Numerical Simulation Of Solar Receiver

Numerical Simulation Of Solar Receiver

Dr.Umashankar1, Ravi Kumar D.S2

1Professor, Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur-572103, Karnataka, India.

2M.Tech Student, Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur-572103, Karnataka, India.

Abstract

The parabolic concentrator reflects the direct incident solar radiation onto a receiver mounted above the dish at its focal point. The conversion of concentrated solar radiation to heat takes place in receiver. The heat transfer characteristics of the receiver changes during the rotation of the receiver which affects thermal performance. The working temperature may also influence the thermal performance and overall efficiency of the dish-Stirling solar electricity generating system.

A heat transfer and flow simulation is performed for four different solar cavity receivers viz.: cylindrical, conical, dome and spherical receivers at various receiver inclinations at constant temperature. The receivers are designed such that they have same surface area and aperture. It is observed that convective heat loss decreases as the inclination changes from 00 to 900. Among these receivers, the convective heat loss is least for conical receiver followed by dome, spherical and cylindrical receivers

1. Introduction

   A central receiver system (CRS) is another concept for a high temperature solar concentrator that aims at the collection of large amounts of highly concentrated solar energy without requiring a piping. In this concept it is expected to achieve economy-of scale benefits, because systems with several 100 000 m2 of reflector surface area can focus to a single receiver approaching several 100 MW of power and thus use conventional power plant technology for the power cycle. Fig. 1 shows the Schematic diagram of the dish solar concentrator/cavity receiver system.

   There are two different receiver designs in CRSs, the external and the cavity design. In cavity receivers, the heat absorbing elements are located inside of an insulated cavity. The focal spot of the heliostat field coincides with the aperture of the cavity. In external receivers the heat transferring surfaces are exposed to

   the ambient and are located directly in the focal point of the heliostat field. Cavity receivers offer the benefit of lower heat losses but generally constrain the direction of the incoming radiation and thus impact the heliostat field arrangement. They also restrict the benefits of selective absorber surfaces when they may become available in the future and are generally costlier. External receivers allow for an easier scale-up to higher power levels. In particular, cylindrical designs offer a flexible design of surround heliostat fields. However, in contrast to cavity receivers, the maximum concentration of the heliostat field that could be exploited is limited by the material constraints of the heat absorbing elements, because they are located directly in the focal point. The Fig. 2 and 3 shows the Stream Lines in Downward and Upward Facing Tilted Cavity Receiver

   Fig. 1 Schematic diagram of the dish solar concentrator/cavity receiver system

   Fig. 2 Stream Lines in Downward Facing Tilted Cavity Receiver

   Fig. 3 Stream Lines in Upward Facing Tilted Cavity

   Receiver

   The important energy loss for the receiver originates from convection and radiation heat transfer to the surroundings. These losses depend on the design of the receiver, whether it is a cavity or external receiver, its heated (or aperture) area, and its operating temperature. Additional factors include the local wind velocity, ambient temperature, and the orientation of the receiver.

   Studies have been made on the combined radiation, free and forced-convection losses from large surfaces, and tilted cavities. Siebers et al. (1982) have performed experiments on large vertical surfaces in horizontal f low, and their data are being used to predict losses from external receivers. Clausing (1981) has developed a method for predicting the natural convective loss from cavity receivers. A summary of these studies may be found in Siebers and Kraabel (1984). Radiation and convection losses are primarily functions of the size of the receiver and the operating temperature of the system. For most currently conceived central receiver system designs, the receiver operates at a constant temperature. Therefore, the rate of energy being lost from the receiver is essentially constant throughout the

   day (and year) and the percentage loss increases in the morning and evening.

   Numerical studies of cavity receiver convective losses are very limited. Numerical investigations on the convective heat transfer in open cavities have been reported in literature (Le Quere et al., 1981, Penot, 1982, Chan and Tien, 1985, Skok et al., 1991). Though the solar cavity receiver is essentially an open cavity, the studies carried out on open cavities cannot be directly extended to the case of solar cavity receivers

   The heat losses from the receiver include three contributions: conductive heat loss from the receiver walls and radiative and convective heat losses through the receiver aperture. Among these contributions, natural convective heat loss contributes a significant fraction of energy loss. The natural convective heat loss in the receiver is an important factor for determining the performance of a fuzzy the overall focal solar dish concentrator. In order to improve system efficiency, natural convection characteristics need to be studied extensively. The main objectives of this work to predict the convective heat loss from the different receiver is shown in Fig.4.

   Fig. 4 Six classical cavity geometries

2. Geometric Model of the Receiver

   Geometric model of solar receiver is created in ANSYS Design Modeller 13.0. Initially for validating the CFD results the geometric model is considered from Taumoefolau [16] as shown in Fig.5and 6.

   Fig. 5 Geometric Model of cylindrical dome receiver

   Initially cylindrical geometry of cylindrical receiver is created as shown in Fig.7. For CFD analysis, the receiver is assumed to be placed in a sufficiently large enclosure with walls at ambient temperature. Due to the symmetrical flow geometry with respect to the middle vertical plane, the computational extent comprises only one half of the physical domain. The size of the enclosure was determined in a preliminary study such that it showed negligible effect on fluid and heat flows in the vicinity of the receiver. It was found that the diameter and height of the enclosure should be approximately twenty times the diameter of the receiver to achieve this

   Fig. 6 Geometric Model of spherical and conical receiver

   Fig. 7 CFD Domain of dome receiver

   Fig. 8 Boundary layer Mesh close to cylindrical receiver wall

   For boundary conditions, the cylindrical enclosure wall was set to ambient temperature of 27oC. The receivers cavity and outer walls were assumed to be isothermal and adiabatic, respectively. The cavity wall temperature for each receiver was set as follows:

   – For the model receiver, the average experimental values of cavity wall temperature data of 450oC is used for the cylindrical section, similar setup is used for all the receivers.

3. RESULTS AND DISCUSSIONS

   The results obtained in this simulation study are presented and discussed in this section. Temperature contours of the cylindrical receiver at surface temperature of 4500 C for inclinations of 00, 300, 600, and 900 are shown in Fig. 9. Red colorrepresents the stagnation zone that has high temperature within the cavity where as blue color represents the convective zone that is near ambient temperature. The zone boundary is the separation of red and yellow colors. Most of the receiver locations have air temperature gradients at 00 inclination (receiver facing

   sideways) of the cavity hence little stagnation zone exists for this particular orientation.

   As the inclination of the receiver increases from 00 to 300 the stagnation zone size increases. A direct consequence of this is the decrease in convective heat loss as shown in fig 11. The convective heat loss values are calculated using FLUENT reporting options. As the inclination of the receiver increased to 900 (facing side ways) the stagnation zone size further increases. The convective heat loss is minimum at 900 inclination of the receiver as shown in Fig10.

   200

   180

   Convection loss (W)

   160

   140

   120

   100

   80

   60

   40

   20

   0

   Experimental Numerical

   0 10 20 30 40 50 60 70 80 90

   Inclination (degree)

   Fig. 10 Comparison of Experimental and Numerical values for cylindrical receivers

   60o

   70o

   Fig. 9 Cylindrical receiver Temperature contours at inclination 0-90o

   Fig. 11 Cylindrical receiver Velocity contours at inclination 0-90o

   Fig. 12 Dome receiver Temperature contours at inclination 0-30o

   Fig12 to 13 shows the temperature contours of the dome receiver for various inclinations at 4500 C. It is observed that the inclination of the receiver increases from 00 to 900 of the receiver, the stagnation zone increases whereas the convective zone decreases. Hence the convective heat loss decreases as the orientation of the receiver changes from 00 to 900

   Fig. 13 Dome receiver Temperature contours at inclination 40-90o

   Fig. 14 Conical receiver Temperature contours at inclination 0-30o

   Fig. 14 to 15 shows the temperature contours of the conical receiver for various inclinations at 4500

   C. It is observed that at 00 inclination the convective heat loss is maximum. The convective heat loss reduces to minimum as the orientation is increased to 900. The stagnation zone increases within the receiver in both cases as the inclination of the receiver increases from 00 to 900. The direct consequence of increase in stagnant zone is decreased in convective heat loss as the orientation changes from 00 to 900.

   Fig. 15 Conical receiver Temperature contours at inclination 40-90o

   200

   180

   Convection loss (W)

   160

   140

   120

   100

   80

   60

   40

   20

   0

   Cylindrical-Numerical Dome-Numerical Conical-Numerical Spherical-Numerical

   0 10 20 30 40 50 60 70 80 90

   Inclination (degree)

   Fig. 16 Spherical receiver Temperature contours at inclination 0-30o

   Fig. 16 to 15 shows the temperature contours of the spherical receiver for various inclinations at 4500 C. It is observed that the maximum convective heat loss occurs at 00 inclination. It is observed that the convective heat loss is minimum as the orientation changes to 900.

