Thermal Performances of U Shape Molten Salt Steam Generator

Transcription

1 Proceedings of the Asian Conference on Thermal Sciences 17, 1st ACTS March 26-, 17, Jeju Island, Korea Thermal Performances of U Shape Molten Salt Steam Generator Canming He, Jianfeng Lu*, Jing Ding *, Weilong Wang, Xiyuan Pan, Hanwen Peng 1 School of Engineering, Sun Yat-Sen University, Guangzhou, , China ACTS-P00467 Presenting Author: @qq.com *Corresponding Author: lujfeng@mail.sysu.edu.cn ABSTRACT Thermal performances of U shape molten salt steam generator were experimentally investigated in this article. The U shape molten salt steam generator contained 3 U shape tubes, water was pumped into the tubes and high temperature molten salt flowed through the shell side, and the overall heat transfer coefficient, steam generation rate and thermal efficiency etc. of the steam generator were further analyzed under different molten salt flow rates and water flow rates. According to the experimental results, as the inlet molten salt flow rate increases, the overall heat transfer coefficient of the steam generator increases, the release heat flow of molten salt and the heat flow absorbed by water/steam both increase. As the water flow rate increases, the overall heat transfer coefficient of the steam generator first increases and then decreases, and the variation of the heat flux is the same. The steam generation rate increases almost linearly as the inlet molten salt flow rate increases, however, as the water flow rate increases, the steam generation rate first increases and then decreases. As the molten salt flow rate and water flow rate increase, the thermal efficiency of the steam generator first increases and then decreases, approaching its maximums with optimal molten salt flow rate and water flow rate respectively. KEYWORDS: Steam generator; molten salt; steam generation rate; thermal efficiency 1. Introduction Molten salt as a heat transfer and storage substance [1] is mainly used in CSP plants and other high temperature heat utilization systems. Steam is commonly used as the working medium for power generation system. As the central hub of molten salt flow loop and steam power cycle, molten salt steam generator is the key technology for energy conversion from molten salt to steam, and is used in solar power tower plants, nuclear reactors and other power generation systems [2, 3]. Vriesema [4] investigated the shell and tube molten salt steam generator with several heat transfer annular duct, and high pressure water with high saturated temperature was adopted to avoid molten salt freezing. Solar Two is the first solar power tower station to applied molten salt steam generator in power generation [5]. Xiao et al. [6] experimentally investigated the heat transfer performance between molten salt and subcritical/supercritical water inside helical annular duct. Gaggioli et al. [7] proposed a single storage tank system integrated with an immersed steam generator, using molten salt as the heat transfer substance. Generally, the thermal performances of U shape molten salt steam generator were seldom studied. In this article, thermal performances of the U shape molten salt steam generator were experimentally investigated, and the effects of molten salt flow rate and water flow rate were further analyzed. 2. Experiment system Molten salt experimental platform was set up in Energy Conservation Centre of Sun Yat-Sen University, as shown in Figure 1a. The experimental molten salt steam generation system mainly consisted of molten salt flow loop, steam generation system, data acquisition system and electronic control system. Molten salt was heated to specified temperature in the molten salt tank, and then it was pumped into the steam generator to work as the high temperature source of the steam generation process. Molten salt was pumped by the molten salt pump, and 1

