Fate of Bi-Facial Solar Modules in India
25th May 2020 | NOIDA | INDIA
Let’s first understand what is a Bifacial Module and how does it work?
“Bi-Facial” as the name itself says, means Two Faced, so when we talk about Bifacial Solar Modules they can be described as a device which transforms sunlight falling on both top and bottom sides to electrical energy. These Modules have solar cells on both sides with a transparent backsheet or dual tempered glass. It allows them to capture the reflected sunlight from ground and convert it to electricity which is not possible with traditional Monofacial solar modules.
Top solar cells of bifacial module work similar to that of a conventional solar module as shown in the picture below. Sunrays of certain wavelengths falling on these cells get absorbed which is then converted to Electrical Energy. A significant portion of sunrays hit ground surface and get reflected which are called Albedo rays. As Bifacial modules have solar cells at the bottom side too, it allows them to absorb these albedo rays and convert it to electrical energy.
Picture Source: Solar Choice
Types of Bifacial Modules?
Bifacial modules are made more commonly from monocrystalline cells, but polycrystalline designs also exist. Monocrystalline cells are preferred for Bifacial Modules because of their higher efficiency. As single silicon crystal is used for producing each solar cell in a monocrystalline module, electrons get more space for movement than polycrystalline counterpart where solar cells are composed of silicon fragments. This extra space result in more electricity and hence enhanced efficiency.
Bifacial modules can be classified according to the backsheet material.
- Dual Glass: In this type of module, glass is used on both top and bottom layer of the module. Owing to lower failure history, longer lasting & lower degradation loss of 0.5 per cent (0.7 per cent in the case of polymer back sheet) it is usually preferred by larger supplier section. The reasons behind durability are;
- Higher Rigidity: It safeguard modules from any damage during handling or installation. It also ensures higher strength against wind, snow and other loads which may cause significant damage.
- Lower Permeability: Lower porosity protects it from rain and other environmental conditions making it more durable.
- Glass on Polymer: Backsheet of such modules are made of polymers. This design definitely has some drawbacks but it also offers certain advantages over the Dual Glass Module.
- Electrical Insulation: Polymer backsheet provides electrical insulation along with enhanced sealing from moisture and UV light.
- Light Weight: Use of Polymer makes module lighter making it more favorable for transportation, handling and installation.
- Lower PID Losses: Data shows that this design has lower Potential Induced Degradation (PID) losses than Dual Glass design.
Why Solar Industry is Excited about Bifacial Modules?
A solar module’s efficiency is defined as its ability to convert solar energy which falls on its surface into electrical energy. It may be thought for a while that Bifacial modules will have doubled efficiency when compared to Monofacial modules as a Bifacial module has double numbers of cells, but it is not the reality.
Study shows that the efficiency may rise by 11% as compared to conventional counterpart. Obviously, the main reason for lower gain is loss of solar energy during reflection and its impingement to the solar cells. However, some manufacturers claim to have achieved gain of as high as 30% by installing bifacial modules on a highly reflective surface such as white thermoplastic polyolefin roof. A reflective surface enables the sunrays to bounce back and enter the module for a second time and generate electricity.
Such a high gain is sufficient to make this a burning topic.
What are the major factors affecting Performance of a Bifacial Module?
- Albedo Value: Albedo is defined as the ratio of reflected light to incident light. It is calculated as a percentage or decimal value, with 1 being a perfect reflector and 0 corresponding to a black body which absorbs all radiation light incident. Thus, it can be said that a highly reflective surface has a high albedo value which simply means installation on such a surface will have a higher yield due to more irradiation impinging the back side of PV module.
Below is the general value of albedo for different surfaces:
Surface of Installtion |
Albedo Value |
| Highly Reflective Surface | 0.9 |
| Fresh Snow | 0.8 |
| Ocean Ice | 0.5 - 0.7 |
| Concrete | 0.16 |
| White Painted Concrete | 0.6 - 0.8 |
| White Gravel | 0.27 |
| White Roofing Metal | 0.56 |
| Light Gray Roofing Foiln | 0.62 |
| White Roofing Foil | 0.8 - 0.9 |
| Desert | 0.35 |
| Green Grass | 0.23 |
| Bare Soil | 0.17 |
| Asphalt | 0.1 - 0.25 |
| Water | 0.08 |
| Earth | 0.2 - 0.4 |
- Latitude of the Location: Ideally, a bifacial module produces highest electricity when vertically oriented. It is because of the maximum absorption of reflected sunrays. In the light of this fact, it can be easily understood that optimum tilt for a bifacial module is greater than its conventional counterpart. Higher the latitude, higher will be the generation of a bifacial module placed at same tilt than other Monofacial module.
