Characteristic analysis of bleeding effect on standing column well (SCW) type geothermal heat exchanger

Thermal performance is the most important factor in the development of a borehole heat exchanger utilizing geothermal energy.The thermal performance is affected by many different design parameters and different operating conditions such as bleeding.This eventually determines the operation and cost efficiency of the borehole heat exchanger system.The thermal performance of an open standing column well (SCW) type geothermal heat exchanger was assessed under the influence of bleeding.For this,a thermal response test rig was established with line-source theory.The test rig also had a bleeding function by releasing fluid while taking additional underground water through the heat exchanger.The thermal response test was performed with an additional constant input heat source.Effective thermal conductivity and thermal resistance were obtained from the measured data.From the measurement,the effective thermal conductivity is found to have 1.47 times higher value when bleeding is applied.The thermal resistance also increases by 1.58 times compared to a non-bleeding case.This trend indicates enhanced heat transfer in the SCW type heat exchanger with a bleeding function.Bleeding,therefore,could be an effective method of achieving a high heat transfer rate in the SCW type heat exchanger with sufficient underground water supply.

MODELING ON THE CYCLIC OPERATION OF STANDING COLUMN WELLS UNDER REGIONAL GROUNDWATER FLOW

Coupled hydrogeological-thermal simulation of the Standing Column Well （SCW） system is essential to provide an optimized configuration and operation schedule for boreholes on the site. This paper presents numerical investigations and thermo-hydraulic evaluation of standing column well system operating under cyclic flow regime. A three-dimensional numerical model for groundwater flow and heat transport is used to analyze the heat exchange in the ground. The model includes the effects of convective and conductive heat transfer, heat loss to the adjacent confining strata, and hydraulic anisotropy. The operation scenario consists of cyclic injection and recovery and four periods per year to simulate the seasonal temperature conditions. For different parameters of the system, performances have been evaluated in terms of variations in recovery temperature. The calculated temperatures at the producing pipe are relatively constant within a certain range through the year and fluctuating quarterly a year. Pipe-to-pipe distance, injection/production rate, ground thickness, and permeability considered in the model are shown to impact the predicted temperature profiles at each stage and the recovery water temperature. The influence of pressure gradient, which determines the velocity of regional groundwater flow, is most substantial.

Simulation of the flow and thermal breakthrough of a forced external circulation standing column well

The flow and thermal breakthrough phenomenon in a forced external circulation standing column well(FECSCW)directly affects heat transfer efficiency and load-carrying capacity.A numerical model for FECSCW is developed to analyze the migration of the temperature and velocity front under the flow and thermal breakthrough.The results indicated that thermal breakthrough began after simulation running 2.5 min and was completely formed after 12 min.The inlet water,which directly entered the production well without heat exchange with the aquifer,accounted for 12.8%.When the porosity of the backfill material decreased from 0.35 to 0,the coefficient of per-formance(COP)of the heat pump unit increased by 1.6%on average,and the thermal breakthrough strength decreased by an average of 45.3%within 25 min.Where seepage velocity near the well wall was greater than 1×10^(−3) m·s^(−1),faster velocity front migration was observed,while the migration advantage of the temperature front was more prominent outside of this region.Through quantitative analysis of flow and thermal breakthrough,temperature and velocity front migration,and COP change of heat pump unit,theoretical suggestions were pro-vided for the thermal transfer mechanism near the thermal well wall.The extended research in this study can be applied to the design and optimization of forced external circulation standing column well system.