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Microencapsulated phase change slurries for thermal energy storage in a residential solar energy system
Renewable Energy 36 (2011) 2932e2939
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Microencapsulated phase change slurries for thermal energy storage in a residential solar energy system M.J. Huang a, *, P.C. Eames b, S. McCormack c, P. Griffiths a, N.J. Hewitt a a
Centre for Sustainable Technologies, School of Built Environment, University of Ulster, N. Ireland BT37 0QB, UK Centre for Renewable Energy Systems Technology, Loughborough University, Leicestershire LE11 3TU, UK c Department of Civil, Structure and Environmental Engineering, Trinity College Dublin, Dublin 1, Ireland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2010 Accepted 4 April 2011 Available online 22 April 2011
Phase change materials (PCMs) are attractive for use in thermal energy storage applications and thermal regulation/control due to their high-energy storage density over a small temperature range. The direct use of phase change materials for energy storage and/or heat transfer applications has been limited due to the low thermal conductivity of the PCM particularly when solidifying on the heat transfer surface. A Phase change slurry (PCS) consists of small micro-encapsulated PCM particles suspended in a carrier fluid which enhances the heat transfer to the PCM. The PCS can serve not only as the thermal storage media but also as the heat transfer fluid, and hence may have many potentially important applications including in the field of heating, ventilation and air-conditioning (HVAC), refrigeration, solar energy and heat exchangers. A test system to examine PCS performance in residential thermal energy storage applications has been developed to both observe and characterise the thermal processes that occur in a thermal store with a helical coil heat exchanger. These test results will be used to improve the system design and identify limitations when used for intermittent application. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Phase change slurry (PCS) Solar thermal energy storage Thermal performance
1. Introduction Most of the available renewable energy is intermittent and seasonal by its nature and is dependent on the meteorological conditions of the location. On sunny days, solar energy systems generally collect significantly more energy than is needed for direct use. To meet heat demand when using an intermittent natural heat supply requires thermal energy storage. Effective heat transfer during both charge and discharge of the store using solar energy and the ability to retain the heat are essential to achieve a significant solar saving fraction. Effective and economic thermal energy storage is essential on at least a daily basis for the effective use of solar energy for heating purposes. Ideally, the thermal store should store all available energy for a long duration and be able to release any stored energy efficiently, independent of the amount of energy actually available in the storage unit [1]. The helical coil tube heat exchangers are widely used in residential hot water storage cylinders. For residential solar heating systems, this type of heat storage system can be used for storing collected solar energy. The literature available on the fundamentals
* Corresponding author. Tel.: þ44 2890366037. E-mail address: [email protected] (M.J. Huang). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.04.004
of heat transfer from such coils to the store fluid is limited. Xin and Ebadian [2] used three different helical coiled pipes to investigate the exterior natural convective heat transfer from the coil to the store. The coils were oriented both vertically and horizontally with a constant heat flux boundary condition. The correlation between the Nusselt number and the Rayleigh number on the outer surface of the tube was studied. Ali [3] performed experiments to measure the average Nusselt number for a whole coil with a constant heat flux boundary condition. Simulations of vertical coils in a hot water store have been studied to predict the heat transfer and outlet temperature from the coils [4]. After studying three coils with different characteristic dimensions to develop correlations, it was concluded that further study was required to develop better Nusselt number relations that cover a wide range of different sizes and coil configurations. In addition to water as the heat transfer medium, experimental studies of steady-state natural convective heat transfer in heat transfer oil have been conducted for vertical helical coil tubes [5]. Correlations of average Nusselt number with Rayleigh number for the specific coil designs have been presented. It is essential to determine how to improve both charge and discharge of a thermal store using a coil heat exchanger for an intermittent heat supply. Latent heat thermal energy storage materials i.e. phase change materials (PCMs) are promising for use in thermal energy storage
M.J. Huang et al. / Renewable Energy 36 (2011) 2932e2939
applications and thermal regulation where heat input/dissipation is periodic, or is subjected to a sudden transient due to their highenergy storage density over a small temperature range. The highenergy storage density will enable the size of the thermal energy storage system to be reduced. However the direct use of phase change materials for energy storage and/or heat transfer applications has been limited to date due to the low thermal conductivity of most PCMs particularly when solidifying on the heat transfer surface. The low thermal conductivity can lead to low rates of heat transfer during charging and discharging of the PCMs, and as a result practical latent heat energy storage systems have been difficult to develop. Metal fins with high thermal conductivity to provide increased heat transfer surface areas or an embedded high conductivity matrix can effectively address this problem [6,7], but the increased cost and weight are issues. Phase change slurry (PCS) consists of small micro-encapsulated PCM particles suspended in a carrier fluid which enhances the heat transfer capacity of the PCMs and still retains the characteristic of high-energy storage density over a small temperature range. The PCM slurry can serve as the thermal storage media and as the heat transfer fluid, hence have many potentially important applications. The thermal performance of PCS using micro-encapsulated paraffin with phase change temperatures at 25e28 C [8] and 18 C [9] as a secondary heat transfer fluid pumped through heat exchangers have been studied experimentally. The slurries also promise advantages in low temperature applications, in the 2e8 C temperature range the physical properties and heat transfer characteristics of PCS for turbulent flow under constant heat flux have been studied [10]. It was found that the phase change process and slurry mass fraction affect the heat transfer process [10]. The drawbacks of using a PCS as a secondary heat transfer fluid in cooling application include clogging of pipes and pumps and a reduction in the heat transfer rates in heat exchangers due to fouling. A heat storage system with a helical coil heat exchanger using PCSs as the heat storage medium and water as the secondary heat transfer fluid has been developed to observe and characterise the thermal charge and discharge processes for constant heat flux with turbulent flow conditions. Developing fluid-to-fluid helical heat exchangers (fluid present on both sides of the tube wall) requires a detailed understanding of the heat transfer effectiveness and fluid characteristics of PCS when used as the heat storage medium. A micro-encapsulated paraffin based slurry with a phase change temperature 65 C (produced by BASF) with three different PCM volumetric concentrations have been tested and compared with water used to provide a reference storage material. These test results are intended to enable improvements to solar energy storage system
