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Numerical study of the geometrically graded metal foam for concentrated photovoltaic solar cell cooling
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Energy (2019) 000–000 761–766 EnergyProcedia Procedia158 00 (2017) www.elsevier.com/locate/procedia 10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, China 10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, China
Numerical study of the geometrically graded metal foam for concentrated photovoltaic cell cooling Numerical study of the geometrically graded metal foam for The 15th International Symposium on solar District Heating and Cooling concentrated photovoltaic solar a cell cooling WengAssessing Cheong Tanathe , Lipfeasibility Huat Sawa,*, of Huiusing San Thiam , Akhildemand-outdoor Gargb, Nugroho Agung the heat c a a b Weng Cheong Tan , Lip Huatfor Sawaa,long-term *, Pambudi Hui San Thiam , Akhil Garg , Nugroho Agung temperature function district heat demand forecast c Pambudi *, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
a Lee Kong Chian Faculty of Engineering and Science, UTAR, Kajang, 43000, Malaysia. Intelligent a,b,c Manufacturing Key Laboratory of Ministry University, Shantou, a a of Education, Shantou b c 515063, China. c c a Mechanical Engineering Universitas Negeri Sebelas Maret, Jl. Ir. Kajang, Sutami 36A, Surakarta 57126, Indonesia. Lee KongEducation, Chian Faculty of Engineering and Science, UTAR, 43000, Malaysia. b Intelligent Manufacturing Key Laboratory of Ministry of Education, Shantou University, Shantou, 515063, China. a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal c Mechanical Engineering Education, Universitas Negeri Sebelas Maret, Jl. Ir. Sutami 36A, Surakarta 57126, Indonesia. b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Abstract c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France b
I. Andrić
Abstract Concentrated photovoltaic cell (CPV) had gained much attention recently due to high efficiency at a competitive cost. However, efficiency of CPV is inversely proportional to the temperature. Hence, it is important to reduce the maximum temperature and Concentrated photovoltaic cell (CPV) hadMetal gainedfoam much attention to high efficiency a competitive However, variation across the CPV. with its highrecently specificdue surface area, thermalatconductivity andcost. tortuous flow Abstractof temperature efficiency of CPV is inversely proportional temperature. Hence,for it is important to reduce the maximum temperature and path to promote mixing is an ideal candidatetoforthethermal management CPV. However, the thermal performance of the metal variation of temperature across the CPV. Metal foam with its high specific surface area, thermal conductivity and tortuous flow foam may drop from upstream to downstream and lead to poor cooling performance near the outlet. In this study, functionally District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the path to promote mixing is an ideal thermal management for CPV. However, thewhich thermal of thegradual metal graded metal gas foam is attached on the CPV for to extract the heat generated. Functionally graded aluminum foamthrough with greenhouse emissions from thecandidate building sector. These systems require high investments areperformance returned the heat foam drop upstream to to downstream and lead to poor cooling performance near the outlet. study, functionally variation porosity modelled investigate the thermal performance and flowheat fielddemand using computational thermal fluid sales.may Due to from the are changed climate conditions and building renovation policies, in In thethis future could decrease, graded metal foam is attached oncorrelation, the CPV to extract the and heatresistance generated.loss Functionally aluminum with gradual dynamics analysis. Heat transfer permeability coefficient graded are extracted fromfoam the literature and prolonging the investment return period. variation porosity are modelled to investigate the thermal performance and flow field using computational thermal fluid used in the simulation. The results showed that functionally graded metal foam with gradual reducing porosity offered a better The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand dynamics Heatof transfer correlation, permeability and will resistance loss coefficient arelife extracted from literature temperature uniformity for the CPV. Therefore, this approach further extend cycle as well as improve the overall forecast. analysis. The district Alvalade, located in Lisbon (Portugal), was used as athe case study. The district isthe consisted ofand 665 used in theofthat simulation. showedperiod that functionally graded metal foam scenarios with gradual reducing porosity offered efficiency thevary CPV.in The buildings bothresults construction and typology. Three weather (low, medium, high) and threea better district temperature for the CPV. Therefore, approach will further extend the as wellheat as improve overall renovation uniformity scenarios were developed (shallow, this intermediate, deep). To estimate the cycle error,life obtained demand the values were efficiency of the CPV. compared with results from a dynamic heat demand model, previously developed and validated by the authors. Copyright © showed 2018 Elsevier Ltd. only All rights reserved. results that when weather change is considered, the margin of error could be acceptable for some applications ©The 2019 The Authors. Published by Elsevier Ltd. Conference onrenovation Applied Selection and peer-review under responsibility of for the all scientific committee of the 10th International (the error in annual demand was lower than 20% weather scenarios considered). However, after introducing This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Copyright © 2018 Elsevier Ltd. All rights reserved. Energy (ICAE2018). scenarios, the error value increased to 59.5% (depending on the weather scenarios combination considered). Peer-review under responsibility of theupscientific committee of ICAE2018 – Theand 10threnovation International Conference on Applied Energy. th International on Applied Selection underincreased responsibility of the within scientific of the The valueand of peer-review slope coefficient on average the committee range of 3.8% up 10 to 8% per decade,Conference that corresponds to the Energy (ICAE2018). Keywords: metal porous solar cell; of cooling; CFDduring the heating season (depending on the combination of weather and decrease in thefoam; number ofmedia; heating hours 22-139h
renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
Keywords: metal foam; porous media; solar cell; cooling; CFD
© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee * Corresponding author. Tel.: +603-9086 0288; fax: +603-9019 3886.of The 15th International Symposium on District Heating and Cooling. E-mail address: [email protected] * Corresponding author. Tel.: +603-9086 0288; fax: +603-9019 3886. Keywords: Heat demand; Forecast; E-mail address: [email protected] 1876-6102 Copyright © 2018 ElsevierClimate Ltd. Allchange rights reserved. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.202
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Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
1. Introduction Solar energy is an important renewable energy source that exists abundantly, free and non-polluting. Therefore, development of the solar energy harvesting system is an essential agenda in promoting renewable energy globally, and solve the depletion of fossil fuel issues for electricity generation as well as global warming. There are various ways to harvest solar energy such as photovoltaic cell, solar thermal collectors, solar concentration systems and solar pond. An efficient ways of converting solar energy to electrical energy is using concentrated photovoltaics (CPV). Recently CPV has gained more attention due to its competitive cost but overheating issue is always a concern. A single solar cell can reach 1400 °C if fully insulated under 500 suns of concentration [1]. Theoretically, only 25% of the absorbed sunlight energy is converted to electrical energy while the remaining energy is converted to heat [2]. The efficiency of a CPV is highly depended on the temperature. It was suggested that the efficiency drops in the gradient of 0.45% per degree [3]. Besides that, high temperature also causes thermal aging of the solar cell. To improve the efficiency, an effective thermal management is required for the CPV. The important requirement for cooling system design are cell temperature, variation of temperature, reliability, pumping power, usability of thermal energy and material efficiency. Numerous studies were conducted to study the effect of cooling on CPV system. Some of the commonly used cooling methods are passive air cooled, forced air cooling, liquid cooling and heat pipe. Passive cooling is the cheapest and simplest method for CPV cooling. However, the cooling performance is limited and strongly depended on the wind velocity and ambient temperature which is not promising especially at summer noon with no wind blow [4]. In addition, it also required heat sink with large area [3]. Active cooling is used to compensate the limitation of passive cooling with a drawback of introducing extra pumping power loss. Usually, air and water cooling are used in the active cooling. Since water has a higher specific heat capacity, it can be used for higher heat flux generation. However, in the case where water cooling is not applicable, air cooling can be used as recent advancement in the photovoltaic solar cell can operate under temperature of 150 °C without affecting the efficiency [5]. Radwan applied microchannel heat sink for the cooling system in the CPV and its performance was analysed experimentally and numerically [6]. The study showed that microchannel cooling achieved the highest cooling performance compared to other cooling method. Siyabi studies the effect of multi-layer microchannel on water cooling of CPV [7]. The results suggested that increasing number of microchannel layer will improve the thermal resistance, pressure drop and temperature uniformity of the CPV. Yang et al. showed that multilayer manifold microchannel is able to reduce the surface temperature variation of the CPV [8]. Metal foam is a material with excellent physical properties especially in the heat transfer characteristics [9]. It has a high surface area per unit volume, thermal conductivity and tortuous flow path to promote mixing is an ideal candidate for thermal management applications. Flitsanov investigated the performance of open cell aluminum foam in the CPV receiver [10]. The results showed that electricity generation and cell efficiency were improved about 1.5% and 0.5%, respectively. The performance can be further improved through compression of metal foam. From the above literature survey, it is clearly shown that temperature control of the CPV is extremely important in the electricity generation and efficiency as well as the cycle life. On the other hand, gradual development of the thermal boundary layer with reduction of cooling capacity as the cooling fluid transfer to the downstream is the root cause of the high variation of temperature across the CPV. Therefore, the objective of this study is to develop a functionally graded metal foam to maintain temperature uniformity across the CPV to optimize the performance of the CPV. Functionally graded metal foam is fabricated by divided the cooling surface into three different segments. Each segment will have different porosity. Cooling air is circulated through the metal foam to extract heat generated. Heat transfer correlation, permeability and resistance loss coefficient are extracted from the literature and used in the computational thermal fluid dynamic simulation. The average temperature, temperature variation and pressure drop will be compared between functionally graded metal foam and single segment metal foam. Lastly, optimized functionally graded metal foam will be suggested from the numerical modeling results.
Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
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2. Numerical modelling 2.1. Numerical model of the CPV cooling system A CPV solar cell (10mm x 10mm) was attached on a heat sink with dimensions of 30mm x 30mm. The CAD model used in the simulation is shown in Fig. 1. The CAD model used in the simulation was simplified by removing all the diode, electrical terminals and ceramic layer which served for electrical insulation. An aluminium heat spreader with dimension of 30mm x 30mm was sandwiched in between CPV solar cell and functionally graded metal foam. Functionally graded metal foam will serve as the heat sink for CPV solar cell. The aluminium metal foam used in this study is from ERG aerospace Corp and its characteristics were summarized in Table 1. Metal foam with 10 PPI of pores density and different porosity were used in this study.
