Stability and nonlinear dynamics in power systems

Power systems are characterized by nonlinear dynamics, and unusual and unexpected behavior has been observed in both simple and complex networks. System behavior is expected to become increasingly complicated because of such factors as greater interconnection, the implementation of high-speed electronic power control devices and wheeling requirements. The author describes how, because other nonlinear systems are known to exhibit chaotic behavior, exploratory research is being conducted to determine if the potential for chaos exists in power networks and, if so, how to avoid or control such behavior. By increasing the fundamental understanding of power system dynamics, these investigations could lead to substantial economic benefits. They could reduce the potential for catastrophic failure as well as allow secure network operation closer to performance margins     

Mass transfer at smooth and rough surfaces in a circular couette flow

An extended set of mass transfer measurements at smooth and rough surfaces in a circular Couette flow has been carried out in the range 20000 < Re < 900000 and 770 < Sc < 8000. Mass transfer measurements for smooth rotating cylinders are in agreement with previous results and confirm that the Sherwood number depends on the power of the Schmidt number. Mass transfer coefficients at rough cylinders show a similar behaviour with Reynolds number as that observed for duct flows and the experimental results for both systems can be generalized in the fully rough region for equally rough surfaces when the maximum velocity of the flow and the surface roughness height are taken as the characteristic velocity and length scale in the Stanton and Reynolds numbers.     

Operating conditions of batteries in off-grid renewable energy systems

Operating conditions in off-grid renewable energy systems (RES) vary significantly in different applications and locations. To describe RES and the operating conditions of their components it is useful to define categories of similar operating conditions. Categories can also be used for lifetime considerations of RES components, for making recommendations and for analysing the properties and performance of a RES and its components. Categories support system designers and an economic analysis. This paper describes the process and the results of creating RES categories of similar operating conditions for batteries. Categories are defined in such a way that batteries belonging to the same category are subjected to similar operating conditions and a similar combination of stress factors. The results provide a comprehensive overview of battery operating conditions in existing off-grid renewable energy systems. This work is part of the EU research project Benchmarking.¹     

Energy and exergy analyses of fabricated solar drying system with smooth and rough surfaces at different conditions: A case study

For this experimental work, the solar dryer system has been fabricated. Various experiments have been performed to evaluate the performance of the fabricated dryer system. Energy and exergy concepts have been used for the assessment. And comparative analyses have also been done with smooth and rough surfaces: Grade 150 and Grade 300. This experimental work has been concluded as the maximum collector (i.e., 74°C) and cabinet temperatures (i.e., 66°C) are attained at 13:00 p.m. with Grade 300 surface. Maximum moisture loss (i.e., 35 g) and percentage moisture loss (i.e., 7.53%) are recorded at 13:00 p.m. again with a rough surface. Minimum exergy destruction rate (i.e., 0.294 W) but minimum exergy efficiency (i.e., 20.82%) are found at 17:00 p.m. with a black painted surface, which is not an acceptable condition. Maximum energy efficiency (i.e., 35.98%) and heat removal factor (i.e., 0.56) are obtained at 13:00 p.m. with a rough surface. The best performance from the fabricated solar system is received between 12:00 and 14:00 p.m. This study recommends rough surfaces and 12:00–13:00 p.m. timings for the solar drying system as the performance of the system is better in terms of energy–exergy.     

Exergy analysis of hypersonic propulsion systems: Performance comparison of two different scramjet configurations at cruise conditions

