Showing papers in "Journal of Thermal Science in 2019"

Review of Molecular Simulation Method for Gas Adsorption/desorption and Diffusion in Shale Matrix

Abstract: Shale gas is becoming an increasingly promising alternative energy resource because of its high efficiency and environment-friendly characteristic. The amount of adsorbed gas on the shale matrix surfaces and dissolved gas in the shale matrix bulk is the dominant factor in the long-term productivity of shale reservoir. Although experimental measurements have been extensively carried out to investigate the gas adsorption and diffusion properties in the shale matrix, they cannot provide the detailed information on the microscopic transport mechanism of shale gas during the gas production process. Molecular simulation can accurately visualize the gas adsorption/desorption and diffusion processes in the shale matrix. In the present study, the recent research advances of molecular simulation on gas adsorption/desorption and diffusion in the shale matrix are reviewed. Firstly, the density functional theory (DFT) for shale gas molecule desorption/adsorption on the surface of the matrix crystal is illustrated. Then, the grand canonical Monte Carlo (GCMC) method predicting the amount of shale gas desorption/adsorption in the shale matrix crystal is introduced. Finally, molecular dynamics simulation (MD) for gas diffusion in the shale matrix is elucidated. Further developments of the molecular simulation method in shale gas production are also discussed.

Supercritical CO 2 : Properties and Technological Applications - A Review

Abstract: The main goal of the present paper is to assess the available information so as to obtain a general procedure for dealing with the critical enhancement of the thermodynamic and transport properties of supercritical CO2 and CO2 containing binary mixtures for practical and scientific applications. The present review provides comprehensive analysis of the thermodynamic and transport properties of supercritical carbon dioxide and CO2 containing binary mixtures (experiment and theory) and their various technological and scientific applications in different natural and industrial processes. The available information for the thermodynamic and transport properties (experiment and theory) enhancement (anomaly) of supercritical carbon dioxide and SC CO2 + solute mixtures is comprehensively reviewed. The effect of long-range order parameter fluctuations on the thermodynamic and transport properties of supercritical fluids (SC CO2) will be discussed. Simplified scaling type equation based on mode-coupling theory of critical dynamics with two critical amplitudes and one cutoff wave number as fluid-specific parameters was used to accurately predict of the transport properties of supercritical carbon dioxide. The recommended values of the specific parameters (asymptotic critical amplitudes) of the carbon dioxide for practical (prediction of the thermodynamic and transport properties of the supercritical CO2 for technological applications) and scientific use were provided. The role of the critical line shapes of the carbon dioxide containing binary mixture (SC CO2+solvent) in determination of the critical behavior of the mixture near the critical point of pure supercritical solvent (CO2) is discussed. Krichevskii parameter concept for a description of thermodynamic behavior of dilute near-critical SC CO2+solute mixtures is also discussed. The structural and thermodynamic properties of the carbon dioxide containing binary mixtures near the critical point of pure solvent (CO2) are discussed.

Energy Loss Analysis of Novel Self-Priming Pump Based on the Entropy Production Theory

Abstract: The conventional method cannot explicitly confirm the location and type of the energy loss, therefore this paper employs the entropy production theory to systematically analyze the category, magnitude and location of hydraulic loss under different blade thickness distribution Based on the analysis, the turbulent entropy and viscosity entropy produced by the separation of boundary layer at the trialing edge are major factors leading to the hydraulic loss In addition, the separation of the boundary layer can not only cause the energy loss, but also block the passage of the impeller and reduce the expelling coefficient of the blade Therefore, the hydraulic performance of the blades with increment thickness distribution is obviously better than the decrement one Further, the flow rate has different influence on the three types of entropy production Meanwhile, the pressure pulsation on the working surface was investigated It was concluded that with flow rates increasing, the amplitude of pressure pulsation firstly decreases and then smoothly improves, and reaches the minimum under design flow rate Finally, the optimal blade was obtained, and the relevant hydraulic performance test was performed to benchmark the simulation result This research can provide the theoretical reference for designing the reasonable thickness distribution of the blades

Design and Development of a Lens-walled Compound Parabolic Concentrator-A Review

Abstract: Compound parabolic concentrator (CPC) is a representative among solar concentrators, one of whose disadvantage is that the concentration ratio limits the half acceptance angle. Based on this, researchers put forward a novel structure, named the lens-walled CPC. This paper reviews the design and development of lens-walled CPC. The structure of the symmetric and asymmetric lens-walled CPC and the improved ones are presented, and their indoor and outdoor performances are also illustrated. The lens-walled CPC has a larger half acceptance angle and a more uniform flux distribution that is suitable for PV application. Furthermore, the life-cycle assessment for building integrated with PV is performed and it shows that the energy payback time of such integrated system has a significant advantage. In addition, future research areas are also indicated that may provide more functions and more stable performance. The design methods and developmental directions given in this study would provide many references in solar optical research and solar concentrator optimization.

