- Email: [email protected]
Software for the analysis of water and energy systems
DESALINATION ELSEVIER
Desalination 156 (2003) 367-378 www.elsevier.com/locate/desal
Software for the analysis of water and energy systems Javier Uche* , Luis Serra, Luis Albert0 Herrero, Antonio Valero, JosCAntonio Turdgano, CCsarTorres International Study Group for Water and Energy Systems (ISG WES), Centre of Research for Energy Resources and Consumption (CIRCE), University of Zuragoza, Zuragoza, Spain Tel. i-34 (976) 76 18 63; Fax +34 (976) 73 20 78; email: [email protected] Received 10 February 2003; accepted 15 February 2003
Abstract
Waterand energy are crucial aspects strongly connected when dealing with the problem of augmenting f?esh water resources. Surprisingly, inthe scientific and technological community there is a marked lack of attention to the combined research of water and energy issues. The present paper outlines a development-oriented object program in the said direction, oriented to the integrated analysis of power and desalination plants, which would hopefully fill the gap in having user-friendly software in the form of building blocks as an appropriate Ii-ameworkfor the analysis, synthesis and design of either individual or integrated systems for energy and water. Such an approach would transcend other traditional approaches for process integration and obviously prove of great help not only to designers but also to researchers, educators and students.For iliustraGvepurposes,in orderto show the scopeofthe softwarebeing developed, an exampleis presentedin whichtwo differentoperationconditionsof a dual-purposepower and desalinationplant are compared.In particular,the effectof the throttlingpart of the steamextractedto the desalinationunit is analyzed,which significantlyincreasesthe cost of water and electricityproduced. Keywordr: Desalination; Simulation; Optimization; Combined production of water and energy; Power generation
1. Introduction
Although the total amount of water on the planet remains constant, fresh water supply is becoming more and more scarce. Hence, augmenting this supply by desalting seawater on a large scale has become inevitable [l]. On the *Correspondingauthor.
other hand, desalination, particularly using thermal methods, requires a great amount of energy [2] (see Table 1). However, desalination is already a means of augmenting fresh water resources in many parts of the world, including the European Union and particularly in Spain and in the Mediterranean countries; it is of continuously increasing
Presented at the European Conference on Desalination and the Environment: Fresh Waterfor All, Malta, 4-8 May 2003. European Desalination Society, International Water Association. 001 l-9164/03/$-
See front matter 0 2003 Elsevier Science B.V. All rights reserved
PII: SO01 l-9164(03)00370-9
J. Uche et al. /Desalination 156 (2003) 367-378
368
Table 1 Specific consumption of more widespread desalination technologies [2] Thermal processes
Specific consumption Qil&&V*,
Mechanical processes
MSF
MED
VC”
RO”
400-50 200-300b
350-400 200-250b
100-200
70-90 30-50’
-
“Electrical energy produced in a conventionalpower plant with 30% effkiency. bDesalinationprocess in cogeneration plant. ‘Including energy recovery system in the RO process.
