A generalized technique for evaluating reflection coefficients for ground-plane systems

General formulae are derived for the reflection of insolation from finite plane surfaces unto tilted sensors. These results are reduced to various special cases which are compared to known results when possible. Both circular and rectangular geometries are considered.     

A filter-independent model identification technique for turbulent combustion modeling

In this paper, we address a method to reduce the number of species equations that must be solved via application of Principal Component Analysis (PCA). This technique provides a robust methodology to reduce the number of species equations by identifying correlations in state-space and defining new variables that are linear combinations of the original variables. We show that applying this technique in the context of Large Eddy Simulation allows for a mapping between the reduced variables and the full set of variables that is insensitive to the size of filter used. This is notable since it provides a model to map state variables to progress variables that is a closed model.As a linear transformation, PCA allows us to derive transport equations for the principal components, which have source terms. These source terms must be parameterized by the reduced set of principal components themselves. We present results from a priori studies to show the strengths and weaknesses of such a modeling approach. Results suggest that the PCA-based model can identify manifolds that exist in state space which are insensitive to filtering, suggesting that the model is directly applicable for use in Large Eddy Simulation. However, the resulting source terms are not parameterized with an accuracy as high as the state variables.     

Analysis of generalized parallel-series ultracapacitor shift circuits for energy storage systems

Ultracapacitors have numerous advantages including a remarkably high energy density as compared to conventional capacitors, long life cycle, temperature stability, and require no maintenance, which make them a good candidate to replace batteries as energy storage devices in renewable energy applications. However, ultracapacitors, just like conventional capacitors, inevitably suffer from constant voltage drop during discharging, which may limit their energy utilization, increase the voltage ripple of the DC bus across which the capacitors are connected and consequently increase the VA stresses of the subsequent converter stage. To alleviate the above limitations, parallel-series ultracapacitor shift circuits are employed to improve the energy utilization and minimize the DC bus voltage ripple. Two generalized parallel-series ultracapacitor shift circuits are presented and analyzed, and some design considerations and optimization methods are discussed.     

A novel approach for modeling the internal behavior of a PEMFC by using electrical circuits

This paper presents a method for modeling a PEMFC by using electrical circuits. In particular, it focuses on temperature and voltage distribution of fuel cell. The current distribution is calculated by using the Newton-Raphson method in order to estimate the physical parameters (connection resistances) of the model. Several test on a single PEMFC cell have been carried out during this study. In order to validate the model, temperature and voltage sensors have been installed in different segments of a single cell. A distinguishing advantage of the developed model is its ability to detect and localize the faults within the PEMFC cell, as well as simulate different faults in all of the three directions of the PEMFC cell.     

A generalized framework for reduced‐order modeling of a wind turbine wake

A reduced‐order model for a wind turbine wake is sought from large eddy simulation data. Fluctuating velocity fields are combined in the correlation tensor to form the kernel of the proper orthogonal decomposition (POD). Proper orthogonal decomposition modes resulting from the decomposition represent the spatially coherent turbulence structures in the wind turbine wake; eigenvalues delineate the relative amount of turbulent kinetic energy associated with each mode. Back‐projecting the POD modes onto the velocity snapshots produces dynamic coefficients that express the amplitude of each mode in time. A reduced‐order model of the wind turbine wake (wakeROM) is defined through a series of polynomial parameters that quantify mode interaction and the evolution of each POD mode coefficients. The resulting system of ordinary differential equations models the wind turbine wake composed only of the large‐scale turbulent dynamics identified by the POD. Tikhonov regularization is used to recalibrate the dynamical system by adding additional constraints to the minimization seeking polynomial parameters, reducing error in the modeled mode coefficients. The wakeROM is periodically reinitialized with new initial conditions found by relating the incoming turbulent velocity to the POD mode coefficients through a series of open‐loop transfer functions. The wakeROM reproduces mode coefficients to within 25.2%, quantified through the normalized root‐mean‐square error.  A high‐level view of the modeling approach is provided as a platform to discuss promising research directions, alternate processes that could benefit stability and efficiency, and desired extensions of the wakeROM.     

