FLOW BEHAVIOR OF ERYTHRITOL SLURRY AS LATENT HEAT STORAGE MATERIAL FOR LOW LEVEL HEAT UTILIZATION

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1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00713 FLOW BEHAVIOR OF ERYTHRITOL SLURRY AS LATENT HEAT STORAGE MATERIAL FOR LOW LEVEL HEAT UTILIZATION Shunsuke ABE 1 Tatsunori ASAOKA 1* Hiroshi MIZUMOTO 1 1 Shinshu University, Wakasato , Nagano-shi, Nagano , Japan Presenting Author: 16w4003g@shinshu-u.ac.jp * Corresponding Author: asaoka@shinshu-u.ac.jp ABSTRACT Recently high efficiency heat transport medium is required for high temperature applications, more than 100 o C, such as utilization of solar heat or unused industrial wasted heat. In this study, erythritol slurry, which is the mixture of erythritol solution and its fine crystals, is proposed as a candidate for the heat transfer medium and heat storage material. In the experiments, flow and heat storage characteristics of erythritol slurry is evaluated. As flow characteristics, it was found that erythritol slurry has good fluidity within the adequate solid fraction range. It is also found that apparent viscosity is significantly higher than that of solution. Additionally, since erythritol slurry contains larger crystals when initial concentration is higher, it may causes blockage of pipe. As a heat storage characteristics, it was found that apparent specific heat of erythritol slurry is significantly higher than that of erythritol solution or water. As a result, it was concluded that erythritol slurry is suitable as a heat storage material and heat transfer medium. KEYWORDS: Slurry, Solid-liquid phase change, Pressure loss, Specific heat, Solar heat, Thermal storage 1. INTRODUCTION Recently high efficiency heat transport medium is required for high temperature applications, more than 100 o C, such as utilization of solar heat or unused industrial wasted heat. Additionally, heat storage/thermal storage is useful for the application, in which the supplied heat from the heat source varies significantly. Erythritol is suitable as the heat storage material for high temperature application, since its melting temperature is around 100 o C and it has large latent heat. [1] Conventionally, erythritol is used without pipe transportation [2]. Using erythritol with pipe transportation as a heat transport medium is useful since it expand the use application. In this study, erythritol slurry, which is the mixture of erythritol solution and its fine crystals, is proposed as a candidate for the heat transfer medium and heat storage material. I was known that slurry; ice slurry for example, is hopeful as a heat transfer medium. [4] In this work, flow and heat storage characteristics of erythritol slurry is investigated. 2.1 PIPE FRICTION MEASUREMENT 2. EXPERIMENT METHOD Pipe friction coefficient was measured to evaluate the flow characteristics of erythritol slurry. Figure 1 shows the experimental apparatus. The apparatus consists of 2 containers, PFA tube and scale. Resolution of the scale is 1g. Cylindrical container of 30mm diameter and 140 depth is used as an upper container, in which sample is stored. PFA tube of 4.35mm diameter and 90mm length is connected to the bottom of the upper container. The lower container is placed 50mm below outlet of the PFA tube to receive the sample. The PFA tube is insulated to keep the temperature of the flowing sample. 1

2 Fig. 1 Experimental apparatus for pipe friction measurement Fig. 2 Experirimental apparatus for observation of sedimentation of erythritol slurry Erythritol solution and erythritol slurry are used as a sample. Erythritol concentration of the solution is 50~80wt%. Erythritol slurry is produced by cooling the solution. The erythritol concentration before crystallization is defined to initial concentration. Beforehand the sample is placed in the upper container, the time spent for 300~500g of sample flowing through the PFA tube is measured. Pipe friction coefficient is calculated from the time and mass variation of the sample. 2.2 OBSERVATION OF SEDIMENTATION OF ERYTHRITOL SLURRY To observe sedimentation of erythritol slurry, following experiment was performed. Figure 2 shows the experimental apparatus. Erythrital is stored in beaker and cooled with stirring. After that, solid fraction increases by the cooling. The sample temperature was measured when sedimentation of erythritol slurry at the bottom of the beaker was observed by visual contact. Platinum resistance temperature detector was used for temperature measurement. Solid fraction is calculated from the measured temperature. First, stirring speed is 200rpm. The stirring speed increases to 500rpm in step of 100rpm. Beaker of 78mm outside diameter, 103mm height and 300ml content manufactured by AGC TECHNO GLASS is used. Erythritol concentration and mass of the solution are 50~80wt% and 300g respectively. Moreover, crystal included in the slurry was extracted and observed. 2.3 SPECIFIC HEAT MEASUREMENT Specific heat is measured to evaluate the heat storage characteristics of erythritol slurry. Figure 3 shows the experimental apparatus. The apparatus consists of Dewar vessel (85mm diameter and 206 depth), stirrer, heater and platinum resistance temperature detector. Erythritol solution and slurry are used as a sample (initial concentration is 70, 75 and 80wt%). The sample which is stored in Dewar vessel is heated by the heater with stirring and the temperature variation of the sample is measured. Specific heat is calculated from the temperature variation and calorific power of the heater. In each experiment, temperature variation is 3.9~4.7. Specific heat of erythritol slurry cannot define strictly like a single-phase material because phase change occurs along with the temperature variation. In this work, apparent specific heat is used to evaluate the characteristics of erythritol slurry, which is defined as heat quantity per unit mass required to rise unit temperature of the slurry. 2

