Experimental study on thermal performance of solar absorber with CuO nano structure selective coating

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Energy Procedia 158 Energy Procedia 00(2019) (2017)1303–1310 000–000 www.elsevier.com/locate/procedia

10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China

Experimental study on thermal performance of solar absorber with Experimental study on thermal performance of solar absorber with The 15th International Symposium on District Heating and Cooling CuO nano structure selective coating CuO nano structure selective coating Assessing the feasibility of using the heat demand-outdoor Yalin Luaa, Zhenqian Chenaa ,b,b,* Yalin , Zhenqian Chen ,* heat demand forecast temperatureSchool function for aLulong-term district of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu, China a

a School of Energy and Environment, Southeast University,Southeast Nanjing 210096, Jiangsu, China Jiangsu Provincial Key Laboratory of Solar Energy Science and Technology, University, Nanjing 210096, Jiangsu, China a,b,c a a b c c b Jiangsu Provincial Key Laboratory of Solar Energy Science and Technology, Southeast University, Nanjing 210096, Jiangsu, China b

a

I. Andrić

*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre

IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal

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 Abstract To improve the efficiency of solar thermal collector, solar absorber with selective coating is essential. Solar absorbers with CuO To improve theselective efficiency of solar collector, solar oxidation absorber with 0.065mol/L selective coating is essential. SolarKOH absorbers with CuO nano structure coating werethermal prepared by chemical K2S2O8 and 2.5mol/L solution at 60℃ 2.5mol/L KOH solution at 60℃ nano structure selective coating were prepared chemical oxidation with in 0.065mol/L 2S2O8 and for 5min, 15min and 60min, respectively, and by then heat treated at 180℃ air for 2h.KTransient calorimetric method was applied Abstract for 5min,the 15min and 60min, and then treatedinatsummer 180℃ ininair for 2h.toTransient calorimetric wasstructure applied to obtain absorptance (α). respectively, And outdoor tests wereheat conducted Nanjing obtain emittance (ε) ofmethod CuO nano to obtain the absorptance (α). And outdoor testsofwere in Nanjing to obtain (ε) of CuOonnano structure selective coating and instantaneous efficiency solarconducted absorber.in It summer shows that the oxidation timeemittance has obvious effect absorptance, District coating heatingand networks are commonly addressed in the literature as one the mosttime effective solutions decreasing the selective instantaneous efficiency of solar It shows that theof oxidation hasThe obvious effectfor onand absorptance, emittance of CuO nano structure selective coating andabsorber. instantaneous efficiency of solar absorber. absorptance emittance greenhouse gas emissions from selective the building sector. These systems require highofinvestments which are returned through the heat emittance of CuO nano structure coating and instantaneous efficiency solar absorber. The absorptance and emittance (α/ε) are measured to be approximately 0.88/0.13 (5min), 0.90/0.11 (15min) and 0.88/0.39 (60min). The instantaneous efficiency sales. to thetochanged climate conditions and building renovation policies, heat demand in future couldefficiency decrease, (α/ε) areDue measured be approximately 0.88/0.13 0.90/0.11 (15min) 0.88/0.39 (60min). Thethe instantaneous measured to be 76.8% (5min), 78.3% (15min) and (5min), 64.9% (60min). CuO nano and structure selective coating showed great potential on prolonging the investment return period. measured toofbesolar 76.8% (5min), 78.3% (15min) and 64.9% (60min). CuO nano structure selective coating showed great potential on application thermal collector. The main scope ofthermal this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand application of solar collector. forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Copyright © 2018 Elsevier Ltd. All rights reserved. ©buildings 2019 The Authors. Published by Elsevier Ltd. and typology. Three weather scenarios vary in both construction period (low, medium, high) and three district Copyright ©that 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were th Selection and peer-review under responsibility of the scientific committee of the 10 International Conference on Applied Energy (ICAE2018). Peer-review under responsibility of the scientific committee ofpreviously ICAE2018developed – The 10th International Conference on Applied Energy. compared with results from a dynamic heat demand model, and validated by the authors. (ICAE2018). The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Keywords: CuO; nano structure; selective coating; thermal performance (the errorCuO; in annual demandselective was lower thanthermal 20% performance for all weather scenarios considered). However, after introducing renovation Keywords: nano structure; coating; scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 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.

