Research on internal structure optimization and energy storage characteristics of electric water heater

doi:10.12034/j.issn.1009-606X.220206

Abstract

Abstract:

The energy consumption of electric water heater in China has a large proportion of total energy consumption. Therefore, it is of great significance to optimize the energy consumption process of electric water heater. The phase change materials (PCM) have the characteristics of high energy storage density and approximately constant temperature in the process of phase transition. In addition, the thermal conductivity of phase change materials can be enhanced by adding nanometer materials such as graphite. Therefore, it has obvious advantages in heat energy storage and utilization of phase change materials. So, the phase change energy storage technology can be applied to electric water heater to adjust the energy storage process and the goal of "peak shifting" also can be achieved. In this work, the energy storage characteristic of energy storage water heater with phase change material were researched employing numerical simulation method. Four different structural models were established and the influent of inlet and outlet piping construction, electric heating tube arrangement, insulation structure on flow and heat transfer characteristics of water in water heater were discussed. And the influence of energy storage layer thickness on energy storage of electric water heater were researched. The results showed that, compared with the vertical heating tube, the water temperature distribution of horizontal heating tube was more uniform during heating process, and the heating efficiency was improved by about 2.2%. When the inlet tube diameter increased to 1.5 times, the output status of hot water can be excellent improved, and the hot water output efficiency can be increased 17.9%. The addition of phase change materials can increase the water temperature by 10.6% under the condition of same heat preservation time (36 h). The results of this study can provide data support for structural optimization of electric water heater and application of phase change energy storage technology.

Key words: electric water heater, phase-change thermal energy storage technology, numerical simulation, peak load shifting

摘要：

将相变储能技术应用于电热水器，并通过添加石墨纳米颗粒改善相变材料的导热特性，对其储能过程进行调节，可以起到“移峰填谷”的作用。建立了四种不同结构的电热水器三维模型，模拟了电热水器内部速度场与温度场分布特性。考察了进出口水管结构、电加热管布置方式、保温层结构等因素对热水器内部流场及传热特性的影响，研究了不同储能层厚度对电热水器储能的影响。结果表明，水平加热管与垂直加热管相比，加热过程中加热效率提高了2.2%；进水管管径增大1.5倍，热水输出率提升了17.9%；加入相变材料可在相同保温时间内(36 h)使水温最大提高10.6%。

关键词: 电热水器, 相变材料储能技术, 数值模拟, 移峰填谷

CLC Number:

TB34

Li WU, Jie HE, Jinli LU, Yafang HAN, Ze HONG. Research on internal structure optimization and energy storage characteristics of electric water heater[J]. The Chinese Journal of Process Engineering, 2021, 21(7): 786-793.

吴丽, 何杰, 鲁进利, 韩亚芳, 洪泽. 电热水器内部结构优化及储能特性研究[J]. 过程工程学报, 2021, 21(7): 786-793.

Figures/Tables 13

Fig.1 Schematic diagram of internal structure of electric water heater

Table 1 Different structure electric water heaters

Case	Structure
1	Vertical heating tube+normal inlet pipe diameter
2	Horizontal heating tube+normal inlet pipe diameter
3	Vertical heating tube+1.5 times diameter of the inlet pipe
4	Horizontal heating tube+1.5 times diameter of the inlet pipe

Fig.2 Schematic diagram of insulation layer of electric water heater with PCM

Table 2 Physical properties of thermal insulation materials

Material

Phase change temperature/℃

Latent heat/(kJ/kg)

Specific heat/

[J/(kg·K)]

Heat conductivity coefficient/[W/(m·K)]

Paraffine+10wt% Graphite^[18]

Fig.3 The effect of different heating powers on heating time

Fig.4 Temperature distribution of hot water after heating (Unit: ℃)

Fig.5 Temperature distribution along height when heating completed

Fig.6 Velocity vector diagrams in the electric water heater for 30 s water intake

Fig.7 Temperature distributions in the electric water heater for 60 s water intake

Fig.8 Variation of outlet temperature of electric water heater with heating time

Table 3 The hot water output rate under different schemes

Parameter	Case 1	Case 2	Case 3	Case 4
Maximum temperature/℃	68.72	68.56	68.86	68.28
Minimum temperature/℃	48.02	47.99	48.47	48.45
Time/s	370	390	430	460
Water quality/kg	30.25	31.89	35.16	37.62
Water temperature/℃	63.53	63.49	63.82	63.50
Shutdown temperature/℃	63.27	63.35	63.27	63.35
Hot water output rate/%	62.00	65.20	72.48	76.90

Fig.9 Variation curve of liquid phase ratio during melting of different phase change materials

Fig.10 The influence of different thicknesses of PCM on the average water temperature of electric water heater during heat preservation process

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