An exergy analysis of an advanced hypersonic vehicle, a scramjet, is presented and discussed with a twofold scope. First, to perfect the exergy approach to the design and optimization of aerospace propulsion systems: the exergy flow diagram can provide aircraft engineers and system designers with additional insight on the avoidable and unavoidable systemic losses, thus allowing for effective design improvements. Second, to explore limits and merits of two different fuelling solutions for a scramjet-powered aircraft. Two configurations are critically compared: one with a direct H₂ injection and one with an on-board kerosene reformer. The present study treats the scramjet-propelled plane as a Large Complex Energy System (“LCES”), and applies system balances (mass, energy, exergy) to calculate the relevant losses. The exergy analysis confirms that the introduction of an on-board reformer is advantageous from the point of view of the thrust efficiency (with a gain of 3 percentage points with respect to the H₂-fuelled engine) and, more importantly, from the point of view of a more correct use of the available resources (the fuel in the tanks). Another advantage of the on-board reforming is that the higher value of the volumetric-specific impulse allows for reducing the fuel tank size. All calculations have been performed with CAMEL^®, a modular simulator for energy conversion processes conceived and developed in the last decade by the Authors’ group at the Mechanical and Aeronautical Engineering Department of the University of Roma 1 “La Sapienza”. Some additional component models have been studied and implemented, and a specific tool dedicated to the analysis of propulsion systems has been created and integrated in the simulation package.     

Total and local heat transfer from a smooth circular cylinder in cross-flow at high reynolds number

The total and local heat transfer from a smooth circular cylinder to the cross flow of air has been measured over the Reynolds number range 3 × 10⁴ < Re < 4 × 10⁶. The interaction between flow and heat transfer is discussed. In particular, the boundary-layer effects on the heat transfer, such as transition from laminar to turbulent flow or boundary-layer separation, are considered in conjunction with the distributions of local static pressure and skin friction.     

Boundary conditions at a planar fluid–porous interface for a Poiseuille flow

The velocity boundary condition that must be imposed at an interface between a porous medium and a free fluid is investigated. A heterogeneous transition zone characterized by rapidly varying properties is introduced between the two homogeneous porous and free fluid regions. The problem is solved using the method of matched asymptotic expansions and boundary conditions between the two homogeneous regions are obtained. The continuity of the velocity is recovered and a jump in the stress built using the viscosity (and not the effective viscosity) appears. This result also provides an explicit dependence of the stress jump coefficient to the internal structure of the transition zone and its sensitivity to this microstructure is recovered.     

Exergy analysis of a PEM fuel cell at variable operating conditions

This paper presents a comprehensive exergy analysis of a 10 kW PEM fuel cell at variable operating temperatures, pressures, cell voltages and air stoichiometrics. The calculations of the physical and chemical exergies, mass flow rates and exergetic efficiency are performed at temperature ratios (T/T₀) and pressure ratios (P/P₀) ranging from 1 to 1.25 and 1 to 3, respectively. In addition, the analysis is conducted on fuel cell operating voltages of 0.5 and 0.6 V and at air stoichiometrics of 2, 3 and 4 in order to determine their effects on the efficiency of the fuel cell. The calculated results illustrate the significance of the operating temperature, pressure, cell voltage and air stoichiometry on the exergetic efficiency of the fuel cell. However, it is recommended that the fuel cell should operate at stoichiometric ratios less than 4 in order to maintain the relative humidity level in the product air and to avoid the membrane drying out at high operating temperatures.     

Exergoeconomic analysis of a PEM fuel cell at various operating conditions

In this paper, a comprehensive exergoeconomic analysis of a 10 kW PEM fuel cell at various operating temperatures, pressures, cell voltages and air stoichiometries is performed. The analysis is performed at fuel cell operating temperatures (T/T_o) and pressures (P/P_o) ranging from 1 to 1.25 and 1 to 3, respectively. In addition, the calculations are performed on typical fuel cell operating voltages of 0.5 V and 0.6 V and at air stoichiometries of 2, 3 and 4 in order to determine their effects on the exergy cost of the fuel cell. The calculated results demonstrated the significance of the operating pressure, cell voltage and air stoichiometry on the exergy cost of the fuel cell. Furthermore, lower capital cost of the fuel cell, annual O & M cost and hydrogen cost could contribute to a drastic reduction in the exergy cost. Thus, a substantial improvement in the overall results could be achieved.     