© 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. * Corresponding author. Tel.: +86-25-83790626; fax: +86-25-83790626. * E-mail Corresponding Tel.: +86-25-83790626; fax: +86-25-83790626. address:author. [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected]

1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility the scientific 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.2019.01.323

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1. Introduction Solar thermal collector is widely applied in utilization of solar energy. To improve the efficiency of solar thermal collector, solar absorber with selective coating is essential. Solar selective coating could achieve higher efficiency for its high absorptance of solar radiation and low emittance to reduce radiative heat loss. The solar selective absorber Nomenclature A area of light exposed surface, m2 cp specific heat capacity, J/kg∙K Gr Grashof Number I solar irradiance, W/m2 L characteristic length, m M mass of sample, kg m mass flow rate, kg/• Nu Nusselt number Pr Prandtl number Q heat transfer rate, W T temperature, K Greek symbols α absorptance ε emittance η instantaneous efficiency of solar absorber λ thermal conductivity, W/m∙K, or wavelength, μm ρ reflectance σ Stefan-Boltzmann constant, 5.67×10-8W/m-2∙K-4 τ time, s Subscripts a ambient c convective heat transfer, cooling h heating r radiative heat transfer mainly collects solar radiation at wavelength range from 0.3-2.5μm, and the radiative heat loss is at long wavelength, which is more than 2.5μm. the computation formulas of solar absorptance and emittance are shown in equation 1 and 2. UV-NIR spectrophotometer and Fourier transform infrared spectrometer are usually applied to determine the reflectance at each wavelength of the absorber in the UV–Vis and IR [1].

 1-  I  d =     I    d  1-  I  d   =    I    d 2.5m

0.3 m

2.5 m

(1)

0.3 m

20 m

2.5 m

s

20 m

(2)

2.5 m s

Based on physical mechanisms, absorbers with selective coating are classified as intrinsic absorber, semiconductor absorber, multilayer absorber, cermet absorber, textured surface absorber and dielectric –metal-dielectric-based absorber [2]. Among above absorbers, textured surface absorber shows great potential by integrating bionic principle. The special nano structure on textured surface has optical trapping ability, which contributes to reduce the reflectance of wavelength ranging from 0.3-2.5μm and improve the solar absorptance. Ruben Hünig et al. [3] replicated the surface texture of rose petal by soft template method. The light tapping element from rose petal was good at light harvesting and achieved integrated reflection of only 7% at incidence angle at 80°. Karthick Kumar S et al. [4] developed CuO thin films made of nanofibers, which achieved absorptance over 0.95 and



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emittance less than 0.07. Cindrella L. et al. [5] fabricated and optimized coating incorporating copper oxide on PANI, and absorptance of 0.94 and emittance of 0.01 were obtained by changing the polyaniline content on the surface of the CuO nanorods. There are varies researches on CuO selective coating [6-11]. However, research on the outdoor operation performance of solar absorber with CuO nano structure selective coating is rare. In this paper, solar absorber with CuO nano structure selective coating was prepared and thermal performance of which was studied experimentally in summer in Nanjing. Solar absorbers with CuO nano structure selective coating were prepared by chemical oxidation with 0.065mol/L K2S2O8 and 2.5mol/L KOH solution at 60℃ for 5min, 15min and 60min, respectively, and then heat treated at 180℃ in air for 2h. To investigate the solar absorptance (α) of CuO nano structure selective coating in actual outdoor condition, transient calorimetric method was applied, which is simple and easy. Outdoor test was conducted as well to obtain emittance (ε) of CuO nano structure selective coating and instantaneous efficiency of solar absorber. 2. Experimental apparatus and procedure 2.1. Preparation of CuO nano structure selective coating CuO nano structure selective coating were synthesized on smooth copper substrate (80mm×80mm×6mm) by chemical oxidation. Firstly, the copper substrates were cleaned by absolute alcohol and rinsed by deionized water, and then dried at room temperature. Secondly, the copper substrates were dipped in 500mL K 2S2O8 (0.065mol/L) and KOH (2.5mol/L) solution, the pH of which was 12. The reaction temperature was fixed at 60℃ by thermostatic oil bath. The oxidative time were 5min, 15min and 60min, respectively. During this process, the main reaction is shown in equation 3 and 4. After the oxidation process, the substrates were rinsed by deionized water and dried at room temperature. Finally, the copper substrates with CuO coating were heated in drying oven at 180℃ for 2h. (3) Cu+2KOH+K2S2O8  Cu OH2 +2K2SO4 Cu OH2  CuO+H2O

