An on-site test method for optical efficiency of large-size parabolic trough collectors

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1 Available onle at ScienceDirect Energy Procedia 105 (2017 ) The 8 th International Conference on Applied Energy ICAE2016 An on-site test method for optical efficiency of large-size parabolic trough collectors Ruil WANG a,b,#, Wanjun QU a,b,#, Jie SUN a,* and Hui HONG a a Institute of Engeerg Thermophysics, Chese Academy of Sciences, Beijg , Cha b University of Chese Academy of Sciences, Beijg , Cha Abstract An on-site test method for optical efficiency of large-size parabolic trough collectors (PTCs) is proposed. This method is based on energy balance of cident solar radiation, heat ga, cose loss, end loss, optical loss and heat loss. The heat loss is calculated based on the correlation between the coolg power and absorber-ient temperature difference obtaed by fourth-order polynomial data-fittg. The cident solar radiation and heat ga are calculated based on the experimental data. The cose and end losses are calculated based on local time and astronomical conditions. Therefore, the optical loss is achievable based on the energy balance and so is the optical efficiency. This method was implemented on the 300kWt PTC experimental rig located Langfang, Hebei, Cha. The optical efficiency is evaluated to be ( ) %, which agrees well with that of the LS-3 collector (77%). On the other hand, a thermohydraulic model for PTC usg heat transfer fluid (HTF) developed before is corporated with the optical efficiency obtaed on the 300kWt PTC experimental rig. The good agreement between the simulation results and experiment data verifies the on-site test method and the thermohydraulic model Published The Authors. by Elsevier Published Ltd. This by is Elsevier an open access Ltd. article under the CC BY-NC-ND license ( Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. Keywords: Concentratg solar power; Parabolic trough collector; Optical efficiency; Heat loss 1. Introduction The parabolic trough collector (PTC) is the earliest and the most mature concentratg solar power (CSP) technology. The annual solar-to-electric efficiency of a PTC-CSP plant is 15.4~16.1%, due to low collector efficiency. The energy losses maly clude cose loss, optical loss and heat loss. The cose loss is caused by the cident angle. The heat loss is due to the convective and radiative heat transfer to the ient. # These authors contributed equally to this study and share first authorship * Correspondg author. address: sunjie@mail.etp.ac.cn. Tel.: ; fax: Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi: /j.egypro

2 Ruil Wang et al. / Energy Procedia 105 ( 2017 ) The optical loss is caused by the optical performances of the mirror reflectance, overall tercept factor, absorptivity of absorber and transmittance of glass envelope. An on-site rather than -door testg method is significant for the practical performance. Kutscher et al.[1] experimentally determed the optical efficiency. However, the optical performance changes with operatg temperature due to deformation of HCE supports. Therefore, this method is not suitable for a loop longer than 100m. Lopez- Mart and Zarza [2] proposed a test method for optical efficiency of large-size PTCs. They calculated the heat loss accordg to the lear correlation and obtaed the optical loss based on energy balance. However, this method, the HTF temperature, rather than the outer surface temperature of absorber, is used to calculate the temperature difference regardg ient, which may affect the accuracy. In the present work, an on-site test method for optical efficiency of large-size PTCs is proposed. This method is based on energy balance and could measure the optical efficiency operatg temperature. The heat loss is calculated based on the correlation between the coolg power and absorber-ient temperature difference obtaed by fourth-order polynomial data-fittg. The optical loss is determed by subtractg the other contributors. This method was implemented on the 300kWt PTC experimental rig located Langfang, Hebei, Cha. On the other hand, a model of PTC usg HTF developed before is corporated with the optical efficiency test result. Good agreement between the simulation and experimental result verifies the proposed method and thermohydraulic model. 2. Methodology The energy balance of PTC can be written as: Q Q Q Q Q Q (1) c ga loss,cos loss,end loss,opt where Q is the cident solar radiation to PTC; Q c ga is the heat ga by PTC; Q loss,cos, Q, Q loss,end and Q are the cose loss, the end loss, the heat loss and the optical loss, respectively. loss,opt 2.1. Evaluation of heat loss For a particular PTC, heat loss ( Q ) is determed by the temperature difference between the outer surface temperature of absorber and the ient temperature ( T ). Heat loss of collector is hard to measure while the PTC is heatg (-focus) mode. Sce the correlation of Q with T holds once it is obtaed. It is possible to carry out coolg (de-focus) mode tests for this correlation. Then the correlation can be used to evaluate the heat loss of PTC heatg tests. The Q T correlation is obtaed by the followg steps: (1) The PTCs before the tested PTCs the same loop is partially de-focus while the tested PTCs are fully de-focus. The HTF is circulated through the loop. Therefore, the HTF is heated when it passes the partially de-focus PTCs while cools down when it passes the fully de-focus PTCs. When the thermal equilibrium state is reached, record the let and outlet HTF temperatures of the tested PTCs, the HTF mass flow rate and the ient temperature. (2) Q is calculated as: Q m c p T T out (2) where m is the HTF mass flow rate, T and T are the let and outlet temperatures respectively; out c is the isobaric thermal capacity of HTF, of which the characteristic temperature is the average of T p and T. T is calculated as: out T T Q Q ln D D tube,out tube, out T T T T (3) tube,out 2 Nu k L 2k L Dtube, HTF tube

