Efficiency Model (efficiency + model)

Distribution by Scientific Domains


Selected Abstracts


Postfire response of North American boreal forest net primary productivity analyzed with satellite observations

GLOBAL CHANGE BIOLOGY, Issue 8 2003
Jeffrey A. Hicke
Abstract Fire is a major disturbance in the boreal forest, and has been shown to release significant amounts of carbon (C) to the atmosphere through combustion. However, less is known about the effects on ecosystems following fire, which include reduced productivity and changes in decomposition in the decade immediately following the disturbance. In this study, we assessed the impact of fire on net primary productivity (NPP) in the North American boreal forest using a 17-year record of satellite NDVI observations at 8- km spatial resolution together with a light-use efficiency model. We identified 61 fire scars in the satellite observations using digitized fire burn perimeters from a database of large fires. We studied the postfire response of NPP by analyzing the most impacted pixel within each burned area. NPP decreased in the year following the fire by 60,260 g C m,2 yr,1 (30,80%). By comparing pre- and postfire observations, we estimated a mean NPP recovery period for boreal forests of about 9 years, with substantial variability among fires. We incorporated this behavior into a carbon cycle model simulation to demonstrate these effects on net ecosystem production. The disturbance resulted in a release of C to the atmosphere during the first 8 years, followed by a small, but long-lived, sink lasting 150 years. Postfire net emissions were three times as large as from a model run without changing NPP. However, only small differences in the C cycle occurred between runs after 8 years due to the rapid recovery of NPP. We conclude by discussing the effects of fire on the long-term continental trends in satellite NDVI observed across boreal North America during the 1980s and 1990s. [source]


Optimal operation of CVT-based powertrains

INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Issue 11 2001
Rolf Pfiffner
Abstract Continuously Variable Transmissions (CVTs) are considered by many as a promising contribution to the improvement of the fuel economy of passenger cars. This paper discusses the fuel-optimal operation of a CVT-equipped powertrain in stationary and in transient conditions. The well-known optimal solution in the stationary case will be given only a short review discussing the importance of a correct CVT efficiency model. For transient vehicle operation so far only heuristic control strategies are known. This work aims to fill this gap. After detailed modelling of the complete powertrain and the introduction of a meaningful cost function, the resulting nonlinear optimal control problem is solved. The optimization is carried out for two different engines: a conventional SI engine and a downsized supercharged (DSC) SI engine. Each can be considered as a typical representative of the corresponding engine class. In both cases the optimal solutions are shown to be superior to standard CVT control algorithms, yielding larger gains in fuel economy for DSC engines. The optimal feedforward controls are then approximated by suboptimal feedback controllers that realize almost the same benefits in fuel economy at a substantially reduced computational effort. Finally, it is shown how the well-known ,single track modified' control approach can be changed such that it produces the same results as the suboptimal controller. Copyright © 2001 John Wiley & Sons, Ltd. [source]


Cost Efficiency in South Asian Banking: The Impact of Bank Size, State Ownership and Stock Exchange Listings,

INTERNATIONAL REVIEW OF FINANCE, Issue 1-2 2007
SHRIMAL PERERA
ABSTRACT This study examines the cost efficiency performance of 111 commercial banks in Bangladesh, India, Pakistan and Sri Lanka over 1997,2004. The primary focus is to assess whether bank size, state ownership and stock exchange listing have significant effects on South Asian banks' efficiency performance. To this end, a translog-form composite-error cost efficiency model, which allows for exogenous environmental influences, is estimated. The results indicate that the overall efficiency of South Asian banks declined over 1997,2004. Larger banks and banks with widespread ownership through stock exchange listings were found to be relatively more cost efficient. In contrast, state-owned banks were less efficient. [source]


Overall efficiency evaluation of commercial distillation columns with valve and dualflow trays

AICHE JOURNAL, Issue 9 2010
T. L. Domingues
Abstract The main objective of this work is to establish appropriated ways for estimating the overall efficiencies of industrial distillation columns with valve trays with downcomer and dualflow trays. The knowledge of efficiencies has fundamental importance in the design and performance evaluation of distillation columns. Searching in the literature, a tree of alternatives was identified to compose the tray efficiency model, depending on the mass transfer models, the liquid distribution and vapor flow models on the tray, the liquid entrainment model, the multicomponent mixture equilibrium model, the physical properties models, the height of froth on the tray model and the efficiency definition. In this work, different methods to predict the overall efficiency of distillation columns with valve and dualflow trays were composed and compared with data from three commercial distillation columns under different operating conditions. The models were inserted in the Aspen Plus 12.1 simulator, in Fortran language, together with tray geometrical data, fluid properties and operating data of the distillation columns. For each column, the best thermodynamic package was chosen by checking the temperature profile and overhead and bottom compositions obtained via simulation against the corresponding actual data of industrial columns. A modification in the fraction of holes evaluation that is jetting parameter of the Garcia's hydraulic model of dispersion above the tray was proposed. This modification produced better results than the original model to predict the fraction of holes that are jetting and in the efficiency of dualflow trays and similar results to Garcia model in the efficiency evaluation of valve trays. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source]


Effective Efficiency as a Tool for Sustainable Water Resources Management,

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 4 2008
Naim Haie
Abstract:, The sufficiency and usefulness of Effective Efficiency (EE) as a water resources index is shown through conceptual formulation of a generalized EE and practical applications. Two EE models are proposed: one is based on water quantity and the other on quantity and quality, with the possibility of considering water reuse (recycling) in both. These models were developed for two scales: the first is called Project EE and the second Basin EE. The latter gives the influence of the project on the water resources systems of the basin while the former does not make such connection to the whole basin. Such considerations give proper signals as to the adequacy of any intervention to increase efficiency. A crucial distinction is made between depletion and diversion water savings. Classical Efficiency (CE) models are analyzed and compared with the various EE models. CE results in values that are less than EE because of not considering water reuse and water quality in its calculation. Some authors, pointing to these problems , particularly the first problem , have advocated the use of hydrological "fractions" instead of efficiency concepts. This paper defends the use of a proper efficiency model such as EE and suggests putting an end to the use of the CE indicators. To test the models, they are applied to five cases of irrigation and city water use in the United States and Egypt. The analysis of the results demonstrates all the points mentioned above and the potential of the EE models to adequately describe the water resources efficiency and sustainability at a location. [source]