PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 11-13, 2013

SGP-TR-198

PERFORMANCE EVALUATION OF DOUBLE-FLASH GEOTHERMAL POWER PLANT

AT DIENG USING SECOND LAW OF THERMODYNAMICS

Nugroho Agung PAMBUDI1, Ryuichi ITOI

1, Saeid JALILINASRABADY

1, KHASANI

2

1Energy Resources Engineering Laboratory, Faculty of Engineering, Kyushu University

No. 418, West Building 2,

744 motooka, Nishi-ku, 819-0395, Japan

2Department of Mechanical and Industrial Engineering, Faculty of Engineering, Gadjah Mada University,

Yogyakarta 55281, Indonesia

e-mail: pambudi@kyudai.jp

ABSTRACT

A Single-flash system has been adopted for power

generation in the Dieng geothermal power plant,

Indonesia, which produces 22 MW of electricity

despite of its installed capacity of 60 MW. To

achieve optimum energy utilization, a modification of

the plant to double-flash system is examined and

evaluated by energy and exergy analysis using actual

and designed data of the plant. The Engineering

Equation Solver (EES) is used to simulate a double

flash system on the basis of the plant model.

The results of a proposed design indicated that the

total maximum net power output of the plant is

obtained to be 34.76 MW from high pressure turbine

(HPT) and low pressure turbine (LPT). The steam is

supplied from five production wells with available

exergy evaluated at the well head to be 84.98 MW.

The separator pressure for HPT is given as 9.87 bar

based on actual data of the plant while that for LPT is

1.43 bar based on the optimal design.

Two gas ejectors are operated for extracting non-

condensable gases and are driven by steam. The total

steam mass flow rates used for these ejectors are

5.207 kg/s. The energy and exergy efficiencies of the

overall power plant are calculated to be 40.90 %.

INTRODUCTION

The world's demand for energy has experienced rapid

growth over the past five decades (Shenga et al.,

2013). This situation is contrary to the limitation of

energy resources reserves especially from fossil fuels

such as coal, oil and gas (Agugliaroa et al., 2013).

Furthermore, using these conventional energy

resources raises the global warming consequence

because of its CO2 emission. This moment is

definitely relevant to explore more extensively

geothermal potential that sustainable and has low

CO2 emission characteristics (Ármannsson et al.,

2005).

Indonesia is the country that has huge resources of

geothermal energy. Its geothermal potential is

estimated to be around 40% of world potential. The

total installed capacity of geothermal power plant in

Indonesia is 1196 MW from seven locations of power

plants: Darajat (260 MW), Dieng (60 MW),

Kamojang (200 MW), Gunung Salak (377 MW),

Sibayak (12 MW), Lahendong (60 MW), and

Wayang Windu (227 MW) (Darma et al., 2010). This

capacity would be increased as the government

policy targeting of 9,500 MWe in 2025.

The massive quantity of geothermal power plant's

development in future should be also endorsed by

performance evaluation of existing power plant.

Therefore, improving an efficiency of the plant can

be achieved using thermodynamic tools of energy

and exergy analyses. The Exergy analysis is useful

for improving the efficiency of energy-resource use,

for it quantifies the locations, types and magnitudes

of wastes and losses (Kanoglu, et al., 2009; Dipippo,

2008). This research is to optimize the utilization of

produced geothermal fluid from reservoir by

modifying existing of single-flash power plant

achieving 22 MW power output and 27.73% second

law efficiency into a double-flash system. The

Engineering Equation Solver (EES) is used to

simulate its double flash system based on the plant

model using actual and designed data.

DIENG FIELD AND POWER PLANT

The Dieng Plateau is located at an elevation of 2000

m above sea level in the southern central of Java's

province in Indonesia with annual ambient

temperature about 20°C. The ambient pressure in

Dieng is 78.06 kPa based on local meteorology

agency (Junaldi et al., 2012). The Dieng plateau is

one of the volcanic paths, where the surface

manifestation of geothermal energy such as hot

springs and fumaroles present. There are also craters

such as Sileri in the north and Sikidang and Pakuwaja

in the south. Dieng was identified as one of the

significant geothermal prospects in Indonesia.

Between 1970 and 1972, the Sikidang sector of the

Dieng volcanic complex was investigated under the

auspices of a USAID program, involving US

Geological Survey staff and VSI/ITB/PLN groups

acting as counterpart. (Sudarman,et al, 2008).

Prasetio et al.(2010) reported that the Dieng reservoir

indicated the two-phase condition with liquid-

dominated reservoir at the temperature ranges

between 240 °C to 333 °C.

