Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 75 (2015) 3148 – 3153
The 7th International Conference on Applied Energy – ICAE2015
Comparative study on heat and moisture transfer in soil heat charging at high temperature for various soils Chen Hongbinga*, Ding Hanwana, Liu Songyua, Wu Weia, Zhang Leia a
Beijing Municipal Key Lab of HVAC, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
Abstract
To investigate the mechanism of heat and moisture transfer in soil heat charging for solar-soil source heat pump system, a testing rig of one-dimension soil heat charging is constructed. Two different soils, sand and loam, are used for the testing. The soil temperature distribution and volumetric water content (VWC) variations are studied under different heat source temperature and soil initial VWC, and the difference between these two soils is analysed as well. The coupled and mutual effect of heat and moisture transfer is discussed. The study shows that sand is better than loam in soil heat charging. The heat source temperature has an obvious impact on soil VWC distribution. Higher heat source temperature leads to higher peak VWC. The initial VWC has a slight impact on soil temperature distribution.
© Published by Elsevier Ltd. This © 2015 2015The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of Applied Energy Innovation Institute
Keywords:Soil heat charging at high temperature; heat and moisture transfer; VWC; soil temperature distribution
1. Introduction Ground source heat pump is a kind of high energy-efficient heat pump system, making use of geothermal resources for both heating and cooling [1]. For northern china, the demand on heating load is much higher than that on cooling load, which means soil source heat pump system absorbs more heat from soil in winter than the heat injected into soil in summer [2]. Therefore, many years' system operation will lead to the decrease of soil temperature gradually, and consequently reducing the system performance
*Chen Hongbing. Tel.: +86-10-68322517; fax: +86-10-68322516. E-mail address:
[email protected].
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.649
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and coefficient of performance (COP). As a result, a novel solar-soil source heat pump compound system (SSHPCS), jointly employing solar energy and geothermal energy, was created [3-4]. The SSHPCS absorbs solar energy and injects heat into soil in summer to provide a better annual soil heat balance between winter and summer. Many studies [5-7] found that soil VWC had a slight impact on heat transfer at low temperature soil heat charging, but the effect of soil VWC on heat transfer seemed to be quite important at high temperature soil heat charging. Meanwhile, different soil features have different effect on heat and moisture transfer in soil heat charging. Many scholars have conducted related studies about this. Chochowski et al. [3] studied the performance of solar-soil source heat pump compound system and discussed the efficient of heat pump in the system and the properties of energy transfer. Hepbasli et al. [4] discussed the soil source heat pump system that using for heating in term of thermodynamic, worked out the balance equation of energy, quality and entropy of underground heat exchanger that using U-tube, and described the working features of ground source heat pump system in term of entropy. Gao et al. [5] conducted a preliminary study on the heat and moisture transfer in underground heat exchanger of ground source heat pump system with heat source and investigate the influence of underground volumetric water content on the heat exchanger features in heat exchanger area. Wang et al. [6] constructed the testing rig of heat and moisture transfer in soil under the condition of high-temperature thermal storage to test the temperature and moisture fields and the amount of heat transfer with different charging temperature and initial VWC. However, these studies failed to provide a clear description and explanation of the coupled and mutual effect of heat and moisture transfer in soil heat charging, and it was difficult to accurately analyze the working principle of heat and moisture transfer and the occurrence of peak temperature and peak VWC. This paper provided a simplified design of testing rig to study the soil heat charging at high temperature in summer working mode and investigated the coupled and mutual effect of heat and moisture transfer under soil temperature gradient and VWC gradient as well as the influence of heat and moisture transfer on soil heat charging for two types of soil sand and loam. The performance difference of soil thermal storage was compared and discussed in this paper. 2. Description of testing rig
