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Thermal performance of myristic acid as a phase change material for energy storage application
Renewable Energy 24 (2001) 303–317 www.elsevier.nl/locate/renene
Technical note
Thermal performance of myristic acid as a phase change material for energy storage application Ahmet Sarı b
a,*
, Kamil Kaygusuz
b
a Department of Chemistry, Gaziosmanpas¸a University, 60100 Tokat, Turkey Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey
Received 5 May 2000; accepted 16 August 2000
Abstract Thermal performance and phase change stability of myristic acid as a latent heat energy storage material has been studied experimentally. In the experimental study, the thermal performance and heat transfer characteristics of the myristic acid were tested and compared with other studies given in the literature. In the present study is included some parameters such as transition times, temperature range, and propagation of the solid–liquid interface as well as heat flow rate effect on the phase change stability of myristic acid as a phase change material (PCM). The experimental results showed that the melting stability of the PCM is better in the radial direction than the axial direction. The variety of the melting and solidification parameters of the PCM with the change of inlet water temperature is also studied. The results show that the better stability of the myristic acid was accomplished at low inlet water temperature compared with the obtained results at high inlet water temperature. We also observed that while the heat exchanger tube is in the horizontal position, the PCM has more effective and steady phase change characteristics than in the vertical position. The heat storage capacity of the container (PCM tube) is not as good as we expected in this study and the average heat storage efficiency (or heat exchanger effectiveness) is 54%. It means that 46% of the heat acrually lost somewhere. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Myristic acid; Energy storage; Phase change material; Thermal performance
* Corresponding author. E-mail address: [email protected] (A. Sarı). 0960-1481/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 0 ) 0 0 1 6 7 - 1
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Nomenclature Cpw CpPCM mw mPCM Tiw Tow Ts,hsc Ts,is T2 T1 ⌬Hmelt Qloss Q QT l L r2 r1
Specific heat of water (kJ/kg °C) Specific heat of PCM (kJ/kg °C) Mass flow rate of water (kg/min) Weight of PCM (kg) Inlet water temperature (°C) Outlet water temperature (°C) Outside surface temperature of heat storage contaner (°C) Outside surface temperature of isolation material (°C) Final temperature of PCM (°C) Initial temperature of PCM (°C) Latent heat of melting of PCM (kJ/kg) Total heat loss from the complete system to the surrounding (J/m) Heat value of the PCM during charging or discharging period at any time Total heat value of the PCM during charging or discharging period Thermal conductivity (J/m °C) Length of the heat storage container (m) Outside radius of the heat storage container (m) Inside radius of the heat storage container (m)
1. Introduction Thermal energy storage has always been one of the most critical components in residential solar space heating and cooling applications. Solar radiation is a timedependent energy source with an intermittent character. The heating demands of a residential house are also time dependent. However, the energy source and the demands of a house, in general, do not match each other, especially in solar heating applications. The peak solar radiation occurs near noon, but the peak heating demand is in the late evening when solar radiation is not available. Thermal energy storage provides a reservoir of energy to adjust this mismatch and to meet the energy needs at all times. It is used as a bridge to cross the gap between the energy source, the sun, the application and the building. So, thermal energy storage is essential in the solar heating system [1–7]. Some fatty acids were investigated as PCM for thermal energy storage by Hasan [8,9]. He observed that stearic and palmitic acids are suitable materials for energy storage in solar heating and cooling applications. He also observed that the melting and solidification times are not affected by the flow rate of the heat transfer fluid in the tested laminar range. In this study, the thermal performance and phase change stability of the myristic
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acid as a PCM were investigated experimentally. The phase change temperatures and times of the myristic acid has been determined. The melting and solidification behavior of the PCM in axial, radial direction and in vertical and horizontal positions was also studied. In addition, the heat storage efficiency of the container (heat exchanger tube) was determined.
