查看更多>>摘要：The process of analysis and design of energy saving measures aimed to reduce building energy consumption, both in the design of new constructions and in the refurbishment of existing ones, has led to the need to solve increasingly complex problems with the consequent demand to develop complex models and new calculation tools, which often entails the coupling of different models each one of them to solve a specific task and always under the requirement of obtaining accurate and reliable results. This article presents a modular dynamic model based on a finite difference scheme with matching conditions between wall layers to record the different thermophysical properties of layer materials and accurately compute the heat flux through the building envelope. The aim of this model is to simulate the energy behavior of building envelopes and to allow the connection, via a co-simulation procedure, with other codes based in models for addressing complex issues usually not included in standard Building Energy Simulation (BES) tools. For that, the simulation model was designed with a modular structure in order to facilitate its connection, when required, to other codes written in C++. The thermal model has been validated experimentally, using data from two full-scale outdoor test cells with different fa?ade constructive solutions for different ventilation and blind opening regimes. An additional code-to-code comparison was also performed between the developed model and the Energy-Plus software to complement the results of the experimental validation. The results obtained in the validation process show the ability of the proposed numerical model to simulate the energy performance of the envelope and of the test cell globally in a wide variety of situations, predicting internal air temperature and envelope internal surfaces temperatures which meet the requirements usually established for the validation of building energy simulation tools. The numerical formulation of the introduced model and its characteristics that combine flexibility, modularity and accuracy for the calculation of the thermal behavior of a fa?ade and is able of an easy connection to external codes written in C++ to solve more complex problems, allow to consider the present work as innovative and novelty on the literature.
查看更多>>摘要：This paper proposes a HVAC system that integrates a thermoelectric heat pump with a double flux ventilation system and a sensible heat recovery unit able to provide heating, cooling and ventilation to a 74.3 m2 Passive House certified dwelling in Pamplona (Spain). This study computationally investigates the energy performance of the system and the comfort conditions of the dwelling for one year long. The thermoelectric HVAC system maintains adequate comfort conditions with an indoor temperature between 20–23 °C in wintertime and 23–25 °C during summer, thanks to the precise control of the voltage supplied to the thermoelectric heat pump that can regulate the heating/cooling capacity from 5 to 100 %. The system consumes 1143.3 kWh/y (15.3 kWh/m2y) of electric energy, that can be provided by 4 photovoltaic panels of 250 Wp each. This system is then compared with a vapor compression heat pump with a COP of 4.5. The vapor compression system reduces the electric energy consumption by 36.1 % with respect to the thermoelectric system, which allows saving only 270 Wp (1–2 PV panels). This demonstrates the promising application of thermoelectricity for HVAC in passive houses.
查看更多>>摘要：In the present work, surface wettability modifications were utilized to enhance the phase change heat transfer in a water-charged two-phase flat thermosyphon. A flat thermosyphon's thermal performance with various surface wettability modifications on evaporator and condenser plates was investigated for various heat inputs and filling ratios in the horizontal orientation. The evaporator and condenser's surface wettabilities were varied to superhydrophilic (contact angle of 0-1°) and superhydrophobic (contact angle of 155.4 ± 3°). Changing the evaporator's surface wettability to superhydrophilic nature increased the thermal resistance of thermosyphon due to the high superheat requirement and delay during bubble nucleation. A 43.74% decrease in the thermal resistance was observed for a thermosyphon with a superhydrophobic condenser due to the dropwise condensation and faster condensate return to the evaporator compared to the bare one. A lumped parameter model was used to predict the thermal resistance of flat thermosyphon with a superhydrophobic condenser and hydrophilic evaporator, which is in good agreement with the experimental results. The experimental results encourage research on a two-phase flat thermosyphon with a superbiphilic evaporator and superhydrophobic condenser as it can further improve thermal performance.
查看更多>>摘要：Exhaust waste heat recovery technology based on power cycles requires high efficiency, low flow resistance, and a compact structure for exhaust gas heat exchangers. It is a huge challenge for traditional heat exchangers, such as double tube or shell and tube heat exchangers. A printed circuit heat exchanger (PCHE), which is a compact heat exchanger, has not been used in the heat exchange process between the exhaust gas and supercritical CO2. Focusing on this application, a novel design concept of PCHE based on a comprehensive performance index is proposed, which fully considers the differences in physical properties and working conditions between the exhaust gas side and the CO2 side. A series of experiments were carried out in a CO2-based transcritical power cycle system. The experimental test results show that the PCHE with a novel structure is stable and feasible. To further optimise the PCHE, a numerical analysis of different structures was conducted. The calculation results show that the comprehensive performance of the PCHE with an optimised structure was improved by 11.92%. This research shows that PCHE has great potential for achieving a compact design with excellent performance, especially for use in engine waste heat recovery.
