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Abstract: A simulation model of a two mode power-split hybrid electric vehicle (HEV) is proposed in this paper for the purpose of HEV dynamics analysis and control system design. The bond graph methodology is used to model dominant dynamic effects of the mechanical part of the HEV transmission. Simple quasi-static battery model, the environment model, the tire and the power losses model of a vehicle are included, as well. A low-level electric generator speed control loop is designed, which includes a PI controller tuned according to the symmetrical optimum tuning procedure. Finally, off-line optimization by conjugate gradient-based BPTT-like optimal control algorithm, which is based on the presented mathematical model, is also given in the paper. [Copyright &y& Elsevier]

Abstract: A new generation of highly efficient absorbers for direct air-cooled LiBr/H2O absorption machines is presented and discussed in this paper. As distinguishing aspects of these absorbers, it is worth mentioning that they are adiabatic units, which improves the heat and mass transfer; besides, they distribute the solution in flat-fan sheets, which allows for compact absorber designs; lastly, they are directly air-cooled units, which eliminates the need of cooling towers. Additionally, the paper includes the development of a mathematical modeling for analysis and simulation of this kind of absorbers. Based on that model, a parametric study of the proposed absorber design is carried out to optimize its use in a particular air-cooled single–double-effect absorption machine. Simulation outcomes of that specific absorber were compared with some experimental results obtained by using the aforementioned absorption machine as testing facility to validate the model. A good agreement was found between predictions and experimental results for most of the characteristic operation parameters of the absorber. Finally, it was observed that the proposed absorber design enables air-cooled LiBr/H2O absorption machines to work far from crystallization limits even at ambient temperatures around 40°C. [Copyright &y& Elsevier]

Abstract: Knocking combustion in spark-ignition (SI) engines is an abnormal combustion phenomenon which can constrain the engine performance and thermal efficiency. It can also result in severe permanent engine damage under certain operating conditions. This paper systematically reviews the engine knock phenomenon, including the mechanisms, influencing factors, consequences and detection methods etc. It introduces the visualization researches, simulations and some judging indexes of engine knock. Three mathematical models which can predict the engine knock, and various kinds of methods of suppressing knock are summarized. Finally, this paper puts forward some new suggestions on the weakness in the researches of knocking combustion. [Copyright &y& Elsevier]

Abstract: This paper reports the study carried out to asses the dynamic behaviour of a real wind farm during fault conditions in the distribution network. The objective is to obtain a transient model valuable for all kinds of operational conditions. The paper begins with a detailed modelling of the distribution network between the wind farm common busbar and the point of common coupling with the HV transmission system, for all types of short-circuits. The state-space network model, together with the electromagnetic transient model of each wind generator, configure a multi-machine wind farm model, with 572 state variables. Simulation results prove that multi-machine modelling is mandatory in order to tackle the wind farm contribution to symmetric and asymmetric short-circuits. [Copyright &y& Elsevier]

In this paper we consider control-oriented modeling of a sensible thermal energy storage (TES) tank with a helical immersed heat exchanger (IHX) coil. A key focus of the modeling approach is to minimize the number of dynamic states required to adequately describe the system dynamics. The resulting model is well-suited for model-based control design, real-time simulation, and hardware-in-the-loop testing aimed at intelligent operation of TES systems. We use a discretized approach to model the temperature dynamics of the water within the storage tank. We use a quasi-steady approach to model the IHX coil dynamics, thereby limiting computational complexity. In simulation, the model runs up to 1200× faster than real-time. A simulated case study of model-based feedback control demonstrates the utility of the modeling approach. The model contains four tuning parameters that are empirically determined using experimental data collected from a commercially available domestic hot water storage tank. The model is then validated, both temporally and spatially, against data collected during the simultaneous charge/discharge mode. Finally, we quantify the trade-off between model fidelity and increased control volume discretization, showing that a 60 node model yields a RMSE value under 4.5%. [ABSTRACT FROM AUTHOR]

