Ocean Wave Energy Potential in Southern Waters of Malang

Ocean or marine energy has immense renewable energy capability, able to provide more than double the world’s current electricity consumption. Ocean energy is considered a non-polluting, renewable source of energy. The research was conducted in the waters of South Malang, East Java Province. This research utilizes wave modeling data that has been picture confirmed and processed within one year, in 2022. This procedure will yield the monthly height, period, and direction of wave arrival. The data used to create a map of the location of wave energy in South Malang comes from previously created wave energy maps. This location map is the result of an overlay of multiple prior maps in order to create a site that can be developed for the usage of wave energy. According to the results, the southern locations of Donomulyo, Bantur, and Sumbermanjing have the most potential for using wave energy for buoy planting. Point 29 is the closest point following point 30 to point 34. However, point 29 is the area covered by the turtle migratory route. Point 34 is the farthest away from the other two places if the distance from the coastline to the potential point is calculated perpendicularly. Despite its distance from the coast, this site experiences the least disturbance when compared to others.


Introduction
Conventional energy sources also known as fossil fuels have been exploited for power generation and giving negative impacts such as carbon emissions that causes global warming.Therefore, other energy sources are needed that can reduce the consumption of conventional energy sources so as to reduce the resulting impact [1].Renewable energy can produce very large amounts of energy and unlimited sources such as energy from wind, geothermal energy, energy from wind, and energy from the sea [2].The ocean is one of the renewable energy sources and the innovation in the utilization of current ocean [3,4] energy technology has the potential to help reduce carbon dioxide emissions due to the impact of electricity generation so that it can be one of the future options for long-term and sustainable energy sources [5].Therefore, ocean energy in Indonesia is considered a promising type of renewable energy [6][7][8][9] that is expected to contribute to the optimization of the energy mix global [10].It's critical to distinguish theoretical potential from technical potential, which is defined as the portion of the theoretical resource that can be exploited given the technologies at hand and while accounting for its current efficiency [11].The Wave Energy Center, in collaboration with the Implementing Agreement on Ocean Energy System (OES), defines ocean energy as energy generated by a variety of technologies that use energy sources such as power waves, ocean currents, tides, ocean thermal differences (Ocean Thermal Energy Conversion), and salinity differences (salt content) to produce electricity.Each maritime energy source has a significant potential for human applications; nevertheless, as indicated in Table 1, sea waves and marine currents have the highest energy potentials.1,000-9,000 8,000-80,000 Source: [12] Research on wave energy conversion technology in Indonesia is starting to be widely carried out.Indonesia offers potential for non-hydro new renewable energy is spreading in diverse regions around Indonesia, including ocean wave energy sources [13].Research on wave energy conversion technology in Indonesia is starting to be widely carried out [14][15][16].The southern part of Java one of the places that have large enough wave potential to be utilized for their energy because their areas directly face the Indian Ocean.The latest new energy potential on the south coast can be used as an energy source for electricity generation [17], one of which is in the waters of South Malang.The south coast of East Java has a continuous and above-average wave height of 2.00 -3.00 m every year [18].South Malang waters are one of the locations in southern Java that border the Indian Ocean, and some of its coastal sections are not electrified.So, this research proposes to be carried out as initial research to quantify and map the potential of wave energy in the waters of South Malang, which may then be exploited as a source of renewable energy for the coastal towns of South Malang.

Materials and Methods
The research was conducted in the waters of South Malang, East Java Province which has extreme wave heights.The figure shown in Figure 1 with station descriptions marked with yellow dots.

Preparation of Data
Preliminary research, problem identification, site surveys, and choosing the appropriate data are all included in this step.The literature that served as the basis for this study's findings focused on wave modeling and the formulas utilized in AquaBuOY, a wave energy producing system.The study's data came from modeling data that was confirmed by satellite imagery, which was made available for one year in 2022 through the website http://marine.copernicus.eu.Next, a monthly period's worth of modeling data were analyzed, yielding results in terms of height, period, and wave coming direction data.The wave energy formula is used to calculate wave energy based on this data and establish the theoretical wave potential.The wave potential will then be practically calculated using further data on the depth of the southern Malang waters gathered from BIG data (Geospatial Information Agency) and a calculation of the efficiency of wave energy conversion technology.The results will be interpreted in the form of a potential map in a monthly period.For more details, the research flow is shown in Figure 2.