2. Experimental set-up The experimental set-up is shown in Fig. 1. It consists of a heat storage container, a high temperature bath (HTB), a low temperature bath (LTB), circulation pumps, piping systems, 14 Pt-100 type thermoresistance (PtRh-Pt) and multimeter to obtain measured temperature data. The heat storage container consists of two concentric cylindrical copper pipes. The inlet cylindrical tube has a 350 mm long and 50 mm diameter while the outlet cylindrical tube has a 200 mm long and 120 mm diameter. Thermoresistances made by platinium, rhodium-platinium were located at the positions of 5, 15, and 25 mm in radial direction and 20, 60, 100, and 140 mm in the longitudinal direction distance from the bottom of the storage container. All thermoresistances were tested before use and their sensibility ranged from ⫺50 to 300°C. In order to measure the temperature distribution in the radial direction, the thermoresistances were selected as 5, 15, and 25 mm long, respectively (Fig. 2). The PCM (950 g) is filled in the annulus of two concentric cylindrical tubes. The used PCM (myristic acid) has 98% purity, 204.5 kJ kg⫺1 fusion enthalpy and 51.5°C
Fig. 1. Schematic diagram of the apparatus. 1, Cold-water tank; 2, hot-water tank; 3, pump; 4, flowmeter; 5, heat exchanger; 6, temperature controller; 7, mixer; 8, data logger; 9, PCM sample; 10, heater; 丢, thermoresistance.
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Fig. 2.
Heat exchanger tube and thermoresisitance location. T.R., thermoresisitance.
m.p., obtained from Merck. These thermopysical properties of the PCM were tested by using DSC analyses and results are shown in Fig. 3. The heat storage container was isolated by 20 mm glass wool. To establish the initial conditions of the melting period, hot water at 75°C was passed through the heat exchanger tube at constant flow rate. After that, the melting period was started by circulating inlet hot water at upper melting temperatures (68– 74°C) of PCM with different flow rates (1.6–6.0 kg min⫺1). During this period, the temperature variations at distance in axial and radial directions were recorded in 5 min time intervals. To calculate the heat losses from PCM and heat exchanger tube to the environment, the temperatures of the heat storage container surface and outer surface of the glass wool were also measured. The solidification period was started directly after the completion of the melting period. For this process, after determining the initial conditions, the low water (40–46°C) at different flow rates (1.6–6.0 kg min⫺1) was circulated through the system and then the temperatures, mentioned above, were recorded in 5 min time intervals. The phase transition behavior of the myristic acid was determined separately in two positions of the PCM container, one is horizontal and the other is vertical. 3. Results and discussion 3.1. Phase transition temperature and times To determine the melting and solidification temperature and times, the measured PCM temperatures during these periods were plotted vs time as shown in Figs 4–7
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Fig. 3.
307
DSC analysis data for investigated PCM.
Fig. 4. Temperature distribution during melting of myristic acid in radial direction (axial distance=60 mm; inlet water temperature=71°C; water flow rate=3 kg/min.
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Fig. 5. Temperature distribution during melting of myristic acid in radial direction (axial distance=100 mm; inlet water temperature=71°C; water flow rate=3 kg/min.
Fig. 6. Temperature distribution during melting of myristic acid in radial direction (axial distance=60 mm; inlet water temperature=71°C; water flow rate=1.6 kg/min.
and Figs. 11 and 12. The melting time range of myristic acid in radial and axial directions is shown in Figs. 4–7. In this study, the temperature change was observed to be high at the initial periods of melting and solidification while the temperature change was low at the same periods. The melting and solidification times, which
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Fig. 7. Temperature distribution during melting of myristic acid in radial direction (axial distance=100 mm; inlet water temperature=71°C; water flow rate=1.6 kg/min.
Fig. 8. Melting behavior of myristic acid in different heat exchanger tube positions (inlet water temperature=71°C; water flow rate=1.6 kg/min).
they can be considered as a nearly constant temperature, were measured as a time of 80 and 50 min by an average, respectively. it was observed that the melting and solidification temperatures occurred at the same temperature range. This temperature range ranged from 49 to 51°C. This result was found to be agreed with given values by literature [10] and given physical properties by the Merck Company.
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Fig. 9. Melting behavior of myristic acid at same water flow rates and different water inlet temperatures (water flow rate=3 kg/min).
Fig. 10. The melting behavior of myristic acid at samew water inlet temperature and different water flow rates (inlet water temperature=71°C).
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Fig. 11. Temperature distribution during solidification of myristic acid in radial direction (axial distance=60 mm; inlet water temperature=44°C; water flow rate=2 kg/min).
Fig. 12. Temperature distribution during solidification of myristic acid in radial direction (axial distance=100 mm; inlet water temperature=44°C; water flow rate=2 kg/min).
