Power Generation Performance of CdTe Thin Film Solar Modules in Lhasa

With the increasing energy consumption of buildings, reducing the energy consumption of buildings has become a key link to achieving the goal of carbon neutrality. Building integrated photovoltaic (BIPV) is an important research direction. Based on the CdTe thin film solar modules power generation system on the south-facing wall of a building in Lhasa, the article collected the annual power generation data for 2022. The characteristics and variation patterns of the total monthly power generation and typical monthly daily power generation are analyzed. Furthermore, the correlation between power generation performance and the main influencing factors was analyzed. The differences in power generation characteristics between CdTe thin films solar modules and single crystal silicon solar modules were compared and analyzed. The results show that the annual power generation of the CdTe thin film solar modules power generation system exhibits a “U” shaped distribution, and the power generation in winter is greatly affected by weather conditions. The peak power of 1kWp CdTe thin film solar modules is lower than that of single crystal silicon solar modules. The generation duration is longer than single crystal silicon solar modules. The total power generation is greater than single crystal silicon solar modules.


Introduction
In recent years, the issue of harmonious coexistence between energy and the environment has received great attention.The energy consumption of buildings in China is increasing day by day; the energy consumption of the entire building process accounts for approximately 46.5% of the total energy consumption at present in China [1].Solar energy has the advantages of on-site collection and application, making it the best carrier for BIPV and the main force for achieving carbon and emission reduction of buildings [2], which has attracted a lot of attention from researchers.
BIPV is the organic combination of solar photovoltaic power generation and building.On the one hand, it provides electrical energy for buildings and reduces building energy consumption; On the other hand, as a part of the architecture, it plays a decorative and functional role [3].Flexible solar cells can be combined with buildings to form new photovoltaic integrated buildings [4].Among them, CdTe thin film solar modules have great advantages, such as good spectral matching, high stability, and good weak light performance, making them one of the most successful industrialized thin film modules [5].Many researchers are accelerating their research, such as the study of ZnSeTe ternary thin film materials [6], the deposition mechanism of CdTe polycrystalline thin films [7], the effects of substrate temperature on the property of CdTe film, and the performance of CdTe thin films solar modules [8], prepared CdTe thin films modules by deposition of CdSnO4, CdS, CdTe, ZnTe:Cu and Au films in turn, 16.73% CdTe solar cell with 0.5 cm2 area has been fabricated by optimizing preparation parameters of main layers [9].
CdTe thin film solar modules are widely used to replace structures such as roofs, curtain walls, windows, and sunshades in some traditional buildings.Many buildings have applied CdTe thin film photovoltaic module technology, such as Zhangjiakou Winter Olympics Village, National Ski Jumping Center, and National Grand Theater Taihu Dance Art Center [10].Photovoltaic windows and curtain walls are a common integrated mode of BIPV.The power generated by photovoltaic windows with appropriate transmittance can compensate for the energy consumption of building air conditioning and lighting, minimizing the overall energy consumption [11].Pei et al. [12] analyzed the impact of photovoltaic window structures on the thermal and optoelectronic comprehensive performance.He et al. [13] compared the thermal performance of ventilated photovoltaic windows with single-layer photovoltaic windows, believing that ventilated photovoltaic windows have better performance.In [14,15], researchers investigated the effects of module coverage, cavity spacing, and transmittance on the heat gain of photovoltaic windows.Zhang et al. [16] analyzed the performance of CdTe thin film photovoltaic modules in practical engineering applications.Carr and Pryor [17] evaluated and compared the outdoor performance of four different photovoltaic modules, including monocrystalline silicon, polycrystalline silicon, amorphous silicon, and CIS.Akhmad et al. [18] studied the operational characteristics of polycrystalline and amorphous silicon photovoltaic modules.The results showed that polycrystalline silicon modules have significant advantages in conversion efficiency and power generation when the temperature is low in winter, while the results are exactly the opposite under hightemperature conditions in summer.Chen et al. [19] analyzed the power generation duration of polycrystalline silicon and amorphous silicon photovoltaic modules and their output characteristics under two different weather conditions, sunny and cloudy.In [20,21], the impact of the solar spectrum on the output power of photovoltaic modules was studied.
In summary, researchers have conducted extensive research on the CdTe thin film solar modules and have achieved certain results.However, there are few studies aimed at analyzing the power generation characteristics of CdTe thin film solar modules used in high-altitude areas.In this study, we collected comprehensive power generation data of a CdTe thin films modules power generation system in Lhasa in 2022 and partial power generation data of a single crystal silicon solar modules power generation system.We analyzed the power generation characteristics of CdTe thin film solar modules at an altitude of 3650 m.Furthermore, we verified the differences in power generation time, efficiency, and low light performance between CdTe thin film solar modules and traditional crystalline silicon modules.This work can provide an important basis for the application selection of CdTe thin film solar modules in high-altitude areas.

