Accepted Manuscript Research Paper Evaluation of Synchronous Execution of Full Repowering and Solar Assisting in a 200 MW Steam Power Plant, a Case Study Gholamreza Ahmadi, Davood Toghraie, Ahmadreza Azimian, Omid Ali Akbari PII: DOI: Reference:
S1359-4311(16)32382-1 http://dx.doi.org/10.1016/j.applthermaleng.2016.10.083 ATE 9285
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
3 July 2016 3 October 2016 12 October 2016
Please cite this article as: G. Ahmadi, D. Toghraie, A. Azimian, O. Ali Akbari, Evaluation of Synchronous Execution of Full Repowering and Solar Assisting in a 200 MW Steam Power Plant, a Case Study, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.10.083
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Evaluation of Synchronous Execution of Full Repowering and Solar Assisting in a 200 MW Steam Power Plant, a Case Study
Gholamreza Ahmadi*1, Davood Toghraie2, Ahmadreza Azimian2, Omid Ali Akbari1 Young Researchers and elite Club, Khomeinishahr Branch, Islamic Azad University, Khomeinishahr, Iran
12-
Department of Mechanical Engineering, Khomeinishahr Branch, Islamic Azad University, Isfahan, Iran.
[email protected]
Abstract This study investigates a full repowering simultaneously with merging solar energy in 200 MW units of Montazeri Steam Power Plant in Iran. A 400 MW gas turbine has been used for full repowering. A part of feed water in the solar field turns into saturated steam. In the repowered cycle without the involvement of solar energy, the energy and exergy efficiencies have increased by 76.8% and 73% reaching to 59.11% and 56.63%, respectively. In power increase mode, the mass flow rate of water in solar field is not to be more than 31.3 kg/s. Under this condition, the power of the steam turbines will experience a 16.8 MW increase. In efficiency improvement mode, with 75 kg/s feed water in solar field and the generation power of the steam turbine to be fixed, the consumption of natural gas by the gas turbine will decrease 2.21 kg/s. This will reduce natural gas consumption and CO2 emissions by 21,481,200 kg and 43,392 ton per year, respectively. Considering the price of natural gas and CO2 to be 0.2 USD/kg and 100 USD/ton respectively, the rewarded profit due to a reduction in both fuel consumption and CO2 emission will be 8,635,440 USD per year. Keywords: Montazeri Steam Power Plant, Full repowering, Heat Recovery Steam Generator, Solar assisting, Exergy efficiency, CO2 emission reduction. Research highlights
Synchronous execution of full repowering and solar assisting for this power plant is evaluated 1
A 400 MW gas turbine is chosen for full repowering
Solar energy is used for evaporating a part of feed water in parallel with HRSG
Annual effects on full consumption reduction and improvement on CO2 emission for different sizes of solar field are investigated
The effects of number of pressure levels of HRSG on cycle performance are evaluated Nomenclature (m2)
A
Area
B
Boiler
BFP
Boiler Feed Pump
C
Condenser
CLFR
Compact Linear Fresnel Reflector
CP-1st
First stage Condensate pump
CP-2st
Second stage Condensate pump
CT
Cooling Tower
CWP
Cooling water pump
DE
Deaerator
DP
Drip pump
DSG
Direct Steam Generator
e
Specific energy
(kJ/kg)
E
Total energy
(kJ)
EC
Economizer
EJ
Ejector
EV
Evaporator
EnPI
Energy Performance Index
ExPI
Exergy Performance Index
EX
Flow exergy
f
Dilution Factor
FRC
Full Repowered Cycle
FRSAC
Full Repowered-Solar Assisted Cycle
FWHR
Feed Water Heating Repowering
G
Generator
GT
Gas Turbine
GTCCPP
Gas Turbine Combined Cycle Power Plant
g
The gravity acceleration
h
Specific enthalpy
HPH
High Pressure Heater
HPT
High Pressure Turbine
(m/s2) (kJ/kg)
2
HR
Heat Rate
HTF
Heat Transfer Fluid
(kJ/kW.h)
Destroyed exergy
(kW)
IPT
Intermediate Pressure Turbine
ISCCS
Integrated Solar Combined Cycle System
LPH
Low Pressure Heater
LPT
Low Pressure Turbine
MSPP
Montazeri Steam Power Plant Mass flow rate
(kg/s)
P
Pressure
(bar)
PTC
Parabolic Trough Collector
Q
Heat
(kW)
World Constant for gases RC
Rankin Cycle
s
Specific entropy
SAPG
Solar Aided Power Generation
SC
Simple Cycle
SD
Solar Dishes
SEGS
Solar Electric Generation System
SH
Superheating
SPP
Steam Power Plant
T
Temperature
TSS
Thermal Storage System
v
Velocity
W
Work
(kW)
z
Elevation
(m)
Heat loss
(kW)
(kJ/kgK)
(°C)
(m/s)
Greek symbols First low efficiency Second low efficiency
Exergy of fuels Specific exergy
(kW/kg)
Subscripts and superscripts a
air
amb
Ambient
c
Solar collector
co
Condenser 3
c ch
Combustion chamber
c.v
Control volume
d
direct
des
destroyed
f
fuel
fw
Feed water
g
Gas
gen
generation
i
Inlet
o
outlet
r
relative
reh
reheater
s
solar
sf
Solar Field
th
Thermal
1- Introduction Iranian power plants had a capacity of 73152 MW, and 47 to 50-thousand-megawatt new power plants are expected to be installed by the next 10 years[1-2]. Currently, 21.6 % of the power plants in Iran are steam cycle, 36.1% Brayton cycle(gas turbine), 25.4% are gas turbine combined cycle power plants(GTCCPP), 14.7% hydroelectric plants, 1.4 % nuclear steam power plant and other 0.8% accounts for diesel, wind and other renewable energy plants. During the same year, 93.2% of the electricity was generated by fossil fuel power plants. Although hydroelectric plants make 14.7% proportion of plants in Iran, these plants have generated only 5.1% of the power due to a fall in precipitation over the past few years. The negative improvement in the efficiency of thermal power plants over the past years[1], which highlights an ever-increasing erosion, calls for investment to improve their efficiency. At the moment, the mean efficiency of Iranian power plants is 37%, which is according to what has been planned by the Energy Department, and is supposed to reach 49% by the year 2041[3]. The carrot and stick policy employed respectively for power plants with high and low efficiency has also speeded up the plans towards the target. The establishment of energy stock market since 2012, which has created a competitive market among power plants, has also encouraged private sector owners to improve the efficiency of their power plants. The reason behind this is the excessive 4