This paper describes a novel design approach of the free piston Stirling engines (FPSEs) based on multiple-scale perturbation method. First, a comprehensive mathematical model for an FPSE possessing nonlinear springs is presented. Then, the method of multiple scales is used to obtain the steady-state response of the engine system. Thus, some useful analytical relationships to predict frequency, strokes of pistons, and phase angle, as well as output power and efficiency of the nonlinear FPSE, are presented. Furthermore, a systematic mathematical approach for estimating the gas temperatures within expansion and compression spaces of the FPSE is proposed based on the perturbation technique. Next, a simulation study is carried out to investigate how much the engine frequency, strokes of pistons, and phase angle of the FPSE are sensitive to the variation of gas temperature. Besides, the effect of changes in the engine design parameters such as mass and stiffness of the pistons on the output power of the FPSE is studied using simulation. Finally, a test engine is developed and experimented to verify the proposed design technique. It is found that the experimental results are in a good agreement with the simulation outcomes of the analytical model through which validity of the proposed design scheme is clearly demonstrated. [ABSTRACT FROM AUTHOR]

In this paper, a promising heat pump based on Stirling technology is examined. Based on the volume variation produced by the oscillating displacement of two pistons, the advantages of this particular system lie in the elimination of a transmission mechanism, as the driving machines are directly coupled to the working pistons, and in the use of environmentally friendly working gases. The system is analysed by coupling the dynamics to an isothermal model of the Stirling cycle. A methodology is proposed to preliminarily size this type of heat pump based on simulations performed on a mathematical model built in MATLAB. The stability of operation is analysed by considering possible changes in working conditions. Actions are proposed to minimize the effect of these changes on the performance of the system. [ABSTRACT FROM AUTHOR]

Abstract: Today’s downsized turbocharged engines mainly focus on improving low end torque and increasing mass flow rate, this is done in order to improve the overall thermodynamic efficiency of the engine and to gain a lower BSFC. An integral part of any combustion engine is the air intake line that has a first order effect on engine filling and emptying. The wave action that takes place is usually simulated using one-dimensional codes. This paper presents a novel technique based on a frequency domain characterization of the intake line. A link over a wide frequency spectrum is identified between the instantaneous mass flow at the valve and the dynamic pressure response. This model is implemented into Simulink via a transfer function and coupled to GT-Power to produce an engine simulation. A shock tube experimental campaign was conducted for a number of tubes with varying lengths and diameters. The parameters of this transfer function are measured for each case then combined with gas dynamic theory and a frequency analysis to identify a law of behavior as a function of pipe geometry. The final model is validated on a single cylinder engine in GT-Power for a variety of pipe geometry. [Copyright &y& Elsevier]

Abstract: Waste heat recovery by means of a Rankine Cycle is a promising approach for achieving significant reductions in fuel consumption and, as a result, exhaust emissions of passenger car engines. This approach is already well established in industrial applications such as gas and steam power plants or ship propulsion systems. While these systems are mainly designed for stationary operation, the behaviour in highly dynamic operating conditions becomes more important when the principle is transferred to a passenger car engine. Knowledge of the dynamic response of the employed heat exchangers plays an important role in performance prediction and control system design of the steam cycle. Hence, a dynamic model of the exhaust gas heat exchanger employing the moving-boundary principle was developed and is presented in this paper. The model describes both design operation and the heat-up procedure of the component. For achieving high model accuracy in the resulting broad range of operating conditions, new approaches for modelling wall temperature distribution and zone switching were developed. Simulations of stationary operating points as well as the response to typical disturbances of the system’s input variables are in good agreement with test bench measurements. The model is used to develop a control system for dynamic operation on the test bench. Further studies of the operating characteristics reveal varying dynamic behaviour depending on the heat flow rate from exhaust gas to working fluid as well as coupling of evaporation pressure and outlet steam temperature. [Copyright &y& Elsevier]

