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A Bi Facial Summary: DC design considerations and financial hurdles

It is important for solar energy professionals to stay informed of the latest advances in technology and solar efficiency. And although we can’t say that bifacial modules is a terribly new advancement, it is not yet mainstream. Bifacial modules is one of the older developments in solar panel technology dating back to the 1960s, but it is also one of the latest latest advances to take hold. And, according to many experts, it is the next trend to sweep the solar industry and soon become the standard.

But what are bi-facial solar modules and what are the advantages? What makes experts say it is a trend of the future? What are the challenges and why hasn’t it already become mainstream? These are the questions we seek to answer in this article.

Bifacial modules are solar modules that are designed to receive photovoltaic energy on the front and back sides of the solar panel. All solar panels have a front, or top side with silicon cells that are ready to receive photovoltaic energy, but the bi facial modules have this material on the backside as well. This is significant because there is always additional energy from the sun that is reflected up onto the backside of the panels and elsewhere in the environment. And there is always diffused solar energy that has been spread out by the clouds and atmosphere that can also be captured with the back side of bifacial modules.

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Bi facial modules are available in many different designs. Some have metal frames and others are frameless, some have glass on the top and bottom while some use clear backsheets, and most are monocrystalline for better performance yet some are polycrystalline for better price. The one constant with all the designs of bifacial modules is that power is produced from both sides of the panel.

Efficient Bifacial modules have been made possible and more efficient with PERC technology, which has also contributed to the efficiency of the standard mono-facial solar cell. The PERC acronym stands for Passivated Emitter and Rear Cell, and although it is too complex to explain in this article, PERC cells use a dielectric passivation film on the rear surface of the cells. This allows increased energy absorption of the scattered and reflected light within the panel. Bifacial modules have been around since the 1960s but this PERC technology has significantly increased the efficiency of bifacial modules.

This sounds great but there are many variables that need to be addressed before bifacial modules can take a significant marketshare. Of course the cost is a big factor—certainly at least the cost above monofacial modules.

We know that the price of solar panels has been falling precipitously over the last two decades; the same is true for bifacial modules. And, notably, as the cost of panels has been decreasing, the cost gap between mono facial modules and bi facial modules has been decreasing as well.

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Source: https://www.pv-magazine.com/module-price-index/

This graph is showing one of the main reasons that bifacial modules are predicted to continue to increase in market share. As the cost gap between becomes smaller and the as the production from bifacial modules increases with more data and better designs, the extra production from the rear side of bifacial modules can make up for the additional cost.

But, today, Is the increased production enough to cover the additional cost?

The answer is… its complicated; sometimes yes, sometimes no. But increasingly yes. While the increased cost of equipment and installation costs of bifacial pv plants is not excessive, there are other factors preventing widespread adoption.

There are many design elements unique to bifacial systems that contribute to overall cost of the installation and especially to the performance of the rear side of the bifacial modules. The DC design, site location, and installation can be much more difficult for a bifacial plant than with monofacial modules, and collectively this creates problems for investors.

With bifacial modules it is quite difficult to accurately predict the increased output for a system design due to the many variables that affect rear side production. Here we will look at some of the major variables.

Environment and albedo

Simulations for bifacial plants must account for the albedo of the surrounding environment, especially beneath the panels. Albedo effect is the name given to the reflectiveness of a surface. It is a measurement between 1 and 0 where a black surface that absorbs all light has an albedo of 0, and a 100% reflective surface has an albedo of 1.

Here is a graph showing the albedo of a few different surfaces.

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It is a challenge for the solar industry, especially those focused on bifacial technology, to understand how albedo is affected by different materials, different seasonal conditions, and other factors that will affect the lifetime production of bifacial pv plants.

System Design

The system design on the DC side for bifacial pv plants requires more effort than in a monofacial plant to achieve optimal performance. The site selection is more important because of the environmental factors related to the albedo. Along with the albedo, the type of mounting structures and the height of panels, panel tilt and row depth play a large factor in predicting the production of the back side of the panels.

Generally, the greater the tilt, the higher the production will be from the backside, but you will be sacrificing production on the front. Studies suggest that the optimum tilt is 2~15 degrees larger with bifacial modules than with monofacial modules.

The row depth, or the distance between the rows is also a factor in bifacial modeling. The greater the space between the rows, the more light will reach the ground and will be reflected onto the backside of the panels for pv generation. But greater distance between rows will increase the land area of the plant, so the cost of land must be included in planning this variable. In areas where land is cheap, bifacial modules can be optimized with greater row depth.

Also the height of the panels is more important in bifacial modeling than for mono facial panels. If bifacial panels are placed very low to the ground there will be little chance for reflected and ambient light to reach the back side of the panels, thereby making bifacials obsolete. Increasing the height of the panel will increase backside production, but too high of a panel height is also a problem. According to several studies, the optimum height is around 1 meter, and although in some conditions installing panels higher than 1 meter will produce more power from their bifacial properties, this height comes at a cost.

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Source: https://www.nrel.gov/docs/fy19osti/74090.pdf

Increased string mismatch

Another challenge in predicting bifacial production in simulations is that there is greater string mismatch on bifacial panels, This has many causes and will result in performance loss that is difficult to predict, but it is partially caused by to non-uniform irradiance at the rear, as well as the current testing methods related to classifying solar module efficiency from the manufacturers.

Bifiacial panels should be installed with special consideration so that the mounting structures do not block the backside of the panels and create shadows from beneath. The DC wiring also requires special consideration so that the wires do not cause shading on the rear side of the panels with the reflected light from below. These shadows will increase the string mismatch and cause additional losses that should be avoided.

And over time, all of the solar cells in any given plant will degrade at a slightly different rate, increasing the string mismatch over time and causing more mismatch in a bifacial pv plant than in a monofacial pv plant.

