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Experimental study using different heat transfer fluid of a packed bed thermal energy storage system during charging process.

1. Introduction

The purpose of the thermal storage applied to chilled water facilities in HVAC systems is to obtain a rationalisation of energy consumption, either by reducing the demand installed and / or the possibility of shutting down water chillers during peak hours. The main advantages of thermal storage are reducing the installed capacity of the chilled water system as well as reduced spending on electricity rates. Another advantage of the system is to allow the shutdown of the chillers, at peak hours, from 18:00 to 21:00 h, leaving only the circulation cold water pumps connected. All cold water during this time is provided by thermal accumulation tank. The three basic types for cooling storage are chilled water, ice and eutectic salts. For sure, there are numerous methods of ice storage with advantages and disadvantages and with consequences in the efficiency of cool storage systems. The performance of thermal ice storage systems is a function of instantaneous loads and the temperatures required. As with all heat exchangers, the log mean temperature difference. In reality, thermal storage systems are similar to large heat exchangers except that the surface area for heat transfer changes as ice melts or as ice is formed.

In this paper, the key controlling variables are: the mass flow rate and the inlet temperature of heat transfer fluid (HTF). According to Bhatia (2010), the encapsulated ice systems are consisted of water contained in plastic containers surrounded by coolant, all contained within a tank or other storage vessel. During the charging cycle subfreezing coolant from a chiller is circulated through the storage tank and the plastic containers, freezing the ice. Discharge is accomplished by circulating warm coolant again through the tank and the containers, melting the ice. The coolant may be routed directly to the load or be isolated from the load via a heat exchanger. The most common form of plastic container is a dimpled ball about 4 inches in diameter. The spherical shape creates a relatively high heat transfer area per unit of water being frozen, while the dimples allow for expansion and contraction while cycling between liquid and solid states.

Cool storage technology can be used to significantly reduce energy costs by allowing energy intensive, electrically driven cooling equipment to be predominantly operated during off-peak hours when electricity rates are lower. In addition, some system configurations may result in lower first costs and/or lower operating costs, Federal Energy Management Program (2000).

Kousksou, Bedecarrets and Dumas Mimet (2005) presented dynamic modelling of the thermal storage of an encapsulated tank. The performance of the cold storage depended on coolant flow rate inlet temperature and the position of the tank as well as the supercooling phenomena. Ismail, Henriquez and da Silva (2003) investigated the study on ice formation inside a spherical capsule where the time for complete solidification increases with the increase of the external fluid temperature. In addition, the thermal conductivity of the capsule material has a strong effect on the time for complete solidification and the solidified mass fraction. In the review by Abdel-Rehim (2011), the author introduced a heat transfer analysis in a packed bed phase change material (PCM) capsules latent heat thermal energy storage system. The conclusions presented by the author, shows that the charging and discharging rates are significantly higher for the capsule of a smaller radius compared to those of a larger radius. Also, the complete solidification time is longer compared to the melting time.

The use of spherical capsules to encapsulate the PCM in air conditioning applications has been analysed by Fang and Shuangmao (2010). Studied experimentally the operation characteristics of cold storage air-conditioning systems with spherical capsules packed bed. The spherical capsules with outside diameter of 100mm and wall thickness of 1mm were filled with water. The experimental results indicated that the cold storage air-conditioning systems with spherical capsules had a better performance and could work during the charging and discharging period. Arnold (1990) analysed the heat transfer during the freezing and melting process by performing a series of charging and discharging experiments.

ElGhnam et al. (2012) presented the results of an experimental study on the heat transfer during freezing (charging) and melting (discharging) of water inside a spherical capsule often found in the beds of thermal (ice) storage systems used for the building air conditioning systems with spherical capsules of different diameters and materials are tested. Bedecarrats et al. (2009) demonstrated an experimental investigation of the performance of an encapsulated phase change energy storage during the charging and the discharging processes. The spherical capsules, containing water with a nucleation agent as a PCM, fill the thermal storage tank.