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design and identification of design limitations when used for residential thermal energy storage applications. It is expected that the characteristics of PCS will improve the density of the thermal energy storage allowing daily surplus collected solar energy or other excess thermal energy to be stored in a more effective and economic way. 2. Experimental system design 2.1. Experimental PCS thermal energy storage system Fig. 1 is a schematic diagram of the thermal energy storage cylinder in which a PCS is used as a thermal storage medium. The system has been designed to be similar to a traditional hot water storage cylinder to permit performance comparison. The interior dimensions of the cylinder were 0.270 m diameter and 1.000 m height. The walls of the storage cylinder and top and bottom covers were made from 0.008 m thick clear Perspex to minimise conduction in the walls, provided some insulation and enable the behaviour of the slurry to be observed. A copper coil helix heat exchanger was located in the lower section of the storage cylinder. Hot water was circulated through the heat exchanger with the return pipe passing through the centre of the top of the storage cylinder. The coil of 200 mm outer diameter was made from copper tube of 22 mm outer diameter. The inner diameter of the pipe fittings used was equal to the inner diameter of the copper tube to minimise disturbance in the flow of the fluid when entering and exiting the heat exchange coil. In order to simulate the heat from solar collector, a 3 kW heater with a proportionaleintegralederivative controller (PID) was used to continuously recharge a water bath with water at temperatures up to 70 C, water from the bath was then circulated through the heat exchanger. The consumed power was recorded to enable analysis of the energy input to the water bath. The rate of heat input to the PCS storage was varied by changing the water flow rate through the heat exchanger and its temperature to provide a range of different constant heat flux values. The storage cylinder was insulated by an 80 mm thick hot water store insulation jacket with aluminium thin film to reduce surface radiation heat loss. 2.2. Employed micro-encapsulated phase change slurries The phase change slurries provided by BASF have 2e8 mm plastic capsules enclosing parafins with melting temperatures of up to 70 C. Capsules of this size range suffer the least degree of damage due to the solid/liquid phase change cycle [10]. The core phase change material is a blend of paraffin compounds. These materials come in powder form and consist of micro-capsules
Fig. 1. Schematic diagram of the PCM slurry thermal test system and thermocouple locations with numbers.
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Fig. 2. The measured temperatures along the central vertical axis of the store and inlet water temperature to the heat exchanger. (The location please refer Fig. 1).
containing the phase change material The heat storage potential of PCSs investigated in this work have volumetric concentrations of micro-capsules of 50, 35 and 25% in water. Extensive cyclic testing has been undertaken to examine the deterioration of the PCSs when used in thermal energy storage applications. 2.3. Temperature measurements A total of 54 T-type copper/constantan thermocouples with an accuracy of 0.1 C were used to measure temperatures at; i) 33 locations within the PCS storage system (as indicated in Fig. 1), ii) 10 locations on the internal and external wall surfaces of the PCS storage system, iii) 3 locations for the ambient temperatures and iv) eight locations to measure temperatures along the water circuit pipeline. The thermocouples located in the central vertical crosssections through the PCS cylinder storage were used to measure the temperature distribution and temperature change with time.
overheating which could lead to the plastic encapsulating material of the PCM micro-capsule bursting and subsequent separation in the store. The data acquisition system was a programmable data logging device, with data from all sensors measured every 60 s and stored prior to transfer to computer for analysis. The indoor ambient temperature was kept constant for the duration of each experiment. The PCS test system was located away from any direct solar radiation in the indoor laboratory environment. Each experiment continued until the PCS in the system reached a steady stage then the heat supply was switched off, the decrease in temperatures in the storage system was recorded to allow heat loss from the store to be analysed. 3. Experimental results and analysis 3.1. The effect of charge rate on the thermal performance of the storage system when employing water as the thermal storage medium
2.4. Experimental procedure The full experimental apparatus consisted of the cylindrical heat storage system with copper coil heat exchanger in the lower section of the store, the hot water supply circuit system with PID control to maintain water temperature at set levels, a data acquisition system and an infrared camera. The maximum operating temperature for the heat supply loop was set at 70 C, this was to prevent
The energy input from the coil heat exchanger is equal to that stored in the storage system and that lost from the store to ambient. The water flow rate in the heat exchanger and temperature difference between the coil and the store fluid will affect the rate of heat supply to the storage system. The inlet and outlet temperatures to the heat exchanger are measured to enable the heat flux to the store to be calculated. The flow in the heat exchanger pipe is turbulent
Fig. 3. Accumulated heat in the store and hot water inlet temperature for three different heat exchanger water mass flow rates.
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Fig. 4. Rate of heat supplied to and lost from the store and heat exchanger water inlet temperature.
with a Reynolds number of more than 4000. The heat supplied Qsupply by the water circuit to the storage is:
_ p ðTinlet Toutlet Þ Qsupply ¼ mc
(1)
The heat loss from the outside wall of the store to the surroundings Qwall is dependent on the U value Uo, the surface area Ao of the cylinder and the temperature difference DT between the external wall of the store and the ambient temperature (Equation (2)). At steady-state conditions when the temperature inside the store is stable the heat loss from the store to ambient equals the heat supplied to the store from the heat exchanger.
Qwall ¼ Uo Ao DT
(2)
Water as a reference thermal storage medium in the store has been carried out. Experiments were undertaken to determine the effect on the heat transfer coefficient inside the heat exchanger and rate of heat input from the heat exchanger for three different water flow rates. The flow with pump inside the heat exchanger is considered to be turbulent due to the calculated Reynolds number exceeding the critical Reynolds number for flow in helices. The temperature variation with time inside the store and heat exchanger water circuit for natural circulation and flow rates of 20 and 30 kg min1
were measured. The temperature of the water inlet was set at 67 C for the heat exchanger loop. The temperature of the hot water inlet was controlled by the PID and a buffer tank to reduce temperature fluctuations. Fig. 2 shows the temperature development at the 33 locations in the store (Fig. 1) and the hot water inlet for water flow rate of 30 kg min2. The oscillation in the inlet water temperature is due to the dead band of the heating system control. With sufficient time, 200 min, the water temperature inside the store can approach 67 C, similar to the water inlet. It was found that the temperature distribution inside of the water store was uniform for the three fluid flow rates. For all 3 different mass flow rates, neglecting the affect of the initial stratification in the water temperature, the temperatures measured inside the stores at different heights are similar indicating a high degree of mixing. The water heated inside the store is uniformly distributed with only 0.29% difference at the measurement points over the duration of the tests. The heat exchanger inlet water temperature and the accumulated heat in the store for three different heat exchanger water mass flow rates are presented in Fig. 3. The increase in hot water inlet temperature is similar for all three flow rates, but lags in the system with natural circulation compared to the 20 kg/min and 30 kg/min flow rates. The rate of heat accumulation in the store is faster with a 30 kg/min flow rate and achieves a store water
Fig. 5. Temperatures measured in the centre of the heat exchanger (point 53) for stores with water, 25% and 50% by volume slurries with heat exchanger water flow rate of 20 kg min1.
