Performance Investigation of Solar ORC Using Different Nanofluids
Round 1
Reviewer 1 Report
The paper provides the results of a simulation environment for a parabolic dish concentrator coupled with an organic Rankine cycle. Three different receivers were tested (hemispherical, cubical, or cylindrical) and four different thermal-oil-based nanofluids were tested as heat transfer fluids. The work is well-written and organized. The mathematical model was validated through experimental tests. However, the paper cannot be accepted in the current form because some relevant information and results are missing (points 3 and 10 of the following list). In addition, some results are strange (point 9). The authors should therefore provide a revision of the paper in order to answer the aforementioned points and the other reported in the following list.
1) Table 1 and 6. There is an error in the line “paraboloid rim angle” (°) and “rim angle” (°), respectively.
2) Page 4, “Conduction heat loss”. Why do you consider conductive and convective losses together? The section should only deal with conductive heat losses.
3) Section 2.3.3. What are the nanofluids concentrations? This quantity is fundamental, however it was not reported.
4) Table 3. What temperature do these properties refer to? Also, dynamic viscosity was not reported.
5) Line 262. Figure 5.
6) Line 314. Figure 9.
7) Line 316. Higher.
8) Figure 9, 10, 11 and so on. Please use the same color to indicate the same receiver geometry.
9) Lines 378-379 and 416-417. This behavior is very, very strange, and in my opinion not correct. The nanofluids viscosity should be higher than the base fluid. As requested in 3), what are the nanofluids concentrations?
10) Results should also analyze the system performance with different nanofluids concentrations.
Author Response
Dear Editor-in-Chief,
"Applied Sciences", https://www.mdpi.com/journal/applsci
Manuscript No. applsci-536755
Title: “Performance Investigation of Solar ORC using Different Nanofluids “
We would like to thank you very much for your positive and interesting comments. We revised our paper according to your suggestions. The revised paper is improved and it is according to the Journal standards. Below, you can find our analytical responses. All the requested modifications have been carried out and highlighted in a separate file.
Response to the reviewers:
Reviewer 1:
The paper provides the results of a simulation environment for a parabolic dish concentrator coupled with an organic Rankine cycle. Three different receivers were tested (hemispherical, cubical, or cylindrical) and four different thermal-oil-based nanofluids were tested as heat transfer fluids. The work is well-written and organized. The mathematical model was validated through experimental tests. However, the paper cannot be accepted in the current form because some relevant information and results are missing (points 3 and 10 of the following list). In addition, some results are strange (point 9). The authors should therefore provide a revision of the paper in order to answer the aforementioned points and the other reported in the following list.
Many thanks for your valuable and positive comments.
Comment 1# Table 1 and 6. There is an error in the line “paraboloid rim angle” (°) and “rim angle” (°), respectively.
Re: Sorry for this problem. Degree symbol was modified in tables 1 and 6 based on your comment.
Comment 2# Page 4, “Conduction heat loss”. Why do you consider conductive and convective losses together? The section should only deal with conductive heat losses.
Re: It should be mentioned that thermal resistance approach was used for estimating cavity heat losses due to the conduction and external convection. This approach was selected because of specified amounts of cavity wall temperature, and ambient temperature. The ambient temperature was assumed equal to 20.2ºC, and the cavity wall temperatures were determined as a variable based on the presented method in the following subsections.
Title of the subsection was changed to “Conduction and external convection heat losses” in the revised manuscript and the following sentences were added to the revised manuscript.
“• Conduction and external convection heat losses
Eq. (5) can be used for the determination of the conduction and external convection heat losses of the cavity. It should be mentioned that thermal resistance approach was used for estimating cavity heat losses due to the conduction and external convection. This approach was selected because of specified amounts of cavity wall temperature, and ambient temperature. The ambient temperature was assumed equal to 20.2ºC, and the cavity wall temperatures were determined as a variable based on the presented method in the following subsections.”
Comment 3# Section 2.3.3. What are the nanofluids concentrations? This quantity is fundamental, however it was not reported.
Re: Sorry for this problem. The following sentence was added to the revised manuscript:
“It should be mentioned that the nanofluids were investigated based on volume fraction equal to 3%.”
