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[en] Highlights: • A skeletal biodiesel reaction mechanism with 112 species was constructed. • The developed mechanism contains the CO, NOx and soot formation kinetics. • It was well validated against detailed reaction mechanism and experimental results. • The mechanism is suitable to simulate biodiesel, diesel and their blend fuels. - Abstract: A tri-component skeletal reaction mechanism consisting of methyl decanoate, methyl-9-decenoate, and n-heptane was developed for biodiesel combustion in diesel engine. It comprises 112 species participating in 498 reactions with the CO, NOx and soot formation mechanisms embedded. In this study, a detailed tri-component biodiesel mechanism was used as the start of mechanism reduction and the reduced mechanism was combined with a previously developed skeletal reaction mechanism for n-heptane to integrate the soot formation kinetics. A combined mechanism reduction strategy including the directed relation graph with error propagation and sensitivity analysis (DRGEPSA), peak concentration analysis, isomer lumping, unimportant reactions elimination and reaction rate adjustment methods was employed. The reduction process for biodiesel was performed over a range of initial conditions covering the pressures from 1 to 100 atm, equivalence ratios from 0.5 to 2.0 and temperatures from 700 to 1800 K, whereas for n-heptane, ignition delay predictions were compared against 17 shock tube experimental conditions. Extensive validations were performed for the developed skeletal reaction mechanism with 0-D ignition delay testing and 3-D engine simulations. The results indicated that the developed mechanism was able to accurately predict the ignition delay timings of n-heptane and biodiesel, and it could be integrated into 3-D engine simulations to predict the combustion characteristics of biodiesel. As such, the developed 112-species skeletal mechanism can accurately mimic the significant reaction pathways of the detailed reaction mechanism, and it is suitable to be used for diesel engine combustion simulations fueled by biodiesel, diesel and their blend fuels
[en] The principal objective of this study is to formulate a calculation process, based on the second law of exergy, for evaluating the thermoeconomic potential of a steam-turbine plant for trigeneration. The plant employs biomass, namely, waste wood as its energy source. Four different plant configurations are presented and assessed. 'Their cost effectiveness is evaluated with varying economic and operating parameters', because only the fuel price and electricity price are varied. In case 1, high pressure superheated steam generated is supplied to meet the demand for process heat as well as chilled water production in an absorption chiller. In cases 2 and 3, steam is extracted at appropriate stages of the turbine and supplied to meet the demand for process heat and chilled water production in an absorption chiller. Steam generated in case 2 produces sufficient power to meet internal demands while case 3 generates excess electricity for sale back to the utility. In case 4, low pressure saturated steam is generated to meet the demand for process heat and electricity is bought from the utilities, including those used to power an electric vapour-compression chiller. For all cases, it was found that exergy destruction is most extensive in the furnace, amounting to nearly 60%. Exergy destruction in the steam drum is the next most extensive ranging from 11% to 16%. It was also observed that the overall production cost decreases with steam pressure and increases with steam temperature. (author)
[en] Desiccant dehumidification technology provides a method of drying air before it enters a conditioned space. When combined with conventional cooling systems, desiccant dehumidification provides an energy-efficient way of supplying thermal comfort air. This paper presents a combined experimental–analytical study on the heat and mass transfer dynamics of composite desiccants during air dehumidification. The composite desiccants are silica gel–calcium chloride, silica gel–lithium chloride, and silica gel–polyvinyl alcohol (PVOH). The derived model is validated against experimental observations of different desiccant types with silica-gel as the host desiccant. Predictions are shown to agree well with extensive experimental measurements conducted using an in-house experimental setup as well as data published in the literature. Experiments were conducted on several promising composite desiccants. The effects of process air velocity, inlet air temperature and humidity on moisture removal capacity, regeneration rates and the associated pressure drops were investigated. Relying on a holistic energy performance index, desiccant coefficient of performance (DCOP), results have indicated that the moisture removal capacity, regeneration rates and the associated pressure drops of composite desiccants outperformed that of pure silica gel by at least 11%. - Highlights: • Heat and mass transfer model dynamics of composite desiccants. • Model is validated against experimental observations of different desiccants. • Experiments performed on moisture adsorption, regeneration and pressure drop. • Energy index indicated composite desiccants outperformed silica gel by 11%.