3.2. Heat transfer behavior during the phase transition period The temperature variation in the PCM is taken place by two different heat transfer rates. One is absorbed sensible heat during melting and the other is heat transfer inside the PCM. So, total heat transfer occurring at any point inside the PCM is
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equal to the two heat transfers at this point. If there is no heat transfer at that point the temperature will increase continuously. For example, the temperature increasing after melting and solidification periods. As shown in Figs. 4–7 and Figs. 11 and 12, melting and solidification takes place from the upper point to the bottom in the axial direction through the container and the melting behavior in the radial direction took place from near the point of the heat transfer fluid to far point of it. On the other hand, the solidification behavior was observed at the opposite direction in the radial position of the container. For this reason why the heat transfer rate at far point from heat transfer fluid (HTF) is higher than that near point of HTF (water) is shown in Figs. 11 and 12. The effective factor for determining the melting and solidification direction is free convection heat transfer increasing at upper space on the myristic acid. Due to the convection heat transfer, at the near points of HTF, the melting is fast or melting time is small. The reason for the longer melting time than the solidification time is free convection which is more effective than that in the solidification process. Namely, the conduction heat transfer during solidification is relatively effective than the convection heat transfer in the PCM. 3.3. Inlet temperature of the heat transfer fluid The effect of the inlet water temperature on both the melting and solidification periods is shown in Figs. 9 and 15. It was observed that the total melting time decreased by 25% by increasing the inlet water temperature from 68 to 74°C. Also observed was that the total solidification time was decreased by 30% as an average by decreasing of inlet water temperature from 46 to 40°C. The main reason for
Fig. 13. The solidification behavior of myristic acid at different heat exchanger tube positions (inlet water temperature=44°C; water flow rate=2 kg/min).
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Fig. 14. The solidification behavior of myristic acid at same water inlet temperature and different water flow rates (water inlet temperature=44°C).
Fig. 15. The solidification behavior of myristic acid at same water flow rate and different water inlet temperatures (water flow rate=2 kg/min).
these two situations is the high temperature difference between PCM and inlet water temperature and therefore the conduction heat transfer is high.
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3.4. Flow rate of the heat transfer fluid The effect of this characteristic factor on the melting and solidification periods is shown in Figs. 10 and 14, respectively. We show that the melting time is decreased 15% by increasing the flow rate as approximately six times, while the solidification time is decreased 35%. 3.5. Position of the heat exchanger tube The position effect of the heat exchanger tube filled by the PCM on the melting process is shown in Fig. 8. On the other hand, the position effect of the tube on the solidification process of myristic acid is given in Fig. 13. According to this data, we obtained that the heat transfer is increased and thus the finishing time of the melting and solidification period is decreased at the horizontal position of the heat exchanger tube. We believed that the large air space on the PCM in the heat exchanger tube which located at the horizontal position is more useful for getting an extra conduction heat transfer than small air space at the vertical position of the tube filled by the PCM. This idea is completely derived from the experimental results. 3.6. Heat flow rate and heat fraction Sensible heat transfer rate, heat storage rate of the PCM, total heat losses rate and heat fractions during the melting and solidification period for myristic acid were calculated by using the following equations [11] and obtained results are given in Figs. 16–18, respectively:
Fig. 16. Heat flow rate vs time during melting of myristic acid (water inlet temperature=71°C; water flow rate=1.6 kg/min).
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Fig. 17. Heat flow rate vs time during solidification of myristic acid (inlet water temperature=44°C; water flow rate=1.6 kg/min).
Fig. 18. Heat flow rate vs time during melting and solidification of myristic acid (water inlet temperature=71°C/44°C; water flow rate=1.6 kg/min).
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mwCpw(Tiw⫺Tow)⫽mPCMCpPCM(T2⫺T1)⫹mPCM⌬Hmelt⫹Qloss Qloss⫽
2plL(Ts,hsc−Ts,is) ln(r2/r1)
Heat fraction⫽Q/QT.