Assis et al. (2007) conducted experimental and numerical study on melting process in a spherical shell. They provided a correlation for the melting fraction versus an appropriate combination of the Grashof, Stefan and Fourier numbers. Also, they performed another combined experimental and numerical study on the solidification of PCM inside a spherical shell with various diameters Assis, Ziskind and Letan (2009).

Medrano et al. (2009) experimentally studied heat transfer characterisation of five small heat exchangers working as latent heat thermal storage systems during the charge and discharge processes. The results indicated that the double-pipe heat exchanger with the PCM embedded in a graphite matrix had the highest values. Erek and Ezan (2007) performed numerical and experimental study for assessing the effects of thermal parameters on the storage performance of an ice-on-coil energy storage system.

Mueller (2005) has developed a numerical model, which allows reasonable predictions of bed porosity when looking at large cylinders packed with spherical capsules. Eames and Adref (2002) studied experimentally the characteristics of heat transfer for water contained in spherical elements in both charging and discharging processes and described a novel method to measure interface position during solidification. Panesi (2015) developed a model for simulation of the process of heat transfer of a latent heat storage system of packed bed of spherical capsules filled with PCM. The author applied the method using a finite difference approach and moving grid technique. The numerical grid was optimised and the predicted results were compared with experimental results to establish the validity of the model. The working fluid entry temperature, the mass flow as well as the capsule temperature were investigated.

2. Materials and methods

A schematic diagram of the experimental set-up is shown in Figure 1.

The thermal storage tank is made of steel of 200 mm internal diameter, 0.8 m in length and 3 mm thick, insulated by a 20-mm thick layer expanded polyurethane. The tank contains spherical capsules of 36 mm external diameter, 1 mm thickness and arrayed internal so that the porosity of the tank filling is 0.53, where porosity or void fraction is a measure of the void 'empty' spaces in a material, and is a fraction of the volume of voids over the total, between 0 and 1 or as a percentage between 0 and 100% and depends on the number of spheres and the diameter of the tank. In practice, the maximum porosity for an ice ball storage system is about 52-60%. Three sphere layers are placed at the bottom at entry of the HTF at mid-height of the tank and at the top of the tank near exit. Each sphere is fitted with a T-type thermocouples localised at the sphere centre. All those thermocouples are calibrated with precision of [+ or -]0.5 [degrees]C and are connected to data acquisition system which delivers the signals to a designated PC.

The primary refrigerant (R22) from the refrigeration machine is allowed to circulate through the coil pipes of the secondary fluid circuit submersed into the tank filled with HTF, where it is cooled until it reaches the required temperature. When the HTF reaches the prescribed temperature, the entry to the storage tank is open to receive it, circulated by the pump. This process is continued until the thermocouples in the three reference spheres indicate the same temperature without any change with time. The HTF is then forced to circulate into the storage tank from the bottom to the top by a designated pump. The flow is controlled by a control valve and measured by a calibrated orifice plate connected to a pressure transducer. The temperature of the HTF is measured by a calibrated thermocouples type-T. The HTF is permitted to circulate until the temperature along the storage tank axis reaches the pre-established value, then the storage tank is considered charged and the HTF is shut down.

Experiments were realised for flow rate: 1.36, 2.24 and 2.75 kg/s, temperatures: -3, -6 and -10 [degrees]C. The experimental conditions are presented in Table 1, while Table 2 shows a summary of estimated uncertainties of the measured quantities.

3. Results and discussions

As shown in the abstract the main objective of this paper is to compare the loading time using as HTF ethylene glycol and ethanol, with experimental observation and measurement related to the thermal performance of an encapsulated thermal storage system by varying the inlet HTF temperature and HTF flow rate. Also, identified through top capsule control, the time of complete charging of the storage system. Through the present method, the experimental results concerning the effect of the variables on the time for complete charging rate are analysed, compared and discussed.

3.1. Effect of inlet working fluid temperature during charging process

Figures 2-4 show the variation of the temperature at the centre of the last sphere layer with time during the charging process under conditions of both HTF (ethanol and ethylene glycol 50%), and inlet HTF temperature ranging from -3 to -10 [degrees]C.

