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cooling: Evaluation of performance variation
Applied Thermal Engineering 86 (2015) 35e42
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
Renovating thermal power plant to trigeneration system for district heating/cooling: Evaluation of performance variation Hasan Huseyin Erdem a, *, Ali Volkan Akkaya a, Ahmet Dagdas a, Suleyman Hakan Sevilgen a, Burhanettin Cetin a, Bahri Sahin b, Ismail Teke a, Cengiz Gungor c, Selcuk Atas c, Mehmet Zahit Basak a a b c
Department of Mechanical Engineering, Yildiz Technical University, 34349 Besiktas, Istanbul, Turkey Department of Naval Architecture, Yildiz Technical University, 34349 Besiktas, Istanbul, Turkey TUBITAK Marmara Research Center, 41470 Gebze, Koceli, Turkey
h i g h l i g h t s Two new performance parameters are defined for renovated power plants. The first parameter is named “The comprehensive thermal efficiency”. The second parameter is named “coefficient of performance for heating/cooling”. According to analysis all examined power plants can be converted to co-tri generation plant.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2014 Accepted 17 April 2015 Available online 27 April 2015
This paper presents performance assessments of thermal power plant-based co/tri-generation systems for district heating/cooling system. The power plants were originally designed exclusively for the generation of electricity. With respect to the renovation of power plants to co/tri-generation systems, the analysis of performance variations in the systems has been undertaken. For the purpose of simulation analysis, thermodynamic models of the eight thermal power plants have been developed. The performance variations have been evaluated with different performance criteria, including electrical power output, classical thermal efficiency, coefficient of performance and comprehensive thermal efficiency. The comprehensive thermal efficiency takes into account all products (electricity, heating and cooling energy) generated from the power plant-based tri-generation system. The results of analysis show that the comprehensive thermal efficiencies of the eight considered systems range from 49% to 61% in the heating mode, although their generated electrical power amounts decrease slightly. As a result, this type of modification for an existing power plant can greatly benefit the cause of energy efficiency and sustainable development. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermal power plant District heating/cooling Cogeneration Trigeneration
1. Introduction Thermal power plants (TPPs) are used globally for the generation of electricity. Statistically, 40% of the global electricity production is provided by coal-fueled power plants, and that percentage is projected to rise in the near future [1,2]. Thus the development of methods to increase the efficiency of fuel conversion is crucial, due to the limitations of fossil-fuel sources and the
* Corresponding author. Tel.: þ90 0212 3832907; fax: þ90 0212 2616659. E-mail address: [email protected] (H.H. Erdem). http://dx.doi.org/10.1016/j.applthermaleng.2015.04.030 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
growing pressure for a cleaner environment. From the perspective of sustainable development, the design of new TPPs as co/trigeneration systems that can produce both electricity and district heating/cooling energy would be a significant step toward the greater utilization efficiency of domestic fossil fuels. Indeed, many power plants were originally designed to produce electricity and district heating/cooling [3e5]. In these types of plants, the extracted steam used for feed-water heating was widely employed for district heating/cooling. Naturally, at the design stage it is possible to specify the most convenient point and amount of steam extraction without negatively affecting plant performance.
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Nomenclature COP _ W _ Net W
Q_ h m_ r
coefficient of performance () power input/output rate (kW) net rate of power output (kW) heat transfer rate (kW) specific enthalpy (kJ/kg) mass flow rate (kg/s) extracted steam rate (%)
Subscripts A auxiliary machines C cooling mode DC district cooling DH district heating
Given this context, the ultimate conversion from the existing TPPs to co/tri-generation systems for the supply of district heating/ cooling energy could be very beneficial in terms of energy efficiency. However, the open literature provides relatively little information about upgrading an existing TPP to a co/tri-generation system and its effects on plant performance. In the cited study [6], some experience with the coal-fired power plant converted to a cogeneration system for the supply of district heating to the city of Kozani, Greece, was given. However, performance analysis was not given for the converted plant in that study. A nationwide research project called TSAD (Utilization of Thermal Power Plant Surplus Heat) was, however, conducted by the authors of this paper during the period of 2006 through 2011 in Turkey. In that project, we systematically examined the question of how the existing power plants generating only electricity could be transformed for district heating/cooling applications [7,8]. Initially, the performance of TPPs under the control of governmental bodies in Turkey were modeled, simulated and analyzed from the energetic and exergetic viewpoints [9]. Then, waste-heat potentials and other heat-extraction capabilities in the power plants were methodically evaluated with respect to co/tri-generation conversion [10,11]. The most effective steam-extraction point for district heating/cooling system was identified [12]. As a result of the detailed technical and economical assessments, the Soma thermal power plant was selected for a pilot application. The conversion process was successfully implemented to compensate the heating requirement of Soma district in Manisa, Turkey [13]. It should be emphasized that if the renovation plans can be applied for all TPPs under government control in Turkey, it will be possible to save annually 7e27 million MWth energy. Each of the TPPs in the project scope has 15,000e140,000 housing district heating potential. The economic potential of heating the residences from the power plants ranges from US$ 100 to $550 million annually. Moreover, the hazardous emissions can be reduced considerably by means of the district heating system. It can provide annually the reduction for 1.5 million to 6 million tons of CO2 and 37,000 to 150,000 tons of SO2. Moreover, it is possible to reduce water consumption by approximately 4 million to 17 million tons/year in the renovated power plants. Some technical conversions in the existing TPPs designed for electricity generation are necessary to use in the district heating application. These conversions will affect the power plant performance. Performing analysis through classical performance criteria (power output and thermal efficiency) considering only electricity generation may lead to wrong evaluations. The effects of converting
H M T th si so s ds c f
heating mode machine used in residential zone for heating/cooling turbine thermal steam inlet steam outlet steam extracted steam condenser fuel
Greek letter difference efficiency
D h
to trigeneration system should be taken into account in the analyses. Therefore, in order to evaluate correctly the performance of power plant-based cogeneration system producing simultaneously both electricity and heat energy, a criterion should be expressed [14e16]. An evaluation of the open literature pertaining to co/trigeneration systems also involved energyeexergy analyses and different objective function-based optimization studies for various regional, commercial, industrial and micro-cogeneration systems [18e26]. In the scope of our study, the conversion of coal-fired thermal power plants (designed to generate only electricity) to co/tri-generation systems was considered for the purpose of district heating and cooling. In the case of such a conversion, the performance indicators have been discussed. There is an important difference in this type power plant conversion study by comparison to the co/tri-generation studies in the cited literature. This difference is that the extracted steam from the turbine, in the case of conversion, is used for a different purpose in place of electricity production, which is the main purpose of the plant. However, the second product (heating or cooling energy) for the co/trigeneration systems considered in the literature is only produced from the energy discharged through the generation of electrical power. This situation does not affect the electricity production in such systems. Therefore, in our study the performance parameters have been defined, taking into account the aforementioned circumstance, and used to analyze the considered thermal power plants. Thus, in this study the changes in performance of the TPPs converted to trigeneration systems to meet the heating/cooling demand in residential areas have been investigated with respect to different criteria. From that perspective, thermodynamic-based simulation models of the eight TPPs under government control were developed. The effects of the energy taken from the specified location of the power plant for the achievement of district heating/ cooling on the performance of trigeneration were analyzed through simulations. The variations in net electrical power output, classical thermal efficiency, utilization factor and comprehensive thermal efficiency considered in this study were analyzed as performance criteria. 2. Renovating existing power plants for district heating applications The authors of this paper have conducted a nationwide project for the purpose of designing a district heating/cooling system using
H.H. Erdem et al. / Applied Thermal Engineering 86 (2015) 35e42
existing TPPs as heat sources [7]. In the scope of the study, the methods that could meet the district area energy demands from TPPs with respect to technical and economic aspects were investigated. The most critical point was to determine the heat-source location in the power plant for district heating application. According to results of these studies [7e13], it has been identified that steam extraction from the inlet stage of the low-pressure turbine is the most convenient location for use as a heat source. At that point the steam is of a temperature suitable for district heating/cooling. Additionally, the physical and technical conditions of the power plant make it possible to extract the steam from this point without an operation problem. Consequently, this renovation will be able to satisfy the heating demand posed by residential areas close to TPPs. Fig. 1 illustrates the layout of a converted system, including the power plant and district heating sections. In this method a portion of the steam going to the low-pressure turbine is extracted by means of a technical modification. The steam thus taken is used to heat the water for the district heating system. For that reason, a condenser is added. The latent heat of this steam is transferred to the water of the district heating system through the process of condensation. The condensed steam, being transferred through an additional pipeline, is connected to the feed-water heater system in the most appropriate location. Hot water through the transmission line is carried to heating zone. The hot water that reaches the houses via regional distribution line is used for heating. The water supplied via the distribution line is sent to the condenser through the transmission line in order to heat it again.
