This paper endeavors to provide the reader with an overview of the various tools needed to forecast photovoltaic (PV) power within a very short-term horizon. The study focuses on the specific application of a large scale grid-connected PV farm. As a matter of fact, the solar resource is largely underexploited worldwide whereas it exceeds by far humans' energy needs. In the current context of global warming, PV energy could potentially play a major role to substitute fossil fuels within the main grid in the future. Indeed, the number of utility-scale PV farms is currently fast increasing globally, with planned capacities in excess of several hundred megawatts. This makes the cost of PV-generated electricity quickly plummet and reach parity with non-renewable resources. However, like many other renewable energy sources, PV power depends highly on weather conditions. This particularity makes PV energy difficult to dispatch unless a properly sized and controlled energy storage system (ESU) is used. An accurate power forecasting method is then required to ensure power continuity but also to manage the ramp rates of the overall power system. In order to perform these actions, the forecasting timeframe also called horizon must be first defined according to the grid operation that is considered. This leads to define both spatial and temporal resolutions. As a second step, an adequate source of input data must be selected. As a third step, the input data must be processed with statistical methods. Finally, the processed data are fed to a precise PV model. It is found that forecasting the irradiance and the cell temperature are the best approaches to forecast precisely swift PV power fluctuations due to the cloud cover. A combination of several sources of input data like satellite and land-based sky imaging also lead to the best results for very-short term forecasting.
Abstract-All possible steady states of a Z-source inverter are identified and analyzed with the objective of deriving design guidelines for the symmetrical impedance network. This paper shows that, in addition to the desired three dynamic states, an operating cycle can contain another three static states that do not contribute to the power conversion process. These three static states can be avoided by selecting suitably large capacitors and inductors. By using the equations derived in the steady-state analysis, this paper presents guidelines to design the impedance network accurately for the case where the inverter is operated only in active and shoot-through states. The proposed design method can also be used to predict the critical values of capacitance and inductance below which static states appear during the operating cycle. Computer simulations and laboratory experiments are used to verify the design method and to demonstrate the appearance of static states when the capacitors and inductors are sized lower than their critical values.
The fastest and yet most prudent ways of changing the output power level of a solid oxide fuel cell power plant connected to the ac-grid are explored. The operating state of the fuel cell power plant is examined in term of the concept of feasible operating area of a cell. The utilization factor of the cell stack is maintained constant in steady-state by feeding natural gas to the fuel processor at a rate proportional to the current drawn from the stack. The fluctuations of the utilization factor in the transient state due to a change in operating power level can be constrained to the allowable range by strategically controlling the current drawn by the power conditioning unit. Based on measured variables and dynamic characteristics of the fuel processor, four strategies of controlling current are compared to arrive at the strategy that results in minimum transient time for a given power change. The proposed control schemes are verified through computer simulations.
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