Review Reports - Experimental Efficiency Analysis of a Photovoltaic System with Different Module Technologies under Temperate Climate Conditions

Round 1

Reviewer 1 Report

In general, the paper is well written, and it presents experimental results. Still, the novelty is not clear.

The performance of three different PV technologies is compared and, from the paper, one understands that the PV systems are located at the same site, for comparison purposes. However, there is no information regarding the MPPT system, and the inverter in the connection to the grid. If they are not the same, this might affect the results. Also, there should be additional information regarding the connection of the PV panels, in particular for the higher power system. Is there only one MPPT of several MPPT systems? This should be better explained.

Author Response

1. In general, the paper is well written, and it presents experimental results. Still, the novelty is not clear.

New text has been added in lines 73-76.

Manuscript modification:

All these investigations add new knowledge on the influence of external conditions on solar modules built with different semiconductor materials and allow to select the best technology type in the given location. At present there are no published results of this kind of comparative study carried in Polish climate.

2. However, there is no information regarding the MPPT system, and the inverter in the connection to the grid. If they are not the same, this might affect the results. Also, there should be additional information regarding the connection of the PV panels, in particular for the higher power system. Is there only one MPPT of several MPPT systems? This should be better explained.

As suggested by the Reviewer information about PV modules connections as well as MPPT systems have been added to the Methodology section. The detailed information on the plant has been added in lines (94-108).

Manuscript modification:

The modules are oriented to the south and tilted at the optimum angle for the given latitude equal to 34o. The installation is connected to the grid using the inverters. Both thin film installations are connected with the use of inverters with one MPPT tracker while for pc-Si PV string the inverter equipped with two MPPT systems is used. The distance between the rows of panels equals to 6.3 m.

Solar radiation sensor based on the monocrystalline silicon solar cell is located in the centre of the installation and tilted at the same angle as modules. Pt1000 resistance temperature detectors are attached on the modules backside.

Inverters as well as both mentioned sensors were connected to the central data-logging computer system for synchronous data collection.

Author Response File: Author Response.pdf

Reviewer 2 Report

The subject of the work is interesting, but I think that some things must be improved in the work in order to have enough quality to be published.

With regard to the PV installation, you can give more details of the plant in general, orientation and tilt angles, whether they are connected to the grid, the layout of the panels, inverters use...

With regard to measurements, irradiance and temperature sensors are specified, but commercial models of these sensors are not given, only one specification for the temperature sensor, where and how they are located in the PV plant. And the electrical measurements that are carried out at work are not indicated how they have been recorded. Andthe period of time in which the data to be analyzed in the work have been recorded must be clearly indicated in section 2 of methodology .

With respect to the editing of the work, the same notation should be always uses, for example, the first time the word photovoltaic appears, put in parentheses its notation (FV), but then from there always use FV. Avoid the use of first people (ie. we characterized...).

The results obtained from the three technologies should be compared with those obtained by other authors in similar works. The results only allude to one comparison.

When reference is made to the IEC 61724 standard, it should be included in the bibliography, and indicate whether it refers to the standard published in 1998, or to its last update in 2017.

The period analysed in the work is from April to September 2015. This is justified in the work indicating that the rest of the months of the year the irradiance is much lower. However, in my opinion it is also very interesting to carry out this study during the whole year, including these months of lower irradiance, given that this type of installations will be operating all year round, and it is precisely in these most critical months that the behaviour of the PV installation must be very well characterised. If data are not really available during these months, then clearly indicate this. But if you have data for the whole year, as shown in figure 2, the analysis must be carried out during the twelve months of the year.

The magnitudes represented in each figure must be very well specified. In some figures it is not clear whether what is represented is total daily, or instantaneous data. Therefore, this must be very well specified.

With respect to figure 6, how the four points of each technology reflected in the graph have been obtained should be explained.

Lines 156-159. This paragraph should be explained better. Allusion is made to a temperature value, 55 º, which does not appear in the figure.

In equation 7, and the explanation of the magnitudes that appear in it, there is some error in the notation of the efficiencies. How the values reflected in table 3 have been calculated, should be better explained.

Many of the results of the work are quite obvious. My recommendation is to extend the study, taking into account more ranges of irradiance values and temperatures, so that the work is more complete.

