Numerical Simulation And Experimental Study Of A Ground Sink Direct Cooling System In Cold Climates

Earth surface energy is a kind of new renewable energy source recognized by people in the past few years. This dissertation presents a novel utilizing manner of it, namely the cold energy extracted from ground by ground heat exchangers (GHEs) is directly supplied to the room by indoor terminals. The system by this manner for air conditioning is called as ground sink direct cooling system (GSDCS).Different from man-made heat sink systems, due to not using refrigerators, GSDCSs, this kind of natural heat sink system, not only have simpler configuration of system, but also own greater coefficient of performance (COP). The working fluid inside the GSDCS is water, which is relatively cheap and does not destroy the environment. Because of direct usage, the supply water temperature of the indoor terminal is evitably comparatively high. This leads to a relatively worse dehumidifying ability. Furthermore, the GSDCS is more suitable to be used to eliminate the sensible heat load of the building. On the other hand, the comparatively high supply water temperature also makes the supply wind temperature of the system comparatively high, thus thermal comfortability is good.First, this dissertation observes the performance achieved of the GSDCS by the experimental setup established, and systematically analyzes the characteristics of its parameters. Vertical U-tube ground heat exchangers are a key component in the GSDCS. In order to be able to accurately predict the performance of the component on condition that short time step (one hour or less) is used and at the same time guarantee high efficiency of the computation, this dissertation presents a temperature response factor (g function) model combining numerical and analytical methods on time for them. Of those, the cumulative constant heat flux model used to develop short time schale response factors is obtained by reconstructing the established Delaunay triangular mesh-based three-dimensional unstructured finite volume conjugate numerical model validated by both analytical and numerical paths. And development of medium and long time schales response factors uses the deduced analytical g function model for multiple boreholes configuration based on finite line-source theory. Besides the ground heat exchanger (GHE) model, this dissertation sets up the fan-coil unit model, on-off control fan-coil unit water system model and pump model. By supplementing to the physical relations between the components, establishment of a model for the GSDCS is finished. An example application is provided using an official building located in Harbin area. On basis of the designed GSDCS, the operating and performance characteristics of the system over one entire year are simulated and analyzed by using the established system model.The GHE model proposed in this dissertation integrates the following six traits: 1) Use Delaunay triangulation method to mesh the borehole field (borefield), thereby intactly retaining the original geometric structure in the borehole; 2) Consider the continuous flow of the fluid in the two pipe legs, thus the conjugate thermal processes occurring between the fluid in the pipes and the soil around it and between the two pipe legs are not taken separately any more; 3) The soil is divided into many layers in the vertical direction in order to account for the effect of changing fluid temperature with depth on the thermal process in the borefield; 4) The cumulative constant heat flux model is developed based on an inverse thought and thus inherits the three-dimensional conjugate characteristic of the original numerical model; 5) The g function model for a multiple boreholes configuration developed by superposition of an analytical g function model for single borehole is not only high efficiency in computation, but also so flexible as to be able to deal with arbitrary borehole arrangement, whether symmetric or not; 6) Joint the numerical g function and analytical g function on time in order to exert their respective advantages in different periods of time. Facing the on-off control fan-coil unit water system having extreme uncertainty in operation, this dissertation presents a so-called'time conservation'principle and on the basis of the principle makes an improvement to the existing scope model, thereby providing a possible path for simulation of this kind of system. A circulating pump is a main component consuming electricity in the GSDCS, so exact establishment of its model is very pivotal for accurate prediction of the system performance. The pump model developed in this dissertation not only can simulate the reduced load conditions at the nominal pump speed, but also can simulate the non-nominal pump speed conditions. This of those includes description to the variations of the pump efficiency, motor efficiency and variable frequency drive (VFD) efficiency at the reduced load conditions.