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Fig. 6. Temperature difference between heat exchanger inlet and outlet water temperatures to for three different PCS concentrations with time.
temperature of 65 C sooner than that for the 20 kg/min flow rate and that with natural circulation by 18% and 32% respectively. The conclusion drawn from this is that the water flow rate in the heat exchanger loop affects the heat transfer to the store. Fig. 4 shows the heat supplied to the store through the heat exchanger and the heat loss from the store surface to the surroundings along with the water inlet temperature to the store with time. The flow rate for the heat exchanger is 30 kg min1. The water temperature in the store starts from 20 C. During the first 120 min with the water inlet temperature increasing the supplied heat to store rises sharply due to the low temperature of the thermal mass in the store and then towards steadily when the store temperature rising. The rate of heat supply depends on the water inlet temperature rising, the average temperature inside of the store and the flow rate of water in the heat exchanger. After 120 min the temperature of the water store reaches a temperature of around 67 C with slight oscillations resulting due to the dead band of the heating system control. With no heat discharge the heat input to the store is equal to the heat loss from the store to the surroundings. The heat loss from the store is around 140 W. The Rayleigh number (Ra) describes the importance of buoyancy forces which drive convection compared to the diffusive processes
(heat and momentum) which act to inhibit convection. The Rayleigh number is given by [11]:
Ra h
g bDTL3 K n rCP
(3)
The Rayleigh number may also be viewed as the ratio of buoyancy forces and (the product of) thermal and momentum diffusivities. In the water store system during the heating period with the 20 kg/ min flow rate the Rayleigh number, using the height of the helical coil as the characteristic length can be up to 3.2 107. The heat transfer when using water as the heat storage medium in a thermal is primarily convection. 3.2. Effect of heat input rate on the thermal performance of the heat store with PCS as the heat storage medium Hot water flow rates of 20 and 30 kg/min have been used to supply heat to the heat store when filled with the 25% by volume concentrated PCS. The central location, (thermocouple 53) in the helical coil heat exchanger was selected to compare the thermal
Fig. 7. Temperatures along the central vertical store axis for a 50% by volume slurry; Point 38 level; Point 48 level.
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response of the store to different water flow rates in the heat exchanger. The coil height is used as the characteristic length for Rayleigh number calculations. The temperature difference DT is the temperature between the bulk fluid in the coil (average of the inlet and outlet temperature) and the cylinder wall. Due to the low thermal conductivity and high viscosity of the PCS, the heat is trapped in the region around the heat exchanger instead of being transferred by natural convection to the upper section of the store. The Rayleigh numbers in the PCS are 7.1 105 and 1.2 105 respectively for heat exchanger mass flow rates of 30 kg min1 and 20 kg min1. It can be seen that the higher flow rate should provide more heat to the PCS store during charging than the lower flow rate.
concentrations has been undertaken. The hot water loop consistently supplies 65 C temperature water at a flow rate of 20 kg/min to the heat exchanger coil in the store. The change in temperature difference between water entering and exiting the heat exchanger with time for the three different slurry concentrations is listed in Fig. 6. The error bands for these values are limited to 5%. Higher PCM mass fractions result in higher fluid viscosity, which cause the rate of heat transfer to be reduced. High slurry concentrations with low thermal conductivity and high viscosity reduce the heat transfer from the heat exchanger to the store. The 50% by volume slurry is not suitable for heat storage in low temperature applications when requiring effective rapid charging.
3.3. Effect of slurry volumetric concentration on the thermal performance of the heat store
3.4. Heat transfer effect inside of the heat storage system with a high-concentration slurry
In order to study the effect of volumetric concentration of slurry on the thermal performance of thermal storage, water as a reference system has been compared to slurry concentrations of 25% and 50%. The water flow rate in the heat exchange loop is 20 kg min1. The measured increase in temperature at the centre of heat exchanger coil (point 53) for these cases are presented in Fig. 5. The temperatures measured for the 25% slurry are similar to those for the water filled system during the initial heat up stage. The water thermal and fluid characteristics determine the measured temperatures in the 25% slurry heat store system. Differential scanning calorimetry (DSC) results indicate that melting of the PCM in the 65 C slurry actually commenced at around 40 C, with a significant amount of energy absorbed from 55 C upwards. This can be seen for the 25% slurry in Fig. 5. At point 53 the 50% slurry takes about 4 times more time than the 25% slurry to reach its phase change stage. For the 50% slurry the temperatures increase significantly slower than for the 25% slurry due to its low thermal conductivity, high viscosity and high specific heat. The effect of the low thermal conductivity of the PCS can also be observed from the rate of heat transfer from the heat exchanger coil to the PCS storage. A comparison of the heat supply rate by the heat exchanger loop to the slurry storage with different slurry
Fig. 7 shows the measured variation in temperature in the test system with the 50% slurry. It can be seen that there is stratification inside the store along the central vertical axis. The temperatures at the lower section of the store are higher than those in the upper section of the store. This is due to the heat flux supplied by the heat exchange coil is more intense than that from the single tube in the centre of the store. As the PCS starts melting at around 40 C, the phase change stage can be seen in the lower section of the store by the low rate of temperature increase. Above the heat exchange coil in the upper section of the PCS store, fluid movement due to natural convection is inhibited due to the low heat flux and high fluid viscosity. Heat transfer in the upper section of the PCS storage is dominated by conduction for the 50% concentration slurry which can be seen from the temperatures at points 18, 28, 33 and 38 in Fig. 7. After 550 min, there is no heat supply to the store. The cooling at this stage is a conduction-dominated process. In order to monitor the temperature distribution inside the store with and without heating, the temperature development at points 38 and 48 which correspond to low and high heat flux areas are presented in Fig. 8. At point 38 the temperature increases by less than 20 C in 600 min remaining below the phase change temperature. After switching the heating system off the temperature at point 38
Fig. 8. Temperature development at layers including points 38 and 48 with heat input for the first 600 min.
