generation of solar modules, where the loss of energy triggered by partial shade can reach up to 5-25% each year. The main reason behind the result is that shade will elevate the electrical impedance within the cells of solar modules, where the blockage in the current path will derive reverse current, and the loss of heat energy converted from current will generate partial heating for the solar cells that will eventually lead to heat spots, yellowing, and glass rupture.
In addition, cell deterioration, module aging and impact, as well as a positive tolerance of 0-3% for module efficiency are also potential factors in affecting the power generation volume of solar energy, despite a solar site with a perfect, seemingly shade-free environment. According to the study of the National Renewable Energy Laboratory (NREL), solar panels manufactured after 2000 are deteriorating at an annual rate of 0.8%, with a median of 0.6%, and the varied degree of module aging within the same array will also result in module mismatch, because although they may look similar, modules on the array do not degrade in performance at a similar rate.
In other words, module mismatch is an unavoidable challenge for solar power plants, for which Israeli solar company SolarEdge has introduced a corresponding solution, in which a patented power optimizer is connected to each solar module, turning each one of them into smart modules. The power optimizers increase the power generation output of the solar system to reduce the potential power loss by constantly tracking the maximum power point (MPPT) of each module individually.
The power optimizers also monitor the performance of each module and communicate performance data to the SolarEdge monitoring platform in real-time. By integrating functions of automatic alerts as well as instant and remote detection, clients are able to swiftly grasp on the power generation status of the solar system, and apart from the ability in checking the daily power generation volume through the App, issues such as shade and cell deterioration can also be located easily at a module level. This avoids paying for third-party monitoring services, as well as saves substantial manpower and time on site to further reduce operating costs and speed. Lastly, each power optimizer is equipped with the unique SafeDC™ feature, which is designed to automatically reduce modules' DC voltage to a safe level whenever the inverter or grid power is shut down. Traditional string inverters typically have limited safety functionality since they do not necessarily reduce the DC voltage when switched off.
(Clients are able to swiftly grasp on the power generation status of the solar system through the SolarEdge monitoring platform in real-time; modules displayed in dark blue have produced less energy)
Applicable on Floating, Ground, and Bifacial Solar Systems besides Rooftops
In order to achieve the solar target of 20GW by 2025, the Ministry of Economic Affairs has drafted the goal of 8.75GW in accumulated installation by the end of 2021, and will be installing solar panels on the rooftops of various industrial locations, public facilities, and agricultural establishments, as well as vigorously implement fishery electricity, photovoltaic set ups on polluted lands, and solar harbor and car parks.
The potential in Rooftop solar, large-scale ground-mount solar, and fishery electricity is being emphasized. Floating solar sites for example, are of particular interest as they appear to kill two birds with one stone: not only do they increase the efficiency and performance of power generation through the cooling effect of water, it has been shown that they also lower the degree of lake water evaporation and eutrophication.
Both ground and floating solar sites can adopt the bifacial solar system that has been developing rapidly over past year. For these solar modules that are able to generate power on either side, the front generates power by converting sunlight, and the back absorbs reflective light and diffused light from the ground or the lake surface, with a power generation volume that is 2-3 times more than that of traditional solar panels, which maximizes land utilization and power generation efficiency regardless of restricted space or narrow lands.
However, these solar sites are prone to respective challenges. Apart from the mismatch problems caused by bird droppings, dust, and gravel, floating solar has to also deal with ripples, typhoons, and temperature differences, where the degree of light received by the modules depends on the water fluctuations, and the scale of reflected light will also be different if a floating solar site adopts bifacial modules. Bifacial solar panels will have to also take into account the uneven optical emission at the front and back. The National Renewable Energy Laboratory (NREL) has recently discovered in its research of bifacial module mismatch that rooftop solar would sustain a 2% mismatch loss each year when installed on a shorter stand, and a sufficient height will also derive a loss of 0.5%.
Solutions such as SolarEdge’s are able to regulate current and voltage according to the specific needs of individual modules without fearing for the array of a solar site being dragged down by the module of the lower performance since the largest potential of the solar system can be exerted instantaneously, which indicates the significant advantages that SolarEdge has in bifacial modules.
Module Mismatch and Shades are Unavoidable
During site planning, MPPT per module with SolarEdge power optimizers allows for flexible installation design and avoidance of shade from nearby trees or buildings from the first moment. It also allows for multiple orientations, tilts and module types to be installed in the same string, infusing flexibility in the planning and expansion of solar systems. Since each power optimizer automatically maintains a fixed string voltage, installers have even greater flexibility with longer strings and strings of different lengths in order to design optimal PV systems.
Take the 50kW project of SolarEdge in Yamanashi Prefecture as an example, the owner of the solar plant was initially troubled by the shade issue caused by the utility poles and wires, and that the surrounding trees of the site were growing increasingly large, which resulted in the continuous declination in the power generation efficiency in the four years after the completion of the site. The 1.6MW rooftop solar set up in the Netherlands had also suffered up to a mismatch of 10% within the same array due to uneven degree of module deterioration. The aforementioned sites have been given a second life after the refurbishment and improvement conducted by SolarEdge.
Module mismatches that are often seen in solar systems during practical situations are induced by shades from clouds, bird droppings, and gravel, whereas possible impacts and uneven cell depletion will also affect the volume of power generation. A solar site, no matter how perfect the environment is, cannot guarantee that it will be free of masking after 20 years of operation, such as new buildings or elevated trees. Hence, flexible site design, optimized module power generation, as well as real-time monitoring and alert system at the module-level, are all crucial to facilitating high-efficiency operation for the modules until the end of their life and the maximization of power generation efficiency for solar sites.
（Photo Credit: SolarEdge）