Research On Heat Transfer Performance Of Vertical Pipe Ground Source Heat Pump In Harbin Area

Heat pump technology is an effective technology for using low temperature renewable energy, it also is one of the effective measures that solve the energy and environmental problems in HVAC. The problem that between buried pipe and soil heat transfer is the key technology of ground source heat pump. So set up buried pipe heat transfer model reasonably, it has very important significance to study ground heat exchanger heat transfer process.This paper established physical and mathematical model of vertical ground heat exchanger temperature field, according to the heat transfer law and restricting factors of vertical ground heat exchanger, and it was simulated by using the CFD fluid simulation software. Second analysis the heat transfer factors that influence on u-shaped ground heat exchanger according to the established model, and put forward the optimized operation method of u-shaped ground heat exchanger. Major research works and results are following:(1) Analysis the simulation results of surrounding soil temperature of heat exchanger when the heat pump running on winter working condition in Harbin, it gets that soil temperature in vertical direction is mainly affected by the ground temperature and the u-shaped tube fluid, and the change of soil temperature is bigger that close to buried pipe in the horizontal direction. The heat action will decrease with the increase of operation time.(2) Compared with continuous operation condition, it can make the soil temperature restore with the intermittent operation mode, and provide higher heat source temperature and lower source temperature under the winter working condition and summer working condition respectively, heat pump units operation efficiency is also get improve. When the ratio of startup-shutdown of intermittent operation mode is12-12, the heat exchange amount of the unit well depth increased by11.7%after running three days later.(3) The heat exchange amount of the unit well depth will increase with the increase of flow velocity. When the flow velocity increase, heat exchange between fluid and heat transfer medium is incomplete, but when the flow velocity is too low, the effect of thermal interference is more obvious, so the flow velocity that should choose from0.3m/s to0.7m/s is appropriate. The coefficient of thermal conductivity of backfill material is bigger, and the heat exchange amount of the unit well depth is bigger. The influence of temperature radius around the U-tube gradually expanded, the interference phenomenon in pipes is more and more obvious. The coefficient of thermal conductivity of buried pipe material is bigger, and the heat exchange amount of the unit well depth will gradually increase. Compared with increasing flow velocity, the increase of heat exchange amount is small, so it can meet the engineering requirements to use high density polyethylene pipe. (4) To add thermal insulation layer at the outlet of ground heat exchanger, the average outlet temperature increased1.1℃when the heat pump units running72h, the heat exchange amount of the unit well depth is about29.3W/m. Compared with no thermal insulation layer, the heat exchange amount is24.4W/m, it increased by20%. The temperature increased about0.8℃at the center of two horizontal pipe corresponding underground35m center, and with the increase of distance from center position, the temperature of the soil around also increase accordingly.(5) After add thermal insulation layer, the heat exchange amount of the unit well depth get improve effectively with the intermittent operation mode. The heat exchange amount is35W/m after the third time running, and the heat exchange amount is29.9W/m without thermal insulation layer. Compared with add thermal insulation layer, it increased by17.1%. The soil temperature recovery rate increased from66.8%to71.6%. With the increase of distance from center position, the growth of the soil temperature recovery rate was less, the distances from the center of the branch pipe1m location, soil temperature recovery rate increased from0.19%to0.5%.