3 488 Ruil Wang et al. / Energy Procedia 105 ( 2017 ) where T and T are the ient temperature and the temperature difference between the ient and the outer surface of absorber, respectively; T tube,out and T are the outer and ner surface tube, temperature of absorber, respectively; D tube,out and D are the outer and ner diameters of absorber, tube, respectively; k and k are the heat conductivities of absorber and HTF, respectively; L is the length tube HTF of PTC; Nu D tube, is the Nusselt number and calculated accordg to Gnielski equation [3]. (3) Change the let temperature of the tested PTCs. Repeat steps (1)~(2) and record a series of groups of T and Q. Fit the heat loss curve depends on T. Considerg the heat loss mechanism of both convection (first-order term) and radiation (fourth-order term), the fittg-curve takes the followg form: 4 Q at a T (4) 1 4 where a is related to conductive and convective heat losses and a is related to radiative heat loss. It is 1 2 noteworthy that the range of T should be wide enough to cover most of the workg temperature range of PTCs Evaluation of optical efficiency The optical loss and other losses are evaluated when the tested PTCs is workg heatg (-focus) mode the followg steps: (1) In the condition of no lateral shadg, drive the tested PTC to track the sun and circulate the HTF through the loop. Keep the system at a thermal equilibrium state and record the let and outlet HTF temperatures of the tested PTC, the mass flow of HTF, the ient temperature, the accurate time, the latitude and longitude of collector loop, the azimuth angle of PTC and the direct normal irradiance. (2) Calculate the cident solar radiation as: Q DNI A c a (5) where DNI is the direct normal irradiance; A a is the overall net PTC aperture area. Calculate The heat ga of PTC focus mode as: f f f Q m c ga p T T out (6) f f f where m is the mass flow rate of HTF; T and T are the let and outlet temperatures, respectively. out Calculate the cose loss Q and end loss Q accordg to Peng[4]. loss,cos loss,end (3) Calculate the heat loss accordg to Eq. (4). The calculation of T is similar to Eq. (3). (4) Accordg to the energy balance of PTC, optical loss is calculated as: Q Q Q Q Q Q (7) loss,opt c ga loss,cos loss,end (5) The optical efficiency of PTC is calculated as: Qloss,opt 1 (8) opt Q Q Q c loss,cos loss,end 3. Results and discussion 3.1. Description of test rig A 300kWt PTC experimental rig is built Langfang, Hebei provce North Cha (39.45ºN, º E). The schematic of rig is shown Fig. 1a. The pump is stalled for circulatg Dowtherm A synthetic oil. The oil-water heat exchanger is stalled for heat exchangg. The water circulation is used for coolg. The two rows of LS-3 PTCs are South-North oriented. The length of each row is 60m. The

4 Ruil Wang et al. / Energy Procedia 105 ( 2017 ) layout of the PTCs experimental rig (see Fig. 1a) is analogue to a sgle loop of the solar field of a PTC-CSP plant (see Fig. 1b). Either of the two rows of PTCs could operate dependently. Experiment for the proposed on-site test method for optical efficiency was conducted on this rig Evaluation of Heat loss Coolg tests for heat loss of PTC have been performed as mentioned Sec The experimental data is shown Table 1 and the data-fittg is shown Fig. 2. The wd speed varied from 0.2 m/s to 3.5 m/s durg the tests, which lies a reasonably small range and has little fluence on the heat loss [1]. T ranged from 25 o C to 185 o C and the maximal fluid temperature was 218 o C the test. (a) (b) Fig. 1. Schematics of (a) 300 kwt PTC experimental rig and (b) solar field of a PTC CSP plant. Fig. 2. Correlation of heat loss with the temperature difference Table 1. Experimental data for evaluation of heat loss m /kg.s -1 T / ο C T / ο C c / kj.kg -1 K -1 p out T / ο C T / ο C tube_out Q / kw The correlation of heat loss is obtaed as: 9 4 Q T T (9) Accordg to Eq.(4), the correlation should obey a fourth order polynomial. The very small fourth-order coefficient accounts for the radiative heat loss because of the low emissivity of the selective coatg on absorber. However, the fourth-order coefficient cannot be neglected even low temperature range. For example, when T equals to 100 o C, the second term is calculated to be up to 8% of the total heat loss accordg to Eq. (9) Evaluation of optical efficiency The heatg tests were performed accordg to the procedure mentioned Sec 2.2. All the results are shown Table 2. The evaluated optical efficiencies of the tested PTCs are consistent and the average value is ( ) %. The result agrees with the published data of LS-3 collector as 77% [5]. The evaluation of heat loss is based on an assumption that the circumferential temperature of absorber is

