Emanuela Colombo

Off-Design Modeling of Natural Gas Combined Cycle Power Plants: An Order Reduction by Means of Thermoeconomic Input–Output Analysis

In a European context characterized by growing need for operational flexibility across the electricity sector, the combined cycle power plants are increasingly subjected to cyclic operation. These new operation profiles cause an increase of production costs and decrease of revenues, which undermines the competitiveness of the combined cycles. Power plant operators need tools to predict the effect of off-design operation and control mechanisms on the performance of the power plant. Traditional Thermodynamic or Thermoeconomic models may be unpractical for the operators, due to their complexity and the computational effort they require. This study proposes a Thermoeconomic Input–Output Analysis model for the on- and off-design performance prediction of energy systems, and applies it to La Casella Natural Gas Combined Cycle (NGCC) power plant, in Italy. It represents a stand-alone, reduced order model, where the cost structure of the plant products and the Thermoeconomic performance indicators are derived for on- and off-design conditions as functions of the load and of different control mechanisms, independently from the Thermodynamic model. The results of the application show that the Thermoeconomic Input–Output Analysis model is a suitable tool for power plant operators, able to derive the same information coming from traditional Thermoeconomic Analysis with reduced complexity and computational effort.

Direct numerical simulation of fully saturated flow in natural porous media at the pore scale: a comparison of three computational systems

Direct numerical simulations of flow through two millimeter-scale rock samples of limestone and sandstone are performed using three diverse fluid dynamic simulators. The resulting steady-state velocity fields are compared in terms of the associated empirical probability density functions (PDFs) and key statistics of the velocity fields. The pore space geometry of each sample is imaged at 5.06−μm voxel size resolution using X-ray microtomography. The samples offer contrasting characteristics in terms of total connected porosity (about 0.31 for the limestone and 0.07 for the sandstone) and are typical of several applications in hydrogeology and petroleum engineering. The three-dimensional fluid velocity fields within the explicit pore spaces are simulated using ANSYS® FLUENT® ANSYS Inc. (2009), EULAG Prusa et al. (Comput. Fluids 37, 1193–1207 2008), and SSTOKES Sarkar et al. (2002). These computational approaches are highly disperse in terms of algorithmic complexity, differ in terms of their governing equations, the adopted numerical methodologies, the enforcement of internal no-slip boundary conditions at the fluid-solid interface, and the computational mesh structure. As metrics of comparison to probe in a statistical sense the internal similarities/differences across sample populations of velocities obtained through the computational systems, we consider (i) integral quantities, such as the Darcy flux and (ii) main statistical moments of local velocity distributions including local correlations between velocity fields. Comparison of simulation results indicates that mutually consistent estimates of the state of flow are obtained in the analyzed samples of natural pore spaces despite the considerable differences associated with the three computational approaches. We note that in the higher porosity limestone sample, the structures of the velocity fields obtained using ANSYS FLUENT and EULAG are more alike than either compared against the results obtained using SSTOKES. In the low-porosity sample, the structures of the velocity fields obtained by EULAG and SSTOKES are more similar than either is to the fields obtained using ANSYS FLUENT. With respect to macroscopic quantities, ANSYS FLUENT and SSTOKES provide similar results in terms of the average vertical velocity for both of the complex microscale geometries considered, while EULAG tends to render the largest velocity values. The influence of the pore space structure on fluid velocity field characteristics is also discussed.

A small-hydro plant model for feasibility analysis of electrification projects in Rural Tanzania

In some rural areas of developing countries grid extension may not be convenient and thus off-grid technologies are frequently the most appropriate option addressing the prevailing lack of electricity access. Among the renewable off-grid technologies, micro and small hydropower often matches local conditions. Nevertheless in order to accomplish reliable and affordable off-grid electrification, techno-economic analyses play a pivotal role. In this paper we describe the model that we developed for assessing the techno-economic feasibility of run-of-river small hydropower plant (up to 2MW) for rural areas in Tanzania. The model makes use of river and load data to generate an optimization of the installed electrical power and to perform an economic analysis of the investment. The capability to search for the best power size while evaluating draft designs and the implementation of Tanzanian-tailored cost functions, make the model useful for the purposes of identifying small-hydropower sites. Application to two real field cases complete the validation of the model.