Data Analysis
The theoretical potential calculations, technical potential calculations, and practical potential calculations are the three operations that make up the data analysis step.Significant wave height, period, and direction statistics are among the data needed in the theoretical potential calculation process.After processing the data in Ms. Excel, the wave energy formula is used to calculate the results, which yield the following equation.: IOP Publishing doi:10.1088/1755-1315/1328/1/012009 After obtaining the wave energy value, an interpolation process in ArcGIS will be carried out based on the theoretical wave energy value which will used as a theoretical wave energy map and calculate its power.Wave energy and wave converter technology efficiency statistics are used to calculate technical potential which in this study used AquaBuOY with an efficiency level of up to 25%.After multiplying the technological efficiency by the theoretical wave energy value from the previous calculation, the results will be interpolated and utilized to create a map of technical wave energy, from which its power will be computed.
The data utilized for practical potential calculations includes energy, technical wave, and bathymetry data.Bathymetry data is utilized to determine water depth in South Malang as a parameter to establish the location of a practical wave energy map, from where the power will be determined.The data used to create a map of the location of wave energy in South Malang comes from previously created wave energy maps.This location map is the result of an overlay of multiple prior maps in order to create a site that can be developed for the usage of wave energy.

Validation Results
The data validation process is carried out by looking for the RMSE value between the main data and comparative data.Validation was carried out at the same four stations between the Global Ocean Wave Analysis and Forecast data and the Global Ocean L 4 Significant Wave Height from Nrt Satellite Measurements data.The time frame used in this process is 2 years from January 2021 to December 2022.The results of the RMSE values can be seen in Table 2.The significant wave height RMSE values were calculated at four separate sites in the South Malang area and ranged from 0.18 to 0.20 m.Station 3 has the lowest RMSE value of 0.18 m.According to prior study [19], the RMSE value between significant wave height data output by the SWAN model and data from altimetry satellites ranges between 0.10 and 0.45 meters.The resultant RMSE values are not significantly different from those found in this investigation.

Significant Wave Height
Significant wave height () refers to the average height of the top one-third of a wave group used for a variety of applications, including ocean energy utilization [20].Accurate significant wave height (Hs) prediction is crucial for marine renewable energy development [19].The research observation point map is shown in Figure 3 as follows.The highest average significant wave height in 2022 is 2.42 m in July, highlighted in brick red (2 m -2.5 m), while the lowest average significant wave height is 1 m in April, marked in blue tosca (1m -1.25m).This is in accordance with research [21] which states that The West Season (December, 6 January, February) and the East Season (June, July, August) have higher average wind speeds than Transitional Seasons I (March, April, May) and II (September, October, November).Wind speed influences the height and direction of the waves.The southern seas of Java are directly opposite the open sea (Indian Ocean), which allows for regular wave heights.The lowest wave height that can be used for alternative energy is 1.6 meters.However, if the wave height in the seas is quite great and consistent, such as in the waters of Southern Java, the power generated tends to be stable [22].

Wave Energy Potential
The following is the result of theoretical wave energy calculations in 2021 and 2022 shown in Figure 5.The maximum theoretical energy potential mapping in 2021 is in August, with an average wave energy of 40 kW/m -50 kW/m marked in orange, while the lowest is in November, with an average wave energy of 4.7 kW/m -10 kW/m shown in dark blue.The maximum theoretical energy potential mapping in 2022 is in July, with an average wave energy of 40 kW/m -50 kW/m shown in orange, while the lowest is in January, with an average wave energy of 5.86 kW/m -10 kW/m marked in dark blue.The wave energy potential is then calculated using the calculation method for the efficiency value of AquaBuOY, and the results are shown in Figure 6 as a technical wave energy potential map.The theoretical and technical mapping of energy potential diverge in terms of the average energy yield that dominates in water.This is evident from the theoretical potential for August 2021, which is dominated by orange with an energy potential of 40 kW/m -50 kW/m, whereas the technical potential is dominated by yellow with an energy potential of 30 kW/m -40 kW/m.The discrepancy between theoretical and technical potential values stems from the efficiency of the method used to determine technical potential.In this study, the tool's effectiveness is assumed to be 90%, meeting the criteria for ideal tool installation features.

Amount of Energy
The results of energy processing are in the form of theoretical and technical energy values that can be utilized in the waters of South Malang which are shown in Table 3.The 36 sample sites that are used to calculate the total energy that can be produced in South Malang Waters are thought to accurately reflect the state of the local waters.The technical potential for 2021 and 2022 is 1352459.992MW and 1195951.119MW, while the theoretical potential is 1502733.324MW and 1319172.735MW.The 149 km of South Malang's coastline is multiplied by the potential value to determine how much energy is produced.Based on the characteristics of the buoy installation, it is effective to plant it at a depth of approximately 50 -70 m with a distance of 2 -8 km from the coastline.Potential points that approach the criteria for installing Buoys are at Station 29, Station 30, and Station 34 as shown in Figure 7 and detail of the values in Table 4.  Based on Table 4, station 30 has the closest criterion for deploying buoys in an efficient manner, based on the information in Table 4 of the three stations.Technically speaking, the total energy in 2021 is 4354.61MW, but the theoretical total is 4838.46MW, with a monthly significant wave height value of 1.69 m.However, with a monthly significant wave height value of 1.65 meters, the total theoretical energy in 2022 is estimated to be 4387.95MW, but technically it is 3977.38MW.Uncovered areas on the map may become possible points if the installation criteria for buoys are taken into account.

