Experimental investigation on thermal behavior of graphene dispersed erythritol PCM in a shell and helical tube latent energy storage system

Abstract

Thermal conductivity of the Phase Change Materials (PCMs) of latent heat storage systems is enhanced by dispersing nanoparticles in base PCM for increased heat transfer rate. Heat transfer characteristics of the newly developed erythritol PCM dispersed with 1 wt% graphene nanoparticles in a newly designed shell and helical tube storage tank during charging and discharging processes was investigated. Both melting and solidification fronts progressed from the outer wall of the shell towards the axis on either side of the axis of the shell due to the helical tube arrangement. At the middle and near the axis of the storage tank, NDPCM melting time was decreased by 21% when inlet temperature of the hot therminol oil was increased from 160 °C to 180 °C and by about 30% when the oil flow rate was increased from 0.5 kg/min to 2 kg/min. Further, NDPCM solidification time was reduced by 11% when the cold therminol oil inlet temperature was decreased from 45 °C to 30 °C and by 20% when the oil flow rate was increased from 0.5 kg/min to 2 kg/min. Complete charging and discharging periods of NDPCM was reduced respectively by 20% at an inlet temperature of 180 °C and by 6% at an inlet temperature 30 °C for 1 kg/min flow rate of therminol oil compared with pure erythritol. This research study confirmed that the helical tube flow of heat transfer oil facilitated more uniform and quicker phase transition of PCM and graphene nanoparticles dispersed erythritol (NDPCM) had superior heat transfer behavior as compared to base erythritol and it can be utilized as a potential PCM for medium temperature thermal energy storage applications.

Introduction

The increasing energy demand and decreasing conventional energy sources push the world to exploit new and renewable energy sources, and to conserve or store energy in every possible means; it amounts to saving equivalent amount of energy from any energy sources. Latent heat storage is one of the thermal energy storage methods which uses phase change materials (PCM) that can store/release a high amount of latent heat during the charging/discharging process [1]. The use of a latent heat storage system is an effective way for storing thermal energy and has the advantages of high-energy storage capacity with less volume and constant temperature heat storage process compared to sensible heat storage systems [2]. Solid-liquid PCMs had been widely applied in LHTES systems to store waste heat and adjust environmental temperatures. The use of solid-liquid PCMs in LHTES systems is regarded as an effective way to contribute energy efficiency solutions to solve the energy crisis [3]. Heat transfer rate is an important factor to determine the efficiency of LHTES applications like solar energy system, buildings, cooling system, textiles and heat recovery system. The enhancement of thermal conductivity is an effective way to improve the overall performance of LHTES [4]. Studies of medium temperature latent heat thermal energy systems to be applied for hot-side thermal energy storage systems are few [5]. Though PCMs offer high energy density, they suffer from slower rates of melting and solidification due to low thermal conductivity [6]. The various techniques used to enhance the thermal performance of the LHTES system are [7] a) using extended surfaces, b) employing multiple PCM methods, c) thermal conductivity enhancement by high conductivity materials, d) micro-encapsulation of PCM. In order to design and develop energy-efficient LHTES, the nanoparticles are used to increase the thermal conductivity and heat transfer rate [8]. The temperature response in paraffin PCM with open-cell metal foam was much higher and temperature distribution was more uniform compared to pure paraffin and it also can dramatically enhance the efficiency of latent heat thermal energy storage system [9]. Embedding metal foam into phase change materials can improve the temperature uniformity of PCMs in the shell and tube thermal energy storage unit [10]. The metal-foam-cored thermal energy storage unit can store more energy compared with the case of plate fin heat exchanger [11]. Erythritol is considered as an energy storage material in LHTES for medium temperature applications [12]. Erythritol is a sugar alcohol and can be characterized as a medium temperature PCM because it has a melting point (T_m) of 120 °C and a large latent heat [13]. Erythritol meets the charge and discharge temperature requirements of a PCM suitable to provide heat to LiBr/H₂O solar absorption cooling system [14]. The thermal stability of erythritol has proven to be excellent [15]. It was found that noon cooking did not affect the evening cooking, and the evening cooking using erythritol PCM heat storage was faster than noon cooking [16]. With the addition of 1% mass fraction of MWCNT, the thermal conductivity of erythritol was increased from 0.1956 W/(mK) to 0.9779 W/(mK). MWCNT also improved the supercooling and solidification enthalpy of erythritol significantly [17]. Cu/Erythritol, Al/Erythritol, TiO₂/Erythritol and SiO₂/Erythritol composites were investigated within the limit of 5% nanoparticle volume fraction. Amongst the composites, Cu/Erythritol composite was suggested based on the best gain in swift discharging operation with only some compromise on the storage capacity [18]. Erythritol–graphite foam as a stable PCM composite was obtained by incipient wetness impregnation method with very high impregnation ratio (75 wt%) and it was reported that the thermal conductivity of the erythritol–graphite composite foam (3.77 W/mK) was enhanced 5 times as compared to that of pure erythritol (0.72 W/mK) [19]. A new kind of PCM (0.4% copper nanoparticle + 99.6% erythritol) with latent heat of 362.2 kJ/kg encapsulated in stainless steel balls (radius of 60 mm, 80 mm and 100 mm) was experimentally examined, and it was found that the increase of ball radius may enhance the effect of heat convection [20]. Heat transfer rate increased in the case of charging as compared to the discharging process because of natural convection occurs in the molten layer of PCM [21]. Shaopeng Guo et al. [22] prepared erythritol with expanded graphite and erythritol with CNT as composite PCMs with additive mass rations of 1 wt%, 3 wt%, 5 wt% and 7 wt% by melting dispersion. The results revealed that the melting point of composites with EG continuously decreased with increasing mass ratio of additive due to the surface energy variation, but for the CNTs composites, it remained nearly constant. The latent heat of both composites gradually decreased as a function of mass ratio because of the replacement of erythritol by additives. The thermal conductivities of the composites also increased continuously with increasing addition of EG/CNTs. At the same mass ratio, EG appeared more effective than CNTs in enhancing the thermal conductivity, especially above 3 wt%. Graphene has the potential to outperform metal nanoparticles, carbon nanotubes, and other carbon allotropes as filler in thermal management materials [23]. The highest values of thermal conductivity of graphene were measured with the Raman optothermal technique compared with other carbon nanomaterials [24]. The supercooling, heat release ability and thermal reliability of encapsulated erythritol changed imperceptibly. The addition of thickening and nucleating agents can be used to suppress the supercooling and improve the thermal behavior of erythritol [25]. The thermal conductivity of the erythritol expanded graphite composites were greatly enhanced by EG, while the thermal stability of the m-Erythritol and D-mannitol eutectic mixture and the EG/eutectic composite could satisfy the demand for it to be applied as PCM [26]. The improved thermal properties, less mass change and only physical interaction make Myoinostol/graphene phase change composites a suitable candidate for solar thermal energy system in the temperature range of 100–260 °C [27]. Preparation of erythritol NDPCM with three different mass fractions of graphene nanoparticles i.e. (0.1, 0.5 and 1 wt%) and their characterization and thermal properties are presented and discussed in detail in our paper [28]. It was found that the addition of 1 wt % graphene in erythritol PCM led to 53.1% increase in thermal conductivity, highest enhancement among the three mass fractions with only 6.1% decrease in latent heat enthalpy. Hence, erythritol with 1 wt% graphene (NDPCM) was selected for further study.

