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ExperimentalStudyofCylindricalLatentHeatEnergyStorageSystemsUsingLauricAcidasthePhaseChangeMaterial

CONFERENCEPAPER·JULY2012

DOI:10.1115/HT2012-58279

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EXPERIMENTAL STUDY OF CYLINDRICAL LATENT HEAT ENERGY STORAGE SYSTEMS USING LAURIC ACID AS THE PHASE CHANGE MATERIAL

Chang Liu Mechanical Engineering, Dalhousie University

Halifax, Nova Scotia, Canada

Robynne E. Murray Mechanical Engineering, Dalhousie University

Halifax, Nova Scotia, Canada

Dominic Groulx

Mechanical Engineering, Dalhousie University Halifax, Nova Scotia, Canada

ABSTRACT

Phase change materials (PCMs) inside latent heat energy

storage systems (LHESS) can be used to store large amounts of thermal energy in relatively small volumes. However, such systems are complicated to design from a heat transfer point of view since the low thermal conductivity of PCMs makes charging and discharging those systems challenging on a usable time scale.

Results of experiments performed on both a vertical and a horizontal cylindrical LHESS, during charging, discharging and simultaneous charging/discharging, are presented in this paper. Both LHESS are made of acrylic plastic, the horizontal LHESS has one 1/2" copper pipe passing through its center. The vertical LHESS has two 1/2" copper pipes, one through which hot water flows, and the other through which cold water flows. Each of the pipes has four longitudinal fins to enhance the overall rate of heat transfer to and from the PCM, therefore reducing the time required for charging and discharging.

The objective of this work is to determine the phase change behavior of the PCM during the operation of the LHESS, as well as the heat transfer processes within the LHESS. Natural convection was found to play a crucial role during charging (melting) and during simultaneous charging/discharging (in the vertical LHESS). However, during discharging, the effect of natural convection was reduced in both systems.

INTRODUCTION

Thermal energy storage (TES) has attracted more and more attention in recent years due to the rising cost of fossil fuels and increasing importance of environmental protection. TES can convert available energy and improve its utilization, which provides a promising solution for smoothing the discrepancy between energy supply and demand. Current TES systems can be categorized by the method they use to store energy, such as sensible heat storage, latent heat storage and thermochemical heat storage [1]. Among these energy storage methods, latent heat energy storage systems (LHESS) show more potential due to their high energy storage density and nearly constant temperature during phase change [2].

LHESS uses phase change materials (PCMs) as energy storage mediums: energy is stored during melting and released during solidification. Various applications found in the open literature include space heating [3], space cooling [4], hot water systems, [5] and incorporating PCMs into building elements [6].

Although a promising medium for energy storage, PCM suffers from low thermal conductivity which limits its wide application in industry. Various heat transfer enhancement methods have been explored by researchers such as adding fins to PCM container [7], inserting metal matrix into PCM [8], PCM encapsulation [9], and combining the PCM with another material which has a higher thermal conductivity [10].

Fins are the most commonly used heat transfer enhancement method, and various studies have compared fin sizes and orientations. It was observed that the shape of fins

Proceedings of the ASME 2012 Summer Heat Transfer Conference HT2012

July 8-12, 2012, Rio Grande, Puerto Rico

HT2012-58279

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have an effect on heat transfer enhancement. In a concentric tube heat exchanger with Erythritol as PCM, longitudinal fins were recommended over circular fins [11]. Moreover, fins with lessing rings and with bubble agitation were tested to study their heat transfer enhancement, and results showed that both could improve thermal performance [12]. Fin parameters were studied both numerically and experimentally. It was reported by Ismail et al. that fin thickness had a relatively small influence on the solidification time; while fin length and the number of fins strongly affected the complete solidification time [13].

Studying phase change numerically is complicated due to the transient characteristics of the process. It was observed that ignoring natural convection in mathematical modeling results in the PCM taking longer to reach its maximum temperature [14]. For that reason, natural convection has to be accounted for and simulated in order to properly describe the physics encountered during phase change, especially during melting [15].