Selection and Evaluation of Dry and Isentropic Organic Working Fluids Used in Organic Rankine Cycle Based on the Turning Point on Their Saturated Vapor Curves

Abstract: The organic Rankine cycle (ORC) is a popular technique used in the utilization of low-grade thermal energy. Among wet, dry, and isentropic organic working fluids, the latter two types are more appropriate for ORC systems. In this paper, the definition of turning point on saturated vapor curve of dry fluid and isentropic fluid was given according to the shape of the saturated curve of working fluids in a T-s diagram. On this basis, the model of near-critical region triangle was established. Using this model, the thermodynamic performance of 57 kinds of dry and isentropic organic working fluids in ORC was evaluated. The performance includes the relation between turning point temperature and cycle thermal efficiency, the relation between near-critical region triangle area and cycle thermal efficiency, the relation between near-critical region triangle area and exergy at turning point temperature, the relation between near-critical region triangle area and reciprocal value of slope of saturated vapor curve. Moreover, working fluid selection was also conducted in terms of heat source type. It was found through theoretical analysis results that the popular R123 is an acceptable choice especially for the utilization of closed type heat source. Considering it will be phased out in near future, then cis-butene, butane, trans-butene, and isobutene are worth studying as its successor. Dodecane is worthy of attention and further research and it can be a good choice for utilization of open type heat source.

Effect of Nanobubble Evolution on Hydrate Process: A Review

Abstract: As a huge reserve for potential energy, natural gas hydrates (NGHs) are attracting increasingly extra attentions, and a series of researches on gas recovery from NGHs sediments have been carried out. But the slow formation and dissociation kinetics of NGHs is a major bottleneck in the applications of NGHs technology. Previous studies have shown that nanobubbles, which formed from melt hydrates, have significant promotion effects on dissociation and reformation dynamics of gas hydrates. Nanobubbles can persist for a long time in liquids, disaccording with the standpoint of classical thermodynamic theories, thus they can participate in the hydrate process. Based on different types of hydrate systems (gas + water, gas +water +inhibitors/promoters, gas + water + hydrophilic/hydrophobic surface), the effects of nanobubble evolution on nucleation, dissociation, reformation process and “memory effect” of gas hydrates are discussed in this paper. Researches on the nanobubbles in hydrate process are also summarized and prospected in this study.

Review of the Working Fluid Thermal Stability for Organic Rankine Cycles

Abstract: The organic Rankine cycle (ORC) is an efficient power generation technology and has been widely used for renewable energy utilization and industrial waste heat recovery. Thermal stability is a significant property of ORC working fluids and is the primary limitation for working fluid selection and system design. This paper presents a review of the working fluid thermal stability for ORCs, including an analysis of the main theoretical method for thermal stability, a summary of the main experimental method for thermal stability, a summary of the decomposition experimental results for working fluids, and a discussion of the decomposition influence on ORC systems. Further research trends of thermal stability are also discussed in this paper.

Design and Performance Simulation of a Novel Liquid CO2 Cycle Refrigeration System for Heat Hazard Control in Coal Mines

Abstract: LCO2 (liquid CO2) can absorb heat and release latent heat via phase transition, which can provide considerable cooling energy and effectively solve the problem of thermal damage in deep coal mining processes. A LCO2 cycle refrigeration system is designed to continuously cool down the working face in a mine, and CO2 is cyclically utilized. Additionally, LCO2 is used not only as a cold source but also to prevent spontaneous combustion of coal in the gob. COMSOL Multiphysics simulation software is used to characterize the thermal performance of the heat exchange system, where the heat transfers between the CO2 and the airflow. For a LCO2 consumption of 13.54 m3/h, the temperature of the airflow in the tunnel decreases by 7.72°C, and the cooling volume of the system reaches 142.99 kW/h; the cooling volume provides a latent heat release of 46.68 kW/h. The main influencing factors of the refrigeration system, such as ventilation flux, LCO2 flow, LCO2 temperature and initial tunnel temperature, are also analyzed quantitatively through the software. The temperature of the steady airflow in the tunnel is proportional to the square of the local fan ventilation flux, and it decreases linearly with an increase in the LCO2 flow but increases with both the temperature of the LCO2 and the initial temperature in the tunnel. When the temperature difference between the LCO2 and wind increases, the heat exchange between the CO2 and wind intensifies, and the cooling volume increases.

