ICAER 2011 AN EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF HEAT LOSSES FROM THE CAVITY RECEIVER USED IN LINEAR FRESNEL REFLECTOR SOLAR THERMAL SYSTEM Sudhansu S. Sahoo* a, Shinu M. Varghese b, Ashwin Kumar b, C. Suresh Kumar b Suneet Singh a, Rangan Banerjee a a Department of Energy Science & Engineering, IIT Bombay, Mumbai, India b KG Design Services Pvt. Ltd., Coimbatore, India *Corresponding author: Tel: +91 9321058464, E-mail: sahoo.sudhansu@iitb.ac.in Abstract This paper presents the analysis of heat losses from the linear Fresnel reflector (LFR) cavity. The performance is obtained via experiment and computational fluid dynamics (CFD) analysis. The effects of parameters including the temperatures of the tube surface, emissivity of tubes, convective heat loss coefficient between glass cover is studied. The experimental study was conducted under laboratory conditions that were especially designed for this purpose. As part of this investigation, the system was modelled and simulated by CFD techniques. The presented technique is easy-to-use approach and offers better result for finding overall heat loss coefficient than previously published works. Computational predictions are shown to be consistent with the experimental observations and make the CFD model a reliable tool for predicting heat loss and overall heat loss coefficient for further use. Keywords: LFR, Solar thermal, CFD, Overall heat loss coefficient 1. Introduction Linear Fresnel Reflector technology relies on an array of linear mirror strips which concentrates light on to a fixed receiver mounted on a linear tower (Fig.1). The receiver is a stationary linear cavity, usually trapezoidal, consisting of a number of tubes. The inside of the cavity, external to the tubes contains air which is not in contact with ambient. The water running through the tubes inside the cavity absorbs heat from the cavity and generates steam inside the tubes. In operation, the absorber tubes in the trapezoidal cavity get heated due to the incident concentrated solar radiation. As it does so, it emits long wavelength radiation into the cavity. This radiation results in heat loss from the tubes. The emitted radiation is absorbed by inner cavity walls and glass cover at the bottom, which in turn raises their temperature. The resulting temperature gradients promote natural convection within the cavity, which lead to convective losses from the tubes. Conduction of heat away from the inner surfaces represents the third mode of heat loss. The cavity receiver heat loss processes involve radiative, convective and conductive heat transfer, and interaction of these three modes makes it difficult to develop an analytical model. Hence, CFD approach is used to predict heat losses. 2. Related work Pye et al. (3) carried out a study of losses in a trapezoidal cavity. They used an analytical model for a trapezoidal cavity and found that radiation accounts for 90% of the heat loss from the top surface. Again using CFD analysis of the cavity, he mentioned losses due to natural convection and radiation. However, for CFD analysis, tubes were modelled as an isothermal plane surface. Reynolds et al. (4) carried out experimental and computational study of heat loss characteristics on trapezoidal cavity. They used flow visualization technique to capture the heat flow patterns within the trapezoidal cavity with a hot plate to investigate the heat losses from the absorber tube. In computational study, flow in the cavity was assumed laminar. They found a reasonable agreement between experimentally observed flow patterns and those 1
predicted by computational model. CFD prediction of heat loss was found 40% less compared with experimental results. Uncertainties in the experimental work were mentioned as the reason for this mismatch. Singh et al. (2010) experimentally studied the thermal performance of the Fresnel reflecting concentrator with trapezoidal cavity at different concentration ratios, with different emissivity of absorber tube and with round as well as rectangular shaped absorber tube. The study revealed that the thermal efficiency was influenced by the concentration ratio and selective surface coating on the absorber. The thermal efficiency decreased with an increase in the concentration ratio of the Fresnel reflecting collector. The selective surface coated absorber had a significant advantage in terms of superior thermal performance as compared to an ordinary black painted absorber. The round pipe