PV modules collect the solar irradiation most efficiently when the sun rays strike the solar module's surface vertically. The orientation of a panel is defined by its tilt and azimuth angle. While the tilt angle, also known as the elevation angle, describes the vertical angle of the PV module, the azimuth is horizontally facing in relation to the equator. For higher latitude locations and under shading conditions in the south, the east-west orientation is more favorable and produces more energy than the south-north.
As a rule of thumb, for optimum harvest of solar irradiation – when using fixed mounting structures –, the modules should face towards the equator, which means south for installations in the northern hemisphere, and their tilt angle should be equal to the geographical latitude. Facing south considers the geographical south, not the magnetic south, meaning not following the compass. Modules that are installed at the equator regions do not need any tilt and can be installed horizontally, while a small tilt angle of 6° to 10° might be beneficial to avoid soiling (See Albedo On-site Measurements).
As the sun's position changes during the day and over the course of the year, optimum tilt angles vary on a daily, seasonal and yearly basis. To determine the optimum tilt angles, the latitude, solar radiation characteristics, climatic conditions and utilization have to be considered case by case. The sun is higher in the summer and lower in the winter. Hence, optimized tilt for harvesting PV generated electricity in the summer time will more likely be latitude minus 10° to 15° for the northern hemisphere and plus 10° to 15° for installations in the southern hemisphere. Anyway, adjusting the tilt of the panels according to the season using tracker systems allows capturing more irradiation during the whole year.
Traditionally, PV modules are generally installed facing the direction of predominant solar irradiation. This is North-South (N-S) for fixed mounting structures and East-West (E-W) for a horizontal single-axis tracking (HSAT) system. When it comes to bifacial modules, it is important to avoid ground-shadowing and self-shading of neighboring panels to achieve the maximum bifacial gain. On the other hand, huge row-to-row distances between module strings result in additional land use and cabling costs, negatively impacting system LCOE. Thus, tilt angles that are 2° to 15° greater than those for monofacial modules can be considered the optimum for bifacial modules, which helps avoid ground-shadowing as much as possible while keeping the row-to-row distance at reasonable levels, according to a LinkedIn article by Nick Lusson from Huawei Digital Power.
Setup matters: The performance of a PV system hinges on its geometry, including module height, spacing, and panel tilt, and orientation.
In addition, an optimized mounting system can also reduce the backside shading and enhance the bifacial gain. One such design with reduced number of horizontal beams increasing the number of purlins was presented in a research paper titled Quantifying the Bifacial Gain Improvement under Optimized Mounting Systems: Comparison of Experimental and Simulated Data by Djaber Berrian from Belectric, presented at the 40th EU PVSEC conference. The additional bifacial gain, evaluated experimentally as well as through modeling, was found to be 0.13% to 1%, which directly reflected as the gain in energy yield.
An interesting alternative to traditional North-South orientation is the East-West alignment. The low cost of solar modules has triggered installations with E-W facing modules which, depending on the location, generate lower yields than the S-N facing panels. For example, a 2023 published scientific paper from Adua et al. of Nigeria's Kebbi State University of Science and Technology estimates that the yield loss for an E-W installation in Cypress is about 9% compared to an optimal south-tilted system, while the gap may be even larger than 20% in Germany.
A blog from solar solution provider Autarco provided more insights into rooftop E-W installation with a comparison analysis. It indicated that while the yield-per-watt peak is better with a south facing system at 7.3% with a practical tilt of 10° to 12° and 13% with the most optimum tilt of 30°, the E-W system provides 22.5% higher yield in the per-square-meter metric, finally resulting in better returns on investment. When typical daily load profiles are taken into consideration, E-W oriented solar modules might actually be a better choice, as the PV generated electricity peak is spread more evenly throughout the day. Such an orientation offers either a double peak (morning and afternoon) or a wider peak, compared to the big peak around midday in south oriented arrays. East-West means that shallower angles can be used for installations, leading to less shading between the panels. Therefore, more panels can be installed at a given place, which is favorable when space is limited or for other reasons. When looking at ground-mount systems, bifacial modules are the most suited for E-W facing vertical installations. There are numerous applications where vertical bifacial installations are the perfect fit, such as sound barriers along roads or agrivoltaic systems. There was also a recent pilot installation at the Frankfurt Airport.
Many studies show that soiling is dramatically reduced for vertically installed solar modules, and snow does not accumulate on the module surface. Altogether, the overall daily loss rate is negligible for vertical installations. As shown in several scientific publications, for example the University of Alaska Fairbanks by Christopher Pike et al, vertical E-W bifacial modules had virtually the same annual production as south-facing latitude tilt bifacial modules, but with different energy production profiles. These vertical East-West PV installations using bifacial modules are increasingly finding a place in agrivoltaic applications. Such installations enable the double use of land – agriculture and power production – as discussed in detail in a technical paper by Roberta Arena et al. from University of Catania, Italy.
While E-W vertical installations are mainly implemented in ground mounts, the 'limited space' argument applies for flat industrial rooftop systems. The yield per roof area with an E-W installation is estimated to be above 20%. Here, the more evenly distributed generation often also better supports the self-consumption of corporates. Although bifacial modules are mostly used for ground-mount applications, there are also examples of E-W rooftop installations, where the technology makes economic sense.
The text is an excerpt from the TaiyangNews Bifacial Solar Systems 2024 Report, which can be downloaded for free here.
.png?w=50&fm=png){kind=logo}