PAM Review: Energy Science & Technology, Vol. 5
ISSN 2205-5231 | Published by UTS ePRESS | https://epress.lib.uts.edu.au/student-journals/index.php/PAMR/index
The Effects of Nanoparticles on the Specific Heat Capacity of Molten Salts
Alexander Foldi1*, Duy Khang Simba Nguyen2 and Yeong Cherng Yap3
University of Technology Sydney, Faculty of Science, PO Box 123, Ultimo NSW 2017, Australia
1 Alexander.Foldi@student.uts.edu.au
2 Simba.Nguyen@student.uts.edu.au
3 Yeong.C.Yap@student.uts.edu.au
Corresponding author: Alexander Foldi, University of Technology Sydney, Faculty of Science, PO Box 123, Ultimo NSW 2017, Australia; Alexander.Foldi@student.uts.edu.au
DOI: http://dx.doi.org/10.5130/pamr.v5i0.1495
Citation: Foldi, A., Nguyen, D. K. S., Yap, Y. C. 2018. The Effects of Nanoparticles on the Specific Heat Capacity of Molten Salts. PAM Review: Energy Science & Technology, Vol. 5, pp. 56-65. http://dx.doi.org/10.5130/pamr.v5i0.1495
© 2018 by the author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) License (https://creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license.
Abstract
The desire to increase the efficiency of existing renewable energy sources has been thoroughly researched over the past years. This meta study aimed to investigate existing methods used by previous researchers to increase the Specific Heat Capacity of Molten Salt used for Concentrated Solar Power Plants. Investigations into nanoparticles were explored because of the effect of particle size and concentration can potentially increase the specific heat capacity of the molten salt. Numerous nanoparticles have shown to improve the thermal properties such as Silica (SiO2), Alumina (Al2O3), Titania (TiO2). Our summation was that the addition of nanoparticles into Molten Salts shows an increase in desired thermal properties of the Molten Salts. An efficiency increase of up to 28% was noted in the SHC (Cp) of the Molten Salts when Nanoparticles of 60nm were introduced.
Keywords
molten salt; specific heat capacity; nanoparticles; concentrated solar power, titanium dioxide
| Nomenclature | |
|---|---|
| SHC (Cp) | Specific Heat Capacity |
| CSP | Concentration Solar Power |
| Wt % | Weight Percentage Concentration |
| TiO2 | Titanium Dioxide |
| HTF | Heat Transfer Fluid |
Introduction
Concentrated solar power (CSP)
In the solar energy industry, the most commonly known solar power systems are solar panels, where solar radiation is used to turn photons into electrical energy, however solar panels are not able to produce electrical energy at night, this causes a lack of energy production at night which needs to be supplemented with conventional energy sources. Concentrated solar power (CSP) is a type of renewable energy that uses solar energy to achieve energy production. Unlike solar panels, CSPs systems work by collecting heat, through reflectors which heats up a Heat Transfer Fluid (HTF) such as Molten Salts.[3] This allows the thermal energy to be transferred to secondary mediums such as water to generate steam to produce electrical energy through a turbine.[2] In addition to using Molten Salts as the HTF, Molten Salts have the ability to store thermal energy, which allows for continuous operations even when no direct sunlight is available.[8] This gives CSP an advantage over solar panels in energy generation.
Nanoparticles
Developing technology at the nano-scale opens possibility to new realms of research. As the world moves into renewable energy sources, new studies into nanoparticle size (typically between 1 and 100nm) and their effect upon properties of materials has been closely investigated. [24-29] Engineered nanoparticles (ENPs) normally possess new or enhanced physico-chemical properties compared to that of the bulk material due to inherent quantum size effects, a large surface to volume ratio, and controlled particle shape and surface coating. [1]
Molten salts
Molten Salts also known as Solar Salts are commonly a binary mixture of sodium and potassium nitrate. [15, 31] While single salts can be used for the same purpose for HTF, the sodium nitrate has a melting point of 307oC and potassium nitrate has a melting point of 333oC. [31] By combining these salts in a certain mixture, the melting point can be reduced to 222oC, this allows for a reduced chance of the molten salts to freeze up the HTF as it moves across the CSP. [31] The advantageous properties of molten salts also include, low toxicity, cost, vapour pressure and thermal stability when in comparison to synthetic oils. [13,15]
When Molten Salts are compared to the conventional HTF of synthetic oils, these oils generally have a maximum operating temperature of 350oC before degradation occurs, this limits the maximum operating temperature of the CSP, whereas molten salts can operate at temperatures of 565oC [31] giving the CSP a much higher heat sink during the process of heating of water into steam for the CSP system. However, with the melting point of molten salts being 300 to 500oC, this increases the chance that a freeze up could occur, relative to oils, [31] where the molten salt would begin to solidify, due to not being able to maintain an operating temperature above the melting point, this would block the molten salts from flowing across the CSP system and effectively stopping the process of heat transfer to the water steam generation system in the CSP.
