Discharging performance enhancement of a phase change material based thermal energy storage device for transport air-conditioning applications

Journal Pre-proofs Discharging performance enhancement of a phase change material based thermal energy storage device for transport air-conditioning applications Binjian Nie, Xiaohui She, Boyang Zou, Yunren Li, Yongliang Li, Yulong Ding PII: DOI: Reference:

S1359-4311(19)34188-2 https://doi.org/10.1016/j.applthermaleng.2019.114582 ATE 114582

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

17 June 2019 11 October 2019 22 October 2019

Please cite this article as: B. Nie, X. She, B. Zou, Y. Li, Y. Li, Y. Ding, Discharging performance enhancement of a phase change material based thermal energy storage device for transport air-conditioning applications, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114582

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Discharging performance enhancement of a phase change material based thermal energy storage device for transport air-conditioning applications Binjian Nie, Xiaohui She, Boyang Zou, Yunren Li, Yongliang Li, Yulong Ding* Birmingham Centre for Energy Storage & School of Chemical Engineering, University of Birmingham, Edgbaston, UK B15 2TT. Corresponding email: [email protected]

Abstract A compact thermal energy storage device containing a phase change material was firstly designed and experimentally investigated which was expected to smooth cooling load of the transport air conditioning systems. The phase change material based device used two different types of fins, serrated fins in the air side and perforated straight fins in the phase change material side, for enhancing the device performance. Focus of the work was experimental investigation on the discharging process of the compact device, which is more important for transportation applications. The processing of the measurement data gave the discharging time, discharging depth, discharging power, accumulated discharging energy, thermal efficiency and exergy efficiency under different inlet air temperatures and velocities over the working period defined on the basis of the cooling requirement. The results show a fast response with the hot air flow cooled down to the desired temperature range of 16-20oC in seconds. The thermal energy storage device showed a high thermal and exergy efficiency at 99.8% and 43.3%, respectively. The discharging depth is higher than 97% which indicates the good heat transfer performance of the device. The PCM based device also shows flexibility for adjusting the output cooling capacity, leading to a great potential to resolving the frequent fluctuations of cooling loads of transport air conditioning system.

Keywords: phase change materials; thermal energy storage; air-conditioning; exergy analysis

1

Nomenclature cp - specific heat, J kg-1 K-1

ΔR - total error

k - thermal conductivity, W m-1 K-1

δXn - error factor

H - latent heat of fusion, J kg-1 m - mass flow rate, kg s-1

Subscripts

t - time, s

l - liquid

T - temperature, °C/K

s - solid

T0 - Ambient temperature, °C/K

a - air

W - power kJ s-1

pcm - phase change material

Q - energy kJ

i - initial

m - mass kg

e - end

ƞ- efficiency

Al - Aluminum

ƞth - thermal efficiency

v - vapor

ƞex - exergy efficiency

ave - average

e - specific exergy kJ kg-1 E - Exergy kJ ω - humidity ratio kg kg-1 D - Discharging depth kJ kJ-1

2

1. Introduction Public transport plays a vital and rising role in urban developments. Mechanical airconditioning systems have been employed as a main mean for the provision of thermal comfort for passengers in transport compartments. Such passenger loading changes frequently, leading to the peak load exceeding some 30% of the designed condition in many cases [1][2]. This requires frequent changes in the cooling loads. Such fluctuating cooling load is also brought in by air infiltration during door opening and closing of the rail carriages [3]. Traditional air-conditioning technologies rely on stop-start operation and are therefore often insufficient to handle such frequent cooling load changes, leading to comfort degradation and increased energy consumption. In addition, traditional air-conditioning systems are designed to deal with peak loads in most cases, which are much lower than the average cooling loads [4]. An effective way is therefore needed to smooth the fluctuating cooling loads and reduce the mechanical overdesign of air-conditioning systems. Latent heat based thermal energy storage (TES) with phase change materials (PCMs) provides an attractive solution to these challenges due to high energy storage densities of PCMs and also the provision of cooling supply at a relatively constant temperature [5][6][7][8]. There have been a number of studies on the use of PCMs for addressing the demand and supply mismatches of air-conditioning systems. The reduction in the peak cooling electrical demand by a maximum of 90% has been observed with the integration of cold storage to a conventional air-conditioning system [9]. The use of cold storage has also been shown to be able