3. Thermal integration of the thermal power plant to district heating/cooling system Our study considered eight coal-fired thermal power plants (TPPs) in Turkey for the analysis of performance change. Table 1 gives the characteristics of the subject coal-fired TPPs and the technical information of the nearby district heating/cooling areas. The average age of these power plants is greater than 15 years. The aforementioned power plants were designed for sub-critical steam conditions. These coal-fired TPPs generally use the lignite type of coal. The maximum heating and cooling requirements of the district area near the considered power plant were determined based on climate characteristics and other important technical parameters.
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The simulation models based on thermodynamic principles were coded by the EES program [17] for every TPP taken into account, and the simulation results were validated with respect to the design conditions [9]. For this study the simulation models were modified in order to evaluate the TPP-based district heating/cooling system. In order to convert the existing plants to co/tri-generation systems, the required components as shown in Fig. 1 were added to the system simulation models. In the altered model developed for analyzing the considered trigeneration systems, a certain amount of steam is extracted from the inlet of the low-pressure turbine, depending on the district heating/cooling load. The effects that the steam extraction at the specified point had on the performance of the considered TPPs could therefore be evaluated according to the simulation models we developed. The extracted steam flow rate ðm_ ds Þ can be expressed for district heating and district cooling, as shown in the following equations:
m_ ds ¼
Q_ DH ðhsi hso Þ
(1)
m_ ds ¼
Q_ DC COPabs ðhsi hso Þ
(2)
where hsi represents the specific enthalpy of the extracted steam, hso denotes the specific enthalpy of the extracted steam at the exit of district heating condenser or absorption chiller. COPabs is the performance coefficient of the absorption chiller. The specific enthalpy of the extracted steam (hsi) varies depending on the design conditions (pressure, temperature) of the thermal power plant. The steam extracted through the district heating condenser is assumed to cool until it is saturated water. Thus the specific enthalpy at the condenser outlet is determined. In other words, the latent heat of the extracted steam is used as an energy source for district heating/cooling. The COPabs value for a two-stage absorption chiller is approximately 1. As seen in Eq. (3), the extracted steam ratio (r) is determined from dividing the steam mass flow rate ðm_ ds Þ extracted from the inlet of the low-pressure turbine for district heating/cooling to the steam mass flow rate ðm_ s Þ at the exit of the intermediate-pressure turbine.
Fig. 1. A simplified schema of renovated thermal power plant for district heating application.
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Table 1 The technical information about the considered thermal power plants and district heating/cooling area. Coal fired thermal power plantsa
Technical data
Total power (MW) Unit number Unit power (MW) Thermal efficiency (%) Exergy efficiency (%) District heating/cooling area near the power plant Distance from the power plant (km) Housing number Maximum heating requirement (MWth) Maximum cooling requirement (MWth) a
Ya
Se
Can
Ca
Ka
AE
So
Tu
630 3 210 37.01 31.95 Yatagan Mugla 4 30 16,800 128 29
600 4 150 38.03 31.50 Kutahya
320 2 160 42.12 37.88 Can Yenice 14 32 13,979 107 19
300 2 150 37.88 35.19 Catalagzi Zonguldak 5 15 45,600 318 29
457 3 157 37.19 28.55 Kangal Sivas 10 37 26,472 329 32
1440 4 360 42.64 32.46 Afsin Elbistan 20 15 33,840 249 68
990 6 165 36.08 32.35 Soma Turgutalp 5 10 24,892 179 49
429 5 160.9 38.44 33.09 Tuncbilek Tavsanli 0 10 25,300 248 30
30 74,678 731 87
Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
r¼
m_ ds m_ s
(3)
The r value varies depending on district heating/cooling load. The steam mass flow rate passed from the low-pressure turbine is reduced, depending on the value of r. As a result, the low-pressure _ net Þ: turbine achieves a power reduction ðDW
_ net ¼ r$m_ s ðh hc Þ DW si
(4)
where hc is the specific enthalpy at the output of the low-pressure turbine.
the same amount of fuel. This is known as a tri-generation system. In this new system electricity, heating and cooling energy are produced concurrently, but the fuel energy given to the system does not change. Thus the performance of the new system isn't measured or evaluated on the basis of a single product. Accordingly, in this study a performance criterion called comprehensive thermal efficiency was used for the evaluating performance variations among co/tri-generation systems, including those of the renovated TPP and district heating/cooling system. Equation (3) expresses the comprehensive thermal efficiency (hth,H) in the case of a system operating in the heating mode:
hth;H ¼
4. Performance evaluation parameters
_ NET W _ Q_ fuel QhDH
(7)
M
4.1. Net power and classical thermal efficiency Thermal power plants (TPPs) usually generate only electricity. Hence, two important performance criteria can be considered, i.e., net power output and thermal efficiency. The net power output can be expressed by subtracting the consumed auxiliary machines power from the obtained turbine power output:
_ Net ¼ W
X
_ T W
X
_ W A
(5)
where the subscripts of T and A represent the turbine and auxiliary machine, respectively. The classical thermal efficiency of a TPP is expressed with the ratio of the obtained net power rate to the fuel heat rate:
hth
_ Net _ DW W ¼ Net _ Q
(6)
f
where Q_ f symbolizes the fuel heat transfer rate given to the boiler of the TPP. When the thermal power plant generates only electricity, the _ net value is zero because the extracted steam mass flow rate DW _ net value can be calculated from Eq. becomes zero. However, the DW (6) in the case of a thermal power plant that functions as a co/trigeneration system. 4.2. Comprehensive thermal efficiency for the co/tri-generation system The addition of a district heating/cooling system to an existing TPP (generating only electricity) increases the number of products (electricity, heating and cooling energy) obtained from the plant for
where hM represents fuel-burning efficiency for heating machines or devices such as stoves, cab heaters, etc. As is shown in Eq. (7), comprehensive thermal efficiency for heating mode is computed by subtracting the thermal heat rate ðQ_ DH =hM Þ of the residential-heating fuel from the rate of fuel combustion ðQ_ fuel Þ at the power plant. Thus the amount of fuel energy used simply to generate electricity can be equally converted to beneficial energy (electricity). Additionally, in regard to the cooling mode, the comprehensive thermal efficiency (hth,C) can be expressed in the following way.