(1) (2) (3)
According to the calculations, we found that the heat losses at the melting period is low while losses at the solidification period is high. We also calculated that the heat storage efficiency (heat exchanger effectiveness) of the PCM tube is 54%. Due to the insufficient isolation of the heat exchanger tube and the construction material, the tube has high heat transfer conduction, this efficiency is found as low and undesired value. 4. Conclusions In the case that the latent heat storage material of myristic acid is put into a vertical cylindrical heat storage container and a vertical single pipe is inserted into the container as the heat exchanger, the heat transfer rates were measured when the heat storage material is melted and solidified. The following conclusions are obtained: 1. Heat transfer from the heat exchanger (heat transfer pipe) to the myristic acid is largely influenced by natural convection at the melting layer section in addition to forced conduction and convection heat transfer. 2. In the horizontal position, the melting and solidification behaviors occurred at the steady state than in the vertical position. 3. The rate of heat transfer and, consequently, the charging and discharging times can be altered by changing the water inlet temperature to the heat exchanger tube. 4. Flow rate and temperature changes of heat transfer fluid are more affected on solidification behavior than the melting behavior of the PCM. 5. A heat exchanger and a storage system which consists of a vertical single pipe as a heat exchanger inserted into another vertical pipe as a heat storage container can be used for energy storage with reasonable charging and discharging times and heat release rate. 6. As a result the myristic acid is a good PCM for energy storage for domestic solar water heating. It has a suitable melting point of 49–51°C, 98% purity and a relatively high latent heat of 204.5 kJ/kg. In addition, it does not exhibit any subcooling.
Acknowledgements This study was supported by the Gaziosmanpas¸ a University Research Fund and the Karadeniz Technical University Research Fund.
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References [1] Hahne E. Thermal conservation technologies. In: Proceedings of the First Trabzon International Energy and Environment Symposium. Turkey: Trabzon, 1996:293–314. [2] Ismail KAR, Gonc¸ alves MM. Thermal performance of a pcm storage unit. Energy Conversion and Management 1998;39:809–18. [3] Kaygusuz K. The viability of thermal energy storage. Energy Sources 1999;21:745–56. [4] Sarı A, Kaygusuz K. Energy and exergy calculations of the latent heat energy storage systems. Energy Sources 2000;22:117–26. [5] Sarı A, Kaygusuz K. Thermal energy storage system using some fatty acids as latent heat energy storage materials. Accepted for publication by Energy Sources. [6] Sarı A. Investigation of usability of some fatty acids and eutectic mixtures as energy storage materials. Ph.D. thesis, Gaziosmanpas¸a University, Tokat, Turkey, 2000. [7] Kaygusuz K. Experimental and theoretical investigation of latent heat storage for water based solar heating systems. Energy Conversion and Management 1995;36:315–23. [8] Hasan A. Thermal energy storage system with stearic acid as phase change material. Energy Conversion and Management, 1994;35:843–56. [9] Hasan A. Phase change material energy storage system employing palmitic acid. Solar Energy 1994;52:143–54. [10] Hasan A, Sayigh AA. Some fatty acids as phase-change thermal energy storage materials. Renewable Energy 1994;4:69–76. [11] Duffie JA, Beckman WA. Solar engineering of thermal processes. 2nd ed. New York: Wiley, 1991.
