SunSolve – Bifacial vs Monofacial

Introduction

Currently, the global market is dominated by mono-facial PV cells and modules. However, the International Technology Roadmap for Photovoltaics 2019[1] predicts that bifacial cells will gain 60% of the global market in 10 years, and they will be used in both bifacial and mono-facial modules. This is primarily due to the expectation that bifacial solar cells would generate more power. However, true bifacial modules with bifacial cells and transparent back covers are expected to make up about 50% of global market share by 2029.

Bifacial modules cannot be rated the same way as mono-facial modules; therefore, further discussion into standard testing conditions are required. Nevertheless, the possible gain from this technology attracts attention from the photovoltaic market.

Learning Objectives

  • Understand the use of PERC architecture in both mono and bifacial solar
  • Be able to perform experiments to observe differences in the optical performance of the different cells
  • Be able to perform an experiment to investigate a possible method of rating bifacial PV cells
  • Observe bifacial and mono-facial module performance differences
  • Identify cell to module performance differences of bifacial technology

Tutorial Exercise

The focus of this tutorial is to observe the optical and electrical advantages/ disadvantages of bifacial PV cells and modules. Please review the pages “recent advances in PV modules” and “PERC Solar Cells” in PVmanufacturing.org before attempting this tutorial. Passivated Emitter and Rear Contact (PERC) solar cells are a hot topic in the PV industry; therefore, you will be using the PERC mono-facial and bifacial SunSolve default templates. Throughout the tutorial, you will investigate their differences in structure and therefore, performance.

REMINDER

Make sure to save and organise any templates/simulations as you proceed throughout this tutorial; any unsaved progress will be lost if the SunSolve page is closed/changed/refreshed.

Part One – Current Standard Testing Conditions

In a modern manufacturing line, international standard testing conditions (STCs) are used to rate PV cells and modules. That is cell temperature of 25 oC, a uniform irradiance of 1000 W/m2 and an air mass spectrum AM1.5G. It is important to note that the irradiance is only applied to the front side of a solar cell or module, i.e. zenith angle 0o.

In this section, you will use the STCs mentioned above (default SunSolve settings) and conduct a simple experiment to observe key differences in cell performance between bifacial and mono-facial PERC cells. The templates you will use are the default c-Si PERC and c-Si Bifi PERC. The aim is to identify the reasons behind these differences.

The responses you will be observing are listed in Table 1 below.

Table 1 - List of responses observed in the comparison experiment.
ResponseUnit
Front reflection photon current density (JR,Front)mA/cm2
Front escape photon current density (JE,Front)mA/cm2
Rear escape photon current density (JE,Rear)mA/cm2
Parasitic absorption at the rear electrode photon current density (JA,RC)mA/cm2
Total rear metal series resistance (RS,Grid)Ω.cm2
Short circuit current density (Jsc)mA/cm2

Conducting the Experiment

  1. Open new simulations using the c-Si PERC cell and c-Si Bifi PERC cell templates
  2. Run both simulations without changing any settings and record the responses outlined in Table 1 above.
  3. In the Outputs -> Photon Currents tab, selecting “Detailed Losses” and unchecking the boxes for “Combine reflection” and “Combine cell components” will allow you to view all the relevant information. Grid resistances can be found under the Outputs -> Cell JV tab
  4. Tabulate the results from Steps 1 and 2, highlighting the differences observed
  5. Make sure to save your simulations; any unsaved data will be lost once SunSolve is closed

Questions

  1. Comment on the difference in rear escape current density.
  2. Which cell shows a lower series resistance of rear metals? Why?
  3. Which cell has a lower parasitic absorption at the rear electrodes? Why?
  4. Comparing the optical losses and Jsc, what is a significant implication in using current STCs in the rating of bifacial PV?

Part Two – Standard Test Conditions of Bifacial Cells

A frequent topic in the discussion of bifacial PV is the standard testing conditions used for rating bifacial PV cells and modules.  In Part One, the standard testing conditions for mono-facial PV were used to observe the performance of both the bifacial and mono-facial cell. However, this does not utilise bifacial cells’ ability to use incoming light from the rear. In this section, you will perform an experiment to test a possible rating method for bifacial PV cells and modules. The responses you will be observing in this experiment are listed in Table 2 below.