demand to buy power from efficient power plants and the dramatic difference between prices these plants offer compared to those offered by less efficient ones [4]. During the past years, the government has provided more support, both financially and politically, to improve the efficiency of fossil fuel power plants. Based on the latest government policy, having put efficiency-improving plans into practice and also keeping the peak of their nominal generation capacity, power plants can sell the extra fuel (the difference between the previous and current consumption) in international markets [2]. Moreover, planning to put supportive policies into practice, the government buys electricity from the power plants that use renewable energies almost 10 times as much as the one generated through fossil fuels [2-5]. For instance, the government pays 90% of the costs of the necessary equipment to install photovoltaic panels in residential as well as administrative sectors and also buys the extra power generated 6 times as much as the legislated cost [2]. Therefore, generating power by using renewable energies is much more thrifty in Iran. As a result of this, the use of these energies has been rapidly increasing, and by the end of the 6th Development Plan, more than 5000 MW of power will be generated by using clean energy resources [2]. Based on what has been identified by research, and with regards to its appropriate geographical location in terms of sunlight, Iran is among the best regions to set up solar power plants and perform solar repowering [6]. However, the central as well as the southern parts of the country are known as the best locations to set up solar power plants. The average number of sunny days is 280 days per year in more than 90% of the regions in Iran [7]. Being located at the geographical width of 32.76°N and a length of 51.87°E and an altitude of 1601 m from sea level, Isfahan is one of the good places to set up solar power plants [8]. Regarding this and the proper location of Montazeri Steam Power Plant (MSPP) [9], a simultaneous administration of full as well as solar repowering of units of this power plant is investigated. 1-1 Literature review In this section, a gist of the studies conducted about repowering of the current thermal power plants and the merging of solar energy in GTCCPP have been reviewed.
5
Szargut and Szczygiel [10] evaluated the hybridization of a gas turbine (GT) with a steam power plant (SPP) using four methods. The result indicated that regarding exergy, the best performance is achieved when the feed water in HRSG is preheated and returned into feed water heaters (FWHs). This leads to a reduction in irreversibility and waste in the HRSG. Tawney et al. [11] conducted a comparative study of FWH, hot wind box and parallel (supply boiler) methods of repowering. They analyzed the benefits and expenses of performing each of the plans. Shahnazari et al. [12] studied the repowering of Lowshan Power Plant in Iran; defining partial methods of reaching a good efficiency in each method. They also put forward all the technical limitations of full and partial repowering besides providing an economic analysis of applying each method in the mentioned power plant. Matthias [13] conducted a study to analyze the best choice to repower a 300 MW SPP in Russia. Having investigated all the methods, he finally recommended the full repowering method due to the old boilers, and SGT4000-F5 was introduced as the best choice for this repowering. He showed that the efficiency of the power plant will improve from 38 to 56.8 percent. Carapellucci and Milazzo [14] studied the repowering of GTCCPPs using steam injection methods in the combustion chamber of the GT. They used a new GT and a one level pressure HRSG to produce steam. The results showed that this can increase the generating power of the main GT, and brought about a raise in the temperature of the combustion products, which led to a 3 to 4 percent increase in efficiency. Haghighi and Tanassan [15] studied the FWHR of Besat Power Plant from technical and economical points of view. In their study, in order to achieve the highest power generation in repowering, gas turbine V94.2 was recommended; while model SGT900 was proposed to reach a new cycle with the best electricity price. Escosa and Romeo [16] studied the impact of manufacturing technology and the capacity of the turbine added to the steam cycle on CO2 emission. Baghestani et al. [17] simulated the cycle in Ghazvin Power Plant using Thermoflex software and proposed the best forms of repowering through performing by using exergy and exergo-economic analysis. Zeki and Durmaz [18] studied the effect of hot wind box repowering on the efficiency of CO2 emissions in a SPP in Turkey. They used GTs with power of a range between 10 to 22 percent of the main SPP. The results indicated an 11 to 27 percent increase in power production and 7 percent 6
reduction in CO2 emission. Marcin et al. [19] simulated FWHR in a 800 MW SPP in India. The results showed that using this method of repowering increases the generation power by 20 percent. Furthermore, efficiency in the whole system increased by 1%; that is from 43.5 to 44.5 percent. Karellas et al. [20] compared FWHR and parallel method in a coal fired power plant. They used exergy and economic analyses. Carapellucci and Giordano [21] investigated the FWHR of a 600 MW coal fired power plant. They presented two different proposals for this. The results showed that this method increased the generation capacity of the plant by 20% with a one-percent increase in total efficiency and an 18-percent reduction in CO2 emission. Grinnan [22] presented an experience of FWHR in an American SPP. In this report, the results of substituting the condenser and FWHs have been reported. Samanta and Ghosh [23] evaluated the effects of using hot gases from an extra GT in a SPP to preheat the boiler feed water and inlet air; reporting the results in energy efficiency and environmental advantages. Bianchi et al. [24] studied repowering of a waste to energy power plant with a GT from a thermodynamic point of view. They proposed the supply boiler method using the information from different GTs with different capacities for simulation. The results showed that if a GT with a 42 MW capacity is used, the first law efficiency increases from 25 to 36 percent. Wolowicz and Badyda [25] studied the selection of best GT for FWHR of a super critical 800 MW SPP in Poland. They tested the safe method of connecting a gas turbine to a steam cycle. The integration of solar energy with GTCCPPs was first proposed by Lus International Co. Ltd. in 1990 [26]. It was first proposed as the use of PTC collectors in GTCCPPs. So far, a lot of research has been conducted on the issue and many papers have been published in this regard. Baghernejad and Yaghoubi [27] optimized a 400 MW ISCCS using energy and economic principles and genetic algorithm. The results showed that the use of exergy and thermos-economic analyses can help improve the efficiency of the power plant. Derbal-Mokrane et al. [28] calculated the annual performance of an ISCCS in Algeria using the simulation software TRANSYS. Finally, they proposed the best conditions of the site and the best phase of power generation.
