Experimental Simulation And Theoretical Investigation On Downhole Heat Exchanger

The abundant low-temperature geothermal resources provide China an opportunity to save conventional fossil fuel resources. If fully developed, the growing demand for coal and oil with the economic and social development could be relieved significantly. The two common used geothermal heating options, i.e. the open loop groundwater system and the ground coupled system, have some limitations in their applications. The former needs to reinject the produced groundwater back to the aquifers to prevent from the drawdown of water level or even the subsidence. The latter generally requires multiple boreholes due to its poor heat transfer performance. Thus, a new option that combines the advantages of the both is necessary to promote the geothermal utilization in China. Downhole Heat Exchanger(DHE) is the very one. It extracts heat from a geothermal well through a U-shaped pipe by natural convection driven by the temperature difference between the water in the pipe and that in the well. No geothermal fluid is taken from the well during the heat transfer process. Thus, DHE is a clean method. However, the keypoint to make an efficient use of DHE depends on both the knowledge of its heat transfer characteristics at given strata conditions and that of its thermodynamic performance.A simulated experiment system was set up in laboratory, which can provide different water temperature at a changeable flow rate to simulate the seepage velocity in the strata. The simulated system meets all requirements on flowing and geometry similarities with the real one. Thus, the experiment results can be used as a reference to deterimne the influencial factors in project design. About 40 series of experiments were carried out under various parameter combinations, such as the geometric parameters, working conditions and reservior permeability, etc. A heat transfer correlation was presented based on the analyses of different factors, which has a relative error of 23.3% compared with the experiment results.A theoretical simulation was performed to study the convective heat transfer in the porous reservoir based on Brinkman-Forchheimer function. With the tested porosity, Darcy number and Rayleigh number, the BGK model of the Lattice Boltzmann Method (LBM) was used to solve the conservation euqations. Its validity was concerned by the comparison with the results obtained by Finite Elemental Method(FEM) for a squared porous medium. The temperature field in the porous reservoir under different temperature grade on the cooling wall was investigated. The results showed that a minus grade was favorable to the natural convective heat transfer.An exergy analysis model was presented considering the common structural features of closed loop ground systems, in which the exergy value at different state was calculated with the hourly ambient temperature as reference. The model was solved for DHE based on experiment correlation presented in this paper. The effects of the ambient temperature, working conditions and geometric parameters were thoroghly analyzed for the thermodynamic performance of DHE. A parameter named exergy ratio on source side, representing the availability of the net exergy from the heat source, was introduced, which can be considered as a criteria of exergy performances for different closed ground systems. Its two critical values, that is, 0 and 1 denote two suitability indexes for applications in different climate and working conditions. Additionally, DHE has a significant advantages in the exergy ratio compared with the ground coupled system even at a very low inlet water temperature.