| In the background of energy shortages and environment pollution,it is particularly important to explore and use renewable energies.Geothermal energy stored in the earth is a kind of clean energy resources,and it is widely used in heating,power generation,bathing and so on.However,The traditional ways in geothermal uses need to pump the underground fluids out of subsurface so that if the underground water were not reinjected back to aquifer properly,those actions would cause some problems such as the quick drawdown of water level underground and even the risk of land subsidence.In addition,the underground pollutants or chemical disposal on the surface can cause the environment pollution on the ground.So it is necessary to develop a new method in geothermal utilization that without pumping out the fluids in aquifers but extract the geothermal energy only.In the present paper,we designed two different deep downhole heat exchanger(DDHE)systems in two prior existing deep wells.In order to get the firsthand data for the available heat extraction rate in DDHE,two in situ field experiments and their corresponding numerical simulations have been done.The first real DDHE experiment was completed in a reformed abandoned oil well located in the northeast of China.That reuse an abandoned oil well can not only reduce the backfill cost of the abandoned oil well but also save the cost for drilling a geothermal well.However,it is difficult to extract the geothermal energy by using such a well since it has in general a small well diameter and a dirty well inner surface.In the present paper,we firstly designed a DDHE system for an abandoned oil well,and then a one-dimensional heat conduction model was established,and it was verified by comparing with the field experimental data.Finally,we investigated some main factors on the extracted heating power.The results show that the system can give a stable heating power after 10 days when the system was run in a flowrate of 15t/h and the inlet temperature of 12?,the stable heating power was around 150 kW.It can be concluded that the main effect factors are the well depth and the local geothermal gradient.The results prove that the deep downhole heat exchanger has a great advantage in geothermal utilization.In terms of system operating parameters,the lower inlet temperature can get more heating power.And the heating power can also be increased by increasing the circulation flow rate,but a high flowrate cause a high pump power so that in a realistic operation we should choose a minimum flowrate for a specific heating power requirement.The second in situ experimental system was designed in Tanggu area,Tianjin.The DDHE test well had once been used as a reinjection well for two years successively.A temperature logging for the well at a static state was performed,and the test DDHE system was specially constructed according to our patent technology.The obtained results show that the DDHE system can give a high heating power at beginning,and the system could reach asymptotically to a stable state in only 4 days.the stable heating power is around 275 kW,it means that the heat power is 152.7W per meter in depth,this value is much higher than that of the first field experiment.In a word,this type of DDHE system can give a higher heating power.In the numerical simulation,an unsteady heat conduction model in a two-dimensional axial coordinate system is established.The result shows that the geothermal temperature can be recovered to its initial state if the system has been running in a heating season but with three times of the heating period stopping or recovering.The simulation results show that a maximum heating power of 500 kW is possible for the tested DDHE system while it was operated in a mode of 8 hours running and 16 hours stopping. |
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