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Turchi Craig and Ma [29] studied an ISCCS plant in the climate of the US both with and without TSS. They used IPS Epro and SAM softwares and concluded that the cost of power generation in ISCCS without TSS is much more economical than the state with TSS. They also found that solar energy can supply up to 64% of the entire feeding heat. Ancona et al. [30] thermodynamically evaluated the steam sector of small GTCCPPs using solar energy. Besides analyzing three different plans, they concluded the necessary thermodynamic evaluation and also proposed some ways to increase the efficiency. They concluded that a combination of solar energy and the current GTCCs can lead to an increase in stabilized profit. Manente [31] studied the methods of merging solar energy into a 390 MW GTCCPP. The calculations were conducted for two different modes: 1- power increase and 2- decreasing fuel consumption (efficiency improvement) modes, with the highest amount of sunshine over a year. The results indicated that the maximum amount of sunshine can be merged in 19 MW to maintain the previous instruments in the plant. Roviria et al. [32] compared the parabolic trough and Linear Fresnel collectors in ISCCSs. They reached a conclusion that the use of linear receivers reduces the costs of electricity generation and the best method to merge solar power into these plants is steam generation. In this paper, full repowering of MSPP in Iran is evaluated. Since full repowering is among the best choices for MSPP, this method is used to increase efficiency in this SPP. Regarding the fact that Isfahan is geographically located in a suitable place to install solar power plants or solar repowering; merging solar energy into the mentioned power plant is also investigated. Firstly, the full repowered cycle including the GT, HRSG and existing part, is completely investigated, and then the effects of using solar energy in the new cycle will be analyzed. We will investigate the final design in energy, exergy, envirinmental and economic points of view. The difference between this study and the previous studies is that repowering through GT and merging solar energy into an existing rankine cycle power plant are simultaneously evaluated. In previous studies, just the effect of GT repowering or solar assistant to an existing Rankine cycle power plant were investigated. The new part of this work is that we tried to do this two changes to be in a compatible conditions with each other. To do this, first of all we evaluate the converting an existing steam cycle power plant to a combined cycle, 8
considering the solar field. Then, we tried to choose the best condition of merging solar field to the new cycle and found the best size of solar field, to optimize the power generation and efficiencies, considering the limitation of cooling capacity and some other limitations. 2- Power Plant Cycle Description
Montazeri Steam Power Plant (MSPP) of Isfahan is located 15 kilometers to the northwest of Isfahan along the Isfahan-Tehran highway next to Isfahan Refinery in a 2.2 million m2 land. This power plant has 8 similar steam units each with a capacity of 200 MW whose technical specifications and operation conditions have been presented in Table. 1. Heat cycle of this power plant is a Rankine cycle whose heat process has been briefly shown in Fig. 1.
3- Repowering of Thermal Power Plants Using renewable energies is becoming more and more popular, but fossil fuels are expected to remain as the main sources of heat and power generation in some countries [34]. Despite the fact that Iran has 9% of the petroleum as well as above 15% of natural gas resources of the world, the government tends to increase the efficiency and reduce CO2 emissions in fossil fuel power plants. Thus, the efficiency of many fossil fuel power plants is to be improved over the next few years [35]. One of the effective ways to do so is to repower the current cycle. Since it is impossible to get legal permission to establish new plants from the environment conservation organizations in many countries and some particular urban areas, repowering the previous power plants can be a more effective choice. It can also be a fact that repowering is the best method to replace boilers in older power plants. Repowering includes adding GT(s) to an existing SC to increase generation as well as efficiency, which can be done to follow several purposes at the same time [36]:
Increase the produced power
Increase the efficiency
Decrease electricity production price
Increase the useful life of the power plant
Change the consumed fuel (usually coal, wast and mazut to natural gas) 9
Decrease the rate of air pollutions (specially NOx and CO2)
Once repowering is performed, depending on the method chosen, power generation increases by 20 to 200% and heat rate (HR) improves by 5 to 40% [37]. In coal fired power plants, the reduction in emissions is much more effective. Repowering in coal fired power plants has some other positive effects including improving flexibility in exploitation, faster operationalization and quicker matching with load fluctuations [37]. For repowering steam power plants, there are some methods [36]:
Hot Wind Box method (HWB)
Supply Boiler method (SB) or Parallel Repowering (PR)
Feed Water Heating method (FWH)
Full Repowering method (FR)
In the first method, the outlet hot gases of GT are led to the boiler (steam generator) as part of the input air flow. This causes a reduction in the heat needed, finally reducing fuel consumption. In this method, power generation and total efficiency can increase by 50 and 15 %, respectively. To perform this method, the whole combustion system (air channels and burners) must be heat-resistant and adapted to new conditions. There is also a need for proper burners that use less O2 to burn. In parallel repowering, the outlet hot gases of the GT not only preheat the feed water, but are also led toward the HRSG to produce superheated steam, too. The increase in power generation and efficiency varies depending on capacity and efficiency of the GT and HRSG. In feed water heating method, a GT and gas-water heat exchangers are used in parallel with FWHs. In this method, regarding the existing heaters, taking the GT out of operation has the least negative impact on the existing part. In this method, there appears to be a 30 to 40 percent as well as a 2 to 4 percent increase in power generation and energy efficiency, respectively [38]. Regarding the upside of the method, we can mention its simple operation, the least amount of added costs and the small amount of manipulations needed in the basic power plant. Full repowering is similar to parallel repowering. The only difference is that the previous boiler is totally removed and the capacity of GT(s) is chosen in 10