hth;C
_ NET þ Q_ DC W COPM ¼ Q_
(8)
fuel
The defined comprehensive thermal efficiency for the cooling mode is calculated by adding the electricity consumed for cooling of the residential zone Q_ DC =COPM to the electricity generated from the power plant. Because electricity generation and the electricity required for district cooling are provided by the power plant, one can identify how much of the fuel energy given to the power plant is converted to total beneficial energy (electricity). 4.3. Coefficient of performance for the co/tri-generation system The performance of a district heating/cooling system implemented through heat extraction from a power plant is compared to that of the mechanical heating/cooling system. Accordingly, a performance criterion for the investigated system is defined as a similar approach with the coefficient of performance for heating (COPDH) and cooling (COPDC) mode, being expressed in the following way:
H.H. Erdem et al. / Applied Thermal Engineering 86 (2015) 35e42
COPDH ¼
Q_ DH _ NET DW
and COPDC ¼
Q_ DC _ NET DW
(9)
_ NET is the power decrease due to the heat extraction from where DW the TPP. The coefficient of performance of mechanical heating/cooling systems shows the obtained heat energy per unit electricity energy. Accordingly, the Eq. (9) expresses the obtained heat energy per electricity power loss in the tri-generation system comprised of a TPP-based district heating system. 5. Results and discussion The various performance criteria used in this study, given the aforementioned generated products, were considered and applied to eight existing coal-fired thermal plants by simulation models based on thermodynamics in order to evaluate the performance of a renovated power plant. The important results have been given and discussed. Fig. 2a shows that the obtained electrical power output from the analyzed plants decreases with the increment of the extracted steam for district heating/cooling applications. The extracted steam rate is defined as the ratio of steam flow for district heating/cooling purposes to the entirety of steam flow that passes through the turbine. This ratio indicates the different district heating/cooling demands. The reason for this decline is that the extracted steam reduces the rate of steam flow that passes through the lowpressure turbine and therefore decreases the power output that can be generated from the turbine. Because the power obtained from the turbine is linear in proportion to the steam mass flow rate, the increased amount of extracted steam causes a corresponding change in power reduction to occur at the turbine. Given the same extracted steam rate, different power-decrease values between the power plants occur due to their different LPT conditions. However, given the lower pressure and temperature of
39
extracted steam, the power decline is also less. For example, in order to meet the 30 MW heating demand (generally equivalent to the heat requirement of 6000 apartments in the climate conditions of Turkey), the ratio of required steam extraction has historically been 10% for the Yatagan thermal power plant (Table 2.) In this steam-extraction condition, it is seen from Fig. 2a that the unit power reduction is approximately 4.46 MW (2.12%). The decrease in power generation is an issue only when heating or cooling demand is at the peak state. When the annual average demand is considered, the annual average reduction in electricity power is significantly less. However, this change at power plants can be assessed as a power reduction, given the coincident peaks of electricity and heating/cooling demand. Such a condition is rarely experienced in Turkey. If it is possible to overlap the demand peaks, the use of a heat-storage system could prevent the imbalance between the electricity demand and the supply. The demand for electrical power for air-conditioning systems in residential zones can be decreased when steam extraction is used as an energy source. This situation produces a reduction in the power required from the subject power plant. In that context, the addition of a district cooling system to the power plant may reduce not only the generation of electrical power but also the demand for electricity. Steam extraction for the purpose of district heating/cooling can influence the classical thermal efficiency of the considered power plants as well as the generation of electricity. Fig. 2b illustrates the change of classical thermal efficiencies for the considered TPPs under conditions of steam extraction. The figure demonstrates that the thermal efficiencies of the power plants decrease proportionally to the increase of extracted steam ratios for all power plants. The reason for this decrease trend can be explained in the following manner: The plant modification made for a district heating/cooling system causes an increase in the feed-water temperature at a point prior to the boiler inlet. Thus it serves to decrease the supply of heat required by the boiler. However, the effect of power reduction at the turbine is greater than that of the reduction in the boiler's heat
Fig. 2. Performance variation of power plants generating only electricity with respect to steam extraction, i.e., (a) net power reduction and (b) classical thermal efficiency.
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Table 2 District heating/cooling energy potential with steam extraction from the power plants. Extracted steam ratio (r) (%)
5 10 15 20 25 30
District heating/cooling energy potential (MW) AE
Can
Ca
Ka
Se
So
Tu
Ya
25.4 50.8 76.2 101.6 126.9 152.3
11.8 23.8 35.8 47.9 60.1 72.4
13.2 26.5 39.8 53.2 66.8 80.3
12.6 25.2 37.8 50.6 63.3 76.2
11.5 23.1 34.8 46.5 58.2 70.1
13.1 26.2 39.3 52.6 65.7 78.9
12.2 24.5 36.7 48.9 61.2 73.4
15.3 30.5 45.8 61.0 76.3 91.6
*Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
supply. When the steam-extraction ratio is changed from 0.0 to 0.3, the classical thermal efficiency reduction becomes an average of 1% for all considered plants. The classical thermal efficiency decreases from 36% to 35.4% with heating for 7000 residences by the Yatagan power plant. The peak value is 7000, but it does not remain constant throughout the year. However, this decrease is less than 0.6% point when one considers annual heating. The classical thermal efficiency takes into account only the efficiency of the power plant; it does not consider other benefits based on steam extraction. Therefore, it isn't a good performance criterion for renovated power plants. Fig. 3 shows the change in the comprehensive thermal efficiencies of the analyzed TPPs with respect to the increase of the extracted steam mass ratio in the heating and cooling modes. In the heating-mode evaluation, the stove, cab heater and lignite-fueled boiler are considered as conventional heating systems. The burning efficiencies of these conventional systems range from 50% to 70%. In this study the efficiency value is taken as 50%. As shown in the heating mode of Fig. 3a, the comprehensive thermal efficiencies defined for the developed trigeneration systems increase with the increment of steam extraction. While the steamextraction ratio is increased from 0.0 to 0.3, the increase variation of the comprehensive thermal efficiency is approximately 4% for all
considered plants in the case of the heating mode. Moreover, it can be said that the comprehensive thermal efficiency is higher at the same steam extraction when the burning efficiency of classical heating systems in residential zone is lower than considered in this study. The reason is that the fuel requirement for a classical heating system grows as its efficiency decreases. In the cooling-mode analysis (Fig. 3b), the coefficient of performance for a classical cooling system in a residential zone is taken as 2. It can be seen from this figure that the comprehensive thermal efficiency of the power plant-based trigeneration system increases with the increment of steam extraction. For example, the efficiency of the Seyitomer power plant increases from 38% to 43% when operated in the cooling mode. As Fig. 3 shows, the rate of increase in comprehensive thermal efficiency with steam extraction is greater than the decrease in the classical thermal efficiency (Fig. 2b) for all power plants. For the 10% extracted steam rate, the comprehensive thermal efficiency increases from 36% to 39% at the Yatagan power plant. The difference is 3%, which is much greater than the decrease in classical thermal efficiency (0.6%). Accordingly, if a power plant is converted to a cotri generation plant, it makes better use of fuel energy, based on the separate generation mode. Therefore, such a transformation is sustainable.
Fig. 3. The variation of comprehensive thermal efficiency with regard to extracted steam, (a) in district heating mode, (b) in district cooling mode.
