Albedo: Sun gathered from the soil

Irradiation was measured in specific tracker points at BiTEC (Figure 7). A comparison of the evolution of the curves throughout the day reveals that, the lower the height of the module, the greater the differences between the rear irradiation in the measured points. That means that East module rear irradiation mismatch is higher in the morning whereas West rear irradiation mismatch is higher in the evening.

Figure 7. Rear side energy absorption depending on tracker angles in 2P geometries. Source: BiTEC

The irradiation mismatch can be approached analyzing rear irradiation standar deviation of the rear irradiation along the module. With this purpose accurated simulations are carried out by using  NREL’s Bifacial Radiance software [2], which enables the use of measurement points all along the tracker width [3]. Results of irradiation distribution for low and high angles in SF7 Bifacial and 1P tracker is represented in figure 8. Assement of standard deviation for high angles shown in table 6 reveals that down module in SF7 2P tracker presents values in the range of 18% that 1P tracker (similar module height on this angle) whereas rear irradiation standard deviation at SF7 bifacial up module is reduced to values around 6%.

Table 6. Rear irradiation mismatch and rear standard deviation comparison at 52 angle
*Relative to total irradiation

Figure 8. Module rear side irradiation distribution with trackers in 2P Vs. 1P in different anglesand hights, with square and round torque tubes. Source: BiTEC

Figure 9. P-V curve in the MPP range per string at 17:00 h August 7th. Source: BiTEC

This irradiation deviation implies that PV cells work on different Power-Voltage curves, and may cause electrical mismatch losses when strings are connected in parallel. Table 7 shows the worst case instantaneous measurements of 8 modules-strings taken in BiTEC during a clear day and a very high inclination angle of the tracker (maximum power difference between strings). The comparison with different connection modes is carried out as follows. Modules in string 1 are Western side and string 2 are Eastern side of the torque tube, whereas string 3 connects 4 modules in the Eastern and 4 modules in the Western side. The addition of Maximum Power Point (MPP) of each string (working independently) compared to the MPP of the 3 P-V curves in parallel allows estimating worst case instantaneous electrical mismatch of 0.3%. BiTEC instantaneous measurements (figure 9) show that these condition variations between the strings involve slight modifications on the MPP Voltage, then the effect on the output power is minimim, below the labelled mismatch between modules in the same string. Long term measurements support that the agregated mismatch effect for a full month results in 0.06% energy loss for bifacial modules, whereas for monofacial modules, results in 0.04%.

4. HOW TO MEASURE ALBEDO?

The word “albedo”, which comes from Latin and means “whiteness”, is defined as “the fraction of the incident sunlight that the surface reflects” [4].

In the area of photovoltaic energy, ground albedo is evaluated as the ratio between global irradiation and reflected irradiation throughout the day. This is a non-dimensional number smaller than “1”, with “1” being total ground reflection and “0” total absorption. It is usually expressed as a percentage [5].

Solar albedo depends on directional irradiation distribution and surface properties. Typical surface albedos vary from 4% for fresh asphalt, 25% for green grass and up to 80-90% for fresh snow [5].

4.1. Albedometer measurements

Using an albedometer is the most reliable method to measure ground albedo. An albedometer is an instrument measuring global solar irradiation and reflected irradiation to calculate the actual albedo. An albedometer is comprised of two pyranometers installed horizontally, with the one facing down measuring reflected solar irradiation. [7].

4.1.1. Factors to be considered when using albedometers

Certain factors need to be considered when using albedometers in the solar field.

1. 1. Selecting the measurement point
      The location where albedo is measured should be representative of the entire plant. That means that albedo should be measured where ground texture and color best represent ground conditions, avoiding areas with large rocks or where grass grows irregularly. Since measuring in angled surfaces leads to error, it is important to measure albedo in a terrain as horizontal as possible, ensuring no shades exist during the day measurements are taken. In the case of heterogeneous terrains, it is best to use multiple measurement points.
   2. Selecting measurement times
      Using an albedometer to measure albedo implies a certain dependency on the Sun’s angle of incidence, as seen in Figure 14. Thus, it is best to measure albedo around midday (± 2 hours), discarding the rest of the day and limiting the shading effect.

