Evaluation of Warm Water (thermal) dispersion using a numerical model at the Binceta Coast (North Sulawesi) in PLTU Development

By Mahatma Lanuru1

1 Department of Marine Science, Faculty of Marine Science and Fishery, Hasanuddin University, Makassar, Indonesia, e-mail: mahat70@gmail.com

Abstract

A numerical study was conducted to model warm water (thermal) dispersion from Steam-Electric Power Plant (PLTU) at Binceta Coast (North Sulawesi). The main objective of the this study was to determine the possibility of warm water discharged from power plant outfall re-entering the power plant intake. Modelling is done with one intake and one outfall on the west and east monsoon. Each monsoon is modeled with two scenarios, i.e. Scenario 1: temperature of the water coming out of outfall is 5 ° C higher than the ambient temperature of the sea water and Scenario 2: The temperature of the water coming out of the outfall is 8 ° C higher than the ambient temperature of the sea water. Result of the simulations of Scenario 1 and Scenario 2 show that warm water plume discharged from power plant outfall will not re-entering the power plant intake both on the west and east monsoon.

Introduction

A Steam-Electric Power Plant (PLTU) to be built on the Village Binceta

(North Sulawesi) to improve the reliability of the electrical system of North-

Sulawesi Gorontalo by taking sea water as cooling power plants, and throw it

back into the sea. To determine the possibility of warm water discharged from

power plant outfall re-entering the power plant intake, it is necessary to study

the thermal dispersion within the framework of those plans. The purpose of this

study is to estimate thermal plume dispersion once the Power Plant run using a

numerical modeling of SMS/RMA2 and RMA4 models.

The Surface Water Modeling System (SMS), which is developed and

enhanced at the Environmental Modeling Research Laboratory(EMRL) at

Brigham Young University, is a comprehensive environment for one-, two-, and

three-dimensional hydrodynamic modeling. It comprise: TABS-MS (GFGEN,

RMA2, RMA4, RMA10, SED2D-WES), ADCIRC, CGWAVE, STWAVE, BOUSS2D

and PTM etc. Each numerical model is designed to address a specific class of

problems. Some of them calculate hydrodynamic data such as water surface

elevations and flow velocities. Others can compute wave mechanics such as

wave height and direction. Still others track contaminant migration or suspended

sediment concentrations (Yin et al., 2010). In this paper, the RMA2 and RMA4

models are adopted.

SMS (Surfacewater Modeling System) has been used elsewhere, e.g., to

model tidal current in in Zhanjiang Harbor, China (Yin et al., 2010), water

elevation distribution, velocity distribution, and BOD (Biological Oxygen

demand) concentration along the Brantas River Stream, Indonesia (Sholichin and

Othman, 2006), water circulation and the distribution of contaminant

concentrations (oil spill) on The northeastern coast of the Sea of Marmara, Turkey

(Kazeziyilmaz et al., 1998), and thermal distribution at The Muria Peninsula

Coast in NPP’s development, Indonesia (Susiati, et al., 2010).

Methodology

Model area includes the coastal waters of power plant construction plans

SULUT-1 Binceta Village, Bolangitang East District, North Bolaan Mongondow

Rengecy, North Sulawesi (0O 53’ 27,98’’ N - 0O 53’ 57,53’’ N, 123O 27’ 58,62’’ E -

123O 28’ 36,74’’ E) with an area of approximately 1,0 km x 0.67 km (Figure 1).

Figure 1. The coverage area of model (red lines)

Current circulation and warm water (thermal) dispersion from the Steam-

Electric Power Plant was performed using SMS 8.1. Two numerical models of

SMS are used to simulate the current circulation and the thermal dispersion. The

first of these models, RMA-2, is a two-dimensional, depth averaged, free surface

finite element model, which can simulate the current circulation. After a finite

element mesh has been constructed and boundary conditions and material

properties have been defined, the water surface elevation and flow velocity at

each grid point can be computed.

Based on the hydrodynamic solution obtained by RMA-2, a second

numerical model, RMA-4 is used to simulate the thermal effluent transport. The

thermal effluent transport model requires as input the initial thermal (warm

water) conditions as a set of point loads in addition to the physical parameters

used in the hydrodynamic model.

