Experimental study on geothermal heat exchangers buried in diaphragm walls
Highlights
► Conducting the first field experiment of heat exchangers in diaphragm wall. ► Finding the difference between heat exchanger in diaphragm wall and in borehole. ► Investigating the influence of 4 factors on heat exchange rate in diaphragm wall.
Introduction
To reduce primary energy consumption and emissions of green house gases, more and more attentions are paid to Ground Source Heat Pump system. The technology of burying heat exchangers in borehole is an early developed technology and its researches are relatively mature now, no matter theoretical study, numerical simulation or experiment researches. The experiment research contents mainly concerned the thermal response tests and the thermal performance under different conditions, such as backfills, tube materials, inlet water temperature, water velocity, heat exchanger types and operation modes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The theoretical researches about exergetic modeling and performance evaluation as well as its experimental investigation of solar assisted ground-source heat pump system were also presented [12], [13], [14]. In recent years, the concept of energy geotechnical engineering was proposed and promoted, which makes up the disadvantages of heat exchanger in boreholes, i.e. great initial investment and large occupied space. Energy geotechnical engineering is an energy saving technology to embed the absorber tubes loop directly in underground structures such as base slabs, piles, and diaphragm walls, forming heat exchanger with part of geotechnical engineering structures. With the advantages of good stability, durability and heat transfer performance, the new technology has the great potential to be widely applied in large population urban areas [15], [16]. Among these energy geotechnical engineering members, energy pile has attracted much attention. Its thermal performance as well as some influence factors, including inlet water temperature, water velocity, tube types and operation modes were studied and tested [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. The technology of embedding heat exchangers in diaphragm wall is a new developing direction of energy geotechnical engineering. In this technology, absorber tubes are buried in diaphragm walls by attaching them to reinforcement cage. However, the application and research of heat exchanger buried in diaphragm wall are so far relatively rare. Only in 1996, absorber tubes were first embedded in diaphragm walls as heat exchanger in Austria and Switzerland [27]. And in 2003, in the sections of Viennese Metro Line Extension U2, absorber tubes were applied as heat exchanger in diaphragm wall, foundation floor and linings of sector tunnel [28].
This paper makes a first attempt to investigate the heat transfer performance of heat exchangers embedded in diaphragm walls based on field experiment. In addition, the influence factors of heat transfer performance, including heat exchangers types, inlet water temperature, water velocity and operation modes were further studied.
Section snippets
System description
The newly built Shanghai Museum of Nature History, under which Shanghai Metro Line 13 passes through, is located in Jingan Figure Garden, Shanghai. The museum is 5 storied, with the building area and total floor area of 12,029 m2 and 45,086 m2, respectively. In order to reduce primary energy consumption and greenhouse gases emission, absorber tubes were embedded in the piles under the base slab of the museum and the diaphragm walls of the museum and metro line 13 as heat exchangers. This paper
Analysis
Constant temperature method, also known as reversible heat pump method [29], was employed in this heat performance in situ test. The base of this method is to keep the temperature of inlet water temperature constant. The heating function was obtained by 4 electrical heaters, while the cooling function was provided by means of a typical R22 refrigeration cycle, which consists of a compressor, a fin condenser, an expansion valve and a coil evaporator. The maximum heating and cooling power were 10
Comparison between heat exchangers in diaphragm wall and in borehole
The heat exchangers in diaphragm walls differ from those in borehole in many respects, such as buried depth, heat exchanger types, temperature of underground, and material surrounding the heat exchangers. Firstly, restricted by the depth of diaphragm wall, the buried depth of heat exchangers in diaphragm wall is about 20–40 m, and it is much smaller than the depth of heat exchangers in borehole which is 80–100 m. And it is well known that the heat exchange rate per meter is relevant to buried
Conclusions
The field experiment at Shanghai Museum of Nature History expanded the understanding of heat exchangers buried in diaphragm wall. Firstly, compared with borehole heat exchanger, some structure and heat transfer characteristics of heat exchanger in diaphragm wall was revealed. Secondly, by the experiment, heat exchanger type, water velocity, inlet water temperature, and operation mode were proven to be important factors influencing the heat exchange rate of heat exchanger in diaphragm wall. From
Acknowledgements
This research was supported by the National Natural Science Foundation of China (No. 50878150), the Major State Basic Research Development Program of China (973 Program, No. 2011CB013800), the National Technology support program from the Ministry of Science and Technology of China (No. 2012BAJ01B02), and the program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1029). The authors gratefully acknowledge the help of Ms. Yan Ping for improving the manuscript.
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