Finding a suitable heat exchanger and material investigation are important for the design of LHTES system. Two types of heat exchangers are commonly used for LHTES, namely, direct-contact heat exchanger and indirect-contact heat exchanger. For the indirect-contact heat exchanger, the plate, shell-and-tube, and packed-bed types are typically used because other complicated forms of heat exchangers can be developed from these types. Shell-and-tube heat exchangers are mainly used in LHTES because of its simple design and relatively very low heat loss [29]. Wang et al. [30] experimentally investigated the melting and solidification behavior of erythritol in a shell and tube LHTES unit using air as the heat transfer fluid. The thermal conductivity of erythritol at solid-state decreases linearly from 0.76 W/mK to 0.68 W/mK but thermal conductivity at liquid state increases as air temperature increases from 15 °C to 60 °C. Natural convection plays an important role during melting and natural convection was enhanced as the molten PCM region and the temperature of the molten PCM increase. Jesumathy et al. [31] conducted an experimental study to investigate the melting and solidification processes of paraffin wax as a phase change material (PCM) in a horizontal double pipe heat latent heat storage unit using water as heat transfer fluid. Heat flow rate during the melting and solidification process increased by 25% and 11% respectively in the case of increase or decrease by 2 °C of the inlet heat transfer fluid temperature. The helical-coil heat exchanger (HCHE) might be a better choice in special applications like LHTES. Latent heat thermal energy storage performance of spiral coil tube with paraffin/expanded graphite high thermal conductivity composite (PCM) were numerically and experimentally studied. It was reported that the temperature difference between the PCM and HTF had a great effect on the performance of the system. Increasing the inlet temperature and the mass flow rate of the HTF during charging can obviously enhance the heat transfer in the PCM [32]. Kousha et al. [33] investigated numerically and experimentally the effect of the shell inclination angle during melting and solidification of Paraffin RT-35 as PCM and water as heat transfer fluid in a shell and tube heat exchanger. The inclination angle of shell had a more effective on the melting process compared to the solidification process. The heat transfer rate in the melting process was higher for the horizontal position compared with the tilted positions. In the current research work, shell and helical tube heat exchanger model was chosen as the storage tank and its orientation to be horizontal.

Thus, some theoretical, numerical, and experimental research studies have been conducted on the melting and solidification characteristics and thermal performance of LHTES systems. The time-wise variations of melting fractions, heat stored, heat transfer rate and efficiency were analyzed as it is important for the assessment and optimization of an LHTES unit [34]. A high inlet temperature leads to a better thermal dynamic of the charging and discharging processes of latent heat thermal energy storage in a coil in PCM storage LHTES [35]. To the best of our knowledge very few experimental investigations based on the thermal behavior of nanoparticle dispersed phase change materials in a shell and helical tube latent energy storage system are reported till date and this could be drawn as a gap in the contemporary research. In the present experimental study, a detailed investigation on the melting and solidification characteristics of erythritol/graphene (1 wt%) NDPCM with Therminol 55 (a synthetic heat transfer fluid used in moderate temperature applications) as heat transfer fluid in a horizontal shell and helical tube LHTES was conducted and the results are discussed in this paper. Further, the effect of operating conditions like inlet temperatures and mass flow rates of heat transfer oil on the heat transfer characteristics of erythritol and NDPCM were also investigated and compared.