Mono vs Poly Solar Panels; Choose the Right Technology for your upcoming Solar Rooftop Projects.

Table of Contents

Executive Summary

In this study, we make a comparison between a mono PERC (Passivated Emitter and Rear Cell) plant and a poly module solar rooftop installed at the same roof and running at an almost similar capacity. The debate around the use of Polycrystalline vs Monocrystalline solar panels is ongoing for the rooftop solar sector. Mono PERC modules are costlier, although it has better performance.

In this study, we present an analysis using the generation data provided to us by Chennai based SolarQuest LLP which does EPC and RESCO projects across India with a portfolio of more than 100 operational projects. 

Challenge 

The challenge is that it is hard to evaluate whether Mono PERC makes sense from a practical standpoint, even though it has been well proved that Mono PERC has higher efficiencies and energy generation.

We attempt to complete an analysis showing the difference in the generation and on the basis of cost showing the financial payback for both technologies. By the end of the study, we trust you will be able to make your own comparison for your projects and can use this study to substantiate to your customers which technology makes better sense for your project 

Brief: Monocrystalline vs Polycrystalline Cells

Monocrystalline Solar Cells

Monocrystalline panels are manufactured from a single silicon crystal structure called an ingot. These panels are solid in their colour and have aesthetic appearances as shown in figure 1.a. Monocrystalline solar panels are made through the Czochralski method, where a silicon crystal ‘seed’ is placed in a container of molten silicon. The seed is then slowly drawn up with the molten silicon forming a solid crystal structure around the seed, called an ‘ingot.’ The ingot of solid crystal silicon that is formed is then finely sliced into a silicon wafer. This is then made into a cell2

Figure 1. Solar modules used for rooftop projects.

Mono PERC solar cells

Passivated Emitter and Rear Cell (PERC) uses dielectric passivation film on the rear surface of the cells as shown in fig. 1 b. Thus, in PERC cells, the front surface absorbs sunlight while the rear surface absorbs the scattered or reflected light. This results in a higher generation than their traditional counterparts due to higher light absorption and internal reflectivity. When we combine two of the best technologies in the market, they supply even higher energy efficiency of up to 22%.

Polycrystalline solar cells

Polycrystalline panels also start as a silicon crystal ‘seed’ placed in a container of molten silicon. However, rather than drawing the silicon crystal seed up as with monocrystalline, the container of silicon can cool. It is the cooling that forms the distinctive edges and grains within the solar cell2 as shown in fig. 1 c.  

Parameter  Monocrystalline Polycrystalline 
Efficiency Higher efficiency rates about 15 – 20% Lower efficiency about 13 – 16% 
Space factor The higher efficiency enables them to deliver better performance in given space. Therefore, very space efficient. Their lower efficiency rate makes them poor in their performance hence require more space. 
Heat Tolerance Performance suffers as temperature goes up but better than polycrystalline panels. They have lower heat tolerance than mono modules and perform slightly lower than them at high temperatures. 
Lifespan Their performance carries for the long run than polycrystalline modules. Their lifespan is lower to that of monocrystalline modules. 
Aesthetics Their uniform and black color makes more attractive. They tend to be less aesthetically pleasing since they have non-uniform patter and speckled blue color 
Cost Cost is higher than the polycrystalline modules. Cost is lower than mono crystalline modules. 
Table 1. Differences between Mono crystalline and poly crystalline solar cells

Solution

At The Solar Labs, we provide software for solar PV design to simplify sales and design process in rooftop solar installations. Through thousands of installations processed through the software and around 100 MW of technical consultancy, we see growing usage of Mono Perc modules.

SolarQuest LLP is a Chennai based company doing EPC and RESCO projects across India with a portfolio of more than 100 operational projects. We have a inverter monitoring data from the plant and a TSL Studio software simulated annual generation report as the base for our analysis.

The details are provided for enhanced understanding of the project. 

Project details

The name of the project is 285 kWp (DC)/240 kVA AC (Alternating Current) SOLAR PV POWER PLANT, set at the location of 13.035477°N, 80.211188° E installed by SolarQuest in Chennai. The plant is running since January 2020. 

Design

The design layout of the plant installed is portrayed in figure 2. Both the type of modules is installed on the same site so that they experience similar radiation and weather pattern.

Figure 2. Mono PERC and poly PV plant layout. 

Single Line Diagram (SLD)  

The Single Line Diagram (SLD) for the plant is shown I figure 2. here, the electrical layout of the plan is depicted well. Inverter 1 has Mono-PERC (Passivated Emitter and Rear Cell) modules and Inverter 2 has Polycrystalline modules connected to it. On comparing these distinct inverters and plant performances, we produce a term ‘Specific Generation per Rupee Invested’. This term takes into account both generation and financial considerations.

Using this term, you can compare the values and see the delta in the specific generation per additional rupee invested in Mono PERC modules. If the value is greater for mono PERC, go for mono PERC. If the value is greater for Polycrystalline, you should ideally opt for Polycrystalline technology to ensure highest ROI (Return Of Investment) for your customer. 