5 490 Ruil Wang et al. / Energy Procedia 105 ( 2017 ) uniform. Eck [6] et al showed that the circumferential temperature difference only lead to up to 3% crease of heat loss heatg mode, which has neglectable fluence on the test results of optical efficiency. Table 2. Evaluation of optical efficiency Time/h:m Q /kw Q /kw Q / kw Q / kw Q / kw Q c ga loss,cos loss,end loss,opt / kw /% opt Relative deviation 10:54: % 10:57: % 11:20: % 11:23: % 12:26: % 12:31: % 12:50: % 12:51: % 15:23: % 15:29: % 3.4. Simulation results The configuration parameters of the 300kWt PTC experimental rig and the test result of optical efficiency are implemented the previous model Sun [3]. Simulations are carried out and good agreement between the simulation and experimental data is seen Fig. 3a~4d with the relative error from 0.04% to 1.48%. In Fig. 3c, the heat loss is calculated three ways, namely based on the simulation results, the correlation Eq. (9), and the energy balance Eq. (1). Q 1 loss,opt opt Q Q Q c loss,cos loss,end (10) where the equals 76.15%. The average relative error between the heat loss calculated by Eq. (9) and opt energy balance Eq. (1) is 5.25%; the average relative error between the heat loss calculated by energy balance Eq. (1) and simulation is 10.57%. The agreement between the heat losses calculated by Eq. (9) and energy balance by Eq. (1) is good, which means that the test result of optical efficiency is reliable. The difference between the heat losses calculated by energy balance Eq. (1) and simulation results is larger, which is probably caused by the idealization the simulation, such as the ignorance of junctions between HCEs. However, considerg the heat loss is only about one tenth of the optical loss therefore even smaller the total losses (<6%), this deviation can be reasonably neglected. (see Fig. 3d). 4. Conclusions In this paper, an on-site test method for optical efficiency of large-size PTCs is proposed and implemented on the 300kWt PTC rig Langfang, Hebei, Cha. The followg conclusions have been drown: (1) The heat loss correlation of the coolg power with the tube-ient temperature difference is obtaed by fourth-order polynomial data-fittg. (2) The optical loss is calculated by subtractg the heat ga, cose loss, end loss and heat loss by the obtaed correlation from the cident solar radiation based on the energy balance equation. The optical efficiency on the 300kWt PTC rig is evaluated to be ( ) %, which agrees well with that of the LS-3 collector (77%). (3) A thermohydraulic model for PTC usg HTF is developed. This model is corporated with the optical efficiency obtaed on the 300kWt PTC rig. The good agreement between the simulation results and experiment data verifies the on-site test method and the thermohydraulic model.

6 Ruil Wang et al. / Energy Procedia 105 ( 2017 ) (a) (b) (c) (d) Fig. 3. (a) Outlet temperature; (b) Overall efficiency; (c) Heat loss; (d) Power proportions. Acknowledgements Fancial supports from the National Natural Science Foundation of Cha ( & ) and the National Basic Research Program of Cha (973 Program) (2015CB251505) are acknowledged. References [1] Kutscher C, Burkholder F, Stynes JK. Generation of a Parabolic Trough Collector Efficiency Curve From Separate Measurements of Outdoor Optical Efficiency and Indoor Receiver Heat Loss. J Sol Energ-T ASME. 2012;134:6. [2] Journal of Solar Energy EngeergProc Thermal Energy Storage Contractors Information Exchange Meetg S, L., Lopez- Mart R, Zarza E. Optical and thermal performance of large-size parabolic-trough solar collectors from outdoor experiments: A test method and a case study. Energy. 2014;70: [3] Sun J, Liu QB, Hong H. Numerical study of parabolic-trough direct steam generation loop recirculation mode: Characteristics, performance and general operation strategy. Energ Convers Manage. 2015;96: [4] Peng S, Hong H, J H, Zhang Z. A new rotatable-axis trackg solar parabolic-trough collector for solar-hybrid coal-fired power plants. Sol Energy. 2013;98: [5] Fernández-García A, Zarza E, Valenzuela L, Pérez M. Parabolic-trough solar collectors and their applications. Renew Sust Energ Rev. 2010;14: [6] Eck M, Feldhoff F, Uhlig R. Thermal Modellg and Simulation of Parabolic Trough Receiver Tubes. International Conference on Energy Sustaability, Es2010. p Biography Dr. Jie SUN is currently associate professor of Institute of Engeerg Thermophysics, Chese Academy of Sciences. He received his BEng and PhD Xi an Jiaotong University, Cha 2005 and 2009, worked as RA Queen Mary, University of London, UK 2010~2013. His terests clude solar thermal energy utilization and heat transfer.