Preliminary assessment of near-shorewave energy potential in the Mozambique Channel

In view of the foreseen increase in energy demand (Hafner et al. 2018; Warner & Jones 2018) and the challenges imposed by climate change, which require the reduction in the use of fossil fuels, the fostering of sources of renewable energy (Deichmann et al. 2011; Ouedraogo 2017; Warner & Jones 2018; United Nations Department of Economic and Social Affairs, Sustainable Development Goals 2021) is becoming a priority for the world. Ocean wave energy, which is being exploited on a trial basis in South Africa (ESI Africa South Africa: wave energy power plant development 2015), has the potential to be a viable alternative source of clean energy to remote coastal households, thus reducing the energy-deficit (Owusu & Asumadu-Sarkodie 2016; Hafner et al. 2018) in Sub-Saharan Africa. Regions of high wave energy in Sub-Saharan Africa have been identified on the coast of South Africa and South Madagascar, as they are near the Southern Ocean, where large swells are generated (Semedo et al. 2011). Apart from generating electricity, through turbines, wave energy can also be used to produce potable water via desalination, while seawater can be used as the feed water in the desalination plants (Abd-Elaty et al. 2022; Panagopoulos 2022a, 2022b), and, further, it can be used for water pumping for irrigation.

The present paper examines in detail the near-shore wave energy potential in the Mozambique Channel, where the authors expect the occurrence of regions of very high waves as a result of the complex interaction of swell entering from both sides of the entrances and locally generated waves (Sci. Behaviour of waves. Science Learning Hub 2011; Støle-Hentschel et al. 2020). Wave energy potential and wave variability have been assessed at ten selected sites along the coast of Mozambique, Western Madagascar, and near the Comoro and Mayotte islands. Potential applications of available wave energy were discussed based on the wave energy density and variability. The present study contributes to the literature on wave energy near the coast of the Mozambique Channel and it is expected that the information presented in this paper will be useful to Government and other relevant organizations in making an informed decision with regard to major investments for harnessing the wave energy resource of the Mozambique Channel.

The Mozambique Channel is an open channel located in the WIO sub-region (Figure 1). The geographic limits of the Mozambique Channel are described by the International Hydrographic Organization (IHO 1953). The channels extend from about 10°S to 26°S. At the northern boundary, the channel is limited by the line joining the Rovuma estuary and the northern border of Mozambique, passing through the RasHabu Ile Grande Comore, on the Comoro archipelago, to the northern tip of the Madagascar Island. The southern limit is a line joining Cap Sainte-Marie, the southern end of Madagascar Island to Ponto do Ouro, the southern border of Mozambique. The eastern limit is Madagascar's west coast and the western limit is the Mozambique coast. The topography is well described by Breitzke et al. (2017). Its width varies from 400 to 950 km and it has a maximum depth of 3,292 m. There are two distinct seasons, the dry and cool winter season between March and August, and the wet and warm season from September to February (Barimalala et al. 2020). The circulation is dominated by anti-cyclonic and cyclonic me so-scale eddies, which transport warm water from the South Equatorial Current to the Agulhas Current in the south (Lutjeharms et al. 2012). The region is influenced by the trade winds and the northern part is also influenced by the seasonal north-eastern monsoon winds (Semedo et al. 2011). During the southern hemisphere winter, when the north-eastern monsoon occurs, the winds enter the channel from both directions, north and south (Semedo et al. 2011). The wave climatology is dominated by swells generated in the Southern Ocean and propagating northwards through the channel (Semedo et al. 2011), and swell waves generated by the northern winds, penetrate the channel from the northern boundary and propagate southwards (Semedo et al. 2011).

In this study, weekly data of daily averaged significant wave height and wave period, measured from satellite altimeters, provided by a marine-analyst (http://www.marine-analyst.eu), were used. The data cover the period from 29 June 2019 to 31 December 2021, in the selected quadrats in the Mozambique Channels, whose coordinates are given in Table 1 (see Figure 1 for further geographical locations).

Modern methods of ocean wave assessment are based on altimetry, with the height and period of waves determined from remote-sensing observations and calibrations using buoy measurements (Hammar et al. 2012). Indeed, altimetry data with higher spatial and temporal resolution was thought to generate more conservative estimates for the WIO coast (Hammar et al. 2012). Despite these advantages, there are two major drawbacks of remote-sensing methods, namely inaccuracy in near-shore waters (Krogstad & Barstow 1999; Cornett 2008a) and difficulties in fully accounting for the energy in ocean swell (Krogstad & Barstow 1999).

Besides estimating the wave energy potential at the sites, the temporal variability of the energy was also calculated. Steady wave energy fluxes are preferred to unsteady wave regimes due to the fact that they are more reliable and display greater efficiency, according to Rusu & Soares (2012).