(4)

2.2. Transient calorimetric method In order to investigate the solar absorptance of CuO nano structure selective coating, transient calorimetric method was conducted. As shown in fig. 1, the transient calorimetric experiment system is composed of the experiment section and data acquisition section. The experiment section consists of solar absorber with CuO nano structure selective coating, shelter and thermocouples. The data acquisition section consists of data logger, solar power meter and computer. The thermocouple is used to measure the temperature of sample, the solar power meter is used to measure the solar irradiance and the mass of sample is measured by electronic balance. The equipment parameters are shown in table 1. The experiment section is well insulated to avoid heat loss. When solar absorber is exposed to solar radiation, temperature of sample will rise and the transient energy balance equation of solar absorber is shown in equation 5. After heating for a while, the absorbers will be covered with shelter to cut the solar radiation, and the temperature of sample will decrease. During the cooling process, transient energy balance equation of solar absorber is shown as equation 6. Whether in heating or cooling process, at a fixed temperature, the thermal state of the solar absorber is supposed to be the same, which means convective heat transfer and radiative heat transfer should be the same. So the absorptance of solar absorber is calculated by equation 7. dT (5) IA (cpM)( )h  Qr  Qc  d dT (6) -(cpM )( ) Qr '  Qc' d c dT dT (cp M )( )h -(cp M )( )c d d   (7) IA

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(a) (b) Fig. 1. (a) Schematic of transient calorimetric experiment system: 1-Shelter 2-Sample 3-Thermocouple 4-Data logger 5-Computer 6-Solar power meter (b) Photo of experimental setup

2.3. Outdoor thermal performance test In order to investigate the thermal performance of solar absorber with CuO nano structure selective coating, outdoor thermal performance test was conducted. The schematic of outdoor test system is shown in fig. 2. The equipment parameters are shown in table 1. The outdoor test system is well insulated to avoid heat loss. The solar absorber and insulation are mounted in a wooden frame. Working fluid is water. The water inflows to the heat exchanger, absorbs heat gained from solar absorber, and outflows from the water outlet. Silicone thermal grease is used between sample and heat exchanger to decrease thermal contact resistance. Thermocouples are used to measure temperature of water inlet, water outlet, solar absorber and ambient. Solar irradiance is measured by solar power meter. Flow rate is measured by flowmeter. The instantaneous efficiency of solar absorber is calculated by equation 8. c m(T  T ) = p o i (8) IA During the outdoor test, transient energy balance equation of solar absorber is shown as equation 9. dT (9) IA cpm( )  cpm(To  Ti )  T 4  hAT  Ta   dt Where h is given by equation 10, following correlation is used to determine Nusselt number for natural convective heat transfer with heated surface upward [12]: Nu (10)  h , Nu  0.54(GrPr)1/4， 104  GrPr 107 L Table 1. Equipment parameters. Equipment

Range

Max uncertainty

Thermocouple (T-type, calibrated) Solar power meter (TM-207) Flowmeter (LZB-6) Electronic balance

-40-125℃ 0-2000W/m2 2.5-25L/h 5

0.2K 10W/m2 2.50% 0.01g

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(a) (b) Fig. 2. (a) Schematic of outdoor thermal performance test: 1-Solar radiation 2-Solar absorber 3-Heat exchanger 4-Insulating layer 5Thermocouple 6-Water pump 7-Flow control valve 8-Flowmeter (b) Photo of experimental setup

2.4. Uncertainty analysis The uncertainty of absorptance is calculated as follows: 



2



2

 T   T   M 2  I 2  c   h       Tc   Th   M   I  2

(11)

2

 To   To   m 2  I 2 (12)           To  Ti   To  Ti   m   I  In this paper, the uncertainties of absorptance and instantaneous efficiency are 0.29% and 11.55%, respectively.  