Comment 4# Table 3. What temperature do these properties refer to? Also, dynamic viscosity was not reported.
Re: Thank you for your comment. It should be mentioned that the reported thermal properties are for the solid nanoparticles that dynamic viscosity is not possible to be reported. Also, refer temperature of the reported properties is ambient temperature of 25ºC. This sentence was added to the revised manuscript.
Comment 5# Line 262. Figure 5.
Re: Sorry for this problem. It was modified in the revised manuscript.
Comment 6# Line 314. Figure 9.
Re: Sorry for this problem. It was modified in the revised manuscript.
Comment 7# Line 316. Higher.
Re: Thank you. It was revised in the modified manuscript.
Comment 8# Figure 9, 10, 11 and so on. Please use the same color to indicate the same receiver geometry.
Re: Many thanks for your comment. Use a same color to indicate the same receiver geometry in Figures 9, 10, and 11 as shown in the revised manuscript.
Comment 9# Lines 378-379 and 416-417. This behavior is very, very strange, and in my opinion not correct. The nanofluids viscosity should be higher than the base fluid. As requested in 3), what are the nanofluids concentrations?
Re: Sorry for this problem and thanks for your valuable comment. All of the models were run again based on constant volume flow rate of 60 ml/s and the results were completely modified. Also, amount of the nanofluid concentration was 3%VF that was mentioned in the revised manuscript.
Comment 10# Results should also analyze the system performance with different nanofluids concentrations.
Re: Influence of different nanofluids with different nanofluid concentrations was investigated as the solar working fluid of the investigated solar ORC system. The following sentences and analyses were added to the revised manuscript:
“Finally, Figure 17 shows variation of overall efficiency improvement of the solar ORC system with variation of nanofluid concentration. The cubical cavity receiver was used as the dish absorber. Different nanofluids including oil+Al2O3, oil+CuO, and oil+SiO2 nanofluids were considered as the solar working fluid. The solar system was investigated at working fluid inlet temperature of 40ºC, and solar radiation as equal to 632.97 W/m2. On the other side, the ORC system was considered at constant turbine inlet temperature of 229ºC, and turbine inlet pressure of 2.5 MPa. Whereas, methanol was used as the ORC working fluid. As seen from Figure 17, thermal efficiency resulted higher improvement using application of oil+Al2O3nanofluid with higher nanofluid concentration. Generally, thermal performance improvement was calculated between 1-2% such as reported results by ref. [41]. Also, similar results were presented by ref. [14] for application of different nanofluids with variation of nanofluid concentration as heat transfer fluid of a cylindrical cavity receiver. In the recent study, application of different oil-based nanofluids as heat source of an ORC system with different shapes of cavity receivers as the ORC heat source is presented as a new subject for study.
Figure 17: Variation of overall efficiency improvement of the solar ORC system with variation of nanofluid concentration at Tin,oil=40ºC, TIT=229ºC, and TIP=2.5 MPa.”
Reviewer 2:
This study investigates the performance of Al2O3/oil, CuO/oil, SiO2/oil, and MWCNT/oil as solar heat transfer fluids. The article is interesting and in general is well presented. Authors have previous experience in the field. I think the article should be accepted after correcting the following minor remarks:
Thank you for your positive comment.
Comment 1# Please, revise the Eq.(44) using to estimate expanded uncertainties according to EA-4/02 guide.
Re: Thank you it was modified based on EA-4/02 guide in the revised manuscript.
Comment 2# Too many significant figures in some tables.
Re: Number of figures was reduced especially in some tables. Most of them were merged together for better understanding in the revised manuscript.
Comment 3# Batchelor model is normally not appropriated for predicted the dynamic viscosity of nanofluids containing MWCNTs.
Re: Sorry for this problem, Analyses related to the MWCNT/oil was removed form the revised manuscript. Application of CNT/oil nanofluid will be investigated in our future research.
Comment 4# How were nanoparticles concentrations selected?
Re: Sorry for this problem. The following sentence was added to the revised manuscript:
“It should be mentioned that the nanofluids were investigated based on volume fraction equal to 3%.”
Comment 5# The maximum percentage improvement of overall efficiency is lower than 2%. Do authors consider it enough? Please, comment.
