Scholarly article on topic 'New Designs of Molten-salt Tubular-receiver for Solar Power Tower'

New Designs of Molten-salt Tubular-receiver for Solar Power Tower Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — M.R. Rodríguez-Sánchez, A. Sánchez-González, C. Marugán-Cruz, D. Santana

Abstract One of the main problems of solar power tower plants with molten salt as heat transfer fluid is the reliability and lifetime estimation of central receivers. The receivers must withstand high working temperatures, molten salt corrosion and important solar flux transients that lead to thermal stresses and fatigue. Therefore, it is necessary optimize the design of these receivers. The aim of this work is to study the thermal behavior of a new design of central receiver. This new external receiver is composed by several vertical panels formed by bayonet tubes. Each bayonet tube presents an inner tube and an outer tube concentric to the internal one. Both are joined in one side by a bayonet end-cap. The heat transfer fluid flows through the inner tube and the annular gap between the two tubes. In addition, the outer tube will be coated in its external surface by a material of high absorptivity of the solar radiation. In absence of experimental data of the receivers both concepts of external receiver, traditional and bayonet, have been characterized using simplified numerical simulations. The results obtained have been the salt, the film and the wall temperature evolutions and the thermal efficiencies. The advantages of the bayonet receiver over the traditional tubular receiver are: soften wall and film temperatures, lower thermal stresses due to a more homogeneous wall temperature, and lower corrosion rate, that allows use more economical materials to built the receiver tubes or even reduce the receiver surface increasing the thermal efficiency.

Academic research paper on topic "New Designs of Molten-salt Tubular-receiver for Solar Power Tower"

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Energy Procedía 49 (2014) 504 - 513

SolarPACES 2013

New designs of molten-salt tubular-receiver for solar power tower

M.R. Rodriguez-Säncheza*, A. Sänchez-Gonzäleza, C. Marugän-Cruza, D. Santanaa

aDepartment of Thermal and Fluid Engineering,University Carlos III of Madrid. Avda. de la Universidad 30, 28911 Leganes, Madrid (Spain).

*Phone number: +0034 916246034, e-mail: mrrsanch@ing.uc3m.es

Abstract

One of the main problems of solar power tower plants with molten salt as heat transfer fluid is the reliability and lifetime estimation of central receivers. The receivers must withstand high working temperatures, molten salt corrosion and important solar flux transients that lead to thermal stresses and fatigue. Therefore, it is necessary optimize the design of these receivers. The aim of this work is to study the thermal behavior of a new design of central receiver. This new external receiver is composed by several vertical panels formed by bayonet tubes. Each bayonet tube presents an inner tube and an outer tube concentric to the internal one. Both are joined in one side by a bayonet end-cap. The heat transfer fluid flows through the inner tube and the annular gap between the two tubes. In addition, the outer tube will be coated in its external surface by a material of high absorptivity of the solar radiation.

In absence of experimental data of the receivers both concepts of external receiver, traditional and bayonet, have been characterized using simplified numerical simulations. The results obtained have been the salt, the film and the wall temperature evolutions and the thermal efficiencies. The advantages of the bayonet receiver over the traditional tubular receiver are: soften wall and film temperatures, lower thermal stresses due to a more homogeneous wall temperature, and lower corrosion rate, that allows use more economical materials to built the receiver tubes or even reduce the receiver surface increasing the thermal efficiency.

© 2013 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/3.0/).

Selectionandpeerreviewbythescientificconference committeeofSolarPACES2013underresponsibilityofPSEAG. Final manuscript published as received without editorial corrections. Keywords: Central-external receiver; bayonet receiver; thermal analysis.

1. Introduction

In a molten salt solar power tower plant (SPT), the receivers are a crucial part of the plant. They cost around the 15-20% of the total capital investment cost of a solar plant [1-3] and they are subjected to extreme working conditions, having uncertain lifetime. During operation of the receiver the main problems are tube corrosion caused

1876-6102 © 2013 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/ 3.0/).

Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG.

Final manuscript published as received without editorial corrections.

doi:10.1016/j.egypro.2014.03.054

by the high corrosive effect of the molten salt at high temperature; cracks in the welded zones and problems related to material resistance due to thermal stresses and fatigue; tube overheating; and salt freezing during unsteady states (passage of clouds).