Optimizing Building Envelope Dimensions for Passive Solar Houses in the Qinghai-Tibetan Region: Window to Wall Ratio and Depth of Sunspace

Abstract: It has been a focus to reduce the energy consumption and improve the space heating performance of high-altitude buildings in winter seasons. In view of the abundant solar energy resources of the high-altitude region, the establishment of passive solar houses should be an effective strategy to deal with the problem of thermal comfort. Both window to wall ratio (WWR) and sunspace depth are of vital importance to determine the thermal comfort level of passive solar houses, while there are limited studies on analyzing their impacts on passive solar houses in high-altitude regions. Therefore, this study is designed to examine how WWR and sunspace depth affect space heating of passive solar houses in the Qinghai-Tibetan region. To be specific, the hourly radiation temperature variations and percentages of dissatisfaction of the residential building with different sunspace depth/WWR (including 0.9m/33%, 0.9m/45%, 0.9m/60%, 1.2m/33% and 1.5m/33%) were quantitatively examined. Results indicated that under the condition of 0.9m/45%, the overall average radiation temperature of the building was approximately 16°C during the entire heating season, which could better satisfy the heating requirements. Meanwhile, the average temperature was higher, and the thermal comfort level was better under the ratio of 45% or the depth of 1.5 m, when only an individual factor in either ratio or depth was considered. These findings can provide references for the determination of dimensions of passive solar houses in high-altitude regions.

Influence of the Addition of Cotton Stalk during Co-pyrolysis with Sewage Sludge on the Properties, Surface Characteristics, and Ecological Risks of Biochars

Abstract: Sewage sludge produced by municipal sewage treatment plants can potentially be used as a biomass energy source because of its high organic content. Presently, the conversion and utilization of rapidly growing amounts of sewage sludge represent an urgent challenge in China. Thermal conversion of sewage sludge to biochar through pyrolysis is a promising solution to this problem. However, biochar produced by pyrolysis of sewage sludge alone has a poor pore structure as a result of its low C content and high ash content. Furthermore, it is enriched in heavy metals that may pose high ecological risks. In this study, we addressed these issues through co-pyrolysis of sewage sludge and cotton stalks (1:1, wt./wt.) at different pyrolysis temperatures ranging from 350°C to 750°C. The properties and surface characteristics of the biochars were investigated. Meanwhile, the transformation behavior of heavy metals during the co-pyrolysis process was studied, and the potential ecological risks of heavy metals in biochars were assessed. The results showed that elevated pyrolysis temperatures reduced the biochar yield and C content of the biochars, whereas such temperatures increased the pH value and ash content of the biochars. The biochars prepared at different pyrolysis temperatures were all mesoporous materials. The elevated temperatures promoted the transformation of heavy metals from mobile fractions to stable ones, thus resulting in a significant decrease in the ecological risks. In summary, co-pyrolysis of sewage sludge with cotton stalks proved to be a feasible method for the conversion and utilization of sewage sludge.

MWCNTs/SWCNTs Nanofluid Thin Film Flow over a Nonlinear Extending Disc: OHAM Solution

Abstract: The aim of this research is the improvement towards the consumption of energy in the field of engineering and industry. The efforts have been paid to the enhancement of heat transmission and cooling process through a nanofluid coating of a nonlinear stretching disc. The combination of Water (H2O) and multiple walled carbon nanotubes (MWCNT) / single walled carbon nanotubes (SWCNT) have been used as a nanofluid. The spreading of a thin nano-layer with variable thickness over a nonlinear and radially stretching surface has been considered. The estimated results of the problem have been accomplished using the Optimal Homotopy Analysis Method (OHAM). The residual errors of the OHAM method have been shown physically and numerically. The important physical parameters of skin friction and Nusselt number have been calculated and discussed. The other embedding parameters like generalized magnetic parameter, Prantl number, nanofluid volume fraction and Eckert number have been intended and discussed. The obtained results have been compared with the Numerical (ND-Solve) method for both sorts of CNTs. The closed agreement of both methods has been achieved.