a way that all the necessary steam in HRSG(s) is produced. This method is the best, regarding the increase in power generation and efficiency. The total efficiency is increased by 30 to 40 percent and the produced power can be improved up to 200 percent [37]. In order to run a repowering plan, we should make sure that all the main equipment of the existing part are safe and sound. This means that the steam turbines and other equipment used after repowering must have enough of their lifespan left. Therefore, the first thing to be done before repowering is to make sure each equipment is sound [39]. 3-1 Choosing Gas Turbine Analyzing the turbines manufactured in reliable factories, we can conclude that the higher the capacity of a turbine is, the more efficient it can be. The increase in capacity often accompanies an increase in temperature of exhaust gases. In GTCCPPs, the higher the input temperature of gas to the HRSG is, the higher the efficiency gets, compared to the phase in which the same amount of energy enters in a lower temperature and higher flow rate. Moreover, the difference in temperatures of the input gases to the HRSG and the output steam gets bigger removing the need for a duct burner. If a duct burner is installed, the inner parts need to be designed to be more resistant against high temperatures [40]. This increases the maintenance costs of the HRSG. Therefore, for full repowering a 200MW steam power plant, a gas turbine with high capacity is better than two with low capacities. Regarding the supposed plant, the GT model SGT5-8000H, a production of siemens company, with a capacity of 400 MW is the best choice. 3-2 Heat Recovering Steam Generator (HRSG) Currently, HRSGs in the newest combined cycles are designed with 3 pressure levels and one steam reheater [41]. They are also designed with 2 pressure levels and less exergy efficiency. Increasing the number of pressure levels causes a reduction in exergy loss during the heat transfer [42]. The important point in choosing the number of pressure levels needed for full repowering is that the choice or design must be based under the conditions of the existing steam turbines. In a one-level pressure HRSG with a single reheater, all the feed water is sent to the drum through a single pump under a particular pressure. The water is heated through the economizers before entering the drum. The steam 11
produced in the evaporator is led toward the superheaters. The dry steam is also sent to the HPT under the desired temperature. The output steam from the HPT is reheated in the HRSG and is led toward the IPT, LPT and finally the condenser. Regarding the above-mentioned, the mass flow rate of steam is the same in all turbines (except the steam used in deaerator), while in dual-pressure HRSGs, some of the feed water is led to the 2nd drum using another pump with a suitable pressure, to produce the necessary steam for reheat part. The steam produced along this route is finally mixed with the output steam from the HPT, and then it is reheated and led toward the IPT and LPT. In a triple-pressure HRSG, the third pump sends a part of feed water toward low pressure economizer and drum number three. The produced steam in this sector is led toward the LPT. Therefore, if the HRSG is dual-pressure, steam flow rate in the HPT is less than that of the IPT and LPT, whereas in triple-pressure HRSGs, this flow rate varies in all turbines. In this state, the mass flow rate of steam in LPT is more than that of the other two. The main point is that the more steam gets into the condenser; the more cooling capacity is required. Two points need to be taken into account to choose the number of pressure levels of HRSG. More pressure levels of HRSG lead to a higher efficiency (less exergy destruction), while the flow of the steam passing through the final stages of the turbine and the input to the condenser is more than the steam entering the HPT (which is not in line with the design of the steam cycle). An increase in the input steam flow also causes a fall in the condenser vacuum, which in turn leads to a reduction in efficiency. As mentioned before, the generation power of the used GT (SGT5-8000H) under standard circumstances (T=15 °C and P=1.013 bar) is 400 MW. The altitude at which the plant is located is 1601 m. As calculated, the generation power of the gas turbine under these circumstances is 329.53 MW. If the air temperature rises to 30°C, power generation will be reduced to 297.32 MW. In this state, the mass flow rate and the temperature of outlet gases are 680 kg/s and 627.4 °C, respectively. Table. 2 shows the variations in some important parameters of the GT at various ambient temperatures. The maximum power of the steam cycle of MSPP, when all the extractions are blocked(like the conditions of one level pressure HRSG in repowered cycle) is 178.5 MW [43]. This amount has been calculated regarding the limitations on the steam flow into the condenser. If dual and multi-pressure 12
HRSGs are used, generation power of the steam turbines gets even less than this. In case of using a triple pressure HRSG, the power decreases to 140 MW, where even the efficiency of the steam turbine falls into a dip. Thus, a single HRSG is used in this study. Under this condition, the power and efficiency of the steam turbine will be close to the designed state and the exhaust heat from the gas turbine will be used in its maximum amount. Besides, the cycle will be less complicated and more convenient to control.
3-3 Solar Repowering There are two states to merge the solar energy into a GTCCPP: a. Combine solar energy with Brayton cycle part (GT) b. Combine solar energy with Rankine cycle part (HRSG) When the capacity of the GT, HRSG, and the ST are designed in synchronize, merging solar energy into GT will lead to making the most use of the solar energy. Because this heat not only contributes to power generation in the GT, but it is also used in the HRSG. In this case, no limitations affect the maximum generation power for the GT. If solar energy gets merged into the water-steam cycle, the capacity of HRSG will increase. Therefore, by maintaining the maximum generation in ST and GT, the temperature of the exhaust gases from the HRSG will increase, which means heat loss and a reduction in efficiency. To keep the temperature of the outlet gases from HRSG, the flow rate of these gases (related to generation power of the GT) should be reduced. During a day the capacity of solar system usually reaches its highest when the ambient temperature rises. A rise in ambient temperature reduces the produced power of the gas turbine, and in turn, the capacity of steam production of the HRSG. Therefore, it seems more reasonable to merge solar field into the water-steam sector (HRSG). Among all configurations that can merge solar energy to HRSG, steam production (evaporation) has been reported to be the best procedure [32, 44]. In this study, the same method is applied, and some of the feed water turns into saturated steam, paralleled with the drum and the evaporator of the HRSG. There are also two ways to transfer heat from the solar collectors to feed water: Direct Steam Generation (DSG) and Heat Transfer Fluid (HTF). The former, which is used in this study, brings 13
about a 3 percent reduction in the cost of electricity generation and a 2.5 percent reduction in greenhouse gas emissions [44]. Fig. 2 shows the final schematic illustration of supposed cycle in this article, which has been drawn using Cycle-Tempo software [45]. In this figure, some thermodynamic properties (include temperature, pressure, energy and exergy flows) are shown in important points, too.
4- Assumptions The following assumptions are considered in simulation and calculations: 1) The reference temperature and pressure for environment are considered as: To=33 oC (306.4 K), Po= 0.94 bar.