- Module Mounting Height: Lesser the module clearance from ground or roof surface, lower is the chance of reflected light to reach the rear solar cells. Analysis using SolarPro suggests that module clearance of 18-36 inches is best suited for bifacial modules and sufficient to boost the output.
- Combination of Latitude & Albedo: An analysis was published in Elsevier in December 2018, which shows the effect of Albedo & Latitude on the financial viability of bifacial modules with respect to conventional solar modules. As per journal “To determine which module technology would be more favorable worldwide, a total of 55 locations around the globe are analyzed considering their weather profiles, market situation, and module electrical performance for 1 MWp systems”.
Below is the summary of analysis;
(Table 1)
Monofacial AMO vs Bifacial AMO (Any Module Orientation)
Latitude |
Albedo |
Viability |
| <40 degree | <0.12 | Monofacial Solar PV Module |
| <40 degree | <0.12 | Monofacial Solar PV Module |
| <40 degree | >0.12 (depending upon location) | Bifacial Solar PV Module (may be) |
| <40 degree | >0.30 | Bifacial Solar PV Module |
| >40 degree | Any Value | Bifacial Solar PV Module |
(Table 2)
Monofacial AMO VS Bifacial VMO (Vertical Module Orientation)
Latitude |
Albedo |
Viability |
| <65 degree | <0.29 | Monofacial Solar PV Module AMO |
| <65 degree | >0.29 - 0.57 (depending upon location) | Bifacial Solar PV Module AMO (may be) |
| >65 degree | Any Value | Bifacial Solar PV Module AMO |
(Table 3)
Suitability of Bifacial Modules for Different Types of Installation
Surface of
Installation
|
Type of
Installation
|
Suitability for Bifacial Modules
|
Comments
|
| Rooftop | Flush Mounted | Not Recommended | Insufficient space b/w module & surface for reflection |
| Rooftop | Elevated/High Clearance | Can be installed but with checks in comments section | 1. Minimum clearance shall be 18 inches 2. Roof surface should have high albedo |
| Ground | Elevated | Suitable | 1. Minimum clearance shall be 18 inches 2. Installation surface should have high albedo |
| Wall | Vertical | Recommended | More impingement of reflected solar rays |
| Water | Floating | Recommended | Double layers of glass reduce micro cracks, snail trails, UV aging, and mitigate vapor penetration |
Future of Bifacial Modules in India
Undoubtedly, Bifacial modules have higher efficiency than Monofacial modules but they come at a higher cost. A bifacial module works well when its rear side is unobstructed. A conventional racking system may cause shadow to the rear cells and therefore a different & special MMS is to be used which is expensive. Yield to Cost ratio varies for different types of installation which is summarized below.
India is situated on north of the equator between 8°4′ N to 37°6′ N latitude. As explained in Table 01, Bifacial modules in India will be beneficial only if it is installed on a surface having albedo greater than 0.12.
- Utility Scaled Ground Mounted SPV: In general, the terrain in India where installation of a solar plant is getting carried out has an albedo greater than 0.12 and therefore bifacial modules will be more cost effective in case of large ground mounted solar installation due to reduction in per Wp cost of Module Mounting Structure and other BOS.
- Floating Solar PV Systems: Gain due to albedo rays is not very significant but still Bifacial Modules may be a better choice due to their durability.
- Residential & Small-Scale Commercial Projects (Rooftop): Use of special MMS & creating a reflective surface for installation becomes economically less viable for smaller systems due to higher cost of bifacial modules. Given the scenario, the best way to get more energy is to install more Monofacial modules rather than using bifacial one. However, with revolutionary technical advancement and price reduction of bifacial modules, we can expect a viability very soon.
About Author
Naresh Kumar is a tech enthusiast having rich experience in Solar PV System Designing. He is available on
[email protected]
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