3 Fig. 3 Experirimental apparatus for specific heat measurement Fig. 4 Pipe friction coefficient of erythritol solution/slurry 3.1 PIPE FRICTION COEFFICIENT 3. RESULTS AND DISCUSSION Figure 4 shows the relationship between temperature and pipe fiction coefficient of 50, 70 and 80wt% erythritol solution and slurry. The relationship between erythritol concentration x and temperature T in the solid-liquid equilibrium condition can be expressed by Eq.(1). 2 x T 1.34 T 5.94 (1) Solid fraction P can be calculated by x, initial concentration x 0 using Eq.(2). x0 x P 100 (2) 100 x Solid fraction of 1wt% was presented by dashed line in Fig.4. Solid line represents the liner approximation of pipe friction coefficient of the solution/slurry. It was found that pipe friction coefficient increases as the temperature decreases. Especially, pipe friction coefficient of slurry increases as the solid fraction increases and it significantly higher than that of solution. It means that apparent viscosity of erythritol slurry is significantly higher than that of solution. Additionally, it was found that pipe friction coefficient increases as initial concentration increases. 3.2 SEDIMENTATION OF ERYTHRITOL SLURRY As a result of the experiment, it was found that sedimentation occur with lower stirring speed and lower solid fraction when initial concentration is higher. We expected sedimentation was inhibited in the solution of higher erythritol concentration, because the density of the solution increases as concentration increases, and the difference in density of solid-liquid phase decreases. However, the opposite trend was presented. This should be because that the particle size of erythritol crystal strongly affects the sedimentation. Figure 5 shows the photographs of the erythritol crystals, which extracted from erythritol slurry (initial concentration is 70 and 80wt%) in order of size. It was found that particle size increases as initial concentration increases. As a result, it was concluded that 3

4 Fig. 5 Differences in particle size of the erythritol crystals between initial concentration of 70wt% and 80wt%. Fig. 6 Specific heat of erythritol solution and apparent specific heat of erythritol slurry. Plots and solid lines represent experimental and theoretical values respectively. sedimentation of erythritol slurry is facilitated under the high initial concentration condition because particle size of the crystals increases. 3.3 SPECIFIC HEAT AND APPARENT SPECIFIC HEAT Figure 6 shows relationship between temperature and specific heat/apparent specific heat of erythritol solution/slurry. Plots and solid line present experimental values and theoretical values respectively. Experimental values are average of 10 times of experiments. Standard variation was presented by error bar. Theoretical values are calculated by latent heat of fusion of erythritol using Eq.(3). dp c c sol L (3) dt It was confirmed that Eq.(3) is appropriate because experimental values roughly agree with theoretical values. It is found that specific heat of erythritol solution decreases as erythritol concentration increases. It was also found that apparent specific heat of erythritol slurry is relatively higher than that of solution or water. The result suggests that erythritol slurry is suitable as a heat storage material and heat transfer medium. 4. CONCLUSION In this study, flow and heat storage characteristics of erythritol slurry is evaluated. As flow characteristics, it was found that erythritol slurry has good fluidity within the adequate solid fraction range. It is also found that apparent viscosity is significantly higher than that of solution. Additionally, since erythritol slurry contains larger crystal when initial concentration is high, it may causes blockage of pipe. As a heat storage characteristics, it is found that apparent specific heat of erythritol slurry is significantly higher than that of erythritol solution or water. As a result, it was concluded that erythritol slurry is suitable as a heat storage material and heat transfer medium. 4

5 ACKNOWLEDGMENT This work was supported by Research Foundation for the Electrotechnology of Chubu ( ). NOMENCLATURE c apparent specific heat (kj/kg k) c sol specific heat (kj/kg K) L latent heat of fusion (kj/kg) P solid fraction (wt%) T temperature ( ) x concentration (wt%) x 0 initial concentration (wt%) REFERENCE [1] H. Hidaka, M. Yamazaki,M. Yabe,H. Kaikuchi,P. Erwin,Y. Kojima, H. Matuda, Evaluation of Fundamental Characteristics of Threitol for Latent Heat Storage for Hot Water Supply, Kagaku Kougaku Ronbunshu, 30(4) (2004) (in Japanese) [2] (Sep., 2016). [3] A. Horibe, H. Jang, N. Haruki, K. Takahashi, Direct Contact Latent Heat Storage System Using Erythritol as a Latent Heat Storage Material, Thermal Science and Engineering, 21(4)(2013) (in Japanese) [4] H. Kumano, Y. Yamanada, Y. Makino, T. Asaoka, Effect of initial aqueous solution concentration on rheological behavior of ice slurry, International Journal of Refrigeration, 68(2016)