2) The relative humidity of the ambient air is taken as 60%. 3) The chemical composition of the reference-environment model constitutes (in mole fraction): N2:0.7562, O2:0.2030, H2O:0.0312, CO2:0.0003 and others: 0.0093. 4) Lost heat in HRSG through radiation, convection and other ways (such as continuous blow down, continuous drain and etc.) is considered equal to 2100 kW. 5) Absolute pressure of the condenser is 0.09 bar [33]. 6) Pressure losses in lines and the equipment are chosen like their catalogues [33]. 7) The isentropic efficiency of the fans and pumps are considered as 82 and 86 percent, respectively. 8) The efficiency of the generator is considered as 98.45 percent. 9) The mechanical efficiency of the steam turbines is considered as 99 percent. 10) Simulations are based on steady state situation. 11) The exergy analysis is based on Lower Heating Value (LHV) of the natural gas. 12) According to the fuel analysis and the combustion products, the rate of CO2 production of the boilers is 2.002 kg/m3 of the consumed natural gas. 13) Number of sunny days in the year is considered as 300 days. 14) The calculations are based on 9 hours use of a solar system in a day.
14
15) Direct irradiance and the efficiency of solar collectors are considered as 500 W/m2 and 60 percent, respectively. 16) The temprature of dew point in outlet of the HRSG is considered as 88 oC [46], and to prevent the production dew on equipment, the outlet temprature of gases from HRSG in simulations and calculations is considered equal to 98 oC (10 oC higher than dew point).
5- Mathematical Formulation
To perform the necessary calculations, we use exergy balance equations in a control volume, mass survival principle and thermodynamics first laws. To use the first law of thermodynamics, energy balance form should be used for standard volume. This equation is as below [47]: dE cv v 2 v 2 m i (hi i gz i ) m o (ho o gz o ) Qcv W cv dt 2 2
)1(
For exergy balance in a control volume, the following equation can be used [47]:
Q r m i (hi r
dE cs v 2 vi2 gz i ) mo (ho o gz o ) W 2 dt 2
)2(
In control volume for irreversibility explanation the following equation is used [47]: I cv ( m i i m o o ) (1
To )Qcv W cv T
)3(
In exergy calculation of all cycle equipment, we should calculate all exergy flows. Exergy calculation for single-phase flows such as water or steam flow is carried out easily. For this action the following equation is used [47]:
(h ho ) To (s so )
)4(
For transferred exergy by heat [48]: Q Q (1
To ) T
)5(
For solar repowered cycle, the fuel consumption, net produced power and energy and exergy efficiencies are investigated. To calculate the energy and exergy efficiencies, the following relations are used [48]:
15
Pgen
1
)6(
m f LHV Pgen
2
)7(
m f LHV
Two performance indices namely, Energy Performance Index (EnPI), and Exergy Performance Index (ExPI) are defined to account for the contribution of energy and exergy, respectively from the solar collectors to the excess power generated over the design rated capacity in “power boosting mode” for FRSAC.The Energy Performance Index (EnPI) is defined as [48]: EnPI
Pexcess Qs
Where
)8(
is excess power generated over the design rated capacity and
is the energy output of
the solar collector field that is specified as [48]: Qs
Qc
c Where, is the energy output of the solar collector field (MWth) and
)9( is the collector efficiency.
The study assumes parabolic trough being used as the solar thermal energy collector with collection efficiency of 60%. The energy output of the solar collector field is specified as: )10(
Qc m .h
is the mass flow rate of the feed water (kg/s), and Δh is the specific enthalpy gain of the feed
Where,
water across the feed water heater (kJ/kg). The collector area (Ac) required transferring the energy output is calculated as [48]: Ac
Qc S d .c
)11(
where, Sd is the direct irradiation in W/m2. The required land for installing the solar collectors is usually three times as large as the area of collectors [49]. The Exergy Performance Index (ExPI) is defined as [48]: ExPI
Where
Pexcess Ex s
)12(
is exergy input through solar irradiation that specified as [49]: 16
4 Ta Ex s 1 (1 0.28ln f ) Q s 3 Ts
)13(
where, Ta is the ambient temperature (306 K), Ts is the temperature of the Sun (5777 K), and f is the dilution factor (1.3×10−5). Dilution factor is a measure of the mixing ratio of radiation from two sources i.e., the radiance of the sun and the radiance of the surroundings. Since Ts≫Ta, the mixing can be regarded as a dilution of ‘hot’ solar radiation with cold ambient radiation [49]. 6- Results and Discussion In this article, two repowering methods were simultaneously evaluated in a 200 MW steam power plant. Frist, full repowering was executed as the main method and the solar repowering for the HRSG in the new cycle was evaluated. The result was ultimately the same as merging the solar energy into a GTCCPP. The difference is that, here, the steam turbines and the cooling tower are the criteria to design the other parts of the plant. Regarding the limitations in the cycle, a GT model: SGT5-8000H and a one-level pressure reheat HRSG are used for repowering. The solar energy can be used to increase the generation power or decrease fuel consumption. The results were achieved for both states. To valid simulations, firstly the simple cycle of the power plant is simulated and the results are shown in Table. 3 and are comparision with data from existing power plant [33]. Some important flows are chosen and their thermodynamic properties from existing plant and results in simulation are reported. These flows include HPT inlet, IPT inlet, Condenser inlet, BFP outlet, Boiler inlet and power production. As it show, most deviation is related to Boiler inlet, that reaches 3.6 percent. However, the results show that the simulation results are almost the same as the existing power plant and valid and can trust on it. Fig. 3 shows the Q-T diagram for full repowered solar assisted cycle (FRSAC). This figure is for the state where the solar energy is merged into the HRSG. In this diagram, Pinch and Approach temperature differences for HRSG can be easily achieved in order to analyze the thermal behavior of the heat exchangers inside it. Table. 4 show several important parameters in three states including SC, FRC and FRSAC. It is worth mentioning that in all simulations, the temperature of outlet gases from the HRSG is considered fixed at 98 °C. The information on FRSAC belongs to the state in which the 17