Technical note
Thermal performance of myristic acid as a phase change material for energy storage application Ahmet Sarı b
a,*
, Kamil Kaygusuz
b
a Department of Chemistry, Gaziosmanpas¸a University, 60100 Tokat, Turkey Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey
Received 5 May 2000; accepted 16 August 2000
Abstract Thermal performance and phase change stability of myristic acid as a latent heat energy storage material has been studied experimentally. In the experimental study, the thermal performance and heat transfer characteristics of the myristic acid were tested and compared with other studies given in the literature. In the present study is included some parameters such as transition times, temperature range, and propagation of the solid–liquid interface as well as heat flow rate effect on the phase change stability of myristic acid as a phase change material (PCM). The experimental results showed that the melting stability of the PCM is better in the radial direction than the axial direction. The variety of the melting and solidification parameters of the PCM with the change of inlet water temperature is also studied. The results show that the better stability of the myristic acid was accomplished at low inlet water temperature compared with the obtained results at high inlet water temperature. We also observed that while the heat exchanger tube is in the horizontal position, the PCM has more effective and steady phase change characteristics than in the vertical position. The heat storage capacity of the container (PCM tube) is not as good as we expected in this study and the average heat storage efficiency (or heat exchanger effectiveness) is 54%. It means that 46% of the heat acrually lost somewhere. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Myristic acid; Energy storage; Phase change material; Thermal performance
* Corresponding author. E-mail address: [email protected] (A. Sarı). 0960-1481/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 0 ) 0 0 1 6 7 - 1
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Nomenclature Cpw CpPCM mw mPCM Tiw Tow Ts,hsc Ts,is T2 T1 ⌬Hmelt Qloss Q QT l L r2 r1
Specific heat of water (kJ/kg °C) Specific heat of PCM (kJ/kg °C) Mass flow rate of water (kg/min) Weight of PCM (kg) Inlet water temperature (°C) Outlet water temperature (°C) Outside surface temperature of heat storage contaner (°C) Outside surface temperature of isolation material (°C) Final temperature of PCM (°C) Initial temperature of PCM (°C) Latent heat of melting of PCM (kJ/kg) Total heat loss from the complete system to the surrounding (J/m) Heat value of the PCM during charging or discharging period at any time Total heat value of the PCM during charging or discharging period Thermal conductivity (J/m °C) Length of the heat storage container (m) Outside radius of the heat storage container (m) Inside radius of the heat storage container (m)
1. Introduction Thermal energy storage has always been one of the most critical components in residential solar space heating and cooling applications. Solar radiation is a timedependent energy source with an intermittent character. The heating demands of a residential house are also time dependent. However, the energy source and the demands of a house, in general, do not match each other, especially in solar heating applications. The peak solar radiation occurs near noon, but the peak heating demand is in the late evening when solar radiation is not available. Thermal energy storage provides a reservoir of energy to adjust this mismatch and to meet the energy needs at all times. It is used as a bridge to cross the gap between the energy source, the sun, the application and the building. So, thermal energy storage is essential in the solar heating system [1–7]. Some fatty acids were investigated as PCM for thermal energy storage by Hasan [8,9]. He observed that stearic and palmitic acids are suitable materials for energy storage in solar heating and cooling applications. He also observed that the melting and solidification times are not affected by the flow rate of the heat transfer fluid in the tested laminar range. In this study, the thermal performance and phase change stability of the myristic
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305
acid as a PCM were investigated experimentally. The phase change temperatures and times of the myristic acid has been determined. The melting and solidification behavior of the PCM in axial, radial direction and in vertical and horizontal positions was also studied. In addition, the heat storage efficiency of the container (heat exchanger tube) was determined.
2. Experimental set-up The experimental set-up is shown in Fig. 1. It consists of a heat storage container, a high temperature bath (HTB), a low temperature bath (LTB), circulation pumps, piping systems, 14 Pt-100 type thermoresistance (PtRh-Pt) and multimeter to obtain measured temperature data. The heat storage container consists of two concentric cylindrical copper pipes. The inlet cylindrical tube has a 350 mm long and 50 mm diameter while the outlet cylindrical tube has a 200 mm long and 120 mm diameter. Thermoresistances made by platinium, rhodium-platinium were located at the positions of 5, 15, and 25 mm in radial direction and 20, 60, 100, and 140 mm in the longitudinal direction distance from the bottom of the storage container. All thermoresistances were tested before use and their sensibility ranged from ⫺50 to 300°C. In order to measure the temperature distribution in the radial direction, the thermoresistances were selected as 5, 15, and 25 mm long, respectively (Fig. 2). The PCM (950 g) is filled in the annulus of two concentric cylindrical tubes. The used PCM (myristic acid) has 98% purity, 204.5 kJ kg⫺1 fusion enthalpy and 51.5°C
Fig. 1. Schematic diagram of the apparatus. 1, Cold-water tank; 2, hot-water tank; 3, pump; 4, flowmeter; 5, heat exchanger; 6, temperature controller; 7, mixer; 8, data logger; 9, PCM sample; 10, heater; 丢, thermoresistance.
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Fig. 2.