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Fig. 9. Isotherms at 100, 300 and 449 min created from measured data for a 50%? PCS heat store while heating.
continues to increase for another 400 min and then starts to reduce. At point 48, the range of temperatures measured is larger than at point 38. The rate of temperature increase is not uniform due to phase change occurring. After the heating is turned off there is a rapid reduction in the temperature at this location. Temperatures measured in the vertical cross-section above the heat exchanger and on the store wall surfaces were used to produce the isothermal contour plots in Fig. 9 for the store filled with 50% PCS. The thermal stratification above the heat exchanger can be seen. Heat transfer is predominantly by conduction with the effects of the central copper pipe clearly evident, illustrated by the increased temperature of the PCS adjacent to it. Fig. 10 shows the measured temperatures along the vertical axis of the heat store when filled with 25% and 50% by volume slurry. The rise in temperature within the store is much faster for the 25% by volume slurry than for the 50% by volume slurry, this is due to the lower viscosity in the more dilute slurry allowing increased fluid movement. The temperature in the lower section of the store when filled with 25% PCS (point 48, 52, 53 and 54) reaches the phase change temperature within 400 min while the store filled with 50% PCS takes 600 min. The effects of convection are more significant in the store filled with 25% PCS than the store filled with 50% PCS in both the upper and lower sections of the store. Due to the different heat inputs to the upper and lower sections, the degree of temperature stratification is different. Convection from
the heating exchanger is greater for the store filled with 25% PCS than the store filled with 50% PCS which results in the temperature at point 48 being greater. Heat transfer to the 25% by volume slurry is more effective, however the low concentration of PCS means a lower thermal storage capacity. Further research is required to optimise concentrations of PCS for different thermal storage applications when using internal heat exchangers. 3.5. Effect of natural convection on PCS thermal energy storage The lower section of the PCS store is heated by the heat exchanger coil. The effective heat transfer from the heat exchanger coil can be inferred from the similarity of the temperatures measured at points 48, 52, 53 and 54 (Fig. 10). The heat transfer to the upper section of the store is conduction dominated. It can be seen that the temperature gradients are in the region of the coil rather than in the upper section of the PCS storage. The driving force for natural convection is buoyancy which results due to differences in fluid density. It can be seen that around the coil buoyancy leads to higher temperatures being measured in the upper region compared to the lower region. The thermal stratification resulting due to natural convection can be seen with the temperature at point 52 being higher than at points 53 and 54, but cooler than at point 48. For the store filled with 25% concentration PCS, the natural convection is clearer with the temperature
Fig. 10. Temperature variation in the vertical axis of the store when filled with 25% and 50% by volume slurry.
M.J. Huang et al. / Renewable Energy 36 (2011) 2932e2939
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at point 48 point being higher than at 52, 53 and 54 points, the PCS surrounding the coil receives heat, becomes less dense and rises and surrounding, cooler fluid flows in to replace it. This cooler fluid is then heated and the process continues, establishing a convective flow. Although there is a convective effect around the heat exchanger in the lower section of the store when filled with 50% slurry it is limited due to the high fluid viscosity.
Acknowledgements
4. Conclusions
[1] Duffie JA, Beckman WA. Solar Engineering of thermal processes. 3rd ed. John Wiley, Inc., USA; 2006 [Chapter 6]. p. 250e291. [2] Xin RC, Ebadian MA. Natural convection heat transfer from helicoidal pipes. J Thermophys Heat Transfer 1996;10:297e302. [3] Ali ME. Laminar natural convection from constant heat flux helical coiled tubes. Int J Heat Mass Transfer 1998;41:2175e82. [4] Prabhanjan DG, Rennie TJ, Vijaya Raghavan GS. Natural convection heat transfer from helical coiled tubes. Int J Therm Sci 2004;43:359e65. [5] Ali ME. Natural convection heat transfer from vertical helical coils in oil. Heat Transfer Eng 2006;27(3):79e85. [6] Zhang Y, Faghri A. Heat transfer enhancement in latent heat thermal energy storage system by using an external radial finned tube. J Enhanced Heat Transfer 1996;3:119e27. [7] Choi JC, Kim SD. Heat transfer characteristics of a latent heat storage system using MgCl2$6H2O. Energy 1992;17:1153e64. [8] Gschwander S, Schossing P, Henning HM. Micro-encapsulated paraffin in phase-change slurries. Sol Energy Mater Sol Cells 2005;89:307e15. [9] Griffiths PW, Eames PC. Performance of chilled ceiling panels using phase change material slurries as the heat transport medium. Appl Thermal Eng 2007;27(10):1756e60. [10] Alvarado J, Marsh C, Sohn C, Phetteplace G, Newell T. Thermal performance of microencapsulated phase change material slurry in turbulent flow under constant heat flux. Int J Heat Mass Transfer 2007;50:1938e52. [11] Kays William, Crawford Michael, Weigand Bernhard. Convective heat and mass transfer. 4 ed. McGraw-Hill Professional, ISBN 0072990732; 2004.
A PCS test system configured as a traditional hot water storage cylinder was developed and performance analysed when filled with water and a 65 C PCS of three different volume concentrations. In this work the heat exchange circuit was constructed to study the store performance with different inlet fluid temperatures and flow rates Slurries with 50% volume concentration are not suitable for heat storage in the application tested due to low rates of heat transfer resulting due to suppression of natural convection and mixing in the store. The thermal performance of PCS heat storage with different rates of heat input have been studied to enable improved system designs to be developed for intermittent thermal energy storage. The size and location of the heat exchanger in the store will influence the thermal performance for heat storage. Different heat exchanger designs and the addition of fins to the heat exchanger coil to increase the heat transfer area to the PCS requires further detailed investigation and should enable improved performance to be realised.