A Quantitative Process-Based Inventory Study on Material Embodied Carbon Emissions of Residential, Office, and Commercial Buildings in China

Abstract: Studies on building carbon emissions focus mainly on the materialization phase of life cycle, as carbon emissions in this stage is intensive and high. This paper proposes a simplified model to calculate embodied carbon emissions in building design stage by conducting a process-based inventory analysis of carbon emissions from materials used in 129 residential buildings, 41 office buildings, and 21 commercial buildings during materialization phase. The results indicate that average carbon emissions per unit area from building materials used in residential buildings, office buildings, and commercial buildings are 514.66 kgCO2e/m2, 533.69 kgCO2e/m2 and 494.19 kgCO2e/m2, respectively. Besides, ten kinds of building materials (namely, steel, commercial concrete, wall building materials, mortar, copper core cables, architectural ceramics, PVC pipes, thermal insulation materials, doors and windows, and water paint) constitute 99% of total carbon emissions in all three types of buildings. These materials are major carbon emissions sources in materialization phase. Thus, embodied carbon emissions can be significantly reduced by limiting the amount of these materials in architectural design as well as by using environmental friendly materials.

Multi-Objective Particle Swarm Optimization (MOPSO) for a Distributed Energy System Integrated with Energy Storage

Abstract: Distributed energy systems are considered as a promising technology for sustainable development and have become a popular research topic in the areas of building energy systems. This work presents a case study of optimizing an integrated distributed energy system consisting of combined heat and power (CHP), photovoltaics (PV), and electric and/or thermal energy storage for a hospital and large hotel buildings located in Texas and California. First, simulation models for all subsystems, which are developed individually, are integrated together according to a control strategy designed to satisfy both the electric and thermal energy requirements of a building. Subsequently, a multi-objective particle swarm optimization (MOPSO) is employed to obtain an optimal design of each subsystem. The objectives of the optimization are to minimize the simple payback period (PBP) and maximize the reduction of carbon dioxide emissions (RCDE). Finally, the energy performance for the selected building types and locations are analyzed after the optimization. Results indicate that the proposed optimization method could be applied to determine an optimal design of distributed energy systems, which reaches a trade-off between the economic and environmental performance for different buildings. With the presented distributed energy system, a peak shaving in electricity of about 300 kW and a reduction in boiler fuel consumption of 610 kW could be attained for the hospital building located in California for a winter day. For the summer and transition seasons, electricity peak shaving of 800 kW and 600 kW could be achieved, respectively.

Review on Development of Small Point-Focusing Solar Concentrators

Abstract: The technology of small point-focusing concentrator of solar energy has been developing rapidly in recent years owing to its compact structure and high collecting efficiency. This report presents important developments of small point-focusing concentrator in the past decade. This kind of solar concentrator refers to the parabolic dish concentrator, the point-focusing Fresnel lens, and the Scheffler reflector. Technological advances of these concentrators and the related performances have been presented. There are three main mirror fabrication technologies for dish concentrator, which are high polishing metal, silver-glass mirror and vacuum-membrane. Polymethyl methacrylate is widely used as material in Fresnel lens. Many scholars have proposed new lens shape to improve the uniformity of focusing. The Scheffler reflector has a characteristic of fixed focus, but its design parameters are not perfect so current research focuses on the theoretical calculation of the mirror. In addition, typical applications of the small point-focusing concentrator in photovoltaic system, solar thermal system, solar chemical system, and day-lighting system are summarized. Upon listing the important publications in open literature, a category of main applications of such kind of solar collector is provided based on the working characteristics of the system.

Dual-Wavelength Laser Flash Raman Spectroscopy Method for In-Situ Measurements of the Thermal Diffusivity: Principle and Experimental Verification

Abstract: This paper presents an in-situ, non-contact, non-destructive “dual-wavelength laser flash Raman spectroscopy method” for measuring the thermal diffusivity. In this method, a heating pulse is used to heat the sample and another pulsed laser with a different wavelength and negligible heating effect is used as a probe to measure the sample temperature changes during the heating and cooling periods from the Raman peak shifts. The sample temperature rise and fall curves are measured by changing the delay between the heating pulse and the probing pulse with the thermal diffusivity then characterized by fitting the temperature curves. The time delay between the heating and probing pulses can be precisely controlled with a minimum step of 100 ps. Hence, the temperature variation can be scanned with an ultra-high temporal resolution of up to 100 ps, which significantly improves the measurement accuracy of transient thermal parameters. The measurement accuracy of this method has been verified using a bulk material model and experiments. The measured thermal diffusivity of a silicon sample has been obtained to be 8.8×10-5 m2/s with a 3% difference between the measured value and the average result for bulk silicon in the literature which verifies the reliability and accuracy of this method.

