چكيده به لاتين
The dynamic behavior of a borehole heat exchanger (BHE) plays an important role in operation of ground source heat pumps. Thus the transient 3D numerical modeling of U-tube and coaxial BHEs are investigated here and their performances in cooling and heating modes of heat pump are compared for different mass flow rates of water. The simulation was performed using ANSYS FLUENT 16.0 software based on the finite volume method to find the temperature distribution around the buried pipes with specific vertical soil temperature profile during the summer. Various parameters are studied and compared for the above two types of ground heat exchangers such as outlet fluid temperature from the ground, heat transfer rate in evaporator and condenser, compressor and pumping power, coefficient of performance of heat pump and ground source heat pump, effectiveness of the two heat exchangers as well as the effect existing tubes in vertical and horizontal temperature distribution inside the soil. The impact of using geothermal energy in fuel and electricity cost reduction, as well as reduction of carbon dioxide production are the other issues that have been investigated in this present research. According to the results, in the cooling state that the hot existing water of condenser enters the inner pipe of coaxial BHE, the performance of coaxial BHE is better than U-tube ones in terms of the above-mentioned parameters. However, the coaxial heat exchanger results in faster ground saturation and, continuous and long-term using of it affects its performance. Results showed that the injection heat transfer rate to the ground in summer, in the coaxial BHE with fluid injection direction to inner pipe at mass flow rates of 0.8, 1, 1.2 kg/s were 5.34%, 11.9%, 16.5% higher than U-tube BHE respectively. In cooling mode that cold existing water of evaporator enters the annular region of coaxial BHE, due to heat dissipation of coaxial BHE, the performance of the U-tube BHE at the corresponding mass flow rate is a little better, and heat extraction rate at mass flow rates of 1, 1.2 kg/s were approximately 2.45% and 3.75% higher in U-tube BHE and at mass flow rate of 0.8 kg/s, both heat exchangers almost have same performance. Also, as a result, the cost-saving on electricity consumption for a year in coaxial BHE in compression to U-tube BHE is greater, so using the former is economical. Higher mass flow rates are recommended if the goal is to reduce electricity consumption, and its cost, but lower mass flow rates are more desirable if the goal is to improve the performance of the heat exchangers and heat pump and to have a low impact on the temperature distribution of the soil.
