A review on heat transfer enhancement of borehole heat exchanger

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1 Available online at ScienceDirect Energy Procedia 104 (2016 ) CUE2016-Applied Energy Symposium and Forum 2016: Low carbon cities & urban energy systems A review on heat transfer enhancement of heat exchanger Jun Zhao a, *, Yang Li a, Junyao Wang a a Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education of China, Tianjin , China Abstract The heat transfer enhancements of heat exchanger () have been intensively studied which strongly influences the performance of ground-coupled heat pump (GCHP) systems. In this paper, some typical heat transfer enhancement methods of are reviewed based on designs, grout materials and the influence of artificial groundwater flow. Features and feasibility of different methods have been discussed for the different applications Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( The Authors. Published Elsevier Ltd. Peer-review Selection and/or under responsibility peer-review of under the scientific responsibility committee of of CUE the Applied Energy Symposium and Forum, CUE2016: Low carbon cities and urban energy systems. Keywords: Borehole heat exchanger; Ground coupled heat pump; Heat transfer enhancement 1. Introduction Ground source heat pump (GSHP) systems have been globally applied in various buildings as heating and cooling could be supplied efficiently with low carbon emissions. Taking China as an example, from 2010 to 2014 the annual average growth rate of geothermal utilization was up to 18.7%.The groundcoupled heat pump (GCHP) system is one of a typical GSHP technologies, which mainly uses heat exchanger () for heat transfer process. As the efficiency of the whole GCHP system is strongly influenced by the performance of, large numbers of researches have been carried out in every aspect for technologies. Basically, there are mainly four factors affecting the heat transfer performance of the : the heat transfer efficiency, the grout material heat transfer coefficient, the underground soil heat transfer efficiency and the groundwater flow influence effect. Therefore, lots of thermal transfer enhancement * Corresponding author. Tel.: ; fax: address: zhaojun@tju.edu.cn Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, CUE2016: Low carbon cities and urban energy systems. doi: /j.egypro

2 414 Jun Zhao et al. / Energy Procedia 104 ( 2016 ) approaches have been proposed and studied as well as already been applied in practical projects. However, the feasibility and compatibility of different enhancement methods under varying application conditions are still remained to be further demonstrated, thus a comprehensive review and comparison is necessary. This paper is aim to review the existing heat transfer enhancement methods of, along with their experiment or simulation results. Different categories of enhancement technologies are reviewed with a comparison and summarize of their technical feasibility, economical efficiency and other features. The suitable method for different application scenario is discussed. 2. Advanced design of 2.1. Advanced design with larger heat transfer area Obviously, with larger heat transfer area can achieve better heat transfer effect. Thus, three types of advanced have been studied: the multi-tube, the helical and the coaxial. The sketches and some typical research results are summarized in Table 1. Table 1. Advanced design of the with larger heat transfer area Type Sketch Source Figures Key findings Multitube Helical Coaxial Aydın al. [1] et Park et al. [2] Zarrella et al. [3] Zhang al. [4] et Zhao et al. [5] Acuña [6] The heat transfer rate of the 3U-tubes increased by 25% than the single U-tube under the designed experimental condition. However the performance improvements of the 4U and 5U-tubes are insignificant indicated by the simulation results. The thermal resistance of the 3U-tubes decreased by 24.8% than the W-shaped based on a TRT test. However, the superior performance could be achieved only for an intermediate operation period rather than a continuous operation time according to a long term simulation. The growth of the heat transfer rate of the helical was up to 40% compared with the double U-tube at peak load. Furthermore, a simulation result shows that when the pitch of the helical decreased by 50%, the peak load could reach an increase by about 14%. The ring-coil model can reflect the real physical configuration of the helical in a more approximation method. It is indicated that the beneficial effect of the groundwater flow became more obviously with the passage of time as well as the increase of the seepage velocity. Due to the effect of natural convection, the heat transfer rate of the coaxial inclines to stabilization at the far-field. The dimensionless temperature gradient along with the outer wall of the has a linear relationship with the non-dimensional height. The theoretical conductive thermal resistance of the HDPE coaxial is about 30.4% lower than that of the single U- tube with the same configuration. Besides, the pressure drop of this coaxial is significantly lower than that of the U-tube.