In the last years, many efforts have been focused on the receiver design optimization in order to reduce heat losses and early failure of the tubes, as well as to increase the energy conversion efficiency of the receiver. Lata et al. [4] focused their research on the optimization of the diameter and wall thickness of the receiver tubes. In addition, they analyzed different tube materials as nickel base alloys 625LCF, 625, 230, 617LCF and the austenitic stainless steel 800H to establish which one could fulfill better the solar power plant requirements. On the contrary, other authors tried modifying the heat transfer fluid (HTF); Jianfeng et al. [5] made a numerical analysis using HIATEC, an eutectic mixture of inorganic salts of NaNO3-KNO3-NaNO2. The latter salt was also used by Yang et al. [6] in their experiments. Yu-ting [7] studied numerically and experimentally the behavior of LiNO3 and Singer et al. [3] proposed as HTF: NaNO3-KNO3, Na, Bi-Pb and LiCl-KCl. Cui et al. [8] even tested solid-liquid phase change materials with high melting point. A further step was taken by other authors that presented novel designs for molten salt solar receivers. It is the case of Yang et al. [6], who tested a solar receiver formed by spiral tubes and Garbrecht et al. [9], who proposed an innovative design composed by many hexagonal pyramid shaped elements instead of circular pipes.

In the present work a new design of molten salt solar receiver based on bayonet tubes is proposed in order to improve its thermal and mechanical behavior under operating conditions with respect to the traditional external receivers. The main goal of this study is to develop a simplified thermal model for the bayonet receiver to compare the results with the traditional receiver's data, obtained from a model previously developed [10]. Both models take into account axial and circumferential variations of the heat flux absorbed by the tubes. Then, the models are able to predict the heat fluxes involved in the receiver, the temperature of the molten salt, the temperature of the tube walls, and the thermal efficiency of the external receiver.

2. Receiver configuration

The traditional molten salt central receiver is configured as a 360° cylindrical receiver, formed by a variable number of vertical blocks of tubes, called panels (see Fig. 1.a). Each panel includes an inlet header, the inlet nozzles, the tubes, the outlet nozzles and an outlet header. The tubes of each panel are individually supported at their top and periodically guided over their entire length by tube clips, allowing unrestricted downward thermal expansion [11]. In order to reduce the heat losses in the back side of the tubes there is a refractory wall thermally insulated, which is built in mineral wool jacketed by a high reflectivity material (White Pyromark).

Receiver's tubes must be built with special materials that support both, high temperatures and salt corrosion. In this study Incoloy 800H coated with a high solar radiation absorptivity material (Black Pyromark) has been used. In the thermal model, the density and specific heat of the tube material are considered to be constant with the temperature, and only the variations of the thermal conductivity variations with temperature are taken into account. The HTF is molten salt 60% KNO3 - 40% NaNO3, whose temperature variations of the density, dynamic viscosity, specific heat, and thermal conductivity are taken into account in the models, using Zavoico's data [ 12].

The inlet flow, at the lowest temperature of the salt, Tsalt{z = 0), enters flowing in parallel through every tube located on the two northern panels of the receiver, where the concentrated solar flux is maximum. The salt is then divided in two parallel flow paths (north-east-south and north-west-south); one or more crossovers in the flow paths are provided to keep the energy capture of the two flows paths in balance over the complete range of operating conditions. To pass from one panel to the next the HTF flows as a serpentine until arriving at the southern panel of the receiver, where the molten salt exits at maximum temperature.

(a) (b) (c)

Fig. 1. (a) Scheme of a traditional external receiver, (b) Scheme of a bayonet receiver, (c) Scheme of one panel of a bayonet receiver. 1. Inner tube; 2. Outer tube; 3. Coating; 4. Inlet fluid; 5. Outlet fluid; 6. Bayonet end-cap; 7. Inlet header; 8. Outlet header.

On the other hand, the new proposed receiver design consists of panels formed by bayonet tubes instead of circular pipes, Fig. l.b. Each bayonet pipe is formed by two concentric tubes joined by a bayonet end-cap in one of its ends, Fig. l.c [13]. The tube with higher diameter will be covered at its external wall with a common coating (Black Pyromark). The tube material is the same as before, although the inner tube could be made of a basic stainless steel due to its working temperatures are lower.