Materials and methods
CdTe thin film solar modules power generation system used in this experiment is in Chengguan District, Lhasa City, Tibet Autonomous Region.The system operates in a mode of integration into the local power grid, and the generated electricity is directly transmitted outward through the power grid.The altitude is 3650 m, with geographical coordinates of N29°41'and E91°1'.Lhasa is extremely rich in solar energy resources, with a total horizontal radiation of over 2000 kWh/m 2 and an average annual sunshine duration of around 3000 hours, making it one of the regions with the most abundant solar energy resources in the world.As shown in Figure 1, the slope area of the system is 141.7 m 2 , with a window area of 26.5 m 2 .The area of CdTe thin film solar modules is 115.2 m 2 .The angle between the CdTe solar cell and the horizontal direction is 85°.Behind the CdTe thin film solar modules is the office, which has a wide space and will not affect the heat dissipation of the system.The system has an installed capacity of 13.8 kW and is composed of 60 CdTe thin film solar modules.Each six CdTe thin film solar module forms a circuit, and there are ten circuits in total.The ten circuits are aggregated and output through a combiner box.The connection method is shown in Figure 2. The specifications of CdTe thin film solar modules used in the system and their various performance parameters under indoor standard testing conditions are shown in Table 1.

Results and discussion
We selected the whole year of 2022 as the period of power generation evaluation.This paper summarizes and analyses the power generation data of the CdTe thin film solar modules power generation system for a whole year.The annual power generation of the system is 17410 kWh.The system runs all year except for isolated island protection caused by power outages in individual periods.The system time is in the International Time Zone Eastern Eight, and the true sun time in Lhasa is about 2 hours behind the Eastern Eight standard time.

Analysis of annual power generation
Figure 3 shows the monthly power generation of the system.The special annual power generation of the CdTe thin film solar modules power generation system exhibits a "U" shaped distribution, showing a phenomenon of high in winter and low in summer.
Figure 3.The monthly power generation of CdTe thin films modules power generation system.
The highest monthly power generation of the year occurred in December, with a power generation of 2306 kWh, followed by January at 2263 kWh, and November at 2240 kWh.The lowest electricity generation occurred in June at 677 kWh, followed by July at 777 kWh, and May at 844 kWh.It shows that the month where the winter solstice occurs and the month near the winter solstice have higher power generation, while the month near the summer solstice has lower power generation.This is because Lhasa, where the test site is located, is located to the north of the Tropic of Cancer.When the sun makes a regression movement between the Tropics of Cancer and Capricorn, the angle between the sun and the incident light changes accordingly.The projection area of the module plane on the vertical plane normal to the incident light also changes with the change, reaching the largest area at the winter solstice and the smallest area at the summer solstice.This leads to obvious differences in power generation.