Fig. 1 CAD design of the CPV with functionally graded metal foam. Sample
PPI
Porosity
Table 1: Properties of aluminium metal foam [11-12]. Surface are per unit volume of foam Permeability (m2/m3) ( x 10-8 K, m2 )
Inertia coefficient, °F (10-1)
A B C
10 10 10
0.918 0.794 0.682
809.1 2053.1 3169.3
0.7 1.28 1.78
10.10 2.20 1.04
2.2. Design of experiment A complete full factorial design of experiment was conducted to investigate the heat transfer characteristics of the functionally graded metal foam. In total there are total 15 combinations as shown in Table 2. All the segments referred to different location of the heat sink. Segment 1 was placed at the upstream while segment 3 was placed at the downstream. When only one segment was used, aluminum foam with size of 30 mm x 30 mm was used. For two segments, two pieces of aluminum foams with different porosity with size of 15 mm x 30 mm each were used as a heat sink. Lastly, three segments consist of three different porosity metal foams with size of 10 mm x 30 mm each were used as a heat sink.
Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
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No 1 2 3 4 5 6 7 8
1 A A A A A B B B
Table 2: Design of experiment. Combination Combination No 2 3 1 2 9 B A C 10 B C B 11 C B C 12 C B C B 13 C A 14 C A C 15 C B A -
3 C A B A
2.3. Parameter extraction The interfacial heat transfer coefficient of metal foam is important in determining the heat transfer characteristics. The heat transfer coefficient was extracted from the literature review [11-12]. The numerical analysis was conducted using commercial CFD software ANSYS-CFX. Heat sources of 50 W is assigned on the CPV solar cell. Air ideal gas was used as a coolant with medium turbulence of 5% and inlet velocity was set at 10 ms-1. The inlet temperature of the air was set as 30 °C. Shear Stress Transport (SST) turbulence model was used to characterize the flow in the metal foam. The convergence criteria was set below 1 x 10-6 for the flow and energy equations while high resolution discretization scheme was used for better accuracy of the result. Besides, some of the assumptions made in the simulation were: i) ii) iii) iv) v) vi)
The sun is under solar radiation of 1000Wm-2 and sun concentration of 500X. Contact resistance between the heat spreader with solar cell and metal foam is neglected. The initialize temperature of all the components is fixed at 30 °C. The homogeneous properties are assumed for the metal foam. The electrical characteristics do not affect the temperature and neglected in the simulation. Heat loss of the solar cell to the surroundings is neglected in the study.
2.4. Grid independent test Grid independent test is conducted to ensure the simulation results are independent of the number of elements and size of the mesh. The relative error should be kept as minimum as possible. Three sets of grid independent test are conducted and the results are tabulated in Table 3. All the errors are maintained relatively close to each other from mesh 1 to mesh 3. When comparing mesh 1 and mesh 2, the maximum error happened at pressure drop which is about 0.48 % only. This error is small enough to provide an accurate result. Parameter No of element Average temperature, °C Variation of temperature , °C Pressure drop, Pa
Table 3: Results of Mesh Independent Test. Mesh 1 Mesh 2 Relative error, % 787347 1619791 51.36 51.25 0.21 5.79 5.79 0.00 333.78 335.39 0.48
Mesh 3 3472805 51.22 5.79 337.0
Relative error, % 0.06 0.00 0.48
3. Results and discussion Full factorial design was conducted to investigate the effect of porosity in affecting the flow field and heat transfer characteristics of the functionally graded metal foam. The simulation results is summarized in Fig. 2. From the
Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
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simulation results, it can be seen that there are no obvious changes on average temperature of the CPV for all design. Most of the average temperature were between 50 °C to 52 °C with 10 ms-1 of flow rate of air with the exception of design 1 which showed 55.5 °C. These showed that metal foam with 91.8% porosity is poor in heat removal. The effects of porosity on different segments of the heat sink on the average temperature was not obvious. In term of temperature uniformity, design 1-5 showed smaller temperature difference as compared to design 6-15. This was attributed to the 91.8% porosity metal foam was used in first zones. As a results, it was recommended that to use 91.8% porosity for segment 1. The temperature difference was always large when 91.8% porosity metal foam was used at last segment further proved that it was poor in heat removal. However, the temperature difference was low when 68.2% porosity metal foam was used at last segments. It can improves the heat transfer at downstream. The effects of porosity is important in affecting the temperature variation. It is necessary to improve the heat transfer at downstream to reduce the temperature variation. Pressure drop is also another important factor to determine the energy consumption of the system, complexity, and operation cost. Therefore, it should be kept as minimum as possible. From the results, porosity affected the pressure in which 68.2% porosity had the highest pressure drop while 91.8% porosity metal foam exhibited lowest pressure drop as compared to 79.4% and 68.2% indicated that it can help to reduce the pumping power. In the views of pumping power, design 1, 3 and 8 required low pumping power. This is due to metal foam with 68.2% porosity was absent from the heat sink. The pressure drop in 3 segments heat sink maintained relatively constant showed that the position of porosity did not affects the pressure drop.