3 Jun Zhao et al. / Energy Procedia 104 ( 2016 ) The multi-tube has simple configuration that consists of at least three connected U-tubes inside one. However, due to its larger horizontal area occupation, it can only be used when the diameter of is large enough such as the energy pile. Aydın and Sisman [1] tested the 3U-tubes with thermal response test (TRT), and investigated the heat transfer effect of 4U-tubes and 5U-tubes by numerical simulation. Park et al. [2] performed a series of TRTs for energy pile with 3U-tubes, and evaluated its performance by three-dimensional finite element analyses under different operation conditions The helical has rather larger heat transfer area than the conventional U-tube, however in order to obtain enough structure strength, it should be fixed on a reinforcement frame when installed, which increases its overall diameter. Therefore, the helical is restricted to be applied in energy pile as well. Zarrella et al. [3] developed a numerical calculation model for the helical to investigate the influence of different design and operation parameters, in which the heat conduction along the axial direction between the ground and the pile were included. In addition, several TRTs have been conducted to examine this model. Zhang et al. [4] developed an analytical calculation model which simplified the helical tube heat source to the form of a ring-coil heat source, which takes into account of the influence of groundwater flow as well. The coaxial includes two coaxial cylindrical tubes and the inner tube is placed inside a larger outer tube, which forms an annular cross-section between them. Zhao et al. [5] experimentally studied the thermal performance of a coaxial in a cylinder filled with micro-balls made of artificial glass, and a theoretical heat transfer model between the surrounding saturated soil and the was established. The conventional coaxial is usually made from steel or other inflexible material, which makes it inconvenient for transportation when the is long. Thus, a new type of coaxial made from HDPE was researched by Acuña [6] experimentally and theoretically. The result indicated that the novel HDPE coaxial can be used in normal directly Advanced design based on phase change principle The heat transfer capacity of the based on the phase change principle is quite better than the conventional. Besides, circulating pump based on the phase change systems is no more needed for source side, which could save the capital and operation costs. Currently, mainly two types of s based on phase change principle are widely investigated: the thermo-syphon and the heat pipe. The thermo-syphon normally uses the conventional U-tube while filled with phase change materials. Acuña et al. [7] tested a 65m U-tube thermo-syphon, which utilized CO2 as the working fluid and cooper as the tube material. The configuration of this is shown in Fig.1 (a). A fiber cable was fixed on the outer wall of the copper tube for the operation temperature measurement. The experiment results showed that the outlet working fluid temperature was almost constant, which indicated that the phase-change process has indeed occurred. The heat transfer rate of this is about 70 W/m under the experiment condition. The heat pipe is a kind of gravity heat pipe which absorbs the heat from the underground soil. The top of the heat pipe is connected to a coil heat exchanger which transfers the absorbed heat from soil to the evaporator. However, it should be noted that the heat pipe could be only used for heating application. Rieberer et al. [8] designed a heat pipe which used CO2 as phase change fluid as shown in Fig.1 (b). An experimental comparison with the U-tube was carried out as well, which indicated that the heat transfer rate of the heat pipe was 41.5% higher than that of the conventional U-tube Advanced design with other enhancement structures