importance. In 2002 just over 30 Mm3/d of fresh
water were being produced all over the world, with 18 Mm3 desalted from seawater [3]. That represents a 30-fold increase in global capacity over three decades. However, desalinated seawater happens to be yet less than 0.2% of the fresh water used worldwide. Desalination, as currently practiced, is driven almost entirely by the combustion of fossil fuels. It is an energy-intensive process. Taking into account that more or less 60% of desalinated water is produced with thermal processes and 40% with mechanical processes [3], then the required primary energy on average is about 150 kJ/kg of desalted water. That is, the value of specific oil consumption is about 3.5 L of oil per m3 of fresh water produced. Correspondingly, producing the total amount of desalted water of all facilities in the world requires a huge amount of energy, which is approximately equivalent to the 0.3% of primary energy in terms of fossil fuels consumed worldwide. Thus, the present situation demands attention to be diverted to the combined research, design and development of efficient water and energy systems. Moreover, power generation and fresh water production systems represent complex networks of mass and energy flows. For the synthesis and design of such systems, it is highly desirable and much more flexible to leave the choice of any
system configuration entirely up to the designer. This, in turn, requires availability of suitable software which should be totally modular in nature. At the same time, it should provide needed computational facilities for modeling, simulation, optimization and control, as well as assessing capability ofthermoeconomic analysis, environmental impact and eco-efflciency. The computational tools incorporated in the software should go down to the level of basic elements, for example, heat exchange devices, expanders, compressors, combustors, reactors, mixers, etc. This paper outlines a development-oriented object program in the said direction, which hopefully will fill the gap in having user-friendly software in the form of building blocks as an appropriate framework for the synthesis and design of either individual or integrated systems for energy and water. Examples of development in the same direction are given by El-Sayed [461. Such an approach would transcend other traditional approaches for process integration and obviously prove of great help not only to designers but also to researchers, educators and students. The subsequent sections outline the features of the program and an example, performed with the software at its present level of development, of an integrated analysis of a combined power and desalination plant.
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2. Software overview The Building Blocks Software for Water and Energy Systems (BBSWES) being developed is a stand-alone application for scientific and educational use containing, among others, the following features: It is amulti-platform (Java language) oriented object program, compatible with Windows OS, Linux and MacOS. It allows for building a system for a given energy and desalination plant using the subsystems representing the various parts of the plant as building blocks. The program automatically simulates the mass and energy balances in the plant, thereby providing a simulator (in its present level of development based on simple models) for any given structure. The program performs a thermoeconomic analysis evaluating the exergy and monetary costs of all significant flows in a given plant. It also allows the simulation of inefficiencies and generates new data for comparison with the design or reference conditions. As a result, the program performs a causal diagnosis of the deviations of the plant behavior with respect to the design conditions. The user has a graphic user interface (GUI) available, which follows the standard windows appearance containing menus, toolbars and shortcuts to access the most important commands; assisted set-up and uninstall; on-line help and customization and preferences. Concerning hardware interfaces, the software should run correctly on Intel Pentium platforms with VGA displays with a minimum resolution of 800x640 a-byte color definition. 2. I. Sofiare
functions
Conceptually, the functions to be performed are the following:
??
. ?? ?? ?? ??
369
graphical design of plant layouts calculation of heat and mass balances (simulator) thermoeconomic analysis parametric calculation report generator ancillary functions
An overview of each is presented in the next paragraphs. 2.1.1. Graphical design ofjlow sheets The GUI allows the user to build up a plant layout by dragging and dropping elements onto a blank sheet from a collection (Fig. 1). These elements represent processes or equipment (e.g., turbine, heat exchanger, gas turbine or different types of desalination units) that are parametrically described by means of some properties (e.g., isentropic efficiencies, GOB, specific energy consumption) that are supposed to behave physically in a particular manner for each type (see Fig. 2). The flow sheet will be complete with the coherent interconnection of the elements and with the definition of control restrictions and parameters. Note that interconnections as well as control restrictions could include properties and methods as well. The GUI contains the normal features in 2-D design. The elements and interconnections include drag and drop, rotation, sizing, move, copy, automatic reposition or collating, doubleclick to modify properties, etc. Numerical results from the heat and mass balances are accessible or visible directly on the screen: mass flow, temperature, pressure, enthalpy, internal energy, entropy, specific volume, exergy, etc. (see Fig. 3). Further, the software has a tool available for the graphical representation of the processes being analyzed in six different thermodynamic charts (see Fig. 3). Once the installation has been depicted and solved, this facility represents the set of thermodynamic processes involved in the
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flow sheet, allowing the user, in an interactive way, to analyze possible design modifications in the proposed system. Extensibility features have been taken into account so as to include new collections of elements through configuration. The basic elements (some of them are not implemented yet in the present version) proposed are shown in Table 2. 2.1.2. Simulator (calculation of heat and mass balances) A coherently defined flow sheet must agree with a set of validity rules, as for instance determination of the equations system: an equal number of variables and equations. Table 2 Simulation units of the Building Blocks Software for Water and Energy Systems . . . . . . . . . . . . . . . . . . . .
