Scientists from the Netherlands have tried to understand the reason for the unexpected advantages of vertical PV systems and have found that these installations have a much higher heat transfer coefficient than their horizontal counterparts.
A group of researchers from the Netherlands Applied Research Organisation (TNO) has carried out a series of tests and simulations to understand why vertical PV systems tend to provide windfall gains compared to horizontal arrays and found that vertical installations have much lower operating temperatures.
"Although better-than-expected results have been reported at conferences and seminars for vertical installation of two-faced PV systems, we have not seen publications showing that this is accompanied by a much lower operating temperature than you would expect for a free-standing PV system with a fixed tilt at the same total irradiance," says study author Bas B. Van Aken told PV Magazine. "These lower operating temperatures have a positive effect on the operating voltage. And thus on the total power production, especially under high radiation conditions."
In the study, "Thermal model in the digital double of a vertical PV system helps explain unexpected yield increase," published in EPJ Photovoltaics, Van Aken and colleagues explain that voltage is affected by both the amount of light and the temperature of the solar cells. As the irradiation conditions increase, the voltage increases logarithmically, while as the temperature increases, the voltage decreases, typically by 0.3-0.4 % C.
"For a standard system, we observed that under high illuminance conditions, the increase due to light is offset by the decrease due to the higher operating temperature," Van Aken emphasises. "For the vertical system, however, we observed that the operating temperature did not increase as much and the increase and decrease in voltage more or less balanced each other out."
For modelling purposes, the group used digital doubles, which are virtual images that connect and visualise real assets, to create doubles consisting of a real PV system and a digital copy of it. Digital twins are commonly used for operation and maintenance (O&M) of solar power plants. "The digital version mimics the power output of PV panels based on a time series of weather and other environmental data. The simulated values are compared with observed data," the researchers explain.
They made their measurements in a vertical PV system located near TNO's facilities in Petten, the Netherlands. The east-west system consists of nine rows, each equipped with eight 315 W double-faced modules, with module row spacing of 2 m, 4 m or 6 m, respectively. Of the 72 modules deployed in the system, 60 rely on n-type M2 TOPCon solar cells. All panels are equipped with power optimizers provided by the Israeli company Solaredge.
For its analysis, the group used a derivation method developed in-house and found that the heat transfer coefficients of the vertical panels were almost twice as high as the values of the horizontal panels. Heat transfer coefficients determine how heat energy transfers from one material to another and are commonly used, for example, to make insulation in homes.
They also found that the vertical array operated at a temperature difference to the ambient that was 'almost half', which they claim results in a 2.5% higher annual energy yield.
"These results show the importance of verifying the actual operating conditions of emerging solar panel applications," Van Aken concludes. "We expect that this effect has a 2-3% increase in annual electricity yield in the Netherlands, the lower operating temperature has a retarding effect on degradation mechanisms, and the effect may be more significant in locations with higher irradiation conditions."
Source: PV Magazine