Electricity tariff design for transition economies: Application to the Libyan power system

This paper presents a general electricity tariff design methodology, especially applicable for transition economies. These countries are trying to modernize their power systems from a centralized environment (with normally, a public vertically integrated electric company) to a liberalized framework (unbundling electricity companies and, eventually, starting a privatization process). Two issues arise as crucial to achieving a successful transition: i) ensuring cost recovery for all future unbundled activities (generation, transmission, distribution and retailing), and ii) sending the right price signals to electricity customers, avoiding cross-subsidies between customer categories. The design of electricity tariffs plays a pivotal role in achieving both objectives. This paper proposes a new tariff design methodology that, complying with these two aforementioned criteria, requires a low amount of information regarding system data and customer load profiles. This is important since, typically, volume and quality of data are poor in those countries. The presented methodology is applied to computing tariffs for the Libyan power system in 2006, using real data.     

Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors: Part 1: Indoor experiment

An especial open thermosyphon device used in high-temperature evacuated tubular solar collectors was designed. The indoor experimental research was carried out to investigate the thermal performance of the open thermosyphon using respectively the deionized water and water-based CuO nanofluids as the working liquid. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and the operating temperature on the evaporating heat transfer characteristics in the open thermosyphon were investigated and discussed. Experiment results show the optimal filling ratio to the evaporator is 60% and the thermal performance of the open thermosyphon increase generally with the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly enhance the thermal performance of the evaporator and evaporating heat transfer coefficients may increase by about 30% compared with those of deionized water. The CuO nanoparticles mass concentration has remarkable influence on the heat transfer coefficient in the evaporation section and the mass concentration of 1.2% corresponds to the optimal heat transfer enhancement.     

High performance storage composite for the enhancement of solar domestic hot water systems: Part 1: Storage material investigation

This work aims to evaluate the performance of a solar domestic hot water (SDHW) system including a latent storage material. The originality of our approach consists to place a composite made of compressed expanded natural graphite (CENG) and phase change material (PCM) directly inside a flat plate solar collector in order to replace the traditional copper-based solar absorber. According to this target, the study is composed of two steps: the composites preparation and characterization; and the analysis of the system to achieve optimal integration of the material in the process.The present paper is focused on the selection of the most promising composite to implement in the solar collector. In order to reach this objective, several composites based on CENG and various storage materials (paraffin, stearic acid, sodium acetate trihydrate and pentaglycerin) have been elaborated and characterized. The synthesis of all these measurements allowed us to select three composites whose characteristics match their integration into a solar thermal collector.     

Calendar life study of Li-ion pouch cells: Part 2: Simulation

The low rate (C/33) discharge data obtained from a calendar life study were numerically analyzed with a single particle model. The simulation showed that the stoichiometric window for the cathode shrank with capacity fade. The change of the stoichiometric window for the anode was more complicated. The aged anode became less charged when the capacity fade was caused mostly by the loss of cyclable lithium ions. The anode would be charged to a higher stoichiometric number (or state of charge, SOC) when the capacity fade became controlled by the loss of active material in the anode.     

Inhomogeneous compression of PEMFC gas diffusion layer: Part I. Experimental

This paper presents a study on the effect of inhomogeneous compression of gas diffusion layer (GDL) caused by the channel/rib structure of flow-field plate. The experimentally evaluated properties are GDL intrusion into the channel, gas permeability, in-plane and through-plane bulk electric conductivities, and contact resistances at interfaces as a function of compressed thickness of GDL. It was found that the GDL is compressed very little under the channel whereas GDL under the rib is compressed to gasket thickness. The compression of GDL reduces gas permeability and contact resistance, and improves bulk conductivity. Hence, the inhomogeneous compression of GDL may lead into significant local variation of mass and charge transport properties in the GDL. These effects have been ignored in most of the published modeling studies. This contribution, part I, covers the experimental setup and measurement results, and a model which takes the inhomogeneous compression of GDL into account is presented in part II.     

A high efficient assembly technique for large PEMFC stacks: Part I. Theory

A high efficient assembly technique for large proton exchange membrane fuel cell (PEMFC) stacks is proposed to obtain the optimal clamping load. The stack system is considered as a mechanical equivalent stiffness model consisting of numerous elastic elements (springs) in either series or parallel connections. We first propose an equivalent stiffness model for a single PEM fuel cell, and then develop an equivalent stiffness model for a large PEMFC stack. Based on the equivalent stiffness model, we discuss the effects of the structural parameters and temperature on the internal stress of the components and the contact resistance at the contact interfaces, and show how to determine the assembly parameters of a large fuel cell stack using the equivalent stiffness model. Finally, a three-dimensional finite element analysis (FEA) for a single PEMFC is compared with what the equivalent stiffness model predicts. It is found that the presented model gives very good prediction accuracy for the component stiffness and the clamping load.     

Direct numerical simulations of the double scalar mixing layer: Part II: Reactive scalars

The reacting double scalar mixing layer (RDSML) is investigated as a canonical multistream flow and a model problem for simple piloted diffusion flames. In piloted diffusion flames, the reacting fuel and oxidizer streams are initially separated by a central pilot stream at stoichiometric composition. The primary purpose of this pilot is to delay the mixing of the pure streams until a stable flame base can develop. In such multistream systems, the modeling of turbulent scalar mixing is complicated by the multiple feed streams, leading to more complex fine-scale statistics, which remain as yet an unmet modeling challenge compared to the simpler two-feed system. In Part I we described how multimodal mixture fraction probability density functions (PDFs) and conditional scalar dissipation rates can be modeled with a presumed mapping function approach. In this work we present an efficient and robust extension of the modeling to a general multistream reacting flow and compare predictions to three-dimensional direct numerical simulations (DNS) of the RDSML with a single-step reversible chemistry model and varying levels of extinction. With high extinction levels, the interaction with the pilot stream is described. Additionally, state-of-the-art combustion modeling calculations including conditional moment closure (CMC) and stationary laminar flamelet modeling (SLFM) are performed with the newly developed mixing model. Excellent agreement is found between the DNS and modeling predictions, even where the PDF is essentially a triple-delta shape near the flame base, so long as extinction levels are moderate to low. The suggested approach outlined in this paper is strictly valid only for flows that can be described by a single mixture fraction. For these flows the approach should provide engineers with fine-scale models that are of accuracy comparable to those already available for binary mixing, at only marginally higher complexity and cost.