Thermal and exergetic analysis of shell and eccentric-tube thermal energy storage
Introduction
With the rapid development of wind power, photovoltaic and solar thermal electric power generation in China, the total installed capacity of renewable energy has been ranked first in the world for many years [1]. However, due to the insufficient consumption of renewable energy, the phenomena of wind and solar power curtailment still exist, which brings serious challenge to power system economic operation. Thermal energy storage technology could store excess renewable energy in the form of heat. Then, the stored renewable energy can be used for household life and industrial production. Latent heat thermal energy storage (LHTES) technique employing PCMs is regarded as the most potential thermal storage technique in solar energy utilization [2], industrial waste heat recovery [3], building energy saving [4], electronics cooling [5], heat pump and air-conditioner [6,7]. Nevertheless, since most PCMs such as paraffin, hydrated salts and erythritol suffer from low thermal conductivity, numerous heat transfer enhancement techniques have been proposed including finned tube [8], [9], [10], [11], open-cell metal foam [12,13], expanded graphite [14], [15], [16] and multiple PCMs [17,18].
It is inevitable that the temperature of HTF reduces with the flow direction, which could be effectively tackled by multiple PCMs. To guide the selection of PCMs in multi-PCMs TES, Xu and Zhao [19] optimized multiple PCMs TES based on entransy and entropy theory. Zhao et al. [20] constructed a thermal storage unit with three-stage PCMs and measured the temperature evolution in each stage. Experimental results showed that phase change process didn't occur simultaneously in the three stages. Tao et al. [21] numerically studied the influence of PCMs melting points on the heat transfer efficiency and heat storage quality. They developed a formula for the selection of the optimum PCM temperature. Tian and Zhao [22] conducted a thermal and exergetic analysis for single-stage LHTES, multi-stage LHTES and metal foam enhanced multi-stage LHTES respectively. The obtained results indicated that the heat transfer efficiency of single-stage LHTES could be improved by adding heat tube and metal foam. However, multi-stage LHTES and metal foam enhanced multi-stage LHTES had little effect on exergy efficiency.
As a typical porous medium, metal foam is widely used in heat transfer enhancement and aerospace field [23]. Joshi and Rathod [24,25] evaluated partial filling strategy of metal foam on the heat transfer enhancement of LHTES. The obtained data showed that metal foam location had a significant effect on thermal behavior. It could be achieved almost the same heat transfer enhancement by adding metal foam at lower portion of the enclosure as that by adding metal foam the whole enclosure. Zhang and He [26], Wang et al. [27], Yang et al. [28] and Sardari [29] found that gradient metal foam showed better heat transfer behavior than uniform metal foam.
Although lots of studies has been conducted on metal foam enhanced TES system and cascaded TES system, few literature focused on investigating the coupling effects of metal foam and multiple PCMs, especially coupling gradient metal foam and multiple PCMs. The combination of gradient metal foam and multiple PCMs could simultaneously deal with the decrease of temperature difference between HTF and PCM and the low heat transfer efficiency of TES.
The heat transfer characteristic of multiple tube TES system out-performed single tube TES system. The result can be explained by assuming that single tube TES system is prone to higher heat loss. Therefore, multiple tubes TES was widespread investigated.
Agyenim et al. [30] compared the heat transfer characteristics of a single tube TES unit set as control group with a multiple tube TES unit. The results indicated that the phase change of erythritol was dominated by natural convection in multiple tube system rather than thermal conduction in single tube system. Esapour et al. [31] also explored the effect of heat transfer tube number on heat transfer characteristic during charging process and they obtained similar results.
Anish et al. [32,33] investigated the operating and design parameters on the heat transfer performance of a multiple finned tube TES. The effects of tube number and fin parameters on heat transfer efficiency were investigated. It was found that buoyancy-driven flow accelerated the PCM in downside. Niyas et al. [34] carried out a lab-scale experiment to explore the phase change process of PCM in a multiple tube heat exchanger. Buoyancy-driven flow has a significant impact on the melting process but could be ignored during solidification process. The melting rate of PCM was faster than solidification rate. Then they conducted numerical simulations to analyze the performance characteristics of the lab-scale experiment. The numerical results were highly consistent with the experiment results [35]. Khan and Khan, [36,37] explored the whole phase change cycle of PCM in a vertical orientation multiple tubes TES for domestic applications. The melting rate of paraffin at top was larger than that at bottom due to buoyancy-driven flow of liquefied paraffin. Joybari et al. [38] conducted experiments to compare the heat transfer behavior of single tube heat exchanger with multiple tube heat exchanger. Results showed that multiple tube heat exchanger out-performed single tube heat exchanger. Then, they validate the common simplifying assumption in numerical simulation that considering an artificial cylindrical boundary around the heat transfer tube. Their experimental results revealed that the simplifying assumption was inaccuracy.