Fig. 2 (a) Average temperature and variation of temperature. (b) Pressure drop.
Fig. 3 (a) Temperature contour plot of the optimized functionally graded metal foam. Through grey relational analysis (GRA), heat sink with 3 segments was recommended as it can find a balance among average temperature, temperature variation and pumping power. Comparing only designs with 3 segments, it can be found that design 4 had the most optimum design. It has low average temperature, low temperature
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Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
uniformity, and low pumping power. This design met with the statements made previously in which 91.8% porosity was used it the first segment while 68.2% porosity was used at the last segment. The average temperature for design 4 was 51.2 ˚C which was much smaller than the allowable operating temperature of a CPV. The allowable operating temperature of a CPV should be kept below 100 ˚C as suggested by Theristics et al. [13]. Siyabi et al. studied the effects of water cooled micro channel heat sink on a CPV [7]. In his work, the operating temperature of the CPV was reduced to roughly 54 ˚C with 4 layers of micro channel. For heat sink under current study, the performance of air cooled is even better than the water cooled heat sink. Compared to the passive cooling of a CPV conducted by Micheli [14], the heat sink with size 110 mm x 110 mm base and 8 fins was able to remove heat of 20 W while maintaining at maximum temperature of 63.3 ˚C. Using metal foam as heat sink required much smaller size and able to further reduce the average temperature in removing heat of 50 W. The performance of air cooled graded metal foam heat sink is much better than passive cooling. These 2 comparisons indicated graded metal foam is ideal to be used as a heat sink. 4. Conclusion This study concluded that functionally graded metal foam can perform better than a single metal foam heat sink. The optimized functionally graded metal foam is able to reduce the average temperature and variation of temperature in the solar cell with minimum pressure drop. The effects of gradually reducing the porosity of the metal foam from the upstream to the downstream helps to enhance the performance of a CPV. Further research work can be conducted by using aluminum metal foam with different PPI to improve the flow field and thermal performance of the functionally graded metal foam. From literature, increasing the PPI of metal foam shown a superior performance in heat transfer performance metal foam can be integrated as a functionally graded metal foam to achieve a higher heat transfer performance for the CPV. Acknowledgements This work is supported by Science fund Grant No. 03-02-11-SF2016 from Ministry of Science, Technology and innovation, Malaysia. References [1] Araki K, Uozumi H, Yamaguchi M. A simple passive cooling structure and its heat analysis for 500 X concentrator PV module. Conf Rec Twenty-Ninth IEEE Photovolt Spec Conf 2002. 2002;(November 2015):2–5. [2] Sinton R a., Kwark Y, Swirhun S, Swanson RM. Silicon point contact concentrator solar cells. IEEE Electron Device Lett. 1985;6(8):405–7. [3] Ye Z, Li Q, Zhu Q, Pan W. The cooling technology of solar cells under concentrated system. In: 2009 IEEE 6th International Power Electronics and Motion Control Conference, IPEMC ’09. 2009. p. 2193–7. [4] Zou Z, Gong H, Wang J, Xie S.Numerical Investigation of Solar Enhanced Passive Air Cooling System for Concentration Photovoltaic Module Heat Dissipation. Vol 5, Journal of Clean Energy Technologies. 2017. p. 206-211 [5] Meneses-Rodríguez D, Horley PP, González-Hernández J, Vorobiev Y V., Gorley PN. Photovoltaic solar cells performance at elevated temperatures. In: Solar Energy. 2005. p. 243–50. [6] Radwan A, Ookawara S, Ahmed M. Analysis and simulation of concentrating photovoltaic systems with a microchannel heat sink. Sol Energy. 2016;136:35–48. [7] Siyabi I Al, Shanks K, Mallick T, Sundaram S. Thermal analysis of a multi-layer microchannel heat sink for cooling concentrator photovoltaic (CPV) cells. In: AIP Conference Proceedings. 2017. [8] Yang K, Zuo C. A novel multi-layer manifold microchannel cooling system for concentrating photovoltaic cells. Energy Convers Manag. 2015;89:214–21. [9] Lefebvre LP, Banhart J, Dunand DC. Porous metals and metallic foams: Current status and recent developments. Adv Eng Mater. 2008;10(9):775–87. [10] Flitsanov Y, Kribus A. A cooler for dense-array CPV receivers based on metal foam. Sol Energy. 2018;160:25–31. [11] Saw LH, Ye Y, Yew MC, Chong WT, Yew MK, Ng TC. Computational fluid dynamics simulation on open cell aluminium foams for Li-ion battery cooling system. Appl Energy. 2017;204:1489–99. [12] Dukhan N, Picón-Feliciano R, Álvarez-Hernández ÁR. Heat transfer analysis in metal foams with low-conductivity fluids. J Heat Transfer. 2006;128(8). [13] Theristis M, O’Donovan TS. An integrated thermal electrical model for single cell photovoltaic receivers under concentration. In 15 th International Heat Transfer Conference (IHTC-15). 2014. [14] Micheli L, Fernandez EF, Almonacid F, Reddy KS, Mallick TK. Enhancing ultra-high CPV passive cooling using least-material finned heat sinks. In: AIP conference Proceedings. 2015.
