Abstract
Tidal currents and rivers are promising sources of renewable energy given that suitable turbines for kinetic energy conversion are developed. To be economically and technically feasible, a velocity distribution that can give a high degree of utilization (or capacity factor) while the ratio of maximum to rated velocity is low would be preferable. The rated velocity is defined as the velocity at which rated power is achieved. Despite many attempts to estimate the resource, however, reports on the possible degree of utilisation from tidal currents and rivers are scarce. In this paper, the velocity distribution from a number of regulated rivers,unregulated rivers, and tidal currents have been analysed regarding the degree of utilisation, the fraction of converted energy, and the ratio of maximum to rated velocity. Two methods have been used for choosing the rated velocity; one aiming at a high fraction of converted energy and one aiming at a high degree of utilisation. Using the first method, with a rated velocity close to the maximum velocity, it is unlikely that the turbine will reach the cut-out velocity. This results in, on average, a degree of utilisation of 23% for regulated rivers, 19% for unregulated rivers, and 17% for tidal currents while converting roughly 30%–40% of the kinetic energy. Choosing a rated velocity closer to the mean velocity resulted in, on average, a degree of utilisation of 57% for regulated rivers, 52% for unregulated rivers, and 45% for tidal currents. The ratio of maximum to rated velocity would still be no higher than 2.0 for regulated rivers, 1.2 for unregulated rivers, and 1.6 for tidal currents. This implies that the velocity distribution of both rivers and tidal currents is promising for kinetic energyconversion. These results, however, do not include weather related effects or extreme velocities such as the 50-year velocity. A velocity factor is introduced to describe what degree of utilisation can be expected at a site. The velocity factor is defined as the ratio Umax/Urate at the desired degree of utilisation, and serves as an early indicator of the suitability of a site.