Regarding operation mode bayonet receivers have two possible flow directions. The cold fluid can be pumped into the inner tube and then the hot fluid flows out through the annular gap (inner bayonet); or else the cold fluid can enter by the annular gap and exits hotter by the inner tube (outer bayonet). Both configurations will be analyzed in the results section. Since in a bayonet tube the inlet and the outlet zones are in the same extreme of the tube, the headers will be at the bottom to allow an easy drainage, see Fig. l.c. In addition, at the top of the bayonet tubes, a venting valve could be placed to avoid gas concentration. Apart from these peculiarities bayonet receiver's configuration is exactly the same that the traditional external receiver.

Table 1. Design parameters.

Parameter Value Parameter Value

Receiver height, H [m] 10.5 Wind velocity, vWind [m/s] 0

Receiver diameter, D [m] 8.4 Ambient Pressure, Pamb [bar] 1

Number of flow paths, 2 Ambient temperature, Tamb [°C] SO

Total number of panels, Np 18 Relative humidity, <p [%] 60

Number of tubes per panel, Nt 22 Global absorptivity of the tubes, at 0.9 S

Total mass flow by the panels, fhsait [kg/s] 290 Tube material emissivity, st 0.84

External diameter of the outer tube, D0 [mm] 60.3 Sky emissivity, £sky(Tamb) 0.895

Wall thickness of tubes, th [mm] 1.65 Refractory wall emissivity, £n+1 0.2

Tube pitch, B [mm] 2 Ground emissivity, sgr 0.955

External diameter of the inner tube (for 52 Temperature of the salt at inlet of 290

bayonet receivers) , dQ [mm] the receiver, Tsalt(z = 0) [°C]

The geometries of the external receivers studied has been selected according to the operating conditions of the receivers which require: high external tube diameters to reduce pressure drop, the number of tubes, the welded zones, and the risk of failure; tubes of small wall thickness to improve the heat transfer in the tubes; moderate

temperature in bayonet receivers is more homogenous in axial and circumferential directions than in a traditional receiver, and hence lower thermal stresses can be found in bayonet receivers.

(a) (b)

Fig. 4. (a) Axial distribution of the external wall temperature in the fifth panel. (b) Circumferential distribution of the external wall temperature in the fifth panel. The colors indicate the type of receiver: blue for traditional receiver, black for inner bayonet and red for outer bayonet. And the symbols indicate the external wall temperature for different axial and circumferential positions in the representative tube of the fifth panel.

The main results of this study have been included in Table 2. The thermal efficiency of the receivers, which is defined as the heat flux absorbed by the tubes divided by the total heat flux reflected by the heliostat, has also been included in Table 2. At the same time it has been calculated the maximum thermal stresses in the receiver and the total pressure drop.

Table 2. Results comparison.

Tube Inner bayonet Outer bayonet

Tsalt(end) [°C] 565 570 570.1

T„aii,e(max) [°C] 723.3 623.2 624

Tfilm(max) [°C] 678.6 594.3 595.4

TWall,e [°C] 508.3 483.8 483.8

T„aii,e(max)-Twalle(min) [°C] 432 326.3 333.2

Eth [ %] 74.8 76.15 76.15

a(max)/UTS [%] 32.15 29.43 29.43

AP [bar] 0.245 9.72 9.72

While for a conventional tube receiver the maximum film temperature is 678 °C and the maximum wall temperature 723 °C, a value that increases quickly the corrosion rate and reduce the lifetime of the receiver, in a bayonet receiver these values are 594 °C and 623 °C, respectively. The maximum thermal stresses are lower in a bayonet receiver too, decreasing from 32% of the ultimate tensile strength (UTS) to 29.5%. However, the pressure drop in bayonet receiver is higher, increasing from 0.24 bars in a traditional receiver to 9.7 bars in the new design. However, this value it is not so significant considering that to fulfill film temperature restrictions it is necessary a traditional receiver with smaller tube diameters, that means higher pressure drop that those showed in Table 2.

5. Conclusions

A new design of external receiver has been proposed in order to improve the thermal and mechanical behavior of the receivers during operating conditions. This new receiver is formed by bayonet tubes. In order to accomplish that goal, a simplified thermal model has been developed. The model takes into account the circumferential and axial

variations of the external tube wall temperature and the results have been compared with the values calculated previously for a traditional receiver. The same external geometry and ambient conditions have been used for both cases.