mass flow rate of feed water in the solar field is 31.3 kg/s. As can be seen in the table, the steam flow rate in the condenser in all three states is the same and equal to the primary design in the SC. The information on the FRSAC column belongs to the efficiency improvement mode. Therefore, the energy and exergy efficiencies are better than FRC. Table. 5 shows some parameters of the FRSAC in the power increase mode, based on different mass flow rates of feed water in solar field. Moreover, the differences between inlet and outlet temperatures of the cooling tower are calculated. The cooling towers are able to condense 124.4 kg/s of inlet steam to the condenser. In this state, the difference between inlet and outlet lines of cooling tower (
) is
10°C. The cooling towers are designed for ambient temperature equal to 16.1°C [43]. Therefore, in ambient temperatures lower than this, the cooling tower works more efficiently. This only happens over a 5-month period during the year, making it possible to increase power generation of steam turbines to reach peak of 183.7 MW. In power increase mode, the more we can increase the power of the STs, the more we can increase the mass flow rate of feed water in solar field. It is worth mentioning that in all modes, the temperatures of the outlet gases from the HRSG are the same and 98 °C. In Table 5, the amount of absorbed solar heat into the cycle is also shown. By increasing the mass flow rate of feed water to 75kg/s, the solar heat will be 103971 kW. Fig. 4 shows the change in energy and exergy efficiencies in relation to various mass flow rates of feed water in solar field. In efficiency improvement mode, the more the mass flow rate of feed water in solar field, the higher the efficiency will be. If the mentioned flow is zero, the energy and exergy efficiencies will be 59.11 and 56.61 percent, respectively. If the mass flow rate goes up to 31.3 kg/s, these numbers will grow up to 61.09 and 58.5 percent, respectively. Fig. 5 shows the variations in the necessary area of solar collectors as well as the aria land needed for this, in relation to various mass flow rates of feed water in solar field. A feed water flow of 31.3kg/s necessitates 14.43 acres of collectors in 43.39 acres of land. Figs.6 shows the variation of inlet energy and exergy to the FRSAC by solar field. As it show, the rate of exergy is sharper, that shows under this conditions (evaporating steam by solar energy), the merging of solar field with another combined cycle power plant is more efficient in exergy point of view than 18
energy point of view. Fig. 7 shows the variation of excess generated power by solar energy in various mass flow rater of feed water in solar field. The more the mass flow rate in solar field, the more excess generated power. However this graph is generated by assuming that there is not any limitation in generating excess power in power increase mode. Fig. 8 shows the variation of two new defined parameters, EnPI and ExPI. As it show, in general the amount of ExPI is more than EnPI in any mass flow rate of feed water in solar field. Moreover, by increasing the mass flow rate of feed water in solar field, both EnPI and ExPI decrease. This decrease is more sensible in low mass flow rates. This variation in mass flow rates of more than 20 kg/s, has almost a flat behavior. Figs. 9, 10, and 11 show the heat rate (HR), total power generation of the whole cycle and rate of CO2 emission in relation to mass flow rate of feed water in solar field, respectively, for both power increase and efficiency improvement modes. With a mass flow rate of 70 kg/s feed water in the solar field, the heat rate in efficiency improvement and power increase modes will be 5549 kJ/kW.h and 5594 kJ/kW.h, respectively. With a zero mass flow rate of feed water in the solar field, the generation power of the whole cycle (GT and ST) will be 496.51 MW (329.51 MW in GT and 167 MW in ST). If the mass flow rate of feed water in the solar field is 31.3 kg/s, the generation power of the cycle in power increases and the efficiency improvement modes will decrease 16.8 MW and 33 MW, respectively. Regarding the fact that a reduction in mass flow rate of feed water in solar field leads to a fall in heat rate, it brings about the same fall in CO2 emissions, too. These changes are observable in Fig. 11. In power increase mode, the higher the mass flow rate of feed water in solar field, the more power is generated. When this mass flow rate is 31.3 kg/s, the power of the whole cycle increases by 16.8 MW. Considering the operation lifespan of the solar panels to be 300 days/year and 9 hours a day, an extra 45.360.000 kW.h power is generated. Fig. 12 shows the annual profit gained through selling the extra generated power compared to different electricity prices. The average fee has been considered to be between 0.02 and 0.2 USD/kW.h.
19
In efficiency improvement mode, the increase of feed water flow in the solar field decreases fuel consumption and CO2 emissions. Figs. 13 and 14 show the saved fuel and deceased emission over a year, respectively, for different mass flow rates of feed water in solar field. To calculate the benefits of reducing fuel consumption, the price of natural gas is estimated to be between 0.02 and 0.2 USD/kg, in order to present the results for different mass flow rates of the feed water in solar field in Fig. 15. The price of CO2 is estimated around 10 to 100 USD/ton, to show the profits from the prevention of its emission for different mass flow rates of the feed water in solar field, and it is shown in Fig. 16. If the price of natural gas per cubic meter and CO2 per tons are 0.2 and 100 USD, respectively, and the mass flow rate of feed water in solar field is 31.3 kg/s, the saved amount over a year will be 3,603,856 USD. With these prices and a mass flow rate of 75 kg/s, the gain over a year will be 8,635,440 USD. 7- Conclusion A reduction in fossil fuel resources, concerns about the consequence of greenhouse effects, the need to increase power generation in Iran and the policies on increasing efficiency of the thermal power plants are all among factors which justify the need to repower the fossil fuel power plants. On the other hand, Isfahan is located in a suitable region to receive solar energy, which necessitates research and investment in this area. In this study, full as well as solar repowering in the units of Montazeri Steam Power Plant was investigated. From among all the common methods of repowering fossil fuel SPPs, full repowering was chosen as a suitable and reliable method. In the combined cycle resulting from this repowering, solar energy was used to produce steam from a part of the feed water in parallel with drum and evaporator of HRSG. Considering the currently available gas turbines, we selected a SGT8000H for full repowering. This is one of the most advanced ones with a nominal capacity of 400 MW and efficiency of 40% under standard conditions. To choose the number of pressure levels and reheaters in HRSG, we need to take some effective factors into account. The bigger the number of pressure levels is, the more efficient the HRSG becomes. On the other hand, steam turbines (HPT, IPT and LPT) and the cooling tower have been designed for certain proportions of the mass flow rate of steam in each of the turbines. These ratios 20