Figure 13. Hourly pattern of albedo measured using an albedometer. Clear day, White ground. Source: BiTEC

4.1.2. Factors to be considered when using pyranometers

It is also necessary to take into consideration factors affecting pyranometer measurements and, in turn, albedometer measurements.

1. 1. Height
      Similarly to bifacial module rear irradiation, height is a determining factor that affects albedo measurements. If the pyranometer is placed too low, a smaller area would be measured and its own shade would interfere. If the pyranometer is placed too high, measurements would be affected by diffuse irradiation, especially in low albedo areas. A height ranging between one and two meters is considered ideal. In areas with vegetation, it is important to ensure the grass is short. In the case of snowy regions, a mechanism for adjusting pyranometer height should be available to ensure pyranometer distance to the snow remains constant. Access to the pyranometer for balancing reasons should be possible without altering the surface area below, especially if it is snow-covered.
   2. Periodic Recalibration
      Device design should ensure a measurement error of less than 2%. Pyranometers should be recalibrated every one to two years, following manufacturer instructions.
   3. Cleaning
      Continuously operated pyranometers should ideally be inspected at least once per day, increasing inspection frequency based on specific factors, such as weather observations. During inspections, pyranometer glass protections should be carefully cleaned and dried to prevent routine measurement distortions. Frozen snow or frost should be carefully removed using deicing fluid before the glass is cleaned. Daily inspections should contribute to ensuring the device is leveled, no condensation is present inside the dome and detection surfaces remain black.

4.2. Measurements Using Satellite Data

Albedo can also be measured using satellite data. Extraterrestrial albedo databases are available [10]. In fact, some models use measurements taken at a distant place, considering that horizontal radiation is not affected by shading at the measurement point.

The advantage of satellite albedo is that it can be used as confirmation measurement for average and maximum albedo values, in addition to offering an idea of albedo evolution. However, its main inconvenience is that extraterrestrial albedo may differ significantly from solar albedo in a specific lot, which is a key parameter to estimate bifacial module power generation in a photovoltaic plant. Furthermore, extraterrestrial albedo provides macroscopic area measurements, with no specific information on the lot housing the photovoltaic project.

To obtain more precise measurements, some specialized companies implement hybrid models. On the one hand, local albedometers are installed to know specific terrain albedo values over a relatively short period. Later, measurements are correlated with historical satellite data, obtaining a rather accurate long-term evolution forecast.

ALBEDO (1 MONTH)

Figure 14. Extraterrestrial Albedo Map (on a per-month basis). Source: NASA. https://neo.sci.gsfc.nasa.gov/view.php?datasetId=MCD43C3_M_BSA

4.3. Databases

Considering the relevance of albedo for photovoltaic projects with bifacial modules, a number of initiatives to draft albedo value maps are being developed. One of these initiatives is being carried out by NREL, which aims to generate a worldwide database for albedo values [11].

4.4. Albedo measurement at a solar plant

Some tracking system trends measure albedo in the rear side of the tracker. This is known as rear albedo, which is basically the ratio between front side irradiation and rear side irradiation. A pyranometer measures irradiation in the plane of its sensor. To measure module rear side irradiation, the pyranometer should be aligned to the photovoltaic panel rear side.  Such measurement cannot be extrapolated to other module distribution geometries because rear albedo is conditioned by various aspects:

• Differ from values in the ends.
• Variations throughout the day, depending on local shading patterns.
• Variations across the seasons, which condition vegetation growth and shading patterns.

Additional inconveniences may affect tracking systems when East-West slopes and tracking algorithms are present.

BiTEC has compared ground-measured albedo and measured tracker rear side albedo, with results showing a considerable variation depending on the type of measurement.

Figure 15. Ground-measured albedo vs. measured tracker rear side albedo Source: BiTEC