Exploring Thermal Barrier and Sedimentation Simulation for Enhanced Performance in Grati Combined Cycle Power Plant

PLTGU is a power generation facility that concurrently utilizes both steam and gas power plant technologies. It necessitates a cooling system to operate efficiently throughout its entire lifespan. If PLTGU Grati plans to increase its power capacity, there is a concern that the water discharged from the Water Outlet channel might not cool down sufficiently before re-entering the Water Intake channel. Additionally, sedimentation in the sea water uptake is causing siltation. Hence, this study focuses on the jetty extension project to address sedimentation and hot water spreading issues in PLTGU Grati. The numerical modeling analysis, conducted using the Delft3D software, indicates that in the case of power addition, Alternative Model 1 can reduce the highest temperature compared to the existing condition by 0.655°C. Similarly, Alternative Model 2 reduces the highest temperature by 0.090°C. Moreover, with power addition, sedimentation rate in Area 1 increases by 261.43 m3/month in Alternative Model 1, while in Alternative Model 2, it decreases by 969.47 m3/month compared to the existing condition. Considering the ability of Alternative Model 2 to effectively reduce the temperature in the inlet canals by 0.090°C, it provides the best solution to contain the spread of hot water in the PLTGU Grati area. Currently, PLTGU Grati employs a Cutter Suction Dredger (CSD) to periodically dredge the water inlet channel. Therefore, Alternative Model 2 is recommended as the optimal choice among the alternatives. Henceforth, detailed studies related to current patterns and sedimentation rates are presented comprehensively in this paper.


Introduction
PLTGU Grati Power Plant, as part of PT.Indonesia Power's portfolio, holds a pivotal role in advancing Indonesia's power generation capacity.Operating as a Gas and Steam Power Plant, PLTGU Grati employs cutting-edge technology to generate electricity.This plant efficiently converts gas and steam into electrical energy, catering to the escalating demands of a burgeoning population and thriving industrial sectors.The reliability and capacity of PLTGU Grati are instrumental in stabilizing and enhancing the national power grid.In December 2014, the Indonesian Ministry of Energy and Mineral Resources (ESDM) authorized the issuance of the Electricity Power Supply Business Plan (RUPTL) 2015-2024 [1].In Java Island, among a total of 35 power plants with a cumulative capacity of 10.681 MW, 8 power plants, with a combined capacity of 4.905 MW, include both existing projects 1298 (2024) 012036 IOP Publishing doi:10.1088/1755-1315/1298/1/012036 2 with completed procurement and new projects.Notably, the Gas & Steam Power Plant (PLTGU) Grati Add On Block 2, located in East Java Province, is among the new projects with an open procurement plan.This plant is planned to have a capacity of 150 MW.
With the increase in power capacity at PLTGU Grati or Grati Combined Cycle Power Plant (CCPP), it can lead to an increased demand for water volume required in the cooling system.Similarly, the volume of hot water discharged from the water outlet channel will also increase and spread into the surrounding waters.One way to increase the water volume for the cooling system is by extending the jetty in the water inlet channel.However, it is essential to consider the impact of sedimentation it may cause.Studies related to sedimentation, including wave movements and currents in coastal waters, have been conducted by e.g.[2, 3, 4, and 5].Moreover, studies related to hydrodynamics, sedimentation, and morphological changes in power plant waters and coastal waters have been conducted by i.e., [6, 7, 8, 9, 10, 11 and 12] showing that coastal structures significantly affect morphological changes and sedimentation patterns.Experimental [13] and numerical simulations can both be employed to anticipate the pattern of fluid flow and the choice of the model of turbulence is cruicial for obtaining accurate predictions [14] including CFD applications [15].
The study of sedimentation rate and thermal circulation at the water inlet channel will reduce the efficiency of the power generation system at the Grati CCPP.Although fuel requirements are inversely proportional to efficiency, a decrease in inefficiency will result in a significant increase in fuel consumption and expenditure.Therefore, to prevent or reduce sedimentation and thermal circulation, a detailed and comprehensive study of the best jetty extension engineering to contain hot water discharge and prevent sediment entering into the inlet channel of water is crucial.In this study, numerical modeling is performed using Delft3D software with environmental data validation obtained through on-site surveys.It is expected that after the increase in power capacity at the PLTGU Grati, there will be no operational challenges that require a systematic and innovative approach to improve overall performance through the selection of the form and extension of the jetty structures in its water inlet channel.

Hydro-Oceanographic Data
The hydro-oceanographic data used in this study includes bathymetric data, tidal information, currents, temperature, wind, waves, river discharge, and sediment.Figure 2 presented the bathymetric data were obtained in November 2014, covering the approximately area 4397m × 2557m.Depths are referenced to the LWS (Lowest Water Spring).The mean depth within the inlet ranges from approximately -0.5 meters to -1.5 meters LWS.Tidal measurements were conducted over 15 days at one-hour intervals.The analysis of tidal, using the method of least squares, yielded a Formzhal value of 1.0725 (Figure 3), indicating that the tidal pattern in the PLTGU area is of the mixed diurnal double high and double low type (two high tides and two low tides occurring within a 24-hour period).Wind and wave data for a period of 10 years (2004-2014) are also provided (figure 4), showing that the dominant wind directions are from the northwest, north, northeast, and east.The prevailing wave orientation originates from the east.The sea in the PLTGU Grati area is enclosed, with Madura Island to the north and Java Island to the south.This implies that the prevailing wave orientation from the eastern direction is quite plausible, as depicted in figure 5.