   Fig. 17 Spherical receiver Temperature contours at inclination 40-90o

   Fig. 18 Convection Heat loss comparison

   Fig.18 shows Comparison of convective heat loss with different inclinations among cylindrical, conical, dome and spherical receivers at 4500C. The receivers are designed such that they have same surface area and aperture. It is observed that convective heat loss decreases as the inclination changes from 00 to 900. Among these receivers, the convective heat loss is least for conical receiver followed by dome, spherical and cylindrical receivers.

4. CONCLUSIONS

   The results of a numerical study of the problem of natural convection in cavity receivers of solar parabolic dish collector have been presented in this thesis. The effect of cavity geometry, inclination, receiver temperature through the aperture of solar cavity receiver has been numerically investigated using CFD package FLUENT 13.0. Modeling and thermal performance characteristics of the solar dish collector system are presented.

   The comparison of convective heat loss with different inclinations among cylindrical, conical, dome and spherical receivers at temperature 4500C is presented. The receivers are designed such that they have same surface area and aperture. It is observed that convective heat loss decreases as the inclination changes from 00 to 900. Among these receivers, the convective heat loss is least for conical receiver followed by dome, spherical and cylindrical receivers.

5. REFERENCES

[1]. Battleson, K.W.(1981), Solar Power Tower Design Guide: Solar Thermal Central Receiver Power Systems, A Source of Electricity and/or Process Heat, Sandia National Labs Report SAND81-8005, April. [2]. Bergeron, K. D., and C. J. Chiang (1980), SCRAM: A Fast Computational Model for the Optical Performance of Point Focus Solar Central Receiver

Systems, Sandia National Labs Report SAND80- 0433, April.

[3]. Biggs, F. and C. M. Vittitoe,(1979), The HELIOS Model for the Optical Behavior of Reflecting Solar Concentrators," Sandia National Labs Report SAND76-0347, March.

[4]. Clausing, A. M. (1981), An Analysis of Convective Losses from Cavity Solar Central Receivers, Solar Energy 27 (4), 295.

[5]. Coggi, J., and H. Eden (1981), Solar 10 Megawatt Pilot Plant Performance Analysis, The Aerospace Corporation, Report No. ATR-81 (7747)-1, February.

[6]. Dellin,T.A., M. J. Fish, and C. L. Yang (1981), A Users Manual for DELSOL2: A Computer Code for Calculating the Optical Performance and Optimal System Design for Solar Thermal Central Receiver Plants, Sandia National Labs Report SAND81-8237, August.

[7]. Holl, R. J. (1978), Definition of Two Small Central Receiver Systems, Sandia National Labs Report SAND78-7001, April.

[8]. King, D. L. (1982), Beam Quality and Tracking Accuracy Evaluation of Second Generation and Barstow Production Heliostats, Sandia National Labs Report SAND82-0181, August.

[9]. Leary, P., and J. Hawkins (1979), A Users Guide for MIRVAL Computer Code for Comparing Designs of Heliostat Receiver Optics for Central Receiver Solar Power Plants, Sandia National Labs Report SAND77-8280, February.

[10]. Lipps, F. W., and L. L. Vant-Hull (1978), A Cellwise Method for the Optimization of Large Central Receiver Systems, Solar Energy 30(6), 505.

[11]. Siebers, D. L., and J. S. Kraabel (1984), Estimating Convective Energy Losses from Solar Central Receivers, Sandia National Labs Report SAND84-8717, April.

[12]. Siebers, D. L., R. G. Schwind, and R. J. Moffat (1982). Experimental Mixed Convection From a Large, Vertical Plate in a Horizontal Flow, in Proceedings of the Seventh International Heat Transfer Conference, Munich, vol. 3, pp. 477-482, September 6-

10.

[13]. Sterns Roger Engineering Company (1979), Tower Cost Data for Central Receiver Studies, Sandia National Labs Report SAND78 -8185, June. [14]. Vittitoe, C. N., and F. Biggs (1978), Terrestrial Propagation Loss, paper presented at the American Section, International Solar Energy Society Meeting, Denver, Colorado, August.

[15]. Vittitoe, C. N., F. Biggs and R. E. Lighthill (1979), HELIOS: A Computer Program for Modeling the Solar Thermal Test Facility, A Users Guide, Sandia National Labs Report SAND76-0346, March. [16]. T. Taumoefolau and K. Lovegrove, An Experimental Study of Natural Convection Heat Loss from a Solar Concentrator Cavity Receiver at Varying Orientation, Centre for Sustainable Energy Systems,