H.H. Erdem et al. / Applied Thermal Engineering 86 (2015) 35e42
Fig. 4 shows the distribution percentage of the fuel-energy amount on one of the power plants with respect to the ratio of steam extracted from the inlet of the low-pressure turbine. It is seen from this figure that the steam-extraction increment decreases in terms of the power output obtained from the turbine (shown in Fig. 2a). However, because the steam pressure and temperature are lower at that point, the decrease in power is relatively slight. Moreover, significant gains are possible despite such a decrease. It is evident from this figure that the extracted steam amount significantly limits the energy discarded from the condenser to the environment. In other words, the majority (approximately 80%) of the thermal energy required for district heating will be met by the energy discarded through the condenser. This situation can effectively reduce the condenser load, whereupon the condenser operation pressure can approach the ideal condition. Thus it is possible to achieve a performance improvement in electrical power output. The COP (Coefficient of Performance) values of the power plantbased tri-generation systems are shown in Table 3. For example, when the extracted steam is used for heating aim, the COPHeating value 5.4 for the Soma power plant. In the conventional method involving a mechanical cooling system, this value is approximately 2.6e3.2. Therefore, it proves that the district heating modification in the power plant can reduce the consumed energy for heating in residential areas. However, the classical method employs a mechanical cooling system to compensate the cooling requirement in residential areas. The COP values of these classical systems are between 2.5 and 2.62. In Table 3, for example, the coefficient of performance (COPCooling) of the Yatagan power plant is 4.79 and 6.84 for the one- and two-stage absorption chillers, respectively. When these values are compared against the mechanical cooling systems used in residential zones, it can be seen that the developed system has a high COP value. With the power plant-based district cooling system developed through our study, it can be observed that the electricity energy consumed for cooling can be dramatically reduced. The COPDH value in Table 3 is identical to the COPDC value of the two-stage absorption chiller. This situation occurs when the COP value of the two-stage absorption chiller is 1. In this case, the QDH
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Table 3 Coefficient of performance values for power plant based tri-generation systems. Thermal power plantsa
COPDH COPDC
One stage Two stage
Ya
Se
Can
Ca
Ka
AE
So
Tu
6.84 4.79 6.84
5.45 3.80 5.45
4 2.8 4
4.2 2.9 4.2
5 3.5 5
5.2 3.63 5.2
5.4 3.7 5.4
4.2 2.9 4.2
a Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
and QDC values are equal. The difference between the COPDH values of the power plants is due to the thermodynamic conditions of steam extraction. The low steam pressure and temperature values increase the COPDH value. Table 4 shows the technical modification results for the considered power plants in the context of the maximum district heating/cooling demands in the related residential areas. Load demands for heating and cooling are expressed as peak values in the table. It is seen that all power plants have capacities sufficient to meet the heating and cooling requirements of their nearby cities. The table shows that all eight power plants, in terms of performance parameters, can be converted to co-tri generation plants. Therefore, the results reveal that all old power plants can be converted to co-tri generation plants. The decrease in the demand for district electric power in the case of compensation of maximum cooling demand from the plant (MWe) is 14.5 for the Yatagan power plant. The reduction of plant electrical power at maximum cooling demand (MWe) is 4.2 for the same power plant. Thus the increased supply of plant electrical power at maximum cooling demand (MWe) is approximately 10.3 MWe. 6. Conclusion The conversions of the eight existing thermal power plants (TPPs) to co/tri-generation systems for district heating/cooling have been evaluated in terms of performance variations through means of the developed thermodynamic simulation models presented in
Fig. 4. Distribution of fuel energy on power plant with regard to the extracted steam ratio.
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H.H. Erdem et al. / Applied Thermal Engineering 86 (2015) 35e42
Table 4 Compensation of district heating/cooling demand from the power plant. Operation Mode Parameters
Thermal power plantsa Ya
Heating
Cooling
a
Se
Can
Ca
Ka
AE
So
Tu
Number of steam extracted power plant unit 2 4 2 2 3 2 3 2 Steam extraction ratio for each unit (%) 21 50 22.5 50 43.5 25 22.8 50 Compensation ratio of heating requirement from the plant (%) 100 63.2 100 83 100 100 100 100 Reduction of plant electrical power at maximum heating demand (MWe) 9.4 21.6 12.15 32.4 22.37 24.5 11.23 29.6 Discharged heat reduction through condenser at maximum heating demand (MWt) 58 96.7 41.7 103.2 88.7 100.2 47.6 99.8 Comprehensive thermal efficiency (%) 44.34 56.34 48.84 53.87 50.36 56.04 44.74 53.9 Number of steam extracted power plant unit 1 2 1 1 1 1 1 1 Steam extraction ratio for each unit (%) 9.5 18.8 8 10.9 12.7 13.4 18.7 12.2 Compensation ratio of cooling requirement from the plant (%) 100 100 100 100 100 100 100 100 Supply increase of plant electrical power at maximum cooling demand(MWe) 10.3 35.4 5.2 7.6 9.6 20.9 15.3 7.8 Discharged heat reduction through condenser at maximum cooling demand (MWt) 26.2 35.8 14.7 22 25.6 53.7 39 24.3 Comprehensive thermal efficiency (%) 37.07 41.72 43.49 38.56 39.24 44.29 39.03 40
Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
this study. It is concluded from the analysis that the modification of TPPs to co/tri-generation systems for district heating/cooling would provide important advantages. This conversion does not significantly affect power plant operation or electricity generation but can actually provide additional production such as heating and cooling energy without increasing the consumption of fuel. Consequently, this conversion serves to increase power plant income. In regard to the district areas, the use of the extracted steam from TPPs as a heat source for district heating will remove the fuel used for the heating of residential centers. This advantage alone could provide a significant contribution to the national economy and benefit sustainable development. Additionally, this renovation could contribute a number of crucial and strategic advantages, such as reducing harmful emission emitted to atmosphere, increasing life quality/comfort, decreasing human healthcare costs due to harmful emissions and reducing the dependence on foreign energy sources. As a result, this type of conversion can be implemented in terms of performance and potential. However, an economical evaluation that takes into account all the investment parameters in the power plant and region should be performed prior to any investment decision. Further, such a study should encompass all the environmental effects on nearby cities.
Acknowledgements The research reported in this paper was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under public research grants (Project No.: 105G099). Also, the authors acknowledge the support provided by the related plant and Turkish Electricity Generation Co. Inc. (EUAS).