BUOY Installation Criteria
Stations 29, 30, and 34 meet the criteria for deploying effective buoys at depths of 50 -70 m and distances of 2 -8 km from the coastline.Stations that meet the requirements Station 30 is the closest to the buoy installation, with a depth of around 93.21 m and a 1.50 km distance from the beach.
In 2021, the total theoretical energy will be 4838.46MW and technically 4354.61MW, with a monthly significant wave height of 1.69 m.Meanwhile, in 2022, the total theoretical energy will be 4387.95MW, while the technical energy will be 3977.38MW, with a monthly significant wave height of 1.65 meters.The map results are based on using National Defense data to create a bathymetric map.Further validation was performed using MAPE calculations.The results of this calculation will demonstrate how accurate the Batnas data is in comparison to the field data from the Pushidrosal Bathymetric Map.Validation was performed at the same four places in the prospective area, specifically Stations 29, 30, and 34.The validation of the bathymetric map yielded a MAPE value of 118%.This indicates that the value has exceeded the MAPE classification's maximum limit.Based on research by [23], a model can be considered accurate/valid if the MAPE value is less than 20%.The significant differences between data are possibly caused by differences in the year of collection where BATNAS data is satellite data collected in 2023, but the Pushidrosal data is from 1950 soundings.Limited sounding sites are also an obstacle to further investigation in the South Malang area.This may occur caused by extreme wave conditions, leading to an important safety concern.

Figure 8. RZWP3K Map of Malang Waters
After validating the bathymetry data, an overlay map was constructed by applying RZWP3K (Figure 8), and the outcome displayed that the places with the best potential for using wave energy for buoy planting were in the southern parts of Donomulyo, Bantur, and Sumbermanjing.This corresponds to previous research [24] in which two of the three proposed possible spots were included as safe areas for placing buoys.Point 29 is the closest point following point 30 to point 34.However, point 29 is the area covered by the turtle migratory route.Visually, there is no substantial influence, but there is still potential for an impact on the buoy installation.Point 34 is the farthest away from the other two places if the distance from the coastline to the potential point is calculated perpendicularly.Despite its distance from the coast, this site experiences the least disturbance when compared to others.If in the future further investigation is carried out regarding accessible resources, it is vital to study further whether the area is a populous area that requires power.

Conclusion
Conclusions that can be drawn from the research about ocean wave energy potential in the Southern Waters of Malang: 1. Validation was performed at the same four places in the prospective area, specifically Stations 29, 30, and 34.The validation of the bathymetric map yielded a MAPE value of 118%.Factors such as the year of collection might cause notable discrepancies between BATNAS and Pushidrosal data.2. Stations 29, 30, and 34 meet the criteria for putting effective buoys at a depth of 50 -70 m and a distance of 2 -8 km from the coast.Point 30 has the highest potential when considering the tool's qualities and practical value.Based on the RZWP3K map and the lack of interference, position 34 has the most potential.

Figure 3 .Figure 4 .
Figure 3. Map of Research PointMapping is used to determine the point of wave energy potential based on the energy generated by waves in numerous sub-districts, including Donomulyo, Bantur, Gedangan, Sumbermanjing, Tirtoyudo, and Ampel Gading.The sampling point region is measured up to 50 kilometers from the coast taking into account aspects such as the expense and efficiency of energy distribution.The following is the result of wave data processing in the form of a significant wave map and wave direction in 2021 and 2022 in Figure4.

Figure 6 .
Theoretical Wave Energy Potential Map in Southern Malang Waters; (a) 2021; (b) 2022 The largest technical energy potential in 2022 is in August, with an average wave energy of 40 kW/m -50 kW/m shown in orange, while the lowest is in November, with an average wave energy of IOP Publishing doi:10.1088/1755-1315/1328/1/0120097 4.22 kW/m -10 kW/m marked in dark blue.The maximum theoretical energy potential mapping in 2022 is in July, with an average wave energy of 40 kW/m -50 kW/m shown in orange, while the lowest is in January, with an average wave energy of 5.27 kW/m -10 kW/m marked in dark blue.

Table 1 .
Potential Energy Production of Marine Energy Resource

Table 2 .
RMSE Calculations of Wave Height

Table 3 .
Energy Value

Table 4 .
Potential Point Value

Table 5 .
Validation Results of Pushidrosal Bathymetry Map with Batnas Data