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Trigeneration: an alternative for energy savings
Applied Energy 76 (2003) 219–227 www.elsevier.com/locate/apenergy
Trigeneration: an alternative for energy savings Joel Herna´ndez-Santoyoa,b,*, Augusto Sa´nchez-Cifuentesb,1 a
Grupo de Exergia, Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas No 152, 07730, Ciudad de Me´xico, Mexico b Facultad de Ingenierı´a, Universidad Nacional Auto´noma de Me´xico, Mexico Accepted 1 January 2003
Abstract The design of new processes focused towards a more efficient use of energy, is nowadays highly desirable. In this paper, the design of a system of trigeneration is presented as an alternative way of improved energy use in cogeneration systems. Savings are observed by the decrease of the fuel fed to the turbogeneration equipment. A regenerative-cycle cogeneration system and a new trigeneration system were studied, showing their benefits as well as the operation criteria for both processes. # 2003 Elsevier Ltd. All rights reserved. Keywords: Cogeneration; Trigeneration; Absorption chiller; Energy saving
1. Introduction The alternative to design a new process for the energy saving in industrial plants (properly say in power plants), is born from the concern for having a bigger yield from the processes, giving as a result, a lower consumption of the natural resources and a more economic performance of the industry. The combined generation of thermal energy and electric power is nowadays one of the technologies often used in industrial processes: however, as with all processes, there are inefficiencies in its operation. Nevertheless, a trigeneration process is an alternative design to increase the efficiency in the thermal and electric generation. In recent years, the trigeneration system has been used like power generation and secondly for air conditioning [1]. Trigeneration, also referred to as district energy,2 achieves a higher efficiency and smaller environmental impact than cogeneration.3 The installation of a * Corresponding author. Tel.: +52-55-30038420; fax: +52-55-30038067. E-mail address: [email protected] (J. Herna´ndez-Santoyo), [email protected] (A. Sa´nchez-Cifuentes). 1 Tel.: +52-55-56223138 or 39. 2 District energy is a system where the generated energy is used elsewhere. 3 With respect to greenhouse gas emission to the environment, as CO2, NOX, and SOX. 0306-2619/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0306-2619(03)00061-8
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J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
Nomenclature Cp E F H HR k P Q0 rp RT S T W
Calorific capacity (kJ/kg C) Electric energy (kWe) Mass flow (kg/h) Enthalpy (kJ/kg) Heat rate (kJ/kW-h) Air’s isentropic constant Pressure (kg/cm2) Thermal power (kWt) Pressure ratio Refrigeration tons Entropy (kJ/kg K) Temperature ( C) Power (kW) quality vapour
Abbreviations and subscript g gases GT gas turbine HP high-pressure steam in input l liquid phase LP low-pressure steam MP medium-pressure steam MRA absorption chiller out output PC power cycle SG steam generator ST steam turbine v vapour phase 0 standard state m mixture
Greeks l Latent heat (kJ/kg) Efficiency (%)
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trigeneration plant can achieve up to a 50% greater system efficiency than cogeneration plant of similar size [2]. A trigeneration plant is often described as a cogeneration plant that has added absorption chillers (MRA), which take the waste heat a cogeneration plant would have wasted, and converts this free energy into useful energy in the form of chilled energy. A cogeneration system is shown in Fig. 1; this is a regenerative-cycle cogeneration system [3]. In the system shown in Fig. 1, the gas-turbine (GT) exhausted gases are taken advantage of to generate high-pressure steam, which is required by another process. The trigeneration system is shown in Fig. 2. The trigeneration energy process produces four different forms of energy from the primary-energy source, namely, hot water, steam, cooling (chilled water) and power generation (electric energy). However, trigeneration has also been referred to as CHCP (combined heating, cooling and power generation). Using an MRA with a water–lithium bromide4 working-fluid takes advantage of the energy (in the form of steam or hot fluid) discarded by a combined-cycle cogeneration system (CCC) to generate chilled water.