Heat exchanger tube and thermoresisitance location. T.R., thermoresisitance.
m.p., obtained from Merck. These thermopysical properties of the PCM were tested by using DSC analyses and results are shown in Fig. 3. The heat storage container was isolated by 20 mm glass wool. To establish the initial conditions of the melting period, hot water at 75°C was passed through the heat exchanger tube at constant flow rate. After that, the melting period was started by circulating inlet hot water at upper melting temperatures (68– 74°C) of PCM with different flow rates (1.6–6.0 kg min⫺1). During this period, the temperature variations at distance in axial and radial directions were recorded in 5 min time intervals. To calculate the heat losses from PCM and heat exchanger tube to the environment, the temperatures of the heat storage container surface and outer surface of the glass wool were also measured. The solidification period was started directly after the completion of the melting period. For this process, after determining the initial conditions, the low water (40–46°C) at different flow rates (1.6–6.0 kg min⫺1) was circulated through the system and then the temperatures, mentioned above, were recorded in 5 min time intervals. The phase transition behavior of the myristic acid was determined separately in two positions of the PCM container, one is horizontal and the other is vertical. 3. Results and discussion 3.1. Phase transition temperature and times To determine the melting and solidification temperature and times, the measured PCM temperatures during these periods were plotted vs time as shown in Figs 4–7
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Fig. 3.
307
DSC analysis data for investigated PCM.
Fig. 4. Temperature distribution during melting of myristic acid in radial direction (axial distance=60 mm; inlet water temperature=71°C; water flow rate=3 kg/min.
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Fig. 5. Temperature distribution during melting of myristic acid in radial direction (axial distance=100 mm; inlet water temperature=71°C; water flow rate=3 kg/min.
Fig. 6. Temperature distribution during melting of myristic acid in radial direction (axial distance=60 mm; inlet water temperature=71°C; water flow rate=1.6 kg/min.
and Figs. 11 and 12. The melting time range of myristic acid in radial and axial directions is shown in Figs. 4–7. In this study, the temperature change was observed to be high at the initial periods of melting and solidification while the temperature change was low at the same periods. The melting and solidification times, which
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Fig. 7. Temperature distribution during melting of myristic acid in radial direction (axial distance=100 mm; inlet water temperature=71°C; water flow rate=1.6 kg/min.
Fig. 8. Melting behavior of myristic acid in different heat exchanger tube positions (inlet water temperature=71°C; water flow rate=1.6 kg/min).
they can be considered as a nearly constant temperature, were measured as a time of 80 and 50 min by an average, respectively. it was observed that the melting and solidification temperatures occurred at the same temperature range. This temperature range ranged from 49 to 51°C. This result was found to be agreed with given values by literature [10] and given physical properties by the Merck Company.
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Fig. 9. Melting behavior of myristic acid at same water flow rates and different water inlet temperatures (water flow rate=3 kg/min).
Fig. 10. The melting behavior of myristic acid at samew water inlet temperature and different water flow rates (inlet water temperature=71°C).
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Fig. 11. Temperature distribution during solidification of myristic acid in radial direction (axial distance=60 mm; inlet water temperature=44°C; water flow rate=2 kg/min).
Fig. 12. Temperature distribution during solidification of myristic acid in radial direction (axial distance=100 mm; inlet water temperature=44°C; water flow rate=2 kg/min).
3.2. Heat transfer behavior during the phase transition period The temperature variation in the PCM is taken place by two different heat transfer rates. One is absorbed sensible heat during melting and the other is heat transfer inside the PCM. So, total heat transfer occurring at any point inside the PCM is
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equal to the two heat transfers at this point. If there is no heat transfer at that point the temperature will increase continuously. For example, the temperature increasing after melting and solidification periods. As shown in Figs. 4–7 and Figs. 11 and 12, melting and solidification takes place from the upper point to the bottom in the axial direction through the container and the melting behavior in the radial direction took place from near the point of the heat transfer fluid to far point of it. On the other hand, the solidification behavior was observed at the opposite direction in the radial position of the container. For this reason why the heat transfer rate at far point from heat transfer fluid (HTF) is higher than that near point of HTF (water) is shown in Figs. 11 and 12. The effective factor for determining the melting and solidification direction is free convection heat transfer increasing at upper space on the myristic acid. Due to the convection heat transfer, at the near points of HTF, the melting is fast or melting time is small. The reason for the longer melting time than the solidification time is free convection which is more effective than that in the solidification process. Namely, the conduction heat transfer during solidification is relatively effective than the convection heat transfer in the PCM. 3.3. Inlet temperature of the heat transfer fluid The effect of the inlet water temperature on both the melting and solidification periods is shown in Figs. 9 and 15. It was observed that the total melting time decreased by 25% by increasing the inlet water temperature from 68 to 74°C. Also observed was that the total solidification time was decreased by 30% as an average by decreasing of inlet water temperature from 46 to 40°C. The main reason for
Fig. 13. The solidification behavior of myristic acid at different heat exchanger tube positions (inlet water temperature=44°C; water flow rate=2 kg/min).