Abstract: This paper presents results obtained from a numerical simulation for the horizontal slinky-loop heat exchanger of a ground-source heat pump system. A three-dimensional numerical model was developed and the results of the thermal performance of various heat exchanger configurations are presented. The investigation was carried out on five types of loop pitch (loop spacing), three types of loop diameter, three values of soil thermal properties, and allowing continuous and intermittent operation. Comparison was made for the heat transfer rate, the amount of pipe material needed, as well as excavation work required for the horizontal slinky-loop heat exchanger. The results indicate that system parameters have a significant effect on the thermal performance of the system. [Copyright &y& Elsevier]

Abstract: An experimental validation for a computational fluid dynamics (CFD) model for tubes coiled in a phase change thermal energy storage system has been conducted. Using the validated CFD model, three CFD models have been developed. The first model was developed having pins embedded on a tube with heat transfer fluid (HTF) flowing in it, with PCM surrounding the tube. Different configurations of pins on the tube have been analysed. The second model developed is similar to the first model; however, fins were embedded instead of pins. Different configurations of fins on the tube were also investigated. The last model developed was a plain copper tube surrounded by PCM with HTF flowing in it. This model was used as a benchmark for comparison for the first two models. The models were analysed for the freezing process. From this study, it was concluded that fins on the tube is better than pins on the tube. The paper gives details of the CFD models and presents the results obtained from simulations carried out using these models. [Copyright &y& Elsevier]

Abstract: This paper presents a power generation and heat recovery model for reciprocating internal combustion engines (ICEs). The purpose of the proposed model is to provide realistic estimates of performance/efficiency maps for both electrical power output and useful thermal output for various capacities of engines for use in a preliminary CHP design/simulation process. The proposed model will serve as an alternative to constant engine efficiencies or empirical efficiency curves commonly used in the current literature for simulations of CHP systems. The engine performance/efficiency calculation algorithm has been coded to a publicly distributed FORTRAN Dynamic Link Library (DLL), and a user friendly tool has been developed using Visual Basic programming. Simulation results using the proposed model are validated against manufacturer’s technical data. [Copyright &y& Elsevier]

Abstract: Earth heat exchangers are essential parts of the ground-source heat pumps and the accurate prediction of their performance is of fundamental importance. This paper presents the development and validation of a numerical model for the simulation of energy flows and temperature changes in and around a borehole heat exchanger when a fluid circulates through a U-tube. Based on the time-dependent convection–diffusion equation, the FlexPDE software package is employed to solve the resulting boundary value problem that model a heat exchanger. First, the mathematical model is validated through a comparison with data obtained from experiments with real borehole set-ups in Cyprus. Then the validated model is used to study the heat flow and the temperature variation in the heat-exchanger. Finally conclusions are extracted on how various parameters like the U-tube diameter, the variation of the ground thermal conductivity and specific heat and the borehole filling material affect the temperature of the inlet and outlet fluid. [Copyright &y& Elsevier]

Abstract: In order to reduce the energy use in the building sector there is a demand for tools that can identify significant building energy performance parameters. In the work introduced in this paper presents a methodology, based on a simulation module and graphical figures, for interactive investigations of the building energy performance. The building energy use simulation program is called TEKLA and is using EN832 with an improved procedure in calculating the heat loss through the floor and the solar heat gain. The graphical figures are simple and are illustrating the savings based on retrofit measures and climate conditions. The accuracy of the TEKLA simulation was investigated on a typical single-family building in Sweden for a period of time in a space heating demand of relatively cold and mild climate. The model was found applicable for relative investigations. Further, the methodology was applied on a typical single family reference building. The climate data from three locations in Sweden were collected and a set of relevant measures were studied. The investigated examples illustrate how decisions in the early stages of the building design process can have decisive importance on the final building energy performance. [Copyright &y& Elsevier]

