Bifacial photovoltaics – The Future has two sides

Solar modules are aligned to or even track the sun. The rear of standard solar modules only receives a small fraction of the energy captured by the front of the module. Nevertheless, the back surface can contribute 5% to 30% of the overall energy balance. The word “bifacial” keeps cropping up nowadays in the photovoltaics field and heralds an emerging trend. Bifacial refers to a solar cell or solar module that is light-sensitive on both sides. This technology is not fundamentally new; bifacial modules have been in use since as early 1994 for applications such as noise barriers on freeways. The fact that bifacial technology is currently enjoying a marked growth in interest can be attributed to several reasons. In general, module manufacturers are keeping up their endeavors to boost module performance and achieve added value.

The underlying thinking is simple: as direct sunshine comes from one direction, the initial focus was on monofacial modules, that is to say modules sensitive on one side only. To date, this has proved entirely satisfactory for most applications because even in the early days of PV, roofs were seen as the primary location for photovoltaic installations. Roofs obviously require some form of cover; they are not as a general rule shaded and there is no solar radiation from the back surface. Single-sided (monofacial) “solar shingles” or solar modules are thus satisfactory in this situation.

Large-scale PV proliferation in the USA, Japan and Germany is in regions where the light is typically 70% direct, approx. 20% diffuse and only approx. 10% from the rear. At the present time, however, a growing number of PV installations are being erected in sunbelt regions such as India. In these regions, the ground frequently exhibits higher reflectance – the so-called albedo. The reflected radiation component therefore becomes more interesting for photovoltaics in these regions. The applications for photovoltaic installations are becoming more diverse:

A1) Normal slanted or tracker installation:

it is important that the modules are not unnecessarily shaded at the back by the mounting systems or cable guides and that the mounting height is not too low. An interesting application lies in floating PV systems, in which the PV modules are mounted on swimming pontoons.

This enables the high reflectivity of the water to be utilized. In addition, the shaded areas beneath the modules provide an agreeable environment for fish because it is cooler there.

In this way, areas of water can be utilized simultaneously (dual-use application) for fish farming and for electricity generation by means of PV systems.

 

A2) Horizontal mounting lends itself to elevated installation variants, for example over usable areas such as fields, for carports and to provide shading.

This mounting variant has advantages in windy regions, for example. The modules themselves do not form continuous surfaces, but are mounted with gaps.

 

A3) Vertical mounting:

Dirt tends not to stick to the module and is washed off very easily by rain.

However, if monofacial modules are mounted in this way in latitudes of up to 45°, the energy yield is greatly reduced. The situation is different with bifacial modules in an east/west configuration.

In desert regions with a high albedo and a serious soiling risk, vertical mounting may be a good option for solving the cleaning problem while attaining the same energy yield.

Noise barriers and dividing walls as well as similar applications in the outdoor area are ideally suited for combination with solar modules. If the modules are mounted in a very high position, plants can also be grown beneath them (agro-photovoltaics). Vertical mounting is another option for dual-use applications.

Bifacial modules have around 3% lower output from the front surface because the light passes through the module between the cells and cannot be reflected by the white backsheet as is the case with monofacial modules. The unit of measure according to which PV modules are costed is Wp (Watt peak). This makes sense because the energy delivered by a monofacial PV installation is roughly proportional to the installed capacity and the balance of system costs are also largely determined by the installed capacity. For this reason, module manufacturers strive to maximize the performance of their PV modules. As the market grows, however, competition becomes more intense and manufacturers exploit niche markets with better prices.

The vast majority of PV installations are erected in an industrial setting, though, where large industrial roofs, for example, or large expanses of land are covered; the energy generated there is frequently traded on the free energy market. This means that in addition to the system costs expressed in USD/Wp, the electricity generation costs of the installations (levelized cost of electricity) are also growing in importance.

Today’s improved cell technology makes it possible to dispense with the back surface metallization of solar cells without performance losses, thereby creating the prerequisite for bifacial cells. Compared to monofacial systems, bifacial systems generate significantly more energy at the same specific capacity. Measurements show an energy gain of 10% to 30% for the same specific installed capacity. Interest in bifacial solutions is growing on account of the concurrent growing demand for solar systems in desert-like areas with very high solar radiation and very high ground reflectance.

Even if the increased demand for bifacial technologies is driven first and foremost by applications in the regions described, PV installations in other regions are also reaping the benefit of the resulting accelerated development and production of bifacial solar modules. Bifacial modules will in future dominate the field of building-integrated installations as well as other combination applications such as agro-photovoltaics, shading or carport solutions.

