Energies2015, 8(12), 13419-13441; doi:10.3390/en81212377 (registering DOI) - published 26 November 2015 Show/Hide Abstract
Abstract: Wind turbines with direct-driven permanent magnet synchronous generators (PMSGs) are widely used in wind power generation. According to the dynamic characteristics of PMSGs, an impact analysis of PMSG-based wind power penetration on the transient stability of multi-machine power systems is carried out in this paper based on the theory of extended equal area criterion (EEAC). Considering the most severe PMSG integration situation, the changes in the system’s equivalent power-angle relationships after integrating PMSGs are studied in detail. The system’s equivalent mechanical input power and the fault period electrical output power curves are found to be mainly affected. The analysis demonstrates that the integration of PMSGs can cause either detrimental or beneficial effects on the system transient stability. It is determined by several factors, including the selection of the synchronous generators used to balance wind power, the reactive power control mode of PMSGs and the wind power penetration level. Two different simulation systems are also adopted to verify the analysis results.
Energies2015, 8(12), 13395-13418; doi:10.3390/en81212374 (registering DOI) - published 25 November 2015 Show/Hide Abstract
Abstract: The motivation for this paper comes from a real need to have an overview of the challenges of modelling and control for very demanding systems, such as wind turbine systems, which require reliability, availability, maintainability, and safety over power conversion efficiency. These issues have begun to stimulate research and development in the wide control community particularly for these installations that need a high degree of “sustainability”. Note that this represents a key point for offshore wind turbines, since they are characterised by expensive and/or safety critical maintenance work. In this case, a clear conflict exists between ensuring a high degree of availability and reducing maintenance times, which affect the final energy cost. On the other hand, wind turbines have highly nonlinear dynamics, with a stochastic and uncontrollable driving force as input in the form of wind speed, thus representing an interesting challenge also from the modelling point of view. Suitable control methods can provide a sustainable optimisation of the energy conversion efficiency over wider than normally expected working conditions. Moreover, a proper mathematical description of the wind turbine system should be able to capture the complete behaviour of the process under monitoring, thus providing an important impact on the control design itself. In this way, the control scheme could guarantee prescribed performance, whilst also giving a degree of “tolerance” to possible deviation of characteristic properties or system parameters from standard conditions, if properly included in the wind turbine model itself. The most important developments in advanced controllers for wind turbines are also briefly referenced, and open problems in the areas of modelling of wind turbines are finally outlined.
Energies2015, 8(12), 13378-13394; doi:10.3390/en81212365 (registering DOI) - published 25 November 2015 Show/Hide Abstract
Abstract: A ground source heat pump system (GSHPS) utilizes a relatively stable underground temperature to achieve energy-saving for heating and cooling in buildings. However, continuous long-term operation will reduce the soil temperature in winter, resulting in a decline in system performance. In this research, in order to improve the system performance of a GSHPS, a ground heat pump system integrated with solar thermal storage was developed. This solar-assisted ground heat pump system (SAGHPS) can both maintain the balance of the soil temperature effectively and achieve higher system performance than the conventional system. In this paper, in order to examine the characteristics of the system, a dynamic simulation was conducted under various conditions. The results of our case study provide specific operation data such as heat exchange rate, heat source temperature, and heat pump COP. As a result, the heat pump COP of SAGHPS was 4.7%, 9.3% higher than that of the GSHPS.
Energies2015, 8(12), 13361-13377; doi:10.3390/en81212373 (registering DOI) - published 25 November 2015 Show/Hide Abstract
Abstract: Parabolic trough solar power plants use a thermal fluid to transfer thermal energy from solar radiation to a water-steam Rankine cycle in order to drive a turbine that, coupled to an electrical generator, produces electricity. These plants have a heat transfer fluid (HTF) system with the necessary elements to transform solar radiation into heat and to transfer that thermal energy to the water-steam exchangers. In order to get the best possible performance in the Rankine cycle and, hence, in the thermal plant, it is necessary that the thermal fluid reach its maximum temperature when leaving the solar field (SF). Also, it is mandatory that the thermal fluid does not exceed the maximum operating temperature of the HTF, above which it degrades. It must be noted that the optimal temperature of the thermal fluid is difficult to obtain, since solar radiation can change abruptly from one moment to another. The aim of this document is to provide a model of an HTF system that can be used to optimize the control of the temperature of the fluid without interfering with the normal operation of the plant. The results obtained with this model will be contrasted with those obtained in a real plant.
Energies2015, 8(12), 13344-13360; doi:10.3390/en81212370 (registering DOI) - published 25 November 2015 Show/Hide Abstract
Abstract: Available transfer capability (ATC) is the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses which needs to be posted in the electricity market to facilitate competition. ATC evaluation is a complicated task including the determination of total transfer capability (TTC) and existing transfer capability (ETC). In the deregulated electricity market, ETC is decided by the independent system operator’s (ISO’s) economic dispatch (ED). TTC can then be obtained by a continuation power flow (CPF) method or by an optimal power flow (OPF) method, based on the given ED solutions as well as the ETC. In this paper, a bi-level optimization framework for the ATC evaluation is proposed in which ATC results can be obtained simultaneously with the ED and ETC results in the deregulated electricity market. In this bi-level optimization model, ATC evaluation is formulated as the upper level problem and the ISO’s ED is the lower level problem. The bi-level model is first converted to a mathematic program with equilibrium constraints (MPEC) by recasting the lower level problem as its Karush-Kuhn-Tucher (KKT) optimality condition. Then, the MPEC is transformed into a mixed-integer linear programming (MILP) problem, which can be solved with the help of available optimization software. In addition, case studies on PJM 5-bus, IEEE 30-bus, and IEEE 118-bus systems are presented to demonstrate the proposed methodology.
Energies2015, 8(12), 13308-13343; doi:10.3390/en81212371 (registering DOI) - published 25 November 2015 Show/Hide Abstract
Abstract: A feasible way to avoid the risk of energy decline and combat climate change is to build a 100% renewable global energy mix. However, a globally electrified economy cannot grow much above 12 electric terawatts without putting pressure on the limits of finite mineral reserves. Here we analyze whether 12 TW of electricity and 1 TW of biomass (final) power will be able to fuel a future post-carbon economy that can provide similar services to those of a contemporary economy. Contrarily to some pessimistic expectations, this analysis shows that the principle economic processes can be replaced with sustainable alternatives based on electricity, charcoal, biogas and hydrogen. Furthermore, those services that cannot be replaced are not as crucial so as to cause a return to a pre-industrial society. Even so, land transport and aviation are at the limit of what is sustainable, outdoor work should be reorganized, metal primary production should be based on hydrogen reduction when possible, mineral production should be increasingly based on recycling, the petrochemical industry should shrink to a size of 40%–43% of the 2012 petrochemical sector, i.e., a size similar to that the sector had in 1985–1986, and agriculture may require organic farming methods to be sustainable.