For the simulation of current circulation, a model grid is developed, i.e. a

mesh for the Binceta Coastal Water. The inputs are the bathymetry of the region,

the coastline, the water level/tide data, warm water discharged from power

plant outfall data, the wind data, turbulent exchange coefficients, friction

coefficient and the boundary conditions. The governing equations for shallow

water circulation model are given as follows (Donnel et al., 2006):

Fluid mass conservation equation:

(Eq.1)

Momentum conservation equation: in the x-direction:

(Eq. 2)

(Eq.3)

where h is water depth; u and v are velocities in the Cartesian directions; x, y and t

are Cartesian coordinates and time; ρ is density of fluid; E is eddy viscosity

coefficient; g is acceleration due to gravity; a is elevation of bottom; n is

Manning’s roughness n-value; ζ is empirical wind shear coefficient; Va is wind

speed; ψ is wind direction; ω is rate of earth’s angular rotation; Φ is local latitude

The warm water (thermal) dispersion modelling is conducted with RMA-4,

a numerical model to simulate the migration and dissipation of the constituent for

a given number of time steps by solving an advection-diffusion type differential

equation. The model uses the following as input; the velocity distribution

computed by the current circulation model (RMA-2), initial mass or concentration

of the pollutant (in this case initial temperature of warm water discharged from

power plant outfall), the decay rate and the dispersion coefficient of the

pollutant.

The governing equations for shallow transport and thermal dispersion

model are given as follows (Letter et al., 2011):

(Eq. 4)

Where h = water depth c = concentration of pollutant for a given constituent, t

=time, u,v = velocities in x direction and y direction, Dx, Dy = turbulent mixing

(dispersion) coefficient , k = first order decay of pollutant, σ = source/sink of

constituent, and R = rainfall/evaporation rate.

Modelling of thermal dispersion is done with one intake and one outfall on

the west and east monsoon. Each monsoon is modeled with two scenarios, i.e.

Scenario 1: temperature of the water coming out of outfallt is 5 ° C higher than the

ambient seawater temperature and Scenario 2: The temperature of the water

coming out of the outfall is 8 ° C higher than the ambient seawater temperature.

All scenarios are simulated using the following data:

• discharge of the water coming out of outfallt is 2,7 m3 /second for

Power Plant with total watt of 2 x 25 MW.

• temperature of the water coming out of outfallt is 35 ° C for normal

condition (Scenario 1) and 38 O C for extreem condition (Scenario 2).

• ambient temperature of the seawater in the study site was 30 O C

that is taken from author observation on 12-13 December 2011.

• Model is simulated for 15 days (360 h) with time step of 1 h.

• Model is simulated in the west monsson (wind speed 5,6 m/second,

wind direction 270 O) and east monsoon (wind speed 5,6 m/second,

wind direction 90 O)

Results

Thermal dispersion model of Scenario 1

In this simulation, hydrodynamic circulation is driven by amplitude and

phase of tide at all open boundry points in the initial condition (t = 0) and

westerly wind with velocity of 5,6 m/second (wind direction of 270 O ) during

west monsson and easterly wind with velocity of 5,6 m/second (wind direction of

90O ) during east monsson. Temperature of the water coming out of outfallt is 5O

C higher than the ambient temperature of the sea water.

In the initial condition (t = 1 hour), thermal plume dispersion is still

limited around the power plant outfall both in the west monsoon and east

monsoon. Temperature at the outfall ranged from 33 to to 35 O C. The temperature

decrease with increasing time distance from the outfall. The further from the

source (outfall) then the temperature is reduced.

At the end of simulation (t = 15 days model), as shown in Figure 2, thermal

plume dispersed further to the east direction approaching power plant intake in

the west monsoon condition. Thermal dispersion raise seawater temperature up

to 31 ºC to a radius of 83 m eastward approaching the intake. It can be seen from

here that aftaer 15 days model simulation the maximum increase in temperature

over the ambient seawater, is close to 4 °C at the outfall, though this decreases to

a maximum of 1 °C at the location 83 m eastward from the outfall. It shows also

from the simulation that seawater temperature in the intake remain the same with

ambient seawater temperature at the end of simulation indicating that intake is

not affected by thermal discharge from outfall.