Re: The calculated results related to enhancement of the solar system performance can be compared with reported results by ref. [41]. Bellos and Tzivanidis [41] investigated performance of a solar concentrator system using different nanofluids including 3% Al2O3/Oil, 3% TiO2/Oil, and 1.5% and Al2O3-1.5% TiO2/Oil. They reported improvement lower than 1% for the investigated solar system using different nanofluids. The cavity receivers have very small thermal losses and so there is not so high thermal enhancement margin. So, the use of nanofluids as a thermal enhancement method can enhance the performance up to 2-3% maximum. These sentences were added to the revised manuscript.
xComment 6# Do not mix different units for the same magnitude. For instance, temperature is given in “ºC” and “K” depending on the part of the article.
Re: Thank you for your comment. Same unit was used for temperature (as ºC) in the revised manuscript.
Comment 7# Some subscripts and superscripts should be controlled. For instance, in the y-axis of Figure 8.
Re: Sorry for this problem. All of the graphs were checked related to this notice.
Reviewer 3:
The work analyses the performance of three tubular cavity receivers (i.e., hemispherical, cubical, and cylindrical) for solar-driven organic Rankine cycle (ORC) systems and the influence of different nanofluids is investigated. To this purpose, a numerical analysis is carried out adopting ideal steady-state conditions.
Major comments:
Thank you for your valuable comments.
Comment 1# The authors should adopt real efficiency values for the turbine, pump, and heat exchangers.
Re: Thank you for your valuable opinion. In this study the main goal was investigation of influence of nanofluid application as the solar working fluid on the ORC performance. Application of the real efficiency values for the turbine, pump, and heat exchangers will be investigated in our next research as an interesting subject for research is the reviewer agrees.
Comment 2# The selection of the ORC operating conditions should be properly motivated.
Re: This sentence was added to the revised manuscript. “Input data were selected based on the reported optimum data by ref. [37].”
Comment 3# The authors should discuss and motivate the adoption of methanol as working fluid. Dry fluids are usually adopted for ORC systems.
Re: The following sentences were added to the modified version of the manuscript:
“Note that methanol was selected for this study, because of its high thermodynamic performance as reported in ref. [34]. As concluded in ref. [34], methanol resulted highest thermodynamic performance with lowest irreversibility as the ORC working fluid among some examined organic fluids including R113, R601, R11, R141b, Ethanol, and Methanol. Also, methanol is presented as an appropriate working fluid for medium to high-temperature system such as the examined solar system in the current research. Related to environmental properties of the methanol, methanol is investigated as environmental friendly fluid with low global warming potential at 100 years (GWP 100) equal to 2.8, and health hazard (H) of 1, and flammability (F) as equal to 3 [35].”
Comment 4# Specific references should be provided for the main assumptions (i.e., line 143: concentrator reflectivity, etc.).
Re: Specific references were added for the main assumptions in the modified version of the manuscript as following:
“All the structural characteristics were selected based on the real built cavities in refs. [10], and [22].
The reflectivity (η_refl) of the dish concentrator is assumed to be 0.84 [23]
Also, the dish optical error and the tracking error of the investigated system are assumed as 10 mrad, and 1º, respectively [23].”
Comment 5# Font style and size should be checked (i.e., lines 102-103, 115-120, 249-250, etc.).
Re: Thank you for your comment. Font style and size of the whole manuscript was checked and modified in the revised manuscript version.
Comment 6# The authors should provide information about the location of elements for each cavity.
Re: Figure 2 was added to the revised manuscript related to the location of elements.
Comment 7# Line 114: References should be in progressive order ([10], [17], and [18]).
Re: Thank you for your comment. References were presented in progressive order in the revised manuscript as following:
“Note that the numerical methodology for using pure thermal oil in the hemispherical, cubical, and cylindrical cavity receivers have been presented in references [10], [17], and [18], respectively.”
Comment 8# Line 208: Check the reference no. 30.
Re: Ref. number was checked and modified in the modified manuscript.
Comment 9# Lines 113 and 179: It is suggested to remove “Note that”.
Re: “Note that” was removed in the revised manuscript.
Comment 10# Table 1: Check values and units of the “paraboloid rim angle”.
Re: Thanks a lot. It was checked.