Comparison of energy yield: monofacial versus bifacial modules In order to compare the technology, measurements were taken on individual modules of the different technologies at a single (hot and sunny) location. In the case of the bifacial modules, the mounting system was installed in such a way as not to shade the back surface. This module comparison contains all factors that might have a bearing on the energy yield: front side and back side irradiation with all spectral effects as well as the temperature. The annual average albedo of the naturally existing ground is around 24%. Derivation is only around 2%. Therefore, this comparative measurement is extremely accurate and much more reliable than currently available simulations. Meyer Burger’s HJT/SWCT (heterojunction cells with SmartWire Connection Technology) modules achieved an average of 37% more specific energy than a monofacial standard module (Al-BSF) and an average of over 12% more than a bifacial PERT module from Tier1 manufacturers.

 

Until recently, all PV installations have followed the same daily pattern: virtually no energy mornings and evenings, maximum feed-in at midday. Bifacial systems can overcome this pattern with an east/west configuration: the east side is active in the morning, the west side in the evening. At midday, when the sun is directly above the solar module, the latter receives the lower, diffuse solar radiation and feeds in somewhat less power. This enables energy to be offered at higher prices at peak load times and avoids clipping at midday when there may already be enough energy in the grid. The cost breakdown for typical largescale PV installations is as follows:

• Module: 47%
• Mounting System: 22%
• Inverter: 19%
• Ground Preparation: 7%
• AC-Connection: 5%

The solar module is still the biggest cost factor, although today it accounts for less than half the total system costs. At the same time, the module offers the greatest potential for optimization, whereas relatively insignificant savings are possible on the mounting system, for example.

Even with monofacial systems, 10% to 20% more energy can be obtained by means of tracking the sun. When tracking using bifacial modules, yet a little more specific energy is generated, since these modules capture the diffuse backlight very well. Here tracking optimizes exploitation not only of the direct sunlight on the front, but also of the diffuse light on the back surface. This is in the order of 3%.

Even if a module manufacturer adds the 3% reflectance losses to the price for the bifacial module, meaning that the system costs for the bifacial system are slightly increased, this can be compensated for by using tracking systems to achieve a higher energy yield. Added to this is the aforementioned energy gain of around 10% to 30%.

Up to now, all-over back surface metallization (Al-BSF) has been applied in the manufacture of solar cells, thereby avoiding electrical series resistance losses on the back surface. The back-surface field improved the efficiency of the solar cells and the metallic back surface enabled a degree of light reflection. The industry is currently experiencing a radical upgrade from Al-BSF technology to PERC (Passivated Emitter and Rear Cell) technology. First of all, this is normal technical progress, because existing systems are enhanced with 2 additional process steps and are thus able to produce solar cells with an output that is 1% higher in absolute terms or 5% higher in relative terms. The important feature is the initial all-over passivation of the cell back surface. Here, all-over back surface metallization is no longer required. The di-electric coatings applied for the purpose of passivation also act as an internal mirror that reflects the long-wave front surface light within the cell. These cells can be manufactured both with all-over metallization and with a finger grid on the back surface. Although the so-called bifaciality factor (front surface output to back surface output) amounts to only around 60% to 80% with this type of cell, the lower metallization reduces costs in cell production.

An example for an upgrade from Al-BSF to PERC with Meyer Burger systems:

To date, Meyer Burger has upgraded more than 30 GW of production equipment to PERC.

Left: PERC process (upgrade with 2 production tools)

Right: Complete PERC production line (A: Wafer inspection; B: Saw damage removal/Texturing; C: Diffusion; D: PSG removal; E: MAiA back side deposition; F: SiNA front side deposition; G: Laser; H: Metallization; I: Test & Sort

These systems can also be upgraded with few changes to the bifacial PERC process (PERC+):

This requires a minor amendment of the production formulation for coating the solar cell in order to increase transparency, as well as a change in back side printing from all-over print to a finger grid. Both can be carried out very quickly using existing production facilities; no additional equipment is required.

The existing facilities are also highly suitable for future upgrade cell technologies such as the combination of n-type wafer material with passivated contacts, including bifaciality. This conversion is extremely costeffective and enables bifaciality factors of approx. 70% to 80%.

PERC+ cell structure

New types of high-efficiency solar cells (e.g. heterojunction cells) are to some extent symmetrical in their design and thus already bifacial. In addition, passivation of the wafer surface enables even better efficiencies, very high bifaciality factors and high cell voltages with very good temperature coefficients.

One challenge is the competitiveness of new technologies compared to products currently on the market. For example, the upgrade of existing production equipment to PERC only became possible when the value chain (production equipment and consumables such as tri-methyl-aluminum [TMA]) was available and customers were convinced that production was reliable. With its MAiA deposition system, of which more than 30 GW have been delivered to date, Meyer Burger ranks among the trailblazers of the upgrade boom. The MAiA production system can also be used for the manufacture of bifacial PERC cells.

