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To date, the tunnel lining GHEs have encountered issues with the relatively low heat exchange rate and the rapid decline of heat exchange rate with time. This is because the huge heat accumulation around the tunnel lining GHEs obstructs the heat transfer between the absorber pipe and the surrounding rock during the GHEs operation. Hence, to improve the heat accumulation and enhance heat exchange rate, the two-phase closed thermosyphon (TPCT), an efficient device for long-distance heat transfer, was employed in the tunnel lining GHEs to build a thermally induced channel between absorber pipes and the surrounding rock, accelerating heat transfer between the absorber pipe and surrounding rock. The heat transfer model of the tunnel lining GHEs using TPCTs was built to analyze the thermal performance enhancement mechanism of tunnel lining GHEs using TPCTs for building cooling with different thermal conductivities of primary lining concrete and surrounding rock, and convective heat transfer coefficients (CHTCs) on tunnel internal walls. The results showed that throughout a 90-day operation, TPCTs dramatically boosted the heat injection rate of tunnel lining GHEs from 16.5 to 27.0 W/m2 with an increase of 63.6%. Thermal conductivities of the primary tunnel lining concrete have significant effects on the heat injection rates of the GHEs with TPCTs. The heat injection rates increased from 27.0 to 36.4 W/m2 as thermal conductivities of the concrete increased from 1.85 to 10.7 W/m K, and the enhancement rate ranged from 63.6% to 94.7% when compared to tunnel lining GHEs without TPCTs. The enhancement rate of tunnel lining GHEs with TPCTs decreased with increasing thermal conductivity of surrounding rock and CHTC, reaching up to 122.1% under the thermal conductivity of 1.4 W/m K and CHTC of 0 W/m2 K. Overall, TPCT greatly enhances the heat injection rate of tunnel lining GHEs, implying that it is a promising technology. [Display omitted] [ABSTRACT FROM AUTHOR]

The application of traditional borehole ground heat exchangers (GHEs) is extremely limited in tropical zones because of the performance degradation induced by thermal imbalances over the years. Unlike traditional borehole GHEs, tunnel-lining GHEs have absorber pipes between the primary and secondary linings, allowing tunnel ventilation to aid in ground temperature recovery during off periods. To date, the application potential of tunnel-lining GHEs in tropical zones has rarely been reported. In this study, a coupled heat transfer model for heat pumps and tunnel-lining GHEs was built to investigate the effects of tunnel ventilation on thermal imbalance and analyze the feasibility of tunnel-lining GHEs in tropical zones under different thermal conductivities of the surrounding rocks, annual average temperatures, and wind velocities. The results showed that tunnel-lining GHEs effectively reduce the influence of thermal imbalance. The maximum outlet temperature increase for absorber pipes induced by thermal imbalance was 0.54 °C for ten years under building cooling, implying that no obvious annual performance degradation of tunnel-lining GHEs occurs in the tropics of China. However, not all tropical zones of China are suitable for tunnel-lining GHEs because of their excessively high outlet temperatures ranging from 31.2 °C to 41.4 °C. Hence, parametric design charts were proposed to determine the outlet temperature for a preliminary evaluation of feasibility. Finally, parametric design charts were employed in an actual tunnel-lining GHEs project in a tropical zone. Energy, environmental, and economic performance were analyzed for this project. The electricity usage and CO 2 emission of tunnel-lining GHEs decreased by 24.6% compared to the cooling towers, and the payback period was 5.9 years. [Display omitted] [ABSTRACT FROM AUTHOR]

The use of phase change materials (PCMs) within the precast high-strength concrete (PHC) energy pile is relatively rare. A study concerning the thermal performance of the PHC energy pile backfilled with PCMs were performed to compare with conventional backfill materials. Field tests of the PHC energy pile were conducted. Then, different types of heat transfer numerical models for the PHC energy pile backfilled with water, ordinary grout, and PCMs were established, and the validities of models were confirmed by the field test data and other published studies. The following results are obtained from this study: (1) the thermal performances of the PHC energy pile backfilled with ordinary grout and PCM-type backfill materials (i.e., PCM, enhanced-PCM, and enhanced-PCM-1) are significantly greater than that backfilled with water under continuous operation; (2) intermittent operation has a larger influence on the heat exchange rate of PCM-type backfilled PHC energy pile than ordinary grout backfill material, and enhanced-PCM and enhanced-PCM-1 backfill materials generate higher heat exchange rates for the PHC energy pile than the ordinary grout under intermittent operation; (3) enhancements in the thermal properties of PCMs and intermittent operation both are important to improve the heat transfer performance of the PHC energy pile backfilled with PCMs, and the lower melting temperature of PCMs helps to enhance the thermal performance of PCM-type backfilled PHC energy pile in the cooling mode; and (4) under higher velocity of groundwater flow, ordinary grout backfill material is more suitable for the PHC energy pile compared with PCM-type backfill materials. • Field thermal response tests of PHC energy pile are carried out. • 3D heat transfer numerical model of PCM backfilled PHC energy pile is established. • Impact of intermittent operation on heat transfer of PHC energy pile backfilled with PCMs is analyzed. • Impact of groundwater on heat transfer of PHC energy pile backfilled with PCMs is assessed. [ABSTRACT FROM AUTHOR]