Thermal-hydraulic-structural Analysis and Design Optimization for Micron-sized Printed Circuit Heat Exchanger

Abstract: The Printed Circuit Heat Exchanger (PCHE) is one of the most promising heat exchangers for Synergetic Air-breathing and Rocket Engine (SABRE). To reduce pressure drop and improve compactness, the micron-sized PCHE made up of rectangular channels of tens of microns in size, is used in SABRE. In present work, we focus on thermal-hydraulic-structural characteristics of micron-sized PCHE by conducting three-dimensional (3-D) numerical simulation. Helium and hydrogen are employed as the working fluids and the Stainless Steel 316 (SS316) as the solid substrate. The thermal-hydraulic performance of the micron-sized PCHE is discussed by using the commercial Computational Fluid Dynamics (CFD) software of Fluent. ANSYS-Mechanical is also employed to simulate stress field of representative PCHE channels. The mechanical stress induced by pressure loading and the thermal stress induced by temperature gradient are found to be equally important sources of stress. To improve comprehensive performances of micron-sized PCHE, two types of channel arrangements and different channel aspect ratios are studied. The double banking is of higher thermal-hydraulic performance compared to the single banking while the stress performance is identical for the two modes. Meanwhile, the effect of channel aspect ratio is investigated by comparing thermal-hydraulic characteristics and structural stress of the model. The rectangular channel with w/h=2 achieves the most balanced stress characteristic and higher thermal-hydraulic performance.

A Comprehensive Evaluation on Energy, Economic and Environmental Performance of the Trombe Wall during the Heating Season

Abstract: Trombe wall is a passive building energy saving technology that uses solar energy to reduce buildings’ heating load and adjust indoor thermal environment. In recent years, much research has been done to increase the thermal efficiency of Trombe wall, but little is focused on the evaluation of Trombe wall from energy, economic and environmental aspects comprehensively. Based on the thermal performance calculation method in ISO 52016-2:2017(E), the authors proposed a concise method to evaluate the energy, economic and environmental performance of ventilated and non-ventilated Trombe walls during a heating season. Firstly, non-iteration calculation methods were introduced for the energy evaluation of Trombe wall and conventional wall during the heating season. Then the economic and environmental evaluation models were brought out according to the energy performance of Trombe wall. After that, a residential building was presented as the case building to evaluate Trombe walls’ performance in five building climate zones of China. The calculation results showed that both heating degree days and solar radiation had significant impact on the energy saving effect of Trombe walls. In comparison with non-ventilated Trombe walls, ventilated ones displayed more obvious energy saving potential in all five climate regions, which can provide averagely 62% more heating for the room in the case study. Though the heating degree days of Guangzhou (hot-summer and warm-winter zone) was the smallest in the five zones, ventilated Trombe wall in the zone had the poorest economic performance due to the scarcest solar radiation during the heating season.

Multi-Objective Optimization for China’s Power Carbon Emission Reduction by 2035

Abstract: Low carbon transformation plays an important role in promoting the energy production and consumption revolution. Currently, the power sector of China still faces a series of challenges, such as the overcapacity of coal-fired power, the renewable energy consumption, the new constraints of carbon-emissions, and fragmented power planning. This study develops a multi-objective optimization model to predict the future trend of China power structure by 2035. The key factors such as network, power, load and storage are taken into account. Besides, the technical feasibility, economic rationality and social acceptable constraints are also fully considered. Through planning and optimization, the premise of low carbon transformation is to ensure the continuity of existing policies for removing inefficient assets, and the core is to develop and utilize non-fossil energy on a large scale. Specifically, the capacity of coal-fired power will be attained in the peak in 2025, and the factor will also transfer from main power supplier to main power and energy supplier. Before 2025, the clean replacement of incremental power installation will be completed. In 2035, 92% of new investment comes from non-fossil energy. The economy and competitiveness of wind power and PV (Photovoltaic) power generation are continuously increasing. By 2020, the coal-fired power and the wind power in eastern of China will be parity firstly. In 2025, the cost of PV and wind power will be the same. Furthermore, the evaluation dimension of modern power system with clean, low-carbon, safety and high efficiency are innovatively constructed, and the index system target of 2035 is quantitatively analyzed and prospected.