2 its flow rate was controlled by the valves and measured by the molten salt flow meter. Molten salt was ternary nitrate salt (KNO 3 +NaNO 2 +NaNO 3 ) [8], its thermal properties were correlated as: λ= T Wm -1 k -1, ρ= t kgm -3, c p = T Jkg -1 K -1, μ= t T T 3 gm -1 s -1, where the temperature of molten salt T ranges from 250 o C to 500 o C. flow meter pump tank control system U shape molten salt steam generation system (a) Experimental system (b) Schematic diagram Fig. 1 Experimental system and steam generation diagram Steam generation system mainly consisted of U shape molten salt steam generator and steam header, as graphically illustrated in Figure 1b. Water was pumped into the steam generation system by the piston pump, absorbing heat from high temperature molten salt along the shell side of the steam generator, and finally changed into saturated vapour. High temperature molten salt was firstly pumped into the molten salt loop, and then entered into the U shape steam generator, heating the water/steam into saturated vapour, finally it flowed back to the molten salt tank. The U shape steam generator consisted of 3 U shape circular tubes, and each tube had a diameter of 32 mm, wall thickness of 2.0 mm and length of 1850 mm. The cylinder tank of the steam generator had an outer diameter of 219 mm, wall thickness of 6 mm and length of 880 mm. The U shape steam generator was made of stainless steel, and insulation with thickness of 100 mm was placed outside the steam generator to reduce the heat loss. The molten salt flow rate was measured by FLEXIM ultrasonic flow meter with the range of 0-4 m 3 /h, and its uncertainty was 1.0%. The steam flow rate was measured by vortex flow meter with the range of m 3 /h, and its uncertainty was 1.0%. Temperatures of molten salt in the inlet and outlet of the steam generator were measured by K type thermocouples with uncertainty of 0.5 K. In this present article, experiments were performed within the range of molten salt flow rate m 3 /h, inlet molten salt temperature o C and water mass flow rate 10- kg/h. 3. Data analyses The release heat flow of molten salt was calculated as: q c T T (1) rel v p in out where q v was the volumetric flow rate of molten salt, ρ and c p were the density and specific heat of molten salt, T in and T out were the inlet and outlet temperatures of molten salt. The heat flow absorbed by water/steam in steam generator was calculated as: q h h (2) ab m, w out,s in,w where was the mass flow rate of water, h in,w and h out,s were the enthalpies of inlet water and outlet vapour. The energy efficiency of steam generator was calculated as: ab (3) rel 2

3 The average heat flux of steam generator was estimated as: q ab S where S was the heat transfer area of steam generator. The average heat transfer temperature difference of steam generator was calculated as: Tin Tout Tlm Tin ln Tout where ΔT in and ΔT out were the temperature differences at the inlet and outlet of steam generator. The overall heat transfer coefficient of steam generator was estimated as: q K T lm (4) (5) (6) 4. Results and Discussions 4.1 Heat transfer performances under different molten salt flow rates Fig. 2 presents the overall heat transfer coefficient and steam generation rate of steam generator under different molten salt flow rates, where T in = 326 o C, u w = m/s., The overall heat transfer coefficient of steam generator almost increased linearly as the inlet molten salt flow rate increased, as a result of the increase of the heat transfer coefficients of molten salt convection and water boiling, and the steam generation rate of steam generator also increased as the overall heat transfer coefficient increased. As the inlet molten salt flow rate increased from m 3 /h, the overall heat transfer coefficient of steam generator increased from W/(m 2 K) to W/(m 2 K), and the steam generation rate increased from 24.5 kg/h to 28.4 kg/h. K (Wm -2 K -1 ) K (kg/h) q v (m 3 /h) Fig. 2 Overall heat transfer coefficient and steam generation rate of steam generator under different molten salt flow rates (T in = 326 o C, u w = m/s) Fig. 3 presents the heat flow and energy efficiency of steam generator under different molten salt flow rates, where T in = 326 o C, u w = m/s. The release heat flow of molten salt and the heat flow absorbed by water/steam both increased as the inlet molten salt flow rate increased, because of the increase of the heat transfer coefficients of molten salt convection and water boiling. As the inlet molten salt flow rate increased, the energy efficiency of steam generator first increased as the overall heat transfer coefficient increased, and then decreased due to the increase of heat loss. As the inlet molten salt flow rate increased from m 3 /h, the release heat flow of molten salt and the heat flow absorbed by water/steam increased respectively from 19.2 kw to 22.0 kw and from 16.7 kw to 19.0 kw, and the energy efficiency with q v = 1.4 m 3 /h reached its maximum 87.7%. 15 3

