Liquid Metal Gallium in Metal Inserts for Solar Thermal Energy Storage: A Novel Heat Transfer Enhancement Technique

doi:10.1016/j.solmat.2019.110365

Solar Energy Materials and Solar Cells

Volume 208, May 2020, 110365

https://doi.org/10.1016/j.solmat.2019.110365 Get rights and content

Highlights

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Thermal response of LHS unit laden with metal inserts with liquid metal gallium.
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Thermal investigation of D-Mannitol based novel TES unit.
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Enhanced Thermal Response without compromising the energy storage density.

Abstract

This paper investigates the heat transfer characteristics of a prototype latent heat energy storage (LHES) system with novel metal insert design configuration. The novel thermal energy storage (TES)system consist ofa vertical cylindrical shell, a helical coil and metalinserts (MI) withliquid metal gallium(Ga), designed for LHES capacity of ~13MJ. 25 kg of D-Mannitol(DM) is investigated as a phase change material (PCM) forenergy storage and property tests were conducted on DM. PCM was cycled 1000 times and checked for suitability for long term energy storage applications. Results confirmed that the addition of MI with Ga enhanced the thermal performance of the TES system. Moreover, the vertical orientation of the shell as well as the metal inserts supported natural convection during charging cycles and acted as a nucleation sites during discharging cycles, thereby assuring rapid charging and discharging of the TES system. High thermal conductivity, low specific heat of Ga and its liquid phase atroom temperatures helped Ga act as thermal energy carriers in TES unit. Maximum power output of 0.64 kW was obtained during solidification cycle and efficiency range of 87–89% for MI configuration. The presented novel LHES design can be used with a wide range of PCM and over a different temperature range of applications, mainly for water heating, high temperature industrial waste heat recovery and solar thermal applications.

Graphical abstract

Introduction

Energy consumption in the world has increased gradually and significantly due to the ever-expanding demand for human activity. In this light, to decrease the gap between energy demand and supply, and to enhance the energy efficiency of current system, thermal energy storage (TES) is a promising candidate [[1], [2], [3]]. Latent heat energy storage is a significant part of TES due to its ability to store large amounts of energy at a constant temperature. Phase change material (PCM) undergo solid-liquid, solid-solid or solid-gas transformations which store huge amounts of energy during phase change at nearly isothermal temperatures [4,5]. Solid-liquid PCMs are widely used in practical application due to their availability over awide temperature range, high latent heat capacity and cheap costs. One of the drawbacks include volumetric expansion during phase change. Moreover, the lower thermal response of the PCM due to lower thermal conductivity values is a bigger challenge to researchers. Many efforts have been done to study the mechanism of heat transfer during solid-liquid phase changes [6]. It is therefore important to improve energy storage/retrieval, which in turn enhances the efficiency of the system. LHES can be broadly classified into low, medium and high temperature storage systems [7]. For low temperatures in the range of 20–90 ^οC, paraffin are the most desired PCM [8]. However, the low thermal conductivity (<0.4 W/m·K) is the drawback which limits its usage [9]. One of the methods to enhance the thermal conductivity of the PCM is by adding high thermal conductivity materials to the PCM. The additives vary from micron to nano sizes and are commonly used in the form of metal oxides, carbon based and pure metal particles [[10], [11], [12], [13], [14]]. However, the addition of these additives leads to a decrease in the latent heat capacity of the PCM. Sari et al. [15] investigated the effect of paraffin with expanded graphite (EG) as a phase change composite and found that the composite showed very good thermophysical properties with stable chemical properties, enhanced thermal conductivity and satisfactory latent heat storage system. Ranjbaret al. [16] studied the heat transfer behaviour in a 3D cavity setup with the addition of nanoparticles. The results showed a substantial increase in heat transfer after the addition of the particles. Hosseinizadeh et al. [17] investigated the melting of nano-enhanced phase change materials inside a spherical container. The PCM used was paraffin laden with copper particles as thermal conductivity enhancer. Results showed that with increase in thermal conductivity of the NEPCM and decreased latent heat, the melting rate of the NEPCM was very high as compared to the case where pure PCM was used. Rabienataj et al. [18] studied the impact of heat transfer tube eccentricity on the performance of the LHES. Melting behaviour of PCM was investigated using a numerical model with shell and tube configuration. Results were compared with concentric and eccentric type heat transfer tubes. The heat exchanger with eccentric configuration wherein the tube was shifted below the axis, showed greater melting performance due enhanced natural convection. Ismail et al. [19] studied the enhancement of heat transfer during solidification using axial fins. They studied the effect of different fin height and fin thickness on the performance of TES. Hosseini et al. [20] conducted a comparison of experimental and numerical investigation of TES and studied the effect of change of inlet temperature on the theoretical efficiency of the heat exchanger. From the results it was seen that by increasing the inlet temperature from 70 °C to 75 °C, the efficiency of the system improved from 81.1% to 88.4% respectively. Vyshak et al. [21] analysed with numerical model the effect of different shape configuration of LHES with the same volume of PCM and the same surface area of Heat transfer. The authors presented the variation in melting time of the PCM in rectangular, cylindrical, shell and tube configurations. From the results obtained they concluded that the cylindrical configuration performed the best as compared to the other two types and the effect of shape was more relevant when the mass of PCM was increased. Adine et al. [22] investigated the use of multiple PCM with different melting points in a heat exchanger. They compared the performance of the latent heat storage unit with single PCM system and two PCM systems. Results showed that the multiple PCM systems works more efficiently when the mass flow rate is in the lower range. Gerard et al. [23] evaluated the concept of multiple phase change materials configuration using PCM with phase transition temperature in the range of 150–200 °C. D-Mannitol and hydroquinone is used as PCM with medium range of phase change temperatures. Tests were conducted with single PCM as well as with multiple PCM in a pilot plant scale setup and results showed that the multiple PCMs configuration enhanced the effectiveness by19.6% as compared to the single PCM configuration. Moreover, there was better uniformity between the HTF temperature difference between inlet and outlet. Rahimi et al. [24] conducted the thermal enhancement study of TES by using fin configuration. Experiments were conducted at various operating conditions and investigation showed that regardless of flow regime, the inclusion of fin enhanced the heat transfer and change in fin pitch did not affect the performance of the TES majorly. Esapour et al. [25] numerically investigated multi tube latent heat storage systems for different geometrical and operational parameters. From the results, it was concluded that as the number of tubes increased inside the shell, the performance of the TES also increased. The design where the tubes occupy the upper part of TES takes the maximum time for melting while with the same inner tube numbers, in the lower part of the TES, melted faster. Zakir et al. [26,27] experimentally investigated the charging/discharging cycle of PCM with a novel design, longitudinal fin tube with paraffin as thermal energy storage material. Results showed that vertical arrangement of longitudinal fins supported the natural convection in PCM which enhanced the charging of TES effectively. The mean power increased by an amount of 50% and 69% as the temperature during charging is changed to 67 °C from 52 °C. Similarly, the power enhanced by 39.05% during solidification as inlet temperature was changed to 5 °C from 15 °C. Ahmad et al. [28] studied the suitability of using D-Mannitol as a PCM for medium temperature TES systems. While characterization using DSC showed DM to have a high enthalpy of phase change of 297 kJ/kg with sufficient amount of subcooling. On the contrary, experimental results in large scale TES showed that the PCM solidified above 150 °C which suggests possibility of usage as PCM with very high latent heat capabilities.

