• Energy Research
• Tsinghua University close

Building occupancy is an important basic factor in building energy simulation but it is hard to represent due to its temporal and spatial stochastic nature. This paper presents a novel approach for building occupancy simulation based on the Markov chain. In this study, occupancy is handled as the straightforward result of occupant movement processes which occur among the spaces inside and outside a building. By using the Markov chain method to simulate this stochastic movement process, the model can generate the location for each occupant and the zone-level occupancy for the whole building. There is no explicit or implicit constraint to the number of occupants and the number of zones in the model while maintaining a simple and clear set of input parameters. From the case study of an office building, it can be seen that the model can produce realistic occupancy variations in the office building for a typical workday with key statistical properties of occupancy such as the time of morning arrival and night departure, lunch time, periods of intermediate walking-around, etc. Due to simplicity, accuracy and unrestraint, this model is sufficient and practical to simulate occupancy for building energy simulations and stochastic analysis of building heating, ventilation, and air conditioning (HVAC) systems.

Extended from the electrochemo-mechanical single particle model, an electrochemo-mechanical impedance model for porous electrode in LIB, which consider the effect of the contact stress, is proposed in this work. A modified coefficient ζ which links the mechanical properties of the electrode and particle material is introduced to describe the effect of contact stress. Firstly, we find that the contact stress can weaken the stress-induced semicircle in the low frequency range through analyzing the Faradaic part of the impedance. Secondly, considering the contact stress is determined by the mechanical bonding between electrode layer and the current collector, we introduce a pseudo-elastic boundary condition to analyze the bonding effect. For rigid substrate, namely the in-plane deformation is totally restricted, the effect of the contact stress is strong. For free substrate, namely the in-plane deformation is not restricted, the contact stress is not induced thus its effect on impedance disappears. Lastly, we discuss the interaction effect of stress and the ionic transport in the electrolyte and find that the interaction is strong for silicon-based material, which indicates the stress effect needs to be considered when interpreting the impedance data in this case.

The electromagnetic coupling between the control cable with metal shielding sheath and metal conductor of grounding grids in the substation of a power system threatens the safe operation of an electronic system connected with the control cable. In order to simplifying the electromagnetic coupling analysis of this system, the core-sheath model of a control cable is replaced by a parallel multi-conductor model with theoretical analysis and numerical computation in this paper. From analysis of the resistance and the self-inductance of the sheath, the mutual inductance and the capacitance between the core and the sheath in both models, four nonlinear equations to solve the four unknown parameters in the parallel multi-conductors model are established. A program has also been coded to solve the equations. The results of the parallel multi-conductors model are compared with the analyzed ones of the actual control cable by the PSCAD software package. They have good agreement with each other.

Abstract Thousands of spherical fuel elements are transported pneumatically in pipelines in a pebble bed reactor every day. The motion of the fuel element has a strong influence on the efficiency and reliability of the reactor. The motion is determined by the force condition of the fuel element which is determined by the characteristics of the flow field in turn. Thus, it is of great significance to analyze the characteristics of the flow field and the force conditions of the fuel element. In this paper, we carry out simulations of the flow field with a structured mesh based on CFD (computational fluid dynamics). We analyze the effect of the motion and position of the fuel element on the flow field. Then we accomplish the force analysis of the fuel element in the flow, and acquire the fitting formulae of the force and torque on the fuel element. The experiment was performed on the experimental platform. The experimental results are in agreement with the theoretical analysis and verify its accuracy. This research may provide an important rationale for the dynamic and kinematic analysis of the fuel element pneumatic transportation as well as the basis for the stable operation of the reactor.

Indoor humidity directly impacts the health of indoor populations. In arid and semi-arid cities, the buildings indoor humidity is typically higher than outdoors, and the presence of water vapor results from water dissipation inside the buildings. Few studies have explored indoor humidity features and vapor distribution or evaluated water dissipation inside buildings. This study examined temperature and relative humidity (RH) changes in typical residential and office buildings. The results indicate a relatively stable temperature with vary range of ±1°C and a fluctuation RH trend which is similarly to that of water use. We proposed the concept of building water dissipation to describe the transformation of liquid water into gaseous water during water consumption and to develop a building water dissipation model that involves two main parameters: indoor population and total floor area. The simulated values were verified by measuring water consumption and water drainage, and the resulting simulation errors were lower for residential than for office buildings. The results indicate that bathroom vapor accounts for 70% of water dissipation in residential buildings. We conclude that indoor humidity was largely a result of water dissipation indoors, and building water dissipation should be considered in urban hydrological cycles.

Abstract Recently, more and more absorption heat pumps have been applied in district heating systems, especially with industrial waste heat used as the driving heat source. This type of heat pumps utilizes flashing as an alternative to generators. The performance of a LiBr solution-based flashing process in an absorption heat pump is discussed in this paper. Flashing experiments were conducted in an absorption heat and mass transfer test bench. Observation results highlighted various types of flashing jets, i.e. the non-shattering jets, partially shattering jets, and completely shattering jets. Experimental results also showed that the initial superheat pressure significantly influenced transitions of the flow regimes. The whole spraying flash phenomenon is divided into three processes, and the flashing performance in each process was discussed. The flashing rate was simplified to be proportional to the superheat pressure in the first stage. And the mass transfer coefficients for the second stage were calculated, which is about 2 × 10−4–7 × 10−4 m/s. It is hoped that these results will help predict the flashing results of LiBr H2O solutions in future research.

The oxidation of coal in supercritical water was explored by using H2O2 as the oxidant. The sulfur-containing components in the effluents were identified. The experiments, which were conducted in a bench scale semi-continuous Supercritical Water Oxidation (SCWO) installation, indicated that the sulfur contained in coal could be gradually oxidized to sulfate in supercritical water medium. The main species containing sulfur in the effluents of coal SCWO were determined as sulfide, thiosulfate, sulfite and sulfate, in which thiosulfate and sulfate were predominant. The effects of the reaction temperature and time on the sulfur transformations during SCWO of coal were also investigated.

In combined electric and heat systems, selecting a suitable testbed for power flow analysis or economic dispatch is not easy: a large number of existing testbeds are not open-source, while others are difficult to be reused by other researchers due to the particularity of system scale, topology, and data. In this paper, we present three open-source testbeds with different scales based on practical combined electric and heat systems. To satisfy researchers' specific demands on the system topology and data, we also discuss how to modify testbeds based on existing topologies and data. Researchers can use the testbeds presented in this paper to test their innovative methods for power flow analysis and economic dispatch, and can also design their own testbeds based on the methodology in this paper, using published topologies and data.