Table 2 - Responses observed when experimenting possible STCs of Bifacial cells
ResponsesUnits
Front escape photon current density (JE,Front)mA/cm2
Rear escape photon current density (JE,Rear)mA/cm2
Front reflected photon current density (JR,Front)mA/cm2
Rear reflected photon current density (JR,Rear)mA/cm2
Short circuit current density (Jsc)mA/cm2
Efficiency (calculated)%

Conducting the Experiment

  1. Open a new simulation using the bifacial PERC cell template
  2. Under Inputs -> Illumination, use the sweep function to setup 2 runs in a single simulation. One with illumination on the front side (zenith angle = 0o) and a second run with illumination on the rear side (180o).
  3. For each run, record the responses as listed in Table 2 above
  4. Graph a single, grouped bar chart of the 4 photon current density losses for both runs
  5. Calculate the ratio of the front-illuminated efficiency to the rear illuminated efficiency, remember irradiance is still 1000 W/m2

General Questions

  1. Comparing this experiment to the default in Part One, what is the importance of running a second illumination experiment on the rear?
  2. Why would it be useless to do this for mono-facial cells?
  3. Does the experiment suggest which sections of the bifacial PERC cell must be optimised for better performance?
  4. What does Step 5 suggest about possible rating methods that could be applied to bifacial PV cells and modules?
  5. This experiment is a simulation of single-sided flash testing of each side, does this mimic how a bifacial cell will be illuminated in the field? Suggest how a combined approach may be used.

Part Three – Rating Bifacial PV Modules

In Monofacial modules, datasheets would include I-V characteristics or thermal loss coefficients. In comparison, bifacial modules require additional information such as bifacial gains. The method of obtaining bifacial gains are one such important discussion into the rating standards of bifacial PV modules. 

One supposed method is IEC 60904-1-2[2], where bifaciality in conjunction with expected bifacial illumination is used to complete 1-sided equivalent illumination tests. The 1-sided equivalent illumination used in these tests are determined by the bifaciality coefficient and the equation is given by:

GEi = 1000Wm-2 + φIsc × GRi

Where:

GEi = the ith equivalent 1-sided illumination

φIsc = short circuit current bifaciality coefficient

GRi = the ith rear illumination

By recording the max power point (Pmp) of each test, a plot can be created to display expected bifacial gains of a bifacial PV module. The template you will use is the default c-Si Bifi PERC Module found under unit cells and modules.

Conducting the Experiment

  1. Open a new simulation using the c-Si Bifi PERC Module template
  2. Under Inputs -> Illumination, use the sweep function to setup 2 runs in a single simulation. One with illumination on the front side (zenith angle = 0o) and a second run with illumination on the rear side (180o)
  3. Recording the short circuit current of both runs, calculate the short circuit current bifaciality coefficient:

φIsc = IscRear / IscFront

  1. Using at least 8 points of GR from 50 – 400 Wm-2, setup a simulation to sweep through the different equivalent 1-sided illumination, GE‘s (Hint: Sweep the scaling factor)
  2. Record the Pmp (W) of each run 
  3. Plot Pmp vs GE (or GR), observing the trend of bifacial gains

Questions

  1. Why is this 1-sided test useful for rating bifacial PV modules? (Hint: How will the results be utilised in the system design stage?)
  2. This true bifacial PV module is Glass-Glass, what are the advantages and disadvantages of using glass on the backside of the module? What other material could be used?

Part Four – Further Understanding of Bifacial PV

  1. In mono-facial PV modules, a reflective back sheet is used to trap light within the module. From an application perspective, propose 2 possible options that true bifacial modules can utilise to achieve a similar result. Therefore, in what type of environment will bifacial PV modules benefit?
  2. When attempting to allow more light to illuminate the rear of bifacial modules, what other factors must also be considered in the installation of bifacial PV modules? (Hint: Consider module tilt angles)
  3. ITRPV predicts that bifacial PV modules will gain half of the worldwide market share. Considering the family of PERC/PERL/PERT cell technology, name 3 types of advanced PV architectures that would allow for this to occur?
  4. It is assumed that STCs used on a manufacturing line mimic real-world application, what are some issues with this assumption? Similarly, what is a major issue with the assumptions of the experiment in Part Two?
  5. Bifacial cells can be used in mono-facial PV modules. How could cell to module gains be possible in this type of module technology?

References

[1] – International Technology Roadmap for Photovoltaics, 10th Edition, 2019, p. 45, fig. 42-43. Available: https://itrpv.vdma.org/

[2] – IEC 60904-1-2: Measurement of current-voltage characteristics of bifacial photovoltaic devices Available: https://www.standards.org.au/standards-catalogue/international/iec-slash-tc–82/iec–ts–60904-1-2-colon-2019