Some of the paragraphs of the section of discussions are neither a consequence nor results of what has been analyzed in the work.

Lines 256-262. The bibliography that supports all this information should be included.

Author Response

1. With regard to the PV installation, you can give more details of the plant in general, orientation and tilt angles, whether they are connected to the grid, the layout of the panels, inverters use...

The detailed information on the plant is added in lines 94 - 108.

Manuscript modification:

The modules are oriented to the south and tilted at the optimum angle for the given latitude equal to 34°. The installation is connected to the grid using the inverters. Both thin film installations are connected with the use of inverters with one MPPT tracker while for pc-Si PV string the inverter equipped with two MPPT systems is used. The distance between the rows of panels equals to 6.3 m.

Inverters as well as both mentioned sensors were connected to the central data-logging computer system for synchronous data collection.

2. With regard to measurements, irradiance and temperature sensors are specified, but commercial models of these sensors are not given, only one specification for the temperature sensor, where and how they are located in the PV plant.

We agree with the Reviewer that models of the irradiance and temperature sensors have not been given. Furthermore, models of the modules used in the analysis have not been published also. It was our intention not to reveal manufacturers of the devices. Our aim was rather to show specific differences of technology modules working under the same conditions than to compare manufacturers. We described specification of the sensors in details. Form literature review one can find that such approach seems to be acceptable and can be found in [1,2] for example.

1. P. Ferrada et al. Solar Energy 114 (2015) 356 – 363

2. M. Basoglu et al. Renewable and Sustainable Energy Reviews 52 (2015) 357–365

Location of the sensors has been described in lines 99-102:

Manuscript:

At the location of the experimental system solar radiation intensity on PV module plane as well as the temperature of the modules were measured. Solar radiation sensor based on the monocrystalline silicon solar cell is located in the centre of the installation and tilted at the same angle as modules. Pt1000 resistance temperature detectors are attached on the modules backside.

3. And the electrical measurements that are carried out at work are not indicated how they have been recorded. And the period of time in which the data to be analyzed in the work have been recorded must be clearly indicated in section 2 of methodology .

To address the reviewers’ concern information about data collection has been provided. Period of the analysis has been also indicated in methodology section (lines 107-110).

Manuscript modification:

Inverters as well as both mentioned sensors were connected to the central data-logging computer system for synchronous data collection.

All parameters such as solar irradiance, DC generated electric power, and module temperature of each photovoltaic technology studied were measured at each 5 minutes during the considered period of the year and stored for the analysis.

4. With respect to the editing of the work, the same notation should be always uses, for example, the first time the word photovoltaic appears, put in parentheses its notation (FV), but then from there always use FV. Avoid the use of first people (ie. we characterized...).

The explanation of abbreviation PV is added in Abstract and now it occurs in lines 16 and 34.

The sentence starting with “we characterized” is changed.

5. The results obtained from the three technologies should be compared with those obtained by other authors in similar works. The results only allude to one comparison.

Following text is added in Discussion section (lines 297-307).

Manuscript modification:

The comparison of the presented results with studies conducted by other authors is rather difficult even taking into account investigations performed at similar latitude since the technical parameters of the modules and other devices differ and also weather conditions in summer and winter are not the same. Exemplary studies performed in UK show better performance of CIGS in winter due to the spectrum [9]. The investigations focused on the assessment of the spectral impact on different PV technologies, performed in Germany [29], indicate spectral gains of 2.4% for CdTe, 1.1% for c-Si (that can be compared with pc-Si), 0.6% for CIGS during the entire year. Temperature coefficients of power estimated in Neatherlands for different PV technologies are higher than technical data for CdTe and CIGS and similar for poly-Si [17]. Considering the results of the investigations in mention countries it is worth to remember that the weather conditions both in winter and summer differ from that in Poland in spite of similar latitude..

6. When reference is made to the IEC 61724 standard, it should be included in the bibliography, and indicate whether it refers to the standard published in 1998, or to its last update in 2017.

As suggested by the Reviewer IEC standard has been added to the bibliography section referring to the publication date:

Manuscript:

IEC 61724, Photovoltaic system performance monitoring-guidelines for measurement, data exchange and analysis. 1^st ed. Geneva. International Electrochemical Commission; 1998

7. However, in my opinion it is also very interesting to carry out this study during the whole year, including these months of lower irradiance, given that this type of installations will be operating all year round, and it is precisely in these most critical months that the behavior of the PV installation must be very well characterised.

