Research on Photovoltaic Carport System Performance

The demand and quantity of photovoltaic carports are increasing rapidly in recent years, and in this study the factors affecting the performance of photovoltaic carport components based on the electricity generation data from a factory in Tianjin, China are analyzed. The research validates that HJT and TOPCon modules outperform PERC double-glass modules and PERC modules in terms of electricity generation efficiency. Practical tests were conducted under various weather conditions such as sunny, cloudy, snowy, and enclosed environments, and the study results demonstrate that under the same low-light conditions, N-type modules exhibit better performance. The comparison of low-light performance among the four module types is as follows: HJT modules > TOPCon modules ≈ PERC double-glass modules > PERC modules. Due to the higher distance between photovoltaic carport modules and the ground, the performance of the bifacial module is significantly affected. In snowy conditions, the cost-effectiveness of PERC double-glass modules in photovoltaic carports is higher than that of TOPCon modules. In sunny and cloudy conditions, HJT modules show a higher increase in electricity generation. This study also examines the impact of enclosing the bottom space of photovoltaic carports on the electricity generation performance of TOPCon modules and PERC double-glass modules, revealing that TOPCon modules experience more power loss. The analysis of the practical operation of the four module types under various conditions are summaried in this paper, which providing valuable insights for the development of future solar cell technologies and the selection strategy of photovoltaic modules in different scenarios.


Introduction
As energy and environmental issues continue to escalate, the development of renewable energy technologies, including photovoltaic (PV) carports and PV cells, has gained significant attention.By the end of May 2021, China's stock of new energy vehicles had reached approximately 5.8 million, accounting for approximately 50% of the global total.With the increasing number of new energy vehicles, there is a pressing need for extensive construction of PV carports, which harness solar energy to provide power support for electric transportation.Currently, mono-crystalline silicon solar cells are among the fastest-developing solar cells, produced using high-purity mono-crystalline silicon rods.These highly efficient cells are widely used in transportation, communication, meteorology, photovoltaic power stations, and integrated photovoltaics for buildings.Common types of mono-crystalline silicon cells include N-type and P-type cells, with the traditional solar PV cells belonging to the P-type category, such as PERC cells, which have achieved conversion efficiencies of up to 22.6%, and even PERC bifacial cells have been introduced.In recent years, many universities, research institutes, and PV companies have conducted in-depth research on solar cells to improve their photoelectric conversion efficiency.Among them, the technology of N-type cells has also become increasingly mature [1] , such as HJT cells and TOPCon cells, which have advantages such as long lifespan, excellent weak light performance, and low-temperature coefficient [2] .The efficiency of N-type cells has reached 26.6% [3] , while the German Solar Energy Research Institute has reported an efficiency of 26.1% for P-type cells [4] .However, most of these reported efficiencies are based on software [5] or model analysis [6] , lacking reliable measured data for validation, and there is a lack of multi-scenario analysis comparing different types of solar cells under the same operating conditions.
This paper aims to verify the power generation performance of the four types of PV modules (N-type HJT modules, N-type TOPCon modules, P-type bifacial PERC modules, and P-type PERC modules) in a photovoltaic carport setting and explore the possibility of selecting different types of PV modules for different scenarios.The research findings can provide important references for future solar cell technology development and the selection of PV module types for various scenarios.

Installation location
The photovoltaic project is located in the Binhai area of Tianjin, China, with an annual average temperature of 14.0°C, an annual average precipitation of 782.6 mm, and an annual average sunshine duration of 2610.8 hours.It is a Class III solar resource area in China, characterized by abundant solar energy resources.
Considering the occlusion of buildings and walls, some sites have insufficient sunlight, and the sunlight analysis of the site is carried out, as shown in Figure 1.On the winter solstice, only the north wall side of the site and the roof of the building have good lighting conditions, the east side of the site is slightly better along the line, and the lighting conditions on the east, west, north and west walls of the office building are poor.The photovoltaic carport composed of four types of photovoltaic modules, which are selected to be arranged and installed on the north side of the site, as shown in Figure 2.