Table 1: Well pad and wells

The Dieng geothermal power plant adopted a single-

flash system with an installed capacity of 60 MWe

and is supplied by steam from eight production wells

at four locations. The production wells located in

Wellpad 7, 9, 28, and 31. There are two reinjection

wells: DNG 10 located in northern part while DNG

17 far in southern part. This plant generates only 22

MW electricity based on the actual data of the plant

in 2011. Table 1 summarizes the production wells at

respective wellpad and reinjection wells.

In the single-flash model in Dieng showed that the

second law efficiency only 27.73% (Pambudi et al.,

2012). The exergy waste that also calculated was

40.62 MW flowing from separator into canals and

ponds.

A PROPOSED DOBLE-FLASH SYSTEM

To make the maximum use of produced geothermal

energy, modification and improvement of energy

extraction system of geothermal fluid is proposed by

introducing a double flash system.. In order to extract

more steam from the fluid, the LPS (low pressure

separator) is added, which employs the lower

separation pressure compared to the first separator of

HPS (high pressure separator). There are two turbines

employed for the system to cover these two

separators: high pressure turbine (HPT) and low

pressure turbine (LPT). The bottoming system

design is selected to increase electricity production.

The new turbine, LPT, is installed without removing

the existing HPT in a single-flash system.

Figure 1 presents a schematic diagram of double-

flash system. The geothermal fluid flows from

reservoir to the wellbore, then to the well head and

flashes in the HPS at 9.87 bars. Separated steam

flows into purification system and continuously

travels to the HPT. The electricity production by the

generator is executed with connecting the axis stator

into the turbine. Separated brine, number 7 in figure 1,

flows into the LPS and for further flashing, then

flows to the LPT together with the steam exhausted

from the HPT and produces more electricity.

Number 24 in figure 24 indicates the NCG flowing

out to environment.

Figure 1: Schematic diagram of proposed double-flash geothermal power plant

Wellpad

7

Wellpad

9

Wellpad

28

Wellpad

31

Reinjection

wells

HCE-7A HCE-7B

HCE-7C

HCE-9A HCE-9B

HCE-28A HCE-28A

HCE-31 DNG 10 DNG 17

ENERGY AND EXERGY ANALYSES

This research is to analyse a double-flash system

design to achieve optimum energy utilization using

energy and exergy analysis which is based on the first

law and second law of thermodynamics. Exergy can

be classified into four components: kinetic exergy

( ), potential exergy ( ), physical exergy ( ),

and chemical exergy ( ). Mathematical expression

of exergy can be expressed as follows (Bejan et al.,

1948):

(1)

The first law of thermodynamics declares that the

energy can neither be created nor destroyed.

Furthermore, this law indicates that work and heat

cannot be distinguished in terms of quality.

Mathematical expression of the first law of

thermodynamics can be described as follows

(Cengel.,1989):

ΔE=Q-W (2)

The energy change (ΔE) is equal to the subtraction of

the amount of heat(Q) which is added to the system,

and the work(W) performed by the system. Then, the

energy change can be expanded as follows (Cengel et

al., 1989):

[( )

(

) ( )] (3)

where h is the specific enthalpy, h0 is the specific

enthalpy of dead state, V is the velocity, g is the

gravitational acceleration and z is the elevation. The

velocity and elevation represent the kinetic and

potential energies, respectively, which are neglected

due to insignificant amount. Then, the mathematical

expression is rewritten into Eqs. (4) and (5) as

follows:

( ) (4)

( ) (5)

The heat flow that added to the system ( ) can be

written as entropy relation in Eq. 6 (Cengel et al.,

1989).

[ ( )] (6)

where To is the ambient temperature, Sgen is the

entropy generation, s is the specific entropy and so is

entropy of the dead state. The Sgen, the entropy

generation, can be written as Eq. (7)

( )

(7)

Exergy is introduced by adopting the first and second

laws of thermodynamics. It shows the valuable

energy could be exploited by the system referring to

the dead state. Combining Eqs. (6) and (7) into Eq.

(5) leads to:

( ) [ ( )] (8)

Simplifying of thermodynamic process, it is assumed

that there is no entropy generation. It is due to the

concept of entropy itself that basically defines as

disorganization of the system. Therefore, in the

reversible process, the system is well organized

meaning that the entropy generation is zero. Then, Eq.

(8) becomes:

[( ) ( ) ] (9)

The work above expressed by is the maximum

work which is equivalent to the exergy (X).

Therefore, Eq. (9) can be expressed as exergy

equation as follows:

[( ) ( ) ] (10)

Equation 10 can be used to calculate the amount of

exergy in any part of the power plant. From those

exergy amounts, the exergy destruction can be

carried out. The results provide valuable information

to improve the thermal system to an optimal

condition.