2. Design basis for the processes The case comparison study systems, consists of analyzing the operation bases for regenerative-cycle cogeneration system (see Fig. 1) and the trigeneration system (see Fig. 2) given in Table 1. Electric-power generation via the GT is shown in Fig. 1; installed in this system is a heat regenerator for heating the air that comes from the exit of the compression stage, thereby causing an efficiency increase in the thermodynamic air cycle (Brayton Cycle) besides a decrease of fuel consumption. The exit regenerator hot gases are
Fig. 1. Regenerative-cycle cogeneration system. 4
Here it is used an absorption liquid chiller as in the York International technology, YIA Model.
222
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
Fig. 2. Trigeneration system.
passed to the recovery boiler or steam generator (SG) to generate high-pressure steam, that later is taken advantage of for heating any other process areas. In the system of Fig. 2, any part of the extracted steam flow from the steam turbine (ST) is taken advantage of by the MRA to provide chilled water. The process Table 1 Conditions of the process Environment conditions Wet-bulb temperature Dry-bulb temperature Pressure Humidity ratio Humidity Dew-point temperature Process characteristics Gas turbinea Model Power Process Fuel Excess air Cooling-water temperature Chilled-water temperature HP and LP steam Steam-extraction temperature a b
21 C 25 C 1.033 kg/cm2 0.014 kg H2O/kg dry air 70.25% 19.23 C
ABB GT 8C, Year 1994 52,800 kW ISOb base rating 15.7 Natural gas (CH4). 100% 32 C 7 C 56.24 and 24.6 kg/cm2 respectively 132 C
Source: Gas Turbine World HANDBOOK 1992-93, Electric Power, p. 3–06. ISO, International Standard Organization.
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characteristics are indicated in Table 1. Furthermore, the MRA requires cooling water from a cooling tower. The purpose of generated chilled water is to cool the humid air that enters the GT stage compression. The compression inter-stage is cooled by means of the cooled water. In the same way as in the cogeneration system, a heat regenerator warms-up the air at entry to the GT stage compression; the air at exit from the regenerator is fed to the SG to generate high-pressure steam and then sent to the ST to generate electric energy. Finally, the energy-recovery stream from the MRA can be used as condenser water or low-pressure steam. For the analyses of the behaviours of both systems, a software program in Excel was developed that allows a quick evaluation and comparison of data [3,4,6,7].
3. Evaluation method The gas power-systems considered include a gas turbine and use a working substance that never changes phase; this remains as a gas. In the steam-power system, the phase of the substance changes [4]. For a thermal process, the Carnot efficiency is the maximum efficiency, which is the efficiency of an ideal cycle, given by: T To T
Carnot ¼
ð1Þ
Naturally, the efficiencies of a real process are lower since losses are involved due to internal and external irreversibilities. Consequently, the thermal efficiency general definition of a real power cycle is: PC ¼
WGT þ WST QGT
ð2Þ
In the general case, the efficiencies of the single cycle can be defined as follows: for GT
GT ¼
for ST
ST ¼
WGT QGT
WST QGT ð1 GT Þ
ð3Þ
ð4Þ
In our case, the trigeneration system efficiency is similar to that for a combinedcycle cogeneration system [Eq. (2)]. However this is modified by the increase in the GT efficiency that is represented by Q0 GT. Hence the trigeneration system’s efficiency is CHCP ¼
WGT þ WST Q0GT
ð5Þ
There are several reasons why the air temperature has a great influence on the power generated and the efficiency [5]: one of them is due to the decrease in density
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
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when the air temperature rises, and this decreases the input mass flow, if the same volume throughput is maintained. For an isentropic process, the powers generated by the GT and ST are [4]: WGT 1 ðk1Þ=kÞ GT : ¼ Cp Tin 1 rp ð6Þ þ Tout 1 ðk1Þ=kÞ rp Fg
ST :
where
WST ¼ Hin Hm out Fsteam
to
ðSin ¼ Sout Þ
v l Hm out ¼ Hout þ ð1 ÞHout
ð7Þ
ð8Þ
The MRA evaluation consists in determining the heat that is removed from the air cooler. The heat is expressed in refrigeration tons (RT).5 The main objective of the MRA consists in taking advantage of the waste heat of the ST to generate chilled water (at about 7 C), having as the working substance the H2O–LiBr mixture [6,7]. The MRA efficiency will be: RTrequired MRA ¼ ð9Þ Qgenerator where Qgenerator ¼ FLiBr CpDT þ Fwater lendo þ levap
ð10Þ
The FLiBr is the mass flow at the concentration of 59.5% of LiBr [6] in the warmup zone, while the Fwater is the mass of the evaporated flow water and in turn the resulting heat is absorbed by the LiBr. The heat absorbed by the water, is identified by the endothermic and evaporation heats lendo and levap. The trigeneration system evaluation will determine the required cooling water and steam flows, fuel consumption and electric-energy generated.