The bayonet receivers include two operation modes depending on the flow direction: inner bayonet and outer bayonet. Both configurations give similar results, finding a small difference in the evolution of the wall and film temperature because the maximum values are located at different heights; and in the salt temperatures due to in the annulus the temperature increment is always higher.

Comparing the bayonet receiver to the traditional receiver the maximum film has been reduced in 84 °C and the maximum wall temperature has fallen 100° C. Consequently, the corrosion rate and the salt decomposition ratio of the molten salt have decreased. Lower wall temperatures reduce the heat losses of the receivers and then their thermal efficiency increases 2%. On the other hand, lower film temperatures allow to use cheaper materials on the tubes or to reduce the size of the receivers that affect positively in the thermal efficiency of the receivers and then it could be possible to reduce the heliostat field. At the same time, the thermal stresses are lower due to a more homogeneous wall temperature, reducing the early failure of the receivers due to fatigue.

Acknowledgements

The authors would like to thank the financial support from CDTI and S2m Solutions for the project MOSARELA (Molten salt receiver lab) whose reference is IDI-20120128. The financial support of the project ENE2012-34255 is also acknowledged.

References

[1] Power Tower Technology Roadmap and cost reduction plan. Sandia report; April 2011.

[2] Renewable energy technologies: cost analysis series. Concentrating solar power. IRENA (international renewable energy agency). June 2012; Vol. 1: power sector, Issue 2/5.

[3] Singer C, Buck R, Pitz-Paal R, Müller-Steinhagen H. Assessment of Solar Power Tower Driven Ultrasupercritical Steam Cycles Applying Tubular Central Receivers With Varied Heat Transfer Media. Journal of Solar Energy Engineering 2010; 132: 041010, 1-12.

[4] Lata JM, Rodríguez M, Alvarez de Lara M. High Flux Central Receivers of Molten Salts for the New Generation of Commercial Stand-Alone Solar Power Plants. Journal of Solar Energy Engineering 2008; 130.

[5] Jianfeng L, Jing D, Jianping Y. Heat transfer performance of an external receiver pipe under unilateral concentrated solar radiation. Solar Energy 2010; 84: 1879-1887.

[6] Yang M, Yang X, Yang X, Ding J. Heat transfer enhancement and performance of the molten salt receiver of a solar power tower. Applied Energy 2010; 87: 2808-2811.

[7] Yu-ting W, Bin L, Chong-fang M, Hang G. Convective heat transfer in the laminar-turbulent transition region with molten salt in circular tube. Experimental Thermal and Fluid Science 2009. 33:1128-1132.

[8] Cui H, Xing Y, Guo Y, Wang Z, Cui H, Yuan X. Numerical simulation and experiment investigation on unit heat Exchange tube for solar heat receiver.Solar Energy 2008. 82: 1229-1234.

[9] Garbrecht O, Al-Sibai F, Kneer R, Wieghardt K. CFD-simulation of a new receiver design for a molten salt solar power tower. Solar Energy 2013; 90:94-106.

[10] Rodríguez-Sanchez MR, Soria-Verdugo A, Almendros-Ibáñez JA, Acosta-Iborra A, Santana D. Thermal design guidelines of solar power towers. Applied Thermal Engineering. Submited for publication.

[11] Falcone PK. A handbook for solar central receiver design. Sandia National Laboratories, Livermore 1986; SAND 86-8809.

[12] Zavoico AB. Solar power tower design basis document. Sandia National Laboratories, San Francisco 2001; Rev.0, SAND 2001-2100.

[13] Wiesenberg R, Serrano E, Ruano A, Santana D, Rodríguez MR, Marugán C, Soria A., Patent name: Receptor Termosolar. Request number: PCT/ES2012/070308. Claiming entities: Sun to Market Solutions S.L. and Carlos III University of Madrid.

[14] Rodríguez MR, Venegas M, Marugán C, Santana D. Nuevo diseño de receptores para centrales termosolares tipo torre: receptor bayoneta. VIII CNIT, 2013.

[15] Modest MF. Radiative Heat Transfer. 2nd ed.: Elsevier; 2003.

[16] Berger X, Buriot D, Garnier F. About the equivalent radiative temperature for clear skies. Solar Energy 1984; 6: 725-733.

[17] Lienhard JH. A Heat Transfer Text Book, 3rd ed.:Phlogiston Press. Cambridge, Massachusetts. 2001.