A comparison between chronological and probabilistic methods to estimate wind power capacity credit

In this paper, the issue of the capacity credit provided by wind energy conversion systems (WECS) is addressed. A chronological method of post-evaluation of the capacity credit is presented and compared with a pre-evaluation probabilistic method. The proposed chronological approach is based on the computation of the WECS capacity factor (ratio between average and total output) over some relevant time period. An appropriate choice of the time interval, for instance the peak load hours, will lead to a closer approximation of the capacity credit. The comparison is illustrated with two case-studies, concerning the Portuguese electric system. The analysis of the theoretical background of both methods and the results obtained allow the conclusion that chronological methods are best designed to assist system operators, whereas probabilistic methods, which are developed within the logic of the public system avoided resources, are a helping tool for system planners.