Experimental study on geothermal heat exchangers buried in diaphragm walls

Abstract

To bury absorber tubes in diaphragm wall as heat exchanger is a new energy saving technology. This paper is dedicated to study this new technology based on the field experiment at Shanghai Museum of Nature History. By the experiment, the heat transfer performance of heat exchanger in diaphragm wall and its impact factors including heat exchanger type, water velocity, inlet water temperature and operation mode were investigated. The test results show that (i) the heat exchange rate of W-shaped heat exchanger reached to 66.3 and 73.7 W/m for tube type (a) and (b) at the inlet temperature of 35 °C, which are approximately 1.2–1.4 times higher than that of single U-shaped type (c); (ii) under the experimental condition, the reasonable water velocity is 0.6–0.9 m/s; (iii) the exchange rate reached to 40.3, 66.3 and 85.0 W/m at the inlet temperature of 32, 35 and 38 °C, respectively, for tube type (a), every 1 °C rise of inlet water will promote 15% heat exchange rate; (iv) in intermittent operation mode, the average heat exchange rate is 82.8 W/m while 72.7 W/m for continuous mode under the experimental condition, i.e., 14.7% higher than that in continuous mode.

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.