4. Results The technology that reduces the fuel consumption in the GT is called trigeneration. In Fig. 3, the comparison of fuel consumptions is observed between the cogeneration and trigeneration systems in relationship (for the analyzed case) to the percentage of humidity air. Increasing the humidity in the GT cogeneration system increases the fuel consumption: this is due to the increase of heat required in the combustion process. The obtained results of both systems are shown in the Table 2.
5
A refrigeration ton is defined as the transfer of heat at the rate of 3.52 kW.
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Fig. 3. Fuel consumptions of both systems. Table 2 Results of both processes
Fuel flow (kg/h) Steam generated (ton/h) kWe generated Air flow (kg/h) GT efficiency (%) Refrigeration tons Cooling-water flow (GPM)b Chilled-water flow (GPM)b Heat rate (kJ/kW-h) a b
Cogenerationa
Trigenerationa
9011 28.85 52,800 457,951 45.38 – – – 2461.89
8098 14.16 54,426 388,729 56.60 1348 2599 1356 2199.17
All results data were obtained by a development program for each system. Gallons per minute.
The decrease in fuel consumption by the use of the trigeneration system leads to approximately a 10% saving economically and of energy. In Table 2, the benefits of using a trigeneration system in comparison with a cogeneration system are observed. Similarly, in Fig. 4, the cooling water consumption, the fuel saving and the RT necessary are presented. The humidity plays an important role. A nomogram (see Fig. 5) is presented that identifies the fuel consumption (for a capacity of fixed generation) in relation to the humidity and ambient temperature. The nomogram, allows one to evaluate quickly, according to the different environmental conditions such as ambient temperature and humidity, the fuel quantity that is required in the system, besides the cost per hour for the energy used.
226
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Fig. 4. Consumption and fuel saving of the trigeneration system.
Fig. 5. Fuel consumption of the trigeneration system for various ambient temperatures and humidities.
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5. Conclusions The two power systems were compared, installing refrigeration equipment for absorption in the cogeneration system results in a decrease of fuel consumption. Nevertheless, different approaches should be considered for the installation without losing view that this refrigeration equipment should take advantage of the waste thermal energy. The trigeneration is a means to achieve energy savings in future installation plants for heat and electricity generation.
References [1] Tozer R, Lozano MA, Valero A. Thermoeconomics applied to an air-conditioning system with cogeneration. Proc CIBSE A, Building Serv Eng Res Technol 1996;17(1):37–42. [2] Trigeneration Consultants IncSM. Available from http://www.trigeneration.com. [3] El-Wakil MM. Power plant technology. New York: McGraw-Hill Book Company; 1984, p. 309–351. [4] Jones JB, Dugan RE. Ingenierı´a termodina´mica. Ultima edicio´n. Editorial Prentice-Hall, 2000. p. 755–808. [5] Kehlhofer R. Combined cycle gas & steam turbine power plants. Lilburn (GA): The Fairmont Press, Inc.; 1991, 388 p. [6] Perry RH. Manual del ingeniero quı´mico, Tomo III, Seccio´n 12. USA: McGraw-Hill, Inc; 1984. [7] York International. YIA Single-Effect Absorption Chillers. Form 155. 16-EG1(597). USA: York International Corporation; 1997.