Authors are involved in the network of COST supported by EU COST TU0802, 2009e2012.
References
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Microencapsulated phase change slurries for thermal energy storage in a residential solar energy system M.J. Huang a, *, P.C. Eames b, S. McCormack c, P. Griffiths a, N.J. Hewitt a a
Centre for Sustainable Technologies, School of Built Environment, University of Ulster, N. Ireland BT37 0QB, UK Centre for Renewable Energy Systems Technology, Loughborough University, Leicestershire LE11 3TU, UK c Department of Civil, Structure and Environmental Engineering, Trinity College Dublin, Dublin 1, Ireland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2010 Accepted 4 April 2011 Available online 22 April 2011
Phase change materials (PCMs) are attractive for use in thermal energy storage applications and thermal regulation/control due to their high-energy storage density over a small temperature range. The direct use of phase change materials for energy storage and/or heat transfer applications has been limited due to the low thermal conductivity of the PCM particularly when solidifying on the heat transfer surface. A Phase change slurry (PCS) consists of small micro-encapsulated PCM particles suspended in a carrier fluid which enhances the heat transfer to the PCM. The PCS can serve not only as the thermal storage media but also as the heat transfer fluid, and hence may have many potentially important applications including in the field of heating, ventilation and air-conditioning (HVAC), refrigeration, solar energy and heat exchangers. A test system to examine PCS performance in residential thermal energy storage applications has been developed to both observe and characterise the thermal processes that occur in a thermal store with a helical coil heat exchanger. These test results will be used to improve the system design and identify limitations when used for intermittent application. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Phase change slurry (PCS) Solar thermal energy storage Thermal performance
1. Introduction Most of the available renewable energy is intermittent and seasonal by its nature and is dependent on the meteorological conditions of the location. On sunny days, solar energy systems generally collect significantly more energy than is needed for direct use. To meet heat demand when using an intermittent natural heat supply requires thermal energy storage. Effective heat transfer during both charge and discharge of the store using solar energy and the ability to retain the heat are essential to achieve a significant solar saving fraction. Effective and economic thermal energy storage is essential on at least a daily basis for the effective use of solar energy for heating purposes. Ideally, the thermal store should store all available energy for a long duration and be able to release any stored energy efficiently, independent of the amount of energy actually available in the storage unit [1]. The helical coil tube heat exchangers are widely used in residential hot water storage cylinders. For residential solar heating systems, this type of heat storage system can be used for storing collected solar energy. The literature available on the fundamentals
* Corresponding author. Tel.: þ44 2890366037. E-mail address: [email protected] (M.J. Huang). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.04.004
of heat transfer from such coils to the store fluid is limited. Xin and Ebadian [2] used three different helical coiled pipes to investigate the exterior natural convective heat transfer from the coil to the store. The coils were oriented both vertically and horizontally with a constant heat flux boundary condition. The correlation between the Nusselt number and the Rayleigh number on the outer surface of the tube was studied. Ali [3] performed experiments to measure the average Nusselt number for a whole coil with a constant heat flux boundary condition. Simulations of vertical coils in a hot water store have been studied to predict the heat transfer and outlet temperature from the coils [4]. After studying three coils with different characteristic dimensions to develop correlations, it was concluded that further study was required to develop better Nusselt number relations that cover a wide range of different sizes and coil configurations. In addition to water as the heat transfer medium, experimental studies of steady-state natural convective heat transfer in heat transfer oil have been conducted for vertical helical coil tubes [5]. Correlations of average Nusselt number with Rayleigh number for the specific coil designs have been presented. It is essential to determine how to improve both charge and discharge of a thermal store using a coil heat exchanger for an intermittent heat supply. Latent heat thermal energy storage materials i.e. phase change materials (PCMs) are promising for use in thermal energy storage
M.J. Huang et al. / Renewable Energy 36 (2011) 2932e2939
applications and thermal regulation where heat input/dissipation is periodic, or is subjected to a sudden transient due to their highenergy storage density over a small temperature range. The highenergy storage density will enable the size of the thermal energy storage system to be reduced. However the direct use of phase change materials for energy storage and/or heat transfer applications has been limited to date due to the low thermal conductivity of most PCMs particularly when solidifying on the heat transfer surface. The low thermal conductivity can lead to low rates of heat transfer during charging and discharging of the PCMs, and as a result practical latent heat energy storage systems have been difficult to develop. Metal fins with high thermal conductivity to provide increased heat transfer surface areas or an embedded high conductivity matrix can effectively address this problem [6,7], but the increased cost and weight are issues. Phase change slurry (PCS) consists of small micro-encapsulated PCM particles suspended in a carrier fluid which enhances the heat transfer capacity of the PCMs and still retains the characteristic of high-energy storage density over a small temperature range. The PCM slurry can serve as the thermal storage media and as the heat transfer fluid, hence have many potentially important applications. The thermal performance of PCS using micro-encapsulated paraffin with phase change temperatures at 25e28 C [8] and 18 C [9] as a secondary heat transfer fluid pumped through heat exchangers have been studied experimentally. The slurries also promise advantages in low temperature applications, in the 2e8 C temperature range the physical properties and heat transfer characteristics of PCS for turbulent flow under constant heat flux have been studied [10]. It was found that the phase change process and slurry mass fraction affect the heat transfer process [10]. The drawbacks of using a PCS as a secondary heat transfer fluid in cooling application include clogging of pipes and pumps and a reduction in the heat transfer rates in heat exchangers due to fouling. A heat storage system with a helical coil heat exchanger using PCSs as the heat storage medium and water as the secondary heat transfer fluid has been developed to observe and characterise the thermal charge and discharge processes for constant heat flux with turbulent flow conditions. Developing fluid-to-fluid helical heat exchangers (fluid present on both sides of the tube wall) requires a detailed understanding of the heat transfer effectiveness and fluid characteristics of PCS when used as the heat storage medium. A micro-encapsulated paraffin based slurry with a phase change temperature 65 C (produced by BASF) with three different PCM volumetric concentrations have been tested and compared with water used to provide a reference storage material. These test results are intended to enable improvements to solar energy storage system
