An investigation of the performance of groundwater-based heat pipes in heating lawn systems

https://doi.org/10.1016/j.enconman.2021.114492Get rights and content

Highlights

  • This study presents a new approach for the application of shallow geothermal energy.

  • Difference in temperatures of heat pipe systems with different diameter of vertical part is insignificant.

  • The heat resistance of the heat pipes only accounted for approximately 1% of the total thermal resistance.

  • The maximum heat transfer power of the heat pipe is 202 W.

Abstract

Owing to the depletion of global fossil energy and the increasing environmental awareness of environmental protection, the use of shallow geothermal energy has been receiving widespread attention. In this study, two sets of groundwater-based heat pipe (GBHP) systems were fabricated to explore the application of heat pipes in shallow geothermal energy and tested through lawn heating experiments during the winter. Results of the experiment showed that the vertical diameter of the heat pipe did not have any significant effect on its heat transfer performance, and the maximum difference in temperature between the horizontal section of the east and west heat pipes during the observation period was 1.6 °C. Furthermore, the heat pipe exhibited good isothermicity with a constant operation, with the average temperature difference between the condensation and evaporation sections were 2.1 °C. The heat pipe also exhibited good thermal conductivity, the thermal resistance accounting for only 1% of the total thermal resistance in the heat transfer process. The maximum heat transfer power of a single heat pipe was found to be 202 W. This study presents a new approach for the application of shallow geothermal energy.

Introduction

Shallow geothermal energy technology, which refers to the energy present approximately 200 m below the earth's surface at a maximum temperature of 25 °C, has gained increasing attention owing to the diminishing fossil energy, emphasis on environmental protection, and maturing ground source heat pump (GSHP) technology [1]. GSHP systems based on the different geothermal energy exchange systems include: ground-coupled heat pump (GCHP) system (vertical U-tube ground heat exchangers), surface water heat pump (SWHP) system (uses river and seawater sources), and groundwater source heat pump (GWHP) system [2], among which, GWHP systems have become increasingly popular.

Over the past decades, significant progress has been made in GWHP systems. In the 1980s, the Chinese government advocated the scientific utilisation of shallow geothermal energy to build an energy-efficient society, and the Ministry of Construction, China, issued the engineering specification for the GSHP system in 2006. Currently, these systems are geographically distributed in almost every Chinese province [1].

Liu et al. [3] studied the suitability and feasibility of GWHP systems in residential buildings in different climate zones in China. Based on economic analysis, the economy of the GWHP system was found to be better than the air source heat pump (ASHP). Furthermore, an optimisation model analysis of the centralised GWHP system in the heating season was performed, by establishing the energy consumption calculation model and water pump to calculate the heating parameters and develop the optimisation method and constraint conditions. The accuracy of the optimisation model was verified by field tests. GWHP systems are also used in high-altitude plateau areas. Zhen et al. [4] studied the application of GWHP in the heating system in the Tibet Plateau airport, with an actual performance coefficient of approximately 5.0. Compared to conventional heat sources, the GWHP system delivered optimal energy conservation and economic efficiency. Li et al. [5] carried out numerical simulations of GSHP systems based on the influence of unsaturated soil characteristics and groundwater flow. It was observed that neglecting variations in moisture content in the unsaturated soil could diminish the heat transfer capacity of the soil. Furthermore, the influence of groundwater table fluctuations should be considered as it is beneficial for the operation of the GSHP system, else, it could result in an error of 3–4% in the outlet fluid temperature. Moreover, the effect of the position of the groundwater table should also be considered when calculating the influence of groundwater flow on the GSHP system performance.

Although many researchers have extensively studied the development [3], [6] and utilisation [4] of GWHP systems, and the effect of groundwater [2], [7], [8], [9], soil properties [5] and natural cold sources [1] on its operation, very few have investigated the direct utilisation of shallow geothermal energy using the heat pipe technology.

Heat pipes are efficient energy converting devices that use the latent heat of fluids to transfer energy from one place to another through the process of evaporation and condensation in a sealed container [10]. Considering heat pipes can transfer large amounts of heat flow over long distances with minimal temperature variations, they are widely used in thermal energy storage systems, thermal management, and various other fields [11]. Therefore, in this study, we investigated the thermodynamic performance of GBHP in a shallow geothermal energy system during the winter. The main objectives of this study are as follows:

(1) Study the thermal behaviour of the GBHP system daily.