Figure 3. SLD layout of the plant. 

Technical information of the Modules and Inverters used in the plant

We provide the datasheet of modules and inverters used.

Mono PERC module datasheet

There are 356 modules of mono PERC (370Wp) installed in the PV layout of the rooftop plant. 

Polycrystalline module datasheet

There are 403 modules of polycrystalline modules (330Wp) installed on the PV layout of the rooftop plant. 

Inverter specifications

Two inverters are installed with each of 110v each and 18 and 19 strings of mono PERC and polycrystalline modules, respectively. 

Inverter specifications

Generation Data

We collected real-time generation data from SolarQuest for the first 6 months of plant life. Since the complete annual data was not available, we then extrapolated data for annual production using the future trend obtained for a similar model designed in the Solar Labs software and create the following table.

Table 2. Monthly and annual generation of mono PERC and poly module plants. 

Key Performance Indicator (KPI) from this case study

The key feature that is considered for decision making in this case study is the ‘Specific Generation per Rupee Invested.’ It quantifies the yield of specific generation per kW of the amount spent /Capital investment per kW. We can see that the plant that has mono PERC panels has the Specific generation per rupee value is higher than that of the modules made of polycrystalline cells. This depicts the increased performance from Mono PERC modules for a long run. 

Analysis 

The analysis is done based on the performance of both the plants and the key features are given in the table below, 

Table 3. Performance Analysis

 Calculation

Before diving down to calculation, let us understand the terms involved in measuring the plant design and performance. 

Measure Criteria Definition 
Specific generation Quality of equipment used and impact of weather. The total annual production divided by the system DC (Direct Current) nameplate power. 
Performance ratio (PR) Quality of the overall design of PV system irrespective of the location. Ratio of the annual production and Nominal Plant Output, where Nominal Plant output is the product of incident energy on the plant and efficiency of modules. 
Specific generation per rupee Cost of generation, panel level. The yield of specific generation per kW of the amount spent /Capital investment per kW on the modules. 
Definition of performance KPIs

Formulas used

1. Specific generation= Total annual production from the plant/DC name plate power. kWh/KWp/year 

2. Spec. Gen per rupee= (Power generation from the panel/Capital investment in that panel) 

         = kWh/kWp * Rupees/kWh   = kWp/Rupees 

3. Performance ratio= Energy measured/ (Total radiation on the panel ∗ Active area of the panel ∗ efficiency) 

Extrapolation details that we followed for understanding this study,

  • The data available from the plant was only up to 6 months until June, 2020. So, we simulated the same model in TSL, The Solar Labs software and estimated the annual generation trend using Excel formulas as shown below. 
  • Having these trend values, we substituted the actual output for February month and extrapolated the other values in that column as given below, 
  • We have the monthly generated trend obtained from TSL for 12 months (say monthly production from TSL as M1, M2, M3…, M12) and 6 months generation data from SolarQuest( January 25 till June 30). Therefore, we extrapolated the annual plant output by having February month’s data as a base. 
  • Annual Trend from TSL = (M1/M1, M2/M1, M3/M1…, M12/M1) 
  • Extrapolation for annual plant output = (February month plant output * trend for month1, February month output * trend for month2 and so on)          
  • Hereby, we achieved the annual generation figures and calculated the specific generation, performance ratio and specific generation per rupee criteria
  • We have knowingly not considered cost of land area as a factor in our analysis. 

Conclusion

Ever since the development of polycrystalline modules with drastic reduction in cost of their modules, there has been this “Monocrystalline Versus Polycrystalline solar cells” debated across the market as well as the researchers.

From this case study, we found some evidence favouring the use of monocrystalline modules for a rooftop solar installation. We have created a KPIs called Generation per Capital Investment which considers generation and financial parameters. This KPI is designed to be a simple factor aiding your decision making process.  

  • Mono PERC modules perform well within the same weather and irradiance pattern with specific generation of 1487 kWh/kWp/year while, the polycrystalline modules lag by lower specific generation values of 1368.31 kWh/kWp/year. 
  • Mono PERC plant has INR 0.068 value more in Specific generation per rupee criteria and 3.55% higher performance ratio than the plant with polycrystalline modules.  
  • Eventually, we could see that Mono PERC system requires 47 panels less than the requirement met by installing polycrystalline modules.  

Bibliography 

  • Barbose, G., Darghouth, N., Elmallah, S., Forrester, S., Kristina S. H., K., Millstein, D., et al. (2019). Tracking the Sun: Pricing and Design Trends for Distributed Photovoltaic Systems in the United States – 2019 Edition. In Tracking the Sun: Pricing and Design Trends for Distributed Photovoltaic Systems in the United States – 2019 Edition. Lawrence Berkeley National Laboratory. 
  • Hawbaker’s, A.Wang, A.M.Milne, J.Zhao, M.A.Green, Efficient on Solar Cell, Appl. Phys. Lett. 55 (1989) 1363–1365. 
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