This paper presents experimental results obtained on two cylindrical LHESS (filled with lauric acid as the PCM) during charging, discharging and simultaneous charging/discharging (vertical cylinder only). The objective of this work is to determine the phase change behavior of the PCM during the operation of the LHESS, as well as the heat transfer processes of importance within the LHESS. The importance of natural convection in the PCM melt will be highlighted. The transient energetic behavior of those systems under the various modes of operation (charging, discharging) is also investigated. Both LHESS use fins to enhance the overall heat transfer rates during the processes.

EXPERIMENTAL SETUP

Phase Change Material (PCM) Lauric acid is the PCM being used in this study, as it is a

promising choice with nearly no supercooling and desirable melting and freezing qualities [16]. The differential scanning calorimeter (DSC) curve for lauric acid (dodecanoic acid; CH3(CH2)10COOH; crude [< 80% pure, from Fisher Scientific]) shows a melting temperature range of 43.3 to 45.7°C and solidification temperature range of 38.8 to 35 °C [16]. The material properties are displayed in Table 1.

Apparatus

Two types of LHESS were studied: a horizontal cylinder

and a vertical cylinder. Both PCM containers are made of acrylic plastic to enable visualization. PCM containers are insulated with fiberglass wool and all hot water pipes are insulated with self-sealing foam pipe wrap to minimize heat losses to the surroundings.

Table 1. Thermal and Physical Properties of Lauric Acid ([17, 18]) Molecular Weight 200.31 (kg/kmol)

Density of Powder at 20°C 869 (kg/m3) Density of Liquid at 45°C 873 (kg/m3)

Fusion Temperature 42.5 (°C) Latent Heat of Fusion 182 (kJ/kg)

Heat Capacities Solid/Liquid 2.4/2.0 (kJ/kg·K) Thermal Conductivities Solid Thermal Conductivities Liquid

0.150 (W/m·K) 0.148(W/m·K)

Viscosity 0.008 (Pa·s)

Horizontal LHESS Experimental Setup

The horizontal cylinder is 12" long and 6" outside diameter, 1/4" thick, with a 1/2" copper pipe passing through its center through which water passes; 4 longitudinal copper fins, also made of copper, are added to the pipe. Figure 1 shows the horizontal LHESS containers. A schematic presenting the experimental setup used for the charging and discharging studies on the horizontal cylinder LHESS is shown in Fig. 2.

In this setup, seven type-T probe thermocouples are connected to a National Instruments 16-channel thermocouple module (NI9213) CompactDAQ data acquisition system. Temperatures are recorded using LabView. As seen in Fig. 3, the probe thermocouples are located inside the lauric acid (T10 to T14) as well as on the inlet and outlet.

Figure 1. a) 3D Solidworks rendering of the horizontal cylinder LHESS, b) Picture of the horizontal cylinder LHESS before

charging containing the solid PCM.

a)

b)

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Figure 2. Schematic of the experimental setup (horizontal system).

One probe was positioned inside each of the four PCM compartments delimited by the fins in order to determine the PCM melting behavior in each. Plus, based on the symmetry of the studied system, twice the amount of information can be obtained this way. In Fig. 3, probe T11, T13 and T14 are 1/2" from the central pipe, while probe T12 is 1" from the pipe; T10 is in the middle of the gap between the end of the fin and the cylinder wall (gap is 8 mm wide). Nine type-T surface thermocouples are attached on the four fins to provide additional temperature information. A pulse counting flowmeter from Omega (model FTB 905) is connected to a counter/pulse generation module (NI9435) on the DAQ system and also read by LabView. In the charging loop, a 500W emersion heater in a water bath (model TSP02793) is used to keep the hot water at a constant temperature and water is circulated by a centrifugal pump from Grundfos (model UPS 15-58 FS).

Figure 3. Positions of probe thermocouples in the horizontal

LHESS.

Figure 4. 3D Solidworks rendering and 2D cross-sectional views of

the horizontal cylinder LHESS.

Vertical LHESS Experimental Setup A similar setup is used for the vertical cylindrical LHESS;

however, the PCM container orientation (Fig. 4) and locations of thermocouples are different. The vertical cylinder is 24" long, 1/4" thick, and has an 8" outside diameter, with two 1/2" copper pipes passing through it, enabling simultaneous charging and discharging. Each pipe has four longitudinal copper fins. In this setup the cold water is circulated by the municipal water pressure, and the hot water is circulated by a magnetic drive centrifugal pump (Cole Parmer model EW-72012-10) from a constant temperature hot water bath. A 2 kW emersion heater (model TSP02794) is used to keep the hot water at a constant temperature. Two OMEGA pulse counting flow meters (model FTB 905) were used to record the flow rate of the hot and cold water. Figure 5 shows a schematic of the experimental setup.