A two-factor saturation model for synchronous machines withmultiple rotor circuits

It is generally felt that no major accuracy breakthrough in predicting the steady-state and transient performance of synchronous machines could be achieved without taking proper account of the iron saturation effects as well as eddy-current losses. Although the two issues were often treated separately in the past, this paper attempts to unite them by developing a general model covering both the main-path magnetic saturation and frequency effects in the dynamic equations. Mathematical analysis in the d-q space pinpoints cross-saturation coupling which, a priori, does not seem to be symmetrical for salient-pole machines. Yet the model is theoretically sound, since it fulfils at least the physical constraints using energy balance principles. Some test points from a 555-MVA turbine-generator are used for an initial assessment the model's capability to predict the field current and internal angle for various loading conditions     

Numerical technique for modeling conjugate heat transfer in an electronic device heat sink

A fast running computational algorithm based on the volume averaging technique (VAT) is developed to simulate conjugate heat transfer process in an electronic device heat sink. The goal is to improve computational capability in the area of heat exchangers and to help eliminate some of empiricism that leads to overly constrained designs with resulting economic penalties.VAT is tested and applied to the transport equations of airflow through an aluminum (Al) chip heat sink. The equations are discretized using the finite volume method (FVM). Such computational algorithm is fast running, but still able to present a detailed picture of temperature fields in the airflow as well as in the solid structure of the heat sink. The calculated whole-section drag coefficient, Nusselt number and thermal effectiveness are compared with experimental data to verify the computational model and validate numerical code. The comparison also shows a good agreement between FVM results and experimental data.The constructed computational algorithm enables prediction of cooling capabilities for the selected geometry. It also offers possibilities for geometry improvements and optimization, to achieve higher thermal effectiveness.     

Thermodynamic modeling based on a generalized cubic equation of state for kerosene/LOx rocket combustion

Based on a generalized cubic equation of state proposed by Cismondi and Mollerup [Fluid Phase Equilib. 232 (2005) 74–89], this study derived formulations for the thermodynamic properties of mixtures that are needed for combustion simulations of liquid rocket engines. The present model was validated thoroughly against reference data provided by NIST in order to assess its validity over a wide range of critical compressibility factors, pressures, and temperatures. The numerical results clearly indicate that the present model is superior in its handling of fluid mixtures with quite different critical compressibility factors, while maintaining the advantages of the cubic equation of state compared to those of the Soave–Redlich–Kwong and Peng–Robinson equations of state. In addition, steady flamelet analysis has been performed to investigate the effects of detailed chemical kinetics, real fluid behavior, and increased pressure on the local flame structures of the kerosene surrogate and liquid oxygen relevant to liquid rocket engines.     

A generalized analysis for liquid-fuel vaporization and burning

A generalized method is presented for vaporization and combustion of multiple-droplet arrays, liquid films, pools, and streams. Conditions are explained for the existence of a mass flux potential function that is independent of fuel type and scalar boundary conditions and satisfies the three-dimensional Laplace’s equation. Gas-phase properties, composition, and flame location are functions only of the potential function, specified scalar boundary conditions, and fuel type. Variable properties are considered. Flame stand-off distances for liquid interfaces near wet-bulb temperatures are predicted more accurately with variable properties. Flame location and transport properties are found for decane, heptane, and methanol fuels, with different ambient conditions. The analysis also applies to vaporization without combustion and to combustion with transient liquid-phase heating.     

A generalized factorization principle for energy conservation analysis

The factorization principle is generalized to express energy consumption associated with any activity in terms of seven generic factors. The validity of this approach is demonstrated via examples involving three diverse activities. Guidelines are provided which permit systematic selection of the seven factors for any other activity. One of the seven factors describes the technical characteristics of the energy consuming device while the other six describe various aspects of the activity and the manner in which the device is utilized.     

A novel approach for modeling deregulated electricity markets

The theoretical framework developed in this study allows development of a model of deregulated electricity markets that explains two familiar empirical findings; the existence of forward premiums and price-cost markups in the spot market. This is a significant contribution because electricity forward premiums have been previously explained exclusively by the assumptions of perfect competition and risk-averse behavior while spot markups are generally the outcome of a body of literature assuming oligopolistic competition. Our theoretical framework indicates that a certain premium for forward contracting is required for efficient allocation of generation capacity. However, due to the uniqueness of electricity and the design of deregulated electricity markets this premium might be substantially higher than its optimal level.     

