Performance Measurement of Amorphous and

Monocrystalline Silicon PV Modules in Eastern U.S. Energy production versus ambient and module temperature

Ulrich Schwabe, Member

Electrical and Computer Engineering

Rowan University

Glassboro, New Jersey USA

u.schwabe@ieee.org

Peter Mark Jansson, Senior Member

Electrical and Computer Engineering

Rowan University

Glassboro, New Jersey USA

jansson@rowan.edu

Abstract—This paper reports new data and findings related to the

decreased performance of a mono-crystalline silicon (c-Si)

photovoltaic (PV) system in the northeastern United States when

compared with an amorphous silicon (a-Si), thin film system.

These findings are based on a kWh per installed kW basis during

a warm summer period with relatively high ambient

temperatures. Electric utilities will become increasingly

dependent upon the performance of renewable energy systems

during peak demand periods to meet their renewable portfolio

standard obligations to public utility commissions as their

investments in these systems expand. At present there is little

data available to correlate the performance of lower efficiency

thin film PV modules with higher efficiency mono-crystalline

modules in a side-by-side test environment. The research findings

demonstrate that while amorphous (a-Si) PV systems are

generally regarded as inferior (due to their lower overall

efficiency on a kW/m2 basis) their performance on a kWh/kW

basis during periods of high ambient temperature is shown

herein to be superior to higher efficiency single crystal silicon

modules. These performance measurements, completed over the

summer of 2008, provide a detailed analysis of energy and

temperature measurements on an hourly basis during the higher

demand periods for summer peaking electric utilities. The

summary data shows a clear correlation where a-Si modules

outperform mono-crystalline PV modules when ambient

conditions lead to increased operating cell temperatures above ca.

30oC. Below this temperature threshold single crystal PV

materials performance generally exceeds that of the thin film

devices. At present, the cost differentials between the two

technologies make a-Si more attractive for many utility scale

applications and these findings indicate that PV power plants of

this construction will outperform their more efficient competitors

during the typical weather conditions of many summer peaking

utility systems.

Keywords-photovoltaics; single crystal and monocrystalline

materials;amorphous materials, performance; high temperature

periods, utility system benefits.

I. INTRODUCTION

In the autumn of 2007 Rowan University College of Engineering was approached by Kaneka [1] (the world’s leading manufacturer of amorphous silicon PV modules) to provide an Eastern U.S. test-bed and performance measurement

system for their a-Si PV modules. This was a perfect project for one of the university’s engineering clinics, a mandatory class where students work with industrial affiliates to provide a useful service while learning in the process [2]&[3]. The site was chosen due to its proximity to nearly 80 million people in the US – living within a 100 mile radius of the university. It was Kaneka’s goal to see how their modules would perform in the unique conditions of the northeastern United States subject to very hot summers and atmospheric aerosols which are typical of industrial and high density traffic regions. These conditions would enable measurement data to be collected and analyzed to asses how these modules will perform in such locations as well as determine their contribution to late afternoon system peaks on hot days. Rowan University researchers proposed the test bed be located at the new South Jersey Technology Park [4] facility in Mantua Township, New Jersey where it could be situated in the vicinity of a newly installed 13 kW single crystal silicon PV system being used to provide electricity to the LEED Platinum rated facility. This location would make it possible to collect insolation data applicable to both systems, ambient temperatures that both systems would be experiencing on an hourly basis and directly compare their performance for all different weather and day types.

The data acquisition and monitoring system chosen for the project consisted of a Sunny WebBox, made by the inverter manufacturer SMA Technology AG [5], which directly collects all ambient environmental and system performance information from the Sunny Boy inverters under test. A direct connection via the internet to the SMA website in Germany allows online retrieval of the data whenever deemed necessary for detailed analysis by the investigators. The nominally 13 kW c-Si PV system was installed in late autumn 2007 and the nominally 1-kW a-Si PV system was installed in early 2008. The Sunny SensorBox was chosen to collect and pre-process ambient and equipment conditions. It includes an onboard pyranometer which was supplemented by a second, more accurate, external pyranometer to measure the incoming solar resource. In addition to measuring insolation, both ambient and module temperatures were sampled by the SensorBox, module temperature was collected via a temperature sensor affixed to the back of one of the a-Si modules in the Kaneka system. The

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module temperatures collected during the entire test period were to provide a realistic estimate of system operating temperatures throughout the variations in solar gain, ambient temperature and local wind speed. The data collection system have been operational since late April 2008, and data was captured reliably for the entire summer test period of interest (May – September 2008). Fig. 1&Fig. 2 which follow show the PV systems that are currently being assessed to determine their performance over the range of environmental conditions found at the site.