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[7] Research and Development of Methods to Benefit from Thermal Power Plant Surplus Heats and District Heating Application, 2007. TUBITAK Project No. 105G099. Progress Report No: 2 (in Turkish). [8] H.H. Erdem, Technical analysis of converting Yatagan thermal power plant for the district heating aim, in: First District Heating and Cooling Conference for Turkey, 01e02 May 2009. Mugla (in Turkish). [9] H.H. Erdem, A.V. Akkaya, B. Cetin, A. Dagdas, S.H. Sevilgen, B. Sahin, I. Teke, C. Gungor, S. Aktas, Comparative energetic and exergetic performance analyses of coal-fired power plants in Turkey, Int. J. Therm. Sci. 48 (11) (2009) 2179e2186. [10] C. Gungor, District heating possibilities around the thermal power plants under the scope of TSAD project, in: 16th International Energy and Environment Fair and Conference, 12e14 May 2010. Istanbul, Turkey. [11] H.H. Erdem, Waste energy and thermal power plants waste energy potential under the scope of TSAD project, in: 16th International Energy and Environment Fair and Conference, 12e14 May 2010. Istanbul, Turkey. [12] H.H. Erdem, A. Dagdas, S.H. Sevilgen, B. Cetin, A.V. Akkaya, B. Sahin, I. Teke, C. Gungor, S. Aktas, Thermodynamic analysis of an existing coal fired power plant for district heating/cooling application, Appl. Therm. Eng. 30 (2e3) (2010) 181e187. [13] H.H. Erdem, C. Gungor, Surplus heat evaluation of Soma thermal power plant : good example for district heating, in: 17th International Energy and Environment Fair and Conference, 15e17 June 2011. Istanbul, Turkey (in Turkish). [14] M.A. Rosen, M.N. Le, I. Dincer, Efficiency analysis of a cogeneration and district energy system, Appl. Therm. Eng. 25 (2005) 147e159. [15] S.J. Nesheim, I.S. Ertesvåg, Efficiencies and indicators defined to promote combined heat and power (CHP), Energy Convers. Manag. 48 (2007) 1004e1015. [16] X. Feng, Y.-N. Cai, L.-L. Qian, A new performance criterion for cogeneration system, Energy Convers. Manag. 39 (15) (1998) 1607e1609. [17] S.A. Klein, EES-engineering Equation Solver, Academic Professional Version V8.172, F-Chart Software, 2008. [18] T. Wakui, R. Yokoyama, Optimal structural design of residential cogeneration systems in consideration of their operating restrictions, Energy 64 (2014) 719e733. [19] A. Rosato, S. Sibilio, G. Ciampi, Energy, environmental and economic dynamic performance assessment of different micro-cogeneration systems in a residential application, Appl. Therm. Eng. 59 (1e2) (2013) 599e617. [20] G. Angrisani, M. Canelli, A. Rosato, C. Roselli, M. Sasso, S. Sibilio, Load sharing with a local thermal network fed by a microcogenerator: thermo-economic optimization by means of dynamic simulations, Appl. Therm. Eng. 71 (2) (2014) 628e635. [21] L. Yang, E. Entchev, M. Ghorab, E.J. Lee, E.C. Kang, Energy and cost analyses of a hybrid renewable microgeneration system serving multiple residential and small office buildings, Appl. Therm. Eng. 65 (1e2) (2014) 477e486. [22] G. Ciampi, A. Rosato, S. Sibilio, Yearly operation of a building-integrated microcogeneration system in south Italy: energy and economic analyses, Int. J. Low-Carbon Technol. 9 (2014) 331e346. [23] E. Açıkkalp, H. Aras, A. Hepbasli, Advanced exergoeconomic analysis of a trigeneration system using a diesel-gas engine, Appl. Therm. Eng. 67 (2014) 388e395. [24] M.H. Bade, S. Bandyopadhyay, Analysis of gas turbine integrated cogeneration plant: process integration approach, Appl. Therm. Eng. 78 (2015) 118e128. [25] X. Luo, J. Hu, J. Zhao, B. Zhang, Y. Chen, S. Mo, Improved exergoeconomic analysis of a retrofitted natural gas-based cogeneration system, Energy 72 (2014) 459e475. [26] K.C. Kavvadias, Z.B. Maroulis, Multi-objective optimization of a trigeneration plant, Energy Policy 38 (2010) 945e954.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
Renovating thermal power plant to trigeneration system for district heating/cooling: Evaluation of performance variation Hasan Huseyin Erdem a, *, Ali Volkan Akkaya a, Ahmet Dagdas a, Suleyman Hakan Sevilgen a, Burhanettin Cetin a, Bahri Sahin b, Ismail Teke a, Cengiz Gungor c, Selcuk Atas c, Mehmet Zahit Basak a a b c
Department of Mechanical Engineering, Yildiz Technical University, 34349 Besiktas, Istanbul, Turkey Department of Naval Architecture, Yildiz Technical University, 34349 Besiktas, Istanbul, Turkey TUBITAK Marmara Research Center, 41470 Gebze, Koceli, Turkey
h i g h l i g h t s Two new performance parameters are defined for renovated power plants. The first parameter is named “The comprehensive thermal efficiency”. The second parameter is named “coefficient of performance for heating/cooling”. According to analysis all examined power plants can be converted to co-tri generation plant.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2014 Accepted 17 April 2015 Available online 27 April 2015
This paper presents performance assessments of thermal power plant-based co/tri-generation systems for district heating/cooling system. The power plants were originally designed exclusively for the generation of electricity. With respect to the renovation of power plants to co/tri-generation systems, the analysis of performance variations in the systems has been undertaken. For the purpose of simulation analysis, thermodynamic models of the eight thermal power plants have been developed. The performance variations have been evaluated with different performance criteria, including electrical power output, classical thermal efficiency, coefficient of performance and comprehensive thermal efficiency. The comprehensive thermal efficiency takes into account all products (electricity, heating and cooling energy) generated from the power plant-based tri-generation system. The results of analysis show that the comprehensive thermal efficiencies of the eight considered systems range from 49% to 61% in the heating mode, although their generated electrical power amounts decrease slightly. As a result, this type of modification for an existing power plant can greatly benefit the cause of energy efficiency and sustainable development. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermal power plant District heating/cooling Cogeneration Trigeneration
1. Introduction Thermal power plants (TPPs) are used globally for the generation of electricity. Statistically, 40% of the global electricity production is provided by coal-fueled power plants, and that percentage is projected to rise in the near future [1,2]. Thus the development of methods to increase the efficiency of fuel conversion is crucial, due to the limitations of fossil-fuel sources and the
* Corresponding author. Tel.: þ90 0212 3832907; fax: þ90 0212 2616659. E-mail address: [email protected] (H.H. Erdem). http://dx.doi.org/10.1016/j.applthermaleng.2015.04.030 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
growing pressure for a cleaner environment. From the perspective of sustainable development, the design of new TPPs as co/trigeneration systems that can produce both electricity and district heating/cooling energy would be a significant step toward the greater utilization efficiency of domestic fossil fuels. Indeed, many power plants were originally designed to produce electricity and district heating/cooling [3e5]. In these types of plants, the extracted steam used for feed-water heating was widely employed for district heating/cooling. Naturally, at the design stage it is possible to specify the most convenient point and amount of steam extraction without negatively affecting plant performance.
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Nomenclature COP _ W _ Net W
Q_ h m_ r
coefficient of performance () power input/output rate (kW) net rate of power output (kW) heat transfer rate (kW) specific enthalpy (kJ/kg) mass flow rate (kg/s) extracted steam rate (%)
Subscripts A auxiliary machines C cooling mode DC district cooling DH district heating
Given this context, the ultimate conversion from the existing TPPs to co/tri-generation systems for the supply of district heating/ cooling energy could be very beneficial in terms of energy efficiency. However, the open literature provides relatively little information about upgrading an existing TPP to a co/tri-generation system and its effects on plant performance. In the cited study [6], some experience with the coal-fired power plant converted to a cogeneration system for the supply of district heating to the city of Kozani, Greece, was given. However, performance analysis was not given for the converted plant in that study. A nationwide research project called TSAD (Utilization of Thermal Power Plant Surplus Heat) was, however, conducted by the authors of this paper during the period of 2006 through 2011 in Turkey. In that project, we systematically examined the question of how the existing power plants generating only electricity could be transformed for district heating/cooling applications [7,8]. Initially, the performance of TPPs under the control of governmental bodies in Turkey were modeled, simulated and analyzed from the energetic and exergetic viewpoints [9]. Then, waste-heat potentials and other heat-extraction capabilities in the power plants were methodically evaluated with respect to co/tri-generation conversion [10,11]. The most effective steam-extraction point for district heating/cooling system was identified [12]. As a result of the detailed technical and economical assessments, the Soma thermal power plant was selected for a pilot application. The conversion process was successfully implemented to compensate the heating requirement of Soma district in Manisa, Turkey [13]. It should be emphasized that if the renovation plans can be applied for all TPPs under government control in Turkey, it will be possible to save annually 7e27 million MWth energy. Each of the TPPs in the project scope has 15,000e140,000 housing district heating potential. The economic potential of heating the residences from the power plants ranges from US$ 100 to $550 million annually. Moreover, the hazardous emissions can be reduced considerably by means of the district heating system. It can provide annually the reduction for 1.5 million to 6 million tons of CO2 and 37,000 to 150,000 tons of SO2. Moreover, it is possible to reduce water consumption by approximately 4 million to 17 million tons/year in the renovated power plants. Some technical conversions in the existing TPPs designed for electricity generation are necessary to use in the district heating application. These conversions will affect the power plant performance. Performing analysis through classical performance criteria (power output and thermal efficiency) considering only electricity generation may lead to wrong evaluations. The effects of converting