Trigeneration: an alternative for energy savings Joel Herna´ndez-Santoyoa,b,*, Augusto Sa´nchez-Cifuentesb,1 a
Grupo de Exergia, Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas No 152, 07730, Ciudad de Me´xico, Mexico b Facultad de Ingenierı´a, Universidad Nacional Auto´noma de Me´xico, Mexico Accepted 1 January 2003
Abstract The design of new processes focused towards a more efficient use of energy, is nowadays highly desirable. In this paper, the design of a system of trigeneration is presented as an alternative way of improved energy use in cogeneration systems. Savings are observed by the decrease of the fuel fed to the turbogeneration equipment. A regenerative-cycle cogeneration system and a new trigeneration system were studied, showing their benefits as well as the operation criteria for both processes. # 2003 Elsevier Ltd. All rights reserved. Keywords: Cogeneration; Trigeneration; Absorption chiller; Energy saving
1. Introduction The alternative to design a new process for the energy saving in industrial plants (properly say in power plants), is born from the concern for having a bigger yield from the processes, giving as a result, a lower consumption of the natural resources and a more economic performance of the industry. The combined generation of thermal energy and electric power is nowadays one of the technologies often used in industrial processes: however, as with all processes, there are inefficiencies in its operation. Nevertheless, a trigeneration process is an alternative design to increase the efficiency in the thermal and electric generation. In recent years, the trigeneration system has been used like power generation and secondly for air conditioning [1]. Trigeneration, also referred to as district energy,2 achieves a higher efficiency and smaller environmental impact than cogeneration.3 The installation of a * Corresponding author. Tel.: +52-55-30038420; fax: +52-55-30038067. E-mail address: [email protected] (J. Herna´ndez-Santoyo), [email protected] (A. Sa´nchez-Cifuentes). 1 Tel.: +52-55-56223138 or 39. 2 District energy is a system where the generated energy is used elsewhere. 3 With respect to greenhouse gas emission to the environment, as CO2, NOX, and SOX. 0306-2619/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0306-2619(03)00061-8
220
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
Nomenclature Cp E F H HR k P Q0 rp RT S T W
Calorific capacity (kJ/kg C) Electric energy (kWe) Mass flow (kg/h) Enthalpy (kJ/kg) Heat rate (kJ/kW-h) Air’s isentropic constant Pressure (kg/cm2) Thermal power (kWt) Pressure ratio Refrigeration tons Entropy (kJ/kg K) Temperature ( C) Power (kW) quality vapour
Abbreviations and subscript g gases GT gas turbine HP high-pressure steam in input l liquid phase LP low-pressure steam MP medium-pressure steam MRA absorption chiller out output PC power cycle SG steam generator ST steam turbine v vapour phase 0 standard state m mixture
Greeks l Latent heat (kJ/kg) Efficiency (%)
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
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trigeneration plant can achieve up to a 50% greater system efficiency than cogeneration plant of similar size [2]. A trigeneration plant is often described as a cogeneration plant that has added absorption chillers (MRA), which take the waste heat a cogeneration plant would have wasted, and converts this free energy into useful energy in the form of chilled energy. A cogeneration system is shown in Fig. 1; this is a regenerative-cycle cogeneration system [3]. In the system shown in Fig. 1, the gas-turbine (GT) exhausted gases are taken advantage of to generate high-pressure steam, which is required by another process. The trigeneration system is shown in Fig. 2. The trigeneration energy process produces four different forms of energy from the primary-energy source, namely, hot water, steam, cooling (chilled water) and power generation (electric energy). However, trigeneration has also been referred to as CHCP (combined heating, cooling and power generation). Using an MRA with a water–lithium bromide4 working-fluid takes advantage of the energy (in the form of steam or hot fluid) discarded by a combined-cycle cogeneration system (CCC) to generate chilled water.
