Experimental study on an EG/paraffin composite thermal storage material and its feasibility for off-peak power heating utilization

doi:10.19799/j.cnki.2095-4239.2021.0358

Abstract

Abstract:

In this study, high-thermal conductivity expanded graphite (EG) is added to prepare a composite phase-change material (PCM) to improve the poor thermal conductivity and the insufficient heating capacity of paraffin. The influence of the EG addition on the composite PCM is then investigated. The results of the thermal performance test show that when the mass fraction of EG increases from 0 to 12%, the thermal conductivity of the composite PCM increases by 12 times that of pure paraffin. Moreover, the latent heat decreases from 190.8 J/g of pure paraffin to 152.1 J/g, and the phase transition temperature is approximately 62 ℃?, showing a fluctuation of approximately 0.43 ℃?. Comprehensively considering the thermal conductivity and the thermal storage density, 8% (mass fraction) composite PCM with 3.059 W/(m·K) thermal conductivity and 159.8 J/g latent heat is selected as the thermal storage material. The novel idea of a prefabricated wall block is applied to design the modular units for the heating system construction. The structure of and the installation method for the modular prefabricated thermal storage wall blocks are also introduced. The experimental study on the charging and discharging performances of different PCMs in the thermal storage unit illustrates that compared with pure paraffin, the 8% (mass fraction) EG/paraffin composite PCM improves the charging efficiency from 93% of pure paraffin to 97%. In addition, the effective discharging efficiency increases from 67.92% of pure paraffin to 84.65%. Accordingly, an economic analysis of the application of the modular prefabricated thermal storage wall blocks in a Nanjing office building with 1500 m² of heating area is performed. Under the current peak and off-peak electricity prices in Jiangsu Province, the net present value of the wall blocks within a 20-year operation period is approximately 139?100 yuan, and the payback period is approximately 14 years. In summary, the application of the prefabricated wall blocks is feasible, but has a long payback period and a low annual income.

Key words: off-peak power utilization, phase change thermal storage, PCM, assembled wall, economic analysis

CLC Number:

TK 02

Huihui YANG, Li ZENG, Bo TANG, Xiaoqing WANG, Yong LU. Experimental study on an EG/paraffin composite thermal storage material and its feasibility for off-peak power heating utilization[J]. Energy Storage Science and Technology, 2022, 11(1): 19-29.

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材料	EG质量分数/%	导热系数/[W/(m·K)]	相变温度/℃	潜热/(J/g)
材料	EG质量分数/%	导热系数/[W/(m·K)]	相变温度/℃	实际值	理论值
EG/石蜡复合PCM	0	0.304	62.12	190.8	190.8
	2	0.452	61.98	183.4	186.9
	4	1.368	62.24	172.6	183.1
	6	2.215	61.83	165.3	179.3
	8	3.059	61.89	159.8	175.5
	10	3.256	62.15	155.5	171.7
	12	3.870	61.69	152.1	167.9

名称	参数	名称	参数
外壳材质	不锈钢	换热管材质	铜
宽度/mm	50	换热管内径/mm	28
高度/mm	650	换热管外径/mm	30

项目	规格	所需数量	单价	费用/万元	备注
复合PCM	石蜡、EG	7.71吨	0.7万元/吨	5.40	—
电加热膜	聚酰亚胺	514 m²	0.006万元/m²	3.08	—
换热管	铜	4.77吨	3.98万元/吨	18.99	密度8.96×103 kg/m³
蓄热箱体	304不锈钢	9吨	1.65万元/吨	14.90	密度7.93×103 kg/m³
保温材料	聚氨酯泡沫	1.8吨	0.45万元/吨	0.81	—
轴流风机	NTF2-8025/DC24S	400	12.5元/个	0.5	单个风机功率为20 W
管路配件及控制系统	—	—	—	4.85	按设备价格的10%计算

因素	数值	变化率/%	NPV/万元	回收年限/年	敏感度/%
初投资/万元	48.53	10	9.05	16	-3.49
		0	13.91	14	—
		-10	18.76	12	3.48
总热效率/%	80	10	17.86	13	2.84
		0	13.91	14	—
		-10	9.22	16	-3.37
峰谷电价差 /[元/(kW·h)]	1.02	10	23.12	12	6.62
		0	13.91	14	—
		-10	4.82	18	-6.53

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