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Fig. 14. The solidification behavior of myristic acid at same water inlet temperature and different water flow rates (water inlet temperature=44°C).
Fig. 15. The solidification behavior of myristic acid at same water flow rate and different water inlet temperatures (water flow rate=2 kg/min).
these two situations is the high temperature difference between PCM and inlet water temperature and therefore the conduction heat transfer is high.
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3.4. Flow rate of the heat transfer fluid The effect of this characteristic factor on the melting and solidification periods is shown in Figs. 10 and 14, respectively. We show that the melting time is decreased 15% by increasing the flow rate as approximately six times, while the solidification time is decreased 35%. 3.5. Position of the heat exchanger tube The position effect of the heat exchanger tube filled by the PCM on the melting process is shown in Fig. 8. On the other hand, the position effect of the tube on the solidification process of myristic acid is given in Fig. 13. According to this data, we obtained that the heat transfer is increased and thus the finishing time of the melting and solidification period is decreased at the horizontal position of the heat exchanger tube. We believed that the large air space on the PCM in the heat exchanger tube which located at the horizontal position is more useful for getting an extra conduction heat transfer than small air space at the vertical position of the tube filled by the PCM. This idea is completely derived from the experimental results. 3.6. Heat flow rate and heat fraction Sensible heat transfer rate, heat storage rate of the PCM, total heat losses rate and heat fractions during the melting and solidification period for myristic acid were calculated by using the following equations [11] and obtained results are given in Figs. 16–18, respectively:
Fig. 16. Heat flow rate vs time during melting of myristic acid (water inlet temperature=71°C; water flow rate=1.6 kg/min).
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Fig. 17. Heat flow rate vs time during solidification of myristic acid (inlet water temperature=44°C; water flow rate=1.6 kg/min).
Fig. 18. Heat flow rate vs time during melting and solidification of myristic acid (water inlet temperature=71°C/44°C; water flow rate=1.6 kg/min).
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mwCpw(Tiw⫺Tow)⫽mPCMCpPCM(T2⫺T1)⫹mPCM⌬Hmelt⫹Qloss Qloss⫽
2plL(Ts,hsc−Ts,is) ln(r2/r1)
Heat fraction⫽Q/QT.
(1) (2) (3)
According to the calculations, we found that the heat losses at the melting period is low while losses at the solidification period is high. We also calculated that the heat storage efficiency (heat exchanger effectiveness) of the PCM tube is 54%. Due to the insufficient isolation of the heat exchanger tube and the construction material, the tube has high heat transfer conduction, this efficiency is found as low and undesired value. 4. Conclusions In the case that the latent heat storage material of myristic acid is put into a vertical cylindrical heat storage container and a vertical single pipe is inserted into the container as the heat exchanger, the heat transfer rates were measured when the heat storage material is melted and solidified. The following conclusions are obtained: 1. Heat transfer from the heat exchanger (heat transfer pipe) to the myristic acid is largely influenced by natural convection at the melting layer section in addition to forced conduction and convection heat transfer. 2. In the horizontal position, the melting and solidification behaviors occurred at the steady state than in the vertical position. 3. The rate of heat transfer and, consequently, the charging and discharging times can be altered by changing the water inlet temperature to the heat exchanger tube. 4. Flow rate and temperature changes of heat transfer fluid are more affected on solidification behavior than the melting behavior of the PCM. 5. A heat exchanger and a storage system which consists of a vertical single pipe as a heat exchanger inserted into another vertical pipe as a heat storage container can be used for energy storage with reasonable charging and discharging times and heat release rate. 6. As a result the myristic acid is a good PCM for energy storage for domestic solar water heating. It has a suitable melting point of 49–51°C, 98% purity and a relatively high latent heat of 204.5 kJ/kg. In addition, it does not exhibit any subcooling.
Acknowledgements This study was supported by the Gaziosmanpas¸ a University Research Fund and the Karadeniz Technical University Research Fund.
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