3. Results and discussion 3.1. Surface morphology Fig. 3. (a) shows the copper substrate before and after oxidation process. After oxidation, the copper substrate was coated by CuO, which was black textured. The difference between CuO coating of copper substrates for different oxidation time can be observed macroscopically. As shown in fig. 3. (b), the CuO coating becomes thicker and more compact as the oxidation time becomes longer. The nano structure of CuO coating is sparse and rather random after oxidation for 5min. With oxidation for 15min, compact flower-type structure (approximately 105 flowers per mm2) is formed, and the thickness of flower-type structure will further increase with longer reaction time [13]

(a)

(b)

Fig. 3 (a) Copper substrate before and after oxidation process (b) Copper substrates of different oxidation time

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42 CuO-5min CuO-15min CuO-60min

40 38 T (℃)

36 34 32 30 28

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 Time (min) Fig. 4. Temperature change of absorbers with time

1.0

0.90

0.88

0.88

Absorptance Emittance

0.8 0.6 0.39

0.4 0.2 0.0

0.13

CuO-5min

0.11

CuO-15min

CuO-60min

Fig.5. Absorptance and emittance of absorbers

3.2. Absorptance The transient calorimetric experiment was conducted on calm clear day in Nanjing. Solar absorbers were positioned horizontally. The temperature change with time is shown in fig. 4. The solar irradiation was 1101±50W/m2. Initial temperature of absorbers was approximately 29℃. After exposing to solar radiation, the temperature of solar absorbers with oxidation time of 5min, 15min and 60min rose by 12.75℃, 12.66℃ and 12.53℃, respectively. After covering by shelter, temperature of solar absorbers fell gradually. Absorptance was shown in fig. 5. There was no significant difference on absorptance among absorbers with different oxidation time, at approximately 0.88-0.90. According to study of Jeong D et al. [6], with the increase of oxidation time, the absorptance will increase. However, the difference was not significant enough to reflect on the result of outdoor test. 3.3. Emittance and instantaneous efficiency The outdoor thermal performance test was conducted on calm clear day in Nanjing. Solar absorbers were positioned horizontally. Before the test, the absorber was covered by shelter and the flow rate was stabilized at 8L/h. The tests of absorbers with oxidation time of 5min, 15min and 60min were conducted from 10:00 to 14:00 on 12 th, 14th and 15th of June, respectively, and the weather of which were very similar. Solar irradiance of each day was shown in fig. 6. The emittance of absorber was shown in fig. 5. There was no significant difference on emittance between absorbers with 5min and 15min oxidation time, at 0.13 and 0.11. There was a sharp rise on emittance when oxidation time



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increased to 60min, at 0.39. Jo H et al. [14] studied the influence of surface roughness and oxidation time on emittance of metal surface. With increased oxidation time which caused thicker coating, the emittance of surface would rise, and also the influenced waveband would be widened. Fig. 7 shows the instantaneous efficiency of absorber. The average instantaneous efficiency of absorbers with 5min and 15min oxidation time were similar, at 76.8% and 78.3%, respectively. The average instantaneous efficiency of absorber with 60min oxidation was 64.9%, much less than former two.

