Energies2015, 8(6), 5018-5039; doi:10.3390/en8065018 (registering DOI) - published 28 May 2015 Show/Hide Abstract
Abstract: This paper describes a method for the control of transient exhaust gas recirculation (EGR) systems. Firstly, a state space model of the air system is developed by simplifying a mean value model. The state space model is linearized by using linearization theory and validated by the GT-Power data with an operating point of the diesel engine. Secondly, a state feedback controller based on the intake oxygen mass fraction is designed for EGR control. Since direct measurement of the intake oxygen mass fraction is unavailable on the engine, the estimation method for intake oxygen mass fraction has been proposed in this paper. The control strategy is analyzed by using co-simulation with the Matlab/Simulink and GT-Powers software. Finally, the whole control system is experimentally validated against experimental data of a turbocharged diesel engine. The control effect of the state feedback controller compared with PID controller proved to be further verify the feasibility and advantages of the proposed state feedback controller.
Energies2015, 8(6), 5000-5017; doi:10.3390/en8065000 (registering DOI) - published 28 May 2015 Show/Hide Abstract
Abstract: Energy Storage Systems (ESSs) have recently been highlighted because of their many benefits such as load-shifting, frequency regulation, price arbitrage, renewables, and so on. Among those benefits, we aim at evaluating their economic value in frequency regulation application. However, unlike previous literature focusing on profits obtained from participating in the ancillary service market, our approach concentrates on the cost reduction from the perspective of a utility firm that has an obligation to pay energy fees to a power exchange. More specifically, we focus on the payments between the power exchange market and the utility firm as a major source of economic benefits. The evaluation is done by cost- benefit analysis (CBA) with a dataset of the Korean market while considering operational constraint costs as well as scheduled energy payments, and a simulation algorithm for the evaluation is provided. Our results show the potential for huge profits to be made by cost reduction. We believe that this research can provide a guideline for a utility firm considering investing in ESSs for frequency regulation application as a source of cost reduction.
Energies2015, 8(6), 4983-4999; doi:10.3390/en8064983 (registering DOI) - published 28 May 2015 Show/Hide Abstract
Abstract: DC flashover performance of ice-covered composite insulators with a parallel air gap (CI/PAG) is an important technical consideration when such insulators are used to isolate ground wires for the purpose of DC ice-melting. Tests on tension and suspension types of CI/PAG were thus carried out in the artificial climate chamber to investigate their DC icing flashover performance. The influences of parallel air gap, ice thickness, pollution severity and air pressure on DC negative 50% flashover voltage (U50%) of CI/PAG were investigated. Test results show that the parallel air gap affected both the discharge path and U50%. With increasing ice thickness, U50% declined by up to 52%; this effect was more evident when the breakdown occurred in the air gap. The pollution severity affected U50% only when the flashover happened along insulator surface. With a decrease of atmospheric pressure, U50% decreased. U50% and the ratio of air pressure were in a power function relationship with a positive characteristic exponent which was relevant to the discharge path.
Energies2015, 8(6), 4963-4982; doi:10.3390/en8064963 (registering DOI) - published 28 May 2015 Show/Hide Abstract
Abstract: A connected foundation is an effective foundation type that can improve the structural performance of electrical transmission towers in soft ground as a resilient energy supply system with improved stability. In the present study, the performance of a connected foundation for transmission towers was investigated, focusing on the effect of connection beam properties and soil conditions. For this purpose, a finite element analysis was performed for various foundation and soil conditions. In order to validate the finite element analysis, the calculated results were compared with measured results obtained from field load tests. The use of connection beams was more effective for uplift foundations that usually control the design of transmission tower foundations. For the effect of soil condition, the use of connected foundation is more effective in soft clays with lower undrained shear strength (su). Smaller amounts of differential settlement were observed in all soil conditions for both unconnected and connected foundations when a bearing rock layer was present. When the foundation was not reinforced by connection beams, the values of lateral load capacity of tower structure (Hu) were similar for both with- and without-rock layers. It was confirmed that introducing haunch-shaped connection beams is effective for increasing connection beam stability.
Energies2015, 8(6), 4943-4962; doi:10.3390/en8064943 (registering DOI) - published 28 May 2015 Show/Hide Abstract
Abstract: As renewable energy sectors evolve and grow within a country, the need for expertise to maintain its infrastructure grows. Such expertise is often provided by foreign industries. It is in the global interest to facilitate expertise to grow domestically, eventually leading to widespread clusters of industries around a renewable energy sector and a global growth of expertise. This ultimately fast tracks the development in the renewable energy sector since more players become active in developing solutions. In this article the factors influencing domestic development are identified from previous studies conducted within the Icelandic geothermal sector. The cause and effect relationships between the identified factors are then mapped. A system dynamics causal loop diagram based on Icelandic case studies is presented to visualise how the formation of industrial clusters in the renewable energy sector can be initiated. This visualisation, based on the Icelandic geothermal sector, can be of use for other industries in the renewable energy sector who are attempting to conduct their maintenance procedures domestically and increase the rate of innovation within a country.
Energies2015, 8(6), 4920-4942; doi:10.3390/en8064920 (registering DOI) - published 27 May 2015 Show/Hide Abstract
Abstract: Bearingless permanent magnet synchronous motors (BPMSMs), with all advantages of permanent magnet motors (PMSMs) and magnetic bearings, have become an important research direction in the bearingless motor field. To realize a stable suspension for the BPMSM, accurate decoupling control between the electromagnetic torque and radial suspension force is indispensable. In this paper, a concise and reliable analysis method based on a magnetic field equivalent current is presented. By this analysis method, the operation principle is analyzed theoretically, and the necessary conditions to produce a stable radial suspension force are confirmed. In addition, mathematical models of the torque and radial suspension force are established which is verified by the finite element analysis (FEA) software ANSYS. Finally, an experimental prototype of a 2-4 poles surface-mounted BPMSM is tested with the customized control strategy. The simulation and experimental results have shown that the motor has good rotation and suspension performance, and validated the accuracy of the proposed analysis method and the feasibility of the control strategy.