On the eastern side of the Mozambique Channel (Figure 3), the significant wave height varied from about 0.4 to 4.7 m, with the southernmost stations experiencing the largest waves. The average significant wave height at Toliara, the southernmost point, was 2.1 ± 0.8 m and at Morandavait was 1.7 ± 0.8 m, whereas at the northern stations the average significant wave height was 1.3 ± 0.5 m for Mayotte, 1.2 ± 0.4 m for Nosy Be, and 1.1 ± 0.4 m for Cape St. Andre. The wave period varied from about 3.7 ± 1.4 to 14.5 ± 1.7 s, with longer period waves observed in the southern part of the channel at Toliara, with an average wave period of about 10 ± 1.6 s and at Morondava with an average period of about 9 ± 1.7 s. At the northern stations, the average wave period was about 6–7 ± 1.4 s. The energy flux varied from 0.3 to 94 ± 17.2 kW m⁻¹, with the highest energy flux, observed at Toliara, the southernmost station, where the average was 23 ± 11.3 kW m⁻¹, followed by Morondava, where the average energy flux was 13 ± 3.5 kW m⁻¹. The average energy flux at the northern stations varied in the range 4.5–6 ± 2.9 kW m⁻¹, with the highest (6 ± 2.9 kW m⁻¹) observed at Mayotte. The significant wave heights were below the minimum threshold (1.5 m) in most cases, except at Toliara and Morondava, where there were long periods when the threshold was exceeded. There is also a seasonal signal here, with the highest waves in the southern hemisphere winter, which coincides with the Indian ocean South-West Monsoon. Again, the southern sites are the most suitable for the installation of wave energy extraction devices.

An understanding of the performance of wave energy converters in different wave states is essential for the constructional details, cost, and efficiency of the installed device. It would be preferable to install a wave energy converter in wave conditions having a maximum total energy flux rather than in one having maximum occurrence.

Tables 2,³–4 present the summary statistics of the wave characteristics and wave energy near the coast in the western and eastern Mozambique Channel. From Table 2, it can be seen clearly that the southern Mozambique Channel experiences the highest waves on both sides of the channel (1.7–2.1 m, on average). The northern part of the channel experiences lower waves (1.2–1.4 m). The probability of the significant wave height attaining and exceeding the minimum threshold values (1.6 m) averages 63% on the western side against 62% on the eastern side. This is considered a marginal difference, though. Both sides of the channel displayed high variability of 34–47% and seasonal variability (1.6–2.5). From Table 3, summary statistics for the wave period, it can be seen that large swell waves were recorded in the southern part of the channel, where the largest (10 s on average) were recorded on the western side, followed by the northern site with an average period of 8.5 s in the western side. The variability in wave period was low (15–24%) and the seasonal variability was also low (1.0–1.4) compared with the variability of the significant wave height. From Table 4, referring to energy flux, it can be seen that the most energetic waves (11–23 ± 4.5 kW m⁻¹) were encountered in the southern part of the channel, with the highest energy (13–23 ± 4.5 kW m⁻¹, on average) observed in the eastern part of the channel. In the northern part of the channel, the energy averaged to 4–7 ±2.5 kW m⁻¹. The variability of the wave energy was highest on the western side of the channel (80–108%) compared with the eastern side (71–99%). However, the probability of the energy flux attaining and exceeding the threshold value (10 ± 2.5 kW m⁻¹) was higher on the western side, averaging 63%, compared to the eastern side where it averaged 61%.

The results of the present study showed clearly that the highest significant wave heights, and hence, the most energetic waves, given the close correlation between the significant wave height and wave energy flux, were observed in the southern part of the channel. Here, significant wave heights reached 4.5 ± 0.7 m and averaged to about 2 ± 0.5 m. Wave energy flux averaged at 16–22 ± 5.6 kW m⁻¹ at Inhambane and Toliara, the southernmost sites studied. The northern opening of the channel was also characterized by relatively high energetic waves with an average significant wave height in the range 1.3–1.4 ± 0.5 m and attaining maxima of 3–4 ± 0.5 m. The energy flux is in the range of 7.0–10.5 ± 5.6 kW m⁻¹ on average peaking at 40–66 ± 9.3 kW m⁻¹.

These results are consistent with the findings of previous studies about wave climatology on a global scale (Semedo et al. 2011; Panagopoulos 2022a) and specifically for the WIO (Hammar et al. 2012). The high-energy waves in the southern part of the channel are attributed to swell from the Extratropical South Indian Ocean (Semedo et al. 2011; Lin & Oey 2020; Colosi et al. 2021; Rossi et al. 2021), whereas those observed in the northern part of the channel are attributed to swell generated in the Tropical South Indian Ocean region (Semedo et al. 2011; Hammar et al. 2012).