A Review about Thermal Comfort in Aircraft

Abstract: Thermal comfort is an important factor which affects both work efficiency and life quality. On the basis of satisfying the normal life of the crew and reliable work of equipment, the thermal comfort is increasingly pursued through the design of the environmental control system of modern craft. Thus, a comprehensive survey of the thermal comfort in the cockpit is carried out. First of all, factors affecting the thermal comfort in aircraft cabin are summarized, including low relative humidity, mean radiant temperature, colored light, human metabolic rate and gender, among which the first three factors are environmental factors and the other two are human factors. Although noise is not a factor affecting thermal comfort, it is an important factor in the overall satisfaction of the aircraft cabin environment. Then the thermal comfort prediction models are introduced, including thermal comfort models suitable for steady state uniform environment and thermal comfort models suitable for transient non-uniform environment. Then the limitations of the typical thermal comfort models applied to aircraft are discussed. Since the concept of thermal adaptation has been gradually accepted in recent years, many field studies on thermal adaptation have been carried out. Therefore, the adaptive thermal comfort models are summarized and analyzed systematically in this paper. At present, mixing ventilation (MV) system is widely used in most commercial aircraft. However, the air quality under the MV system is very poor, and contaminants cannot be effectively eliminated. So a noticeable shift is the design of ventilation system for cabin drawing lessons from the surface buildings. Currently, the most interesting question is that whether the traditional mixing ventilation (MV) system in an aircraft can be replaced by or combined with displacement ventilation (DV) system without decreasing thermal comfort. A reduction of energy consumption is a valuable gain. Additionally, various seat personalized ventilation systems have also been proposed which could effectively reduce the risk of infectious diseases. At present, optimal design of airflow in aircraft cabin is the most commonly used method to enhance thermal comfort and save energy. The optimal design of the aircraft cabin colored lighting system, however, is also worth trying.

Performance of S-CO 2 Brayton Cycle and Organic Rankine Cycle (ORC) Combined System Considering the Diurnal Distribution of Solar Radiation

Abstract: This paper researches the performance of a novel supercritical carbon dioxide (S-CO2) Brayton cycle and organic Rankine cycle (ORC) combined system with a theoretical solar radiation diurnal distribution. The new system supplies all solar energy to a S-CO2 Brayton cycle heater, where heat releasing from the S-CO2 cooler is stored in the thermal storage system which is supplied to the ORC. Therefore, solar energy is kept at a high temperature, while at the same time the thermal storage system temperature is low. This paper builds a simple solar radiation diurnal distribution model. The maximum continuous working time, mass of thermal storage material, and parameter variations of the two cycles are simulated with the solar radiation diurnal distribution model. 10 organic fluids and 5 representative thermal storage materials are compared in this paper, with the mass and volume of these materials being shown. The longer the continuous working time is, the lower the system thermal efficiency is. The maximum continuous working time can reach 19.1 hours if the system provides a constant power output. At the same time, the system efficiency can be kept above 38% for most fluids.

Optimization Potentials for the Waste Heat Recovery of a Gas-Steam Combined Cycle Power Plant Based on Absorption Heat Pump

Abstract: A new waste heat recovery system is presented to recover exhausted steam waste heat from the steam turbine by absorption heat pump (AHP) in a gas-steam combined cycle (GSCC) power plant. The system can decrease energy consumption and further improve the energy utilization. The performance evaluation criteria are calculated, and exergy analysis for key components are implemented in terms of the energy and exergy analysis theory. Besides, the change of these criteria is also revealed before and after modification. The net power output approximately increases by 21738 kW, and equivalent coal consumption decreases by 5.58 g/kWh. A 1.81% and 1.92% increase in the thermal and exergy efficiency is respectively obtained in the new integrated system as the heating load is 401095 kJ at 100% condition. Meanwhile, the appropriate extraction parameters for heating have been also analyzed in the two systems. The proposed scheme can not only save energy consumption but also reduce emission and gain great economic benefit, which is proven to be a huge potential for practical application.

Thermal and Electrical Modelling of a CPV/T System Varying Its Configuration

Abstract: In this paper, the main aim is the performances modelling from the electrical and thermal point of view of a concentrating photovoltaic and thermal (CPV/T) system in order to evaluate the primary energy and economic savings respect to a traditional system, when the same energy loads are satisfied. This study is realized by both varying the CPV/T system configuration and considering two different users. In particular, the point-focus (PF), and linear focus (LF) configurations of the CPV/T system are considered in order to match the residential user and hotel energy loads. The CPV/T system is sized adopting as input data: the Direct Normal Irradiance (DNI) modelled by an artificial neural network and the users’ energy demands. In these hypotheses, the performances of the PF and LF systems are evaluated and then compared for the two users located in Southern Italy, in terms of electrical and thermal energy production, cells number, space occupied, energy and economic savings and CO2 emissions avoided. Finally, the PF system shows a lower simple pay-back and a higher primary energy saving, while the space occupied by a LF system results to be lower respect to the PF configuration.