From the literature survey, it is reviewed that most of the research work was concentrated more on enhancing the thermal performance of TES mostly by two methods (1) By adding high thermal conductivity additives to PCM (2) By incorporating fins to increase the heat transfer area between the HTF and PCM. However, in the former case, it is seen that the enthalpy of PCM is comprised by addition of particles while the later increases the weight of the TES and complexity of assembly and maintenance of the heat exchanger.

In this work, the aim is to conduct a series of experiments with DM, to draw an effective conclusion about the performance and applicability of DM as PCM. Thermo physical property study is done using various characterization techniques and the effect of thermal cycling on DM is discussed in this paper. Moreover, a novel TES design is proposed wherein gallium in metal inserts is assembled inside the TES and the effects on performance of TES is tested. Thermal behaviour in macro scale TES system is studied and the feasibility of using the novel design TES with any PCM available, for any temperature range of operation is investigated for real world conditions.

Section snippets

Material purchased

D-Mannitol (99% Pure) was supplied by SC limited, India. Gallium, 99.97% pure, was supplied by CDH Limited, India. All samples were reagent analytical grade and no further processing was done before use. The pictorial view of materials used in this project is shown in Fig. 1.

CharacterizationTechniques

The SEM technique (Make: Vega 3 Tescan) was used to study the morphology of the samples. Polymorphic structure of the DM was studied using XRD diffraction technique and comparison of the results with JCPDS. The XRD was

PCM characterization

Fig. 3a shows the SEM image of DM. From the images it can be seen that DMis very irregular in shape in microstructure level. Fig. 3b showsthe XRD pattern of pristine DM. Two major peaks are seen at 23.71 and 19.08. By comparing the data obtained from JCPDS (22–1797), the β phase polymorphic state can be inferred. This is the most stable phase of DM.

Fig. 3c shows the TGA data for uncycled and 1000 thermal cycled DM. DM was stable up to 300 °C with a mass loss of less than 2%. One step mass loss

Conclusion

In this paper, the thermal performances of novel TES unit with metal inserts were experimentally investigated at various operating conditions using D-Mannitol as phase change material. Liquid metal gallium in metal inserts were assembled in the TES to check its effect on the thermal performance of TES. Based on experimental results, the following conclusions were derived.

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D-Mannitol showed a latent heat capacity of 285 kJ/kg and has a melting temperature range of 165–168 °C. After 1000 thermal

Acknowledgment

This work is sponsored by DST-SERI, Government of India, Ref. No:DST/TM/SERI/DSS/275(G); dated September 9, 2015. In addition, the authors would like to acknowledge the funding acquired from UKIERI-DST (IND/CONT/GA/18-19/16), which made this research possible.

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Liquid Metal Gallium in Metal Inserts for Solar Thermal Energy Storage: A Novel Heat Transfer Enhancement Technique

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Material purchased

CharacterizationTechniques

PCM characterization

Conclusion

Acknowledgment

Prog. Mater. Sci.

Nano Energy

J. Clean. Prod.

J. Clean. Prod.

Sol. Energy

J. Clean. Prod.

J. Taiwan Inst. Chem. Eng.

Mater. Des.

Renew. Sustain. Energy Rev.

Renew. Sustain. Energy Rev.

Thermochim. Acta

Renew. Sustain. Energy Rev.

Energy Convers. Manag.

Appl. Therm. Eng.

Int. J. Therm. Sci.

Appl. Math. Model.