Discrepancies in brine density databases at geothermal conditions

The study is aimed at evaluating the influence of brine density data on the assessment of pumping requirements for geothermal installations. Five databases were compiled and compared, based on an evaluation of buoyancy effect in a producing well at the site of the Soultz-sous-Forêts EGS/HDR project. The results show large discrepancies between the different references, which lead to severe inaccuracies in the evaluation of the buoyancy effect, and thus of the pumping requirements. It is therefore recommended that care be taken when using such databases, in particular when determining the size of the pumping systems. It is also recommended that new measurements of brine densities be carried out, focussing specifically on geothermal conditions.     

Laser diagnostics of soot precursors in a heavy-duty diesel engine at low-temperature combustion conditions

To better understand in-cylinder soot formation processes for modern, low-emissions, low-temperature combustion (LTC) operating conditions in diesel engines, soot and its precursors are imaged by laser diagnostics in a heavy duty optical engine. Virtually simultaneous images of planar laser-induced incandescence of soot (soot-PLII) using 1064 nm excitation and combined soot-PLII and planar laser-induced fluorescence of poly-cyclic aromatic hydrocarbons (PAH-PLIF) using 532 nm excitation reveal the temporal and spatial evolution of soot and its precursors during combustion. With increasing dilution of the intake air stream by various levels of nitrogen to simulate the use of exhaust-gas recirculation (EGR) to achieve LTC, the residence time of PAH increases as soot formation is delayed. At zero dilution (21% intake oxygen), soot appears nearly simultaneously with PAH, while at higher dilution (12.7% or 9.5% intake oxygen), soot formation is delayed by hundreds of microseconds to a millisecond or more. At all dilution levels, at its onset, PAH rapidly fills most of the downstream cross-section of the jet, with a relatively uniform fluorescence intensity distribution. The spatial evolution of soot, however, shifts from a relatively rapid filling of the downstream head of the jet at low dilution, to inception at the midstream periphery of the jet followed by a gradual progression downstream at high dilution. Effects of fuel-bound aromatics are also explored by comparing soot and PAH formation for a representative diesel fuel containing 27% aromatics by weight to that for neat n-heptane. The residence time of PAH before the onset of soot is slightly longer with the non-aromatic fuel, but otherwise the spatial and temporal evolution of PAH-PLIF and soot-PLII are indistinguishable from that of the diesel fuel at LTC conditions. Spectral analysis shows a characteristic soot emission spectra from 1064 nm excitation, while 532 nm excitation yields significant short-wavelength emission that cannot be attributed solely to soot incandescence, and is likely due to fluorescence of large PAH soot precursors. No narrow spectral features, such as from C2 or C3 emission, are apparent in the spectra, even at high laser fluence.     

Stability of ceramic matrix materials in molten hydroxide under oxidizing and reducing conditions

The degradation mechanism of ceramic matrix materials used as molten hydroxide electrolyte support for steam electrolysis cells, is not well documented. In this study, we have investigated the structural and chemical stability of non-oxide and oxide ceramics (SiC, YSZ, Li₂ZrO_3, LiAlO₂, and SrZrO₃) in the presence of molten hydroxide under oxidizing and reducing conditions with humidity levels of 3 and 85% at 550 °C. SiC, YSZ, Li₂ZrO₃ and LiAlO₂ showed structural and chemical changes due to dissolution of the ceramic particles in the hydroxide melt followed by precipitation. The SrZrO₃ powder, on the other hand, remained both chemically and structurally stable during 500 h test under both oxidizing and reducing atmospheres. The findings of a stable ceramic matrix in molten hydroxide contribute to the enhancement of the durability and performance of intermediate temperature water electrolysis system.     