Design and Analysis of S-CO 2 Cycle and Radial Turbine for SOFC Vehicle Waste-Heat Recovery

Abstract: Solid oxide fuel cell (SOFC) vehicles are considered to have broad prospects for development, and the high operating temperature of SOFC results in great potential for waste-heat recovery. Many concepts for utilizing waste heat of SOFC have been suggested and studied, and most of them directly couple an SOFC to a gas turbine, which require the SOFC to operate at an elevated pressure and make the system less flexible and thus harder to operate. In recent years, with the development of turbine and heat exchanger technology, the supercritical carbon dioxide (S-CO2) power cycle has raised widespread attractions for the waste recovery. This study explores the potential of S-CO2 Brayton cycle to realize waste-heat recovery for an SOFC vehicle. The SOFC can operate at atmospheric pressure, and the hybrid system is easier to operate than the directly coupled systems. In this paper, a simple recuperated S-CO2 Brayton cycle is proposed and the key component, radial inflow turbine is designed and focused. The flow state of the designed turbine is analyzed in detail based on computational fluid dynamics (CFD) numerical simulation. Five cases with different impeller tip clearances are numerically simulated to study its influence on the turbine performance. In addition, off-design performance analysis of the radial inflow turbine is conducted considering the temperature fluctuation of SOFC in practical applications.

Numerical Study of Pressure Pulsation of Centrifugal Pumps with the Compressible Mode

Abstract: The compressible effect of water is often neglected in the simulation of hydraulic machinery. However, based on experimental and numerical study, it is found that the compressibility of water could influence the magnitude of the pressure pulsation at some frequency in the pump. Therefore, in order to investigate the influence of water compressibility, compressible model is established by using Tait equation. The internal flow of centrifugal pump under different conditions is calculated by this model. The calculated results are compared with the incompressible results, and it is indicated that the compressibility of water has little effect on the performance parameters. But it affects the amplitude of pressure fluctuations at some discrete frequency, especially at the outlet of impeller and volute tongue where significant jet-wake and rotor/stator interaction appears respectively. Meanwhile, water compressibility makes greater influence on the flow pulsation under off-design condition. Therefore, it is necessary to consider the compressibility of working medium in the numerical simulation of unsteady flow in centrifugal pumps, especially in area with strong unsteady flow and at off-design condition.

Investigation on the Influence of Refrigerant Charge Amount on the Cooling Performance of Air Conditioning Heat Pump System for Electric Vehicles

Abstract: The application of air conditioning heat pump (ACHP) in electric vehicles could lead to significant electrical power saving effect. As for an air conditioning heat pump system for electric vehicles, the influence of refrigerant charge amount should be investigated during the design phase. In this study, experimental method was employed to investigate the influence of the refrigerant charge amount on the performance of the ACHP system. The results showed that the refrigerant charge amount had different influence on the refrigerant properties at various locations within the system. The coefficient of performance and pressure-enthalpy diagram were calculated, and showed a close relationship with refrigerant charge amount under different compressor speeds. The degree of subcooling and the degree of superheating were recorded and the critical charge amount was determined by a typical practical test method. In addition, the critical refrigerant charge amount determined by the experimental method was also compared with two typical void fraction correlation models, and the model with consideration of two phase stream reaction of the refrigerant showed a good estimation accuracy on the critical charge amount.

Performance Analysis of a Multistage Centrifugal Pump Used in an Organic Rankine Cycle (ORC) System under Various Condensation Conditions

Abstract: In an organic Rankine cycle (ORC) system, the working fluid pump plays an important role in the system performance. This paper focused on the operating characteristics of a multistage centrifugal pump at various speeds and condensation conditions. The experimental investigation was carried out to assess the influence of the performance of the pump by the ORC system with special attention to actual net power output, thermal efficiency as well as back work ratio (BWR). The results showed that an increase in the pump speed led to an increase in the mass flow rate and expand in the operating range of the outlet pressure. The mass flow rate decreased nonlinearly with the increase of the outlet pressure from 0.22 to 2.41 MPa; the electric power consumption changed between 151.54 and 2409.34 W and the mechanical efficiency of the pump changed from 7.90% to 61.88% when the pump speed varied from 1160 to 2900 r/min. Furthermore, at lower pump specific speed the ORC system achieved higher thermal efficiency, which suggested that an ultra-low specific speed pump was a promising candidate for an ORC system. The results also suggested that the effects of condensation conditions on the pump performance decreased with the pump speed increasing and BWR was relatively sensitive to the condensation conditions, especially at low pump speed.