[en] Research highlights: → New building energy-estimating equation that accounts for varying outdoor air rate. → New correlation compares building energy consumption operating with either VAV or CAV systems. → New energy-estimating equation estimates well the cooling energy consumption in buildings of varying aspect ratio and orientation. → Concept of a design-day weather file predicts well the cooling energy consumption. -- Abstract: Building energy standards provide control over excessive use of energy in buildings, promoting energy efficiency and mitigating detrimental environment impacts brought by high energy consumption. The central objective of this paper is to develop correlations that will predict building heat gains and cooling energy consumption for commercial buildings in tropical climates. The energy performance index OTTV was first revised to obtain a new performance index, ETTV for commercial buildings. We developed new correlations to investigate the impact ventilation rates and building aspect ratios had on building cooling energy consumption. Comparing estimated and simulated results, we demonstrated good agreement, even for buildings having different aspect ratios. A study was further conducted to investigate the impact of weather conditions on the developed methodology for estimating the energy consumption of buildings. We employed a design-day weather file to provide simplicity, flexibility and greater ease of use. The design day concept is pivotal in providing key inputs to the cooling energy-estimating methodology yielding good agreement with DOE-2.1E simulated results. We believe that the results presented in this study will benefit building authorities in their pursuit of developing and refining stringent building energy standards in order to realize better energy efficient buildings.
[en] Energy consumption of buildings takes up about a third of Singapore's total electricity production. In this paper, we present a pioneering study to investigate the energy performance of residential buildings. Beginning with an energy survey of households, we established the air-conditioning usage patterns and modelled residential buildings for computer simulations. An ETTV equation for residential buildings was developed. Employing this equation, we demonstrated how to achieve improved energy efficiency in residential buildings. Two types of residential buildings, namely, point block and slab block, were modelled and parametric runs performed. ETTV impacts the energy consumption of residential buildings and thus lowering the ETTV will result in reduced building heat load. Results from the developed equation showed that a unit decrease in ETTV resulted in 4% and 3.5% reduction in annual cooling energy for point block and slab block residential buildings, respectively. In addition, a set of simple energy and load estimating equations were developed using computer simulation and local climatic data. These equations provided a means of estimating the annual cooling energy consumption of residential buildings in Singapore.
[en] Heat pump systems offer economical alternatives of recovering heat from different sources for use in various industrial, commercial and residential applications. As the cost of energy continues to rise, it becomes imperative to save energy and improve overall energy efficiency. In this light, the heat pump becomes a key component in an energy recovery system with great potential for energy saving. Improving heat pump performance, reliability, and its environmental impact has been an ongoing concern. Recent progresses in heat pump systems have centred upon advanced cycle designs for both heat- and work-actuated systems, improved cycle components (including choice of working fluid), and exploiting utilisation in a wider range of applications. For the heat pump to be an economical proposition, continuous efforts need to be devoted to improving its performance and reliability while discovering novel applications. Some recent research efforts have markedly improved the energy efficiency of heat pump. For example, the incorporation of a heat-driven ejector to the heat pump has improved system efficiency by more than 20%. Additionally, the development of better compressor technology has the potential to reduce energy consumption of heat pump systems by as much as 80%. The evolution of new hybrid systems has also enabled the heat pump to perform efficiently with wider applications. For example, incorporating a desiccant to a heat pump cycle allowed better humidity and temperature controls with achievable COP as high as 6. This review paper provides an update on recent developments in heat pump systems, and is intended to be a 'one-stop' archive of known practical heat pump solutions. The paper, broadly divided into three main sections, begins with a review of the various methods of enhancing the performance of heat pumps. This is followed by a review of the major hybrid heat pump systems suitable for application with various heat sources. Lastly, the paper presents novel applications of heat pump systems used in select industries.
[en] Highlights: ► Group contribution methods from molecular level have been used for the prediction. ► Complete prediction of the physical properties for 5 methyl esters has been done. ► The predicted results can be very useful for biodiesel combustion modeling. ► Various models have been compared and the best model has been identified. ► Predicted properties are over large temperature ranges with excellent accuracies. -- Abstract: In order to accurately simulate the fuel spray, atomization, combustion and emission formation processes of a diesel engine fueled with biodiesel, adequate knowledge of biodiesel’s physical properties is desired. The objective of this work is to do a detailed physical properties prediction for the five major methyl esters of biodiesel for combustion modeling. The physical properties considered in this study are: normal boiling point, critical properties, vapor pressure, and latent heat of vaporization, liquid density, liquid viscosity, liquid thermal conductivity, gas diffusion coefficients and surface tension. For each physical property, the best prediction model has been identified, and very good agreements have been obtained between the predicted results and the published data where available. The calculated results can be used as key references for biodiesel combustion modeling.