to

increase

the

system

efficiency

of

the

air

conditioning

systems

[10][11][12][13][14][15][16]. Zhao et al. [17] studied the integration of the PCM integration into a water-cooled air-conditioning system and showed that a coefficient of performance (COP) improved by ~25.6%. Allouche et al. [15] simulated the dynamics of an air

3

conditioning system with a PCM cold storage and concluded that the application of the PCM could contribute to the stabilisation of the system operation. There are numerous PCMs and paraffin based PCMs are of great specific interest for transport related air-conditioning applications due to their chemical inertness and stability, long thermal cycling life-span, high latent heat and with little super-cooling associated with their self-nucleating characteristics [18][19]. It is therefore considered in this work. The main disadvantage of such a type of PCMs lies in their low thermal conductivity, particularly during the discharging process which determines the dynamics of cooling supply at peak hours [20] [21][22]. Various methods have been used to address the low thermal conductivity issue [23][24]. Al-Abidi et al. [25] used fins in a triplex tube heat exchanger containing a PCM and found that the presence of the fins reduced the total melting time by 34.7%. Ping et al. [26] claimed that the use of fins could improve natural convection and heat conduction within a PCM structure. Wang et al. [27] showed that the use of copper foam could decrease the total energy storage time of a PCM by 40%, whereas Li et al. [28] infiltrated PCMs into a metal foam and observed a heat conduction enhancement reflected by a reduction in the wall temperature by up to 38%. The work reported in [29] suggested the use of highly thermally conductive fins could dramatically decrease the total cost of PCM based TES systems [29]. As a result, this work focuses on the use of fins. There is an additional reason for such a choice –unlike the normal air-conditioners, transport air-conditioning system requires lightin-weight and compact-in-volume, and the plate-fin type of heat exchangers have been proven to have a high degree of compactness with an excellent thermal performance [30][31]. There are numerous studies on the use of PCM based TES for thermal comfort and the reduction of cooling load fluctuation. These studies, to the best of our knowledge, are on building environment and little has been focused on air-conditioning for transport applications. Besides, our previous work showed that the system COP could be improved 4

with the cost reduction when integrating such a device with a conventional air conditioning system[32]. The charging performance of the TES device has been studied as well[33]. The obtained results showed that the device had a maximum charging rate at 1.3 kJ/s with the charging thermal and exergy efficiency up to 87% and 70%, respectively. When using the TES for air conditioning system, the system COP was improved by 19.05% with the electrical cost reduced by ∼17.82%, leading to a payback period of 1.83-3.3 years. Hence, it is essential to investigate the discharging performance of the device in depth. It also indicated that the melting and solidification ranges of the PCM locating between 17-19oC were suited to the working condition of the transport air-conditioning system. The latent heat capacity of the PCM could meet the variable cooling load of the vehicle caused by flexible passenger load and frequent door opening, which lead to a drop on ON-OFF times of the compressor. Hence, in this paper, we report a study on a compact TES device integrated with a paraffinbased PCM for the transport air-conditioning applications. An experimental rig was set up to experimentally measure the performance of the device. 2. Experimental set up 2.1. Phase Change Material A commercial PCM (RT 18 HC), purchased from Rubitherm, Germany, was used in the work. The thermal properties of the PCM (melting point, latent heat and specific heat capacity) were characterised by using a differential scanning calorimeter (DSC). An aluminium crucible was used to hold the sample in the DSC, which was approximately 10 mg. Three replicas were taken for the measurements with each sample measured three times in order to obtain reliable results. A sapphire standard was used as a reference to determine the specific heat capacity of the samples. The starting temperature of the measurements was set at 0 °C, the sample was then heated up to 50°C at a rate of 1°C min-1 in a nitrogen atmosphere and then cooled down to 0°C at the same rate at the heating rate. The DSC analyses showed that 5