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Energy (2019) 000–000 761–766 EnergyProcedia Procedia158 00 (2017) www.elsevier.com/locate/procedia 10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, China 10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, China
Numerical study of the geometrically graded metal foam for concentrated photovoltaic cell cooling Numerical study of the geometrically graded metal foam for The 15th International Symposium on solar District Heating and Cooling concentrated photovoltaic solar a cell cooling WengAssessing Cheong Tanathe , Lipfeasibility Huat Sawa,*, of Huiusing San Thiam , Akhildemand-outdoor Gargb, Nugroho Agung the heat c a a b Weng Cheong Tan , Lip Huatfor Sawaa,long-term *, Pambudi Hui San Thiam , Akhil Garg , Nugroho Agung temperature function district heat demand forecast c Pambudi *, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
a Lee Kong Chian Faculty of Engineering and Science, UTAR, Kajang, 43000, Malaysia. Intelligent a,b,c Manufacturing Key Laboratory of Ministry University, Shantou, a a of Education, Shantou b c 515063, China. c c a Mechanical Engineering Universitas Negeri Sebelas Maret, Jl. Ir. Kajang, Sutami 36A, Surakarta 57126, Indonesia. Lee KongEducation, Chian Faculty of Engineering and Science, UTAR, 43000, Malaysia. b Intelligent Manufacturing Key Laboratory of Ministry of Education, Shantou University, Shantou, 515063, China. a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal c Mechanical Engineering Education, Universitas Negeri Sebelas Maret, Jl. Ir. Sutami 36A, Surakarta 57126, Indonesia. b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Abstract c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France b
I. Andrić
Abstract Concentrated photovoltaic cell (CPV) had gained much attention recently due to high efficiency at a competitive cost. However, efficiency of CPV is inversely proportional to the temperature. Hence, it is important to reduce the maximum temperature and Concentrated photovoltaic cell (CPV) hadMetal gainedfoam much attention to high efficiency a competitive However, variation across the CPV. with its highrecently specificdue surface area, thermalatconductivity andcost. tortuous flow Abstractof temperature efficiency of CPV is inversely proportional temperature. Hence,for it is important to reduce the maximum temperature and path to promote mixing is an ideal candidatetoforthethermal management CPV. However, the thermal performance of the metal variation of temperature across the CPV. Metal foam with its high specific surface area, thermal conductivity and tortuous flow foam may drop from upstream to downstream and lead to poor cooling performance near the outlet. In this study, functionally District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the path to promote mixing is an ideal thermal management for CPV. However, thewhich thermal of thegradual metal graded metal gas foam is attached on the CPV for to extract the heat generated. Functionally graded aluminum foamthrough with greenhouse emissions from thecandidate building sector. These systems require high investments areperformance returned the heat foam drop upstream to to downstream and lead to poor cooling performance near the outlet. study, functionally variation porosity modelled investigate the thermal performance and flowheat fielddemand using computational thermal fluid sales.may Due to from the are changed climate conditions and building renovation policies, in In thethis future could decrease, graded metal foam is attached oncorrelation, the CPV to extract the and heatresistance generated.loss Functionally aluminum with gradual dynamics analysis. Heat transfer permeability coefficient graded are extracted fromfoam the literature and prolonging the investment return period. variation porosity are modelled to investigate the thermal performance and flow field using computational thermal fluid used in the simulation. The results showed that functionally graded metal foam with gradual reducing porosity offered a better The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand dynamics Heatof transfer correlation, permeability and will resistance loss coefficient arelife extracted from literature temperature uniformity for the CPV. Therefore, this approach further extend cycle as well as improve the overall forecast. analysis. The district Alvalade, located in Lisbon (Portugal), was used as athe case study. The district isthe consisted ofand 665 used in theofthat simulation. showedperiod that functionally graded metal foam scenarios with gradual reducing porosity offered efficiency thevary CPV.in The buildings bothresults construction and typology. Three weather (low, medium, high) and threea better district temperature for the CPV. Therefore, approach will further extend the as wellheat as improve overall renovation uniformity scenarios were developed (shallow, this intermediate, deep). To estimate the cycle error,life obtained demand the values were efficiency of the CPV. compared with results from a dynamic heat demand model, previously developed and validated by the authors. Copyright © showed 2018 Elsevier Ltd. only All rights reserved. results that when weather change is considered, the margin of error could be acceptable for some applications ©The 2019 The Authors. Published by Elsevier Ltd. Conference onrenovation Applied Selection and peer-review under responsibility of for the all scientific committee of the 10th International (the error in annual demand was lower than 20% weather scenarios considered). However, after introducing This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Copyright © 2018 Elsevier Ltd. All rights reserved. Energy (ICAE2018). scenarios, the error value increased to 59.5% (depending on the weather scenarios combination considered). Peer-review under responsibility of theupscientific committee of ICAE2018 – Theand 10threnovation International Conference on Applied Energy. th International on Applied Selection underincreased responsibility of the within scientific of the The valueand of peer-review slope coefficient on average the committee range of 3.8% up 10 to 8% per decade,Conference that corresponds to the Energy (ICAE2018). Keywords: metal porous solar cell; of cooling; CFDduring the heating season (depending on the combination of weather and decrease in thefoam; number ofmedia; heating hours 22-139h
renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
Keywords: metal foam; porous media; solar cell; cooling; CFD
© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee * Corresponding author. Tel.: +603-9086 0288; fax: +603-9019 3886.of The 15th International Symposium on District Heating and Cooling. E-mail address: [email protected] * Corresponding author. Tel.: +603-9086 0288; fax: +603-9019 3886. Keywords: Heat demand; Forecast; E-mail address: [email protected] 1876-6102 Copyright © 2018 ElsevierClimate Ltd. Allchange rights reserved. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.202
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Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
1. Introduction Solar energy is an important renewable energy source that exists abundantly, free and non-polluting. Therefore, development of the solar energy harvesting system is an essential agenda in promoting renewable energy globally, and solve the depletion of fossil fuel issues for electricity generation as well as global warming. There are various ways to harvest solar energy such as photovoltaic cell, solar thermal collectors, solar concentration systems and solar pond. An efficient ways of converting solar energy to electrical energy is using concentrated photovoltaics (CPV). Recently CPV has gained more attention due to its competitive cost but overheating issue is always a concern. A single solar cell can reach 1400 °C if fully insulated under 500 suns of concentration [1]. Theoretically, only 25% of the absorbed sunlight energy is converted to electrical energy while the remaining energy is converted to heat [2]. The efficiency of a CPV is highly depended on the temperature. It was suggested that the efficiency drops in the gradient of 0.45% per degree [3]. Besides that, high temperature also causes thermal aging of the solar cell. To improve the efficiency, an effective thermal management is required for the CPV. The important requirement for cooling system design are cell temperature, variation of temperature, reliability, pumping power, usability of thermal energy and material efficiency. Numerous studies were conducted to study the effect of cooling on CPV system. Some of the commonly used cooling methods are passive air cooled, forced air cooling, liquid cooling and heat pipe. Passive cooling is the cheapest and simplest method for CPV cooling. However, the cooling performance is limited and strongly depended on the wind velocity and ambient temperature which is not promising especially at summer noon with no wind blow [4]. In addition, it also required heat sink with large area [3]. Active cooling is used to compensate the limitation of passive cooling with a drawback of introducing extra pumping power loss. Usually, air and water cooling are used in the active cooling. Since water has a higher specific heat capacity, it can be used for higher heat flux generation. However, in the case where water cooling is not applicable, air cooling can be used as recent advancement in the photovoltaic solar cell can operate under temperature of 150 °C without affecting the efficiency [5]. Radwan applied microchannel heat sink for the cooling system in the CPV and its performance was analysed experimentally and numerically [6]. The study showed that microchannel cooling achieved the highest cooling performance compared to other cooling method. Siyabi studies the effect of multi-layer microchannel on water cooling of CPV [7]. The results suggested that increasing number of microchannel layer will improve the thermal resistance, pressure drop and temperature uniformity of the CPV. Yang et al. showed that multilayer manifold microchannel is able to reduce the surface temperature variation of the CPV [8]. Metal foam is a material with excellent physical properties especially in the heat transfer characteristics [9]. It has a high surface area per unit volume, thermal conductivity and tortuous flow path to promote mixing is an ideal candidate for thermal management applications. Flitsanov investigated the performance of open cell aluminum foam in the CPV receiver [10]. The results showed that electricity generation and cell efficiency were improved about 1.5% and 0.5%, respectively. The performance can be further improved through compression of metal foam. From the above literature survey, it is clearly shown that temperature control of the CPV is extremely important in the electricity generation and efficiency as well as the cycle life. On the other hand, gradual development of the thermal boundary layer with reduction of cooling capacity as the cooling fluid transfer to the downstream is the root cause of the high variation of temperature across the CPV. Therefore, the objective of this study is to develop a functionally graded metal foam to maintain temperature uniformity across the CPV to optimize the performance of the CPV. Functionally graded metal foam is fabricated by divided the cooling surface into three different segments. Each segment will have different porosity. Cooling air is circulated through the metal foam to extract heat generated. Heat transfer correlation, permeability and resistance loss coefficient are extracted from the literature and used in the computational thermal fluid dynamic simulation. The average temperature, temperature variation and pressure drop will be compared between functionally graded metal foam and single segment metal foam. Lastly, optimized functionally graded metal foam will be suggested from the numerical modeling results.
Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
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2. Numerical modelling 2.1. Numerical model of the CPV cooling system A CPV solar cell (10mm x 10mm) was attached on a heat sink with dimensions of 30mm x 30mm. The CAD model used in the simulation is shown in Fig. 1. The CAD model used in the simulation was simplified by removing all the diode, electrical terminals and ceramic layer which served for electrical insulation. An aluminium heat spreader with dimension of 30mm x 30mm was sandwiched in between CPV solar cell and functionally graded metal foam. Functionally graded metal foam will serve as the heat sink for CPV solar cell. The aluminium metal foam used in this study is from ERG aerospace Corp and its characteristics were summarized in Table 1. Metal foam with 10 PPI of pores density and different porosity were used in this study.