查看更多>>摘要：In this study, an airfoil fin printed circuit heat exchanger (PCHE) was first employed as a micro gas turbine recuperator for extended-range electric vehicles. Optimal heat transfer rate, pressure drop, and compactness are important requirements for a PCHE to serve as a recuperator; therefore, in this study, a multi-objective optimal design was proposed using the aforementioned three targets. The considered factors included three airfoil fin structure parameters (arc height, maximum arc height position, and airfoil thickness), two airfoil fin arrangement parameters (horizontal and vertical spacings), and the velocity inlet condition, for both cold and hot sides. A series of 53 numerical tests were designed using the central composite surface method. And the numerical work was performed by the realizable k-ε model with the maximum validation error is less than ± 5%. Moreover, regression formulas for the three targets were established based on the results of the 53 numerical tests, using the high-determine factors. The maximum arc height position was found to be an insignificant factor; thus, it was neglected in the regression formulas. The Pareto fronts for the cold and hot sides were obtained based on nondominated sorting genetic algorithm II; decision-making could be achieved on the Pareto front, according to the design requirements. A comprehensive optimal solution was recommended for the recuperator PCHE with an overall heat transfer rate of 181.29 W/m2/K, a total pressure drop of 26.63 kPa/m, and compactness of 8.5 kw/m2.
查看更多>>摘要：Induction heating is commonly used in laboratory-scale facilities to replicate the heating conditions of the receiver tubes of concentrated solar power plants. This work aims at shedding light at the induction heating characteristics for such applications through the development of a multiphysics numerical model capable of replicating the experimental conditions of a molten salt loop locally heated by an induction heater. In the experiments, a stainless steel pipe is heated on its external surface by the induction heater, which is switched on and off during the experimental data acquisition while molten salts are continuously circulating in its interior. These conditions are replicated, for the first time, in a two-dimensional numerical domain fully coupling the electromagnetic and thermal physics, including thermally dependent material properties of the heated pipe. Once validated against the experiments, the numerical results revealed that the volumetric nature of the induction heating shall be considered for an accurate representation of the temperature profile inside the tube. As a novelty, different equivalent surface boundary conditions are presented and, despite the Gaussian-like behavior of the induction heating on the surface of the tube, the results indicate that there exists no equivalent wall boundary condition to fully replicate the temperature profile obtained with the induction heater. The effect of independently varying experimental parameters such as the geometry of the pipe (i.e., diameter and thickness) and its distance to the induction heating system is also evaluated. Using large diameters of the tube reduces the difference between the angular temperature profile obtained using induction heating and a simplified wall boundary condition. For small wall thicknesses, the induction heating is capable of penetrating along the whole thickness of the tube, the total heat generated in the volume of the tube being exposed to the counteracting effects of the volumetric generation and the enhancement of the heat dissipation by the molten salt, as both of them increase for small thicknesses. The distance of the inductor to the pipe wall appears to maintain the volumetric characteristics of the heating and only affects the induction heating magnitude and efficiency.
查看更多>>摘要：The traditional combined cooling, heating and power (CCHP) system can only provide cooling power, heating power and electricity. However, when the CCHP system is used in places with high humidity requirements such as data centers, it cannot meet the humidity control requirements. Based on this, this study proposes a CCHP system with independent control of refrigeration and dehumidification based on a PEMFC. The system model is developed by MATLAB/Simulink, and its performance and efficiency are comparatively analyzed. The results show that the current density and hot water distribution ratio are critical to the performance of the system. Meanwhile, the mass flow rate of the cooling water and chilled water of the adsorption chiller and the parameters of the process air also have an impact on the performance of the system. When the mass flow rates of cooling water and chilled water increase from 0.2 kg·s?1 to 0.8 kg·s?1, the cooling power of the system increases from 5.3 kW and 5.12 kW to 6.52 kW and 6.64 kW, respectively. Moreover, the addition of the dehumidification system reduces the cooling power by 1.28 kW but removes the latent heat of the process air by about 1.84 kW.