A dynamic switching strategy for air-conditioning systems operated in light-thermal-load conditions

Recently, modern air-conditioners have begun to incorporate variable-speed compressors and variable-opening expansion valves, together with feedback control to improve the performance and energy efficiency. However, for the compressor there usually exists a low-speed limit below which its speed can not be continuously modulated unless it is completely turned off. When the air-conditioning system is operated in light-thermal-load conditions, the low-speed limit causes the compressor to run in an on–off manner which can significantly degrade the performance and efficiency. In this paper, a dynamic switching strategy is proposed for such scenarios. The strategy is basically an integration of a cascading control structure, an intuitive switching strategy, and a dynamic compensator. While the control structure provides the nominal performance, the intuitive switching strategy and the dynamic compensator together can account for the compressor’s low-speed limitation. Theoretical analysis reveals that when the output matrix of the dynamic compensator is chosen properly, the proposed strategy can effectively reduce the output error caused by the on–off operation of the compressor. Experiments also demonstrate that the proposed strategy can simultaneously provide better regulation for the indoor temperature and improve the energy efficiency at steady state.     

Effects of non-linear protective devices in cable systems under energisation conditions

This paper investigates the transient performance of cable systems under energisation conditions, including the non-linear effects of surge divertors which are used as sheath voltage limiters. The method of solution developed and used is based on the modified Fourier transform technique in conjunction with model analysis. The non-linear effects of surge divertors are included by the use of Duhamel's integral. Mathematical formulations of the transient responses are also given. Both the Cross-bonded and Single-point bonded electrical power cable systems are considered in the study. In the study, the frequency-dependent characteristics of system parameters, in particular skin-effect and effect of the earth-return path, are taken into account in obtaining the transient solutions. Particular attention is given to systems which include cross-bonded cables. The effects of various system parameters, such as source impedance, system length and sheath earthing-resistance, on the transient performance of the systems, as well as on the surge divertor responses, are investigated.     

Measured and modelled improvement in solar energy yield from flat plate photovoltaic systems utilizing different tracking systems and under a range of environmental conditions

This work is performed to investigate the effect of using different sun tracking mechanisms on the flat plate photovoltaic system performances and the main parameters affecting the amount of their electrical energy output as well as those affecting their gains compared to the traditional fixed photovoltaic systems. To this end, five configurations of sun tracking systems and two traditional fixed panels have been considered. The sun tracking systems effect on the PV system performances is improved by using the hourly data collected over 18 days for different seasonal sky conditions. The daily cumulative electrical energy produced by the different systems have been quantified separately for each sky state and the corresponding electrical gains have then been compared to those experienced with two traditional fixed photovoltaic systems. It is found that for a completely clear day, the highest obtained gains are those related to the two-axis sun tracker systems, which decrease gradually from the inclined to the vertical rotating axis when the same optimum slope is applied and from the seasonal to the yearly optimum slope if the same rotating axis is considered. On the other hand, for the partially clear days, the gain amounts are mainly dependant on the clearness index and on the seasonal variation of day length values. For a completely cloudy day, the results show that all considered systems produced closely the same electrical energy and the horizontal position of the photovoltaic panel presented the best performance.     

Performance of a scroll compressor with R134a at medium temperature heat pump conditions

The isentropic and volumetric efficiency of a scroll hermetic compressor is measured using R134a under medium temperature heat pump conditions. The evaporating temperature ranges from 3 to 36°C and the condensing temperature from 34 to 78°C. The efficiency parameters are fitted to functions of the suction and discharge pressures. At the same port pressures, there are only small differences between the isentropic and volumetric efficiency parameters for R134a and those for R22, the latter determined from the manufacturer's data. The efficiency parameters for R134a are used to compare the performance of the compressor with R12, R134a and R152a in a medium temperature heat pump cycle. The COP and heating capacity exhibit trends similar to those in previous experimental data for a reciprocating compressor.     

Stability of liquid CO2 in seawater at high pressures

The stability of liquid CO₂, in seawater at 3°C under high pressures were investigated by observing the pH and temperature changes around liquid CO₂ as well as its state. There was no distinct change of the state of liquid CO₂ at 37 MPa, while the liquid CO₂ at 32 MPa was unstable. Our experimental investigations suggest that a huge amount of liquid CO₂ can be sequestered for a very long time with reducing CO₂ dissolution and its impacts on ecosystems, if we dispose liquid CO₂ at the depth greater than 3700 m in an ocean.