4 416 Jun Zhao et al. / Energy Procedia 104 ( 2016 ) Acuña [6] investigated the performance of U-tube with spacers between the two tubes, as shown in Fig.1 (c), The simulation results showed that the thermal resistance of the s with 38mm interval decreased considerably by 33.8%. The U-tube with grooves is also investigated by Acuña [6] as listed in Fig.1 (d), which had a 12.3% lower thermal resistance indicated by the test results. Bouhacina et al. [9] investigated a finned U-tube by means of numerical simulation, as shown in Fig.1 (e). The heat transfer rate between the water and the wall surface was 7% higher than the smooth tube on the basis of the simulation results. Fig.1 (f) shows the s with insulation at the outer side of the outlet tube studied by Pan [10]. The numerical simulation results showed that the heat transfer rate increased by about 8% under optimal design. Fig. 1. (a) Thermo-syphon, (b) Heat pipe, (c) U-tube with spacers, (d) U-tube with grooves, (e) Finned U-tube, (f) U-tube with insulation 3. Novel grout materials The conventional grout materials normally consist of slurry and some additives, or only cement when used for energy pile. However their heat transfer properties leave much to be desired. Thus, in recent years, a few of enhanced grout materials have been invented and researched. Bottarelli et al. [11] investigated a phase change material (PCM) grout made from paraffin. The numerical simulation results showed that the thermal wave in the ground could be smoothed when employ the PCM, thus the performance of the heat pump could be improved. Wang et al. [12] investigated a PCM grout made from enhanced n-decanoic acid and lauric acid. Based on the numerical simulation results, the land use for s when applying the novel materials can be reduced significantly by 29% compared to the normal materials. Focaccia et al. [13] experimentally investigated a novel approach which immersed the inside a brine water-filled tube instead of filling the conventional grout materials. The results showed that there is an increase in heat transfer rate due to the enhanced natural convection effect. 4. Artificial groundwater flow In general, the groundwater flow can ease the accumulated heat/cold around the s. However, in some regions like Tianjin, there are numbers of aquifers, while the groundwater flow is weak. As for such kind of geological structure, an artificial groundwater flow produced by pumping/injecting wells can be used for heat transfer enhancement, as shown in Fig.2 (a). Zhao et al. [14] investigated this method, which was based on the geological structure of Tianjin. As can be seen from Fig.2 (b) six pumping/injecting wells were set for a 6 6 s group, and the enhancement effect was simulated by software FEFLOW. The results showed that the average heat transfer rate of the overall s increased considerably by 33% after a 90-days heat injection. As shown in Fig.2 (c), under the influence of the artificial groundwater flow, the accumulated heat around the s could extend to the farther soil, which enhanced the overall heat transfer efficiency of the s.

5 Jun Zhao et al. / Energy Procedia 104 ( 2016 ) Fig. 2. Artificial groundwater flow: (a) principle, (b) simulation set, (c) simulation result 5. Discussion A summary of reviewed enhancement technologies is listed in Table 2 as a guidance for the enhancement method choice under different application conditions taking account into their enhancement level, investment, technical feasibility, etc. Basically, The HDPE coaxial is suitable for most scenarios, due to its good comprehensive properties such as good enhancement effect, relatively low investment. For energy pile, the enhancement effect of the helical is superior due to its larger heat transfer area. However the fixing process requires much skilled labor work, thus it is more suitable for regions with low labor cost. The with spacers or insulation can increase the heat transfer rate effectively, nevertheless some extra time consuming fixing works are needed as well, which indicates that they are more suitable for small scale projects. In particular, as for a GCHP system in use which needs more heat/cold from the existing s, the artificial groundwater flow enhancement method may be the only choice, since all of the other methods need to be performed during the construction period. Table 2. Features and suitable scenarios of different enhancement methods Enhancement methods Enhancement level Helical HDPE coaxial Multitube Thermosyphon / Heat pipe With spacers / insulation With grooves / fins PCM grout / Liquid grout Artificial groundwater flow Medium