2. Design basis for the processes The case comparison study systems, consists of analyzing the operation bases for regenerative-cycle cogeneration system (see Fig. 1) and the trigeneration system (see Fig. 2) given in Table 1. Electric-power generation via the GT is shown in Fig. 1; installed in this system is a heat regenerator for heating the air that comes from the exit of the compression stage, thereby causing an efficiency increase in the thermodynamic air cycle (Brayton Cycle) besides a decrease of fuel consumption. The exit regenerator hot gases are
Fig. 1. Regenerative-cycle cogeneration system. 4
Here it is used an absorption liquid chiller as in the York International technology, YIA Model.
222
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
Fig. 2. Trigeneration system.
passed to the recovery boiler or steam generator (SG) to generate high-pressure steam, that later is taken advantage of for heating any other process areas. In the system of Fig. 2, any part of the extracted steam flow from the steam turbine (ST) is taken advantage of by the MRA to provide chilled water. The process Table 1 Conditions of the process Environment conditions Wet-bulb temperature Dry-bulb temperature Pressure Humidity ratio Humidity Dew-point temperature Process characteristics Gas turbinea Model Power Process Fuel Excess air Cooling-water temperature Chilled-water temperature HP and LP steam Steam-extraction temperature a b
21 C 25 C 1.033 kg/cm2 0.014 kg H2O/kg dry air 70.25% 19.23 C
ABB GT 8C, Year 1994 52,800 kW ISOb base rating 15.7 Natural gas (CH4). 100% 32 C 7 C 56.24 and 24.6 kg/cm2 respectively 132 C
Source: Gas Turbine World HANDBOOK 1992-93, Electric Power, p. 3–06. ISO, International Standard Organization.
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
223
characteristics are indicated in Table 1. Furthermore, the MRA requires cooling water from a cooling tower. The purpose of generated chilled water is to cool the humid air that enters the GT stage compression. The compression inter-stage is cooled by means of the cooled water. In the same way as in the cogeneration system, a heat regenerator warms-up the air at entry to the GT stage compression; the air at exit from the regenerator is fed to the SG to generate high-pressure steam and then sent to the ST to generate electric energy. Finally, the energy-recovery stream from the MRA can be used as condenser water or low-pressure steam. For the analyses of the behaviours of both systems, a software program in Excel was developed that allows a quick evaluation and comparison of data [3,4,6,7].
3. Evaluation method The gas power-systems considered include a gas turbine and use a working substance that never changes phase; this remains as a gas. In the steam-power system, the phase of the substance changes [4]. For a thermal process, the Carnot efficiency is the maximum efficiency, which is the efficiency of an ideal cycle, given by: T To T
Carnot ¼
ð1Þ
Naturally, the efficiencies of a real process are lower since losses are involved due to internal and external irreversibilities. Consequently, the thermal efficiency general definition of a real power cycle is: PC ¼
WGT þ WST QGT
ð2Þ
In the general case, the efficiencies of the single cycle can be defined as follows: for GT
GT ¼
for ST
ST ¼
WGT QGT
WST QGT ð1 GT Þ
ð3Þ
ð4Þ
In our case, the trigeneration system efficiency is similar to that for a combinedcycle cogeneration system [Eq. (2)]. However this is modified by the increase in the GT efficiency that is represented by Q0 GT. Hence the trigeneration system’s efficiency is CHCP ¼
WGT þ WST Q0GT
ð5Þ
There are several reasons why the air temperature has a great influence on the power generated and the efficiency [5]: one of them is due to the decrease in density
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
224
when the air temperature rises, and this decreases the input mass flow, if the same volume throughput is maintained. For an isentropic process, the powers generated by the GT and ST are [4]: WGT 1 ðk1Þ=kÞ GT : ¼ Cp Tin 1 rp ð6Þ þ Tout 1 ðk1Þ=kÞ rp Fg
ST :
where
WST ¼ Hin Hm out Fsteam
to
ðSin ¼ Sout Þ
v l Hm out ¼ Hout þ ð1 ÞHout
ð7Þ
ð8Þ
The MRA evaluation consists in determining the heat that is removed from the air cooler. The heat is expressed in refrigeration tons (RT).5 The main objective of the MRA consists in taking advantage of the waste heat of the ST to generate chilled water (at about 7 C), having as the working substance the H2O–LiBr mixture [6,7]. The MRA efficiency will be: RTrequired MRA ¼ ð9Þ Qgenerator where Qgenerator ¼ FLiBr CpDT þ Fwater lendo þ levap
ð10Þ
The FLiBr is the mass flow at the concentration of 59.5% of LiBr [6] in the warmup zone, while the Fwater is the mass of the evaporated flow water and in turn the resulting heat is absorbed by the LiBr. The heat absorbed by the water, is identified by the endothermic and evaporation heats lendo and levap. The trigeneration system evaluation will determine the required cooling water and steam flows, fuel consumption and electric-energy generated.