Analysis

There is no doubt that bifacial modules will increase your power production. The question is how to accurately measure the cost of the bifacial plant, and how to accurately predict the power production with all of the variables accounted for

Results and studies have shown that bifacial modules can produce additional power between 10-20% over monofacial panels. If conditions are optimized and single axis trackers are adopted the additional power can be as high as 30-40% more than monofacials.

It is important to bear in mind that we are looking for the optimum LCOE (levilized cost of electricity, not the maximum power possible. There are several ways to increase power production, but many of these options are not cost effective and therefore not practical in the market. For example dual axis tracking can increase the power production, but it is still too costly to be recommended.

So a major obstacle for bifacial modules in the market is the difficulty to create accurate simulations and thereby satisfy financial queries regarding the additional costs.

Improved testing and improved modeling has been underway for years, and improvements in available data regarding irradiance, and geospatial data have contributed greatly to improvements in these simulations. There have been many bifacial test sites built, many bifacial studies undertaken, and many bifacial installations completed providing real data. In fact despite all the obstacles and uncertainties bifacial installations have grown rapidly in the last half decade, from only 97MW of installed global capacity in 2016, to almost 6000MW in 2019. And the largest bifacial plant to date of 224MW was completed late 2019. And the growth is predicted to continue. According to Wood Mackenzie Consultancy bifacial modules will account for 17% of the global market for solar panels in 2024.

With all of the data from tests and completed installations we do have a model of how to make bifacial panels work for a lower LCOE.

Some of the best practice recommendations to optimize your bifiacial installation and achieve increased ROI and lower LCOE are as follows:

  • Site selection: the cost of land affects if bifacial can be optimized. For places where land is scarce and expensive, panels should be laid flat on the ground to ensure maximum energy collection over a given land area, but where land is cheap bifacial modules can have optimal spacing and therefore higher yields. Also bifacial yields are greater where the diffuse light energy is greater, this means at higher latitudes the bifacial yield will be greater than at lower latitudes.
  • High albedo: the environment selected should have a high albedo. Desert sand is a good option. The best option is white concrete or highly reflective roof foil. Snow and ice also has very high albedo.
  • Panel height: This will vary from site to site but 1m has provided good benefit to cost ratios. Increasing panel height requires other variables to be measured as well, such as wind speed and lift from the tilt and therefore requires stronger ground mounts.
  • Tilt: this will vary from site to site but generally 2~15 degrees more than the monofacial tilt has been shown to be effective.
  • Row Distance: Again this will vary from site to site but 6 to 8 meters row distance has been shown to produce good results. Of course the cost of land or the available space must be considered, and if the cost of land is too great then a greater row distance will increase your LCOE. Ideally somewhere that land is very cheap can be used to increase row distance cost effectively.
  • Greater MPPT density: Using string inverters with more MPPTs is an effective way to reduce string mismatch and ensure efficient performance. The more MPPTs per watt the better.
  • Single axis tracker: Researchers from the Solar Energy Research Institute of Singapore have concluded that bifacial installations with single axis tracking can increase energy yield by 35% and reach the lowest LCOE for most of the land area on the planet. Although dual-axis trackers achieve the highest energy generation, their costs are still too high and are therefore not as cost effective. The researchers wrote: “In general, with the same mounting structure, bifacial configuration outperforms monofacial configuration. Tracker configurations outperform fixed-tilt configurations significantly, with dual-axis tracker installations having marginally higher yield than one axis.”

Cost analyses have shown that the cost of equipment and installation of a bifacial PV plant will be around 5% higher than a comparable monofacial plant. Furthermore the data shows that the cost of adding a single axis tracking system for optimal yields adds another 10% to the equipment and installation costs over an equivalent installation with no tracking.

Therefore, a bifiacial, single axis tracking (1T) pv plant can cost approximately 15% more than a comparable monofacial non tracking pv plant.

The yields from a bifacial 1T plant can be well above 20%, up to 35 to 39% in some cases if conditions can be optimized.

After working with many calculations and reviewing many production models and spreadsheets, here is a table of reference values and examples to help understand some of the variables:

BiFacial PV Plant Examples

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This sounds like a simple calculation, but we have one more obstacle to discuss, and that is the view from the finance side of this equation.

Due to the uncertainty in modeling the costs and production of bifacial pv plants and the relatively small sample size of reliable performance data, investors are often unwilling to finance the full amount of predicted rear side production.

Therefore, investors might only value a portion of the modeled bifacial gain.

For example, if a project had projected a gain of 10% over an equivalent monofacial project, a lender could be expected to provide 10% more debt financing than for the monofacial project. However that lender may not value the full projected bifacial gain due to the uncertainties we have discussed. They may only value the projected bifacial gains at 50%, meaning they would only offer 5% more debt financing over the monofacial project. And because debt financing is generally the cheapest form of financing, the uncertainties in the modeling and simulation stage end up adding financing costs to the project, thereby increasing the LCOE.

Conclusion

In summary, the industry is currently still struggling to manage the variables of predicting the output of bifacial modules, and also to manage the costs of optimizing the power produced by bifacial modules. These obstacles are being defeated slowly every day with every advance in data and every advance in technology. And despite these obstacles bifacial modules are almost universally accepted to be the future of the solar market.

Further Reading

https://www.nrel.gov/docs/fy19osti/74090.pdf

https://www.cell.com/action/showPdf?pii=S2542-4351%2820%2930188-4

https://www.greentechmedia.com/articles/read/bifacial-or-bust-engineering-solar-financings-for-the-future

https://www.pv-magazine.com/2020/06/05/single-axis-bifacial-pv-offers-lowest-lcoe-in-93-1-of-worlds-land-area/