[en] Highlights: • A novel thermal desalination technology driven by low temperature heat source. • A rigorous thermodynamic model taking into account key input parameters. • An in-depth analysis revealing the system thermal performance and the effects of key parameters. - Abstract: Thermally-driven desalination technologies are ideal for mitigating the problem of water scarcity using sustainable and renewable energy sources. This paper presents a novel technology for thermal desalination: a spray assisted low-temperature desalination. It applies direct contact heat and mass transfer mechanism for both evaporation and condensation, which markedly enhances heat and mass transfer while eliminating the need for a metallic surface inside the chambers. To perform an in-depth analysis on this novel technology, a detailed thermodynamic model has been developed. The model is based on the principles of appropriate heat balance, mass balance and salt balance, and judicious heat transfer and mass transfer in the system. Key input parameters that affect system performance are taken into account in formulating the model. Simulations were conducted for top brine temperatures spanning 55–90 °C for low-grade heat utilization. Key results from the model revealed that more operating stages and higher top brine temperatures are essential to enable higher production rates and realizing better thermal efficiencies, while a cooling water flowrate close to the feed water flowrate is at the optimal. A system performance ratio of 6.5 can be achieved for a 14-stage system with a top brine temperature of 90 °C.
[en] Highlights: • We made judicious modification to the Penne’s equation in the process of developing our model. We consider the liver to consist of tumor and health tissue. The model has been validated with experimental data. • The proposed electrode system can reduce the tissue volume damage outside the electrodes. The designed building unit with 10 mm inter-electrode distance is the optimal choice to achieve desired ablation zone. • The influence of blood vessel is relatively small for using this electrode system. A spatial distance of 13 mm is deemed as the safe distance between the wall of the central probe and the large vessel. • This proposed electrode system demonstrated higher ablation stability even for tissue regions that are close to blood vessels. The system is better suited for matrix-type RFA. - Abstract: Radiofrequency ablation (RFA) is becoming an effective treatment method for both primary tumors and tumors that have metastasized. Large tumors in difficult anatomic locations can be treated by RFA technologies. However, constant size and regular shape of damage zones cannot be obtained by recent RFA technologies. The aim of this study is to optimize the stability of RFA treatment by employing a newly proposed bipolar electrode system. A hepatic RFA mathematical model is developed by the finite element method approach. The model is validated with the experimental data. This model is then used to verify the reliability and stability of the proposed electrode system. Simulated results showed the cross section of the ablation zone utilizing designed electrode system approximates a square. In addition, the fraction of the necrosed tissue with this electrode pattern turned out to be larger than the fraction with single-probe RFA techniques. This system demonstrated higher ablation stability even for tissue regions that are close to blood vessels. The proposed electrode system is better suited for matrix-type RFA.
[en] Highlights: • A spray-assisted low-temperature desalination system powered by solar energy was evaluated. • A non-steady-state mathematical model has been developed and validated using experimental data. • The feasibility and potential of the desalination system was evaluated under tropic climatic conditions. • Sources of energy inefficiencies inside the system have been identified. - Abstract: The use of solar energy has huge potential for desalination application due to the geographical coincidence between high solar irradiance and fresh water scarcity. This paper investigates the performance of a spray-assisted low-temperature desalination system powered by solar thermal energy. The proposed system applies a spray evaporator and a coil condenser that operate under low-pressure conditions, which increases evaporation rate and promotes productivity. A numerical model was developed to predict the dynamical system performance. Concurrently, an experimental setup was designed and commissioned to demonstrate the feasibility of the spray-assisted low-temperature desalination system and to validate the model. Applying the developed model, the long-term desalination performance of the system coupled with a flat plate solar thermal collector was evaluated under Singapore’s climatic conditions. Additionally, the energy flow inside the system is analyzed in order to highlight the sources of energy losses. Results revealed that the inefficiency of the system is attributed to the losses of both the solar thermal collector and the desalination unit. There exists an optimal feed flowrate that promotes the solar collector performance while minimizing the inefficiency of the desalination unit. The system is able to provide uninterrupted fresh water supply of 30 L per day with a solar collector area of 7.6 m2 and a water storage tank of 305 L. The contributions of this paper include: (1) the development of a validated non-steady-state model via the dual experimental and numerical approach; (2) identifying the sources of inefficiencies inside the system through a detail energy flow analysis; and (3) evaluating and optimizing the system based on long-term performance calculated from annual weather data, which provides a more accurate and robust design basis for this type of standalone solar desalination system.