4. Results The technology that reduces the fuel consumption in the GT is called trigeneration. In Fig. 3, the comparison of fuel consumptions is observed between the cogeneration and trigeneration systems in relationship (for the analyzed case) to the percentage of humidity air. Increasing the humidity in the GT cogeneration system increases the fuel consumption: this is due to the increase of heat required in the combustion process. The obtained results of both systems are shown in the Table 2.
5
A refrigeration ton is defined as the transfer of heat at the rate of 3.52 kW.
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
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Fig. 3. Fuel consumptions of both systems. Table 2 Results of both processes
Fuel flow (kg/h) Steam generated (ton/h) kWe generated Air flow (kg/h) GT efficiency (%) Refrigeration tons Cooling-water flow (GPM)b Chilled-water flow (GPM)b Heat rate (kJ/kW-h) a b
Cogenerationa
Trigenerationa
9011 28.85 52,800 457,951 45.38 – – – 2461.89
8098 14.16 54,426 388,729 56.60 1348 2599 1356 2199.17
All results data were obtained by a development program for each system. Gallons per minute.
The decrease in fuel consumption by the use of the trigeneration system leads to approximately a 10% saving economically and of energy. In Table 2, the benefits of using a trigeneration system in comparison with a cogeneration system are observed. Similarly, in Fig. 4, the cooling water consumption, the fuel saving and the RT necessary are presented. The humidity plays an important role. A nomogram (see Fig. 5) is presented that identifies the fuel consumption (for a capacity of fixed generation) in relation to the humidity and ambient temperature. The nomogram, allows one to evaluate quickly, according to the different environmental conditions such as ambient temperature and humidity, the fuel quantity that is required in the system, besides the cost per hour for the energy used.
226
J. Herna´ndez-Santoyo, A. Sa´nchez-Cifuentes / Applied Energy 76 (2003) 219–227
Fig. 4. Consumption and fuel saving of the trigeneration system.
Fig. 5. Fuel consumption of the trigeneration system for various ambient temperatures and humidities.
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5. Conclusions The two power systems were compared, installing refrigeration equipment for absorption in the cogeneration system results in a decrease of fuel consumption. Nevertheless, different approaches should be considered for the installation without losing view that this refrigeration equipment should take advantage of the waste thermal energy. The trigeneration is a means to achieve energy savings in future installation plants for heat and electricity generation.
References [1] Tozer R, Lozano MA, Valero A. Thermoeconomics applied to an air-conditioning system with cogeneration. Proc CIBSE A, Building Serv Eng Res Technol 1996;17(1):37–42. [2] Trigeneration Consultants IncSM. Available from http://www.trigeneration.com. [3] El-Wakil MM. Power plant technology. New York: McGraw-Hill Book Company; 1984, p. 309–351. [4] Jones JB, Dugan RE. Ingenierı´a termodina´mica. Ultima edicio´n. Editorial Prentice-Hall, 2000. p. 755–808. [5] Kehlhofer R. Combined cycle gas & steam turbine power plants. Lilburn (GA): The Fairmont Press, Inc.; 1991, 388 p. [6] Perry RH. Manual del ingeniero quı´mico, Tomo III, Seccio´n 12. USA: McGraw-Hill, Inc; 1984. [7] York International. YIA Single-Effect Absorption Chillers. Form 155. 16-EG1(597). USA: York International Corporation; 1997.