Increasing the Specific Heat Capacity (SHC) of the Molten Salts would allow the Molten Salts to retain this high operating temperature, through rainy/cloudy weathers, which would allow for continuous operation of the CSP. Therefore, this meta-study aims to determine what properties of nanoparticles increase the SHC of the Molten Salts.
Methods
Google Scholar and PubMed were accessed through UTS Library and served as the locations of the sources retrieved for this study. Different databases were used to search for scientific papers that were used as reference for the meta-study. PubMed and Google Scholar were primarily used to search for keywords that were relevant to Molten Salt with nanoparticles. Keywords that were used were nanoparticles, molten salt, specific heat capacity, thermal storage, concentrated solar power and thermodynamics. Large quantities of papers were obtained from the search result, using different combination of the keywords were utilized to narrow down the search results to find papers that were most relevant to the topic. Reading through the various abstracts were done to determine how relevant the papers were towards the topic.
The sources used in this meta-Study were only taken from Scientific Journals as independent papers house risks for credibility and relevance. The Articles that focused first and foremost upon Molten Salts and Nanoparticles were used as they served as the most relevant and informative. Reports that were backed by Universities and large number of citations were preferable as to limit the bias due to the pertinent nature of the Nanotechnology field. We also utilized this database with Journal articles that are from 2005 onwards, as these are more relevant to our meta study.
This meta-study is based solely on how nanoparticles added into Molten Salts change the SHC of the Molten Salt and does not account into cost and toxicity against conventional HTFs. Comparison between properties of the nanoparticles, which are particle size, concentration and nanolayers formed by the particles within the Molten Salts. By comparing each property to the change in specific heat of the molten salt, a relationship between how each property contributes directly to the amount of change in specific heat can be observed. This would allow for the best contributing factor between the properties looked into, to be concluded.
Because this study does not focus on the manufacturing of the nanoparticles themselves we only looked at the impact the particles had upon the Molten Salts and the systems that they are introduced into.
In order to find efficiency we looked at the initial SHC of the Molten Salts systems and how this changed when differing sized particles were introduced.
Due to the nature of our analysis we found that sifting through larger amounts of papers and finding unanimous results/conclusions added to the comprehensive nature of this Meta-Study and allowed for our conclusions to be made with high certainty.
Results and Discussion
Nanolayers
The addition of nanoparticles in Molten Salts have shown to increase the SHC. It was observed that there were multiple factors that contributed to what specific property of the nanoparticle would lead to the enhancement of the Cp of Molten Salts. One common explanation that has shown to be consistent throughout the various papers was the formation of Nano-layers being the key factor for the rise in Cp observed. [5-6, 13-14, 16] Nano-layers occurs at the interface of the liquid salt and solid nanoparticles, where the production of layers would be the consequent of the Molten Salts ions being constrained into a semi-solid phase. [13] Lasfargues. M, Bell. A, Ding. Y. (2015) theorized that the shape of the nanoparticles would greatly affect the adsorbed layers of fluid or solvent molecules with Nano-scale thickness. They concluded that the partial incorporation of the enthalpy of fusion in the semi-solid structure could be a viable explanation to the rise in Cp. [13]
Using this concept, Lasfargues. M, et al, (2016) hypothesized that during the liquid phase of the Molten Salts, interaction between the ionic liquid and the solid nanoparticles can enhance the SHC through the production of Nano-layers. This concept is further elaborated as it was concluded that the layer has higher thermal properties compare to the bulk liquid, which allows for the enhancement of the specific heat for nanofluids. [6,13,14]
Particle Size/Surface Area