Fig. 1 CAD design of the CPV with functionally graded metal foam. Sample
PPI
Porosity
Table 1: Properties of aluminium metal foam [11-12]. Surface are per unit volume of foam Permeability (m2/m3) ( x 10-8 K, m2 )
Inertia coefficient, °F (10-1)
A B C
10 10 10
0.918 0.794 0.682
809.1 2053.1 3169.3
0.7 1.28 1.78
10.10 2.20 1.04
2.2. Design of experiment A complete full factorial design of experiment was conducted to investigate the heat transfer characteristics of the functionally graded metal foam. In total there are total 15 combinations as shown in Table 2. All the segments referred to different location of the heat sink. Segment 1 was placed at the upstream while segment 3 was placed at the downstream. When only one segment was used, aluminum foam with size of 30 mm x 30 mm was used. For two segments, two pieces of aluminum foams with different porosity with size of 15 mm x 30 mm each were used as a heat sink. Lastly, three segments consist of three different porosity metal foams with size of 10 mm x 30 mm each were used as a heat sink.
Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
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No 1 2 3 4 5 6 7 8
1 A A A A A B B B
Table 2: Design of experiment. Combination Combination No 2 3 1 2 9 B A C 10 B C B 11 C B C 12 C B C B 13 C A 14 C A C 15 C B A -
3 C A B A
2.3. Parameter extraction The interfacial heat transfer coefficient of metal foam is important in determining the heat transfer characteristics. The heat transfer coefficient was extracted from the literature review [11-12]. The numerical analysis was conducted using commercial CFD software ANSYS-CFX. Heat sources of 50 W is assigned on the CPV solar cell. Air ideal gas was used as a coolant with medium turbulence of 5% and inlet velocity was set at 10 ms-1. The inlet temperature of the air was set as 30 °C. Shear Stress Transport (SST) turbulence model was used to characterize the flow in the metal foam. The convergence criteria was set below 1 x 10-6 for the flow and energy equations while high resolution discretization scheme was used for better accuracy of the result. Besides, some of the assumptions made in the simulation were: i) ii) iii) iv) v) vi)
The sun is under solar radiation of 1000Wm-2 and sun concentration of 500X. Contact resistance between the heat spreader with solar cell and metal foam is neglected. The initialize temperature of all the components is fixed at 30 °C. The homogeneous properties are assumed for the metal foam. The electrical characteristics do not affect the temperature and neglected in the simulation. Heat loss of the solar cell to the surroundings is neglected in the study.
2.4. Grid independent test Grid independent test is conducted to ensure the simulation results are independent of the number of elements and size of the mesh. The relative error should be kept as minimum as possible. Three sets of grid independent test are conducted and the results are tabulated in Table 3. All the errors are maintained relatively close to each other from mesh 1 to mesh 3. When comparing mesh 1 and mesh 2, the maximum error happened at pressure drop which is about 0.48 % only. This error is small enough to provide an accurate result. Parameter No of element Average temperature, °C Variation of temperature , °C Pressure drop, Pa
Table 3: Results of Mesh Independent Test. Mesh 1 Mesh 2 Relative error, % 787347 1619791 51.36 51.25 0.21 5.79 5.79 0.00 333.78 335.39 0.48
Mesh 3 3472805 51.22 5.79 337.0
Relative error, % 0.06 0.00 0.48
3. Results and discussion Full factorial design was conducted to investigate the effect of porosity in affecting the flow field and heat transfer characteristics of the functionally graded metal foam. The simulation results is summarized in Fig. 2. From the
Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
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simulation results, it can be seen that there are no obvious changes on average temperature of the CPV for all design. Most of the average temperature were between 50 °C to 52 °C with 10 ms-1 of flow rate of air with the exception of design 1 which showed 55.5 °C. These showed that metal foam with 91.8% porosity is poor in heat removal. The effects of porosity on different segments of the heat sink on the average temperature was not obvious. In term of temperature uniformity, design 1-5 showed smaller temperature difference as compared to design 6-15. This was attributed to the 91.8% porosity metal foam was used in first zones. As a results, it was recommended that to use 91.8% porosity for segment 1. The temperature difference was always large when 91.8% porosity metal foam was used at last segment further proved that it was poor in heat removal. However, the temperature difference was low when 68.2% porosity metal foam was used at last segments. It can improves the heat transfer at downstream. The effects of porosity is important in affecting the temperature variation. It is necessary to improve the heat transfer at downstream to reduce the temperature variation. Pressure drop is also another important factor to determine the energy consumption of the system, complexity, and operation cost. Therefore, it should be kept as minimum as possible. From the results, porosity affected the pressure in which 68.2% porosity had the highest pressure drop while 91.8% porosity metal foam exhibited lowest pressure drop as compared to 79.4% and 68.2% indicated that it can help to reduce the pumping power. In the views of pumping power, design 1, 3 and 8 required low pumping power. This is due to metal foam with 68.2% porosity was absent from the heat sink. The pressure drop in 3 segments heat sink maintained relatively constant showed that the position of porosity did not affects the pressure drop.
Fig. 2 (a) Average temperature and variation of temperature. (b) Pressure drop.