receiver had higher surface area to absorb solar energy as compared to rectangular pipe receiver. Thermal efficiency of the solar device with round pipe absorber was found to be higher than a rectangular pipe absorber. Facão et al. (2010) analysed and optimized trapezoidal cavity receiver for a linear Fresnel solar collector concentrator using ray trace and CFD simulations. They improved the CFD model of Pye et al. (8) by including lower half of the pipes with no gap between those, instead of modeling them as a plane surface. It was concluded that effects of the absorber tube should not be neglected as 25% more heat loss happens than that of plane wall. It was found from literature review that few numerical and experimental investigations have already done. However, considering the complete absorber tube instead of just the lower half of the tube will result in further changes in the heat transfer characteristics. Here in this paper en effort has been made to conduct experiment with a model of an actual cavity used for LFR system. Heat losses that occur from the tube to the atmosphere have been found out. Further investigations related to heat losses at different emissivities and external heat transfer coefficient has been made using computational approach. 3. Physical problem The proposed Linear Fresnel Reflector system under consideration consists of a trapezoidal cavity receiver made of steel with black painted (Fig.2) filled with air and it houses eight parallel pipes having Nominal Pipe Size (NPS) 1 which are made of SS304 material. The pipe is coated with black nickel selective surface by the process of electroplating. Tubes are placed below the inner top surface of the cavity. A gap is provided between each of the tubes as allowance for thermal expansion. At bottom portion plane glass is provided to allow entry of solar radiation. Outer cavity is enclosed with glass wool insulation. This receiver receives reflected radiation from all eight parallel reflectors along the entire length of reflection. The schematic of the LFR setup is shown in Fig.2. Figure 1. Schematic sketch of the LFR set up Figure 2. Schematic sketch of the Cavity receiver 4. Experimental Investigation The schematic sketch of the experimental rig of LFR cavity receiver is shown in Fig.3. This cavity receiver is a replica of proposed cavity having dimensions as per the Fig. 2 and length is 0.5m (perpendicular to the paper). As shown in Fig.2, the enclosure has eight tubes at the top portion of it. The top and side walls are made of steel sheets and the bottom cover is made of glass and hence it becomes a closed cavity. Bottom of the glass cover is exposed to 2
the outside environment. To prevent heat loss from the side and top walls, the outer surface of the cavity receiver is covered with insulating material made of glass wool of 45 mm which is surrounded by plywood of 4mm. The enclosed cavity is not evacuated and filled with non absorbing air. Each tube contains an electrical heater inside it and heat output can be controlled by changing input voltage to the heaters. A voltage is set and eight heaters uniformly heat up the receiver tubes to a particular temperature. K-type thermocouples were used for temperature measurements. These thermocouples were bonded over the outer surface at mid length of each tube. Three thermocouples are located at three points of the glass cover as shown in Fig 3 to measure the glass temperatures. Thermocouple channels were logged on to a digital thermometer and data logger for monitoring the temperatures. Experiments were carried out by increasing the voltage to the heater step by step and constantly monitoring the temperatures so that a predefined final temperature is achieved without air flow at the bottom i.e off condition. Temperature readings are monitored every 15 min until steady values are achieved. A steady-state condition was also judged to have been attained when the surface temperature of the heater was seen not to vary significantly for example, less than 0.3 C per hour. The experiments are repeated for various tube temperatures of the tubes. For each set of experiments, after reaching steady temperature values, the corresponding voltage, current and the temperatures are logged. At steady state, when there is no further increase in temperature, the energy consumption of the heaters is the heat loss of the receiver, at that temperature. As the top and side walls are insulated, only heat interaction takes place with surrounding was with the bottom glass cover only. Calibrated thermocouples were used for the measurements and there exists a deviation of ±1.25% of the actual reading. Air temperature was measured with an uncertainty of ±1%, as specified by the manufacturer. An estimate in the experimental data has been carried out based on standard techniques.