Researchers from Bangladesh’s East West University have developed a novel method for analyzing bifacial PV systems on sloped terrains. The novel technique is a precursor formulation that is then inserted into existing PV simulation models that do not account for hilly solar farms. “A complete reformulation for bifacials would need a lot of effort, so instead we proposed an initial calculation step that can be used followed by any conventional PV model,” corresponding author Mohammad Ryyan Khan told pv magazine.
“The proposed method involves recalculating the sun path and irradiance (direct and diffuse) to that of ‘as seen from a sloped surface’ and inputs these values into a conventional PV model. The updated inputs ensure that the overall outputs are the results for a PV array on a sloped surface,” said the researchers. “We are therefore able to design and analyze PV farms for any slope and orientation without reformulating any other part of the existing PV models.”
The technique has three parts: the modeling of the Sun path on a given hill; the correction of the sunlight values; and the integration into existing models. In the first step, the global coordinate is rotated along with the slope’s orientation and angle. This process yields a modified sun path from the point of view of the tilted surface.
In the second step, the diffuse horizontal irradiance (DHI) is reduced based on the slope’s view factor to the sky, while the direct normal irradiance (DNI) stays as is. The new DHI value accounts for the limited sky exposure of the hilly surface. In the last step, the corrected incidence and sun angles are inserted into available PV models, which treat them as flat ground.
The team has validated their model in a few ways. A self-validation found an error of up to 2% in most cases, and benchmarking against the PVsyst software, which can account for sloped trees, has yielded similar results. Finally, the team also built a small-scale experimental setup including a panel monofacial array placed on an east-facing hill with a 20◦ slope. The panels were placed flat against the slope, at a height of 51.5 cm and a width of 28.5 cm.
“This experiment was conducted on the rooftop of East West University, Dhaka (23.8◦N, 90.4◦E) for 10 days of February-March 2022. The output was measured every 2 minutes from 6:30 am to 6:00 pm, then integrated over the day to get daily energy output,” the team explained “We have used the measured global horizontal irradiance (GHI) as the input to the combined model to simulate the output of the same configuration and location. The comparison between the energy output from our combined model and that from the experiment showed a daily error of less than 3%.”
Finally, the team demonstrated their model, using their precursor formulation with the PV-MAPS simulation software. They tested a slope angle of 20◦, on a hill facing either north, south, east, or west. The panel, monofacial or bifacial, was mounted in parallel to the slope. In the bifacial simulation, they assumed a bifaciality of 1 (unity) and an albedo of 0.2. All panels had an efficiency of 16.8%, with panel height and fixture height of one meter.
The analysis showed that the bifacial PV system yields higher output, as expected, with the annual energy from the bifacial PV systems on various hills ranging between 211.33 and 290.45 kWh/m2/year and the monofacial PV system spanning from 187.18 to 259.72 kWh/m2/year.
“Overall, the developed model simplifies the implementation of numerical models for bifacial panel arrays on sloped terrain. The models for mono/bifacial farms, mono/bifacial (single junction and tandem) tracking farms, and agrivoltaics systems can all now be implemented for corresponding systems on sloped surfaces for predictions and analysis,” the team concluded.
The novel approach was presented in “Modeling any bifacial solar panel array configuration on sloped terrain: Generalization using a precursor formulation,” published in Energy Conversion and Management: X.
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