Thank you for this suggestion. According to Reviewers’ recommendation the whole year has been taken into account in revised manuscript. Figs. 3, 4, 10 and 11 have been replaced by new Figures with all months of 2015 included. Detailed analysis of new results as well as the discussion have been provided. New values of median for irradiance and temperature have been computed and showed (including all months of the year). New sentences has been added to Results section (lines 255-273):

Manuscript modification:

In summer period (June - August) CIGS modules produced about 3 % more energy than polycrystalline Si modules which is in a good agreement with observations of temperature influence in this period. CdTe modules exhibit the lowest energy production, even in summer, in spite of small value of temperature coefficient. Energy production by CdTe technology is lower in comparison with pc-Si of 1.1%-8.9% which is probably caused by its small efficiency. The exception is June when CdTe produced 0.2% more energy per kWp than pc-Si.

Higher energy yield and performance ratio of pc-Si modules were noticed in colder period of the year, i.e. from March to May (up to 4.8%) and from October to November (up to 4.6%). In terms of CdTe modules lower monthly PR values were noticed in comparison to pc-Si technology for the whole year (from about 1% in May to even 14% in October) with exception of June. In winter months (December – February) PR values drop significantly due to occurring of snow cover and decrease of inverters efficiency caused by low temperatures. These two problems make difficult the interpretation of the PR results obtained in winter and thereby the assessment of particular PV technologies in this period. Most of the months (March – October) are characterized by better insolation (Fig. 2) and PR value above 80% (even 90% for pc-Si).

8. The magnitudes represented in each figure must be very well specified. In some figures it is not clear whether what is represented is total daily, or instantaneous data. Therefore, this must be very well specified.

The following lines have been changed:

179, 189, 192, 221, 244

9. With respect to figure 6, how the four points of each technology reflected in the graph have been obtained should be explained.

Caption of the Figure as well as the text in lines 191-192 have been modified.

Manuscript:

Figure 6. Daily energy yield vs. daily average temperature of the different technology modules for four chosen sunny days of 2015.

For different analyzed PV technologies the dependency of daily energy output as a function of daily average module temperature was shown in Figure 6.

10. Lines 156-159. This paragraph should be explained better. Allusion is made to a temperature value, 55 º, which does not appear in the figure.

The text is changed in lines 184-187 to provide better explanation of Fig. 5.

Manuscript:

The example of hot sunny day, characterized by the high solar irradiance level reaching 1000 W/m2 when the modules are heated by solar radiation up to 55 °C, shows that under this kind of realistic conditions the power generated by pc-Si installation is lower comparing to thin film technologies (Figure 5).

11. In equation 7, and the explanation of the magnitudes that appear in it, there is some error in the notation of the efficiencies. How the values reflected in table 3 have been calculated, should be better explained.

The text in lines 220-230 and titles of columns in Table 3 is changed.

12. Many of the results of the work are quite obvious. My recommendation is to extend the study, taking into account more ranges of irradiance values and temperatures, so that the work is more complete.

We agree with the Reviewer that some of the results can be known form other researches. However, to our best knowledge, there are not many research studies which compare different technology PV systems working under moderate climate. For this reason we found our analysis new and interesting for readers.

According to the Reviewers suggestion revised study has been extended, including all months of the year. Irradiance and temperature distributions have been computed and showed in new Figs. 3 and 4. PR values as well as daily energy output have been calculated for all months (including winter) and showed in new Figs. 10 and 11. Detail discussion of the results has been provided.

13. Some of the paragraphs of the section of discussions are neither a consequence nor results of what has been analyzed in the work.

The last paragraph of Discussion is canceled.

14. Lines 256-262. The bibliography that supports all this information should be included.

Bibliography is added in lines 289-296.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper intended to evaluate a number of parameters listed in the manufacturer's sheet under ambient environment. This exercise doesn't make too much sense in my opinion. It is well know that those parameters for PV panels are tested under lab environment, and the discrepancy will be inevitable if measured in ambient condition. Meanwhile, those parameters are for guidance only.