(kline andmcclintock,1953). The estimated error on the heat losses, due to errors in the measurements of basic parameters is as follows. The total heat can be represented in terms of input voltage, current. Power supply UPM Solid state array Thermocouples Cavity receiver Tubes with heater inside Data logger Thermocouples Computer Figure 3. Schematic sketch of LFR cavity lab experimental test up 5. Computational Investigation A 2D model has been adopted in the present study to predict the total heat loss from the receiver. The 2D modelling and grid generation was carried out in the GAMBIT 2.3.16 package. The grid independence study was carried out with fine grid size of 173246 Quad, Pave cells inside the cavity receiver. The laminar natural convection model equations and radiosity vector equations were solved using the software package FLUENT 6.3. In this model, 2D 3
governing equations with laminar, incompressible and steady state problem were solved using an implicit solver. Boussinesq approximation was considered while solving the momentum equation. The average radiative heat flux leaving from the internal surface of the modified cavity receiver is obtained directly from the FLUENT results. Air properties inside the cavity were used by piecewise approximation method. Operating temperature was chosen as 350-K which is approximately average of tube and glass temperature. For pressure velocity coupling, SIMPLE algorithm was used with second order upwind scheme for the discretization of equations. A convergence criterion of 10 5 was imposed on the residuals of the continuity and momentum equations. The convergence criterion of 10 6 was given on the residual of energy equation. Boundary conditions imposed are as follows. Tubes: T = T h, ε = 0.25 Top wall: u = 0, v = 0, φ = 0, ε i = 0.1 Side walls: u = 0, v = 0, φ = 0, ε i = 0.1 Bottom wall: u = 0, v = 0, ε inner =ε outer = 0.9, h ext = 5-10W/m 2 K, Atmospheric temperature = 30 C 6. Results Fig. 4 and Fig. 5 shows the experimental data obtained from heat loss study using LFR cavity test setup. The heat loss obtained experimentally is compared to the numerical values obtained from CFD simulations. It can be seen from the Fig.6 that the numerical results obtained were in good agreement (difference is between 3-9%) with those obtained from experiment. In the higher tube temperature conditions, the CFD results under predicted the experimental results. This variation may be due to the fact that heat loss occurs through the insulated side and top side of the cavity which has been neglected in CFD analysis. After investigating heat loss, it was found that, radiative heat loss plays a dominant role. Total heat loss (W) 1 Depth of cavity= 100mm ε tubes =0.25 ε cover =0.9 T cover ( C) 240 220 180 160 140 Depth of cavity= 100mm ε tubes =0.25 ε cover =0.9 Total heat loss,q 250 300 350 120 100 Cover glass temperature 250 300 350 ( C) ( C) Figure 4.Total heat loss from the LFR test rig Figure 5.Glass cover temperature of the LFR test rig 2 2 0 1 1 1 1 y Expt =0.005x 2.152 (R 2 =0.998) y CFD =0.009x 2.06 (R 2 =0.998) Experimental results CFD results 250 300 350 Absorber tube temperature ( C) ε tubes =0.25 h ext =5W/m 2 K ε =0.25 tubes h ext =5W/m 2 K Q convection Q radiation Q total 0 50 100 150 250 300 -T atm (K) Figure 6. Comparison of heat loss between Experimental and CFD data Figure 7. Comaparison of convective and radiative heat losses from the cavity 4
Total heat loss comprising sum of convective and radiative heat losses from the cavity is shown in Fig.7. Proportion of convective and radiative heat losses for different external heat transfer coefficient (due to wind) is given in Table 1. It was noticed that, the natural convection is plays a least role which is only 7-15% of the total losses. The effect of different emissivities of the tube on the heat losses is shown in Fig. 8. As expected, there is significant effect of the change in emissivity in the radiative heat transfer and almost insignificant effect on the convective heat transfer. 