Performance Evaluation of Double-flash Geothermal Power ...€¦ · performance evaluation of...
Transcript of Performance Evaluation of Double-flash Geothermal Power ...€¦ · performance evaluation of...
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: [email protected]
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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