Bifacial solar PV modules give increased power output potential

Both mono and polycrystaline solar cells are reaching the limit of their efficiency with current technology and no major improvement is expected in the current family of products. A new development, the bifacial (BF) module, offers to increase the power output of solar modules  without major efficiency improvement, by using reflected and diffuse radiation on the reverse side of a module.

Historically, bifacial solar cells were targeted towards building integrated PV applications or in areas where much of the available solar energy is diffuse sunlight which has bounced off the ground and surrounding objects, i.e. extreme latitudes and snow‑prone regions. However, the combination of plateauing peak efficiencies from standard screen-printed solar cells and significant reductions in the cost of solar glass in recent years, making the use of DG encapsulation viable, has pushed bifacial solar modules back into the spotlight [2].

The aim of BF technology is not to increase the efficiency of the solar module or panel but to capture more solar energy per module. Gains of up to 30% are projected, depending on several factors, such as the reflectivity of the ground surface, height above ground, tilt angle and several others. The radiation received by the module consists of several components:

• Direct radiation from the sun
• Indirect diffuse radiation caused by air particles, clouds, etc.
• Reflected radiation from surfaces close to the solar module

Reflected radiation is generally not taken into account in solar energy calculations. Diffuse radiation measurements refer to radiation sources above the horizontal plane. The usual method of measuring solar radiation makes use of a pyranometer which is horizontally mounted and only measures radiation above the horizontal plane. Even in a tilted configuration the pyranometer will not measure radiation below the plane of measurement (Fig. 1).

Diffuse radiation can contribute a substantial amount of the total radiation, but much of this will not be captured in a tilted or horizontally mounted module . Tilting the module increases the intensity of the direct radiation but blocks off a large portion of the indirect radiation . Diffuse radiation is isoptropic in nature, i.e. has the same value irrespective of the source, whereas reflected radiation will depend on the nature of the surface surrounding the solar array, the angle of the array and other factors. The front panel will receive both direct and diffuse radiation, the ratio depending on the tilt angle of the panel.

The rear side of the module will receive light from two sources:

• Near field scatter: reflected direct and diffuse radiation
• Diffuse radiation: non-reflected radiation directly from diffuse sources

Different surfaces reflect light at different rates, and the  reflective properties are described by the albedo factor . The albedo describes the reflectivity of a non-luminous surface — it is determined by the ratio between the light reflected from the surface and the incidental radiation.

 Table 1 gives some values of albedo measured [2] 

Table 1: Albedo values for various surfaces [4].

Surface type	Albedo
Green field (grass)	10-25%
Concrete	20-40%
White painted concrete	60-80%
White gravel	27%
White roofing material	56%
Grey roofing membrane	62%
White roofing membrane	80%
Sand	20-40%
White sand	60%
Snow	45-95%
Water	8%

The ratio of diffuse light to direct light will vary with conditions. Under low radiance due to cloud the percentage of diffuse light will be higher than under sunny conditions and hence the gain compared to monofacial PV can be higher  than under sunny conditions [5].

Construction of BF modules

Cell construction

Monofacial PV cells are usually constructed with a reflective layer on the rear face of the cell, to allow better absorption of light falling on the front surface. Photons which do not get absorbed in the front layer can be absorbed on the return trip, thus increasing the efficiency of the cell.  This means that photons travelling in the opposite direction to normal can generate electricity, and if photons falling on the back face can be allowed to enter the cell they can be used effectively to generate electricity. This is achieved by removing the reflective layer, which also acts as a conductor, and  it is not possible to remove it completely.

Reduction of the conductive layer at the rear of the cell increases the resistance and more conductors are required at the rear of the cell than at the front to compensate for this. This reduces the area of the rear of the cell available to radiation.

The construction of the different types of PV cell is are more complex than that shown, and the conversion is not quite as simple. There are other steps required to make a BF cell which  works efficiently, and several designs have emerged which make use of the BF principle. Most involve modification of existing cells, but there are several which have been designed specifically as BF cells.