(2) Investigate the thermal performance and temperature difference in the GBHP system.

(3) Analyse the heat transfer performance of the GBHP system.

Section snippets

Working principle of GBHP system

The GBHP is a gravity heat pipe [12] comprising evaporation, insulation, and condensation sections, as seen in Fig. 1.In the lawn heating system, the high temperature evaporation section was buried in the groundwater of the shaft, the insulation section was placed in the shallow soil foundation, and the low temperature horizontal condensation section section was placed on the root of the lawn. Due to the existence of geothermal gradient in winter, the temperature of soil and groundwater below

Experiment

Fig. 3 shows the exterior of the GBHP located in Ningxia University Science and Technology Park in Yinchuan, the capital of the Ningxia Hui Autonomous Region. The average altitude of Yinchuan city, located at 38.5 N and 106.2 E, is 1080 m above sea level with an average annual temperature of 9 °C, and extreme maximum and minimum temperatures of 38.7 °C and −27.7 °C, respectively.

Results and discussion

The measurements were conducted for 114 days, from 16 November 2020 to 10 March 2021.

Conclusion

We conducted a lawn heating test using a groundwater-based heat pipe (GBHP) system in winter conditions to explore the application of the heat pipe in shallow geothermal energy. We fabricated two sets of GBHP with different diameters and a series of tests were conducted to evaluate their performance during the observation period, including thermodynamics, thermal performance comparison, the temperature difference of GBHP, and heat transfer performance of the GBHP system. This study provides a

CRediT authorship contribution statement

Gaige Deng: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing Ningbo Kang: Methodology, Validation, Formal analysis, Investigation. Jianguo He: Project administration, Funding acquisition. Songlei Wang: Investigation. Guishan Liu: Investigation, Funding acquisition. Ning Liu: Supervision, Conceptualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge the Key R&D Program of Ningxia Hui Autonomous Region in 2018, “Research and development of key technology and equipment for cold chain storage of typical fruits and vegetables in Ningxia” (Grant No. 2018BCF01001).

References (21)

There are more references available in the full text version of this article.

Cited by (8)

  • Heat extraction from hot dry rock by super-long gravity heat pipe: Selection of working fluid

    2022, Energy
    Citation Excerpt :

    The super-long gravity heat pipe (SLGHP) system provides an alternative novel technical route for HDR heat extraction to the enhanced geothermal system that has not been really implemented because of the grand technical risks [7–10], and to the downhole heat exchangers [11–14] that need to consume pump work to circulate working fluid. Research on the use of large-scale heat pipes in geothermal energy extraction has been conducted for more than 20 years; notwithstanding, development of heat pipes for kilometers-deep geothermal energy exploitation is still in its early steps [15–20]. Several creative and inspiring designs were proposed [15,16] for hundreds-meter long gravity heat pipes.

  • Temperature response and thermal performance analysis of a super-long flexible thermosyphon for shallow geothermal utilization: Field test and numerical simulation

    2022, International Journal of Heat and Mass Transfer
    Citation Excerpt :

    With the aid of phase change and flow circle of the working fluid, a thermosyphon can efficiently transfer heat over a long distance at a relatively small temperature difference, without any extra energy consumption [3]. The high efficiency and reliability of the thermosyphons make them more attractive heat transfer devices, particularly in the super-long distance heat transfer fields such as the shallow geothermal energy utilization [4,5] and heavy oil recovery [6]. Shallow geothermal energy is available in rock and soil mass, groundwater, and surface water within a certain depth of 400m below the surface, and the temperature generally is below 25 °C [7].

  • Heat extraction from hot dry rock by super-long gravity heat pipe: A field test

    2022, Energy
    Citation Excerpt :

    The technical characteristics of SLGHP yield lower energy consumption and reduce maintenance cost, which in turn, widens the viability of using this technology for the HDR heat extraction. It should be noted that the heat pipes currently used in many applications do not meet the specifications required by kilometers-deep geothermal energy applications [30–35]. The literature reports several attempts to use large scale heat pipes in geothermal energy extraction in recent 20 years.

View all citing articles on Scopus
1

Gaige Deng and Ningbo Kang contributed equally to this work.

View full text