Fig. 3 (a) Temperature contour plot of the optimized functionally graded metal foam. Through grey relational analysis (GRA), heat sink with 3 segments was recommended as it can find a balance among average temperature, temperature variation and pumping power. Comparing only designs with 3 segments, it can be found that design 4 had the most optimum design. It has low average temperature, low temperature
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Weng Cheong Tan et al. / Energy Procedia 158 (2019) 761–766 Author name / Energy Procedia 00 (2018) 000–000
uniformity, and low pumping power. This design met with the statements made previously in which 91.8% porosity was used it the first segment while 68.2% porosity was used at the last segment. The average temperature for design 4 was 51.2 ˚C which was much smaller than the allowable operating temperature of a CPV. The allowable operating temperature of a CPV should be kept below 100 ˚C as suggested by Theristics et al. [13]. Siyabi et al. studied the effects of water cooled micro channel heat sink on a CPV [7]. In his work, the operating temperature of the CPV was reduced to roughly 54 ˚C with 4 layers of micro channel. For heat sink under current study, the performance of air cooled is even better than the water cooled heat sink. Compared to the passive cooling of a CPV conducted by Micheli [14], the heat sink with size 110 mm x 110 mm base and 8 fins was able to remove heat of 20 W while maintaining at maximum temperature of 63.3 ˚C. Using metal foam as heat sink required much smaller size and able to further reduce the average temperature in removing heat of 50 W. The performance of air cooled graded metal foam heat sink is much better than passive cooling. These 2 comparisons indicated graded metal foam is ideal to be used as a heat sink. 4. Conclusion This study concluded that functionally graded metal foam can perform better than a single metal foam heat sink. The optimized functionally graded metal foam is able to reduce the average temperature and variation of temperature in the solar cell with minimum pressure drop. The effects of gradually reducing the porosity of the metal foam from the upstream to the downstream helps to enhance the performance of a CPV. Further research work can be conducted by using aluminum metal foam with different PPI to improve the flow field and thermal performance of the functionally graded metal foam. From literature, increasing the PPI of metal foam shown a superior performance in heat transfer performance metal foam can be integrated as a functionally graded metal foam to achieve a higher heat transfer performance for the CPV. Acknowledgements This work is supported by Science fund Grant No. 03-02-11-SF2016 from Ministry of Science, Technology and innovation, Malaysia. References [1] Araki K, Uozumi H, Yamaguchi M. A simple passive cooling structure and its heat analysis for 500 X concentrator PV module. Conf Rec Twenty-Ninth IEEE Photovolt Spec Conf 2002. 2002;(November 2015):2–5. [2] Sinton R a., Kwark Y, Swirhun S, Swanson RM. Silicon point contact concentrator solar cells. IEEE Electron Device Lett. 1985;6(8):405–7. [3] Ye Z, Li Q, Zhu Q, Pan W. The cooling technology of solar cells under concentrated system. In: 2009 IEEE 6th International Power Electronics and Motion Control Conference, IPEMC ’09. 2009. p. 2193–7. [4] Zou Z, Gong H, Wang J, Xie S.Numerical Investigation of Solar Enhanced Passive Air Cooling System for Concentration Photovoltaic Module Heat Dissipation. Vol 5, Journal of Clean Energy Technologies. 2017. p. 206-211 [5] Meneses-Rodríguez D, Horley PP, González-Hernández J, Vorobiev Y V., Gorley PN. Photovoltaic solar cells performance at elevated temperatures. In: Solar Energy. 2005. p. 243–50. [6] Radwan A, Ookawara S, Ahmed M. Analysis and simulation of concentrating photovoltaic systems with a microchannel heat sink. Sol Energy. 2016;136:35–48. [7] Siyabi I Al, Shanks K, Mallick T, Sundaram S. Thermal analysis of a multi-layer microchannel heat sink for cooling concentrator photovoltaic (CPV) cells. In: AIP Conference Proceedings. 2017. [8] Yang K, Zuo C. A novel multi-layer manifold microchannel cooling system for concentrating photovoltaic cells. Energy Convers Manag. 2015;89:214–21. [9] Lefebvre LP, Banhart J, Dunand DC. Porous metals and metallic foams: Current status and recent developments. Adv Eng Mater. 2008;10(9):775–87. [10] Flitsanov Y, Kribus A. A cooler for dense-array CPV receivers based on metal foam. Sol Energy. 2018;160:25–31. [11] Saw LH, Ye Y, Yew MC, Chong WT, Yew MK, Ng TC. Computational fluid dynamics simulation on open cell aluminium foams for Li-ion battery cooling system. Appl Energy. 2017;204:1489–99. [12] Dukhan N, Picón-Feliciano R, Álvarez-Hernández ÁR. Heat transfer analysis in metal foams with low-conductivity fluids. J Heat Transfer. 2006;128(8). [13] Theristis M, O’Donovan TS. An integrated thermal electrical model for single cell photovoltaic receivers under concentration. In 15 th International Heat Transfer Conference (IHTC-15). 2014. [14] Micheli L, Fernandez EF, Almonacid F, Reddy KS, Mallick TK. Enhancing ultra-high CPV passive cooling using least-material finned heat sinks. In: AIP conference Proceedings. 2015.