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Performance of a Coil-pipe Heat Exchanger Filled with Mannitol for Solar Water Heating System
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 75 (2015) 827 – 833
The 7th International Conference on Applied Energy – ICAE2015
Performance of a coil-pipe heat exchanger filled with mannitol for solar water heating system Ziye Linga, Guohao Zenga, Tao Xua, Xiaoming Fanga, Zhengguo Zhanga* a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation, the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China
Abstract Solar water heating systems that incorporate latent heat storage unit deals with instabilities of insolation. But most previous studies focus on phase change materials (PCMs) with phase change temperature (Tm) below 80 oC. In this study, we investigate the performance of mannitol (Tm: 166.7, enthalpy of phase change γ: 323 kJ kg -1) in storing solar thermal energy and producing hot water. Results show mannitol can store high-level energy and the thermal energy storage performance are affected by the inlet flow rate and temperature of heat transfer fluid. 14kg mannitol with latent heat activated can heat 100L water from 30 up to 50 oC in 6 hours © 2015 2015The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This Selection and/or peer-review under responsibility of ICAE (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute Keywords: solar water heater; solar thermal energy; phase change material; latent heat storage; mannitol;
1. Introduction Solar water heating systems use free heat from the sun to heat water. But the intermittent solar radiation is an unreliable source of energy. Thermal energy storage (TES) system - that stores heat during high solar intensity and releases the heat energy when needed - solves the mismatch between the supply and demand of energy. Latent heat storage system utilizes the phase change enthalpy (γ) of phase change materials (PCMs). It stands out among TES systems because its wide operating temperature range, more efficient use of solar energy, large storage capacity, and long-term stability. Previous studies on PCMs focused on materials with low phase change temperatures (Tm) below 80 [1-4], which could not heat water quickly. Recent studies have found PCMs like erythritol, Dmannitol, hydroquinone, or other sugar alcohols possess large γ over 200 kJ/kg and high Tm (100~200 ) - they have huge capacity store the energy at high level [5-9]. We believe in applying PCMs with Tm over 100 will turn on faster access to hot water.
* Corresponding author. Tel.: +86-20-87112845; fax: +86-20-87113870. E-mail address: [email protected].
1876-6102 © 2015 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/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.08.001
828
xxxx / Energy Procedia 75 (2015) 827 – 833
In this study, we evaluate the performance of a solar water heating system- incorporating with a coil pipe heat exchanger that contains mannitol (Tm: 167 ć, ¤: 323kJ/kg)-in providing 100 L hot water up to 55 ć 2. Experiment Table I Thermo-physical properties of mannitol
Liquid 1340 Solid 1512 0.6 Thermal conductivity/W m-1K-1 166.7 Tm /oC -1 323 Ȗ/kJ kg Specific heat/J kg-1 oC-1 1500 Figure 1 (a) shows how this solar water heating system works. The system is mainly divided into three parts: an oil bath, a water tank and a thermal energy storage system. In charging, thermal oil is pumped through one of two coil pipes and heat is transferred to PCM. Oil is heated in the oil bath to ensure a constant inlet temperature. Charging continued until temperature inside the PCM container reached the equilibrium. Then turn off the high temperature pump for oil and turn on low temperature pump for water. Water is circulated to acquire heat from PCMs until its temperature reached 55 oC. A buffer tank is placed between the TES unit and water tank to prevent backflow. Charging performances at different inlet temperature of thermal oil are compared. Detailed parameters for each case are listed in Table II. The inlet temperature and flow rate of water are fixed to 30 o C and 60 L/h. Density/kg m-3
Fig.1 Schematic diagram of experimental system 1 water tank 2 thermal energy storage unit 3 oil bath 4 buffer tank 5 Agilent data acquisition unit 6 circulating pump for oil 7 circulating pump for water 8 flowmeter for water 9 flowmeter for oil 10 valve 11 thermo-couples 12 coiled pipes
xxxx / Energy Procedia 75 (2015) 827 – 833
Table II Operating parameters of different cases
Case
Flow rate of thermal oil
Inlet temperature of thermal oil
L/h
°C
Fig.2 Physical structure of thermal energy storage unit Figure 2 demonstrates the internal structure of TES unit. A ijîPPF\OLQGHUVWDLQOHVVFRQWDLQHU is filled with 14kg mannitol. TZRij îPPWXEHVVZLUOHGLQ a diameter of 140mm, with a pitch 50mm. Thermal oil flows in the red tube and water flows in the blue one. Twelve K-type thermo-couples (measurement error: ±0.5oC) were used to monitor the temperature evolution in the thermal energy storage tank. Fig.3 illustrates positions of thermo-couples. Thermocouples positioned at top, bottom surface and mid-plane (Fig. 3 b); In each horizontal section, four thermo-couples located at O, A, B, C in Fig.3 a ; Another three thermo-couples were used to monitor the temperature of water (15#) in the buffer tank, and the temperature of hot fluid at the inlet (13#) and outlet (14#).