查看更多>>摘要：This work presents a dynamic lumped parameter model to predict the transient and steady-state cabinet temperature, energy consumption and efficiency parameters of a small capacity magnetic refrigerator equipped with a retrofitted thermally insulated wine cooler cabinet. The model is divided into independent, experimentally validated sub-models for the active magnetic regenerator (AMR), magnetic and hydraulic circuits, heat exchanger/fan assemblies and refrigerated cabinet. Primary inputs to the model are the geometric features and component dimensions, frequency and speed of power sources (hydraulic pump, axial fans and induction motor), and composition and properties of magnetic materials. Special emphasis is put on the lumped AMR model, which accurately predicts the performance of first- and second-order materials in single- and multi-layer configurations in terms of the cooling capacity, heat rejection rate and magnetic work. The model predicts the experimental steady-state cabinet temperature of a compact magnetic wine cooler prototype to within 1.5 oC, while estimating the coefficient of performance with a mean error of 6.5%. The time response of the magnetic wine cooler during a temperature pull-down test is also correctly reproduced by the model, enabling its use in the design of future magnetic heat pumping systems.
查看更多>>摘要：Aiming at the power improvement of the aviation kerosene engine caused by the mixture preparation, and slow combustion rate, this paper introduced the design and application of the asymmetric jet pre-chamber on a spark ignition piston-type aviation engine equipped with the kerosene low-pressure air-assisted direct injection (AADI) system. Simulation calculation was conducted on combustion differences between the asymmetric pre-chamber and the traditional symmetrical pre-chamber. Meanwhile, the impacts on the flame combustion duration and heat release process in the main combustion chamber, which was divided into zone A and zone B, by the different jet nozzle orifice diameters and spatial distribution angles in the asymmetric jet pre-chamber were analyzed in the paper; and engine bench tests were performed on the influence of the combustion characteristics and power performance by the asymmetric jet pre-chamber ignition scheme under different control strategies. The simulation results showed that the asymmetric pre-chamber could further improve the combustion reaction rate by 7% and the cumulative heat release rate by 17%, thereby effectively promoting the engine combustion efficiency. And an optimal orifice diameter by 2.00 mm of the two jet nozzle in the zone B existed. Compared with the spatial angle α of these two jet nozzle in zone B by 80°, the spatial angle α of 50° can further be helpful to strengthen the heat transfer between the jet flame in the pre-chamber and the mixture in the main combustion chamber. The test results showed that the asymmetric pre-chamber was beneficial to improve the engine combustion performance by 11.4% under low load and by 16.7% under heavy load conditions. The optimal combustion performance can be obtained by the asymmetric pre-chamber at the condition of λ = 0.7, which resulted in higher jet flame ignition energy and faster combustion progression at the condition of richer mixture. As a conclusion, it is benefit to increase the initial heat release and enhance heat work transfer efficiency in the main combustion chamber of the AADI aviation kerosene engine.
查看更多>>摘要：The design and application of dynamic fa?ades have gained attention in recent years as new high-performance building fa?ade alternatives. In this study, the optimization of a novel opaque dynamic fa?ade, the MICRO-V (Multifunctional Integrated Climate-responsive Opaque and Ventilated) fa?ade, is investigated. This fa?ade is comprised of different components to regulate the flow of heat, air, and moisture into buildings dynamically with daily and seasonal responses using an integrated ventilation module, phase change materials (PCM), and an adjustable insulation system. This fa?ade acts as a decentralized ventilation system, in which the fresh air is pre-conditioned using the thermal energy storage provided by the PCMs, and the heat recovery inside the ventilation module. The aim of this paper is to optimize the conceptual fa?ade design using parametric simulations by quantifying the performance of the MICRO-V facade. Multiple parameters, among which the geometry, the material properties, and the airspeed were tested. CFD simulations were performed in both the heating and the cooling seasons in a continental climate (Toronto, Canada). The pre-conditioning efficiency of fresh air in the fa?ade was 73% in the summer week and 65% in the winter week. Including the PCMs to condition the air resulted in an average increase of air temperature in the winter by 3 °C, through solar gain storage, and helped to reduce the extreme temperature by 5 °C on extremely hot summer periods. The study showed the thermal resistance in the fa?ade could be increased and decreased as a function of airflow in the fa?ade, which is a potential way to control the overall heat gain and heat loss through the fa?ade annually. The fa?ade-scale simulations showed the necessity of this type of analysis prior to whole-building simulations to accurately represent the fa?ade's performance.
NSTL
Elsevier