2500 1 0 =548K 1 1 1500 500 h ext =5W/m 2 K Q radiation Q convection 0 0.0 0.2 0.4 0.6 0.8 1.0 Emissivities of the tube 0 350 450 500 550 Tube temperature (K) Pye et al (3) Facão et al (2011) Present code Figure 8.Variation of heat losses at different emissivities of the tube surface Figure 9. Comparison of total heat loss with published results The current CFD results have also been compared with the results obtained without considering the gaps between, [methodology adopted by Facão et al.(2011)] as well as by replacing tubes by a hot absorber plate [methodology adopted by Pye et al.(3)]. Fig. 9 shows that, present method calculates 24-29% more heat loss than those obtained by method used by Pye et al.(3) and 4-13% more heat loss than methodology proposed by Facão et al. (2011), respectively. The isotherms and streamlines for the proposed cavity is shown in Fig. 10. Due to symmetry, only one side of the cavity is shown for isotherm and stream contour plots. From the isotherms it can be seen that temperatures are almost uniform in the horizontal direction. However, there is sufficient temperature difference so as to result in natural convection with air rising from the centre of the cavity and then coming down from the sides. From flow pattern figure, it can be seen that vortices are formed between side walls and adjacent tubes. In the lower half of the cavity, additional vortices can be seen which are almost symmetric to the mid-plane. It can be observed further that stream function gradient is more top side of the cavity than bottom plane Figure 10. Isotherm contours and Isotherm contours inside the cavity (when tubes are at 548K) 5
Table1. Percentage heat loss coefficient of convective and radiative component with variation in external convective heat transfer coefficient h_ext 5 W/m 2 K 10W/m 2 K T (K) Convection Radiation Convection Radiation 40 90 140 190 240 9.86 90.14 15.45 84.55 8.1 91.9 12.6 87.4 6.7 93.3 9.95 90.05 5.67 94.33 8.1 91.9 4.48 95.52 6.13 93.87 7. Conclusion Total heat losses using a lab experimental setup for trapezoidal cavity with eight tubes was found out. A steady state modelling and simulation of trapezoidal cavity with eight tubes was carried out using CFD. The results obtained by the model are compared with the experimental data. The comparison shows a good match between experimental and numerical results, hence validating the model. The validated model was used for simulation of the cavity for various parameters. The computations have been carried out for different convective heat transfer coefficients related to different wind speeds near the outer side of glass cover. Total heat losses due to different emissivities of the tubes were also performed. It has been observed that the dominant mode of heat losses from the cavity is radiation. Hence, using selective coating on tubes and cavity inside wall, the overall losses can be minimized. Although the dominant mode of losses is radiation, the losses by natural convection at 8-15% are also significant. The use of evacuated cavities may be recommended to minimize convection losses. Acknowledgement: We would like to thank KG Design Services Private Ltd., Coimbatore for providing the laboratory set up for experimentation for finding heat loss from the LFR cavity, which was then used for validation of the computational model. 8. References Ansys Fluent manual,2010. Facão, J., Oliveira, A.C., 2011, Numerical simulation of trapezoidal cavity receiver for a linear Fresnel solar collector concentrator, Renewable Energy, 36, 90-96. Kline, S.J., McClintock, F.A., 1953, Describing uncertainties in single sample experiments, Mechanical Engineering, 75, 3 8. Mills, D.R., Morrison, G.L., 0, Compact linear Fresnel reflector solar thermal power plants, Solar Energy 8, 263 283. Pye, J.D., Morrison, G., Behnia. M., 3,Convection inside the Cavity Receiver of the CLFR Concentrating Solar Power System, 7th Natural Convection Workshop, Sydney, Australia. Reynolds, D.J., Jance, M.J., Behnia, M., Morrison, G.L., 4, An experimental and computational study of the heat loss characteristics of a trapezoidal cavity absorber, Solar Energy, 76, 229 234. Singh, P.L., Sarviya,R.M, Bhagoria,J.L., 2010, Thermal performance of linear Fresnel reflecting solar concentrator, Applied Energy, 87, 541-550. Singh, P.L., Sarviya,R.M, Bhagoria,J.L., 2010, Heat loss study of trapezoidal cavity absorbers for linear solar concentrating collector. Energy Conversion and Management, 51, 329-337. Sukhatme, S. P., Nayak, J.K.,, Solar Energy, Principles of Thermal Collection and Storage- 3e, 8, TMH, New Delhi. 6