Yi Man

Development of spiral heat source model for novel pile ground heat exchangers

The extensive application of ground coupled heat pump system (GCHP) is restricted by the installation cost of conventional borehole ground heat exchangers (GHE), which is not only expensive but also requires additional ground area. The idea of combining the GHE with building foundation piles, i.e., the “energy pile,” has arisen in recent years. The novel pile GHEs consisting of vertical spiral coils buried in the piles of a building, are proposed for their flexibility and convenience for applications. Evolved from existing models, a new spiral source model and its analytical solutions are established in this article for thermal analysis and design of the pile GHEs. The temperature field of the pile and its surrounding soil is simulated; in addition, the temperature responses of the coil pipe wall and the circulating water entering/effusing the pile GHE to the short time-step heat transfer loads are deduced based on the new spiral source model. The study indicates that the heat transfer features of the novel pile GHE can be described adequately by the proposed new spiral heat source model, which provides an appropriate tool for thermal analysis and design of the novel pile GHE or other potential similar engineering problems.

Research on Design and Simulation of Folded Baffle Solar Air Collector

In order to promote the outlet temperature and heat collection efficiency of solar air collector, this study puts forward a folded baffle solar air collector, which can improve the heat collection efficiency of the collector by enhancing the air turbulence inside the flow passage and increasing the heat exchange area. In this paper, the outlet temperature and heat collection efficiency of solar air collector with different baffle angles and different inlet flow velocities are simulated and calculated by the fluent software. The results show that under the conditions of inlet temperature of 278 K and ambient temperature of 274 K, the collector with inlet flow rate of 2 m/s and folding angle of 60° is more suitable. Introduction Using clean energy such as solar energy for heating is an effective way to actively promote the national energy administrations northern winter clean heating plan ( 2017 2021 ), and the solar air collector heating system is simple in structure, free from frost protection problems, low in cost and convenient to maintain compared with the conventional solar water system [1,2]. In order to improve the heat collection efficiency of solar air collector, some efforts have done in this area. Cheng etc [3]. Proposed a new type of solar air collector with parabolic absorption plate structure. The instantaneous efficiency and pressure loss of different inclination angle are discussed. The crossed v-shaped solar air collector with wave plate proposed by Chen [4] changes the height of the air flow channel to enhance the heat conversion efficiency between air and the heat absorbing plate. In order to further improve the photo-thermal conversion efficiency of solar air collector, a folded baffle solar air collector with different baffle angles are designed and simulated in this paper. Physical Model of Solar Air Collector The physical model of the folded baffle air collector designed in this article is shown in Fig. 1. 48 Copyright

Design on Nocturnal Cooling Radiator for Cooling Provision

As a natural passive cooling technology, nocturnal radiator can reject heat into the sky by electromagnetic infrared radiator exchange. In order to take full use of nocturnal radiator for cooling provision at night, this paper put forward two types of novel nocturnal cooling radiators in panel structure with different forms which is covered with high emissivity film installed horizontally against on the building roof. Both ends of the panels are connected to sub-catchment and thermal insulation material is spread below the panels. Fluent software is utilized to simulate the cooling performance of radiators and analysis the outlet temperature of water. Based on simulation results, the proposed radiator can satisfy sizable cooling requirement as the heat rejecter with low initial cost and operation fee for practical project utilization. Introduction It is well known that cooling energy consumption has a large proportion in the total energy use especially in hot climate countries. In order to achieve indoor thermal comfort, a great amount of energy need to be consumed for cooling and heating. Consequently, it is of great importance to search renewable energy, which can restrict energy expenditure in the buildings. Under the circumstance, the use of passive cooling techniques can well improve the current situation. Radiative cooling techniques are based on the principle of heat loss by long-wave radiator emissions, from a body towards another body of lower temperature, which is considered as the heat sink [1]. By exploiting the “atmospheric window” in the range of wavelengths 8-13μm to achieve the objectives of cooling. Long-wave radiator happens during the daytime and nighttime, which is referred to as solar radiator in daytime and nocturnal radiator cooling in the nighttime. By applying cooling radiator in the buildings to achieve the the purpose of cooling. But as it is pointed out by Eicker and Daliband [2], nocturnal radiative cooling is still not applied widely in today’s buildings. Sikula et al use hybrid roof panel and solar energy to evaluate the usability [3]. By Hollick’s paper, night cooling radiator is not just limited to the night time [4]. Man studied a novel nocturnal cooling radiator which was used as a supplement heat sink [5]. Matsuta et al. studied a solar heating and radiative cooling system using a solar collector-sky radiator [6]. The performance of nocturnal radiative surfaces with the convective heat gain inhibited by a polyethylene film covering the radiative surface has been studied by Mostrel and Givoni [7]. Research focused on the high-emissivity paints for nocturnal radiative surfaces was studied by Harrison and Walton [8]. Based on different application, the passive nocturnal cooling system can be classified into passive system and hybrid system. But the direct application of radiative cooling has several limitatiffons. Firstly, the cooling of capacity of nocturnal radiative cooling can not match the building cooling loads and can not be used at daytime when the peak building cooling loads exists. And furthermore, the cooling performance of the radiative cooling is influenced by cloud and ambient temperature. Therefore the first system is rarely applied in the structure. The existing buildings usually work with the cooling tower to exclude the unbalanced cooling load. But in actual running processes, the 54 Copyright

Design and Analytical Analysis of Foundation Pile Ground Heat Exchanger with Spiral Coils

Recently, utilization of building foundation piles as the ground heat exchanger (GHE) received more and more attention since it can reduce the initial cost and land area requirement compared with the borehole GHE. This study designs a foundation pile GHE with spiral coil (FPGHE) by intertwing the circulating coil pipe tightly in spiral shape against the reinforcing steel of a pile. The distinct advantage of this proposed FPGHE is that it can offer higher heat transfer efficiency, reduce pipe connection complexity, prevent air blocking and decrease the thermal “short-circuit” between the feed and return pipes compared with other existing configurations. In order to analyze its heat transfer characteristic, analytical models are established for the proposed FPGHE. Analytical thermal analysis is carried out to simulate temperature responses of the coil pipe wall and the circulating water entering/effusing the FPGHE to the short time step heat transfer loads based on the established analytical model. Furthermore, the operation performance and heat exchange capacity of the FPGHE is investigated. INTRODUCTION The ground heat exchangers (GHE) with vertical boreholes (Bose, et al. 1985) have been the mainstream technology for the ground coupled heat pump systems, but the high initial cost and land area requirement to install the borehole GHE remain the major obstacles of this technology. Since the foundation pile is commonly used in high rise buildings, combining the heat exchanger and building foundation pile can eliminate the drilling expense and land area requirement of borehole GHE. Therefore, utilization of building foundation piles as the GHE has received more and more attention(Mehrizi, et al. 2016; Luo, et al. 2016; Huerta and Krarti 2015; Loveridge and Powrie 2014). In existing studies, pipes are usually buried in foundation piles in configurations of U-tubes, W-tubes or spiral coils. For the first two configurations, the heat transfer area inside foundation pile is small and the air blocking may occur in pipes. In order to overcome these drawbacks, this study focus on the third type and designs a foundation pile GHE with spiral coil (FPGHE). The circulation coil pipe is intertwined tightly in spiral shape against the reinforcing steel of a pile, and is disposed within about 0.1m of the pile’s outer surface. The distinct advantage of FPGHE is that it can offer higher heat transfer efficiency, reduce pipe connection complexity, prevent air blocking and decrease the thermal “short-circuit” between the feed and return pipes compared with other existing configurations. The schematic diagrams of a conventional single U-tube vertical borehole GHE and the FPGHE with spiral coils are compared in Figure 1. Figure 1 Schematic diagram of a vertical borehole and a FPGHE with spiral coil Modeling the FPGHE with spiral coils is complex and existing studies concentrated on the numerical or experimental methods. Suryatriyastuti carried out a 3-D numerical heat transfer analysis on the energy pile under the constant heat load case (Suryatriyastuti, et al. 2012). Xiang built a numerical model includes a 1-D transient convection–diffusion submodel for the fluid domain and a 1-D transient diffusion submodel for the solid domain (Xiang, et al. 2015). Luo conducted the thermal performance test to analyze the operation performance of energy pile under an intermittent condition (Luo, et al. 2016). In the present study, an analytical method is explored as it can provide a more practical and convenient tool for engineering design, as well as thermal analysis of the FPGHE, compared with existing numerical and experimental methods. For the proposed FPGHE, its diameter is much thicker and depth is usually shorter compared with the borehole GHE. Obviously, classical heat transfer models for the borehole GHE fail for the FPGHE. By analyzing the heat transfer process of proposed FPGHE, the analytical finite spiral heat source model is established in this study based on the Green’s function theory, the virtual heat source theory, and the superposition method. The temperature responses of the spiral heat source, the coil pipe wall, and the circulating water entering/effusing the FPGHE to the short time step heat transfer loads are deduced based on the established analytical model. Then the operation performance and the heat exchange capacity of the FPGHE is investigated. DESIGN OF FOUNDATION PILE GHE WITH SPIRAL COIL The high-density polyethylene (HDPE) pipe with exterior and interior diameters of 25mm and 20mm, respectively, are selected as the circulation pipe of proposed FPGHE. First hydrostatic test with pressure of 0.8Mpa and last for 15min are required in order to prevent the leakage before the pipe are installed. Then the pipe is intertwined tightly in spiral shape against the reinforcing steel cage of a pile, as shown in Figure 2(a). The second hydrostatic test with pressure of 0.8Mpa and last for 15min should be carried out followed. Then the combination of reinforcing steel cage and coil pipe are put into the hole of pile. After the third hydrostatic test with pressure of 0.8Mpa and last for 2 hours, the last step is the concrete pouring of the pile foundation, as shown in Figure 2(b). Extreme caution should be paid during the concrete pouring, and coarse aggregate in concrete must be smooth and non-angular particles. The conduit are utilized to lead the concrete into the bottom of pile hole, and it should be extracted gradually from the bottom to top according to grouting speed. During the placement and extraction process of conduit, it is important to keep the vertical and center for preventing the hanging cage of concrete, ensuring the compaction of pouring and decreasing the heat transfer resistance. The concrete pouring process is finished when the density of return slurry is identical with which of pouring concrete. grout U-tube concrete pile spiral coil