Type-T thermocouples are connected to a National Instruments 16-channel thermocouple CompactDAQ module (NI9213). Four of the thermocouples are located at the inlets and outlets of the copper pipes, and probe thermocouples are located at three heights, at three different sections around the container, as seen in Fig. 6.

Figure 5. Schematic of the experimental setup (vertical system).

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Figure 6. Position of probe thermocouples in the vertical LHESS.

Thermocouple probe positions were selected to obtain information required to investigate the effect natural convection in the liquid melt has on the overall phase change heat transfer. The position of the probes relative to the center of the container can be varied to increase the number of temperature measurements, as seen in the two top views in Fig. 6.

EXPERIMENTAL PROCEDURE Horizontal LHESS Experimental Procedure

For the horizontal cylinder, two modes are studied:

charging and discharging. At the beginning of the charging process, lauric acid is solid in the container at room temperature. Hot water from the constant temperature water bath is pumped through the finned copper pipe, eventually entirely melting the lauric acid. The charging portion of the experiment is completed when the system reaches steady state. Temperatures are recorded every minute. At this point in the experiment, cold water from the municipal water supply is passed through the system to solidify the lauric acid and recover the stored thermal energy (discharging process). This second leg of the experiment is concluded when the lauric acid is entirely solid at room temperature. Table 2 presents the experimental parameters used for those studies.

Table 2. Experimental Parameters for horizontal LHESS studies

Hot Flow Rate 1.5 and 5 L/min Hot inlet temperature 60 oC

Cold Flow Rate 18 L/min Cold inlet temperature 6 oC

Table 3. Vertical Consecutive Charging/discharging Experimental Parameters

Hot Flow Rate 0.55 L/min Hot inlet temperature 60 oC

Cold Flow Rate 3.5 L/min Cold inlet temperature 6 oC

Table 4. Vertical Simultaneous Charging/discharging

Experimental Parameters

Hot Flow Rate 5 L/min Hot inlet temperature 60 oC

Cold Flow Rate 5 L/min Cold inlet temperature 6 oC

Vertical LHESS Experimental Procedure

A similar experimental procedure is used for the vertical

LHESS. Preliminary experiments are performed to determine complete charging and discharging time, as well as the total energy storage capacity and phase change behavior of the PCM. The PCM container is fully charged and then fully discharged using the parameters in Table 3.

The system is then charged and discharged simultaneously to replicate a solar domestic hot water system with domestic water demand during sunshine hours. In this experiment the container is fully charged first, and then run simultaneously for 24 hours using the parameters in Table 4.

RESULTS AND DISCUSSION Horizontal LHESS: Charging

Figure 7 shows the measured temperatures by the five

probes inserted inside the PCM when the flow rate during charging is 1.5 L/min. Some fluctuations can be seen in the measured temperatures, which are due to fluctuations in the inlet temperature caused by the on/off function of the temperature controller of the immersion heater. Probe T10, being situated very close to the end of a fin, is the first to reach and surpass the melting temperature of lauric acid (42.5ºC). Probes T12 and T14 are situated in the top half of the container, above the horizontal fins and on either side of the vertical fin. As such, one would expect natural convection to be present, and play a large role during heat transfer and melting in that part of the system. It was observed that the PCM melted faster and reached higher temperatures in those two quadrants compared to the bottom quadrants where probes T11 and T13 are located. In the bottom quadrants, natural convection stems only from the vertical fin, diminishing the impact of natural convection in the liquid melt on the overall melting process. Similar observations can be made in Fig. 8, although over a shorter time scale since the flow rate was higher (5 L/min).

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Figure 7. Measured temperature of probe thermocouples during charging (flow rate of 1.5 L/min).

Figure 8. Measured temperature of probe thermocouples during charging (flow rate of 5 L/min).

a)

b)

Figure 9. Picture of PCM container after a) 2 hours of charging, and b) 5 hours of charging (flow rate of 5 L/min).

