Shallow Geothermal Exploitation In Karst Area:Methods To Curb Thermal Accumulation

With the increase of energy consumption,environmental protection,sustainable development,energy saving and emission reduction are the themes of today's society.According to the requirements of the 13th Five-Year Plan,the promotion and application of renewable energy as a supplement to traditional energy is an indispensable way for a country's environmentally friendly and sustainable development.Among various sectors,the energy consumption for buildings has accounted for a considerable proportion of the total energy consumption of the society,and the ground source heat pump(GSHP)is a system that efficiently utilizing the shallow geothermal resources to satisfy heating and cooling demand for the building.The great merit of the GSHP is its low energy consumption and easy for maintenance,which has been recognized by the society.While in the application of GSHPs,due to the different geological conditions,different climates and building types,plus the year-round operation strategy of the units,all these factors will affect the proportion of heat extraction from the ground and heat injection to the ground,and subsequently affect the service life and efficiency of the heat pump system in consequence of the thermal imbalance.The karst landforms are widely distributed in southern China,with the specific hydrogeological conditions and abundant groundwater.Not only the thermophysical characteristics of the rock mass,but also the special karst geological structures underground,and dynamic groundwater transformation,all exert a significant influence over the ground heat transfer efficiency of the ground heat exchanger(GHE)and ground heat balance.Therefore,specific studies on the heat transfer characteristics of GSHPs in karst areas for dealing with the thermal imbalance(heat accumulation)problem,will help the promotion and application of GSHPs in karst areas.This article concentrates on addressing the heat accumulation in the application of GSHPs located in karst areas,and innovatively proposes two solutions to cope with the heat accumulation with the long-term operation by GSHP units.First of all,a novel hybrid ground source heat pumps(Hy GSHP)that mainly uses karst water was proposed,and correspondingly,a sizing method for the novel Hy GSHP was also established.Based on a real project,economic comparison between the proposed Hy GSHP solutions with the solo ground coupled heat pump(GCHP)and analysis were presented.Then,another potential solution firstly proposed in this paper is to take advantage of the existence of the karst groundwater by fracturing the rock in the vicinity of the GHEs,which is a mature technique in the field of oil exploitation.The optimized GHEs are called“enhanced ground heat exchanger”(EGHE).To introduce the EGHE,a numerical study on the performance of GHE influenced by three factors including different waterflow velocities,variable numbers of fractures and differentiated fractures distribution was carried out.The numerical simulation software COMSOL Multiphysics was used to conduct the simulation of the long-term operation of the GSHP system,recording a series of indicators to evaluate the influences by waterflow in fractures,including temperature field distribution,near-field average temperature,GHE outlet water temperature,coefficient of performance for cooling(COPc),and heat transfer per unit length were obtained.In this way,to quantify and assess the performance of various influencing factors on GHE and GSHP buried pipes in karst areas.Finally,at the basis of the study of the simulation work,the concept of"enhanced GHE"was proposed in this paper.Likewise,simulation work was taken again to evaluate the influence of artificial fractures on the performance of EGHEs.It is found that the Hy GSHP system selection solutions proposed in this paper can ensure that the annual ground heat balance,and theoretically,the heat imbalance rate can be controlled to close to zero.One of the solutions(groundwater source heat pump+soil source heat pump+air source heat pump in summer,groundwater source heat pump+ground coupled heat pump in winter)is more cost effective and is more potential than using a GCHP alone,which can reduce the annually cost up to 16.10%.Another solution is to use ground water-source pumps and air source heat pumps in summer,ground water-source heat pump and gas boilers in winter.When amount of karst water is sufficient,the total annualized cost of this solution is equivalent to that of using solo GCHP,but the initial investment 73%reduction.This is a good choice for companies to reduce initial investment but cater the cooling and heating demand.On the other hand,through simulation,it is established that the velocity of fracture water flow has limited effect on the underground temperature field and the efficiency of the heat pump when the water flow velocity is beyond a certain threshold value;it is also identified that the more fractures the better performance of the units.It is worth noting that the impact of horizontal fractures have a remarkable effect on the performance of the units than vertical fractures do.The heat transfer per unit length of the buried pipe in the fractured rock mass can be up to 78.83%higher than that in the unfractured rock mass,and the COP_c is correspondingly 4.5%higher than that of the buried pipes in no fracture rock mass.On this basis,the concept of“Enhanced Ground Heat Exchanger”(EGHE)has been first proposed,which aims to increase the permeability of matrix mass encompassed buried GHEs by hydraulic fracturing near the wellbore,enhance the flow of groundwater transportation,and accelerate the heat exchange between GHEs and matrix mass,reducing the risk of heat accumulation and increasing the service life of the GSHP system.By comparison to natural fractures,man-made fractures can be controlled to stick to the wellbore wall,taking more heat away.According to the research,when there were 1 vertical pair of fractures and 1horizontal pair,the heat transfer rate per unit length of the EGHE of the two scenarios are increased by 12.46%and 25.38%respectively compared with the GHE buried in intact rock bodies.When the fractures pair added to 5,vertical fracture pair and horizontal fracture pair increased by 112.07%and 65%respectively under same conditions.It is suggested that artificial fractures can maximize the heat exchange efficiency and performance increase.