Bifacial tracker pitch an important design factor in maximizing energy gain

Last week, JinkoSolar answered your questions regarding the use of bifacial PV technology in large-scale solar installations, following on from a webinar pv magazine held last month. Now Soltec, which spoke about how its tracker technology can, in particular, help boost energy yields in such installations, has taken the time out to respond to your queries.

Read on to discover more in-depth information on the topic; you can also listen to the webinar and download the presentations.

What is your recommended ground coverage ratio (GCR) range with bifacial trackers?

Soltec: What we have found is a sweet spot greater than 9m but less than 13m. If we were to layout the tracker between 10 and 12m, we would have the best case scenario for maximum bifacial energy capture. So the ideal range is between 40-33% GCR, making pitch an important design factor in maximizing energy gain. It is also important to note that wider aisles will not only maximize reflected solar energy but also optimize O&M practices.

How does terrain affect albedo on different types of surfaces?

Albedo is a determinant factor in bifacial gain. It is dependent on reflective surface color, texture and extension. Maximum gain comes from smooth white surfaces and greater reflective areas free of disrupting obstructions. At La Silla, experience highlighted a measurable seasonal variability of albedo as vegetation colors change. Different soil conditions will have a different associated albedo rate. For instance, if we take humid soil usually associated with fall – dark in color – its tendency is to absorb light. This soil condition is not conducive to bifacial energy gain. In contrast, if we take snow usually associated with winter, its tendency is to reflect. Snow will provide the highest albedo. Although dry grass or sandy conditions, which have a relatively high reflectivity rate, will also make for a strong case for bifacial applications.

Albedo values vary seasonally and from location to location. Was this taken into consideration for the simulated example? Can you share the simulated results for a lower albedo of 0.15-0.2, which is more common for several locations?

Yes. We will be publishing energy gains using different albedo conditions as a result of testing at the Bifacial Tracking Evaluation Center (BiTEC).

Have you analyzed the exact effect on the rear side yield in an installation with trackers versus an installation with fixed tilt? What were the results?

Yes, this evaluation is currently taking place at BiTEC. Results will soon be published.

If you compare when it is better to use trackers compared to an installation with fixed tilt, does this comparison change for bifacial modules? For example, when you compare tracking for sites with high irradiation and sites with more diffuse irradiation (cloudy sites)?

It is difficult to make a blanket statement as a response to this question, but it is true that trackers offer yield gains over fixed tilt in all scenarios, and yield boosts from bifacial panels over monofacial panels further increases that overall yield gain. If you have an especially cloudy site, the benefits of using trackers with bifacial panels over using fixed-tilt with bifacial panels might offer less advantages than they would in other locations, but they will still offer a boost in yield.

As the pitch increases, the quantity of cables and ohmic losses in the power cables also increase. How does this increased loss and cost compare to the energy gains?

Although aisles are wider, reducing O&M costs, our 2x tracker configuration results in a more compact layout, reducing the DC cabling required to reach inverter locations, resulting in less voltage drop.

How much does a Soltec tracker tailored for bifacial increase the energy yield compared to a standard Soltec non-bifacial tracker, or other similar non-bifacial trackers on the market? What is the cost difference between Soltec trackers for bifacial compared to Soltec trackers for monofacial modules?

Soltec’s SF7 standard features provide for drop-in bifacial compatibility with higher mounting height, shadow-free backside and wide-aisle reflecting surfaces. In addition to intrinsically optimizing bifacial gain, the standard features enable other economic and performance benefits compared to the leading competitors.

SF7 double-wide service aisles between tracker rows increase bifacial reflected albedo capture from the ground and from solar modules of adjacent tracker rows. The absence of dampers and the above-mentioned cabling management solution DC Harness StringRunner also eliminates bifacial backside shading.

No cost increase is associated between Soltec trackers.

What is the albedo of the location of the La Silla plant that you have shown in the presentation?

An average of 55%.

One important point is the accuracy of yield prognosis for installations with bifacial modules. Have you verified with real measurements how accurate the PVsyst estimates for bifacial gains are, particularly for systems with trackers?

PVsyst needs to incorporate a lot of additional inputs currently considered in order to properly make energy assumptions for bifacial plants and 2x trackers. There is not nearly enough consideration put into the variety of mounting or tracking equipment on the market and the influence that it has on bifacial gain.