the PCM was in a solid state below 17°C and became liquid totally at temperatures over 19°C. The PCM was in a phase transformation process between 17 and 19°C. The measured latent heat was 220 kJ kg-1 and the specific heat capacities were 1.9 ± 0.2 and 2.0 ± 0.2 kJ kg-1 K-1, at solid (10°C) and liquid (30°C) states respectively. These data are summarised in Tab. 1. 2.2. PCM based TES device The PCM based TES device was made of aluminium and had an outside dimension of 300 mm in length, 400 mm in width, and 300 mm in height. It was covered by an insulation layer to prevent the heat transfer with the environment. The device had a total mass of 28 kg with 10 kg of PCM and 18 kg of aluminium. Fig. 1 shows the internal structure of the TES device, illustrating the air flow channel arrangement, channels filled with PCM, fins in both PCM and air sides, and clapboards. The PCM channels rectangularly shaped formed by the straight perforated fins. Serrated fins were adopted in the air side channels. There are 15 air channels and 16 PCM channels totally in the TES device. Each one of the air channels neighbours two PCM channels with the clapboard as a separation. The thickness of the fins and clapboards are 0.2mm and the height of the fins and the pitch between the fins are 12.8 mm and 2.5 mm, respectively, in both channels. The offset length of the serrated fins in the air flow direction is 10 mm. The holes on perforated fins have two purposes, to enhance the PCM fluidity and hence natural convection, and to facilitate the PCM filling process. As the two clapboards and the neighbouring fins divide the whole PCM channel into single chambers, if there are no holes on the fins, the single chambers will be separated without allowing the fluidity of the liquid PCMs. Hence, we introduced the perforated fins in the PCM side, in this way, the liquid PCMs inside each PCM channel could flow among the all the single chambers which could improve the natural convection of the PCMs during the phase transition.

6

For the system integration with the transport application, the TES device is proposed to locate behind the evaporator. The hot air is cooled down by the evaporator and then the TES device before entering the compartment[34]. 2.3. Experimental rig Fig. 2 shows schematically the experimental rig for measuring the performance of PCM based TES device. It consisted mainly of a commercial air-conditioner, a heater, a chamber for temperature and humidity regulation and stabilisation, a data acquisition unit, a fan, the PCM based TES device and various valves and sensors. The air heater was used to provide hot air, while the cold air came from the commercial air-conditioner. The chamber had a dimension of 800 mm in height, 800mm in width and 1000 mm length. However, this would lead to an increased turbulence of the air flow. Hence, we added two straight air ducts with the same cross-section size with the TES device before and after the device. The mixed air was driven through a long enough air duct before entering the device. The length of the duct should be at least 2 times higher than the equivalent diameter[35]. The length of the air ducts before and after the device is 800mm and 600mm, receptively. There were a total of 28 K-type thermocouples installed for measuring the air and PCM temperatures. Fig. 3 illustrates the locations of these thermocouples with 8 for air temperatures measurements (4 at the inlet and 4 at the outlet), and 20 distributed within the PCM. The thermocouples were arranged in 4 layers in air flow direction with 5 thermocouples in each layer; see Fig. 3(a). The distances of these thermocouples, measured from the top surface of the TES, are 250 mm, 200 mm, 150 mm, 100 mm and 50 mm, respectively for T6-9, T10-13, T14-17, T18-21 and T22-25; see Fig. 3(b). An Orchestrator data logger unit with 64 channels, including a 7320 measurement module and a 7020 Analog I/p module, was used to record the data. Two RH transmitters (3.0 % accuracy) were used to measure air relative humidity at the inlet and outlet. A thermal 7