Fig.3 Schematic of positions of thermo-couples
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xxxx / Energy Procedia 75 (2015) 827 – 833
3. Results and Discussions
Fig. 4 Temperature history at different positions at mid-plane in Case 2(inlet temperature of thermal oil: 210 oC and its flow rate: 360 L/h) Figure 4 describes the temperature evolution with time at mid-plane in Case 2. During charging, the constant heat flux from thermal oil melted mannitol in 5 hours. During discharging, the melted mannitol heated 100L water from 30 oC to 55 oC within 6 hours. Compared with the work done by [5], where 8kg water was heated by 45 kg erythritol (Tm: 118 oC, Ȗ:339 kJ kg-1) in 5 hours, the heating efficiency is much better. In the charging process, temperature fluctuated around the Tm. Flow of melted mannitol may explain: mannitol melted from the area near the coil pipe; the melted mannitol flowed around due to density difference between solid and liquid; if the liquid at high temperature flowed away and the solid at low temperature took the place, temperature fluctuated. But the flow was suppressed as mannitol solidified radially from inside out; temperature dropped smoothly in discharging. We observed a typical platform around 150 oC – 17 oC below the Tm, showing mannitol subcooled. 220 200 180
140 120 100
䉝 㻕
Temperature (
160
80 60
330L/h 360L/h 390L/h
40 20 0
100
200
300
400
500
600
700
800
Time (min)
Figure 5 Temperature elevation rates at 11# under different inlet flow rates of thermal oil (Inlet temperature: 210oC) Figure 5 shows the effect of flow rate of thermal oil on charging. Increasing flow rates of thermal oil decreased the time to reach equilibrium state very little, but increased the temperature at equilibrium state. Faster speed reduced the heat transfer resistance between thermal oil and mannitol.
xxxx / Energy Procedia 75 (2015) 827 – 833
Fig. 6 Temperature elevation rates at 11# under different inlet temperature of thermal oil (Flow rate: 330L/h)
Fig.7 Time to heat water up to 55 oC with different inlet temperature of thermal oil (flow rate: 330L/h) Temperature of mannitol rises faster if we increase the temperature of thermal oil (Fig. 6). Bigger temperature difference offered higher driving forces and accelerates heat transfer. Higher inlet temperature of thermal oil also intensified the natural convection - temperature fluctuated more significantly around Tm as the inlet temperature rose from 200 to 220 oC – which may also accelerate the endothermic process. The temperature at equilibrium state in charging also has an effect on the efficiency of heating water. Figure 7 shows the time to heat water from 30 oC up to 55 oC decreased as the inlet temperature of thermal oil increased. But the time decreased by very small amount due to the decisive factors: total latent heat and Tm, were kept constant. Therefore, input/output power of thermal energy storage tanks increase with temperature difference between heat transfer fluid and PCMs. Using higher efficient solar collector may improve efficiency of this kind of thermal energy storage tank. 4. Conclusion
1. 14 kg mannitol that have been fully melted can heat 100L water from 30 oC up to 50 oC in 6 hours, much more efficient than PCMs with lower Tm.