must be kept as far as possible. So a one-level HRSG with one reheat phase is the best choice. After these choices, the generation power of the ST and GT will be 167 and 329.51 MW, respectively. In this state (FRC without solar energy), the energy and exergy efficiencies will increase by 76.8 and 73 percent; reaching 59.11 and 56.61 percent, respectively. The effect of merging solar energy into the new FRC, which results a SAFRC, in both power increase and efficiency improvement modes has been analyzed. The mass flow rate of feed water in solar field has been considered to be in a range of 0 to 75 kg/s, in order to study the effects that it may have on different parameters of the cycle, including total power generation, energy and exergy efficiencies, input solar heat, area of the collectors and the required land, decrease in CO2 emission and some other parameters. The results indicated that in power increase mode, to prevent the adverse effect of increase in the steam flow rate to the condenser, the mass flow rate of feed water in solar field must not exceed 31.3 kg/s. Under this conditions, the power of the STs increases by 16.8 MW. If the mass flow rate of feed water exceeds the mentioned number, it will need an increase in the cooling capacity of the cooling tower; which is only possible when the ambient temperature is less than 16.1°C. Otherwise, the cooling capacity should increase proportional to inlet solar energy. This can do by installing new packages of cooling cells. In efficiency improvement mode, there are no limitations for the mass flow rate of feed water in solar field. The results show that in this mode, if the mass flow rate of feed water is 75 kg/s, with a constant generation power for the ST, in order to keep the outlet temperature of the gases from HRSG, the generation power of the GT should be reduced by 33 MW; otherwise, the heat loss through stack will increase. As a result, fuel consumption of the GT also decreases by 2.21 kg/s; which reduces natural gas consumption and CO2 emissions by 21,481,200 kg and 43,392 tons per year, respectively (estimating that the solar field is to function 300 days a year and 9 hours a day). Considering the price of natural gas and CO2 to be 0.2 USD/kg and 100 USD/ton, respectively, the profit gained through these savings will be 8,635,440 USD per year. Overall, the follow conclusions are made:
21
1- To increase the efficiency of Montazeri Steam Power Plant it is a very good suggestion to convert the steam cycles of this power plant (eight similar units) to the combined cycles. 2- To lead to use solar energy as a renewable energy in Iran, merging a solar field with this power plant can be a very good and a rational solution. It can decrease the investment and eliminate some related limitations. 3- By considering the solar irradiation potential in Isfahan in Iran, this power plant can be a good choice for solar repowering. 4- Considering all mentioned limitations in this power plant, using a 400 MW gas turbine in full repowering is a very good choice. 5- To keep the ratio of mass flow rates in all turbines and condenser, using a one level pressure HRSG can be the best choices for full repowering. 6- While using a two or three pressure level HRSG is more effective in exergy point of view, but a one level pressure HRSG had some superior advantages. 7- Under the condition with no increase in cooling capacity of the cooling tower, the mass flow rate of feed water in solar field should not be more than 31.3 kg/s
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26
Table Captions Table 1. Operating conditions of the power plant [38] Table 2. The effect of ambient temperature on operational conditions of SGT5-800H GT Table 3. Comparision of simulation results with existing power plant Table 4. Some major parameters of simple, full repowered and full repowered-solar assisted cycle of the power plant Table 5. Some parameters of full repowered-solar assisted cycle of the power plant in power increase mode
Figure Captions Fig1. Schematic diagram of the power plant [43] Fig2. Schematic of final design for full repowered-solar assisted cycle of the power plant, (simulated in Cycle Tempo) Fig3. Q-T Diagram of the HRSG in Full repowered-solar assisted cycle of the power plant Fig4. Net energy and exergy efficiencies vs. the flow rate of feed water in solar field in both power increase and efficiency improvement modes Fig5. Area of solar colectors and requirement land in both power increase and efficiency improvement modes Fig6. Inlet energy and exergy to the cycle by solar field vs. the mass flow rate of feed water in solar field Fig7. Excess generated power by solar enery vs. the mass flow rate of feed water in solar field Fig8. Variation of EnPI and ExPI parameters vs. the mass flow rate of feed water in solar field Fig9. Variation of heat rate of the cycle (kJ/kW.h) vs. the flow rate of feed water in solar field in both power increase and efficiency improvement modes Fig10. Variation of power production vs. the flow rate of feed water in solar field in both power increase and efficiency improvement modes Fig11. Variation of rate of CO2 production (kg/kW.h) vs. the flow rate of feed water in solar field in both power increase and efficiency improvement modes Fig12. Annual income with electricity sell vs. the electricity price in different flow ratees of feed water in solar field; in power increase mode Fig13. Variation of annual fuel saving vs. the rate of feed water in solar field in efficiency improvement mode Fig14. Annual decrease in CO2 production in efficiency improvement mode Fig15. Annual income with fuel conservation vs. the price of natural gas in efficiency improvement mode Fig16. Annual income with CO2 emission prevention vs. the price of CO2 in efficiency improvement mode