The typical curve of temperature change of pure water inside a capsule along with the time history during a cooling process is shown in Figures 2-4. Also shows that the lower the temperature of the HTF inlet, lower the time of the complete solidification of PCM. It is evident also at the end of the process, the temperature approximates to the initial temperature of the HTF. It can be seen also the lower HTF temperature can reduce the supercooling of water and the total charging time. In practical situations, a low coolant temperature to promote nucleation is often required in order to establish reasonable heat exchange rates during period of charging. As can be seen for inlet HTF temperature of -3 [degrees]C, did not occurred the nucleation and dendritic ice growth, since, the lower temperatures of the working fluid are responsible for stronger convection processes inside the capsules that decrease the solidification time and consequently inducing the occurrence of the nucleation phenomenon.

The effect of the entry temperature of the HTF is shown in Figure 5. It can be seen that the decrease of the HTF temperature from -3 to -10 [degrees]C reduces greatly the time for complete charging in the same way that for higher mass flow rates. The charging time is lower for ethanol when compared with ethylene glycol. This difference in charging time is due the high specific heat of ethanol, and a lower kinematic viscosity in the same way to ethylene glycol. Because the temperature of the HTF decreases as it moves through the thermal storage, heat rates to the capsules are the largest and the smallest at the entrance and exit, respectively, of the thermal storage.

3.2. Storage tank freeze rate

The importance of freeze rate in the process is that during ice-making mode, the ice storage tank is often the only load being served by the ice-making chiller and must be selected for a freeze rate that is fast enough to make the desired amount of ice in the time available for ice making. The freeze rate is defined by the ratio of ice storage capacity to time available for making ice. Therefore, In Figure 6 depicts the freeze rate of ice storage tank by ethylene glycol. When the entering fluid temperature is -3 [degrees]C, and the fluid flow rate is 1.36 kg/s, the freeze rate of this tank is 700 W. By reducing the entering fluid temperature to -6 [degrees]C, with the same fluid flow rate, the freeze rate increases to about 900 W. However, to achieve the equivalent, higher freeze rate with the original entering fluid temperature of -3 [degrees]C would require an increase more than double the fluid flow rate. In doing so, it is important that the inlet temperature be the lower as possible.

4. Conclusions

This paper investigated the thermal performance of a thermal storage tank utilising spherical capsules during charging process varying inlet HTF temperature, coolant flow rate and HTF type. In addition, the process of water cooling inside spherical capsules involves several phenomena, particularly super cooling and nucleation mainly in charging process. The following conclusions can be drawn from the results:

(1) The lower the inlet working fluid temperature and the larger the coolant flow rate, the faster the cold storage rate.

(2) The time of complete solidification in charging process depends on flow rate and inlet temperature, since that the lower inlet coolant temperature; the lower is the time for complete solidification.

(3) The solidification time was lower using ethanol as HTF for the same mass flow rate.

(4) After complete freezing begins to occur in the capsules, progressing downstream with increasing time, heat transfer to the HTF will decrease the temperatures of the capsules, thereby decreasing the heat rate.

(5) The freeze rate is bigger at low inlet temperatures of the HTF with the same fluid flow rate.

Notes on contributor

Ricardo Panesi is a professor in the Department of Thermal and Fluids Engineering, Federal Institute of Education, Science and Technology of Sao Paulo, working with thermodynamics, refrigeration and air conditioning, with 25 years of teaching experience in technical and higher education. For the moment, the author has have undertaken research on energy recovery, such as photovoltaics and mechanical automation.

References

Abdel-Rehim, Z. S. 2011. "Heat Transfer Analysis of a Packed Bed-PCM Capsules Latent Heat Thermal Energy Storage System." Energy Sources, Part a: Recovery, Utilization, and Environmental Effects 33 (14): 1326-1343.

Arnold, D. 1990. "Dynamic Simulation of Encapsulated Ice Stores-Part I: The Model" ASHRAE Transactions 97 (2): 1170-1178.

Assis, E., L. Katsman, G. Ziskind, and R. Letan. 2007. "Numerical and Experimental Study of Melting in a Spherical Shell." International Journal of Heat and Mass Transfer 50: 790-1804.