831
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xxxx / Energy Procedia 75 (2015) 827 – 833
2. Subcooling can be observed during releasing latent heat. 3. Both the inlet temperature and flow rate of thermal oil have effects on accelerating storage process, but the effect were limited. 4. This study has confirmed the availability of mannitol on solar heater with thermal energy storage system. The data on thermo-physical properties of mannitol and further analysis on heat transfer rate of this coil pipe heat exchanger will help the design of thermal energy storage system. Acknowledgements The authors are grateful for the support from the Joint Project of JST-MOST˄2013DFG60080˅, the Joint Funds of NSFC-Guangdong of China (U0934005) and the Fundamental Research Funds for the Central Universities (2014ZM0054) Reference [1] Mahfuz MH, Anisur MR, Kibria MA, Saidur R, Metselaar IHSC. Performance investigation of thermal energy storage system with Phase Change Material (PCM) for solar water heating application. International Communications in Heat and Mass Transfer. 2014;57:132-9. [2] Al-Kayiem HH, Lin SC. Performance evaluation of a solar water heater integrated with a PCM nanocomposite TES at various inclinations. Solar Energy. 2014;109:82-92. [3] Mazman M, Cabeza LF, Mehling H, Nogues M, Evliya H, Paksoy HÖ. Utilization of phase change materials in solar domestic hot water systems. Renewable Energy. 2009;34:1639-43. [4] Al-Hinti I, Al-Ghandoor A, Maaly A, Abu Naqeera I, Al-Khateeb Z, Al-Sheikh O. Experimental investigation on the use of water-phase change material storage in conventional solar water heating systems. Energy Conversion and Management. 2010;51:1735-40. [5] Sharma SD, Iwata T, Kitano H, Sagara K. Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage unit. Solar Energy. 2005;78:416-26. [6] Gil A, Barreneche C, Moreno P, Solé C, Inés Fernández A, Cabeza LF. Thermal behaviour of dmannitol when used as PCM: Comparison of results obtained by DSC and in a thermal energy storage unit at pilot plant scale. Applied Energy. 2013;111:1107-13. [7] Gil A, Medrano M, Martorell I, Lázaro A, Dolado P, Zalba B, et al. State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization. Renewable and Sustainable Energy Reviews. 2010;14:31-55. [8] Gil A, Oró E, Miró L, Peiró G, Ruiz Á, Salmerón JM, et al. Experimental analysis of hydroquinone used as phase change material (PCM) to be applied in solar cooling refrigeration. International Journal of Refrigeration. 2014;39:95-103. [9] Gil A, Oró E, Peiró G, Álvarez S, Cabeza LF. Material selection and testing for thermal energy storage in solar cooling. Renewable Energy. 2013;57:366-71.
xxxx / Energy Procedia 75 (2015) 827 – 833
Biography Prof.Zhengguo Zhang, Deputy Dean, South China University of Technology Research Areas: Thermal management of electronic devices based on phase change materials. Heat transfer enhancement and novel heat exchanger development. Composite phase change thermal energy storage material.
833
ScienceDirect Energy Procedia 75 (2015) 827 – 833
The 7th International Conference on Applied Energy – ICAE2015
Performance of a coil-pipe heat exchanger filled with mannitol for solar water heating system Ziye Linga, Guohao Zenga, Tao Xua, Xiaoming Fanga, Zhengguo Zhanga* a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation, the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China
Abstract Solar water heating systems that incorporate latent heat storage unit deals with instabilities of insolation. But most previous studies focus on phase change materials (PCMs) with phase change temperature (Tm) below 80 oC. In this study, we investigate the performance of mannitol (Tm: 166.7, enthalpy of phase change γ: 323 kJ kg -1) in storing solar thermal energy and producing hot water. Results show mannitol can store high-level energy and the thermal energy storage performance are affected by the inlet flow rate and temperature of heat transfer fluid. 14kg mannitol with latent heat activated can heat 100L water from 30 up to 50 oC in 6 hours © 2015 2015The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This Selection and/or peer-review under responsibility of ICAE (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute Keywords: solar water heater; solar thermal energy; phase change material; latent heat storage; mannitol;
1. Introduction Solar water heating systems use free heat from the sun to heat water. But the intermittent solar radiation is an unreliable source of energy. Thermal energy storage (TES) system - that stores heat during high solar intensity and releases the heat energy when needed - solves the mismatch between the supply and demand of energy. Latent heat storage system utilizes the phase change enthalpy (γ) of phase change materials (PCMs). It stands out among TES systems because its wide operating temperature range, more efficient use of solar energy, large storage capacity, and long-term stability. Previous studies on PCMs focused on materials with low phase change temperatures (Tm) below 80 [1-4], which could not heat water quickly. Recent studies have found PCMs like erythritol, Dmannitol, hydroquinone, or other sugar alcohols possess large γ over 200 kJ/kg and high Tm (100~200 ) - they have huge capacity store the energy at high level [5-9]. We believe in applying PCMs with Tm over 100 will turn on faster access to hot water.
* Corresponding author. Tel.: +86-20-87112845; fax: +86-20-87113870. E-mail address: [email protected].