The central part of the channel was characterized by relatively low-energy waves, probably due to the fact that this region is less exposed to swell. However, in the western central part of the Mozambique Channel, on the Mozambique coast, large waves are observed. These have an average significant wave height in the range of 1.6–1.8 ± 0.5 m. The significant wave height here reaches a maximum of 3.8–4.3 ± 0.8 m and the average wave energy flux is 10.5–11.3 ± 5.6 kW m⁻¹ at Angoche and Beira, compared to values of 1 m and 4.5 ± 0.8 kW m⁻¹ for Cape St. Andre on the central-eastern side of the channel. There was no evidence of larger waves resulting from constructive interference of waves inside the channel. The increase in wave height and wave energy on the western side of the channel may be attributed to Kelvin waves (Griffiths 2013).

The wave patterns in the whole of the Mozambique Channel exhibited a seasonal cycle, with large waves occurring during the time of the southern monsoon, during the southern hemisphere winter, as can be seen in the time series plots (Figures 2 and 3). This result is consistent with previous studies by Colosi et al. (2021), who attributed the observed high waves in the winter to seasonal changes in high-latitude storm patterns that generate swell which propagates equatorward.

The waves in the Mozambique Channel are highly variable with a proportion of the variability of 34–47% for significant wave height and over 70% for wave energy. This high variability may be attributed to the effect of storms and cyclones which frequently affect the region (Matyas 2015; Muthige et al. 2018). High variability may affect the sustainability of any wave energy exploitation device (Rusu & Soares 2012). However, the thresholds for the viable exploitation of waves for energy production of 1.5 m for significant wave height and 10 kW m⁻¹ for energy flux, are exceeded most of the time in the southern Mozambique Channel. This means that, though there is high wave variability, there is usually sufficient power to generate electricity and the southern part of the channel is suitable for wave energy harvesting for electricity production. On the western central and northern parts of the Mozambique Channel, the thresholds are exceeded from July to November, making these sites suitable for seasonal electricity production.

There are many applications of wave energy, including electricity generation, water desalination, and pumping of water. Concerning electricity production, the threshold for technical and economic viability is set based on the converter devices. Wave exploitation for electricity production is recommended in areas with wave energy density ≥25 ± 5.6 kW m⁻¹ (Lavidas & Blok 2021). It has been argued that moderate wave resources (<25 ± 5.6 kW m⁻¹) may be economically viable if an optimal device is employed (Lavidas & Blok 2021). The threshold of 10 kW m⁻¹ chosen in this study, based on Hammar et al. (2012), would assure sustainable electricity production in the southern part of the channel. But, based on the argument by Lavidas & Blok (2021), we recommend the development of appropriate devices for harvesting wave energy for electricity production in the western central Mozambique Channel, where the energy density is about the average for the world ocean (Colosi et al. 2021). Furthermore, the whole coastline of the Mozambique Channel has wave energy potential for water desalination as stated by Abd-Elaty et al. (2022) and further argued by Panagopoulos (2022a, 2022b) and for water pumping for irrigation, as these applications are the least demanding in terms of energy power and variability. Both desalination and irrigation would have a significant positive impact on the livelihood of people living in coastal areas and on coastal area development in the region, considering the fact that more than half of the population of the region lives in coastal areas (Taylor et al. 2019).

This study was based on averaged data over space and time, and therefore, the results must be interpreted with caution. The wave and wave energy climate near the coast of the Mozambique Channel are highly variable, changing as storms and cyclones pass through the area. However, the minimum threshold of significant wave height (1.5 m) and wave energy flux (10 kW m⁻¹) for electricity production is exceeded most of the time in the southern part of the channel. The southern winter is the time for the highest waves and greatest energy, and is, therefore, the most promising season for the deployment of wave energy harvesting devices. Although one should interpret with caution, the finding in this study suggests that the whole coast of Mozambique Channel is suitable for applications of wave energy resources for desalination and water pumping, with the potential to enhance the livelihood of more than half of the population of the region. The present study recommends studies on the development of appropriate wave energy harvesting devices for electricity production to match the power available in the central western and northern Mozambique Channel, where the energy density is at the level of the world ocean average. Further studies should be directed towards the exploitation of applications of the available wave resources for coastal development.

The research was part of PhD studies on renewable energy, financially supported by the Eduardo Mondlane University post-graduate research grant, under the SIDA-SAREC Project No 2.1.6. Thanks, are also due to Mrs Faith Bowers and Prof. D.G. Bowers for the constructive remarks and suggestions, which significantly improved the quality of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.