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design and identification of design limitations when used for residential thermal energy storage applications. It is expected that the characteristics of PCS will improve the density of the thermal energy storage allowing daily surplus collected solar energy or other excess thermal energy to be stored in a more effective and economic way. 2. Experimental system design 2.1. Experimental PCS thermal energy storage system Fig. 1 is a schematic diagram of the thermal energy storage cylinder in which a PCS is used as a thermal storage medium. The system has been designed to be similar to a traditional hot water storage cylinder to permit performance comparison. The interior dimensions of the cylinder were 0.270 m diameter and 1.000 m height. The walls of the storage cylinder and top and bottom covers were made from 0.008 m thick clear Perspex to minimise conduction in the walls, provided some insulation and enable the behaviour of the slurry to be observed. A copper coil helix heat exchanger was located in the lower section of the storage cylinder. Hot water was circulated through the heat exchanger with the return pipe passing through the centre of the top of the storage cylinder. The coil of 200 mm outer diameter was made from copper tube of 22 mm outer diameter. The inner diameter of the pipe fittings used was equal to the inner diameter of the copper tube to minimise disturbance in the flow of the fluid when entering and exiting the heat exchange coil. In order to simulate the heat from solar collector, a 3 kW heater with a proportionaleintegralederivative controller (PID) was used to continuously recharge a water bath with water at temperatures up to 70 C, water from the bath was then circulated through the heat exchanger. The consumed power was recorded to enable analysis of the energy input to the water bath. The rate of heat input to the PCS storage was varied by changing the water flow rate through the heat exchanger and its temperature to provide a range of different constant heat flux values. The storage cylinder was insulated by an 80 mm thick hot water store insulation jacket with aluminium thin film to reduce surface radiation heat loss. 2.2. Employed micro-encapsulated phase change slurries The phase change slurries provided by BASF have 2e8 mm plastic capsules enclosing parafins with melting temperatures of up to 70 C. Capsules of this size range suffer the least degree of damage due to the solid/liquid phase change cycle [10]. The core phase change material is a blend of paraffin compounds. These materials come in powder form and consist of micro-capsules
Fig. 1. Schematic diagram of the PCM slurry thermal test system and thermocouple locations with numbers.
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Fig. 2. The measured temperatures along the central vertical axis of the store and inlet water temperature to the heat exchanger. (The location please refer Fig. 1).
containing the phase change material The heat storage potential of PCSs investigated in this work have volumetric concentrations of micro-capsules of 50, 35 and 25% in water. Extensive cyclic testing has been undertaken to examine the deterioration of the PCSs when used in thermal energy storage applications. 2.3. Temperature measurements A total of 54 T-type copper/constantan thermocouples with an accuracy of 0.1 C were used to measure temperatures at; i) 33 locations within the PCS storage system (as indicated in Fig. 1), ii) 10 locations on the internal and external wall surfaces of the PCS storage system, iii) 3 locations for the ambient temperatures and iv) eight locations to measure temperatures along the water circuit pipeline. The thermocouples located in the central vertical crosssections through the PCS cylinder storage were used to measure the temperature distribution and temperature change with time.
overheating which could lead to the plastic encapsulating material of the PCM micro-capsule bursting and subsequent separation in the store. The data acquisition system was a programmable data logging device, with data from all sensors measured every 60 s and stored prior to transfer to computer for analysis. The indoor ambient temperature was kept constant for the duration of each experiment. The PCS test system was located away from any direct solar radiation in the indoor laboratory environment. Each experiment continued until the PCS in the system reached a steady stage then the heat supply was switched off, the decrease in temperatures in the storage system was recorded to allow heat loss from the store to be analysed. 3. Experimental results and analysis 3.1. The effect of charge rate on the thermal performance of the storage system when employing water as the thermal storage medium
2.4. Experimental procedure The full experimental apparatus consisted of the cylindrical heat storage system with copper coil heat exchanger in the lower section of the store, the hot water supply circuit system with PID control to maintain water temperature at set levels, a data acquisition system and an infrared camera. The maximum operating temperature for the heat supply loop was set at 70 C, this was to prevent
The energy input from the coil heat exchanger is equal to that stored in the storage system and that lost from the store to ambient. The water flow rate in the heat exchanger and temperature difference between the coil and the store fluid will affect the rate of heat supply to the storage system. The inlet and outlet temperatures to the heat exchanger are measured to enable the heat flux to the store to be calculated. The flow in the heat exchanger pipe is turbulent
Fig. 3. Accumulated heat in the store and hot water inlet temperature for three different heat exchanger water mass flow rates.
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Fig. 4. Rate of heat supplied to and lost from the store and heat exchanger water inlet temperature.
with a Reynolds number of more than 4000. The heat supplied Qsupply by the water circuit to the storage is:
_ p ðTinlet Toutlet Þ Qsupply ¼ mc
(1)
The heat loss from the outside wall of the store to the surroundings Qwall is dependent on the U value Uo, the surface area Ao of the cylinder and the temperature difference DT between the external wall of the store and the ambient temperature (Equation (2)). At steady-state conditions when the temperature inside the store is stable the heat loss from the store to ambient equals the heat supplied to the store from the heat exchanger.
Qwall ¼ Uo Ao DT
(2)
Water as a reference thermal storage medium in the store has been carried out. Experiments were undertaken to determine the effect on the heat transfer coefficient inside the heat exchanger and rate of heat input from the heat exchanger for three different water flow rates. The flow with pump inside the heat exchanger is considered to be turbulent due to the calculated Reynolds number exceeding the critical Reynolds number for flow in helices. The temperature variation with time inside the store and heat exchanger water circuit for natural circulation and flow rates of 20 and 30 kg min1
were measured. The temperature of the water inlet was set at 67 C for the heat exchanger loop. The temperature of the hot water inlet was controlled by the PID and a buffer tank to reduce temperature fluctuations. Fig. 2 shows the temperature development at the 33 locations in the store (Fig. 1) and the hot water inlet for water flow rate of 30 kg min2. The oscillation in the inlet water temperature is due to the dead band of the heating system control. With sufficient time, 200 min, the water temperature inside the store can approach 67 C, similar to the water inlet. It was found that the temperature distribution inside of the water store was uniform for the three fluid flow rates. For all 3 different mass flow rates, neglecting the affect of the initial stratification in the water temperature, the temperatures measured inside the stores at different heights are similar indicating a high degree of mixing. The water heated inside the store is uniformly distributed with only 0.29% difference at the measurement points over the duration of the tests. The heat exchanger inlet water temperature and the accumulated heat in the store for three different heat exchanger water mass flow rates are presented in Fig. 3. The increase in hot water inlet temperature is similar for all three flow rates, but lags in the system with natural circulation compared to the 20 kg/min and 30 kg/min flow rates. The rate of heat accumulation in the store is faster with a 30 kg/min flow rate and achieves a store water
Fig. 5. Temperatures measured in the centre of the heat exchanger (point 53) for stores with water, 25% and 50% by volume slurries with heat exchanger water flow rate of 20 kg min1.