Solar irradiance(W/m2)

1200 1000 800 600 12th June 14th June 15th June

400 200 0 10:00

11:00

12:00 Time

13:00

14:00

Fig. 6. Solar irradiance

Instantaneous efficiency

1.0 0.8 0.6 0.4 CuO-5min CuO-15min CuO-60min

0.2 0.0 10:00

11:00

12:00 Time

13:00

14:00

Fig. 7. Instantaneous efficiency

4. Conclusion Solar absorbers with CuO nano structure selective coating were prepared by chemical oxidation. Absorptance (α), emittance (ε) of CuO nano structure selective coating and instantaneous efficiency of solar absorber were investigated experimentally. The oxidation time has obvious effect on emittance of CuO nano structure selective coating and instantaneous efficiency of solar absorber. The absorptance and emittance (α/ε) are measured to be approximately 0.88/0.13 (5min), 0.90/0.11 (15min) and 0.88/0.39 (60min). The instantaneous efficiency measured to be 76.8% (5min), 78.3% (15min) and 64.9% (60min). CuO nano structure selective coating showed great potential on application of solar thermal collector.

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Yalin Lu et al. / Energy Procedia 158 (2019) 1303–1310 Yalin Lu et al. / Energy Procedia 00 (2018) 000–000

Acknowledgements This work is supported by the National Natural Science Foundation of China 50776015, the Research Funds of Key Laboratory of Heating and Air Conditioning and the Education Department of Henan Province. References [1] Wang K, Khan S, Yuan G, Hua C, Wu Z, Song C, Han G, Liu Y. A facile one-step method to fabricate multi-scaled solar selective absorber with nano-composite and controllable micro-porous texture. Sol Energ Mat Sol C 2017;163:105-12. [2] Dan A, Barshilia HC, Chattopadhyay K, Basu B. Solar energy absorption mediated by surface plasma polaritons in spectrally selective dielectric-metal-dielectric coatings: A critical review. Renew Sust Energ Rev 2017;79:1050-77. [3] Hünig R, Mertens A, Stephan M, Schulz A, Richter B, Hetterich M, Powalla M, Lemmer U, Colsmann A, Gomard G. Flower Power: Exploiting Plants' Epidermal Structures for Enhanced Light Harvesting in Thin-Film Solar Cells. Adv Opt Mater 2016;4(10):1487-93. [4] Karthick Kumar S, Suresh S, Murugesan S, Raj SP. CuO thin films made of nanofibers for solar selective absorber applications. Sol Energy 2013;94:299-304. [5] Cindrella L, Prabhu. S. CuO-PANI nanostructure with tunable spectral selectivity for solar selective coating application. Appl Surf Sci 2016;378:245-52. [6] Jeong D, Lee J, Hong H, Choi D, Cho J-W, Kim S-K, Nam Y. Absorption mechanism and performance characterization of CuO nanostructured absorbers. Sol Energ Mat Sol C 2017;169:270-9. [7] Xiao X, Miao L, Xu G, Lu L, Su Z, Wang N, Tanemura S. A facile process to prepare copper oxide thin films as solar selective absorbers. Appl Surf Sci 2011;257(24):10729-36. [8] Alami AH, Allagui A, Alawadhi H. Microstructural and optical studies of CuO thin films prepared by chemical ageing of copper substrate in alkaline ammonia solution. J Alloy Compd 2014;617:542-6. [9] Karthick Kumar S, Murugesan S, Suresh S. Preparation and characterization of CuO nanostructures on copper substrate as selective solar absorbers. Mater Chem Phys 2014 143(3):1209-14. [10] Shehayeb S, Deschanels X, Karamé I, Ghannam L, Toquer G. Spectrally selective coatings obtained from electrophoretic deposition of CuO nanoparticles. Surf Coat Tech 2017;322:38-45. [11] Siddiqui H, Qureshi MS, Haque FZ. Valuation of copper oxide (CuO) nanoflakes for its suitability as an absorbing material in solar cells fabrication. Optik 2016;127(8):3713-7. [12] Yang S, Tao W. Heat transfer. 4th ed. Beijing: Higher education press; 2006. [13] Qian BT, Shen ZQ. Super-hydrophobic CuO nanoflowers by controlled surface oxidation on copper. J Inorg Mater 2006;21(3):747-52. [14] Jo H, King JL, Blomstrand K, Sridharan K. Spectral emissivity of oxidized and roughened metal surfaces. Int J Heat Mass Tran 2017;115:106571.