Research on the Performance of Supercritical CO 2 Dry Gas Seal with Different Deep Spiral Groove

Abstract: The performance of supercritical CO2 (SCO2) dry gas seal (DGS) with different deep spiral groove is investigated with the thermal-fluid-solid coupling method. The performance parameters of DGSs with five different kinds of grooves are obtained. The influence of inlet temperature, inlet pressure, velocity and film thickness on performance is analyzed compared with air DGS. The average film pressure, open force and leakage decrease while the average face temperature and flow velocity increase as the spiral groove number increases. The average film pressure, average face temperature, open force and leakage of DGS with radial different deep groove are higher than those of DGS with circumferential different deep groove respectively under the same spiral groove number while the average flow velocity is the opposite. SCO2 DGS can generate larger average film pressure, open force and leakage with lower average face temperature than air DGS. SCO2 DGS could maintain better sealing performance despite larger leakage with the variations of inlet temperature, inlet pressure, velocity and film thickness. The variables hold a more remarkable influence on SCO2 DGS compared with air DGS.

Experimental Investigation on the Temperature Distribution Characteristics of the Evaporation Section in a Pulsating Heat Pipe

Abstract: With the increasing demand for heat dissipation in the electronics industry, pulsating heat pipe (PHP) has attracted wide attention due to its simple structure and excellent heat transfer ability. However, due to the unique operational mechanism of PHP, the temperature distribution in the evaporation section is obviously not even during the operational process of PHP. When the PHP is used as a heat dissipater, the evaporation section of the PHP directly contacts with the chips and has great influence on the performance of the chips, so it is very important to investigate the temperature distribution characteristics in the evaporation section. In this paper, both the effects of the filling ratio and heat flux on these characteristics were investigated. The experimental results indicated that the temperatures of the middle “U” turn were the highest. When the heat flux and the filling ratio were 364 W/cm2 and 36.3%, respectively, the maximum temperature difference between the middle “U” turn and the other “U” turns could be as high as 18.92 K. Furthermore, the temperature differences between the middle “U” turn and the other “U” turns firstly increased and then decreased with the increase of heat flux and filling ratio.

Study on Unstable Characteristics of Centrifugal Pump under Different Cavitation Stages

Abstract: In order to reveal the regularity of unsteady flow of centrifugal pump under different cavitation stages, a visual closed test-bed is built to collect signals such as the distribution of cavitation bubbles at the impeller inlet and external characteristics, etc. in the process of cavitation of centrifugal pumps. Combined with the shape and distribution of bubbles captured by high-speed photography, the cavitation stage of the centrifugal pump is divided. In addition, the variation of vorticity distribution, pressure pulsation and radial force of centrifugal pump under different cavitation stages are studied using the standard k-e turbulence model and the Kunz cavitation model. Main contributions are as follows: The cavitation bubbles can absorb the energy of vortex core to a certain extent and increase the volume of vortex core. Cavitation bubbles can also block the flow-path and induce the distortion of the internal flow field, resulting in unstable pressure waves that cause a significant increase in pressure pulsation rate. Besides, with the development of cavitation, the radial force on the impeller tends to remain invariable first and then decrease, and trajectory of the radial force changes from closed to open.

System Analysis on Supercritical CO2 Power Cycle with Circulating Fluidized Bed Oxy-Coal Combustion

Abstract: Supercritical carbon dioxide (S-CO2) Brayton power cycle is a competitive technology to achieve high efficiency in a variety of applications. However, in coal power applications, the CO2 generated from coal combustion still discharges into the atmosphere causing a series of environment problems. In this work, an 300 MWe S-CO2 power cycle with circulating fluidized bed (CFB) oxy-coal combustion was established including air separation unit (ASU), CFB boiler, recuperator system and carbon dioxide compression and purification unit (CPU). Based on the material and energy conservation, the cycle efficiency of S-CO2 (620°C, 25 MPa) Brayton power cycle with CFB oxy-coal combustion is evaluated compared to the oxy-coal combustion steam Rankine cycle and S-CO2 Brayton power cycle with the 31.65 kg/s coal supply. After that, the influence of several factors, e.g., exhaust flue gas temperature, split ratio in recuperator system and the oxygen supply on the cycle efficiency was investigated and analyzed. The results show that the net efficiency of S-CO2 power cycle with CFB oxy-coal combustion (32.67%) is much higher than the steam Rankine cycle utilizing CFB with 17.5 Mpa, 540°C steam (27.3%), and 25 Mpa, 620°C steam (30.15%) under the same exhaust flue gas temperature. In addition, lower exhaust flue gas temperature and higher split ratio are preferred to achieve higher cycle efficiency. Lower oxygen supply can reduce the energy consumption of ASU and CPU, further increasing the system net efficiency. However, the energy losses of ASU and CPU are still very large in oxy-coal combustion and need to be improved in further work.