System layout and related parameters
The design of the photovoltaic carport power generation system is shown in Figure 3, with an installed capacity of 97.2 kW, including 20 pieces of 580 W HJT modules, 20 pieces of 550 W TOPCon modules, 20 pieces of 540 W PERC double-glass modules and 120 pieces of 540 W PERC modules.The parameters of the photovoltaic modules are shown in Table 1.The 100 kW solar inverter is used.The parameters are shown in Table 2.The input is 9 and 18 strings, and the output is 1 channel.The best inclination angle is 15° for installation.The photovoltaic carport is installed on a hardened concrete floor, the lowest point is 2.84 m from the ground, and the highest point is 5.356 m from the ground.

Characteristics and test selection of photovoltaic cells
(1) HJT, HJT cells are often more efficient than single-material cells of the same size and mass, and they can respond to different wavelengths in the solar spectrum, from ultraviolet to infrared, allowing cells to generate electricity across a wider spectral range.Its energy conversion efficiency has fast response characteristics and can quickly respond to changes in light.This property allows it to generate electricity even in partial sunlight or brief periods of sunlight [7] .Risen Titan RSM120-8-580M single-sided HJT module was selected and used in this test.
(2) TOPCon, TOPCon has high conversion efficiency and high stability, can work in high temperature and humidity environments, and has a long life, the reason for which is that the double-layer passivation layer and molybdenum back reflection layer can reduce the corrosion and aging of materials [8] .The JinKO Solar Tiger Neo JKM550N-72HL4-BDV double-sided TOPCon cells was selected and used in this test.
(3) PERC, the more mature solar cell technology used in the solar power market is the boron-doped p-type cell.There is limited room for improvement in the future, but the cost is relatively low [9] [10] .In this test, JinKO Solar Tiger Pro JKM540-72HL4-BDVP double-sided PERC cells and JinKO Solar Tiger Pro JKM540-72HL4-V single-sided cells were selected and used.(1) The power generation data of the four types of photovoltaic modules in a photovoltaic carport in a factory in Tianjin from November 2022 to April 2023 are obtained through the intelligent management system of the factory, and data screening is performed.According to the screened data, the power generation efficiency of different photovoltaic modules is calculated, and the power generation gains of N-type HJT modules, N-type TOPCon modules, and P-type PERC double-glass modules are compared with those of P-type PERC modules.

Research Methods
(2) Since the low-light performance of N-type photovoltaic modules is better than that of P-type photovoltaic modules, the power generation performance of different photovoltaic modules is explored by comparing the fill factors of the four types of modules under low-light conditions (irradiance).
(3) Since the power generation performance of N-type photovoltaic modules is more stable than that of P-type photovoltaic modules with temperature changes, by comparing the effects of different weather conditions (cloudy and sunny) on the power generation gain of the four types of photovoltaic modules, we explored the performance of different photovoltaic modules in different weather conditions.Power generation performance under certain conditions is studied, and then typical snowy day data are selected.The power generation performance improvement of bifacial photovoltaic modules on snowy and cloudy days is explored by comparing the single-watt power generation gain of two bifacial photovoltaic modules on typical days when snowy days are more cloudy.
(4) The dual power generation efficiency of the two modules under the bottom space occlusion is explored by comparing the reduction of the power generation of two types of double-sided power generation modules, namely N-type TOPCon modules and P-type PERC double-glass modules, before and after the bottom space is enclosed.

Test results and analysis
From November 2022 to April 2023, the power generation data of four types of photovoltaic modules in a photovoltaic carport in a factory in Tianjin were collected.The original data of 0 and non-morning and afternoon continuous operation data are excluded.After deletion, 142,094 groups of relatively accurate data are left.Some current and voltage data will change suddenly, which is an unstable factor.After the screening, 40, 205 groups of relatively stable data are selected.