anemometer (1% accuracy) was employed to measure the inlet air velocity. The air velocity measurements were done at different positions for obtaining an average value. 3. Definition of the TES performance indexes 3.1. The working period For a traditional transport air conditioning system, the supply air temperature is required to be in the range of 16-20oC. Therefore, the working period, t, is defined as the period when the air outlet temperature of the TES device is between 16 and 20oC during the discharging process. 3.2. The discharging power The discharging power of the TES device, Wa, is defined as the amount of cold energy absorbed by the air, and can be calculated as follows: t  t  Wa  ma   H a ,in   H a , out  0  0 

(1)

where, ma is the average air mass flow rate; t is the discharging time; Ha,in and Ha,out are the inlet and outlet air enthalpies, respectively. 3.3. The discharging thermal efficiency The total energy absorbed by the air during discharging, Qa, is calculated by Eq. (2), t

Qa   Wa dt 0

(2)

whereas the total energy released by the PCM, QPCM, is calculated by Eq. (3): QPCM  mPCM c p , s Tm,low  Ti  +H PCM  c p ,l Te  Tm,high 

(3)

where the first part is the subcooling capacity, the second part is the latent heat capacity and the third part is the superheating capacity; the mPCM and ΔHPCM are the mass and latent heat of the PCM, respectively; cp,l and cp,s are the specific heat capacities at the liquid and solid states respectively; Te and Ti are respectively the end temperature and initial temperature of

8

the PCM. The Tm,low and Tm,high are the lower (17 oC) and upper (19 oC) bounds of the PCM melting temperatures. The total energy released by the aluminium of the TES device, QAl, can be calculated by Eq. (4):

QAl  mAl c p , Al (Te  Ti )

(4)

where, mAl and cp,Al are the mass and specific heat capacity of aluminium, respectively. The discharging thermal efficiency, ƞth, is therefore defined by Eq. (5): th 

Qa QPCM  QAl

(5)

3.4. The discharging exergy efficiency The specific flow exergy of the moist air, ea, is given by Eq. (6): ea  (c p , a  c p ,v )T0 (

Ta ,out -Ta ,in T0

 ln

Ta ,out Ta ,in

)

(6)

where, cp,a and cp,v are respectively the specific heat capacities of dry air and vapor; T0 is the ambient temperature; Ta,in and Ta,out are respectively the air temperatures at the inlet and outlet of the TES device. The total flow exergy of the moist air, Ea, is given by Eq. (7): t

Ea  ma  ea dt

(7)

0

The total exergy released by the PCM, EPCM, is obtained by Eq. (8): EPCM  mPCM c p , s [(Ti  Tm,low )  T0 ln(

Ti Tm,low

)]  mPCM H PCM [(

T0 Tm, ave

)  1]  mPCM c p,l [(Tm, high  Te )  T0 ln(

Tm, high Te

)]

(8)

where, the first part is the subcooling exergy; the second part is the latent heat exergy, and the last part is the supheating exergy; and Tm,ave is the average melting temperature (18 oC). The total exergy released by the aluminium within the TES device, EAl, is calculated in the following:

9

E Al  mAl c p , Al T0 [(

Ti  Te T )  ln( i )] T0 Te

(9)

The discharging exergy efficiency, ηex, is therefore defined by Eq. (10): ex 

Ea EPCM  E Al

(10)

3.5. The discharging depth Given the air temperature requirement of 16-20oC, the discharging process will stop when the air outlet temperature of the TES device is equal or higher than 20oC. This can occur due to two cases – either or both of insufficient cold energy in the storage device and the heat transfer process do not allow the exit air to be cooled down the air to below 20oC. Considering that there may still be cold energy left, it is therefore of interest to understand the amount of cold energy remaining in the TES device. The discharging depth, D, defined as the ratio of exergy released for cooling the air to the exergy stored in the TES device, can be used as an indicator for this purpose, which is discussed in the following. The total exergy stored in the PCM, EPCM,store, is determined by: EPCM , store  mPCM c p , s [(Ti  T0 )  T0 ln(