Some fancy equations are used but there are already concepts describe such situation, for instance, Equation (3) and (4) are effectively the capacity factor and sun hour.

The temperature effect demonstrated in Fig 6, 8 are well understood. As for Figure 7, there will be a mismatch between the efficiency shown in datasheets and the experimental measured values.

I think the paper needs to identify the fundamental question, which is “what’s the novelty and the true contribution of the work”.

Author Response

1. This paper intended to evaluate a number of parameters listed in the manufacturer's sheet under ambient environment. This exercise doesn't make too much sense in my opinion. It is well know that those parameters for PV panels are tested under lab environment, and the discrepancy will be inevitable if measured in ambient condition. Meanwhile, those parameters are for guidance only.

To address the reviewers’ concern our aim was rather to show specific differences of technology modules working under the same conditions than to evaluate parameters delivered by manufacturers. In the literature one can find many similar studies carried out for different climates [1-3]. To our best knowledge, there are not many research studies which compare different technology PV systems working under moderate climate. For this reason we found our analysis new and interesting.

[1] C. Canete et al. Energy 65 (2014) 295 – 302

[2] S. Rehman et al. Energy 46 (2012) 451 - 458

[3] G. Markides et al. Renewable Energy 43 (2012) 407-417

What is more, the discrepancy between manufacturer sheet and real conditions is obvious but which parameters change and what is the measure of their change is not obvious.

The text is added (lines 73-76):

2. Some fancy equations are used but there are already concepts describe such situation, for instance, Equation (3) and (4) are effectively the capacity factor and sun hour.

To address the reviewers’ concern the performance evaluation procedure has been carried out according to the International Standard for photovoltaic system performance monitoring IEC 61724. Equations used for computing have been chosen similarly to approach used by other researchers in this area of expertise, for example [1,2].

[1] C. Canete et al. Energy 65 (2014) 295 – 302

[2] V. Sharma et al. Energy 59 (2013) 511 - 518

3. The temperature effect demonstrated in Fig 6, 8 are well understood. As for Figure 7, there will be a mismatch between the efficiency shown in datasheets and the experimental measured values.

We agree with the Reviewer. However, these Figures are part of the whole analysis and in our opinion they were necessary to be included. They show clearly effects of temperature influence on efficiency of polycrystalline modules that results in lower performance ratios in summer months comparing to CIGS thin film modules as shown in Fig. 11. Similar effects can be noticed in Fig. 10.

4. I think the paper needs to identify the fundamental question, which is “what’s the novelty and the true contribution of the work

Thank you for this question. As it was mentioned before there are not many research studies which compare different technology PV systems working under moderate climate. For this reason we found the performance analysis of different technology modules working under polish climate conditions new and interesting.

See test added in lines (73-76)

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have done all the requested changes.

Author Response

We, the Authors, would like to thank you for the time you spent on our manuscript and appreciate the provided comments and suggestions to improve the quality of the manuscript.

Reviewer 2 Report

The work has improved with the changes made, following the proposed indications. However, I believe that there are aspects that still need to be improved so that the paper can be published.

The use of the acronym PV must be homogeneous in the document, it is used only sometimes, and the term photovoltaic is included in other occasions.

Although in the new version of the manuscript the analysis of the measured data in the PV installation is done for one year, in the abstract it still appears that it is done only from April to August.

We must try to avoid the use of the first person when writing, for example, "our work".

English grammar should be checked.

When reference is made to the European directive 2009/28/EC, reference is being made to a directive from nine years ago, there are more recent agreements of the European Union that could be better included, such as the agreements of June 2018) [1], which propose to materialize the measures already agreed in the so-called Winter Package of November 2016 [2].

When reference is made to the IEC 61724 standard, it should include the year in which the standard was published, and it would be logical to refer to the most recent publication.

With regard to figure 6, it refers only to the values found in 4 days of the whole period analysed. The results would be more representative if a longer period of time were used, because generalizing the behavior to what happens in four days, having recorded data for a whole year, is not very correct. And that there are only four days is only included in the figure caption, but not in the text of the manuscript that alludes to that figure.

Figure 7 refers only to the months of April to September. Could the data corresponding to the whole year be collected in some other graph? If it is done only for these months, justify why. If you want to have results to compare with the manufacturer's data, you can filter the data and calculate the instantaneous efficiencies at values of 1000 W/m2 and 25 degrees, but you would also have to give the values of these efficiencies to the rest of the irradiance values and temperatures in which the modules work.