Comparison of power generation efficiency of photovoltaic modules
The power generation per watt is the power generation capacity of solar modules per watt-hour under standard test conditions, which can reflect the power generation efficiency of the modules.In the recording period τ, the calculation formula of DC power generation is as follows: where  , -power generation, kWh; -DC power, kW.
Since the nominal power of the four types of photovoltaic modules is not the same, the data is normalized, and the single-watt power generation of each module is calculated according to Formula (2), and then compared.
where -Power generation per watt，kWh/kW;  -Module power under standard conditions.Table 3 shows the single-watt power generation and power generation gain of N-type HJT modules, N-type TOPCon modules, and P-type PERC double-glass modules compared with P-type PERC modules.Since the power of the four components is different, it needs to be converted into a single-watt power generation according to the formula before making a comparison.It can be seen that under the same weather data and the same module installation form, the power generation per watt of the four modules is compared as follows: TOPCon> HJT> PERC double glass > PERC module, that is, the power generation performance of N-type photovoltaic modules is better than P-type.TOPCon, HJT and PERC double-glass have a power generation gain of about 10% compared with PERC.

Comparison of low-light performance of photovoltaic modules
Affected by climate and natural conditions, photovoltaic modules often work under non-standard test conditions.It is extremely important to study the performance of modules under low light conditions to evaluate the power generation performance of photovoltaic modules.Under weak light conditions, the fill factor can reflect the cell conversion efficiency of photovoltaic modules, and the calculation formula of the fill factor is as follows: where I mp is the actual maximum output power point current of the module, V mp is the actual maximum output power point voltage of the module, and (I sc *V oc ) is the ideal maximum output power of the module.
The actual working power of all-day modules under typical low-light weather is selected and the fill factor is calculated simultaneously to analyze the low-light performance of four types of photovoltaic modules.Figure 4 shows the power generation power of each PV module in typical weak light weather, and Table 4 shows the fill factor of each PV module in typical weak light weather.Under the same low-light conditions, the N-type has better low-light performance.The comparison of the low-light performance of the four modules is as follows: HJT >TOPCon/PERC double-glass >PERC.The reason is that the performance of TOPCon is greatly reduced under low-light conditions, while HJT will be relatively stable.

The influence of different weather conditions on the power generation of photovoltaic modules
The power generation performance of N-type modules is more stable than the P-type modules with temperature changes.The actual power generation data of the modules in two typical weather conditions (sunny and rainy) were selected to compare the power generation of the four types of modules and they are analyzed in comparison with the temperature power factor.
As can be seen in Figure 5 and Figure 6, HJT modules have more power generation gains under different weather conditions, since the HJT modules are more stable in winter and spring when there is no hot weather and the TOPCon modules are more suitable for hot weather.The N-type modules have more power generation gain and are more stable than the P-type modules.The power temperature coefficient of HJT, TOPCon, PERC double-glass and PERC modules is -0.34%/℃,-0.30%/℃-0.35%/℃and -0.35%/℃ respectively, which is also in line with the power temperature coefficient of each type of module.Due to the high distance between the photovoltaic modules of the photovoltaic carport and the ground, the power generation performance of the double-sided photovoltaic modules is greatly affected.The typical snowy days data are selected, and the power generation performance of double-sided photovoltaic modules in snowy days is better than that on cloudy days are explored by comparing the single-watt power generation gains of the two double-sided photovoltaic modules on typical days with snowy days compared with cloudy days.It can be seen from Table 5 that on cloudy days, the power generation gain of TOPCon modules is higher than that of PERC double-glass modules, while on snowy days, the power generation gain of TOPCon modules does not increase but decreases compared with cloudy days, while the power generation performance of PERC double-glass modules is lower.Part of the increase is because the PERC double-glass module of the photovoltaic carport is relatively high from the ground, which cann't effectively reflect light on the ground, resulting in ineffective use of both sides.On snowy days, the drop in temperature leads to unstable power generation performance of the TOPCon module, and the effect of snow causes PERC The double-glass module receives more reflected light, and the power generation performance is improved to a certain extent.Therefore, PERC double-glass modules for photovoltaic carports are more cost-effective than TOPCon modules in snowy weather.