Ti )] T0

(11)

The total exergy stored by the aluminium (frame of the energy storage device), EAl,store, is calculated by: E Al , store  mAl c p , Al [(Ti  T0 )  T0 ln(

Ti )] T0

(12)

The discharging depth, D, is therefore given as: D

EPCM  E Al EPCM , store  E Al , store

10

(13)

3.6. Uncertainty analysis In this study, the uncertainty of the working period comes from the measuring error of the thermocouples. Through the calibration carried out by the supplier recently, the accuracy is 1%. The uncertainly of the discharging depth and power, discharging thermal efficiency and discharging exergy efficiency is caused by the measuring accuracy of the thermocouples, relative humidity sensors and velocity meter. Theses sensors have been calibrated and show a 0.3%, 3% and 1% measuring error, respectively. In order to avoid the operating error of the experimental work, we repeated the experiments for three times. Particularly, for measuring the inlet and outlet air velocity, we divided the cross-section of the duct into 20 equally sized measurement areas, with the measurement position being in the centre of each area. The air velocity is the average number of the 20 measuring values. The uncertainty analyses were performed by using a method described by Moffat [36] who derived the following equation for the estimation of the overall uncertainty of measured parameter, δR, as a function of uncertainty of variables: R 

2

 R  2    X n   n 1  X n  N

(14)

By using the above Eq. (14), the overall uncertainties of the working period, the discharging depth, and the discharging power were estimated to be 6.47%, 9.25% and 8.28%, respectively. The overall uncertainly of the discharging thermal efficiency and discharging exergy efficiency were found to be 8.98% and 8.64%, respectively. These results indicate a good reliability of the experiment results. 4. Results and Discussions In a typical experiment, the TES device was charged by flowing the cold air flow from the air conditioner, which has a temperature of approximately 11oC. The air flow from the TES outlet was vented to the environment. The air temperature from the outlet of the TES was 11

monitored for controlling the charge process, which was stopped when there is no change to the air outlet temperature. The discharging process was then started with all the sensors and data-logging unit checked. The discharging process involved flowing an air through the TES device with the air temperature controlled by mixing the cold air from air-conditioner and hot air from the air heater in the chamber. The inlet air from the chamber was cooled down when flowing through the TES device. To study the performance of the TES device, three different inlet air temperatures (25, 28 and 30 °C) and velocities (0.70 m/s, 0.85 m/s and 1.2 m/s) were examined. 4.1. Time evolution of PCM temperature Fig. 4 shows the time dependence of PCM temperatures at two depths of 50mm and 250mm from the TES device surface in the air flow direction for an air temperature of 25oC and air flow velocity of 0.85m/s. The results at the 250mm depth are shown in Fig. 4(a). One can see that, with time, the first layer (T6), second layer (T7), third layer (T8) and fourth layer (T9) start melting in turn. In less than 25 s, the first layer starts to melt, whereas the other layers remain in a sub-cooling state. The second layer, third layer and fourth layer start to melt at ~150, ~225 and 315 s, respectively. The first layer completes the melting process, reaching the superheating state at ~660 s, whereas it takes ~1525 s, ~1780 s and ~2150 s respectively for the second layer, third layer, and fourth layer to reach the superheating states. Similar results are observed for the 50mm depth, as shown in Fig. 4(b). Fig. 5 shows the time dependence of PCM temperature in the direction perpendicular to the air flow direction at two layers. In the figure, the thermocouples T6 (8), T10 (12), T14 (16), T18 (20) and T22 (24) are located at the depth of 250, 200, 150, 100 and 50 mm, respectively. One can see that the PCM at the bottom part (T22 and T24) takes a longer melting time, compared with that in other positions. This may be associated with convective flow in the air