SECOND LAW EFFICIENCY

To evaluate the performance of power plant related to

the exergy, the second law efficiency analysis can be

used. It can be expressed mathematically in the whole

system as follows (Cengel et al, 1989):

(11)

where is the produced work by the plant and

is the total exergy entered to the plant. The

exergy entered to the system can be calculated using

information of fluid property at production wellhead.

The second law efficiency can also be calculated

from each component in the system. It uses exergy

input and output from those components as given in

such mathematical expression as follows (Cengel et

al., 1989):

∑

∑ (12)

where ∑ is the total of exergy output, ∑ is

the total input of exergy. Each component of power

plant has several exergy input and output, and they

are summarized as shown in Eq.(12). For example in

separator, exergy input comes from the production

well, number 1, in the form of steam-water two-phase

mixture while exergy output counts for steam in

number 2 and brine in number 3 that flow out from

the separator as shows in Fig. 2.

Figure 2: Exergy flow of the separator

OPTIMAL FLASHING PRESSURE

For evaluating a proposed double-flash system, the

present single-flash actual data are used such as at the

HPS pressure and the HPT mass flow and other unit

data that exist in the single-flash system. In the

double-flash system, several data designed are

assumed such as isentropic efficiency of the LPT,

isentropic efficiency of pumps. This LPT pressure is

determined in such a way that produces maximum

total output both in the HPT and LPT. Using the LPS

pressure as a variable, the optimization is carried out

by EES.

Figure 3: Pressure of Low Pressure Separator (LPS)

vs total output

Figure 2 presents the relationship between pressure of

the LPS and the total output. To find out an optimal

pressure, the LPS pressure was given in a specified

pressure range and repeated calculation with EES.

This pressure determines the temperature of of LPS

with saturated water condition. The HPT exhausted

fluid has same temperature with the LPS. Therefore

the pressure of the LPS influence of electricity

produced.. From this figure, an optimal value of

pressure in the LPS can be found as 1.43 bar resulting

34.756 MW of the total output of the power plant.

THERMODYNAMICS PROCESS

Figure 4 shows temperature-entropy diagram of

double-flash system. The geothermal fluid is assumed

to flow in isenthalpic manner from the reservoir to

the wellhead through the wellbore. During this

process, the fluid starts flashing in the well and both

temperature and pressure decrease as the fluid

reaching the well head in two-phase condition.

The steam and brine are separated at the HPS in

isobaric condition at actual pressure of the plant. The

enthalpy of fluid flowing from reservoir into the

wellbore is estimated using well head pressure data.

From those actual data, enthalpy on each location in

the LPS can be found. Therefore dryness fraction can

be calculated as 13.78%. The separated steam flows

to the turbine through a purifier unit by assuming no-

pressure drop, and continuously travels to the HPT.

The inlet properties at the HPT, indicated by number

2 in Figure 3, such as temperature, pressure and mass

flow rates is based on actual data of the plant. The

temperature of turbine exhaust is determined from

optimal flashing pressure in saturated water

condition. Then, the enthalpy of fluid on this turbine

exhaust is determined by temperature and isentropic

turbine efficiency which is calculated from single-

flash system, ηt, with 0.72.

The brine discharged from the HPS flashes in the

LPS under the optimal pressure 1.43 bar as indicated

number 4. The actual steam exhausted from the HPT

indicated solid arrow number 3 is then flow into the

LPT together with the steam from the LPS. The

enthalpy of steam at the LPT inlet is determined by

mixture of these steams that generate 34.756 MW of

electricity. The isentropic efficiency in the LPT is

assumed to be 0.85. The exhausted fluid from the

LPT indicated number 5 then flows to the condenser

for cooling and extracting non-condensable gases

(NGG) using the gas ejector.

Figure 4 : Temperature (T)-entropy(s) diagram

representing of double-flash system

EXERGY OF COMPONENTS

Table 2 summarizes the amount of exergies at several

components such as separator, turbine, condenser and

overall plant, including the exergy loss and second

law efficiency.

In the HPS, the source of exergy input is the fluid

from reservoir. It has 84.67 MW and produces 40.62

MW steam. The second law efficiency of this

component is calculated to be 94 %. If a double-flash

system is not applied on this geothermal plant, some

amount of exergy waste is released. In this double-

flash system case, that of exergy waste from single-

flash system is utilized. The steam produced at the

HPS flows to the HPT and generates 7.23 MW

electricity. The second law efficiency of the HPT

itself is 83.64%. The LPT receives amount of steam

from both the HPT exhausted and from the LPS. The

total amount of this steam generates 27.52 MW

electricity. The waste of exergy from the LPS that

flows to reinjections system is 12.33 MW.