Assis, E., G. Ziskind, and R. Letan. 2009. "Numerical and Experimental Study of Solidification in a Spherical Shell." Journal of Heat Transfer 131 (2): 24502-24507.

Bedecarrats, J. P., J. C. Lasvignottes, F. Strub, and J. P. Dumas. 2009. "Study of a Phase Change Energy Storage Using Spherical Capsules. Part I: Experimental Results." Energy Conversion and Management. 50: 2527-2536.

Bhatia, A. 2010. Air Conditioning with Thermal Energy Storage. PDHengineer. https://pt.scribd.com/document/280149512/Air-Conditioning-with-Thermal-Energy-Storage-pdf.

Eames, I. W., and K. T. Adref. 2002. "Freezing and Melting of Water in Spherical Enclosures of the Type Used in Thermal (Ice) Storage Systems" Applied Thermal Engineering 22: 733-745.

ElGhnam, R. I., R. A. Abdelaziz, M. H. Sakr, and H. E. Abdelrhman. 2012. "An Experimental Study of Freezing and Melting of Water inside Spherical Capsules Used in Thermal Energy Storage Systems." Ain Shams Engineering Journal 3: 33-48.

Erek, A., and M. A. Ezan. 2007. "Experimental and Numerical Study on Charging Processes of an Ice-on-coil Thermal Energy Storage System"' International Journal of Energy Research 31: 158-176.

Fang, G., and W. X. L. Shuangmao. 2010. "Experimental Study on Cool Storage Air-conditioning System with Spherical Capsules Packed Bed" Energy and Buildings 42: 1056-1062.

Federal Energy Management Program. 2000. Thermal Energy Storage for Space Cooling Technology for Reducing on-Peak Electricity Demand and Cost. https://www.osti.gov/scitech/servlets/purl/770996.

Ismail, K. A. R., J. R. Henriquez, and T. M. da Silva. 2003. "A Parametric Study on Ice Formation inside a Spherical Capsule." International Journal of Thermal Sciences 42: 881-887.

Kousksou, T., J. P. Bedecarrets, and J. P. Dumas Mimet. 2005. "A. Dynamic Modeling of the Storage of an Encapsulated Ice Tank" Applied Thermal Engineering 22: 1705-1716.

Medrano, M., M. O. Yilmaz, M. Nogues, I. Martorell, J. Roca, and L. F. Cabeza. 2009. "Experimental Evaluation of Commercial Heat Exchangers for Use as PCM Thermal Storage Systems." Applied Energy 86: 2047-2055.

Mueller, Gary E. 2005. "Numerically Packing Spheres in Cylinders." Powder Technology 159: 105-110.

Panesi, Andre. 2015. "Numerical and Experimental Investigation of a Fixed Bed Latent Heat Storage System during Charging Processes." Australian Journal of Mechanical Engineering. 14: 2204-2253.

Ricardo Panesi

Department of Thermal and Fluids Engineering, Federal Institute of Education, Science and Technology of Sao Paulo, Sao Paulo, Brazil

CONTACT Ricardo Panesi [??] panesi@ifsp.edu.br

KEYWORDS

PCM spherical capsules; supercooling; thermal energy storage

ARTICLE HISTORY

Received 30 October 2015

Accepted 26 January 2017

https://doi.org/10.1080/14484846.2017.1290515
Table 1. Experimental conditions.

HTF flow  Inlet HTF       Inlet HTF     Inlet HTF
rate      T,([degrees]C)  [T.sub.2]     [T.sub.3]
(kg/s)                    ([degrees]C)  ([degrees]C)

1.36      -3              -6            -10
2.24      -3              -6            -10
2.75      -3              -6            -10

Table 2. summary of estimated uncertainties.

Parameter             Uncertainty

Temperature           [+ or -]0.505 [degrees]c
Flow rate             [+ or -]0.02%
Mass of the capsules  9.7 x [10.sup.-3%]
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Author:Panesi, Ricardo
Publication:Australian Journal of Mechanical Engineering
Date:Oct 1, 2018
Words:2871
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