High Medium High Medium Low Medium Medium Investment Medium High Medium High Low Medium High Medium High Medium High Low Medium Medium Low Medium Advantage Simple Short length Disadvantage Suitable scenario Techfeasibility Widediameter Energy pile Fixing needed Energy pile applicable Extra investment Most scenarios Low operation cost Large investment Heating application U-tube applicable Hard for installing Small scale applicable Extra investment High heat transfer coefficient Potential environment impact Laboratory research Existing project available Aquifer needed Retrofit existing projects 6. Conclusion The is a widely used and mature industrial equipment, while there are still many enhancement methods worth further studied. Some typical researches on the advanced design of s in terms of configurations, novel grout materials and the effect by groundwater flow are reviewed. A comparison of

6 418 Jun Zhao et al. / Energy Procedia 104 ( 2016 ) the features and suitable application scenarios of some typical enhancement methods are summarized, which indicates: 1) The HDPE coaxial is applicable for most scenarios; 2) The helical is feasible for energy piles when the labor cost is low; 3) The with spacers or insulation is more suitable for small scale projects; 4) The artificial groundwater flow is the only choice for existing GCHP project. Further work is needed to develop more approximation analytical model for different approaches and carry out pilot and demonstration research projects as well. The tech-economic, environment impact should be further analyzed based on regional differences. Acknowledgements This work is sponsored by National Key Technology R&D Program of China (No. 2013BAJ09B04). References [1] Aydın M, Sisman A. Experimental and computational investigation of multi U-tube s. Applied Energy 2015;145: [2] Park H, Lee SR, Yoon S, Choi JC. Evaluation of thermal response and performance of PHC energy pile: Field experiments and numerical simulation. Applied Energy 2013;103: [3] Zarrella A, De Carli M, Galgaro A. Thermal performance of two types of energy foundation pile: helical pipe and triple U- tube. Applied Thermal Engineering 2013;61(2): [4] Zhang W, Yang H, Lu L, Cui P, Fang Z. The research on ring-coil heat transfer models of pile foundation ground heat exchangers in the case of groundwater seepage. Energy and Buildings 2014;71: [5] Zhao J, Wang H, Li X, Dai C. Experimental investigation and theoretical model of heat transfer of saturated soil around coaxial ground coupled heat exchanger. Applied Thermal Engineering 2008;28(2): [6] Acuña J. Improvements of U-pipe Borehole Heat Exchangers. Licentiate Thesis, Stockholm: The Royal Institute of Technology KTH [7] Acuña J, Palm B, Khodabandeh R, Weber K. Distributed temperature measurements on a U-pipe thermosyphon heat exchanger with CO2. 9th IIF/IIR Gustav Lorentzen Conference on Natural Working Fluids. Sydney, Australia [8] Rieberer R, Mittermayr K, Halozan H. CO2 Heat Pipe for Heat Pumps. IIR/IIF Commission B1,B2,E1 and E2. ss Guangzhou, China [9] Bouhacina B, Saim R, Oztop HF. Numerical investigation of a novel tube design for the geothermal heat exchanger. Applied Thermal Engineering 2015;79: [10] Pan YK. Research on Heat Transfer Enhancement of the Vertical U-tube in Ground-Source Heat Pump System (in Chinese). Master Thesis, Zhengzhou: Zhengzhou University [11] Bottarelli M, Bortoloni M, Su Y, Yousif C, Aydın AA, Georgiev A. Numerical analysis of a novel ground heat exchanger coupled with phase change materials. Applied Thermal Engineering 2015;88: [12] Wang JL, Zhao JD, Liu N. Numerical Simulation of Borehole Heat Transfer with Phase Change Material as Grout. Applied Mechanics and Materials 2014;577: [13] Focaccia S, Tinti F. An innovative heat exchanger configuration with improved heat transfer. Geothermics 2013;48: [14] Zhao J, Zhang ZY, Liu JL, Li Y, Sun T. Simulation Study on Heat Transfer of Buried Pipe Banks Under the Influence of Artificial Flow (in Chinese). Journal of Tianjin University (Science and Technology) 2016;49(8): Biography Yang Li, born in 1990, PhD candidate in thermal engineering at Tianjin University. Current research direction: heat exchanger, ground-coupled heat pump, multienergy heating & cooling system.