A generalized window energy rating system for typical office buildings

Detailed computer simulation programs require lengthy inputs, and cannot directly provide an insight to relationship between the window energy performance and the key window design parameters. Hence, several window energy rating systems (WERS) for residential houses and small buildings have been developed in different countries. Many studies showed that utilization of daylight through elaborate design and operation of windows leads to significant energy savings in both cooling and lighting in office buildings. However, the current WERSs do not consider daylighting effect, while most of daylighting analyses do not take into account the influence of convective and infiltration heat gains. Therefore, a generalized WERS for typical office buildings has been presented, which takes all primary influence factors into account. The model includes embodied and operation energy uses and savings by a window to fully reflect interactions among the influence parameters. Reference locations selected for artificial lighting and glare control in the current common simulation practice may cause uncompromised conflicts, which could result in over- or under-estimated energy performance. Widely used computer programs, DOE2 and ADELINE, for hourly daylighting and cooling simulations have their own weaknesses, which may result in unrealistic or inaccurate results. An approach is also presented for taking the advantages of the both programs and avoiding their weaknesses. The model and approach have been applied to a typical office building of Hong Kong as an example to demonstrate how a WERS in a particular location can be established and how well the model can work. The energy effect of window properties, window-to-wall ratio (WWR), building orientation and lighting control strategies have been analyzed, and can be indicated by the localized WERS. An application example also demonstrates that the algebraic WERS derived from simulation results can be easily used for the optimal design of windows in buildings similar to the typical buildings.     

A generalized non-isothermal tank model for liquid dominated geothermal reservoirs

We present a generalized non-isothermal tank model for predicting the pressure and temperature behaviors of liquid dominated geothermal reservoirs. A geothermal system is represented by a single or multiple tanks. These tanks can represent the reservoir, multiple reservoirs, aquifers or any other component of a geothermal system. The mass and energy balance equations for each tank are solved jointly. One of the main advantages of the model is that only a small number of tanks are used for modeling which avoids over parameterization of a geothermal system and results in faster run times when compared with fully discretized numerical simulators. Synthetic examples are used for studying the effects of heat conduction on reservoir performance, an analysis of the location of injection wells, recovery times of depleted geothermal fields and the benefits of using temperature data for a better characterization of the geothermal system.     

Design development of a static sunshade using small scale modeling technique

The rising global demand for energy has triggered emphasis on conservation of energy. Buildings are one of the important energy consuming sectors. Passive solar architecture encompasses a wide range of strategies and options resulting in energy efficient building design and increased occupant's comfort. Passive solar design, aiming at increasing direct solar gains during winter period and to avoid overheating during summer period should make use of specific shading devices over energy efficient window. The static sunshades are most effective for solar control inside the buildings.Countries like India have composite climate, which can be classified under summer, winter and rainy season. Depending on the seasonal requirements, this paper introduces a new geometry of a static sunshade, designed by calculating the sun angles for the two dates. The static sun shading design methodology is validated with the help of small scale modeling experimentation technique, carried out in Pilani, Rajasthan (India). Although insulating materials can be used as a part of a building structure, its feasibility should be checked before particular application. In the present paper, the two small-scale experimental models of actual construction material with varying static sunshades, i.e. horizontal and the proposed one have been constructed and analyzed with the models of insulating material (Polyurethane Foam [PUF]). Depending upon the solar intersection over south facade wall, sunlit area and shaded area have been correlated with temperature inside the models to decide the effectiveness of the proposed sunshade.     

A technique for mismatched PV array simulation

In this paper a computationally efficient approach to the modeling of a photovoltaic array is presented. It is especially suited for simulating mismatched operating conditions, in which the different modules of the array work at different irradiation levels and temperatures. The model used allows to keep into account parametric tolerances and drifts among the modules. The proposed technique is based on the use of the Lambert W-function, which allows to obtain an explicit relationship between the voltage and the current of any photovoltaic module. The non linear system of equations describing the photovoltaic array is easily solved thanks to the explicit symbolic calculation of the inverse of the Jacobian matrix. The performances of the proposed numerical approach are discussed especially in cases where a deep mismatch affects the photovoltaic array.