Does Soltec use its own simulation software for tracking yield estimates prior to installation, or an external software?

We use U. S. National Renewable Energy Laboratory (NREL) calculations for bifacial installations and PVsyst for monofacial.

Would you advise using backtracking technology on PVsyst nowadays, or is it not a mature simulation variant yet?

We find that it is not nearly mature enough. It only applies to sites that are perfectly flat, for example.

Backtracking is a routine that adapts the orientation of the panels, in order to minimize shading losses. Some manufacturers have started mentioning that for bifacial technology there might be a different tracking algorithm, mainly related to backtracking. The reason for that is that they have observed that inter-row reflection might give some gain versus traditional backtracking. Has Soltec observed something similar?

Yes, but results are inconclusive. This is an area of active-research within our bifacial testing site, BiTEC.

Is there any evidence that backtracking always results in more yield? Could it be that with bifacial modules diffuse light can lead to a higher yield gain without backtracking?

Could be. This is what we are trying to determine at BiTEC.

Have you tried different strategies for backtracking with bifacial modules and what sort of routine do you recommend?

This is an area of active-research within BiTEC.

Do the trackers use backtracking technology?

Yes. Most  terrains are not perfect, they usually have rolling slopes. Soltec’s 3D modelling considers the terrains' characteristics. The resulting model enables energy production maximization of 6%. Soltec’s algorithm includes the TeamTrack asymmetric backtracking control, a smart system that calculates all the shadow projections on the whole plant. With this information, Tracker Monitoring System (TMS) by Soltec, places all trackers in the most optimal positions enabling the highest energy production. TeamTrack avoids all interrow shading during sunrise and sunset. This solution was awarded by MIREC under the category Innovative Technology.

Do O&M costs increase when using bifacial modules with tracking compared to monofacial modules with tracking or fixed tilt installations (cleaning of the back side, height of the modules is increased to 2.3m)?

Yes, the O&M cost of bifacial modules maintenance can be increased by the rear side cleaning, but it would be minimal. The rear side won’t need to be cleaned so often as the front side, because the dust won’t be accumulated as much as it does on the front side. Bifacial application won’t affect the maintenance of the tracker.

Where can the combiner box on the bifacial tracker be placed?

Our DC Harness completely eliminates the use of DC Combiners. The Combiner Box is replaced with a DC Switch Box included in the DC Harness StringRunner. DC Harness is a PV string installation solution that combines and fuses PV source circuits. The traditional combiner box is replaced by tracker level fusing. Combined with StringRunner, a wire management solution which runs PV source-circuit string wiring within the tracker torque tube, reduces dramatically the costs of material, installation, and maintenance while the wiring is protected against UV radiation, weathering conditions, rodents, and insects.

Does the portrait orientation of the Soltec tracker lead to more non-uniform irradiance on the rear side?

Soltec’s SF7 trackers have a standard height of 2.35 m, while 1x-configuration trackers have a height of 1.35 m. Preliminary electric performance measures taken at BiTEC over bifacial modules reveal a short-circuit current difference of over 2.3 percent between 1x and 2x trackers.

What is the shading in percent due to the mechanical structure of the Soltec trackers?

0%.

If the pitch has to increase to get more energy gain, then there is no true land cost saving. Or is the land cost saving presented by Jinko earlier taken into consideration of the increased pitch?

No, the pitch is increased in proportion with the width of the tracker. In other words, the GCR is the same. It occupies less land per installed megawatt than other trackers with the same GCR.

Is this tracker suitable for both framed and frameless modules?

Yes, the SF7 is compatible with all module types and applications from thin film, to crystalline to bifacial.

Could you please clarify the stub height and spacing between modules (across the torque tube and along the string)?

Torque tube height is 2.2 m (7 ft), and the gap between modules to avoid backside shading is 15 cm (6 in). There is no specific gap for the string as it is placed within the torque tube.

What is the maximum span between foundations that you can install?

The distance between piles is usually around 7-8 m (22-26 ft).

Can one install your trackers on concrete foundations?

Yes, we have already made it with good results.

Can you decrease the number of piles for a specific application?

We assure a cost-effective installation with the lowest piles-per-MW spec on the market: seven piles per tracker, 225 piles per megawatt. This means 46% less piles per MW than our leading competitor.

How many motors per tracker does the Soltec tracker structure have?

One.