Two types of Bifacial cell construction are in common use in the market: the heterojunction and the passivated emmiter rear cell (PERC). Heterojunction cells use  monocrystalline silicon while the PERC cell is available in both  mono- and polycrystalline silicon versions. Bifacial cells are more complex to manufacture and this adds to the cost of the module.

The efficiency of the rear illumination is lower than the front illumination, as shown in Table 2. This is largely due to the increased area occupied by conductors on the rear of the cell compared to the front.

Table 2: Front and rear efficiencies of several BF solar modules [1].

Product	Front efficiency%	Rear efficiency %
ISFH	21,5	16,7
Jinko solar	20,7	13,9
Longi solar	21,6	17,3
Big sun energy	20,7	13,9

Module construction

MF crystalline silicon panels are usually encased in opaque encapsulant at the rear, and this method cannot be used with BF systems, which results in the double glass encapsulation method which has other advantages as well. The module needs to have transparent back and front surfaces that provide mechanical strength. In addition the cells need to be enclosed in a layer of protective material. The most common configuration adopted is a double layer of photovoltaic glass enclosing the cells which are encapsulated in a protective polymer material.

To allow light to shine on the back-side of a bifacial cell, either a UV-resistant transparent backsheet material or an additional layer of solar glass is required .  In most cases, as shown in Fig. 4, manufacturers have opted for a glass-on-glass package that generally improves field durability as compared to glass-on-film options. The glass-on-glass package is more rigid, Which reduces mechanical stress on cells during transportation, handling and installation, as well as stress due to environmental conditions such as wind or snow. The configuration is also less permeable to water, which may reduce annual degradation rates. Bifacial modules are frameless, and eliminating the aluminum frame effectively reduces opportunities for potential-induced degradation (PID) [3].

Dual glass (DG) mounting has a number of advantages:

• Reduction in microcracking, delamination and moisture corrosion
• Lower cell temperature
• No potential induced degradation as there is no metal frame which required earthing
• Lower degradation rate
• Higher flameproof rating
• Higher mechanical strength and less flexing

Market products

Table 3 lists some of the BF systems available on the market at the moment with their characteristics.

Table 3:  BF solar PV module  characteristics.

Product	Type	Rating (Wp)	Efficiency at zero BF gain (%)	Efficiency at 30% BF gain (%)
Jinko solar Eagle Dual 72	Polycrystaline	315	16,13	20,969
Canadian Solar BiKu	Polycrystaline	350	17,54	22,8
JA solar JAN60D00	Monocrystaline	290	17,3	22,49
Trina solar Duomax	Monocrystaline	285	17,2	22,36
Yingli Panda 144HCF	Monocrystaline	360	17,6	22,88

Peformance parameters

Several parameters are used in the industry to describe the characteristics of BF solar modules.

Bifaciality factor

This is the ratio between the rear side and front side efficiencies, or the ratio of the front to the rear power measured under standard test conditions (STC).

Bifacial gain

This is additional power gained from the back of the module compared to the power from the front of the module at standard test conditions. The bifacial gain depends on the mounting (structure, height, tilt angle etc.) and the albedo of the ground surface.

Bifacial gain = (??? − ? )/???

Where:

YB_i = The power from the BF module

YM_o = The power from an MF module under the same conditions.

Albedo

This is the ratio of light reflected from a surface to the incident light and varies with different surface types.

Ground coverage ratio

This is the ratio of the ground area covered by the PV modules to the total ground area occupied by the installation. This ratio has an effect on the reflected light and can influence the performance of the BF panel.

Optimum mounting of BF modules

Since bifacial modules absorb solar radiation from both sides, they allow a variety of tilt and installation options and are ideal for elevated ground, rooftop, desert and snowy area installations or on-water applications. Mounting systems designed to optimise the backscatter and reflection from roofs and ground mount installations raise the structure above the ground or roof to capture more scattered or reflected light.