H M T th si so s ds c f
heating mode machine used in residential zone for heating/cooling turbine thermal steam inlet steam outlet steam extracted steam condenser fuel
Greek letter difference efficiency
D h
to trigeneration system should be taken into account in the analyses. Therefore, in order to evaluate correctly the performance of power plant-based cogeneration system producing simultaneously both electricity and heat energy, a criterion should be expressed [14e16]. An evaluation of the open literature pertaining to co/trigeneration systems also involved energyeexergy analyses and different objective function-based optimization studies for various regional, commercial, industrial and micro-cogeneration systems [18e26]. In the scope of our study, the conversion of coal-fired thermal power plants (designed to generate only electricity) to co/tri-generation systems was considered for the purpose of district heating and cooling. In the case of such a conversion, the performance indicators have been discussed. There is an important difference in this type power plant conversion study by comparison to the co/tri-generation studies in the cited literature. This difference is that the extracted steam from the turbine, in the case of conversion, is used for a different purpose in place of electricity production, which is the main purpose of the plant. However, the second product (heating or cooling energy) for the co/trigeneration systems considered in the literature is only produced from the energy discharged through the generation of electrical power. This situation does not affect the electricity production in such systems. Therefore, in our study the performance parameters have been defined, taking into account the aforementioned circumstance, and used to analyze the considered thermal power plants. Thus, in this study the changes in performance of the TPPs converted to trigeneration systems to meet the heating/cooling demand in residential areas have been investigated with respect to different criteria. From that perspective, thermodynamic-based simulation models of the eight TPPs under government control were developed. The effects of the energy taken from the specified location of the power plant for the achievement of district heating/ cooling on the performance of trigeneration were analyzed through simulations. The variations in net electrical power output, classical thermal efficiency, utilization factor and comprehensive thermal efficiency considered in this study were analyzed as performance criteria. 2. Renovating existing power plants for district heating applications The authors of this paper have conducted a nationwide project for the purpose of designing a district heating/cooling system using
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existing TPPs as heat sources [7]. In the scope of the study, the methods that could meet the district area energy demands from TPPs with respect to technical and economic aspects were investigated. The most critical point was to determine the heat-source location in the power plant for district heating application. According to results of these studies [7e13], it has been identified that steam extraction from the inlet stage of the low-pressure turbine is the most convenient location for use as a heat source. At that point the steam is of a temperature suitable for district heating/cooling. Additionally, the physical and technical conditions of the power plant make it possible to extract the steam from this point without an operation problem. Consequently, this renovation will be able to satisfy the heating demand posed by residential areas close to TPPs. Fig. 1 illustrates the layout of a converted system, including the power plant and district heating sections. In this method a portion of the steam going to the low-pressure turbine is extracted by means of a technical modification. The steam thus taken is used to heat the water for the district heating system. For that reason, a condenser is added. The latent heat of this steam is transferred to the water of the district heating system through the process of condensation. The condensed steam, being transferred through an additional pipeline, is connected to the feed-water heater system in the most appropriate location. Hot water through the transmission line is carried to heating zone. The hot water that reaches the houses via regional distribution line is used for heating. The water supplied via the distribution line is sent to the condenser through the transmission line in order to heat it again.
3. Thermal integration of the thermal power plant to district heating/cooling system Our study considered eight coal-fired thermal power plants (TPPs) in Turkey for the analysis of performance change. Table 1 gives the characteristics of the subject coal-fired TPPs and the technical information of the nearby district heating/cooling areas. The average age of these power plants is greater than 15 years. The aforementioned power plants were designed for sub-critical steam conditions. These coal-fired TPPs generally use the lignite type of coal. The maximum heating and cooling requirements of the district area near the considered power plant were determined based on climate characteristics and other important technical parameters.
37
The simulation models based on thermodynamic principles were coded by the EES program [17] for every TPP taken into account, and the simulation results were validated with respect to the design conditions [9]. For this study the simulation models were modified in order to evaluate the TPP-based district heating/cooling system. In order to convert the existing plants to co/tri-generation systems, the required components as shown in Fig. 1 were added to the system simulation models. In the altered model developed for analyzing the considered trigeneration systems, a certain amount of steam is extracted from the inlet of the low-pressure turbine, depending on the district heating/cooling load. The effects that the steam extraction at the specified point had on the performance of the considered TPPs could therefore be evaluated according to the simulation models we developed. The extracted steam flow rate ðm_ ds Þ can be expressed for district heating and district cooling, as shown in the following equations:
m_ ds ¼
Q_ DH ðhsi hso Þ
(1)
m_ ds ¼
Q_ DC COPabs ðhsi hso Þ
(2)
where hsi represents the specific enthalpy of the extracted steam, hso denotes the specific enthalpy of the extracted steam at the exit of district heating condenser or absorption chiller. COPabs is the performance coefficient of the absorption chiller. The specific enthalpy of the extracted steam (hsi) varies depending on the design conditions (pressure, temperature) of the thermal power plant. The steam extracted through the district heating condenser is assumed to cool until it is saturated water. Thus the specific enthalpy at the condenser outlet is determined. In other words, the latent heat of the extracted steam is used as an energy source for district heating/cooling. The COPabs value for a two-stage absorption chiller is approximately 1. As seen in Eq. (3), the extracted steam ratio (r) is determined from dividing the steam mass flow rate ðm_ ds Þ extracted from the inlet of the low-pressure turbine for district heating/cooling to the steam mass flow rate ðm_ s Þ at the exit of the intermediate-pressure turbine.
Fig. 1. A simplified schema of renovated thermal power plant for district heating application.
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Table 1 The technical information about the considered thermal power plants and district heating/cooling area. Coal fired thermal power plantsa
Technical data
Total power (MW) Unit number Unit power (MW) Thermal efficiency (%) Exergy efficiency (%) District heating/cooling area near the power plant Distance from the power plant (km) Housing number Maximum heating requirement (MWth) Maximum cooling requirement (MWth) a
Ya
Se
Can
Ca
Ka
AE
So
Tu
630 3 210 37.01 31.95 Yatagan Mugla 4 30 16,800 128 29
600 4 150 38.03 31.50 Kutahya
320 2 160 42.12 37.88 Can Yenice 14 32 13,979 107 19
300 2 150 37.88 35.19 Catalagzi Zonguldak 5 15 45,600 318 29
457 3 157 37.19 28.55 Kangal Sivas 10 37 26,472 329 32
1440 4 360 42.64 32.46 Afsin Elbistan 20 15 33,840 249 68
990 6 165 36.08 32.35 Soma Turgutalp 5 10 24,892 179 49
429 5 160.9 38.44 33.09 Tuncbilek Tavsanli 0 10 25,300 248 30
30 74,678 731 87
Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
r¼
m_ ds m_ s
(3)
The r value varies depending on district heating/cooling load. The steam mass flow rate passed from the low-pressure turbine is reduced, depending on the value of r. As a result, the low-pressure _ net Þ: turbine achieves a power reduction ðDW
_ net ¼ r$m_ s ðh hc Þ DW si
(4)
where hc is the specific enthalpy at the output of the low-pressure turbine.