12

flow channel with cold air going downwards and hot air going upwards due to density difference, making the PCM at the bottom melts slower. 4.2. Outlet air temperature and working period In transport field, such as rail carriages, the desired supply air temperature is in the range of 16-20 oC. Hence, the outlet air temperature will be a key parameter to evaluate the performance of the energy storage device. As shown in Fig. 6, the outlet air of the TES device could reach the desired temperature (16oC) in seconds for all the studied inlet air temperature and velocity conditions, illustrating good heat transfer performance of the TES device. With time, the outlet air temperature is seen to increase first quickly from 16 to 17oC, then very slowly from 17 to 19 oC, and, finally quickly again to reach the inlet air temperature. Both initial and final fast increases in the temperature is mainly due to the release of a small amount of sensible heat in the sub-cooling (solid) and sup-heating (liquid) regions (specific heat capacity ~2kJ/kg·K over a small temperature difference), respectively. The slow increase is due to the latent heat of the PCM in the phase change region (220 kJ/kg). At a given air velocity, the outlet air temperature reaches 20oC more quickly at a higher inlet air temperature. This is due to greater temperature difference between the air and PCM, leading to a faster heat transfer. Given the inlet air temperature, a shorter duration is needed for the outlet air temperature to reach 20oC at a higher air velocity. This is mainly due to an increased heat transfer coefficient due to the increased air velocity. The results indicate that the TES device could supply a stable air temperature of ~18oC for ~1 hour under suitable inlet air temperatures and/or air velocities, thus enabling an easy control to achieve thermal comfort of passengers. The working period is an indicator of the length the TES device could supply air at the suitable air temperature range. Fig. 6 shows the working period under different air inlet temperatures and velocities. One can see that the higher the inlet air temperature or the 13

velocity, the lower the working period. For a given air velocity of 0.7 m/s, the working period decreases from 3080 to 1775 s when the inlet air temperature increases from 25 to 30oC. This is because a higher inlet air temperature gives a greater temperature difference, leading to faster consumption of the cold energy stored in the TES device. At an inlet air temperature of 30oC, the working period decreases from 1775 to 815 s when air velocity increases from 0.7 to 1.2 m/s, which results mainly from the increase in the heat transfer coefficient and air mass flow rate. The obtained working periods under various conditions can be used as the design guideline of the system operation strategy. 4.3. Discharging depth Fig. 7 shows the discharging depth at different inlet air temperatures and velocities. One can see that the discharging depth is above 97% under all investigated conditions, indicating a high heat transfer performance of the TES device. At a given inlet air velocity of 1.2 m/s, the discharging depth slightly decreases from 98.1% to 97.10% when the air temperature increases from 25oC to 30oC. Such a trend is also noticed when the inlet air velocity is increased at a given inlet air temperature. This is because an increase in either the inlet air temperature or the air velocity increases the air heat load. However, the change in the discharge depth is small under the conditions investigated in this study. These indicate the high heat transfer performance of the TES device that can work efficiently over a fairly wide range of operating conditions, providing relatively high tolerance for the design of the integrated PCM based TES device with the air-conditioning system. 4.4. Discharging power The discharging power is of great importance to the TES device, which reflects the cooling dynamics of the TES device. Fig. 8 shows the transient discharging power during the working period at different inlet air temperatures.

