Researchers from South Africa's Stellenbosch University have conducted a long-term experiment to study heat dissipation factors in fixed-tilt (FT) and single-axis tracked (SAT) PV modules.
“The novelty of our research lies in the development of heat dissipation factors for SAT open-rack PV systems,” corresponding author Dr. Hannes Pretorius told pv magazine. “While previous studies have quantified heat dissipation factors for FT, this is the first study, to our knowledge, to provide heat dissipation values specifically for SAT systems.”
The researchers have focused on the constant heat dissipation factor (U0) and the wind-dependent heat dissipation factor (U1), which the popular Faiman model uses to calculate module temperature. U0 and U1 depend on factors such as plane-of-array (POA) irradiance, wind speed, and the model's direction.
“The Faiman model has been widely adopted as basis for the calculation of module temperatures. The commercial software PVSyst, for example, utilizes it. The user is required to input specific heat dissipation factors based on the module configuration, whereafter the software calculates annual power output based on the annualised meteorological data at the location,” the group noted. “It is also worth noting that PVsyst employs default (unaccented) heat dissipation factors of U0= 29 W/m2K and U1= 0 Ws/m3K for any open-rack installation. The is effectively inflated to account for the contribution of wind, due to the potential unreliability of site-specific wind data.”
The researchers have analyzed the actual U0 and U1 factors based on measurements of four panels outside Stellenbosch, South Africa. They were all 420 W polycrystalline modules, with two of them being FT and two SAT. The SAT modules were in landscape orientation, with east-west tracking, while the FT had a fixed 31-degree tilt angle and a portrait orientation.
Measurements were taken during a four-month period in 2023, although only 44 days met the criteria of clear skies and were, therefore, used. Thermocouples were used to measure module and ambient temperatures, and pyranometers followed POA, while other sensors recorded wind speed and direction and the tilt angle of the SATs. All data was filtered to the time frame between 10:00 and 14:00 to reduce noise.
Image: Stellenbosch University, in Progress in Photovoltaics, CC BY 4.0
“The results of the FT experiments compare favourably to results from literature, producing U0 = 25.7 W/m2K and U1 = 9.8 Ws/m3K from the linear data regression analysis for the heat dissipation factors,” the scientists said. “Experiments on SAT modules extract heat dissipation factors of U 0 = 29.9 W/m2K and U 1 = 9.7 Ws/ m3K, which represent a novel contribution in the field. These values indicate that enhanced heat dissipation is experienced at no/low wind conditions for SAT modules compared to FT modules.”
When using the site-specific U0 and U1 measurements to calculate the predicted annual power production, the results show a 2.9% and 3.3% enhancement for the FT and SAT configurations, respectively, in comparison to the PVsyst default heat dissipation factors of U0= 29 W/m2K and U1= 0 Ws/m3K.
“These findings carry significance, particularly in the context of growing competition in tendering for the construction of contemporary PV power plants and the heightened importance of precise techno-economic calculations,” the team said. “An exploration into selected days and averaged data over the test period reveals a clear relationship between module temperature, POA irradiance, and wind speed. Evaluations plainly confirm lower module temperatures due to boosted heat dissipation from SAT modules in relation to FT modules … Interestingly, wind direction plays a secondary but not insignificant role in the heat dissipation of FT modules, whereas heat dissipation from SAT modules seems to be insensitive to wind direction.”
Pretorius added that the “most surprising result” was the significantly more stable heat dissipation from SAT modules compared to FT modules. Even though SAT modules change angle throughout the day, the more consistent irradiance on their surface makes these systems far less sensitive to daily fluctuations in environmental conditions.
The scientists presented their results in “Understanding Heat Dissipation Factors for Fixed-Tilt and Single-Axis Tracked Open-Rack Photovoltaic Modules: Experimental Insights,” which was recently published in Progress in Photovoltaics.
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