Figure 2a) shows the horizontal LHESS when all the PCM

inside is solid, before charging. Figure 9 shows pictures taken after 2 hours (a) and 5 hours of charging (b) with a flow rate of 5 L/min. After two hours of charging the PCM around the edge of the fin melted and more PCM inside the container next to the pipe has melted. After five hours, even more PCM melted around the fins; with more melting in the top quadrants because of natural convection. Also, knowing that the water flows from left to right in Fig. 9b), more molten PCM can be observed close to the inlet at left compared to the outlet at right.

Horizontal LHESS: Discharging

Figure 10 shows the temperatures recorded by the probes

during discharging with a cold water flow rate of 18 L/min. In this case, natural convection is nearly non-existent. Each thermocouple recorded a drop in temperature, in the order of their position from the inlet; except for T12, which is farther from the central pipe than the other probes. This explains the additional time required to cool down and solidify the PCM.

Vertical LHESS: Complete charging

Figure 11 presents the temperatures recorded by the nine

probes in the PCM inside the vertical LHESS during charging with a flow rate of 0.55 L/min.

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Figure 10. Measured temperature of probe thermocouples during

discharging (flow rate of 18 L/min).

From Figs. 11a) and b), for the probes on the hot side of the container and the probes situated between the hot and cold pipes, the PCM starts melting rapidly closer to the top of the container. It takes a longer time for the PCM near the bottom to melt, because most of the energy given off by the pipe and fins is displaced upward by natural convection. The expected temperature plateau leading to the melting temperature can be observed on Fig. 11c); the PCM is again melting in layers, from top to bottom, the process dominated by natural convection but taking longer since the heat source is at the opposite side of the container.

With the given flow rate, it took close to 48 hours to melt the entire PCM, noting that the last layer of PCM at the bottom of the tank and below the fins takes a considerable amount of time to melt because natural convection from the melted PCM layer above does not assist in the process.

The total amount of energy stored over time in the LHESS with a charging flow rate of 0.55 L/min is presented in Fig. 12. The rate of energy storage is nearly constant. During the charging process, at any given time, some of the solid PCM is being heated to the melting temperature (sensible heat), some of the PCM is melting (latent heat), and some of the liquid PCM is being heated beyond the melting temperature (sensible heat). All these simultaneous contributions result in this nearly constant rate of energy storage and result in a total of 5 MJ of energy stored. As seen in Fig. 12, the error in the energy calculations increases over time as the uncertainty on the measured temperature between the water at the inlet and outlet gets compounded over time. When this temperature difference is small enough, the calibration uncertainty is the same order of magnitude as the temperature difference. For this reason the theoretical energy storage capacity is used as a benchmark to increase confidence in the energy values calculated.

Figure 11. Measured temperature of probe thermocouples during charging (flow rate of 0.55 L/min): a) Hot-side probes T2, T5 and T8; b) Middle probes T1, T4 and T7; c) Cold-side probes T3, T6

and T9.

c)

a)

b)

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Figure 12. Energy stored in the vertical LHESS as a function

of time (flow rate of 0.55 L/min).

Vertical LHESS: Complete discharging Figure 13 presents the temperature recorded by the nine

probes in the PCM of the vertical LHESS during discharging with a flow rate of 3.5 L/min. Complete discharging of the system took 24 hours.

The PCM solidified rapidly close to the cold pipe, as expected. It took longer in the middle section of the container, and ever longer in the hot section, on the other side of the container. It is important to note that the temperature profiles recorded at all three sections of the container are weakly dependent on the probe height, showing that natural convection did not play an important role in the solidification process.

Figure 13. Measured temperature of probe thermocouples during

discharging (flow rate of 3.5 L/min): a) Hot-side probes T2, T5 and T8; b) Middle probes T1, T4 and T7; c) Cold-side probes T3, T6

and T9. The liquid PCM cooled faster than the post-solidification

solid PCM. This is due to the large initial temperature difference between the liquid PCM and the cold water in the pipe, leading to an increased heat transfer rate. Also, as solidification progresses around the pipe and fins, the extra layer of solidified PCM acts as an ever increasing thermal barrier, impeding heat transfer.

a)

b)

c)

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Figure 14. Measured temperature of probe thermocouples during

simultaneous charging/discharging (flow rates of 5 L/min): a) Hot-side probes T2, T5 and T8; b) Middle probes T1, T4 and T7;

c) Cold-side probes T3, T6 and T9.