Comment 11# Line 125: It is recommended to add the reference for the software.
Re: Thank you for your comment. The following refs. were added to the revised manuscript related to the software.
The cavity receivers are optically investigated with a ray-tracing software, SolTrace [8].
It should be mentioned that thermal modeling was developed on the Maple software [18].
It should be mentioned that thermodynamic analyses were conducted using the Maple and REFPROP.8 software [14].
Comment 12# Line 262: Modify "Figure 6" in Figure 5.
Re: Sorry for this problem. It was modified in the revised manuscript.
Comment 13# Figure 8: It is suggested to reduce the maximum value of the y-axis.
Re: In the revised manuscript, the maximum value of the y-axis was reduced in Fig. 8.
Comment 14# Line 362: Modify "Figure 15" in Figure 14.
Re: Thank you. It was modified in the revised manuscript.
Reviewer 4:
This paper investigates the performance of ORC systems which use receivers with different cavity shapes and employ various types of nano fluids as working fluid. Numerical thermal model of the cavity receivers was developed and validated experimentally. Generally, the reported results in the paper are good but many issues still exist in the current paper and need to be addressed before recommending the paper for any possible publication.
Thank you for your valuable comments.
Comment 1# The source of many parameters mentioned in the paper and used in the calculation are missing and should all be added. For example, The source of data in Table 3, Table 4, and Table 5 is not mentioned. Please check and add the sources across the whole paper.
Re: Thank you for your comment. Whole the manuscript was checked and references were added for them.
Comment 2# In section 3, please add a schematic of box diagram which explains the system used for the experimental testing. Figures 3,4, and 5 are good but hard to follow so it is better to add a schematic.
Re:The following schematic was added to the revised manuscript as a compact view of the experimental setup. Also, the material and method section was rewritten and rearranged for better understanding.
Comment 3# In section 3, how many experiments were conducted? It is better to show the results of more than one experiment in order to ensure the repeatability of the experimental work
Re: The test was conducted during different times of a day, the number of tests were 12 during the tests. Many researches were conducted based on this method for experimental tests with variable ambient conditions. Of course we will repeat our tests in our future works. Thank you for your comment.
Comment 4# Results and findings section need more discussion to relate the results of this paper with those already published in the literature and to emphases the new knowledge generated from the paper.
Re: Thank you for your valuable comment. More discussion and refs. was added to the modified manuscript as following:
“The presented results in Figure 9 can be compared with the reported results by refs. [17] and [18] for rectangular and cylindrical cavity receiver as solar dish absorber, respectively. In this research, variation of cavity surface temperature was presented and compared for three shapes of cavity receiver including hemispherical, cubical, and cylindrical cavities as a new achievement.
Similar results were presented by refs. [17] and [18] for rectangular and cylindrical cavity receiver. In the current study, a comparison heat flux distribution was presented for three investigated cavity receiver including hemispherical, cubical, and cylindrical cavities as a new achievement.
Figure 12 depicts variation of the net heat transfer rate along the cavity receivers using weather conditions of 20th October 2016, Tehran, Iran. As observed from Figure 13, the maximum solar power is at the 7th element in the hemispherical cavity receiver. On the other side, variation of available solar power along the cavity tubes for different investigated cavity receives is presented in Figure 13 based on the calculated results by the SolTrace software.
Similar achievements are reported by other papers including [17] and [18] for a solar dish concentrator with rectangular and cylindrical cavity receiver. A comparison study was presented in this research for different thermal performance parameters such as solar heat flux, absorbed heat, and outlet temperature for three investigated cavity receivers including the hemispherical, cubical, and cylindrical cavities as a new result.
In this section, performance of a solar ORC system with different shapes of the cavity receivers is considered including the hemispherical, cylindrical, and cubical cavity receivers. Thermal oil and methanol were used as the solar working fluid, and ORC working fluid. Inlet temperature of the solar heat transfer fluid was assumed equal to 40ºC, and 632.97 W/m2, respectively.
Similar results were concluded by ref. [34] for a cubical cavity receiver as heat source of an ORC system. In the current study a performance comparison of different shapes of cavity receiver including hemispherical, cylindrical, and cubical cavity receivers is presented as heat source of the ORC system for selecting the best system for power generation.