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Fig. 6. Temperature difference between heat exchanger inlet and outlet water temperatures to for three different PCS concentrations with time.
temperature of 65 C sooner than that for the 20 kg/min flow rate and that with natural circulation by 18% and 32% respectively. The conclusion drawn from this is that the water flow rate in the heat exchanger loop affects the heat transfer to the store. Fig. 4 shows the heat supplied to the store through the heat exchanger and the heat loss from the store surface to the surroundings along with the water inlet temperature to the store with time. The flow rate for the heat exchanger is 30 kg min1. The water temperature in the store starts from 20 C. During the first 120 min with the water inlet temperature increasing the supplied heat to store rises sharply due to the low temperature of the thermal mass in the store and then towards steadily when the store temperature rising. The rate of heat supply depends on the water inlet temperature rising, the average temperature inside of the store and the flow rate of water in the heat exchanger. After 120 min the temperature of the water store reaches a temperature of around 67 C with slight oscillations resulting due to the dead band of the heating system control. With no heat discharge the heat input to the store is equal to the heat loss from the store to the surroundings. The heat loss from the store is around 140 W. The Rayleigh number (Ra) describes the importance of buoyancy forces which drive convection compared to the diffusive processes
(heat and momentum) which act to inhibit convection. The Rayleigh number is given by [11]:
Ra h
g bDTL3 K n rCP
(3)
The Rayleigh number may also be viewed as the ratio of buoyancy forces and (the product of) thermal and momentum diffusivities. In the water store system during the heating period with the 20 kg/ min flow rate the Rayleigh number, using the height of the helical coil as the characteristic length can be up to 3.2 107. The heat transfer when using water as the heat storage medium in a thermal is primarily convection. 3.2. Effect of heat input rate on the thermal performance of the heat store with PCS as the heat storage medium Hot water flow rates of 20 and 30 kg/min have been used to supply heat to the heat store when filled with the 25% by volume concentrated PCS. The central location, (thermocouple 53) in the helical coil heat exchanger was selected to compare the thermal
Fig. 7. Temperatures along the central vertical store axis for a 50% by volume slurry; Point 38 level; Point 48 level.
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response of the store to different water flow rates in the heat exchanger. The coil height is used as the characteristic length for Rayleigh number calculations. The temperature difference DT is the temperature between the bulk fluid in the coil (average of the inlet and outlet temperature) and the cylinder wall. Due to the low thermal conductivity and high viscosity of the PCS, the heat is trapped in the region around the heat exchanger instead of being transferred by natural convection to the upper section of the store. The Rayleigh numbers in the PCS are 7.1 105 and 1.2 105 respectively for heat exchanger mass flow rates of 30 kg min1 and 20 kg min1. It can be seen that the higher flow rate should provide more heat to the PCS store during charging than the lower flow rate.
concentrations has been undertaken. The hot water loop consistently supplies 65 C temperature water at a flow rate of 20 kg/min to the heat exchanger coil in the store. The change in temperature difference between water entering and exiting the heat exchanger with time for the three different slurry concentrations is listed in Fig. 6. The error bands for these values are limited to 5%. Higher PCM mass fractions result in higher fluid viscosity, which cause the rate of heat transfer to be reduced. High slurry concentrations with low thermal conductivity and high viscosity reduce the heat transfer from the heat exchanger to the store. The 50% by volume slurry is not suitable for heat storage in low temperature applications when requiring effective rapid charging.
3.3. Effect of slurry volumetric concentration on the thermal performance of the heat store
3.4. Heat transfer effect inside of the heat storage system with a high-concentration slurry
In order to study the effect of volumetric concentration of slurry on the thermal performance of thermal storage, water as a reference system has been compared to slurry concentrations of 25% and 50%. The water flow rate in the heat exchange loop is 20 kg min1. The measured increase in temperature at the centre of heat exchanger coil (point 53) for these cases are presented in Fig. 5. The temperatures measured for the 25% slurry are similar to those for the water filled system during the initial heat up stage. The water thermal and fluid characteristics determine the measured temperatures in the 25% slurry heat store system. Differential scanning calorimetry (DSC) results indicate that melting of the PCM in the 65 C slurry actually commenced at around 40 C, with a significant amount of energy absorbed from 55 C upwards. This can be seen for the 25% slurry in Fig. 5. At point 53 the 50% slurry takes about 4 times more time than the 25% slurry to reach its phase change stage. For the 50% slurry the temperatures increase significantly slower than for the 25% slurry due to its low thermal conductivity, high viscosity and high specific heat. The effect of the low thermal conductivity of the PCS can also be observed from the rate of heat transfer from the heat exchanger coil to the PCS storage. A comparison of the heat supply rate by the heat exchanger loop to the slurry storage with different slurry
Fig. 7 shows the measured variation in temperature in the test system with the 50% slurry. It can be seen that there is stratification inside the store along the central vertical axis. The temperatures at the lower section of the store are higher than those in the upper section of the store. This is due to the heat flux supplied by the heat exchange coil is more intense than that from the single tube in the centre of the store. As the PCS starts melting at around 40 C, the phase change stage can be seen in the lower section of the store by the low rate of temperature increase. Above the heat exchange coil in the upper section of the PCS store, fluid movement due to natural convection is inhibited due to the low heat flux and high fluid viscosity. Heat transfer in the upper section of the PCS storage is dominated by conduction for the 50% concentration slurry which can be seen from the temperatures at points 18, 28, 33 and 38 in Fig. 7. After 550 min, there is no heat supply to the store. The cooling at this stage is a conduction-dominated process. In order to monitor the temperature distribution inside the store with and without heating, the temperature development at points 38 and 48 which correspond to low and high heat flux areas are presented in Fig. 8. At point 38 the temperature increases by less than 20 C in 600 min remaining below the phase change temperature. After switching the heating system off the temperature at point 38
Fig. 8. Temperature development at layers including points 38 and 48 with heat input for the first 600 min.