Figure 8 is also calculated with only four days? It is said that for a few days chosen with a higher irradiance value, but it would have to be specified. In any case, I think that choosing only four days is not representative, authors should use all possible days in which these values have been recorded, and also with the rest of values, if authors want to find a tendency representative of the behavior of the different technologies of the panels. I think that the value of the temperature coefficient is obtained from this figure with data from only four days, which I do not think is very representative, having recorded data for a full year.

On the y-axis of figure 8 indicate that it refers to daily efficiency.

The same comments can also be made with respect to figure 9 of the manuscript, I think you can include the data of more days in the figure, so that the data are more representative of what actually happens in the environmental conditions recorded during the year analyzed.

In the figure 9 it is necessary to indicate if what is represented are average values or daily total values. It is necessary to specify very well in all the graphs what is really being represented.

Line 270: "decrease of inverters efficiency caused by low temperatures". Does this really happen? I believe that this assertion should be supported, either with some bibliographical reference, or with experimental results that clearly corroborate it.

Author Response

The work has improved with the changes made, following the proposed indications. However, I believe that there are aspects that still need to be improved so that the paper can be published.

1. The use of the acronym PV must be homogeneous in the document, it is used only sometimes, and the term photovoltaic is included in other occasions.

As suggested by the Reviewer text has been modified.

2. Although in the new version of the manuscript the analysis of the measured data in the PV installation is done for one year, in the abstract it still appears that it is done only from April to August.

Thank you for this comment. It was our mistake not to correct this sentence. Now, first line of the Abstract (line 15) is changed.

3. We must try to avoid the use of the first person when writing, for example, "our work".

Line 75 - the text is changed

4. English grammar should be checked.

As suggested, manuscript has been checked for grammar and typo mistakes. Corrections have been made.

5. When reference is made to the European directive 2009/28/EC, reference is being made to a directive from nine years ago, there are more recent agreements of the European Union that could be better included, such as the agreements of June 2018) [1], which propose to materialize the measures already agreed in the so-called Winter Package of November 2016 [2].

Thank you for this suggestion. According to Reviewers’ recommendation the text has been changed in lines 31-33. Directive (EU) 2015/1513 has been added to the References section (lines 350-352).

6. When reference is made to the IEC 61724 standard, it should include the year in which the standard was published, and it would be logical to refer to the most recent publication.

As suggested by the Reviewer, the most recent standard has been cited in reference section. The year of publication has been included.

7. With regard to figure 6, it refers only to the values found in 4 days of the whole period analysed. The results would be more representative if a longer period of time were used, because generalizing the behavior to what happens in four days, having recorded data for a whole year, is not very correct. And that there are only four days is only included in the figure caption, but not in the text of the manuscript that alludes to that figure.

To address the Reviewers’ concern more points have been added to the plot. Longer period of time was analyzed and included in Figs. 7-9, but in the case of daily energy yield (Fig. 6) only days with similar (daily) solar irradiation could have been taken into account. For this reason, it was impossible to find sunny days with low modules temperature (winter or early spring sunny days) and similar to summer irradiation level. However, Fig. 6 has been updated. Change in line 189 has been proposed.

8. Figure 7 refers only to the months of April to September. Could the data corresponding to the whole year be collected in some other graph? If it is done only for these months, justify why. If you want to have results to compare with the manufacturer's data, you can filter the data and calculate the instantaneous efficiencies at values of 1000 W/m2 and 25 degrees, but you would also have to give the values of these efficiencies to the rest of the irradiance values and temperatures in which the modules work.

Thank you for this suggestion. Experimental efficiencies (averages of daily efficiencies of all days in studied period) have been calculated for longer period of time (February – November of 2015). Winter has been excluded because of poor data due to snow. Fig. 7 has been updated. Text has been modified in lines 207-215.

9. Figure 8 is also calculated with only four days? It is said that for a few days chosen with a higher irradiance value, but it would have to be specified. In any case, I think that choosing only four days is not representative, authors should use all possible days in which these values have been recorded, and also with the rest of values, if authors want to find a tendency representative of the behavior of the different technologies of the panels. I think that the value of the temperature coefficient is obtained from this figure with data from only four days, which I do not think is very representative, having recorded data for a full year.

