Developing a methodology for the assessment of wave energy along the South African Coast

Gweba, Bafana (2018-03-19)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT : Ocean wave energy can become one of the alternative energy resources for fossil-fuelled power generation in South Africa. Due to global warming, several studies about the generation of wave energy have been done to find cleaner and sustainable renewable energy resources. An array of Wave Energy Converters (WECs) in a form of a wave farm may be used to harness the energy resource to generate electricity. Nearshore wave field effects due to the presence of a wave farm must be investigated particularly at the coastline as it will be affected. The principal objective of this thesis is to investigate the impacts induced by a wave farm on the nearshore wave field region through numerical modelling. Another objective is to give guidance about some of the parameters and input conditions for numerical modelling of wave transformations. In the present study, wave conditions have been assessed at selected locations of the South African coast. The JONSWAP model, which is the most frequently used spectral model to describe wind-generated waves, was used to represent wave energy spectrum along chosen locations. The JONSWAP model was fitted into the measured data along the coast to obtain the peak enhancement factor (gamma) values for chosen locations. The measured data was found to consist of bimodal spectra, local winds and distant storms and also multiple peaks in the spectra were observed. The spectral decomposition method was then applied to split the data into wind sea and swell to assess a more realistic description of the wave system. It was found that the method is effective in splitting bimodal spectra but is not successful in multi-peaked spectra. Saldanha Bay was chosen as the case study for installation of a wave farm due to its abundance of wave energy. A nested numerical wave model, referred to as SWAN (SimulatingWAves Nearshore), was used to simulate the nearshore wave field conditions in Saldanha Bay. The obtained gamma value for Saldanha Bay was used to set the wave model. Two model simulations in the study were considered, model simulations in the presence of a wave farm and model simulations in the absence of a wave farm. The difference in significant wave height and wave energy spectrum with and without the wave farm was assessed. The results show a reduction in significant wave height and a change in wave energy spectrum at the selected output locations. A gradual redirection of waves induced by the presence of wave farm has been observed for all selected boundary wave direction conditions. The overall results of the study indicate the change in the nearshore wave field during the presence of wave farm. A sensitivity assessment was conducted to investigate the change in wave energy due to the orientation of the original wave farm layout and the addition of two devices in the original wave farm layout. A proposed methodology for the assessment of wave energy was presented to evaluate the wave energy resource along the South African coast. The proposed methodology is based on analysis that was conducted in the study.

AFRIKAANSE OPSOMMING : Oseaangolfenergie kan een van die alternatiewe energiebronne op fossielbrandstowwe word vir kragopwekking in Suid-Afrika. Aardverwarming het meegebring dat verskeie studies oor die benutting van golfenergie gedoen is ten einde skoner en meer volhoubare hernubare energiebronne te vind. Daar is bevind dat ’n versameling golfenergie-omsetters (GEOs) in ’n golfplaas aangewend kan word as energiebron vir die opwekking van elektrisiteit. Die effek van sulke golfplase op die golfveld van die aangrensende kuslyn word in hierdie proefskrif ondersoek ten einde te bepaal hoe die kus geraak sal word. Die hoofdoel van hierdie proefskrif is om die impak van so ’n golfplaas op die omliggende golfveld van die kuslyn te ondersoek. In die huidige proefskrif is ’n beraming van golftoestande op geselekteerde plekke aan die Suid-Afrikaanse kus gedoen. Die JONSWAP model, wat die mees gebruikte spektrale model vir die beskrywing van windgegenereerde golwe is, is gebruik om golfenergie spektra te verteenwoordig by die gekose liggings. ’n Afskatting vir die piekverbeteringsfaktor-waardes is verkry deur gemete golfdata met die JONSWAP model te pas. Die gamma waarde vir Saldanhabaai is gebruik om die golfmodel vir hierdie studie op te stel. As gevolg van verafgeleë storms en plaaslike winde, is die gemete data bimodaal of multimodaal van aard. ’n Spektraleontbindingsmetode is toegepas om die data in wind- en deiningskomponente te verdeel ten einde ’n meer realistiese beskrywing van die golfstelsel te bekom. Die ontbindingsmetode was egter slegs suksesvol vir die ontbinding van bimodale golfstelses aangesien multimodale data met geen dominante piek nie verdeel kon word nie. Gebasseer op hierdie studie asook vorige studies oor golfenergie aan die Suid-Afrikaanse kus, is Saldanhabaai gekies as ’n potensiële terrein vir die installering van ’n golfplaas. ’n Geneste numeriese golfmodel, SWAN (Simulating WAves Nearshore), is gebruik om die nabygeleë golfveldtoestand na te boots. Twee model- simulasies is in die studie oorweeg: Model simulasies in die teenwoordigheid van ’n golfplaas en model simulasies in die afwesigheid van ’n golfplaas. ’n Beraming van die verskil in beduidende golfhoogte en die golfenergiespektrum met en sonder die golfplaas is gedoen. Die resultate toon ’n vermindering in beduidende golfhoogte en ’n verandering in golfenergiespektra by die geselekteerde plekke. ’n Geleidelike verandering in golfrigting is waargeneem vir alle geselekteerde grensgolfrigting-voorwaardes. Die resultate van die studie dui op ’n verandering in die kusgolfveld weens die teenwoordigheid van ’n golfplaas. Die motivering vir die studie is om ’n metodologie vir die installering van GEOs langs die Suid-Afrikaanse kus met baie golfenergie daar te stel.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103308
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