Improving the SHC could be associated with the highly specific energies related to the high surface area of the nanoparticles per unit volume. [6] It was theorized that the diameter of the nanoparticles could play an essential role in increasing the SHC [6,7], this was achieved due to smaller nanoparticles being able to give greater solid/liquid interface area, resulting in an increasing contribution to the interfacial effect in the corresponding suspensions. [6] Changing the solid/liquid interfacial changes the phonon vibration mode near the surface area of the nanoparticle, allowing it to be able to change the SHC of a nanofluid. [6] The highly specific interfacial area of the nanoparticle is capable of absorbing liquid to the molecule’s surface, forming liquid layers. [6] The layers formed were capable of constraining the nanoparticles and control the free-boundary surface atoms into non-interior atoms. [6]
It was observed that decreasing the size of the nanoparticles was able to increase the SHC of the solar salt with nanomaterials in both solid phase (Table 1, Table 3) and liquid phase (Table 2, Table 4) [8], this was supported when it was suggested that larger nanoparticles have smaller SHC which would decrease the SHC of the molten salt overall. [16]
They were able to conclude that the increase in SHC was a result from the solar salt being able to form small nanostructure near the nanoparticles, this would effectively increase the surface area compare to the bulk material. [8] However, Tiznobaik, H and Shin, D. 2012, observed that the increase of the SHC observed was independent of the different nanoparticle size, they have observed that the solar salt would increase about 25% regardless of the size (Table 3, Table 4). [19] Tiznobai, H and Shin, D. 2012, were able to observe needle like structures forming over the nanomaterial which contributed to increasing the surface area when compared to the bulk material. (Figure 1) [19]
Figure 1 Backscattered electron micrograph of nanomaterial. Figure shows the Needle-like structure being brighter than the bulk salt. [19]
They had two explanations for the effects of reducing particle size had, on SHC. The first explanation was that reducing the particle size would increase the SHC surface area, this would affect the surface energy on the effective SHC. The second explanation is that since nanoparticles have very large surface area, the specific interfacial area between the nanoparticle and the surrounding solar salt are also large. This would also greatly increase the effect of the interfacial thermal resistance between the nanoparticle and the salt molecules. The interfacial thermal resistance would act as additional thermal energy storage which would increase the SHC of the nanomaterial. [19]
Concentration
From Figure 2 that 1wt% of TiO2 was the optimal concentration to obtain the highest Cp percentage increase. Different types of nanoparticles were also tested with the molten salt and it was found that 1wt% of nanoparticle also obtained the highest value for Cp. [4, 6, 9] Although most of the results from figure2 are in the negative region, it should be noted that at 1wt%, the results are closer towards the positive region, this would further support that 1wt% of TiO2 is the optimal concentration.
As opposed to Figure 2, Increasing the concentration of the nanoparticle decreases the SHC of the Molten Salt. [9, 14, 16] For given nanoparticle concentration, the nanolayer effect increases as the size of the particle decreases since the number of particles increases with the decreasing particle size. As a result, the SHC decreases as the particle size decreases and particle concentration increases due to the augmentation of the nanolayer effect [16]. This was also supported by Lasfargues, M. et al, 2015. [14, 16] They were able to conclude that the SHC of the molten salt was independent of the concentration of the TiO2. [14]
In conclusion, the effect of the concentration of the nanoparticle on the solar salt affects the SHC of the solar salt, with 1wt% of TiO2 being able to provide the greatest increase. It should be noted there were findings that suggest that the concentration of the nanoparticle was not significant enough to affect the SHC of the solar salt. [16] Therefore, further research is required to accurately determine the effect of the concentration of the nanoparticle has on the molten salt.
Figure 2 Effects of different concentration of TiO2 in Solar Salt
Conclusions
In this Meta-Study, Molten Salts have a singular purpose in solar power technology; to replace conventional synthetic oils currently used throughout most solar power energy generation facilities. Upon investigation into molten salt technology, and from this Meta-Study we can say that utilizing nanoparticles within Molten Salts warrants enough of a SHC increase that further study and investigation is required. An increase of 28% in the SHC of the fluid phase [28] is more than enough to increase study within this area.
Adding nanoparticles to the molten salt was shown to increase efficiency within the system. The main reason for this was the formation of Nano-layers which was the reason for the SHC to increase. TiO2 was the most commonly used nanoparticle due to its relatively low cost and high malleability.