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Fig. 9. Isotherms at 100, 300 and 449 min created from measured data for a 50%? PCS heat store while heating.
continues to increase for another 400 min and then starts to reduce. At point 48, the range of temperatures measured is larger than at point 38. The rate of temperature increase is not uniform due to phase change occurring. After the heating is turned off there is a rapid reduction in the temperature at this location. Temperatures measured in the vertical cross-section above the heat exchanger and on the store wall surfaces were used to produce the isothermal contour plots in Fig. 9 for the store filled with 50% PCS. The thermal stratification above the heat exchanger can be seen. Heat transfer is predominantly by conduction with the effects of the central copper pipe clearly evident, illustrated by the increased temperature of the PCS adjacent to it. Fig. 10 shows the measured temperatures along the vertical axis of the heat store when filled with 25% and 50% by volume slurry. The rise in temperature within the store is much faster for the 25% by volume slurry than for the 50% by volume slurry, this is due to the lower viscosity in the more dilute slurry allowing increased fluid movement. The temperature in the lower section of the store when filled with 25% PCS (point 48, 52, 53 and 54) reaches the phase change temperature within 400 min while the store filled with 50% PCS takes 600 min. The effects of convection are more significant in the store filled with 25% PCS than the store filled with 50% PCS in both the upper and lower sections of the store. Due to the different heat inputs to the upper and lower sections, the degree of temperature stratification is different. Convection from
the heating exchanger is greater for the store filled with 25% PCS than the store filled with 50% PCS which results in the temperature at point 48 being greater. Heat transfer to the 25% by volume slurry is more effective, however the low concentration of PCS means a lower thermal storage capacity. Further research is required to optimise concentrations of PCS for different thermal storage applications when using internal heat exchangers. 3.5. Effect of natural convection on PCS thermal energy storage The lower section of the PCS store is heated by the heat exchanger coil. The effective heat transfer from the heat exchanger coil can be inferred from the similarity of the temperatures measured at points 48, 52, 53 and 54 (Fig. 10). The heat transfer to the upper section of the store is conduction dominated. It can be seen that the temperature gradients are in the region of the coil rather than in the upper section of the PCS storage. The driving force for natural convection is buoyancy which results due to differences in fluid density. It can be seen that around the coil buoyancy leads to higher temperatures being measured in the upper region compared to the lower region. The thermal stratification resulting due to natural convection can be seen with the temperature at point 52 being higher than at points 53 and 54, but cooler than at point 48. For the store filled with 25% concentration PCS, the natural convection is clearer with the temperature
Fig. 10. Temperature variation in the vertical axis of the store when filled with 25% and 50% by volume slurry.
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at point 48 point being higher than at 52, 53 and 54 points, the PCS surrounding the coil receives heat, becomes less dense and rises and surrounding, cooler fluid flows in to replace it. This cooler fluid is then heated and the process continues, establishing a convective flow. Although there is a convective effect around the heat exchanger in the lower section of the store when filled with 50% slurry it is limited due to the high fluid viscosity.
Acknowledgements
4. Conclusions
[1] Duffie JA, Beckman WA. Solar Engineering of thermal processes. 3rd ed. John Wiley, Inc., USA; 2006 [Chapter 6]. p. 250e291. [2] Xin RC, Ebadian MA. Natural convection heat transfer from helicoidal pipes. J Thermophys Heat Transfer 1996;10:297e302. [3] Ali ME. Laminar natural convection from constant heat flux helical coiled tubes. Int J Heat Mass Transfer 1998;41:2175e82. [4] Prabhanjan DG, Rennie TJ, Vijaya Raghavan GS. Natural convection heat transfer from helical coiled tubes. Int J Therm Sci 2004;43:359e65. [5] Ali ME. Natural convection heat transfer from vertical helical coils in oil. Heat Transfer Eng 2006;27(3):79e85. [6] Zhang Y, Faghri A. Heat transfer enhancement in latent heat thermal energy storage system by using an external radial finned tube. J Enhanced Heat Transfer 1996;3:119e27. [7] Choi JC, Kim SD. Heat transfer characteristics of a latent heat storage system using MgCl2$6H2O. Energy 1992;17:1153e64. [8] Gschwander S, Schossing P, Henning HM. Micro-encapsulated paraffin in phase-change slurries. Sol Energy Mater Sol Cells 2005;89:307e15. [9] Griffiths PW, Eames PC. Performance of chilled ceiling panels using phase change material slurries as the heat transport medium. Appl Thermal Eng 2007;27(10):1756e60. [10] Alvarado J, Marsh C, Sohn C, Phetteplace G, Newell T. Thermal performance of microencapsulated phase change material slurry in turbulent flow under constant heat flux. Int J Heat Mass Transfer 2007;50:1938e52. [11] Kays William, Crawford Michael, Weigand Bernhard. Convective heat and mass transfer. 4 ed. McGraw-Hill Professional, ISBN 0072990732; 2004.
A PCS test system configured as a traditional hot water storage cylinder was developed and performance analysed when filled with water and a 65 C PCS of three different volume concentrations. In this work the heat exchange circuit was constructed to study the store performance with different inlet fluid temperatures and flow rates Slurries with 50% volume concentration are not suitable for heat storage in the application tested due to low rates of heat transfer resulting due to suppression of natural convection and mixing in the store. The thermal performance of PCS heat storage with different rates of heat input have been studied to enable improved system designs to be developed for intermittent thermal energy storage. The size and location of the heat exchanger in the store will influence the thermal performance for heat storage. Different heat exchanger designs and the addition of fins to the heat exchanger coil to increase the heat transfer area to the PCS requires further detailed investigation and should enable improved performance to be realised.
Authors are involved in the network of COST supported by EU COST TU0802, 2009e2012.
References