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The potential of Bifacial PV

17 Jul 2018

ECN part of TNO have developed a unique and versatile simulation model, called BIGEYE, to predict the yield of a bifacial PV system. Scientists of ECN part of TNO have been leading in the field of development of bifacial solar cells and modules through experimental work and modelling.

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Calculated contributions in BIGEYE

With this computational tool it is now possible to make accurate predictions of the energy output, in kWh, that a bifacial PV plant at a specific location will give. These predictions based on (hourly resolved) climate data can be used to give stakeholders trust in the plant design, the lower cost of electricity generation of bifacial PV, and therefore on the anticipated return of investment.

Predicting the yield of a bifacial PV system requires more than knowing the Wp of the installation and the local climate conditions. The layout of the system strongly determines how much extra ground-reflected and diffuse irradiance the rear of the panels will receive. The optimal tilt angle, ground clearance and ground coverage ratio of the panels will depend on site-specific parameters such as climate conditions and the ground-reflection coefficient (albedo) as well as on project-specific economic parameters, e.g. cost of land, capital and electricity price. The ECN part of TNO model BIGEYE can determine the optimal layout and expected yield using an advanced irradiance model that uses fully 3-dimensional view factors for each panel to the sky and the ground. The predicted electrical output of the PV system considers the bifaciality factor of the panels, the effect of bifacial irradiance on the temperature of the panels and the electrical interconnection of the solar cells. All direct shading effects caused by panels on other panels are treated by an algorithm that takes the bypass circuit layout into account.

BIGEYE result: diffuse irradiance on rear

The ECN part of TNO model effectively treats well-known issues that make the prediction of the yield of a bifacial system complicated, such as the shade that is created on the ground by the panels in the system and the presence of e.g. a mounting structure at the rear. This reduces the irradiance on the rear of the panels and makes the total irradiance in the system non-uniform. Both the reduced irradiance and the mismatch losses resulting from the non-uniformity of the irradiance are treated by the model. BIGEYE can also work with partially transparent modules, which would have less self-shading. The calculation of the temperature includes the recent insights obtained at ECN part of TNO on how the additional irradiance on the rear, the bifacial cell architecture and the module bill of materials cause the temperature of bifacial panels to be different from conventional panels at the same location.

The ECN part of TNO model is very versatile and can predict the yield of both fixed tilt systems and single-axis tracking systems. There are no limitations to tilt angles and orientations. So, in addition to conventional equator-facing systems, it is possible to consider e.g. east-west facing systems either with vertically placed panels or in a high-packing, low-tilt angle configuration. Quite unique is that tilt angles and orientations can be mixed within a system, and that realistic system geometries can be used with, e.g., irregular inter-row or intra-row spacings and the presence of mounting structure.

With the model BIGEYE ECN part of TNO can share their expertise of bifacial PV with all stakeholders in the value chain, from module manufacturer to investor and end-user. ECN part of TNO specialists can assist with:

  • Comparison of different (bifacial) system designs, e.g. single-axis tracking, east-west or vertical orientation
  • Optimization of system designs: ground coverage ratio, tilt angle, spacing within rows, ground-clearance of system
  • Choosing between module types with lower or higher bifaciality factor, different bill of materials or thermal coefficients
  • Adapting design for better matching to demand profile or value profile

ECN part of TNO recognises the importance of bifacial energy yield prediction through modelling and is already collaborating with partners on a range of applications to improve their energy yield and business cases. In a continued commitment to addressing these issues ECN part of TNO have developed the bifacial energy yield model BIGEYE.

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