Analysis of total power generation in typical months
Figure 4 shows the daily power generation in March, June, September, and December for typical days such as the spring equinox, summer solstice, autumn equinox, and winter solstice.The daily power generation in June is relatively stable.The maximum daily power generation of 25.3 kWh, and the minimum daily power generation of 19.2 kWh, accounting for 75.9% of the maximum daily power generation.The D-value between the maximum and minimum daily total power generation is only 6.1 kWh.The optical loss caused by fixed angles at this time is the main reason for the low daily power generation, and the impact of weather factors is relatively small.Therefore, the overall daily power generation is relatively stable.
The daily electricity generation in December, where the winter solstice is located, is generally stable, but the difference in the extreme values of the daily power generation is the greatest.The daily maximum power generation is 79.9 kWh, and the daily minimum power generation is 29.8 kWh, which is only 37.3% of the daily maximum power generation.The D-value between the maximum and minimum daily power generation is 50.1 kWh.This indicates that daily power generation is more significantly affected by weather factors at this time.At the same time, the optical loss caused by fixed angles has a relatively small impact on daily power generation.Therefore, there is a significant difference in the extreme values of the daily total power generation.
The fluctuation of daily power generation in March, where the vernal equinox is located, is extremely significant, showing an overall downward trend.The daily maximum power generation is 66.8 kWh, and the daily minimum power generation is 31 kWh, accounting for 46.4% of the daily maximum power generation.The D-value between the highest and lowest daily total power generation is only 35.8 kWh.The daily power generation in September, where the autumnal equinox is located, shows a significant contrast with the daily power generation in March, with significant fluctuations and an overall upward trend.The daily maximum power generation is 59.9 kWh, and the daily minimum power generation is 27.5 kWh, which is 45.9% of the daily maximum power generation.The D-value between the maximum and minimum daily power generation is only 32.4 kWh.The ratio between the daily minimum and maximum power generation in March and September is smaller than that in March and larger than in December.This is because, at this time, the sun is moving near the equator, and the daily power generation is not only affected by optical losses caused by fixed angles but also significantly affected by weather factors, resulting in significant fluctuations in the daily power generation.

Analysis of typical daily real-time power
Figure 5 shows the daily power generation, real-time power, and duration of the summer solstice and winter solstice.There is a significant difference in the power generation duration and real-time power between the summer solstice day and the winter solstice day.Within 8 hours before and after noon of local true solar time in Lhasa, the real-time power on the winter solstice day is significantly higher than that on the summer solstice day.The daily power generation of the summer solstice is 22.1 kWh, while the winter solstice is 78.3 kWh, which is 3.54 times that of the summer solstice.The maximum realtime power on the summer solstice is 3.97 kW, while on the winter solstice, the maximum real-time power of the system is as high as 11.44 kW, which is 82.9% of the system's installed rated power, and 2.9 times that of the summer solstice.
At the same time, there are also significant differences in the duration of power generation.On the summer solstice, power generation starts at 7:00, stops at 21:35, and lasts up to 14 hours and 35 minutes.On the winter solstice, the start time for power generation is 8:45, which is 1 hour and 45 minutes later than the summer solstice.The stop time for power generation is 19:25, which is 2 hours and 10 minutes earlier than the summer solstice.The daily power generation duration is 10 hours and 40 minutes, which is 73% of the power generation duration on the summer solstice and 3 hours and 55 minutes less than the summer solstice.However, on the winter solstice, it only takes 30 minutes from the start of system power generation to reach 1.0 kW.In contrast, on the summer solstice, it takes 2 hours and 30 minutes.Taking 20% of the system's installed capacity as the measurement standard, the required power generation time for the summer solstice is 12:35-16:40, with a duration of 4 hours and 5 minutes, accounting for 28% of the daily power generation time.The winter solstice lasts from 9:35 to 18:25, with a duration of 8 hours and 50 minutes, accounting for 83% of the electricity generation time on that day.
Figure 5.The real-time power of the 21st of June and the 22 nd of December.