Heat conduction along axial direction couldn't be involved in the two-dimensional model used by Anish et al. [33]. Niyas et al. [35] established a lab-scale three-dimensional physical model. To save computation resources, Yang et al. [39,40] designed a concentric artificial cylinder boundary. However, the selection of the artificial cylinder boundary has a significant impact on the thermal behavior of shell-and-tube TES. The concentric artificial boundary may seriously underestimate the heat transfer efficiency due to ignoring buoyancy-driven flow.
In conclusion, the following drawbacks were identified in previous studies regarding the utilization of horizontal multiple tube thermal energy storage systems:
- 1)
Natural convection was not considered when selecting the artificial cylindrical boundary around each heat transfer tube in horizontal multiple tube heat exchanger.
- 2)
Few literature investigated the utilization of gradient metal foam in horizontal thermal energy storage.
- 3)
The combination of gradient metal foam and multiple PCMs has not been studied.
In this paper, two artificial cylindrical boundaries used in single tube thermal energy storage were designed. A comparative study was conducted to investigate the accuracy of the proposed artificial cylindrical boundaries. Based on the accurate artificial boundary, the effects of multiple PCMs and gradient copper foam on thermal behavior and exergetic efficiency were investigated.
Section snippets
Physical model
The schematic diagram of multi-tube TES is shown in Fig. 1. To save the computation resources in numerical computation of multi-tube TES, the common solution is choosing a single tube as the computation zone. The red circles are two kinds of artificial boundaries, named concentric artificial cylinder boundary and eccentric artificial cylinder boundary as indicated by Fig. 1(b) and (c). The corresponding TES is named as eccentric tube TES and concentric tube TES.
Fig. 2 depicts the schematic
Effect of the artificial cylindrical boundary
The PCM zone was divided into three parts including solid-phase area, phase-transition area and liquid-phase area based on the different temperature of each part. In the solid-phase area, the PCM temperature is lower than 327 K, which is the lower limit of the phase change temperature range. In the liquid-phase area, the temperature of PCM is higher than 337 K, which is the upper limit of the phase change temperature range. In the phase-transition zone, the temperature of PCM is between 327 K
Conclusions
This paper compared the heat transfer behavior in the single tube TES with concentric/eccentric artificial boundary. Then, energy and exergetic analysis were carried out to explore the influences of multiple PCMs and gradient copper foam on the heat transfer efficiency based on the selected artificial cylinder boundary. The conclusions are as follows:
- 1)
The artificial cylindrical boundary has a great effect on the melting process of the PCMs because of the natural convection. The selection of
CRediT authorship contribution statement
Shengqi Zhang: Investigation, Methodology, Software, Data curtion, Writing – original draft. Liang Pu: Supervision, Conceptualization, Methodology, Writing – review & editing, Funding acquisition, Project administration. Lingling Xu: Validation, Formal analysis, Data curtion, Writing – review & editing. Zhenjun Ma: Methodology, Formal analysis, Writing – review & editing.
Declaration of Competing Interest
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.
Acknowledgments
This research work is jointly supported by the National Natural Science Foundation of China (No. 51641608) and the Fundamental Research Funds for the Central Universities of China (Nos. xzy022020026, 022019058).
References (53)
- et al.
Numerical study and experimental validation of the effects of orientation and configuration on melting in a latent heat thermal storage unit
J. Energy Storage
(2019) - et al.
Performance evaluation of nano-enhanced phase change materials during discharge stage in waste heat recovery
Renew. Energy
(2018) - et al.
Economic optimization of PCM and insulation layer thickness in residential buildings
Sustain. Energy Technol. Assess.
(2016) - et al.
Conjugate heat transfer in the PCM-based heat storage system with finned copper profile: application in electronics cooling
Int. J. Heat Mass Transf.
(2018) - et al.
Design and optimization of a hybrid air conditioning system with thermal energy storage using phase change composite
Energy Convers. Manage.
(2018) - et al.
Melting performance analysis of phase change materials in different finned thermal energy storage
Appl. Therm. Eng.
(2020) - et al.
Thermal performance optimization and evaluation of a radial finned shell-and-tube latent heat thermal energy storage unit
Appl. Therm. Eng.
(2020) - et al.
Numerical modeling of large-scale finned tube latent thermal energy storage systems
J. Energy Storage
(2020) - et al.
An experimental and numerical study on effect of longitudinal finned tube eccentric configuration on melting behaviour of lauric acid in a horizontal tube-in-shell storage unit
J. Energy Storage
(2020) - et al.
Thermal characterization of a heat exchanger equipped with a combined material of phase change material and metallic foams
Int. J. Heat Mass Transf.
(2020)