Fig.1 Testing rig of one-dimension soil heat charging at high temperature
Fig.1 shows the diagram of testing rig for the investigation of one-dimension horizontal heat and moisture transfer at high temperature in soil heat charging. The rig was made up of soil pillar, water bath,
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thermocouples and VWC sensors as well as data logger. The testing rig was simplified to investigate the one-dimension horizontal coupled and mutual effect of heat and moisture transfer without considering vertical moisture transfer under the gravity effect. The soil pillar, 1200 mm in long and 110 mm in diameter, was made of a PVC pipe with good external insulation filled with soil. Spiral copper tube, connected to water bath, was buried at one side end of soil pillar, acting as the heat source for soil heat charging. The water bath could provide hot water of various temperatures ranging from 0 to 100oC, with an accuracy of ±0.1oC and kept constant temperature as controlled during a period of time. Thermocouples with an accuracy of ±0.1oC and VWC sensors with an accuracy of ±2%, connected to data logger, were installed for the testing of soil temperature and VWC. The soil pillar was fixed on a horizontal plane. Considering the soil area near heat source (spiral copper tube) could be more affected by heat and moisture transfer than the further end side, more testing points were set at the area. As shown in Fig. 1, seven testing points for VWC sensors, marking I, II, III, IV, V, VI and VII, were set with a distance to heat source of 30 mm, 100 mm, 200 mm, 300 mm, 460 mm, 600 mm and 750 mm, respectively. Eight testing points for thermocouples, marking I, II, III, IV, V, VI, VII and VIII were set with a distance to heat source of 5 mm, 55 mm, 125 mm, 225 mm, 325 mm, 475 mm, 625 mm and 775 mm, respectively. Sand and loam were used for the testing. Hot water of 60ćand 80ć was employed for the testing in this paper. The time of each testing lasted for 48 h. The list of testing modes is shown in Table 1. Table 1. List of testing modes. Mode
Heat source temp. (ć)
Initial soil temp. (ć)
Initial VWC (m3/m3)
A
60
22.9
0.10
B
60
24.5
0.16
C
60
24.7
0.21
D
80
22.7
0.10
E
80
24.7
0.16
F
80
24.6
0.21
3. Results and discussions Fig.2 shows the effect of different initial VWC at testing point I for sand and loam. It can be seen from Fig.2 that the time consumption for peak VWC at testing point I increases with increasing initial VWC. The time consumption for peak VWC of sand is 2h and 6h while for loam it is 1.5h and 3h at the initial VWC of 0.10 m3/m3 and 0.16 m3/m3, respectively. Fig.3 shows the effect of different heat source temperature on peak VWC at testing point I for sand and loam. It can be seen from Fig.3 that the time consumption for peak VWC increases with the increasing heat source temperature under the same initial VWC. For loam, the time consumption for peak VWC increases from 4.5h to 5h while the value of peak VWC increases from 0.18 to 0.2251 as the heat source temperature increases from 60 ć to 80 ć. For sand, the time consumption for peak VWC increases from 2h to 3h while the value of peak VWC increases from 0.1876 m3/m3 to 0.2161 m3/m3 as the heat source temperature increased from 60 ć to 80 ć. The time consumption for peak VWC under each testing mode is different. That is because the same heat source temperature leads to the same soil internal temperature difference and consequently the same VWC difference caused by soil temperature difference at the same initial VWC. But the VWC difference could increase with the increasing initial VWC, therefore the time for the increasing VWC difference to
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reach equilibrium increases and the time consumption for peak VWC under coupled and mutual effect of heat and moisture transfer increases as well. Similarly, the soil temperature difference increases with the increasing heat source temperature, and consequently the VWC difference increases as well at the same initial VWC. As a result, the time for the increased soil temperature difference and VWC difference to reach equilibrium increases. Moreover the time consumption and value for peak VWC under coupled and mutual effect of heat and moisture transfer increase as well.