A. Amorphous PV

As a newer addition to the commercial PV market, amorphous (a-Si) technology has been subject to a wide variety of criticism and various exorbitant claims. In the present case, manufacturer claims are that these a-Si modules will convert between four and six percent of incoming solar energy into usable power. This compares to mono- and poly-crystalline module claims commonly in the range of 12-17% efficiency, with a-Si at first glance apparently much more inferior.

Fig. 1. 12.95 kW PV System (SunTech-Power 175W c-Si Modules)

Fig. 2. 0.96kW PV System (Kaneka 60W a-Si Modules)

However, due to several reasons presented in this research the a-Si technology does provide other important benefits. Their ease of manufacture means a-Si materials and modules are significantly less expensive than other PV materials, while the nature of the thin-film method increases the overall area of the module per watt. As a result while a-Si is possibly the cheapest type of PV module available commercially at present, this technology requires the most amount of roof space or area for a ground mounted system. Increased balance of system costs is a direct consequence of using a-Si technology. However, one real advantage of these amorphous materials lies

in their low thermal mass and resulting higher tolerance for heat which has been attributed to their extremely thin (~3µm) layer of silicon. In the normal operation of PV materials efficiency drops significantly as module temperatures increases, but a-Si cells suffer less losses from increased ambient temperature and solar gain as is demonstrated in the following sections of our research. Our study has collected substantial data on-site to aid us in determining specifically when a-Si modules have a performance advantage over c-Si counterparts. This specific field data may aid future system designers in selecting the most appropriate PV technology for their application given the needs for summer performance as well as economic value.

II. DATA COLLECTION AND ANALYSIS

A. Temperature Data May-September

Correlations of hourly average measurements of module versus ambient temperatures are shown in Fig. 3-Fig. 5 for the months of May, July and September. Noted on each of the charts is the Normal Operating Cell Temperature (NOCT) for the a-Si modules at 47C°. The crystalline modules’ NOCT lies at 45C°±2C°. While R

2 values of simple polynomial fits were

between .75 and .85 which indicate a relatively strong correlation between the two data, module temperatures varied greatly at higher ambient temperatures. Several meteorological factors were the likely cause of this spread at the higher end. The two most important environmental factors are ambient solar and local wind speed. Unfortunately no wind measurements were avaible for this specific site, so data from a small airport about 25 miles south east was chosen as a proxy for this environmental effect. Average daily windspeeds were separated between above and below the mean for the month, and datapoints plotted accordingly.

Fig. 3. Ambient and Module Temperatures for May

One would expect to see a decrease in module temperature

when strong winds are prevalent at the site. This seems to be

at least partially true, as illustrated in Fig. 4-10, note that

higher average windspeeds nearly always correlate to lower

module temperatures during times of increased solar radiation.

This seems to be a general trend that holds for most months

where the lowest module temperatures at high insolation hours

occur at above average windspeeds, as given in Fig. 6Fig. 9.

The amorphous silicon modules and system were funded by Kaneka, the

world’s leader in the manufacture and distribution of amorphous silicon

photovoltaic modules

This is not always true, as seen in Fig. 7, presumably when

module

Fig. 4. Ambient and Module Temperatures for July

Fig. 5. Ambient and Module Temperatures for September

and ambient temperatures are high enough where the weak winds were not adequate to cool the modules effectively. What is still true as seen in Fig. 8 is that high winds always correspond to below average module temperatures, even if the inverse is not always true.