An example of a fully developed technology ready for mass production is the combined HJT/SWCT technology. The example shows that a holistic approach is called for in order to achieve competitive production methods. HJT stands for Heterojunction Technology, a cell structure that has been mainly marketed by Sanyo/Panasonic for around 18 years. Up to now, the cost-effective mass production of HJT cells with a high yield has proved challenging. Like MAiA, the HELiA deposition systems and a corresponding integrated process were developed by the technology company Meyer Burger. The HELiA system enables the deposition of intrinsic and doped a-Si coatings with high uniformity and quality in a 24/7 industrial process. The average efficiencies achieved are >23% Gt, while efficiencies >24% have already been reached on the series production lines. Gt stands for “Grid touch” measurement, because these are busbarless cells. These “gt” cells can be calibrated by ISE CalLab, among others. HJT cells must be metallized with low-temperature pastes, because the a-Si coatings crystallize at conventional firing process temperatures and would thus lose their special passivation properties. These pastes, however, can only be adapted to the soldering processes forming part of the cell connection procedure with considerable effort and the use of a large quantity of paste, thus incurring additional costs. To counteract this problem, Meyer Burger already developed a connection technology 6 years ago, under the name SmartWire Connection Technology (SWCTTM). Here, a large number of thin round wires are used to connect the cells. The technology reduces the minimum contact finger conductivity required to such an extent that low-temperature pastes can be used without problem. In addition, unlike the competing multi-wire technology, SWCTTM places no requirements on the positioning of the wires. More than 1 GW of solar modules with SWCTTM will be installed worldwide until end of 2018. At this year’s Renewable Energy India show, Meyer Burger exhibited a 480 W solar module. This bifacial glass/glass module incorporates 72 heterojunction cells with SmartWire Connection Technology. According to Meyer Burger, the production costs are competitive compared to the Al-BSF or PERC standard technologies.

 

With bifacial systems, other factors have to be considered that do not need to be taken into account in the design of monofacial systems, such as ground albedo, module installation height, shading caused by the mounting system, etc.

Bifacial modules capture light from both sides, however. If such modules have a high bifaciality factor, the energy gain is markedly higher than with monofacial PV systems that have the same output. The energy gain can be maximized if a small number of design rules are borne in mind.

Albedo: If light is absorbed by rather than reflected from the ground, this means less energy on the back surface. Even grass and reddish soil have a reflectance of 10%, however. For gray-white gravel it is approx. 15% to 20%, for desert sand up to 40%, and white surfaces have an albedo of around 60%. By selecting a suitable ground, it is possible to achieve a favorable albedo and thus increase the energy yield. At the same time, vegetation or snow for example can have a seasonal influence on albedo.

A greater installation height enables the module back surface to see a larger area, that is to say to capture more light. Trials and simulations have shown minimal increase in energy gain for installation heights in excess of approx. 0.8 m. Placing the module directly on the ground creates a very uneven background lighting, however. As a result, the lower cells typically have a 10% lower maximum current than the uppermost cells in a module. For bifacial systems, this imbalance should be taken into consideration and minimized at the planning stage. Simulation programs with the capacity to do this have been developed particularly in the last two years. The light irradiation per unit of area is given. If the surface area is not a cost factor, it is more advantageous to space the module/ tracker rows a little further apart. Whereas this makes no difference in the case of monofacial systems, a slightly larger spacing achieves gains for bifacial systems because the diffuse back surface light can be utilized. During the overall installation planning, more aspects have to be taken into account for bifacial systems than for monofacial systems.

Summary

Bifacial PV systems are highly compatible with already existing PV systems and generally achieve a markedly higher energy yield than monofacial systems. At the same time, bifacial systems are competitive because the manufacturing costs for the solar cells are slightly lower and the modern passivated cell types are inherently bifacial and do not involve additional costs.

Certified production technologies for the large-scale manufacture of bifacial cells and modules are already available on the market. They include bifacial PERC (PERC+) as an upgrade or bifacial HJT/ SWCT in newly constructed production facilities.

Bifacial systems can be planned in exactly the same way as monofacial systems, with a few factors demanding extra attention, for example the properties of the reflective ground. This attention will, however, be rewarded with a higher energy yield.

Bifacial modules are opening up new application possibilities, often arising from dual use of the installation area.

All in all, bifacial modules can be employed to good advantage for most applications in terms of energy yield, dual use of areas and distribution of feed-in times, thus contributing to the ongoing reduction of energy generation costs for electricity produced using photovoltaics.