Fig.2 The effect of different initial VWCs on peak VWC at testing point I for sand and loam
Fig.3 The effect of different heat source temperatures on peak VWC at testing point I for sand and loam
Fig.4 shows the temperature variation of sand and loam at testing point ĉ’, Ċ’ and ċ’. It can be seen from Fig.4 that the soil temperature for each testing point increases firstly, reaching a peak value and after that keeping relatively stable. That is because the temperature difference between heat source and soil is high at the preliminary stage of soil heat charging, so that the soil temperature increases sharply at the beginning. With soil temperature going up, the temperature difference decreases and therefore the soil temperature increase tends to be slight later. The peak temperature at testing point I of loam is 66.4 ć,
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12.4 ć higher than that of testing pointing II. The time consumption of peak temperature at testing point I is 7h, 2.5h less than that of testing point II. It is obvious that the peak soil temperatures of testing points near heat source are higher than that of other testing points and the time consumption for peak soil temperatures of testing points near heat source are less as well due to soil thermal resistance. The peak temperature at testing point I of sand is 75.2 ć, 8.8 ć higher than that of loam. The time consumption for peak temperature at testing point I of sand is 5h, 2h less than that of loam. It’s obvious that the temperature of sand is higher than that of loam, and the time consumption before temperature stabilization is less than that of loam.
Fig.4 Temperature variation of sand and loam at testing point I’, II’ and III’
Fig.5 The effect of initial VWC on soil temperature variation at testing point I’ and II’ for loam
Fig.5 shows the effect of initial VWC on soil temperature variation at testing point I’ and Ċ’ for loam. It can be seen from Fig.7 that the increase of initial VWC leads to a slight increase of loam temperature. The time consumption for loam temperature to be stabilized at testing point I’ is shown in Table 2. It can
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be seen that the time consumption for loam temperature to be stabilized increases with the increasing heat source temperature and initial VWC.
4. Conclusions
1) 2)
3)
The occurrence of peak VWC is found in soil heat charging at high temperature and it happens firstly at the area near heat source. With further heat transfer in soil, the position of peak VWC moves towards the other cold end of soil pillar. Under the same condition, the speed of moisture transfer in sand is faster than that in loam, and the amount of moisture transfer in sand is also more than that in loam. At the same position, the time consumption for peak VWC of sand is less than that of loam, and the value of peak VWC of sand is higher than that of loam.
The heat source temperature has an obvious impact on soil VWC distribution. Higher heat source temperature leads to higher peak VWC. The initial VWC has a slight impact on soil temperature distribution.
Acknowledgements The work of this paper is fully supported by Funding Project for New Star of Scientific and Technical Research of Beijing (2011029), The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD201304067) and Academic Innovation Team of BUCEA (21221214104). References [1] Gabrielsson A, Bergdahl U. Thermal energy storage in soils at temperature reaching 90oC[J]. Journal of Solar Engineering; 2000;122( 3):1-8. [2] Zhang H-F, Ge X-S, Ye H, et al. Heat conduction and heat storage characteristics of soils [J]. Applied Thermal Engineering; 2007; 27(2-3):369-373. [3] Czekalski Dariusz, Mirski Tomasz, Chochowski Andrzej. The analysis of energy flux flow in the hybrid system of renewable sources. Proceedings of the Ninth International Symposium on Heat Transfer and Renewable Sources of Energy; 2002, p.245-250. [4] Hepbasli, Arif. Thermo dynamic analysis of a ground-source heat pump system for district heating. International Journal of Energy Research; 2005; 29(7): 671-687. [5] Gao Qing, Li Ming, et al. Heat transfer influenced by containing water in wet soil. Journal of Thermal Science and Technology; 2005; 4(2): 136-141. [6] Wang Huajun, Qi Chengying, et al. Experimental study of heat and moisture transfer process for high-temperature heat storage of soils. Acta Energiae Solaris Sinica; 2010; 31(7): 824-828. [7] Chen Baoming, Wang Buxuan, Zhang Liqiang, et al. Numerical study on soret-effect and dufour-effect in porous enclosure[J]. Journal of Engineering Thermophysics; 2004; 25:123-126.
Biography Dr Hongbing Chen is an associate professor at Beijing University of Civil Engineering and Architecture. His research area is focused on PV/T technology, heat and moisture transfer in soil heat charging and discharging and building energy-efficient technology. He has published more than 70 academic papers.
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