Analysis of power generation characteristics of CdTe thin film solar modules and single crystal silicon solar c modules
To analyze the different output characteristics of CdTe thin film solar modules and single crystal silicon solar cells, a single crystal silicon power generation system with a capacity of 2.48 kW was built as a comparison system.The comparison system and CdTe thin film solar modules power generation system used the same layout and grid connection method.The parameters of the single-crystal silicon solar module are shown in Table 2. To more intuitively compare the differences in operating characteristics between CdTe thin film solar modules and single crystal silicon solar modules, the power generation data of the two systems on January 1, 2022, were normalized.Figure 6 shows the power generation curves of 1 kWp CdTe thin film solar modules and single crystal silicon solar cells.From the perspective of power generation, the CdTe thin film solar modules have good power generation performance, with a power generation of 5.282 kWh at 1 kWh.In contrast, the single crystal silicon solar modules have poor power generation performance, with a power generation performance of 4.315 kWh.In terms of real-time power generation, the peak power of single crystal silicon solar modules is relatively high, with a peak power of 882.3 W at 1 kW, and the peak power of CdTe thin film solar modules is relatively low, with a peak power of 873.2 W. Around noon during the local accurate solar time in Lhasa, the power generation of single crystal silicon solar modules is higher than that of CdTe thin film solar modules.When sunlight is obstructed in the afternoon, the power generation is weaker than that of CdTe thin film solar modules.From the perspective of power generation duration, single-crystal silicon solar cells start generating power from 9:40 am to 19:25 pm, with a power generation duration of 9 hours and 45 minutes.CdTe thin film solar modules start generating power from 8:50 am to 20:25 pm, with a power generation duration of 11 hours and 35 minutes.This is consistent with the statement reported in the literature that amorphous silicon has a stronger power generation performance than single-crystal silicon solar modules at low irradiance [22].This also explains the high-power generation per unit power of CdTe thin film solar modules.

Conclusions
Taking the CdTe thin film solar modules power generation system in the city of Lhasa as the research object, the power generation performance of CdTe thin film solar modules in high-altitude areas is studied.The results showed the following: 1.The annual power generation of CdTe thin film solar modules power generation system presents a "U" shaped distribution with high winter and low summer, which is completely opposite to the "ŀ" shaped distribution of traditional power generation systems.The highest monthly power generation of the year occurred in December, with a power generation of 2306 kWh.The lowest electricity generation occurred in June at 677 kWh.
2. The daily power generation of typical months is analyzed.It is found that the daily power generation in June is relatively stable.The optical loss caused by fixed angles is the main reason for the low daily power generation.The daily power generation in December is generally stable, but the difference in the extreme values of the daily power generation is the greatest, with the lowest power generation being only 37.3% of the highest.At this point, daily power generation is more significantly affected by weather factors.The daily power generation in March and September fluctuates significantly.But it shows an overall downward trend in March and an overall upward trend in September.The ratio between the minimum daily electricity generation and the maximum daily power generation in March and September is smaller than that in March and larger than that in December.At this time, the daily power generation is not only affected by the optical loss caused by fixed angle but also by weather factors, causing the fluctuation of daily total power generation is very obvious.
3. The power generation and duration of the summer solstice and winter solstice are analyzed.It shows that during the 8 hours before and after noon of local accurate solar time in Lhasa, the power generation on the winter solstice was significantly higher than that on the summer solstice.The power generation on the winter solstice is 3.54 times that on the summer solstice.And the maximum real-time power is 2.9 times that of the summer solstice.Taking 20% of the system's installed capacity as the measurement standard, the summer solstice accounts for 28% of the day's power generation time, and the winter solstice accounts for 83%.
4. The different output characteristics of CdTe thin film solar modules and single crystal silicon solar cells are analyzed.From the perspective of power generation, the CdTe thin film solar modules are better than the single crystal silicon solar modules.In terms of real-time power generation, the CdTe thin film solar modules are relatively lower than single crystal silicon solar cells.From the perspective of power generation duration, the CdTe thin film solar modules are 1.19 times that of single crystal silicon solar modules.

Figure 1 .
Figure 1.CdTe thin film solar modules power generation system.

Figure 2 .
Figure 2. The connection method of CdTe thin film solar modules.

Figure 4 .
Figure 4.The daily power generation of typical months.

Figure 6 .
Figure 6.The real-time power of the CdTe thin film solar module and single crystal silicon solar module.

Table 1 .
Parameters of CdTe thin film solar modules.

Table 2 .
Parameters of single crystal silicon solar module.