4 (kw) rel ab q v (m 3 /h) Fig. 3 Heat flow and energy efficiency of steam generator under different molten salt flow rates (T in = 326 o C, u w = m/s) 4.2 Heat transfer performances under different water flow rates Fig. 4 presents the overall heat transfer coefficient and heat flux of steam generator under different water flow rates, where T in = 346 o C, q v =2.0 m 3 /h. As the water flow rate increased, the overall heat transfer coefficient of steam generator first increased and then decreased, because of the short boiling region at low water flow rates and the low wall temperature at high water flow rates, and the heat flux of steam generator also first increased and then decreased. As the water flow rate increased from m/s to m/s, the overall heat transfer coefficient of the steam generator first increased from W/(m 2 K) to its maximum W/(m 2 K) at u w = m/s, and then decreased to W/(m 2 K), and the heat flux first increased from 31.5 kw/m 2 to its maximum 45.4 kw/m 2 at u w = m/s, and then decreased to 44.1 kw/m (-) q ab (kwm -2 ) q ab K K (Wm -2 K -1 ) u w (m/s) Fig. 4 Overall heat transfer coefficient and heat flux of steam generator under different water flow rates (T in = 346 o C, q v =2.0 m 3 /h) Fig. 5 presents the steam generation rate and energy efficiency of steam generator under different water flow rates, where T in = 346 o C, q v =2.0 m 3 /h. As the water flow rate increased, the steam generation rate first increased and then decreased, as the result of the variation of the overall heat transfer coefficient. As the water flow rate increased, the energy efficiency first increased because of the increase of the overall heat transfer coefficient, and then began to drop as the result of the increase of the heat loss and the decrease of the overall heat transfer coefficient. Because the heat loss of steam generator increased as water flow rate increased, the energy efficiency reached its maximum at lower water flow rate than that of steam generation rate. As the water flow rate increased from m/s to m/s, the steam generation rate first increased from 17.5kg/h to its maximum 35.7 kg/h at u w = m/s, and then 100 4

5 decreased to 34.8 kg/h, the energy efficiency first increased from 69.6% to its maximum 82.4% at u w = m/s, and then decreased to 70.0%. (kg/h) (-) u w (m/s) Fig. 5 Steam generation rate and energy efficiency of steam generator under different water flow rates (T in = 346 o C, q v =2.0 m 3 /h) 5. CONCLUSIONS In this paper, a U shape molten salt steam generator was set up, and its thermal performances were experimentally investigated and analyzed. Some conclusions are as follows. 1) As the inlet molten salt flow rate increases, the overall heat transfer coefficient of the steam generator increases, the release heat flow of molten salt and the heat flow absorbed by water/steam both increased. As the water flow rate increases, the overall heat transfer coefficient of the steam generator first increases and then decreases, and the variation of the heat flux is the same. 2) Steam generation rate increases almost linearly as the inlet molten salt flow rate increases, however, as the water flow rate increases, the steam generation rate first increases and then decreases. 3) As the molten salt flow rate and water flow rate increase, the thermal efficiency of the steam generator first increases and then decreases, approaching its maximums with optimal molten salt flow rate and water flow rate respectively. ACKNOWLEDGMENT This paper is supported by National Natural Science Foundation of China ( , ), National Key Technology Support Program (14BAA01B01), and the Fundamental Research Funds for the Central Universities. REFERENCE [1] Zavoico AB. Solar power tower design basis document. Sandia National Laboratories Report, SAND , 01 [2] Serp J, Allibert M, Beneš O, et al. The molten salt reactor (MSR) in generation IV: Overview and perspectives. Progress in Nuclear Energy, 14, 77: [3] Yuan Y, He C, Lu J, et al. Thermal performances of molten salt steam generator. Applied Thermal Engineering, 16, 105: [4] Vriesema B. Aspects of molten salt fluorides as heat transfer agents for power generation. Doctoral Thesis, 1979 [5] Bradshaw RW, Dawson DB, Wilfredo DLR, et al. Final Test and Evaluation Results from the Solar Two Project. Energy Storage, 01 [6] Xiao P, Guo L, Zhang X. Investigations on heat transfer characteristic of molten salt flow in helical annular duct. Applied Thermal Engineering, 15, 88: [7] Gaggioli W, Fabrizi F, Fontana F, Rinaldi L, Tarquini P. An innovative concept of a thermal energy storage system based on a single tank configuration using stratifying molten salts as both heat storage medium and heat transfer fluid, and with an integrated steam generator. Energy Procedia, 14, 49: [8] Lu JF, Shen XY, Ding J, Peng, Wen YL. Convective heat transfer of high temperature molten salt in transversely grooved tube. Applied Thermal Engineering, 13, 61: