Showing papers by "Dengjia Wang published in 2022"

Long-term thermal performance analysis of a large-scale water pit thermal energy storage

Abstract: Large-scale water pit thermal energy storage (PTES) promotes solar district heating (SDH) system as one of the most potential renewable applications for carbon neutrality. PTES needs vast investment and operates in a complicated system with numerous components, highlighting the need for a suitable simulation tool for tech-economic and feasibility investigations. This paper experimentally and theoretically investigated the long-term thermal performance of a 60,000 m 3 PTES in Dronninglund, Denmark. Five years measurements were analyzed to investigate the development of temperatures, heat flows, and thermal stratification in heat storage. A modified 2D model was proposed to calculate the thermal performance of the large-scale PTES based on the XST model in TRNSYS. The results showed that the developed model predicts well the storage temperatures and the heat flows. For one-year validation, the deviations of annual charged/discharged energy, internal energy content, and annual thermal loss between the model and the measurements were 2.0%/1.8%, 2.8% and 1.3%, respectively. The Dronninglund PTES showed 90.1% storage efficiency in the five-year investigation due to its high storage utilization cycle of 2.16. Even without any insulation on the sidewall and the bottom of the PTES, the average thermal loss from the two parts only accounted for 32.3% of total heat loss. Meanwhile, the soil region needed four years to stabilize. Approx. 24.4% of the heat loss in summer from the sidewall and the bottom is regained by the PTES in winter, when there is a low temperature in the heat storage. The findings of the paper serve as a good reference for designers and practitioners of water pit heat storage. • A 60000 m 3 water pit heat storage investigated experimentally and theoretically. • Annual charged/discharged energy and annual thermal loss obtained. • Trnsys model of the PTES developed and validated. • Long term thermal performance of the PTES investigated in 5 consecutive years. • Development of soil temperatures around the PTES elucidated.

Experimental study on the influence of temperature and humidity on the thermal conductivity of building insulation materials

Abstract: At present, thermal conductivity is usually taken as a constant value in the calculation of building energy consumption and load. However, in the actual use of building materials, they are exposed to the environment with continuously changing temperature and relative humidity. The thermal conductivity of materials will inevitably change with temperature and humidity, leading to deviations in the estimation of energy consumption in the building. Therefore, in this study, variations in the thermal conductivity of eight common building insulation materials (glass wool, rock wool, silica aerogel blanket, expanded polystyrene, extruded polystyrene, phenolic foam, foam ceramic and foam glass) with temperature (in the range of 20–60 °C) and relative humidity (in the range of 0–100%) were studied by experimental methods. The results show that the thermal conductivity of these common building insulation materials increased approximately linearly with increasing temperature with maximum growth rates from 3.9 to 22.7% in the examined temperature range. Due to the structural characteristics of materials, the increasing thermal conductivity of different materials varies depending on the relative humidity. The maximum growth rates of thermal conductivity with humidity ranged from 8.2 to 186.7%. In addition, the principles of selection of building insulation materials in different humidity regions were given. The research results of this paper aim to provide basic data for the accurate value of thermal conductivity of building insulation materials and for the calculation of energy consumption.

Ionic Liquids as Thermal Fluids for Solar Energy Storage: Computer-Aided Molecular Design and TRNSYS Simulation

Abstract: Due to the great potential of ionic liquid (ILs) for solar energy storage, this work combines computer-aided ionic liquid design (CAILD) and a TRNSYS simulation to identify promising IL candidates as simultaneous thermal storage media and heat transfer fluids. First, a mixed-integer nonlinear programming (MINLP) problem is formulated to search for optimal IL structures, where the thermal storage density integrating the IL density, heat capacity, melting point, and desorption temperature is employed as the objective function and the physical properties, i.e. thermal conductivity and viscosity, are implemented as optimization constraints. After that, TRNSYS simulations of a solar energy power system with the designed ILs as thermal fluids are conducted during a typical meteorological year to further study the practical performance of the IL candidates. Through the analyses of annual system operation performance as well as the long-term cost savings of such a system, the application feasibility and sustainability of the designed ILs for solar energy storage are well interpreted.

Theoretical, experimental and numerical study on the influence of connectors on the thermal performance of precast concrete sandwich walls

Abstract: The thermal performance of precast concrete sandwich walls is in direct relation to energy efficiency of prefabricated buildings. This paper presents the theoretical, experimental and numerical studies on the thermal performance of precast concrete sandwich walls. The equivalent heat transfer coefficient model was proposed by theoretical analysis, and verified by experimental and numerical study. Six specimens with three types of connectors were fabricated, tested and modeled. The theoretical, experimental and numerical results agreed well, the accuracy of the equivalent heat transfer coefficient model was verified. It revealed how the material and geometrical parameters of connectors influenced the thermal performance of walls. The results showed that as the thickness of insulation increased from 30 mm to 90 mm, the heat transfer coefficient decreased by about 56.8%, 58.8% and 59.9% for theoretical, experimental and numerical results respectively. Meanwhile, an increased spacing and decreased connector length led to a better thermal performance, which further indicated the necessity to research the impact on thermal performance caused by connectors. It was also found that the proposed model was more suitable for the walls with stainless steel connector in this paper, for the walls with low thermal conductivity of connector like GFRP connector, the equivalent heat transfer coefficient could be obtained based on a prediction model of thermal bridge influence area and revised by correction factor in further research.

Impacts of low atmospheric pressure on distribution of indoor hot air jet in plateau area

Abstract: The existing design parameters of hot-air heating for local buildings do not consider the distinctive low-pressure that can be experienced in the plateau regions, resulting in significant inefficiency. In this study, a numerical model based on the Reynolds time-averaged method coupled with the RNG k–ε turbulence model was established by considering the air thermal properties influenced by pressure. Four atmospheric conditions of 101.325 kPa, 84.547 kPa, 70.093 kPa and 57.708 kPa were selected to simulate the velocity and temperature fields of the horizontal and vertical hot-air jets. The influence of low pressure on the flow characteristics of hot-air jets was analyzed subsequently. The results suggest the jet flow patterns are essentially similar under various pressures. The horizontal hot-air jets bent upward more significantly and the diffusion range of the vertical jets was reduced under low pressure. The jet velocity decayed faster at low pressure, and the axial velocity of a vertical jet at 57.708 kPa was 0.26 m/s lower than that at 101.325 kPa. The temperature gradient of the room with a hot-air jet was increased with decreasing pressure, which was 5.0 K/m at 57.708 kPa. This study should be helpful to the detailed design and efficient operation of hot-air heating in buildings in the plateau regions.