Vertical LHESS: Simultaneous charging/discharging Figure 14 shows the temperatures recorded during

simultaneous charging/discharging with hot and cold flow rates of 5 L/min. The PCM in the container was initially fully melted. In the ensuing 24 hours, during simultaneous charging and discharging, only the bottom 2/3 of the PCM on the cold side of the container solidified (see Fig. 16).

At the beginning of simultaneous charging and discharging, Figs. 14a) and b) show an initial decrease in temperature in the liquid PCM at every height. This seems to be due to an initial thermal shock in the system when warmer water resting in the cold pipe is quickly replaced by cold water. The liquid PCM in the LHESS reacts by first dropping 2 to 4ºC, with larger temperature drops at the bottom of the tank. With natural convection keeping hotter liquid at the top, stratified liquid PCM layers are observed in the container.

From Fig. 14c), energy is removed much faster from the cold side of the system, with the temperature of the now solid PCM dropping below 20ºC at the bottom and middle probes. The temperature near the top, even on the cold side, remains above the melting temperature as hot liquid PCM is displaced from the lower part of the container to the top.

Figure 15 presents all nine temperature measurements shown in Fig.14 on the same plot. This makes it easier to visualize the overall temperature variations within the PCM container. Shown in blue are the three temperatures measured next to the cold water pipe, it can be seen that a large amount of energy is extracted from this side of the system compared to the middle portion (in green) and the hot side (in red).

Figure 15. Measured temperature of probe thermocouples during simultaneous charging/discharging (flow rates of 5 L/min): red is associated with hot-side probes, green with the middle probes and

blue with cold-side probes.

a)

b)

c)

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Figure 16. Picture of vertical PCM container after 24 hours of simultaneous charging and discharging.

The photograph in Fig. 16 shows the solid PCM on the

cold side of the LHESS (right side of the photograph), while most of the PCM on the left side is still liquid. Part of one of the fins on the cold pipe can be seen near to top. Also, the insulation used in the experiment can be seen in the photograph. Notice the blue color of lauric acid. This coloring comes from a reaction of the lauric acid with the copper pipe and fins [16]. This reaction is mild and the lauric acid becomes saturated by the copper rapidly, putting a stop to the reaction.

CONCLUSIONS

Results of experiments performed on both vertical and

horizontal cylindrical LHESS during charging, discharging, and simultaneous charging/discharging have been presented. Each pipe passing through the cylindrical PCM containers had four longitudinal fins to enhance the overall rate of heat transfer in and out of the systems.

Natural convection was found to play a crucial role during charging (melting) and during simultaneous charging/discharging (in the vertical LHESS).

During charging in the horizontal LHESS, the PCM in the two upper quadrants melted faster due to the presence of natural convection enhanced by the lower and side fins enclosing each of the two quadrants. The PCM in the lower quadrants, benefiting only from natural convection stemming from the side

fin (the other fin being above the side fin and not contributing to natural convection) remained solid much longer.

In the vertical system, the upper region melted more rapidly because of the increased energy carried to it by convective movement of the liquid melt.

During discharging, the effect of natural convection was reduced in both systems. Finally, during simultaneous charging/discharging, a stratified state was found inside the remaining liquid PCM.

Future work will include performing additional experiments at different flow rates, as well as repeating these same experiments with different thermocouple probe positions. This will lead to more information about the temperature profiles inside the LHESS, resulting in a better understanding of the impact of natural convection. Phase change heat transfer numerical models will also be created to increase the understanding of heat transfer in LHESS and help in future LHESS design. Those numerical models will be validated using the experimental results.

ACKNOWLEDGEMENTS

The authors are grateful to the Canadian Foundation for

Innovation (CFI), ecoNovaScotia and Scotian WindField inc. for their financial assistant in procuring some of the infrastructures, as well as the Natural Science and Engineering Research Council of Canada (NSERC), the Dalhousie Research in Energy, Advanced Materials and Sustainability (DREAMS) program (an NSERC CREATE program) and Dalhousie University for their financial Support.

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