CFD Study of Ejector Efficiencies

This paper presents a method to evaluate ejector efficiency in function of local flow parameters. The paper is divided into two parts. In the first part, a Computational Fluid-Dynamics (CFD) approach for convergent nozzle ejectors is presented and computational results are validated using experimental velocity and temperature profiles at different sections. The validation process includes the evaluation of seven Reynolds-Averaged Navier–Stokes (RANS) turbulence models: the Spalart-Allmaras and the k–omega SST models show better performance in terms of convergence capability and flow and thermal field prediction. In the second part, local flow phenomena and their influence on ejector component efficiencies are investigated. The validated CFD approach is used to determine the efficiencies of the ejector primary nozzle, suction chamber, and mixing zone. Efficiency maps, regressing equation linking efficiencies, and local flow quantities are proposed and discussed. Finally, global ejector performance is mapped and considerations are outlined.Copyright

Renewable Energies to Promote Local Development

Energy has been deeply linked to the history of mankind and to its development. Some of the natural forces which are today referred to as renewable energies, such as solar, wind, hydro and biomass have been known and used for centuries. Among all the different sources of energy, renewables were the first to satisfy human energy needs. Today the opportunity to overcome the development divide strongly depends on the availability of energy, and hence, the nexus between energy and sustainable development needs to be better explored in order to understand how energy can contribute to poverty reduction. In this scenario, in the chapter it is shown how renewable energies and distributed generation play a key role as they allow the utilization of local resources while preserving the environment, creating employment, promoting income generation, capacity building and local empowerment.

Computational fluid dynamic model of a tapered Holweck vacuum pump operating in the viscous and transition regimes. I. Vacuum performance

Holweck molecular drag pumps are used as high-pressure stages in hybrid turbomolecular vacuum pumps, where they can operate in the transition and the viscous regime. In this article we develop a Navier-Stokes model of a Holweck pump with tapered pumping channels, applying slip-flow boundary conditions, to predict vacuum performances with and without gas flow. The commercial computational fluid dynamic code FLUENT is used to solve the model equations and to predict the pressure profile along the grooves. A specifically designed experiment is presented, whose arrangement provides the boundary conditions as input to the model and whose results are used to validate it.

Numerical analysis of fluid dynamics and thermal characteristics inside a wavy channel

Purpose – This paper aims to present the results of a numerical investigation of the fluid dynamics and heat transfer behavior of forced incompressible flow inside a rectangular wavy channel. Reynolds numbers, based on hydraulic inlet diameter and bulk velocity, ranging from 500 to 10000 are investigated.Design/methodology/approach – The numerical analysis is performed by means of a finite volume commercial CFD code. A Reynolds Averaged Navier‐Stokes (RANS) approach is applied to a three‐dimensional fluid domain over a single module with periodic conditions. Further analysis over six modules is also performed to validate the periodic numerical domain.Findings – Mean velocity and temperature fields are obtained. The global values of Nusselt number are compared with data obtained by an experimental facility with the same geometry and operating with Re from 1000 to 10000.Research limitations/implications – Some limitations related to the numerical approach used are observed in laminar‐turbulent transitional ...

Numerical Experiments with a New Dynamic Mixed Subgrid-Scale Model

In the present work we introduce an LES framework which does not require any commutation property between filtering and derivative operators. A consistent redefinition of the SGS tensor in the new framework introduces several differences in the classical modeling strategies, including a modified, cheaper, form of Germano dynamic procedure. This dynamic procedure is exploited trough a new form of mixed model, whose scale-similar part is derived by a Taylor series analysis of the SGS tensor. The model is implemented in a commercial unstructured finite volume solver and numerical tests are performed on the turbulent channel flow at \(\mathrm {Re}_\tau = 590\), showing the flexibility and accuracy of the proposed modeling strategy.

Global Dimension of Universal Access to Energy

The development of human society has been marked all throughout history by the role of energy resources. The importance of energy in the global scenario has constantly risen and the interconnections with the environment and society have become more evident. The need to fight both poverty through eradication of energy insufficiency and to increase access to modern energy service is recognized worldwide. The designation of Year 2012 as the International Year for Sustainable Energy for All and the subsequent Sustainable Energy for All initiative [1] promoted by the UN Secretary General Ban Ki-moon to foster access to energy, energy efficiency and renewable energies has raised awareness in the international agenda on these issues and has opened a wide window on the problem for the coming decades. The size of the problem and some of the key challenges are discussed in the chapter.

The Role of Academia for Sustainable Development

Access to energy is one of the pillars of sustainable development and therefore may be considered as part of this wider research topics. Today the role of university within sustainable development is quite debated at the international level. A double volume has recently been dedicated to this issue by the Journal of Cleaner Production, representing the most updated review. In some circumstances, universities are recognised to have contributed in transforming the society and promoting the common good. At the same time, other examples proving that universities have contributed to the dissemination of unsustainable practises may also be found. Nevertheless, the responsibility of universities and higher education institutions becomes increasingly important when knowledge, skills and innovation are needed to deal with today’s global challenges such as those linked to energy. The role of Academia and some of the main related issues are discussed in the chapter.