Abstract: This paper proposes a new maximum power point (MPP) model based on datasheets of photovoltaic (PV) panels and on a specific equivalent circuit. Initially, a graphical approach is implemented in order to express the characteristic parameters of PV panels as a function of solar irradiance and panel temperature. These values are used further in a mathematical approach, which implements the equations of the equivalent circuit of a PV panel to calculate MPP voltage, current and power. Furthermore, the proposed maximum power point model is compared and verified against data obtained in field measurements, and also compared with the fill factor method and with respective data for 14 PV panels. The model can be used for PV plant simulations where an accurate and easy to use model of MPP is required to be deployed under operating conditions which are not covered by the datasheets. [Copyright &y& Elsevier]

Abstract: High temperature steam gasification (HTSG) is a gasification technology which utilizes super-heated steam at a temperature above 1273K. This paper addresses the performance analysis of an updraft HTSG gasifier using a numerical model. The experimental data obtained from a demonstration-scale gasifier was successfully simulated by the developed model. The calculation results showed 150–300K temperature difference between gas phase and solid phase throughout the bed. Among a number of reactions, char gasification and water–gas shift reaction at char gasification zone played a major role to determine the syn-gas composition. Steam temperature, the ratio of steam to biomass and biomass feed rate affected the syn-gas composition while biomass particle diameter showed no significant effect. For the steam temperature and the ratio of steam to biomass, the difference of solid temperature at the bottom of gasifier determined the syn-gas composition. For biomass feed rate, the ratio of unreacted char extracted from the bottom of gasifier to supplied biomass determined the syn-gas composition. [Copyright &y& Elsevier]

Abstract: In this paper, a three-dimensional hydrogen absorption model is developed to precisely study the hydrogen absorption reaction and resultant heat and mass transport phenomena in metal hydride hydrogen storage vessels. The 3D model is first experimentally validated against the temperature evolution data available in the literature. In addition to model validation, the detailed 3D simulation results show that at the initial absorption stage, the vessel temperature and H/M ratio distributions are uniform throughout the entire vessel, indicating that hydrogen absorption is very efficient early during the hydriding process; thus, the local cooling effect is not influential. On the other hand, non-uniform distributions are predicted at the subsequent absorption stage, which is mainly due to differential degrees of cooling between the vessel wall and core regions. In addition, a parametric study is carried out for various designs and hydrogen feed pressures. This numerical study provides a fundamental understanding of the detailed heat and mass transfer phenomena during the hydrogen absorption process and further indicates that efficient design of the storage vessel and cooling system is critical to achieve rapid hydrogen charging performance. [Copyright &y& Elsevier]

Abstract: This paper investigates the transient heat conduction around the buried spiral coils which could be applied in the ground-coupled heat pump systems with the pile foundation as a geothermal heat exchanger. A transient ring-coil heat source model is developed, and the explicit analytical solutions for the temperature response are derived by means of the Green’s function theory and the image method. The influences of the coil pitch and locations are evaluated and discussed according to the solutions. In addition, comparisons between the ring-coil and cylindrical source models give that the improved finite ring-coil source model can accurately describe the heat transfer process of the pile geothermal heat exchanger (PGHE). The analytical solutions may provide a desirable and better tool for the PGHE simulation/design. [Copyright &y& Elsevier]

Abstract: The improvement of the energy efficiency of belt conveyor systems can be achieved at equipment and operation levels. Specifically, variable speed control, an equipment level intervention, is recommended to improve operation efficiency of belt conveyors. However, the current implementations mostly focus on lower level control loops without operational considerations at the system level. This paper intends to take a model based optimization approach to improve the efficiency of belt conveyors at the operational level. An analytical energy model, originating from ISO 5048, is firstly proposed, which lumps all the parameters into four coefficients. Subsequently, both an off-line and an on-line parameter estimation schemes are applied to identify the new energy model, respectively. Simulation results are presented for the estimates of the four coefficients. Finally, optimization is done to achieve the best operation efficiency of belt conveyors under various constraints. Six optimization problems of a typical belt conveyor system are formulated, respectively, with solutions in simulation for a case study. [Copyright &y& Elsevier]