The hemispherical, cylindrical, and cubical cavity receivers were used as the ORC heat source. Different nanofluids including oil+MWCNT, oil+Al2O3, oil+CuO, and oil+SiO2 nanofluids were considered as the solar working fluid with nanofluid concentration as equal of 3% volume fraction. The solar system was investigated at working fluid inlet temperature of 40ºC, and solar radiation as equal to 632.97 W/m2. On the other side, the ORC system was considered at constant turbine inlet temperature of 229ºC, and turbine inlet pressure of 2.5 MPa.
Similar studies were conducted by other refs. such as [39] and [40] that energetically and energetically investigated influence of nanofluid application as solar working fluid of a dish concentrator with a spiral cavity receiver. Similar results were reported by refs. [39] and [40]. In the recent study, application of different oil-based nanofluids as heat source of an ORC system with different shapes of cavity receivers as the ORC heat source is presented as a new subject for study.
The calculated results related to enhancement of the solar system performance can be compared with reported results by ref. [41]. Bellos and Tzivanidis [41] investigated performance of a solar concentrator system using different nanofluids including 3% Al2O3/Oil, 3% TiO2/Oil, and 1.5% and Al2O3-1.5% TiO2/Oil. They reported improvement lower than 1% for the investigated solar system using different nanofluids. The cavity receivers have very small thermal losses and so there is not so high thermal enhancement margin. So, the use of nanofluids as a thermal enhancement method can enhance the performance up to 2-3% maximum. The calculated results can be compared with the reported results by other refs. such as [42]. Researchers were considered effect of alumina/oil nanofluid with different size and volume fractions as solar working fluid of a solar ORC system. In the recent research, performance of the solar ORC system using different nanofluids including oil+MWCNT, oil+Al2O3, oil+CuO, and oil+SiO2 nanofluid of the solar working fluid a new subject for assessment.
Finally, Figure 17 shows variation of overall efficiency improvement of the solar ORC system with variation of nanofluid concentration. The cubical cavity receiver was used as the dish absorber. Different nanofluids including oil+MWCNT, oil+Al2O3, oil+CuO, and oil+SiO2 nanofluids were considered as the solar working fluid. The solar system was investigated at working fluid inlet temperature of 40ºC, and solar radiation as equal to 632.97 W/m2. On the other side, the ORC system was considered at constant turbine inlet temperature of 229ºC, and turbine inlet pressure of 2.5 MPa. Whereas, methanol was used as the ORC working fluid. As seen from Figure 17, thermal efficiency resulted higher improvement using application of MWCNT/oil nanofluid with higher nanofluid concentration. Generally, thermal performance improvement was calculated between 1-2% such as reported results by ref. [41]. Also, similar results were presented by ref. [14] for application of different nanofluids with variation of nanofluid concentration as heat transfer fluid of a cylindrical cavity receiver.”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This study investigates the performance of Al2O3/oil, CuO/oil, SiO2/oil, and MWCNT/oil as solar heat transfer fluids. The article is interesting and in general is well presented. Authors have previous experience in the field. I think the article should be accepted after correcting the following minor remarks:
- Please, revise the Eq.(44) using to estimate expanded uncertainties according to EA-4/02 guide.
- Too many significant figures in some tables.
- Batchelor model is normally not appropriated for predicted the dynamic viscosity of nanofluids containing MWCNTs.
- How were nanoparticles concentrations selected?
- The maximum percentage improvement of overall efficiency is lower than 2%. Do authors consider it enough? Please, comment.
- Do not mix different units for the same magnitude. For instance, temperature is given in “ºC” and “K” depending on the part of the article.
- Some subscripts and superscripts should be controlled. For instance, in the y-axis of Figure 8.
Author Response
Please find attached the response. Thank you.
Author Response File: Author Response.pdf
Reviewer 3 Report
The work analyses the performance of three tubular cavity receivers (i.e., hemispherical, cubical, and cylindrical) for solar-driven organic Rankine cycle (ORC) systems and the influence of different nanofluids is investigated. To this purpose, a numerical analysis is carried out adopting ideal steady-state conditions.