14

At inlet air temperatures of 28 and 30oC, the average discharging powers are respectively 1.10 and 1.30 kW; see Fig. 8(a). A higher inlet air temperature gives a higher discharging power. The air velocity also has a large effect on the discharging power, as shown in Fig. 8(b). For example, at an inlet air temperature of 28oC, the average discharging powers are 1.10, 1.20, and 1.70 kW respectively at air velocities of 0.7, 0.85 and 1.2 m/s. A larger air velocity can exact more quickly the cooling energy. The results also indicate that the designed TES device has a flexibility in regulating the output cooling power, with a maximum of 2.6 kW and a turndown ratio of ~2.4 under the conditions of this study. The results also indicate that the PCM based TES can also be a backup on top of peak shaving for mechanical refrigeration systems. 4.5. Accumulated discharging energy The accumulated discharging energy during the working period as function of inlet air temperatures and velocities are shown in Fig. 9. It can be seen that, for a given air velocity, the higher inlet air temperature leads to a quicker rising rate of the accumulated discharging energy. For a given inlet air temperature, the higher inlet air velocity causes a quicker increasing speed of the accumulated discharging energy as well. Both the higher inlet air temperatures and velocities contribute to the higher heat transfer of the thermal energy storage device. The higher heat transfer leads to the quicker rising rate of the accumulated discharging energy. 4.6. Discharging thermal efficiency The discharging thermal efficiency provides an indicator for the thermal insulation performance of a TES device. Tab. 2 shows the results. It can be seen that the discharging thermal efficiency is high, ranging between 86.79% and 99.80%. For a given inlet air temperature, the higher the air velocity, the lower the thermal efficiency; for a given air velocity, the higher the inlet air temperature, the lower the thermal efficiency. The higher 15

inlet air temperatures lead to a higher device temperature and a quicker temperature rising rate. This caused a higher heat loss of the device to the ambient which resulted in a lower thermal efficiency. 4.7. Discharging exergy efficiency The discharging exergy efficiency measures the irreversibility of energy losses due to temperature difference during heat transfer. Fig. 10 shows the influences of air velocity and temperature on the exergy efficiency. Given the inlet air velocity, the discharging exergy efficiency drops with the increase of the inlet air temperature. At a given inlet air temperature, the higher the air velocity, the lower the exergy efficiency. The trends of the exergy efficiency with the varied inlet air temperatures and velocities are similar with those of the thermal energy efficiency. The higher heat loss to the ambient under higher inlet air temperatures and velocities determined the trends. 5. Conclusions This paper concerns a compact thermal energy storage (TES) device containing a phase change material (PCM) for transport air-conditioning applications. The PCM based device used two different types of fins, serrated fins in the air side and perforated straight fins in the PCM side, for enhancing the storage device performance. The focus of the work was on the discharging process of the compact device, which is more important for transportation applications. This was done by measuring PCM temperature as a function of time in both air flow direction and the direction perpendicular to the air flow. The processing of the measurement data gave the discharging time, discharging depth, discharging power, accumulated discharging energy, thermal efficiency and exergy efficiency under different inlet air temperatures and velocities over the working period defined on the basis of the cooling requirement. The main conclusions are summarized in the following: 

The PCM based TES device has a good flexibility for the regulation of the output 16

cooling power with a turndown ratio of ~2.4, thus enabling the smoothing the frequent fluctuations and peak shaving of the cooling load of the transport compartments. 

Hot inlet air can be cooled down by the PCM based TES device to the desired temperature range of 16-20oC in seconds, demonstrating a very fast response. This enables the TES device to be compatible with the transport air conditioning system;



The outlet air of the TES device can be stabilized at ~18oC for ~1 hour, suggesting that the PCM-based TES can provide a back-up / emergency services.



Both the higher inlet air temperature and velocity leads to the higher discharging power and accumulated discharging energy. This is due to that the higher inlet air temperatures and velocities contribute to the higher heat transfer of the TES device.



The discharging thermal and exergy efficiency can reach up to 99.80% and 43.6% which indicates the high heat transfer performance of the designed device. Overall, both the drop of the inlet air temperature and the rise of the inlet air velocity contribute to the discharging efficiency;



The PCM based TES device has a high thermal performance with a discharging depth higher than ~97%.The obtained discharging depth and working time can be used as the guideline of the operation strategy, which is expected to improve the system efficiency.