1876-6102 © 2015 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/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.08.001
828
xxxx / Energy Procedia 75 (2015) 827 – 833
In this study, we evaluate the performance of a solar water heating system- incorporating with a coil pipe heat exchanger that contains mannitol (Tm: 167 ć, ¤: 323kJ/kg)-in providing 100 L hot water up to 55 ć 2. Experiment Table I Thermo-physical properties of mannitol
Liquid 1340 Solid 1512 0.6 Thermal conductivity/W m-1K-1 166.7 Tm /oC -1 323 Ȗ/kJ kg Specific heat/J kg-1 oC-1 1500 Figure 1 (a) shows how this solar water heating system works. The system is mainly divided into three parts: an oil bath, a water tank and a thermal energy storage system. In charging, thermal oil is pumped through one of two coil pipes and heat is transferred to PCM. Oil is heated in the oil bath to ensure a constant inlet temperature. Charging continued until temperature inside the PCM container reached the equilibrium. Then turn off the high temperature pump for oil and turn on low temperature pump for water. Water is circulated to acquire heat from PCMs until its temperature reached 55 oC. A buffer tank is placed between the TES unit and water tank to prevent backflow. Charging performances at different inlet temperature of thermal oil are compared. Detailed parameters for each case are listed in Table II. The inlet temperature and flow rate of water are fixed to 30 o C and 60 L/h. Density/kg m-3
Fig.1 Schematic diagram of experimental system 1 water tank 2 thermal energy storage unit 3 oil bath 4 buffer tank 5 Agilent data acquisition unit 6 circulating pump for oil 7 circulating pump for water 8 flowmeter for water 9 flowmeter for oil 10 valve 11 thermo-couples 12 coiled pipes
xxxx / Energy Procedia 75 (2015) 827 – 833
Table II Operating parameters of different cases
Case
Flow rate of thermal oil
Inlet temperature of thermal oil
L/h
°C
Fig.2 Physical structure of thermal energy storage unit Figure 2 demonstrates the internal structure of TES unit. A ijîPPF\OLQGHUVWDLQOHVVFRQWDLQHU is filled with 14kg mannitol. TZRij îPPWXEHVVZLUOHGLQ a diameter of 140mm, with a pitch 50mm. Thermal oil flows in the red tube and water flows in the blue one. Twelve K-type thermo-couples (measurement error: ±0.5oC) were used to monitor the temperature evolution in the thermal energy storage tank. Fig.3 illustrates positions of thermo-couples. Thermocouples positioned at top, bottom surface and mid-plane (Fig. 3 b); In each horizontal section, four thermo-couples located at O, A, B, C in Fig.3 a ; Another three thermo-couples were used to monitor the temperature of water (15#) in the buffer tank, and the temperature of hot fluid at the inlet (13#) and outlet (14#).
Fig.3 Schematic of positions of thermo-couples
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xxxx / Energy Procedia 75 (2015) 827 – 833
3. Results and Discussions
Fig. 4 Temperature history at different positions at mid-plane in Case 2(inlet temperature of thermal oil: 210 oC and its flow rate: 360 L/h) Figure 4 describes the temperature evolution with time at mid-plane in Case 2. During charging, the constant heat flux from thermal oil melted mannitol in 5 hours. During discharging, the melted mannitol heated 100L water from 30 oC to 55 oC within 6 hours. Compared with the work done by [5], where 8kg water was heated by 45 kg erythritol (Tm: 118 oC, Ȗ:339 kJ kg-1) in 5 hours, the heating efficiency is much better. In the charging process, temperature fluctuated around the Tm. Flow of melted mannitol may explain: mannitol melted from the area near the coil pipe; the melted mannitol flowed around due to density difference between solid and liquid; if the liquid at high temperature flowed away and the solid at low temperature took the place, temperature fluctuated. But the flow was suppressed as mannitol solidified radially from inside out; temperature dropped smoothly in discharging. We observed a typical platform around 150 oC – 17 oC below the Tm, showing mannitol subcooled. 220 200 180
140 120 100
䉝 㻕
Temperature (
160
80 60
330L/h 360L/h 390L/h
40 20 0
100
200
300
400
500
600
700
800
Time (min)
Figure 5 Temperature elevation rates at 11# under different inlet flow rates of thermal oil (Inlet temperature: 210oC) Figure 5 shows the effect of flow rate of thermal oil on charging. Increasing flow rates of thermal oil decreased the time to reach equilibrium state very little, but increased the temperature at equilibrium state. Faster speed reduced the heat transfer resistance between thermal oil and mannitol.