Thank you for this comment. Fig. 8 has been updated including all representative sunny days of 2015. The range of avg. modules’ temperature has been extended. More points have been added. Lines 217 – 222 have been modified. Coefficients presented in Tab. 3 have been updated.

10. On the y-axis of figure 8 indicate that it refers to daily efficiency.

Y-label of Fig. 8 has been changed to ‘Daily efficiency [%]’.

11. The same comments can also be made with respect to figure 9 of the manuscript, I think you can include the data of more days in the figure, so that the data are more representative of what actually happens in the environmental conditions recorded during the year analyzed.

According to the Reviewer suggestion, more points have been added. Fig. 9 has been updated. Text has been changed in lines of 244 – 249.

12. In the figure 9 it is necessary to indicate if what is represented are average values or daily total values. It is necessary to specify very well in all the graphs what is really being represented.

According to the Reviewer suggestion explanation has been given in lines 244 – 249.

13. Line 270: "decrease of inverters efficiency caused by low temperatures". Does this really happen? I believe that this assertion should be supported, either with some bibliographical reference, or with experimental results that clearly corroborate it.

Typically the highest efficiency of inverter is reached when the load is in the range of the 20-80% of its nominal power. Below 15-20% of the inverter power the efficiency rapidly drops. This situation occurs in winter because of the low insolation level. Line 276 (present form of the manuscript) – the text is changed.

Author Response File: Author Response.pdf

Reviewer 3 Report

The revised version addressed a number of comments raised in the previous review. However, there are still points not very clear to me.

In the abstract, it mentioned the study is over six months, from April to September. However, later in the manuscript, it presented data for entire year. It causes confusion to the readers.

Data presented in Figure 6, 8 and 9 are not clear. How those data points are generated should be clearly addressed.

Some arguments are questionable. For instance, last paragraph of section 3, "decrease of inverters efficiency caused by low temperature", is it right?

The manuscript argued that this study is new and add new knowledge, although not very convenience to me, it is better to include the impact of the work in more detail, as well as drawbacks of the research and further recommendation.

Finally, proofreading is required to remove all typos and grammar mistakes.

Author Response

The revised version addressed a number of comments raised in the previous review. However, there are still points not very clear to me.

1. In the abstract, it mentioned the study is over six months, from April to September. However, later in the manuscript, it presented data for entire year. It causes confusion to the readers.

Thank you for this comment. Line 15 – the text is changed

2. Data presented in Figure 6, 8 and 9 are not clear. How those data points are generated should be clearly addressed.

To address the Reviewers’ concern we would like to inform that Figs. 6 – 9 have been modified as suggested by one of the Reviewers. Longer period of time was analyzed and included in Figs. 7-9. In the case of daily energy yield (Fig. 6) only days with approximate (daily) solar irradiation could have been taken into account. For this reason, it was impossible to find sunny days with low modules temperature (winter or early spring sunny days) and similar to summer irradiation level. Explanation in line 189 has been proposed.

Fig. 8 has been updated including all representative sunny days of 2015. The range of avg. module temperature has been extended. More points have been added. Lines 217 – 221 have been modified explaining the methodology. Coefficients presented in Tab. 3 have been recalculated.

More points have been also added to graphs in Fig. 9. Text has been changed in lines of 244 – 249 to explain how the points have been generated.

3. Some arguments are questionable. For instance, last paragraph of section 3, "decrease of inverters efficiency caused by low temperature", is it right?

4. The manuscript argued that this study is new and add new knowledge, although not very convenience to me, it is better to include the impact of the work in more detail, as well as drawbacks of the research and further recommendation.

To address the Reviewers’ concern impact of the work has been presented in our opinion in lines: 72-78. More can be found in 11-14 or 25-27 and in the Conclusion part (334-340)

5. Finally, proofreading is required to remove all typos and grammar mistakes.

As suggested, manuscript has been checked for grammar and typo mistakes. Corrections have been made.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

I think the paper has improved and can be published. However, the authors are encouraged to extend the study by one or two more years, if possible, in order to try to have more experimental data to give more basis to the trend values obtained in some of the graphs.

Reviewer 3 Report