the same amount of fuel. This is known as a tri-generation system. In this new system electricity, heating and cooling energy are produced concurrently, but the fuel energy given to the system does not change. Thus the performance of the new system isn't measured or evaluated on the basis of a single product. Accordingly, in this study a performance criterion called comprehensive thermal efficiency was used for the evaluating performance variations among co/tri-generation systems, including those of the renovated TPP and district heating/cooling system. Equation (3) expresses the comprehensive thermal efficiency (hth,H) in the case of a system operating in the heating mode:
hth;H ¼
4. Performance evaluation parameters
_ NET W _ Q_ fuel QhDH
(7)
M
4.1. Net power and classical thermal efficiency Thermal power plants (TPPs) usually generate only electricity. Hence, two important performance criteria can be considered, i.e., net power output and thermal efficiency. The net power output can be expressed by subtracting the consumed auxiliary machines power from the obtained turbine power output:
_ Net ¼ W
X
_ T W
X
_ W A
(5)
where the subscripts of T and A represent the turbine and auxiliary machine, respectively. The classical thermal efficiency of a TPP is expressed with the ratio of the obtained net power rate to the fuel heat rate:
hth
_ Net _ DW W ¼ Net _ Q
(6)
f
where Q_ f symbolizes the fuel heat transfer rate given to the boiler of the TPP. When the thermal power plant generates only electricity, the _ net value is zero because the extracted steam mass flow rate DW _ net value can be calculated from Eq. becomes zero. However, the DW (6) in the case of a thermal power plant that functions as a co/trigeneration system. 4.2. Comprehensive thermal efficiency for the co/tri-generation system The addition of a district heating/cooling system to an existing TPP (generating only electricity) increases the number of products (electricity, heating and cooling energy) obtained from the plant for
where hM represents fuel-burning efficiency for heating machines or devices such as stoves, cab heaters, etc. As is shown in Eq. (7), comprehensive thermal efficiency for heating mode is computed by subtracting the thermal heat rate ðQ_ DH =hM Þ of the residential-heating fuel from the rate of fuel combustion ðQ_ fuel Þ at the power plant. Thus the amount of fuel energy used simply to generate electricity can be equally converted to beneficial energy (electricity). Additionally, in regard to the cooling mode, the comprehensive thermal efficiency (hth,C) can be expressed in the following way.
hth;C
_ NET þ Q_ DC W COPM ¼ Q_
(8)
fuel
The defined comprehensive thermal efficiency for the cooling mode is calculated by adding the electricity consumed for cooling of the residential zone Q_ DC =COPM to the electricity generated from the power plant. Because electricity generation and the electricity required for district cooling are provided by the power plant, one can identify how much of the fuel energy given to the power plant is converted to total beneficial energy (electricity). 4.3. Coefficient of performance for the co/tri-generation system The performance of a district heating/cooling system implemented through heat extraction from a power plant is compared to that of the mechanical heating/cooling system. Accordingly, a performance criterion for the investigated system is defined as a similar approach with the coefficient of performance for heating (COPDH) and cooling (COPDC) mode, being expressed in the following way:
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COPDH ¼
Q_ DH _ NET DW
and COPDC ¼
Q_ DC _ NET DW
(9)
_ NET is the power decrease due to the heat extraction from where DW the TPP. The coefficient of performance of mechanical heating/cooling systems shows the obtained heat energy per unit electricity energy. Accordingly, the Eq. (9) expresses the obtained heat energy per electricity power loss in the tri-generation system comprised of a TPP-based district heating system. 5. Results and discussion The various performance criteria used in this study, given the aforementioned generated products, were considered and applied to eight existing coal-fired thermal plants by simulation models based on thermodynamics in order to evaluate the performance of a renovated power plant. The important results have been given and discussed. Fig. 2a shows that the obtained electrical power output from the analyzed plants decreases with the increment of the extracted steam for district heating/cooling applications. The extracted steam rate is defined as the ratio of steam flow for district heating/cooling purposes to the entirety of steam flow that passes through the turbine. This ratio indicates the different district heating/cooling demands. The reason for this decline is that the extracted steam reduces the rate of steam flow that passes through the lowpressure turbine and therefore decreases the power output that can be generated from the turbine. Because the power obtained from the turbine is linear in proportion to the steam mass flow rate, the increased amount of extracted steam causes a corresponding change in power reduction to occur at the turbine. Given the same extracted steam rate, different power-decrease values between the power plants occur due to their different LPT conditions. However, given the lower pressure and temperature of
39
extracted steam, the power decline is also less. For example, in order to meet the 30 MW heating demand (generally equivalent to the heat requirement of 6000 apartments in the climate conditions of Turkey), the ratio of required steam extraction has historically been 10% for the Yatagan thermal power plant (Table 2.) In this steam-extraction condition, it is seen from Fig. 2a that the unit power reduction is approximately 4.46 MW (2.12%). The decrease in power generation is an issue only when heating or cooling demand is at the peak state. When the annual average demand is considered, the annual average reduction in electricity power is significantly less. However, this change at power plants can be assessed as a power reduction, given the coincident peaks of electricity and heating/cooling demand. Such a condition is rarely experienced in Turkey. If it is possible to overlap the demand peaks, the use of a heat-storage system could prevent the imbalance between the electricity demand and the supply. The demand for electrical power for air-conditioning systems in residential zones can be decreased when steam extraction is used as an energy source. This situation produces a reduction in the power required from the subject power plant. In that context, the addition of a district cooling system to the power plant may reduce not only the generation of electrical power but also the demand for electricity. Steam extraction for the purpose of district heating/cooling can influence the classical thermal efficiency of the considered power plants as well as the generation of electricity. Fig. 2b illustrates the change of classical thermal efficiencies for the considered TPPs under conditions of steam extraction. The figure demonstrates that the thermal efficiencies of the power plants decrease proportionally to the increase of extracted steam ratios for all power plants. The reason for this decrease trend can be explained in the following manner: The plant modification made for a district heating/cooling system causes an increase in the feed-water temperature at a point prior to the boiler inlet. Thus it serves to decrease the supply of heat required by the boiler. However, the effect of power reduction at the turbine is greater than that of the reduction in the boiler's heat
Fig. 2. Performance variation of power plants generating only electricity with respect to steam extraction, i.e., (a) net power reduction and (b) classical thermal efficiency.
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H.H. Erdem et al. / Applied Thermal Engineering 86 (2015) 35e42
Table 2 District heating/cooling energy potential with steam extraction from the power plants. Extracted steam ratio (r) (%)
5 10 15 20 25 30
District heating/cooling energy potential (MW) AE
Can
Ca
Ka
Se
So
Tu
Ya
25.4 50.8 76.2 101.6 126.9 152.3
11.8 23.8 35.8 47.9 60.1 72.4
13.2 26.5 39.8 53.2 66.8 80.3
12.6 25.2 37.8 50.6 63.3 76.2
11.5 23.1 34.8 46.5 58.2 70.1
13.1 26.2 39.3 52.6 65.7 78.9
12.2 24.5 36.7 48.9 61.2 73.4
15.3 30.5 45.8 61.0 76.3 91.6
*Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
supply. When the steam-extraction ratio is changed from 0.0 to 0.3, the classical thermal efficiency reduction becomes an average of 1% for all considered plants. The classical thermal efficiency decreases from 36% to 35.4% with heating for 7000 residences by the Yatagan power plant. The peak value is 7000, but it does not remain constant throughout the year. However, this decrease is less than 0.6% point when one considers annual heating. The classical thermal efficiency takes into account only the efficiency of the power plant; it does not consider other benefits based on steam extraction. Therefore, it isn't a good performance criterion for renovated power plants. Fig. 3 shows the change in the comprehensive thermal efficiencies of the analyzed TPPs with respect to the increase of the extracted steam mass ratio in the heating and cooling modes. In the heating-mode evaluation, the stove, cab heater and lignite-fueled boiler are considered as conventional heating systems. The burning efficiencies of these conventional systems range from 50% to 70%. In this study the efficiency value is taken as 50%. As shown in the heating mode of Fig. 3a, the comprehensive thermal efficiencies defined for the developed trigeneration systems increase with the increment of steam extraction. While the steamextraction ratio is increased from 0.0 to 0.3, the increase variation of the comprehensive thermal efficiency is approximately 4% for all
considered plants in the case of the heating mode. Moreover, it can be said that the comprehensive thermal efficiency is higher at the same steam extraction when the burning efficiency of classical heating systems in residential zone is lower than considered in this study. The reason is that the fuel requirement for a classical heating system grows as its efficiency decreases. In the cooling-mode analysis (Fig. 3b), the coefficient of performance for a classical cooling system in a residential zone is taken as 2. It can be seen from this figure that the comprehensive thermal efficiency of the power plant-based trigeneration system increases with the increment of steam extraction. For example, the efficiency of the Seyitomer power plant increases from 38% to 43% when operated in the cooling mode. As Fig. 3 shows, the rate of increase in comprehensive thermal efficiency with steam extraction is greater than the decrease in the classical thermal efficiency (Fig. 2b) for all power plants. For the 10% extracted steam rate, the comprehensive thermal efficiency increases from 36% to 39% at the Yatagan power plant. The difference is 3%, which is much greater than the decrease in classical thermal efficiency (0.6%). Accordingly, if a power plant is converted to a cotri generation plant, it makes better use of fuel energy, based on the separate generation mode. Therefore, such a transformation is sustainable.