In the solar energy industry increasing the SHC means to increase efficiency within the Solar Cell System itself and because this technology is inefficient at the moment, any optimization of any magnitude is highly sought after and the potential for growth is high. Due to these factors, it is recommended that, further research should be focused onto how the mechanics of how Nanolayers within the Molten Salts increases the SHC.
Acknowledgments
We would like to express our gratitude towards Dr Jurgen Schulte, Blake Regan and the University of Technology, Sydney for their support and guidance.
References and Notes
1. Bing Tan and and Yiying Wu Dye-Sensitized Solar Cells Based on Anatase TiO2 Nanoparticle/Nanowire Composites, The Journal of Physical Chemistry B 2006 110 (32), 15932- 15938. https://doi.org/10.1021/jp063972n
2. Xu C, Wang Z, Li X, Sun F. Energy and exergy analysis of solar power tower plants. Applied Thermal Engineering. 2011;31(17):3904-13. https://doi.org/10.1016/j.applthermaleng.2011.07.038
3. Zhao Z, Arif MT, Amanullah MTO. Solar Thermal Energy with Molten-salt Storage for Residential Heating Application. Energy Procedia. 2017;110:243-9. https://doi.org/10.1016/j.egypro.2017.03.134
4. Andreu-Cabedo P, Mondragon R, Hernandez L, Martinez-Cuenca R, Cabedo L, Julia JE. Increment of specific heat capacity of solar salt with SiO(2) nanoparticles. Nanoscale Research Letters. 2014;9(1):582. https://doi.org/10.1186/1556-276X-9-582
5. Awad A, Navarro H, Ding Y, Wen D. Thermal-physical properties of nanoparticle-seeded nitrate Molten Salts. Renewable Energy. 2018;120:275-88. https://doi.org/10.1016/j.renene.2017.12.026
6. Chieruzzi M, Adio M, Crescenzi T, Torre L, Kenny J. A New Phase Change Material Based on Potassium Nitrate with Silica and Alumina Nanoparticles for Thermal Energy Storage 2015. 984 p.
7. Chieruzzi M, Cerritelli GF, Miliozzi A, Kenny JM. Effect of nanoparticles on heat capacity of nanofluids based on Molten Salts as PCM for thermal energy storage. Nanoscale Research Letters. 2013;8(1):448. https://doi.org/10.1186/1556-276X-8-448
8. Dudda B, Shin D. Effect of nanoparticle dispersion on specific heat capacity of a binary nitrate salt eutectic for concentrated solar power applications. International Journal of Thermal Sciences. 2013;69:37-42. https://doi.org/10.1016/j.ijthermalsci.2013.02.003
9. Hentschke R. On the specific heat capacity enhancement in nanofluids. Nanoscale Research Letters. 2016;11:88. https://doi.org/10.1186/s11671-015-1188-5
10. Hickin C, Li H, Kemp S. An Analysis of the Effect of Molten Salt Thermal Storage on Parabolic Trough Concentrated Solar Power Plant Efficiency. PAM Review: Energy Science & Technology. 2015.
11. Kotyla J. Molten salt composition and composites for improved latent heat thermal energy storage. PAM Review: Energy Science & Technology. 2017.