xxxx / Energy Procedia 75 (2015) 827 – 833
Fig. 6 Temperature elevation rates at 11# under different inlet temperature of thermal oil (Flow rate: 330L/h)
Fig.7 Time to heat water up to 55 oC with different inlet temperature of thermal oil (flow rate: 330L/h) Temperature of mannitol rises faster if we increase the temperature of thermal oil (Fig. 6). Bigger temperature difference offered higher driving forces and accelerates heat transfer. Higher inlet temperature of thermal oil also intensified the natural convection - temperature fluctuated more significantly around Tm as the inlet temperature rose from 200 to 220 oC – which may also accelerate the endothermic process. The temperature at equilibrium state in charging also has an effect on the efficiency of heating water. Figure 7 shows the time to heat water from 30 oC up to 55 oC decreased as the inlet temperature of thermal oil increased. But the time decreased by very small amount due to the decisive factors: total latent heat and Tm, were kept constant. Therefore, input/output power of thermal energy storage tanks increase with temperature difference between heat transfer fluid and PCMs. Using higher efficient solar collector may improve efficiency of this kind of thermal energy storage tank. 4. Conclusion
1. 14 kg mannitol that have been fully melted can heat 100L water from 30 oC up to 50 oC in 6 hours, much more efficient than PCMs with lower Tm.
831
832
xxxx / Energy Procedia 75 (2015) 827 – 833
2. Subcooling can be observed during releasing latent heat. 3. Both the inlet temperature and flow rate of thermal oil have effects on accelerating storage process, but the effect were limited. 4. This study has confirmed the availability of mannitol on solar heater with thermal energy storage system. The data on thermo-physical properties of mannitol and further analysis on heat transfer rate of this coil pipe heat exchanger will help the design of thermal energy storage system. Acknowledgements The authors are grateful for the support from the Joint Project of JST-MOST˄2013DFG60080˅, the Joint Funds of NSFC-Guangdong of China (U0934005) and the Fundamental Research Funds for the Central Universities (2014ZM0054) Reference [1] Mahfuz MH, Anisur MR, Kibria MA, Saidur R, Metselaar IHSC. Performance investigation of thermal energy storage system with Phase Change Material (PCM) for solar water heating application. International Communications in Heat and Mass Transfer. 2014;57:132-9. [2] Al-Kayiem HH, Lin SC. Performance evaluation of a solar water heater integrated with a PCM nanocomposite TES at various inclinations. Solar Energy. 2014;109:82-92. [3] Mazman M, Cabeza LF, Mehling H, Nogues M, Evliya H, Paksoy HÖ. Utilization of phase change materials in solar domestic hot water systems. Renewable Energy. 2009;34:1639-43. [4] Al-Hinti I, Al-Ghandoor A, Maaly A, Abu Naqeera I, Al-Khateeb Z, Al-Sheikh O. Experimental investigation on the use of water-phase change material storage in conventional solar water heating systems. Energy Conversion and Management. 2010;51:1735-40. [5] Sharma SD, Iwata T, Kitano H, Sagara K. Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage unit. Solar Energy. 2005;78:416-26. [6] Gil A, Barreneche C, Moreno P, Solé C, Inés Fernández A, Cabeza LF. Thermal behaviour of dmannitol when used as PCM: Comparison of results obtained by DSC and in a thermal energy storage unit at pilot plant scale. Applied Energy. 2013;111:1107-13. [7] Gil A, Medrano M, Martorell I, Lázaro A, Dolado P, Zalba B, et al. State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization. Renewable and Sustainable Energy Reviews. 2010;14:31-55. [8] Gil A, Oró E, Miró L, Peiró G, Ruiz Á, Salmerón JM, et al. Experimental analysis of hydroquinone used as phase change material (PCM) to be applied in solar cooling refrigeration. International Journal of Refrigeration. 2014;39:95-103. [9] Gil A, Oró E, Peiró G, Álvarez S, Cabeza LF. Material selection and testing for thermal energy storage in solar cooling. Renewable Energy. 2013;57:366-71.
xxxx / Energy Procedia 75 (2015) 827 – 833
Biography Prof.Zhengguo Zhang, Deputy Dean, South China University of Technology Research Areas: Thermal management of electronic devices based on phase change materials. Heat transfer enhancement and novel heat exchanger development. Composite phase change thermal energy storage material.
833