Fig. 6. Module Temperature and Insolation for May

Fig. 7. Module Temperature and Insolation for June

Fig. 8. June Module Temp. vs Insolation with 3m/s or higher

windspeeds noted

Fig. 9. Module Temperature and Insolation for July

Fig. 10. Module T. vs Insolation (high)

B. Module Temperature Prediction

Since there was no obvious single factor that could

adequately account for predicting module temperatures, a

multi-variate prediction model was built using both ambient

temperatures and insolation. This was done with the help of a

simple statistics Excel add-on. Provided with all the data

collected over the summer, it can generate a regression (in our

case linear) of several independent variables in the form of (1).

y=α+β1x1+β2x2+β3x3+….+ βnxn (1)

The specific variables of solar insolation and ambient

temperature were chosen to run the program to create a better

predictive model for module temperature (2).

y=3.334+1.17x1+19.12x2 (2)

This model did quite well when tested over the available

summer data, giving us an R2 of .97, when comparing

modeled temperature for the modules and actual module

temperatures. Fig. 11 shows a scatter plot of the comparison.

This relationship may be helpful in determining module

temperatures and with that, as in the following section, overall

performance of the PV system and its efficiency.

Fig. 11. Predicted versus actual module temperatres

C. Comparison of Energy Generation

Relatively high ambient (and module) temperatures were recorded in these summer months regardless of the time of day, which caused the a-Si modules to have higher kWh outputs per kW than the mono-crystalline type. This is clearly shown in Fig. 12Fig. 15, where a-Si gain is graphed versus module temperature for each month. Equation (3) is used to calculate these gains represented in terms of a percentage by simply dividing the difference of the two kWh/kWP ratings by the rating of the crystalline module.

(3)

Another important measure of photovoltaic performance is the

actual efficiency of the modules when converting sunlight to

electricty. The easiest way to calculate an approximation of

this value is to take the energy produced by the system (kWh

Array), divide it by the total area of the modules (Area Array), and

dividing this value by the available solar insolation (Solar

Radiation) measured via the pyranometer,as shown in (4).

(4)

Fig. 12. a-Si gain compared to module temperature for June

Fig. 13. a-Si gain compared to module temperature for July

Fig. 14. a-Si gain compared to module temperature for August

Fig. 15. a-Si gain compared to module temperature for September

Fig. 16. Amorphous Gain versus Module Temperature (summer 2008)

It is clear that the gain in output per WDC of the a-Si over the c-Si modules correlates with their measured temperature. Of course, these “gains” are not truly an increase in efficiency on part of the a-Si modules, but rather a significantly larger drop in efficiency in the c-Si modules. Positive gains are found quite low on the temperature scale, with the point of equiproduction lying centered around 30C°. This was quite unexpected, as NOCT values for both modules were approximately 45C°, suggesting that normal operation (and with that, performance) should be found in that range. Once module temperatures reach higher on the scale, we have gains that come close to 25%, a substantial increase in generated kWh per WDC. As given by Fig. 16, the amorphous PV produced more energy per kW throughout almost all days of the summer, the slight dip in output on the third day can be attributed to cloud cover. On average, they worked close to 10% better in the observed temperatures than the mono-crystalline type.

To reinforce this statement visually, Fig. 17 gives a comparison of the output per kW for both systems for three days in July. The a-Si array generated on the order of 0.15 kWh/kW more than the reference system at the hottest times of the day. The chart also shows the congruency of the output for the two systems quite nicely.

To some extent what is more important to compare, as mentioned in the previous sections of the paper, is the actual efficiency of the photovoltaic modules. This is determined by (2), and calculates how much of the incoming solar radiation is actually converted into usable energy. Fig. 18 gives a snapshot for July, and Fig. 19 is a compilation of these module efficiencies calculated throughout the entirety of the summer

data. The blue markers correspond to the SunTech-Power modules, and the red to Kaneka’s amorphous modules. Visually we can attribute a much larger variation to the c-Si efficiencies, as they are disperesed over a much wider range than those of the a-Si type.