Abstract: This paper presents a study on optimization of a fixed bed tri-reformer reactor (TR). This reactor has been used instead of conventional steam reformer (CSR) and auto thermal reformer (CAR). A theoretical investigation has been performed in order to evaluate the optimal operating conditions and enhancement of methane conversion, hydrogen production and desired H2/CO ratio as a synthesis gas for methanol production. A mathematical heterogeneous model has been used to simulate the reactor. The process performance under steady state conditions was analyzed with respect to key operational parameters (inlet temperature, O2/CH4, CO2/CH4 and steam/CH4 ratios). The influence of these parameters on gas temperature, methane conversion, hydrogen production and H2/CO ratio was investigated. Model validation was carried out by comparison of the reforming model results with industrial data of CSR. Differential evolution (DE) method was applied as a powerful method for optimization. Optimum feed temperature and reactant ratios (CH4/CO2/H2O/O2) are 1100K and 1/1.3/2.46/0.47 respectively. The optimized TR has enhanced methane conversion by 3.8% relative to industrial reformers in a single reactor. Methane conversion, hydrogen yield and H2/CO ratio in optimized TR are 97.9%, 1.84 and 1.7 respectively. The optimization results of tri-reformer were compared with the corresponding predictions from process simulation software operated at the same feed conditions. [Copyright &y& Elsevier]

Abstract: This paper analyses how to apply mist pre-cooling coupled with condensing temperature control to enhance the coefficient of performance (COP) of an air-cooled chiller system and hence achieve electricity savings. A modified DOE-2.1E chiller model was developed to predict the change of chiller COP due to various set points of condensing temperature and pre-cooling of air stream entering the condenser. The model was calibrated by using manufacturer’s data and used to estimate the annual electricity consumption of a chiller system serving an office building under four operating schemes: traditional head pressure control (HPC); HPC with a fixed mist generation rate; condensing temperature control (CTC) with a fixed mist generation rate; CTC with an optimal mist generation rate. It was estimated that using optimal mist control with CTC could achieve a 19.84% reduction in the annual electricity consumption of the system. Considerations when using mist pre-cooling to maximize electricity savings have been discussed. [Copyright &y& Elsevier]

Abstract: This paper presents an approach to obtain the income of solar irradiation within partitioned enclosures partially transparent to solar radiation. This model is mathematically exact and it is function of the outside solar irradiation, the orientation of the enclosure, the properties of the enclosure envelope, the geometry and properties of partitions. From the physical point of view it is founded on the assumption that the envelope and partitions surfaces are fully diffusive and that the radiation diffused through the atmosphere is fully isotropic. The model was applied to assess the solar irradiation at the ground of a hemicylindrical tunnel fitted with inclined partitions. The results of this study demonstrate, among other things, that: (i) the solar irradiation inside the enclosure, with any cladding material and with any orientation, is mainly determined by the transmittance of the partitions and to a lesser extent by its reflectance, (ii) the solar radiation inside an enclosure with highly transparent partitions (i.e., transmittance>0.5) is noticeable influenced by its orientation, but for a lower transmittance the influence of orientation becomes negligible; and (iii) to prevent the overheating within the enclosure it is advisable to install partitions with a low transmittance and a high absorptance on their surface facing downward. [Copyright &y& Elsevier]

Abstract: This paper describes two new strategies for controlling air economizers. The model-based control strategy uses a model to estimate the load on the cooling coil for 100% outdoor air and minimum outdoor air and transitions dampers between these two positions to minimize the load. The optimization-based control strategy uses a cooling coil model and a one-dimensional optimization routine to determine the fraction of outdoor air that minimizes the coil load. A coil model based on the contact–mixture analogy was adopted for this study and validated with experimental data. Simulations were used to compare the new economizer control strategies with traditional strategies. Simulations evaluated the sensitivity of the annual and peak cooling loads to outdoor air and return air sensor errors and climatic conditions. Incorrect control decisions stemming from sensor errors reduce the achievable energy savings and lead to higher peak hourly loads. For a scenario representative of poorly maintained sensors, the lowest annual coil loads of the economizer strategies investigated are 3.0–40.3% higher than those achieved for the ideal scenario of optimization-based control and ideal sensors (i.e., no sensor errors), and the lowest peak hourly loads for those cases are 6.8–84.1% higher than those for the ideal scenario. [Copyright &y& Elsevier]