Structure height and spacing  

Raising the structure above the ground increases the amount of radiation reaching the back of the panel and hence improves the performance and bifacial gain. Increasing the spacing between rows also improves the bifacial gain.

The gain increase seems to flatten out at a height of about 1 m. Increasing the height of the structure has a very pronounced effect on roof mount arrays, especially where flat roofs are involved. The danger of increased wind loading can be a problem. Several mounting structure manufacturers have produced elevated structures for both ground and roof installations.

The gains obtained with increased height can be put to good use in open shed type structures such as parking lots and open air storage bays, as well as entertainment and hospitality areas. The transparent encapsulant allows some light to filter through the module.

Vertically orientated BF panels

One of the most interesting applications to emerge from the BF array is the possibility of a vertical mounted array. Vertically mounted BF have been used effectively in the past as sound and light barriers on highways. A vertically mounted panel occupies much less space than a horizontal or tilted panel. Two options exist, classical north/south orientation and alternative east/west facing.

In order to better match onsite demand with PV generation profiles throughout the day, there is a trend to use an east-west panel orientation, where half of the panels are tilted eastwards to create a generation peak in the morning and the remaining half is tilted westwards to also allow for a generation peak in the afternoon. This “double-peak” profile can better match onsite electricity use, especially for residential and commercial installations.

This unconventional approach can go one step further if vertically mounted E/W facing bifacial modules are used, which would more than halve the number of modules needed for an equivalent installation. This configuration would again produce two generation peaks but would also benefit from the additional diffuse light entering the module from the indirect . The use of BF panels allows a vertical mounted E/W orientation with potential to give a higher energy production than monofacial panels.

In the N/S orientation the front panel receives direct and diffuse radiation and the rear of the panel diffuse radiation. The E/W orientation, with opposite sides facing east and west, both sides receive direct and reflected radiation at different times of the day (Fig. 7). At first site the mounting method seems to be inefficient, as at midday, the sun is at right angles to the panels and there should be no output. Significant output is due to the fact that both the front and rear surfaces are receiving the maximum amount of diffuse and reflected radiation.

The radiation received by a module will depend to a large extent upon the reflectivity (albedo) of nearby objects and the ground. This is particularly important for vertical modules around noon in summer, when direct beam sunshine is most intense but when the angle of the sun means that the direct beam sunshine received by the modules is relatively small. A vertical bifacial panel reduces dust and snow accumulation and provides two output peaks during the day, with the second peak aligned to the peak electricity demand.

One of the reasons for greater energy production is that the temperature of the east/west module is lower during the time of maximum irradiance as compared to the south oriented module.  Many networks with a high penetration of solar suffer from a surplus of energy during the midday peak production times and a shortage during the off peak period . Shifting the peaks by using vertical mount E/W orientation for new PV gives a more even energy production curve (Fig. 9).

Future prospects

Although there are several projects using BF modules, the percentage of BF modules in the market is very small at the moment but is expected to increase significantly in future as more and more products come onto the market and more installations take place. The possible improvement of up to 30% in output is expected to be far more attractive than the few percentage points increase in efficiency that might be achieved with technology development.

References

[1] T Dullweber, et al: “Bifacial PERC+ solar cells: status of industrial implementation and future perspectives”,  bifiPV2017 workshop, Konstanz, October 2017.
[2] W Herman: “Performance characteristics of bifacial PV modules and power labelling”, bifiPV2017 workshop, Konstanz, October 2017.
[3] D Brearly: “Bifacial PV Systems”, Solarpro magazine  Issue 10.2, Mar/Apr ’17
[4] Solarworld: “How to maximise energy yield with bifacial technology” ,White paper SW9001US 160729
[5] EPRI: “Bifacial solar PV modules”, www.epri.com

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