Delft3D Numerical Modelling
Delft3D is a widely used numerical modeling software package for simulating various physical and environmental processes in aquatic systems, including oceans, rivers, lakes, and coastal areas.It is developed by Deltares, a research institute in the Netherlands, and is particularly well-suited for hydrodynamic, sediment transport, and water quality modeling.When applied to the context of PLTGU Grati, which is interested in exploring thermal and sedimentation barriers, Delft3D can be used to perform numerical simulations to analyze and optimize these aspects.In this study, the modeling of heat distribution and sedimentation rate is conducted both for the initial jetty condition and for the modified layout and length of the jetty.

Validation of the model with Tide Data
Simultaneously, the validation process at a given time step revealed an error of 8.97% when comparing observational data with model output data.Refer to figure 7 for the graphical representation of this comparison.

Results of the Existing condition model
The distribution of currents velocity, which is a combination of tidal currents and currents generated by wave breaking, is presented in this model.Figure 8 and figure 9 show the distribution of warm water exiting the outlet canal, which is highly dependent on the flow rate.It can be observed that an increase in flow rate (an increase in power) leads to a broader distribution of hot water flow.Meanwhile, the characteristics of current distribution show that during high tide, the dominant current direction is towards the coastline, whereas during low tide, the dominant current direction moves away from the coast, as shown in figure 10 and figure 11.During low tide, the dominant current direction moves away from the coast, allowing the distribution of warm water to reach the mouth of the inlet canal.A comparison of temperature differences at the inlet canal under each condition is presented in table 1.It is shown that the temperature difference between the existing condition and the condition with increased power (increased flow rate) is greater than 0.149°C for the lowest temperature and greater than 0.181°C for the highest temperature, this outcome aligns with the findings presented in [16].

5.3.The proposed jetty extension
In this study, two forms of jetty extension for the PLTGU Grati's canal water intake are proposed.The first is Alternative 1, which involves an extension turning to the right at 51 o degrees relative to the Xaxis, with an additional length of 200 meters on the eastern side and 272 meters on the western side of the jetty, as shown in figure 12(a).The second is Alternative 2, which involves an extension turning to the left at 58 o degrees relative to the X-axis, with a length of 200 meters on the western side and 272 meters on the eastern side of the jetty (figure 12(b)).Subsequently, both of these alternatives are simulated to assess the distribution of hot water, the patterns of current velocity, and the sedimentation rate, in order to evaluate which alternative better meets the needs of the PLTGU Grati Power Plant.
Alternative 2 are quite effective in containing the spread of hot water compared to the average temperature simulation results for the existing jetty condition (28.640°C).This indicates that the hot water distribution in the simulation results for Alternative 1 and Alternative 2 is still below the existing condition, which is 27.792°C and 28.617°C, respectively.However, for sedimentation barriers, Alternative 2 shows better results than Alternative 1.The average sediment inflow rate into the canal water intake for Alternative 1 is 4207.08 m3/month, while for Alternative 2, it is 3751.10m3/month.In contrast, the existing jetty condition with the addition of flow rate results in an average sedimentation rate of 5579.67 m3/month (as shown in figure 15 and figure 16).Based on the analysis of hot water distribution and sedimentation rate, it can be suggested that modifying the shape and length of the jetty for Alternative 2 can provide satisfactory results to support the operation of the Grati Combined Cycle Power Plant with the planned additional capacity.However, the selection of sediment transport formula and modifying the jetty extension are also very important to increase the accuracy of modeling results and the sedimentation barriers [17, 18, 19 and 20].

Conclusions
A study on sedimentation and thermal barriers related to the expansion of the Grati Combined Cycle Power Plant (PLTGU) has been conducted through alternative forms and extensions of the jetty.From the discussion above, it can be concluded that Alternative 2 is capable of reducing the average temperature significantly compared to the Existing condition, by approximately 0.023°C.As for sedimentation rates, Alternative 2 can reduce the sedimentation rate to 3751.10 m3/month, whereas Alternative 1 results in a sedimentation rate of 4207.08 m3/month.In contrast, the sedimentation rate for the existing jetty condition is significantly higher, at 5579.67 m3/month.It is evident that Alternative 2 can reduce the sedimentation rate by approximately 1828.57m3/month compared to the existing jetty condition.Therefore, with the construction of the modified extension of jetty in Alternative 2, it is expected that this condition will increase the water intake capacity for cooling through the water intake canal, in line with the need for increased power and capacity at the Grati Combined Cycle Power Plant.Furthermore, dredging activities in the water intake canal are expected to decrease significantly, as Alternative 2 can reduce sedimentation rates by up to 32.77%.

Figure 7 .
Tide Validation Result

Figure 8 .Figure 9 .
Figure 8. Water Temperature during High Tide in Existing Condition

Figure 10 .Figure 11 .
Figure 10.Current Direction during High Tide in Existing Condition

Figure 12 .
The proposed jetty extension, Alternative 1 and Alternative 2

Figure 13 .
Figure 13.Distribution of hot water dispersion during Rising Tide Conditions of Power Addition Alternative Model

Figure 14 .Figure 15 . 9 Figure 16 .
Figure 14.Distribution of hot water dispersion during Low Tide Conditions of Power AdditionAlternative Model

Table 1 .
Comparison of Temperature in the Existing Condition Before and After Power Increase