Fig. 3. The variation of comprehensive thermal efficiency with regard to extracted steam, (a) in district heating mode, (b) in district cooling mode.
H.H. Erdem et al. / Applied Thermal Engineering 86 (2015) 35e42
Fig. 4 shows the distribution percentage of the fuel-energy amount on one of the power plants with respect to the ratio of steam extracted from the inlet of the low-pressure turbine. It is seen from this figure that the steam-extraction increment decreases in terms of the power output obtained from the turbine (shown in Fig. 2a). However, because the steam pressure and temperature are lower at that point, the decrease in power is relatively slight. Moreover, significant gains are possible despite such a decrease. It is evident from this figure that the extracted steam amount significantly limits the energy discarded from the condenser to the environment. In other words, the majority (approximately 80%) of the thermal energy required for district heating will be met by the energy discarded through the condenser. This situation can effectively reduce the condenser load, whereupon the condenser operation pressure can approach the ideal condition. Thus it is possible to achieve a performance improvement in electrical power output. The COP (Coefficient of Performance) values of the power plantbased tri-generation systems are shown in Table 3. For example, when the extracted steam is used for heating aim, the COPHeating value 5.4 for the Soma power plant. In the conventional method involving a mechanical cooling system, this value is approximately 2.6e3.2. Therefore, it proves that the district heating modification in the power plant can reduce the consumed energy for heating in residential areas. However, the classical method employs a mechanical cooling system to compensate the cooling requirement in residential areas. The COP values of these classical systems are between 2.5 and 2.62. In Table 3, for example, the coefficient of performance (COPCooling) of the Yatagan power plant is 4.79 and 6.84 for the one- and two-stage absorption chillers, respectively. When these values are compared against the mechanical cooling systems used in residential zones, it can be seen that the developed system has a high COP value. With the power plant-based district cooling system developed through our study, it can be observed that the electricity energy consumed for cooling can be dramatically reduced. The COPDH value in Table 3 is identical to the COPDC value of the two-stage absorption chiller. This situation occurs when the COP value of the two-stage absorption chiller is 1. In this case, the QDH
41
Table 3 Coefficient of performance values for power plant based tri-generation systems. Thermal power plantsa
COPDH COPDC
One stage Two stage
Ya
Se
Can
Ca
Ka
AE
So
Tu
6.84 4.79 6.84
5.45 3.80 5.45
4 2.8 4
4.2 2.9 4.2
5 3.5 5
5.2 3.63 5.2
5.4 3.7 5.4
4.2 2.9 4.2
a Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
and QDC values are equal. The difference between the COPDH values of the power plants is due to the thermodynamic conditions of steam extraction. The low steam pressure and temperature values increase the COPDH value. Table 4 shows the technical modification results for the considered power plants in the context of the maximum district heating/cooling demands in the related residential areas. Load demands for heating and cooling are expressed as peak values in the table. It is seen that all power plants have capacities sufficient to meet the heating and cooling requirements of their nearby cities. The table shows that all eight power plants, in terms of performance parameters, can be converted to co-tri generation plants. Therefore, the results reveal that all old power plants can be converted to co-tri generation plants. The decrease in the demand for district electric power in the case of compensation of maximum cooling demand from the plant (MWe) is 14.5 for the Yatagan power plant. The reduction of plant electrical power at maximum cooling demand (MWe) is 4.2 for the same power plant. Thus the increased supply of plant electrical power at maximum cooling demand (MWe) is approximately 10.3 MWe. 6. Conclusion The conversions of the eight existing thermal power plants (TPPs) to co/tri-generation systems for district heating/cooling have been evaluated in terms of performance variations through means of the developed thermodynamic simulation models presented in
Fig. 4. Distribution of fuel energy on power plant with regard to the extracted steam ratio.
42
H.H. Erdem et al. / Applied Thermal Engineering 86 (2015) 35e42
Table 4 Compensation of district heating/cooling demand from the power plant. Operation Mode Parameters
Thermal power plantsa Ya
Heating
Cooling
a
Se
Can
Ca
Ka
AE
So
Tu
Number of steam extracted power plant unit 2 4 2 2 3 2 3 2 Steam extraction ratio for each unit (%) 21 50 22.5 50 43.5 25 22.8 50 Compensation ratio of heating requirement from the plant (%) 100 63.2 100 83 100 100 100 100 Reduction of plant electrical power at maximum heating demand (MWe) 9.4 21.6 12.15 32.4 22.37 24.5 11.23 29.6 Discharged heat reduction through condenser at maximum heating demand (MWt) 58 96.7 41.7 103.2 88.7 100.2 47.6 99.8 Comprehensive thermal efficiency (%) 44.34 56.34 48.84 53.87 50.36 56.04 44.74 53.9 Number of steam extracted power plant unit 1 2 1 1 1 1 1 1 Steam extraction ratio for each unit (%) 9.5 18.8 8 10.9 12.7 13.4 18.7 12.2 Compensation ratio of cooling requirement from the plant (%) 100 100 100 100 100 100 100 100 Supply increase of plant electrical power at maximum cooling demand(MWe) 10.3 35.4 5.2 7.6 9.6 20.9 15.3 7.8 Discharged heat reduction through condenser at maximum cooling demand (MWt) 26.2 35.8 14.7 22 25.6 53.7 39 24.3 Comprehensive thermal efficiency (%) 37.07 41.72 43.49 38.56 39.24 44.29 39.03 40
Ya e Yatagan; Se e Seyitomer; Can e Can; Ca e Catalagzi; Ka e Kangal; AE e Afsin Elbistan; So e Soma; Tu e Tuncbilek.
this study. It is concluded from the analysis that the modification of TPPs to co/tri-generation systems for district heating/cooling would provide important advantages. This conversion does not significantly affect power plant operation or electricity generation but can actually provide additional production such as heating and cooling energy without increasing the consumption of fuel. Consequently, this conversion serves to increase power plant income. In regard to the district areas, the use of the extracted steam from TPPs as a heat source for district heating will remove the fuel used for the heating of residential centers. This advantage alone could provide a significant contribution to the national economy and benefit sustainable development. Additionally, this renovation could contribute a number of crucial and strategic advantages, such as reducing harmful emission emitted to atmosphere, increasing life quality/comfort, decreasing human healthcare costs due to harmful emissions and reducing the dependence on foreign energy sources. As a result, this type of conversion can be implemented in terms of performance and potential. However, an economical evaluation that takes into account all the investment parameters in the power plant and region should be performed prior to any investment decision. Further, such a study should encompass all the environmental effects on nearby cities.
Acknowledgements The research reported in this paper was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under public research grants (Project No.: 105G099). Also, the authors acknowledge the support provided by the related plant and Turkish Electricity Generation Co. Inc. (EUAS).
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