Major comments:
1. The authors should adopt real efficiency values for the turbine, pump, and heat exchangers.
2. The selection of the ORC operating conditions should be properly motivated.
3. The authors should discuss and motivate the adoption of methanol as working fluid. Dry fluids are usually adopted for ORC systems.
4. Specific references should be provided for the main assumptions (i.e., line 143: concentrator reflectivity, etc.).
5. Font style and size should be checked (i.e., lines 102-103, 115-120, 249-250, etc.).
6. The authors should provide information about the location of elements for each cavity.
7. Line 114: References should be in progressive order ([10], [17], and [18]).
8. Line 208: Check the reference no. 30.
9. Lines 113 and 179: It is suggested to remove “Note that”.
10. Table 1: Check values and units of the “paraboloid rim angle”.
11. Line 125: It is recommended to add the reference for the software.
12. Line 262: Modify "Figure 6" in Figure 5.
13. Figure 8: It is suggested to reduce the maximum value of the y-axis.
14. Line 362: Modify "Figure 15" in Figure 14.
Author Response
Please find attached the response. Thank you.
Author Response File: Author Response.pdf
Reviewer 4 Report
This paper investigate the performance of ORC systems which use receivers with different cavity shapes and employ various types of nano fluids as working fluid. Numerical thermal model of the cavity receivers was developed and validated experimentally. Generally, the reported results in the paper are good but many issues still exist in the current paper and need to be addressed before recommending the paper for any possible publication.
- The source of many parameters mentioned in the paper and used in the calculation are missing and should all be added. For example, The source of data in Table 3, Table 4, and Table 5 is not mentioned. Please check and add the sources across the whole paper.
- In section 3, please add a schematic of box diagram which explains the system used for the experimental testing. Figures 3,4, and 5 are good but hard to follow so it is better to add a schematic.
- In section 3, how many experiments were conducted? It is better to show the results of more than one experiment in order to ensure the repeatability of the experimental work
- Results and findings section need more discussion to relate the results of this paper with those already published in the literature and to emphases the new knowledge generated from the paper.
Author Response
Please find attached the response. Thank you.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The paper has improved much with the revision provided by the authors. However, the numbers reported in Table 10-15 have too many significant digits, thus they should be corrected.
Author Response
Dear Editor-in-Chief,
"Applied Sciences", https://www.mdpi.com/journal/applsci
Manuscript No. applsci-536755
Title: “Performance Investigation of Solar ORC using Different Nanofluids “
We would like to thank you very much for your positive and interesting comments. We revised our paper according to your suggestions. The revised paper is improved and it is according to the Journal standards. Below, you can find our analytical responses. All the requested modifications have been carried out and highlighted in a separate file.
Response to the reviewers:
Reviewer 1:
The paper has improved much with the revision provided by the authors. However, the numbers reported in Table 10-15 have too many significant digits, thus they should be corrected.
Re: Many thanks for your comment. Reported data in Table 10-15 were modified based on your comment.
Author Response File: Author Response.pdf
Reviewer 2 Report
Authors have successfully performed the changes proposed by this reviewer. I think the article can be accepted for publication.
Author Response
Reviewer 2:
Authors have successfully performed the changes proposed by this reviewer. I think the article can be accepted for publication.
Thank you for your positive comment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Authors revised the manuscript following the reviewer suggestions and recommendations.
Minor comments:
1. The quality of Figure 1 should be improved.
2. Figure 14: Add units.
3. Table 10: Adopt subscripts.
4. Lines 460-462, Figure 14 caption: Check the “°” symbol.
Author Response
Reviewer 3:
Authors revised the manuscript following the reviewer suggestions and recommendations.
Thank you for your positive comments.
Minor comments:
Comment 1# The quality of Figure 1 should be improved.
Re: The quality of Figure 1 had been improved in the revised manuscript.
Comment 2# Figure 14: Add units.
Re: Thank you for your comment. Units were added to Figure 14 in the revised manuscript.
Comment 3# Table 10: Adopt subscripts.
Re: Sorry for this problem, subscriptions were modified in the revised manuscript.
Comment 4# Lines 460-462, Figure 14 caption: Check the “°” symbol.
Re: Thank you for your comment. The “°” symbol was modified in the revised manuscript.
