Impact of Tidal Level Variations on Wave Energy Absorption at Wave Hub

Abstract

Abstract:The energy absorption of the wave energy converters (WEC) characterized by a limited stroke length -like the point absorbers developed at Uppsala University-depends on the sea level variation at the deployment site. In coastal areas characterized by high tidal ranges, the daily energy production of the generators is not optimal. The study presented in this paper quantifies the effects of the changing sea level at the Wave Hub test site, located at the south-west coast of England. This area is strongly affected by tides: the tidal height calculated as the difference between the Mean High Water Spring and the Mean Low Water Spring in 2014 was about 6.6 m. The results are obtained from a hydro-mechanic model that analyzes the behaviour of the point absorber at the Wave Hub, taking into account the sea state occurrence scatter diagram and the tidal time series at the site. It turns out that the impact of the tide decreases the energy absorption by 53%. For this reason, the need for a tidal compensation system to be included in the design of the WEC becomes compelling. The economic advantages are evaluated for different scenarios: the economic analysis proposed within the paper allows an educated guess to be made on the profits. The alternative of extending the stroke length of the WEC is investigated, and the gain in energy absorption is estimated.

Sea level variability in the Swedish Exclusive Economic Zone and adjacent seawaters: influence on a point absorbing wave energy converter

“…The tension in the connection line and the distance between the buoy and the sea bed is influenced by low-frequency SL variations: for a significantly low SL, the connection line is slack and the translator rests on the bottom of the generator, while for a significantly high SL, the translator continuously hits the upper end stop, which results in additional stresses on the hull of the generator and in a reduced stroke of the translator itself. In both cases, the energy absorption decreases drastically, together with the lifetime and survivability of the WEC (Castellucci et al, 2016). The same problem is experienced by other technologies, such as oscillating water columns, as suggested by Muetze and Vining (2006) and by López et al (2015), and in more general terms by WECs which have a part that is fixed in position relative to the sea bed and a part that moves with the waves.…”

“…The following features are assumed: a cylindrical buoy of radius 3 m and draft 0.6 m; a translator stroke length of about 2.5 m; a total weight of the moving parts except the buoy of 10 t; a damping factor of about 135 kNs m −1 . For more details regarding the model and its limitations, see Castellucci et al (2016). For the mere purpose of providing an example of WEC energy absorption at different SLs, a sea state characterized by a significant wave height (H s = 1 m) and energy period (T e = 5 s) is used as input to the model.…”

“…In Castellucci et al (2016), the hydro-mechanic model that analyses the behaviour of a point absorber is described. In particular, the model evaluates how SL variations influence the power absorption, and hence the energy production, of the Uppsala WEC across a representative scatter of wave climates.…”

“…An example is presented in Fig. 8 with the purpose of pointing out the effect of SL changes on the performance of the Uppsala WEC denoted L12 (Castellucci et al, 2016). Let us assume that the hypothetical wave energy developer is interested in deploying a wave energy park where the significant wave height is not greater than 1 m. The normalized annual energy absorption for different SLs in the range of ±0.8 m is close to 100 % and it drops drastically for |SL| > 0.8 m, as illustrated in Fig.…”

Study of an Altered Magnetic Circuit of a Permanent Magnet Linear Generator for Wave Power

“…Figure 2 shows a photo of buoys and translators on the harbor before offshore deployment in Norway. Since 2006, the WEC system developed at Uppsala University has undergone several offshore experiments at the research site in Lysekil, Sweden [7][8][9][10][11], covering different aspects on wave energy conversion including the environmental aspects [12,13], wave power park layout analysis [14], manufacturing [15,16] and deployment [6] processes, WEC survivability in extreme seas [17], and efficiency of WECs subjected to tides [18]. An application of a direct drive PMLG for wave energy conversion (mainly used for the Archimedes Wave Swings) was suggested in [19].…”