Advances in ground heat exchangers for space heating and cooling:Review and perspectives

As a renewable energy source,geothermal energy has been widely used to provide space heating and cooling for buildings.The thermal performance of ground heat exchanger(GHE)is significant for the operating efficiency of the ground source heat pump(GSHP)systems.This paper presents a comprehensive review of developments and advances of three kinds of GHE,including vertical borehole GHE(VBGHE),Pile GHE(PGHE),and deep borehole GHE(DBGHE)which are currently popular in larger GSHP systems.Firstly,analytical models proposed to ana-lyze heat transfer process of VBGHE with different geological conditions are summarized,such as homogenous or heterogeneous ground,with or without groundwater advection.Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed.Secondly,a summary of research advances in PGHE is provided,which includes the heat transfer models of PGHE,the effects of geometric structure,oper-ation modes,pile spacing,use of phase change material(PCM),thermal properties of PCM,thermo-mechanical behavior and/or thermal performance of PGHE.The effects of groundwater flow direction and velocity on PGHE are also summarized in brief.Lastly,models of three kinds of DBGHEs,i.e.,deep coaxial GHE(DCGHE),deep U-bend GHE(DUGHE)and super-long gravity heat pipe(SLGHP),are reviewed.The physical bases of the dif-ferent analytical models are elaborated and also their advantages and disadvantages are described.Advances in numerical modelling and improving numerical model calculation speed of DCBHE,DCBHE array,and DUBHE are summarized.The review provides a meaningful reference for the further study of GHEs.

Submesoscale-enhanced filaments and frontogenetic mechanism within mesoscale eddies of the South China Sea

Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade,but in the eddy-active South China Sea(SCS)the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood.This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution(∼500 m)simulation.The satellite images show that the elongated highly productive stripes with a typical lateral scale of∼25 km and associated filaments are frequently observed at the periphery of mesoscale eddies.The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d)due to submesoscale instabilities.The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m^(2).This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments.Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity.The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy,but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence.In this context,these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy,as well as enhanced vertical heat transport.

Insights into multidimensional transport flux from vertical observation and numerical simulation in two cities in North China

This study represents the first quantitative evaluation of pollution transport budget within the boundary layer of typical cities in the Beijing-Tianjin-Hebei (BTH) region from the perspective of horizontal and vertical exchanges and further discusses the impact of the atmospheric boundary layer (ABL)-free troposphere (FT) exchange on concentration of fine particulate matter (PM_(2.5)) within the ABL during heavy pollution.From the perspective of the transport flux balance relationship,differences in pollution transport characteristics between the two cities is mainly reflected in the ABL-FT exchange effect.The FT mainly flowed into the ABL in BJ,while in SJZ,the outflow from the ABL to the FT was more intense.Combined with an analysis of vertical wind profile distribution,BJ was found to be more susceptible to the influence of northwest cold high prevailing in winter,while sinking of strong cold air allowed the FT flowing into the ABL influence the vertical exchange over BJ.In addition,we selected a typical pollution event for targeted analysis to understand mechanistic details of the influence of ABL-FT exchange on the pollution event.These results showed that ABL-FT interaction played an important role in PM_(2.5)concentration within the ABL during heavy pollution.Especially in the early stage of heavy pollution,FT transport contributed as much as 82.74% of PM_(2.5)within the ABL.These findings are significant for improving our understanding of pollution transport characteristics within the boundary layer and the effect of ABL-FT exchange on air quality.