The influence of space enclosure at the bottom of the photovoltaic carport
The power generation data after the space enclosure at the bottom of the photovoltaic carport (after March 24, 2023, when the weather temperature has shown a warming trend) are selected, and then the data are calculated and analyzed through Formula (1) and Formula (2).Next, the two types of dual Surface power generation components, namely N-type TOPCon components and P-type PERC double-glass components are compared, which shows a reduction effect on power generation compared with PERC components.Finally, the power generation efficiency of the two components under the shadow of the bottom space is explored.
As can be seen in Table 6, on the basis of warmer weather and increased irradiance, the enclosed power generation of TOPCon modules and PERC double-glass modules still have a greater reduction effect than before the enclosure.The increase in irradiance cann't make up for the loss caused by the double-sided utilization of double-sided photovoltaic modules caused by enclosure.In this case, the use of double-sided photovoltaic modules is not cost-effective.

Conclusion
This paper studies the power generation performance of four types of photovoltaic modules (N-type HJT, N-type TOPCon, P-type PERC double-glass and P-type PERC) of the photovoltaic carport.The conclusions are as follows: 1) In the case of the same weather data and the same module installation form, the power generation per watt of the four types of modules is compared as follows: TOPCon > HJT> PERC double glass > PERC, N-type HJT, N-type The power generation of TOPCon and P-type PERC double-glass is about 10% higher than the average power generation of P-type PERC modules.
2) Under the same low-light conditions, the N-type has better low-light performance.The comparison of the four types of modules is as follows: HJT > TOPCon / PERC double-glass > PERC.
3) Due to the high distance between the photovoltaic modules of the photovoltaic carport and the ground, the power generation performance of the double-sided photovoltaic module is greatly affected.In snowy weather, PERC double-glass of PV carports are more cost-effective than TOPCon.On sunny and cloudy days, HJT has more power generation gain.There is no high-temperature weather in winter and spring, HJT is more stable, and TOPCon is more suitable for high-temperature weather.
4) On the basis of warmer weather and increased irradiance, the space enclosure at the bottom of the PV carport still has a relatively large adverse effect on the power generation efficiency of double-sided photovoltaic modules, so it is not recommended to use double-sided photovoltaic modules.
5) For the possibility of selecting different components, a variety of factors should be fully considered, such as regional normal weather (more cloudy, rainy, snowy, sunny, less rainy, etc.), regional temperature, field environment (site area, distance from the reference surface, etc.) distance, shading, construction site, etc.), photovoltaic functionality (single power generation, carport, etc.), terminal power consumption, etc.The possibility of selecting some components is given here as a reference: when the climate in the area is mainly cloudy, rainy, snowy or shaded, it is recommended to use PERC double-glass modules, which have good low-light performance and high-cost performance; when the climate in the area is mainly sunny and the site area is limited, it is recommended to use HJT modules, which have the highest power generation efficiency; when the climate in the area is mainly sunny, the site area is not tight, and the terminal power consumption is large, it is recommended to choose PERC modules, which are the most cost-effective.Considering the functional requirements of photovoltaics, such as being used as a carport, and the distance between photovoltaic modules and the ground is far away, it is not recommended to use TOPCon modules and PERC double-glass modules, because the advantages of the double-sided cann't be fully utilized and the cost performance is relatively low.
This study has made a certain contribution to the power generation performance of photovoltaic carport components, but there are still limitations and deficiencies in data collection, such as the lack of summer and autumn power generation data as experimental support.In follow-up research, the time of data collection can be increased to improve the accuracy of the research.

Figure 2 .
Figure 2. Photovoltaic carport on the north side of the site.

Figure 4 .
Figure 4. Power generation power of each photovoltaic module in typical weak light weather.

Figure 5 .
Figure 5. Power generation on sunny days.Figure6.Power generation on cloudy days.

Figure 6 .
Figure 5. Power generation on sunny days.Figure 6.Power generation on cloudy days.Table 5. Comparison of power generation performance on typical snowy days and cloudy days Type TOPCon PERC double glass PERC Power generation per watt on snowy days (kWh/kW) Power generation per watt on cloudy days (kWh/kW) Power generation gain of PERC modules in snowy weather Power generation gain of PERC modules on cloudy days 0.0711 0.0214 6.28% 9.26%

Table 1 .
Parameters of the foure typess of photovoltaic modules

Table 3
Power generation of each module

Table 4 .
Fill factor of each photovoltaic module in typical weak light weather

Table 6 .
The power generation of three types of modules under the enclosure of the bottom space