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Study on Stagnant Water Body and Heat Transfer Performance Ofclosed-Loop Surfacewater Source Heat Pump System

Author: ChenJianPing
Tutor: LianZhiWei
School: Shanghai Jiaotong University
Course: Refrigeration and Cryogenic Engineering
Keywords: Surface water source heat pump system Stagnant Water Body Closed-loopHeat Transfer Performance Heat carrying load
CLC: TU831
Type: PhD thesis
Year: 2012
Downloads: 95
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As a renewable energy technology that saves energy and pollutes less, ground sourceheat pump can save HVAC energy consumption. So energy-saving buildings has vast markets,and has to be built according to its urgent needs, especially for our country is wealthy inresources. But there are some problems in application during the promoted applicationprocess: because the surface water source heat pump has been seldom used, the key designparameters of surface water temperature basic data and coil heat transfer performanceparameters is not adequate, the system integration lacks of maturity, the evaluation for thecarrying capacity of environment lacks of basis etc.. This paper aims at the problems ofsurface water source heat pump especially the stagnant closed-loop system, as their theoreticalsystems are imperfect and lack of basic data, and to reflect the system’s study principles of theorganic interaction, long-term dynamics and3D realistic characteristic, and then this paperfocuses on the theoretical calculation, experimental research and application analysis of thewater temperature, the coil heat transfer performance and the water heat carrying load in thesystem. The main research and conclusion are as follows:1) By studying the temperature transfer rule of stagnant surface water, a3D waterdynamic coupling model had been completely established and CFD software has beensecondary developed, and then the annual dynamic meteorological parameters, airtemperature, solar radiation, evaporation heat transfer, geotechnical wall and bottomtemperature etc. were inputted. By simulating a large number of numerical value, the all-yearhourly temperature distribution results of water has been obtained, and then we analyzed theusage of surface water source heat pump from the vertical temperature distribution law;because of the influence of solar radiation, the fluctuation of temperature gradient within3mof water meter was great by the influence of outside, which accounted for more than70%ofthe vertical temperature distribution, while it was relatively gentle and stable below5m, andthe temperature is about22℃~24℃in summer. Therefore, if use surface water as cold andheat source, the minimum recommended water depth is3m, and the ideal depth is5m ormore.2) Under the condition of Shanghai typical meteorological year, calculating the all-yearhourly water temperature of5m,10m and20m deep water, we got the results that: the annualaverage temperature of the entire water was close to the air temperature, the fluctuationgreatly deduced, and the maximum environmental temperature was36.8℃, while the highesttemperature of water was only27.2℃(5m water),25.7℃(10m water), and24.5℃(20mwater); when the minimum environmental temperature was-4.5℃, the minimum water temperature was only3.1℃(5m water)3.9℃(10m water), and6.4℃(20m water), whichshowed that the characteristics of good energy storage of water and relatively stabletemperature. Therefore, surface water source is a good air conditioning cool/heat sourcecomparing to the air source, and its advantages is obvious in summer; as to different depths ofwater, it can also be found that the deeper the water, the stronger the hysteresis property ofenergy storage changing with the environmental temperature, and the smaller the temperaturefluctuations, so the advantages of cold and heat source can be much more significant. By thecomparison of measured data of long-term and short-term, it indicated that the long-termannual data had statistical laws, and simulated values was closed to the measured values, so itcould be used as fundamental theoretic data for research applications.3)3D model had been established for different coils, which was relatively systematic andcomplete; had segmentation on the interface of hybrid grid with the hybrid grid method,which efficiently solved the precision, capacity, and speed of data calculation; using Ansyssoftware to solve multi-physical coupling, then simulated and obtained the heat transferresults of different types of coil heat exchanger (vertical spiral coil, horizontal spiralcoilmodel and horizontal U-shaped coil). Calculating and analyzing the simulation results byTecplot and Matlab software, and comparing and analyzing heat transfer property parametersof different coils, we can get that: the channel of the spiral coil is curve that enhances thedisturbing of circulation, which benefits the heat transfer of fluid in the coil. However, thethermal resistance of internal coil heat transfer only occupies about3.5%of total thermalresistance, so it has minimal impact with this advantage. Because of the shape arrangement ofthe vertical spiral coil, the heat storage which after the cooling of heat transfer coil can bewith two contradictory effects: the positive effect was that, as the increasing of watertemperature, the small density of fluid buoyancy was improved and the coefficient of naturalconvection heat transfer was increased; the negative effect was that, it may cause the watertemperature of external coil become higher, which led to the heat exchange of the internal andexternal coil became worse, while in turn, it could also affect the heat transfer. We shouldhave a comprehensive analysis of these two effects of different coils, because different effectslead to different heat transfer effects. And if the heat storage of vertical spiral coil was toohigh would lead to the negative effect rather than the positive effect, so the highest integratedheat transfer coefficient of the vertical spiral coil was25.97W/(m.℃), while the horizontalspiral coil of that was slightly higher than the horizontal coil, were225.81W/(m.℃) and25.58W/(m.℃), respectively; while at last, we can find that the heat transfer of vertical spiral coilwas the lowest, only67.00W/m; the positive effect of heat storage of horizontal spiral coilwas higher than negative effect, the integrated heat transfer effect was the best, which was73.02W/m; the integrated unit length heat transfer of the horizontal U-shaped coil was69.62W/m. Through the verification of multiple conditions testing, the results were close to theoretic calculating deviation, and0.9was the recommended correcting coefficient used astheoretic calculation parameter for model, which also could provided theoretical basis andreference data for the application of the closed coil heat exchanger and to guide theapplication of engineering.4)By coupling the indoor load and outdoor meteorological parameters, the5m,10m and20m water temperatures were simulated and calculated, and then the variation of water loadand the indicators of heat carrying load were analyzed. And we found that, the shallow waterwithin10m was prone to generate heat rather than cold, so that the average temperature of thewater was relatively high, which was an unfavorable factor for summer cooling-based systemwhile it was benefit for winter heating-based system; while as for deeper than10m water, itscold storage capacity was small, so after long-term running the water temperature wouldreduce year by year, which was benefit for summer cooling-based system, but wasunfavorable to the winter heating-based system.5) Under the limitation of environmental indicators, we try to work out that themaximum water heat carrying load of different depth of water were120W/m~2(5m water),140W/m~2(10m water) and190W/m~2(20m water), respectively. This paper puts forward forthe first time that the minimum water load of integrated environmental standard and energysupply property is the indicator of water heat carrying load: if the water is deeper than10m,the temperature of load water under the limitation of the environmental standards could alsoreach the energy supply requirements, which is the maximum indicator of water heat carryingload; if the water is of5m deep, the temperature of load water under the environmentalindicator could also reach the temperature of28.45℃, which could not meet the demand ofenergy supply property, therefore, the maximum heat carrying load is31W/m~2according tothe energy supply requirement. We can see from above that, the differences of maximum heatcarrying load is obvious under the conditions of different depth of water and different regionalclimate, therefore we should calculate different heat carrying load indicators according todifferent depths, areas, and regions.6) By combining with the application of Shanghai Dianshan Lake demonstration project,and by comprehensive studying of Water model, underground heat transfer model, systemenergy consumption model as well as the evaluation model of heat carrying load, wedeveloped the evaluation software of closed-loop surface water source heat pump, andcompleted the evaluation and implementation of system design, water heat carrying load, andestablished the monitoring platform of surface water source heat pump, and at last evaluatedthe property coefficient of seasonal energy efficiency.

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