12. Lai C-C, Chang W-C, Hu W-L, Wang ZM, Lu M-C, Chueh Y-L. A solar-thermal energy harvesting scheme: enhanced heat capacity of molten HITEC salt mixed with Sn/SiOx core-shell nanoparticles. Nanoscale. 2014;6(9):4555-9. https://doi.org/10.1039/c3nr06810b
13. Lasfargues M, Bell A, Ding Y. In situ production of titanium dioxide nanoparticles in molten salt phase for thermal energy storage and heat-transfer fluid applications. Journal of Nanoparticle Research. 2016;18(6):150. https://doi.org/10.1007/s11051-016-3460-8
14. Lasfargues M, Geng Q, Cao H, Ding Y. Mechanical Dispersion of Nanoparticles and Its Effect on the Specific Heat Capacity of Impure Binary Nitrate Salt Mixtures. Nanomaterials. 2015;5(3):1136- 46. https://doi.org/10.3390/nano5031136
15. Lasfargues M, Stead G, Amjad M, Ding Y, Wen D. In Situ Production of Copper Oxide Nanoparticles in a Binary Molten Salt for Concentrated Solar Power Plant Applications. Materials. 2017;10(5):537. https://doi.org/10.3390/ma10050537
16. Lu M-C, Huang C-H. Specific heat capacity of molten salt-based alumina nanofluid. Nanoscale Research Letters. 2013;8(1):292. https://doi.org/10.1186/1556-276X-8-292
17. Pournorouz Z, Mostafavi A, Pinto A, Bokka A, Jeon J, Shin D. Enhanced thermophysical properties via PAO superstructure. Nanoscale Research Letters. 2017;12(1):29. https://doi.org/10.1186/s11671- 016-1802-1
18. Shavel A, Guerrini L, Alvarez-Puebla RA. Colloidal synthesis of silicon nanoparticles in Molten Salts. Nanoscale. 2017;9(24):8157-63. https://doi.org/10.1039/C7NR01839H
19. Tiznobaik H, Shin D. Enhanced specific heat capacity of high-temperature molten salt-based nanofluids. International Journal of Heat and Mass Transfer. 2013;57(2):542-8. https://doi.org/10.1016/j.ijheatmasstransfer.2012.10.062
20. Wang W, Lee D, Murray RW. Electron Transport Dynamics in a Room-Temperature Au Nanoparticle Molten Salt. The Journal of Physical Chemistry B. 2006;110(21):10258-65. https://doi.org/10.1021/jp060598i
21. Xie Q, Zhu Q, Li Y. Thermal Storage Properties of Molten Nitrate Salt-Based Nanofluids with Graphene Nanoplatelets. Nanoscale Research Letters. 2016;11(1):306. https://doi.org/10.1186/s11671-016-1519-1
22. Zhao Z, Arif MT, Amanullah MTO. Solar Thermal Energy with Molten-salt Storage for Residential Heating Application. Energy Procedia. 2017;110:243-9. https://doi.org/10.1016/j.egypro.2017.03.134
23. Medrano, M.; Gil, A.; Martorell, I.; Potau, X.; Cabeza, L.F. State of the art on high-temperature thermal energy storage for power generation. Part 2—Case studies. Renew. Sustain. Energy Rev. 2010, 14, 56–72. https://doi.org/10.1016/j.rser.2009.07.036
24. Shin, D.; Banerjee, D. Effects of silica nanoparticles on enhancing the specific heat capacity of carbonate salt eutectic (work in progress). Int. J. Struct. Chang. SOLIDS 2010, 2, 25–31.
25. Shin, D.; Banerjee, D. Enhanced Specific Heat of Silica Nanofluid. J. Heat Transf. 2010, 133, 024501. https://doi.org/10.1115/1.4002600
26. Shin, D.; Banerjee, D. Enhancement of specific heat capacity of high-temperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications. Int. J. Heat Mass Transf. 2011, 54, 1064–1070. https://doi.org/10.1016/j.ijheatmasstransfer.2010.11.017
27. Lu, M.-C.; Huang, C.-H. Specific heat capacity of molten salt-based alumina nanofluid. Nanoscale Res. Lett. 2013, 8, 292. https://doi.org/10.1186/1556-276X-8-292
28. Tiznobaik, H.; Shin, D. Enhanced specific heat capacity of high-temperature molten salt-based nanofluids. Int. J. Heat Mass Transf. 2013, 57, 542–548. https://doi.org/10.1016/j.ijheatmasstransfer.2012.10.062
29. Ho, M.X.; Pan, C. Optimal concentration of alumina nanoparticles in molten Hitec salt to maximize its specific heat capacity. Int. J. Heat Mass Transf. 2014, 70, 174–184. https://doi.org/10.1016/j.ijheatmasstransfer.2013.10.078
30. Jo, B.; Banerjee, D. Enhanced specific heat capacity of molten salt-based nanomaterials: Effects of nanoparticle dispersion and solvent material. Acta Mater. 2014, 75, 80–91. https://doi.org/10.1016/j.actamat.2014.05.005
31. McMullen P. Using MOLTEN SALTS AS A HEAT TRANSFER FLUID AND FOR THERMAL- STORAGE MEDIUM. Process Heating [serial on the Internet]. (2016, Nov), [cited May 20, 2018]; 23(11): 24-27. Available from: Business Source Complete.