Fig. 17. System generation for July 18-20

If we take a more statistical approach, the means lie around 5.65% and 11.22% for them respectively, while standard deviation was calculated to be .18 for the Kaneka and .62 for the SunTech-Power modules. It is interesting to see both types narrowing in on a specific efficiency at high temperatures, values which lie just above 10% for the crystalline and around 5.8% for the amorphous modules. If we compare these numbers with all efficiencies observed over the summer, this accounts for only about 75% of the maximum output for SunTech’s modules, and 95% of Kaneka’s. It is once again clear that the a-Si modules have a significant advantage in terms of efficiency retention at high temperatures. A drop in output of 25% for c-Si is quite significant when contrasted with the much lower reductions (5%) of the a-Si system.

Fig. 18. Module efficiencies vs. Module Temperatures for July

While the losses observed throughout the summer by the c-Si with respect to the a-Si system will most likely be recoverd during the winter months, it may not be in the best economic interest of the system owner to have the highest losses during the summer. Energy prices increase significantly in the hottest months of the year in large part to peak demands caused by the operation of air conditioning systems across northeastern U.S. utilities. During these times of intense demand, energy prices have been shown to increase between 100% to 1000% [7].

Thus, keeping a PV system’s losses low during high ambient temperatures means it can generate energy closer to its full potential, which in turn produces the highest revenues for the owner. In this respect the amorphous modules have definitely shown to be superior to the more common crystalline types.

Fig. 19. Module efficiencies vs. Module Temperatures

III. FINDINGS AND CONCLUSIONS

The paper gives an analysis of the differences in operational efficiency for two distinctly different types of photovoltaic modules. The effective performance of the two types of PV materials studied depends directly on the module temperature.

It is clear that the amorphous silicon (a-Si) modules have the potential for substantially lower efficiency decreases when contrasted with mono-crystalline modules during the summer periods of key interest to electric utilities. In the five month (summer) period for which data has thus far been collected and analyzed, it was not uncommon to see better performance by a-Si materials (increases in production in kWh/kW of over 10%). These results always occurred in the hottest hours of the day, where mono-crystalline efficiency drops dramatically.

Overall, the comparison showed that there is a significant advantage that can be realized by using thin film materials when loss of system efficiency during the hot summer period is undesirable. While these materials require larger areas to compete with similar single crystal PV they experience much lower degradations in output during system utility system peak demand periods in the hot summer. If sufficient area is available, their low price and high heat tolerance makes them a potentially viable competitor for large scale generation.

ACKNOWLEDGMENT

The authors wish to acknowledge the research funding provided by Kaneka and the support of Conergy[8] in the monitoring of the single crystal system installed at the South Jersey Technology Park. In addition, the installations of both PV systems and the SMA Sensor Box and Web box were completed by SunTechnics Energy Systems (now Conergy) who we gratefully acknowledge here. We would also like to acknowledge Jonathan Bucca, a student worker on this project, for providing the regression model and information in section B. Finally, online data for these systems has been maintained

by SMA’s SunnyPortal [9] without whose assistance this analysis and data resource would not be possible.

REFERENCES

[1] Kaneka Corporation [Online Available] http://www.kaneka.com/

[2] J. L Schmalzel, A. J. Marchese, J. Mariappan and S. A. Mandayam, "The Engineering Clinic: A four-year design sequence," presented at the 2nd An. Conf. of Nat. Collegiate Inventors and Innovators Alliance, Washington, D.C., 1998.

[3] J. L Schmalzel, A. J. Marchese and R. P. Hesketh, "What's brewing in the Clinic?," HP Engineering Educator,2:1, Winter 1998, pp. 6-7.

[4] South Jersey TechPark [Online Available] http://www.sjtechpark.org/

[5] SMA Solar Technology [Online Available] http://www.sma.de/

[6] SunTechnics Energy Systems [Online Available] http://www.suntechnics.com/

[7] New Jersey Energy Master Plan [Online Available]

http://nj.gov/emp/docs/pdf/081022_emp.pdf

[8] Conergy [Online Available] http://www.conergy.us

[9] SMA SunnyPortal [Online Available]

http://www.sunnyportal.com/Templates/Start.aspx