Table 2. Exergy values at several components of the plant

Component Exergy input Exergy output

Second Law

efficiency

(MW) (MW) ηII (%)

HPS 84.97 80.55 94.79

HPT-Generator 35.58 29.76 83.64

LPT-Generator 45.13 34.81 77.12

LPS 40.62 34.94 86.02

Main condenser 8.05 5.57 69

Intercondenser 3.73 0,30 8

After Condenser 2.43 0,33 13

Overall plant 84.97 34.76 40.90

The condensers consist of main condenser,

intercondenser and aftercondenser that work to

condense the fluid, and all of them create exergy loss.

There are also two ejectors installed at the condensers

to remove NCG from the power plant. The total plant

produces 34.76 MW of exergy desired and this is

40.90 % of second law efficiency. The electricity

generated from this double-flash system increased by

57.98 % compared that from the present single-flash

power plant generating 22 MW electricity. The

ejectors that extract NCG from condensers are driven

by steam. First ejector works in the main condenser

while the second ejector in the intercondenser. The

total amount of steam consumed with these two

ejectors is 5.21 kg/s that are equivalent to 10.66% of

total amount of steam

A Grassman diagram that indicates exergy loss and

net power output of the proposed double-flash power

plant is presented in Figure 5. The total exergy input

to the system is 84.97 MW that arrives from the

reservoir in the form of fluid and produces net power

output of 34.76 MW. The pump loss represents

consumed exergy of 1.454 MW at a hot well pump

and two auxiliary pumps. The hot well pump sends

condensate from condenser into a cooling tower, and

the auxiliary pumps send cooling water into

condenser, the intercondenser and the aftercondenser.

The purifier loss is not drawn in the Grassman

diagram because of an insignificant value. The

exergy destroyed at the HPS and the LPS are 4.42

MW and 5.69 MW, respectively. Meanwhile, turbines

have 5.80 MW and 10.33 MW, respectively, for the

HPT and the LPT loss.

1 2

4 3

5

Figure 5: Grassman diagram for exergy flow of the double-flash geothermal power plant

In condensers, furthermore, that consist of main

condenser, intercondenser and aftercondenser have a

total exergy loss of 6.59 MW. The amount of brine

that being send back to the formation still contains

exergy of 16.03 MW. There are opportunities to

utilize this brine for Dieng village in the form such as

district heating of houses and greenhouse for

agriculture. If these utilization can be realized the

waste brine loss in terms of exergy can be minimized.

CONCLUSION

Performance evaluation of double-flash system was

examined using energy and exergy analyses. The

results from optimization of the Low Pressure

Separator (LPS) showed that the optimal pressure is

1.43 bar. The total available exergy from production

wells was calculated to be 84.97 MW generating

34.76 MW electricity with two turbines.

Grassman diagram clearly shows the losses of exergy

at several components in the system. Pump losses is

1.454 MW from total exergy available from hot well

pump and auxiliary pumps. In separator losses, the

high pressure separator (HPS) has 4.42 MW and the

LPS 5.68 MW In generating electricity unit loss,

turbines, have 5.80 MW and 10.39 MW for the high

pressure turbine (HPT) and the low pressure turbine

(LPT), respectively. This corresponds to 6.83 % and

12.2 % of available exergy.

Two gas ejectors driven by steam consume 5.21 kg/s

of steam. The overall power plant second law

efficiency was calculated to be 40.90%. The result of

energy and exergy calculation showed that

modification current single-flash by introducing a

double-flash system will improve the power plant

capacity as well as efficiency of plant.

NOMENCLATURE

G gravitational acceleration (m/s2)

h enthalpy (kJ/kg)

mass flow rate (kg/s)

s entropy (KJ/Kg-K)

T temperature (K)

V velocity (m/s)

W work (MW)

net power output (MW)

exergy (MW)

kinetic exergy (MW)

potential exergy (MW)

physical exergy (MW)

. chemical exergy (MW)

Z elevation (m)

ηt isentropic efficiency

second law efficiency

AKNOWLEDGEMENT

The authors would like to thank to the PT. Geodipa

Energy for their support in research activities on

Dieng power plant. The authors would also like to

thank to GCOE program, Kyushu University to their

financial support.

REFERENCES

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Total exergy from

geofluid 84.97 MW

1. Pump loss 1.45 MW, 1.7%

2. HPS loss 4.42 MW, 5,2%

3. LPS loss 5.69 MW, 6.69%

4. HPT loss 5.80 MW, 6.83%

5. LPT loss 10.33 MW, 12.16%

6. Condenser loss 6.59 MW, 7.73%

7. Waste brine loss 16.03MW, 18.87%

8. Net power output 34.76 MW, 40.91%

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