Gambar 2. Thernal plume dispersion after 15 model days (360 hours) during west moonson condition. Colors indicating temperature between 30 ºC and 35 ºC.

A different thermal dispersion pattern was obtained for east monsoon

condition at the end of simulation (after 15 days model) where thermal plume

dispersion moved westward away from the intake. As shown at Figure 3, after 15

days run seawater temperatur incresed up to 1ºC above ambient seawater

temperature to a radius of 117 m from the outfall. Simulation results of scenario

1 show that warm water plume discharged from power plant outfall will not re-

entering the power plant intake both on the west and east monsoon.

Figure 3. Thernal plume dispersion after 15 model days (360 hours) during

east moonson condition. Colors indicating temperature between 30 ºC and 35 ºC.

Thermal dispersion model of Scenario 2

In Scenario 2 simulation, temperature of the water coming out of outfallt is

8O C higher than the ambient seawater. Other input parameters, i.e. tide and

wind velocity and direction remain unchanged. Simulation results showed that

thermal plume dispersion at the initial condition (t = 1 hour) is still limited

around the power plant outfall both in the west monsoon and east monsoon as

shown at Scenario 1. Temperature at the outfall ranged from 35 to to 38 O C. The

temperature decrease with increasing time distance from the outfall.

As in Scenario 1, at the end of simulation (after 15 days model), thermal

plume dispersed further to the east direction approaching power plant intake in

the west monsoon condition. Thermal dispersion raised seawater temperature up

to 31,6 ºC to a radius of 84 m eastward approaching the intake (Figure 4). In

contrast, thermal plume dispersion moved westward away from the intake

during east monsoon condition. After 15 days model, seawater temperatur

incresed up to 1.6 ºC above ambient seawater temperature to a radius of 117 m

from the outfall (Figure 5). Simulation results of scenario 2 also confirm that

warm water (thermal) plume discharged from outfall will not re-entering the

power plant intake both on the west and east monsoon.

Figure 4. Thernal plume dispersion after 15 model days (360 hours) during

west moonson condition in Scenario 2. Colors indicating temperature between 30 ºC and 38 ºC.

Figure 5. Thernal plume dispersion after 15 model days (360 hours) during

east moonson condition in Scenario 2. Colors indicating temperature between 30 ºC and 38 ºC.

Discussion

Binceta (North Sulawesi) Power Plant (PLTU) will utilizes intake sea water

at ambient conditions from a depth of 6 m and discharges the thermal effluent at

a depth of 1.5 m. Thermal dispersion model using RMA-2 and RMA-4 of SMS

were performet to asses the possibility of warm water discharged from power

plant outfall re-entering the power plant intake.

The results of the thermal dispersion simulation show that maximum

increase in temperature over the ambient seawater, is close to 4 °C at the outfall

for Scenario 1 and 6,4 °C for Scenario 2. However, the temperature decrease with

increasing time distance from the outfall. The further from the source (outfall)

then the temperature is reduced.

The extent of the thermal plume movement is slightly higher during the

east monsoon than that during west monsson. However, the extent of the

temperature increase over the ambient seawaters is remain the same duing east

and west monsoons indicating that wind play an importang role in affecting the

extent plume movement but no the extent of the temperature increase.

At the end of simulation (after 15 days model), seawater temperature in

the intake remain the same with ambient seawater temperature indicating that

intake location is not affected by thermal discharge from outfall during the west

and east monsoon both in Scenario 1 and Scenario 2. The simulations resuts

confirm that warm water (thermal) plume discharged from outfall will not re-

entering the power plant intake. This study shows that numerical models play an

important role in determining the extent of thermal plume movement and

temperature increase over the ambient seawater. The model is an useful tool to

select a correct location of intake structure with respect to discharge point but it

needs calibration to make more meaningful.

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