27
Table Legends Table 1. Operating conditions of the power plant [33] Operating conditions Power produced Power consumption Volumetric flow rate of fuel(Natural gas) Heat rate Stem flow rate, main line Steam pressure, main line Steam temperature, main line Water temperature, to boiler Stack gas temperature Volumetric flow rate of inlet air to burners Number of induced and draft fans Number of burners Combined pump/motor efficiency
value 200 14 54*103 10448.6 670 130 540 247 160 9.6*105 2 12 95
unit MW MW Nm3/h kJ/kW.h Ton/h Bar °C °C °C Nm3/h %
Table 2. The effect of ambient temperature on operational conditions of SGT5-800H GT Tamb (oC)
Power MW
Efficiency( 1) %
-11 -5 1 5 11 15 21 25 31 35 41
410.73 395.56 379.9 352.9 341.02 329.53 318.4 307.73 297.39 287.3 277.7
41.25 41.09 40.91 40.23 40.04 39.84 39.6 39.44 39.23 39 38.8
Combustion products T (oC) (kg/s)
794.9 779.76 705 743 729.6 716.76 714 092 680.5 009 658.8
031 629.7 629.3 628.7 628.4 628.2 627.9 627.6 627.4 027 626.8
Table.3 Comparision of simulation results with existing power plant flow
parameter
unit
Existing plant[33]
simulation
Deviation (%)
Produced Power
---
MW
200
200
0
kg/s o C bar kg/s o C bar kg/s o C bar kg/s o C bar kg/s o C bar
173.08 541 131 149.03 541 24.05 173.08 254.07 171 120.97 43.9 0.094 173.08 166.6 185
174.313 535.41 131 153.16 541 24 174.313 245.37 172 124.4 44.59 0.094 174.313 168.27 183.8
0.7 0.8 1 2.5 1 0.2 0.7 3.6 1.17 2.8 1.58 1 0.7 1 0.64
HPT inlet
IPT inlet
Boiler inlet Condenser inlet BFP outlet
28
Table 4. Some major parameters of simple, full repowered and full repowered-solar assisted cycle of the power plant parameter PST PGT QS Ptot
unit MW MW MW MW %
1
%
2
Kg/s Kg/s
HR
kJ/kW.h
SC
211 ----------211 33.4 32.6 125/4 181.6 11133
FRC
FRSAC
107 329.51 -----496.51 59.11 56.61 113.7 118.79 0111
183.8 298.57 43.39 482.37 61.09 58.5 125.4 130.99 5893
Table 5. Some parameters of full repowered-solar assisted cycle of the power plant in power increase mode
Kg/s
1 5 11 15 21 25 31 31/3 35 41 45 51 55 01 05 71 75
PST MW 167 169.8 172.4 175.1 177.7 180.4 183 183/8 185.7 188.3 190.9 193.5 196.2 198.8 201.4 204 206.5
MW 0 6931 13862 20794 27725 34656 41588 43390 48519 55451 62382 69314 76245 83176 90108 97039 103971
29
Kg/s
Kg/s
113.73 115.55 117.46 119.33 121.14 123.01 124.84 125.4 126.74 128.58 130.41 132.25 134.16 136 137.84 139.68 141.47
9.06 9.21 9.36 9.51 9.66 9.8 9.95 11 10.1 10.25 10.39 10.54 10.69 10.84 10.99 11.13 11.28
Figure Legends
Fig1. Schematic diagram of the reference power plant [33]
Fig2. Schematic of final design for full repowered-solar assisted cycle of the power plant with thermodynamic properties of the flows, (simulated in Cycle Tempo)
30
Fig3. Q-T Diagram of the HRSG in Full repowered-solar assisted cycle of the power plant
66 Energy - Power Increase Mode Energy - Efficiency Improvement Mode Exergy - Power Increase Mode Exergy - Efficiency Improvement Mode
Efficiency (%)
64
62
60
58
56 0
20
40
60
Feed Water to Solar Field (kg/s) Fig4. Net energy and exergy efficiencies vs. The mass flow rate of feed water in solar field in both power increase and efficiency improvement modes
31
120 Required Solar Collectors Required Land
100
Area (ha)
80
60
40
20
0
0
20
40
60
80
Feed Water to Solar Field (kg/s) Fig5. Area of solar colectors and requirement land in both power increase and efficiency improvement modes
1e+5
Energy Exergy
Inlet (kW)
8e+4
6e+4
4e+4
2e+4
0
0
20
40
60
80
Feed Water to Solar Field (kg/s) Fig6. Inlet energy and exergy to the cycle by solar field vs. the mass flow rate of feed water in solar field
32
Excess Power Generated (MW)
40
30
20
10
0 0
20
40
60
80
Feed Water to Solar Field (kg/s) Fig7. Excess generated power by solar enery vs. the mass flow rate of feed water in solar field
0.8
EnPI ExPI
Performance Index
0.7
0.6
0.5
0.4
0
20
40
60
80
Feed Water to Solar Field (kg/s) Fig8. Variation of EnPI and ExPI parameters vs. the mass flow rate of feed water in solar field
33
6000 Power Increase Mode Efficiency Improvement Mode
Heat Rate (kJ/kW.h)
5900
5800
5700
5600
5500
0
20
40
60
Feed Water to Solar Field (kg/s) Fig9. Variation of heat rate of the cycle (kJ/kW.h) vs. the mass flow rate of feed water in solar field in both power increase and efficiency improvement modes
Total Power Production (MW)
540
520
500 Power Increase Mode Efficiency Improvement Mode 480
460
440
420 0
20
40
60
Feed Water to Solar Field (kg/s) Fig10. Variation of power production vs. The mass flow rate of feed water in solar field in both power increase and efficiency improvement modes
34
Rate of CO2 Production (kg/kW.h)
0.41 Power Increase Mode Efficiency Improvement Mode 0.40
0.39
0.38
0.37
0
20
40
60
80
Feed Water to Solar Field (kg/s) Fig11. Variation of rate of CO2 production (kg/kW.h) vs. the mass flow rate of feed water in solar field in both power increase and efficiency improvement modes
Fig12. Annual income with electricity sell vs. the electricity price in different mass flow rates of feed water in solar field; in power increase mode
35
7e+7
5e+7
3
Annual Fuel Saving (m /year)
6e+7
4e+7 3e+7 2e+7 1e+7 0
0
20
40
60
Feed Water to Solar Field (kg/s) Fig13. Variation of annual fuel saving vs. the mass flow rate of feed water in solar field in efficiency improvement mode
Annual Decrease of CO 2 Production (ton/year)
1.4e+5
1.2e+5
1.0e+5
8.0e+4
6.0e+4
4.0e+4
2.0e+4
0.0 0
20
40
60
Feed Water to Solar Field (kg/s)
Fig14. Annual decrease in CO2 production per year in efficiency improvement mode
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Fig15. Annual income with fuel conservation vs. the price of natural gas in efficiency improvement mode
Fig16. Annual income with CO2 emission prevention vs. the price of CO2 in efficiency improvement mode
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Research highlights
Synchronous execution of full repowering and solar assisting for this power plant is evaluated A 400 MW gas turbine is chosen for full repowering Solar energy is used for evaporating a part of feed water in parallel with HRSG Annual effects on fuel consumption reduction and Decrease in CO2 emission for different sizes of solar fields are investigated The effects of number of pressure levels of HRSG on cycle performance are evaluated
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