Abstract: Direct hot water production consumes about 4% of the total energy use in Hong Kong, and about 20% when considering only the domestic sector. For water heating the energy sources are mostly town gas, liquefied petroleum gas and electricity. The use of heat pump or solar water heating, particularly the solar-assisted heat pump options, is not popular. In this paper, the potential application of a unitary type direct-expansion solar-assisted heat pump (DX-SAHP) system was examined. A numerical model of the DX-SAHP system was first introduced. From the simulation results with the use of the Typical Meteorological Year (TMY) weather data of Hong Kong, the system was found achieving a year-average coefficient of performance (COP) of 6.46, which is much better than the conventional heat pump system performance. The potential use of DX-SAHP therefore deserves further evaluation. [Copyright &y& Elsevier]

Abstract: In this paper, the simulation of the performance of solar-assisted heating and cooling systems is analyzed. Three different plant layouts are considered: (i) the first one consists of evacuated solar collectors and a single-stage LiBr–H2O absorption chiller; here in order to integrate the system in case of insufficient solar radiation, an electric water-cooled chiller is activated; (ii) configuration of the secondly considered system is similar to the first one, but the absorption chiller and the solar collector area are sized for balancing about 30% of the building cooling load only; (iii) the layout of the thirdly considered system differs from the first one since the auxiliary electric chiller is replaced by a gas-fired heater. Such system configurations also include: circulation pumps, storage tanks, feedback controllers, mixers, diverters and on/off hysteresis controllers. All such devices are modelled for maximizing the system energy efficiency. In order to simulate the systems’ performance for dynamic heating/cooling loads, a single-lumped capacitance building is also modelled and implemented in the computer code. A cost model is also developed in order to calculate the systems’ operating and capital costs. All the models and the relative simulations are carried out by TRNSYS. A design of experiment procedure is also included. By such tool the effects of the system operating parameters’ variation on the relative energy efficiency are analyzed. In addition, the set of synthesis/design variables maximizing the system’s energetic performance can be also identified. The annual primary energy saving is chosen as the optimization objective function, whereas collector slope, pump flows, set-point temperatures and tank volume are selected as optimizing system design variables. A case study was developed for an office building located in South Italy. Here, the energetic and the economic analysis for all the three considered system layouts are carried out. The simulations’ results are referred to both the initial and the optimized systems configurations. The highest primary energy saving vs. the reference traditional HVAC system is reached by the first considered system layout. The economic performance of the investigated solar heating/cooling systems is still unsatisfactory. The economical profitability of the considered solar heating and cooling systems can be improved only by significant public finding. From this point of view, the best results were achieved by the second above mentioned system configuration. [Copyright &y& Elsevier]

Abstract: This paper presents experimental and numerical studies of the pressure drop in ceramic foams for solar air receiver applications. There are three main aims in this study. The first is to measure the pressure drop in the studied ceramic foams, and to build an empirical model based on the experimental results and a parametric numerical simulation. The second aim is to study flow field characteristics in the ceramic foams, especially in the vicinity of the interface. The third is to study the pressure drop characteristics of two modified structures (by manufacturing holes on the ceramic foams) that are expected to decrease the pressure drop in ceramic foams, but maintain good heat transfer properties. The experimental results from the samples, including two modified structures, along with the simulation results, show that the pressure drop in the ceramic foams follows a modified Darcy relationship. The experimental results also show that the two modified structures dramatically decrease the pressure drop (with pressure drop decreases up to 70% at a superficial velocity of 5m/s). Based on both the experimental and the simulation results, a generalized model for predicting the pressure drop in ceramic foams was proposed. [Copyright &y& Elsevier]