In summary, the designed energy storage device is feasible to be applied on the air conditioning system to meet the fast-changing cooling demand and improve the thermal performance of compartments. According to the results, prototypes have been designed and applied in commercial applications.

17

Acknowledgments The authors would like to acknowledge the financial support provided by UK EPSRC projects under EP/S016627/1, EP/N021142 and N000714/1 and FCO GPF on Cold Chain Technologies.

18

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Figures: Fig .1 Inner structure of the PCM based TES device. Fig. 2 A schematic diagram of the experimental rig for performance measurements of the PCM based TES device. Fig. 3 The locations of the thermocouples in the TES device: (a) Top view and (b) cross sectional view (unit in mm). Fig. 4 PCM temperature as a function of time on stream at an inlet air temperature of 25oC and an air velocity of 0.85 m/s (a: Depth 250 mm, b: Depth 50 mm). Fig. 5 Time dependence of PCM temperature perpendicular to the air flow direction at inlet air temperature of 25 °C and a velocity of 0.85 m/s (a: Layer 1, b: Layer 3). Fig. 6 Time dependence of outlet air temperature at different inlet air temperatures and velocities. Fig. 7 Discharging depth at different inlet air temperatures and velocities. Fig. 8 Discharging power during the working period as function of inlet air temperature and velocity (a: 0.70 m/s; b: 0.85 m/s; c:1.20 m/s). Fig. 9 Accumulated discharging energy during the working period as function of inlet air temperature and velocity (a: 0.70 m/s; b: 0.85 m/s; c:1.20 m/s). Fig. 10 Discharging exergy efficiency at different inlet air temperatures and velocities.

Tables: Tab.1 Thermal-physical properties of RT18 HC for the experiments. Tab.2 Discharging thermal efficiency of the TES device.

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Figures:

Fig .1 Inner structure of the PCM based TES device.

Fig. 2 A schematic diagram of the experimental rig for performance measurements of the PCM based TES device.

25

b

a

Fig. 3 The locations of the thermocouples in the TES device: (a) Top view and (b) cross sectional view (unit in mm).

b

a

Fig. 4 PCM temperature as a function of time on stream at an inlet air temperature of 25oC and an air velocity of 0.85 m/s (a: Depth 250 mm, b: Depth 50 mm).

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a

b

Fig. 5 Time dependence of PCM temperature perpendicular to the air flow direction at inlet air temperature of 25 °C and a velocity of 0.85 m/s (a: Layer 1, b: Layer 3).

Fig.6 Time dependence of outlet air temperature at different inlet air temperatures and velocities.

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Fig. 7 Discharging depth at different inlet air temperatures and velocities.

b

a

28

c

Fig. 8 Discharging power during the working period as function of inlet air temperature and velocity (a: 0.70 m/s; b: 0.85 m/s; c:1.20 m/s).

b

a

c

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Fig. 9 Accumulated discharging energy during the working period as function of inlet air temperature and velocity (a: 0.70 m/s; b: 0.85 m/s; c:1.20 m/s).

Fig. 10 Discharging exergy efficiency at different inlet air temperatures and velocities.

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Tables: Tab.1 Thermal-physical properties of RT18 HC for the experiments. Latent heat

220 kJ/kg

Melting range (main peak)

17-19°C (18°C)

Congealing range (main peak)

19 -17°C (17°C)

Specific heat capacity (10°C)

2 kJ/(kg·K))

Tab.2 Discharging thermal efficiency of the TES device. Air velocity (m/s) Inlet air temperature (oC)

0.70

0.85

1.20

25

99.47%

91.15%

86.79%

28

99.80%

90.40%

88.60%

30

96.30%

95.56%

89.90%

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Highlights • A fast response with the hot air flow cooled down to the desired temperature range of 1620oC in seconds. • The discharging energy efficiency can reach up to 99.8%. • The discharging exergy efficiency varies between 40.3% and 43.3%. • The discharging depth is above 97.0%.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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