General gas turbine Steam turbine: condensing, non-condensing Heat recovery steam generator: tired or unfired (tired not included yet in the present version) Open heater Closed heater General heat exchanger Furnace/combustion chamber (not included yet in the present version) Absorption unit, general (not included yet in the present version) Furnace/combustion chamber (not included yet in the present version) Absorption unit, general (not included yet in the present version) Compressor Expander Valve Pump Distributor AC generator (not included yet in the present version) MSF unit/element MED unit/element RO distiliation unit VC distillation unit
156 (2003)
367-3
78
Once the flow sheet is checked against these rules (compilation), the resolution of the complete equations system can be conducted (simulation). As a result, mass flows, composition, temperature and pressure of every interconnection are calculated, which is called thermodynamic analysis or heat and mass balance (I-IhW. Flow-sheet-defined variables are also calculated. These correspond to an algebraic formula where the parameters are taken from the flow sheet, for instance, the definition of net or raw efficiency as a function of the input fuels and the output products. 2.1.3. Thermoeconomic analysis Thermoeconomic analysis is a powerful energy analysis tool combining thermodynamics with economic aspects (71.Thermoeconomic cost and profitability calculations based on the Structural Theory of Thermoeconomics [8,9] are proposed. The proposed method will allow using the software to analyze any water and energy system. 1. Calculate the costs [7, lo] of the flows and products of a plant based on physical criteria (Second Law of Thermodynamics), i.e, the amount of fuel consumed for producing a given internal flow or a plant product. 2. Assessment of alternatives for energy savings. 3. Energy audits and assessment of fuel impact of malfunctions (operation diagnosis [7,11]). The operation diagnosis gives information about the cost of the inefficiencies occurring in the different plant units, and also how the inefficiency of a given unit affects the behavior of the other devices, quantified in monetary units or in terms of additional fuel consumption. 2.1.4. Parametric caIcuIations Following a successful simulation and defining a free variable (control parameter or element
J. Uche et al. /Desalination I56 (2003) 367-3 78
property), a set of simulations can be conducted varying stepwise the free variable in a range, yielding new HMBs, one for each simulation. The parametric calculation can be coupled with the Structural Theory of Thermoeconomics cost calculation method to obtain the impact on fuel from the comparison of the base HMB with the set of HMBs generated through the parametric calculations. This facility can be used also for optimization purposes; however, it is planned in near future to implement optimization facilities, based on the definition of an objective function from a predefined set (maximizing outputs, profitability, revenue, minimizing global or specific cost/ greenhouse emissions) allowing the optimization of the original HMB, yielding another HMB. 2.1.5. Report generator The complete set of results for a HMB are accessible in an organized way: 1. Boundary conditions: control parameters and element properties. 2. Interconnection properties: mass flows, composition, temperature, pressure by default. Other thermodynamic properties on user request: enthalpy, entropy, specific volume, exergy. 3. List of flow sheet defined variables. 4. Results of the cost calculation performed (if any) and comparison between different HMB. 2.1.6. Ancillary
finctions
Among others, the following functions are also provided: 1. New/open flow sheet and Quit, with option to save the current work. 2. Generation of different plant states. 3. Different calculation options. 4. Direct accesses to the last flow sheet. 5. Printing current form. 6. On-line help 7. Customization and preferences command
371
2. I. 7. User characteristics The users may range between technical, scientific or academic personnel, who should have a relatively good knowledge of thermodynamics, economics, power plant technology and desalination processes.
3. Simulation and analysis of a dual plant In this section an example of a possible analysis that can be performed with the software presented in this paper is presented for. Despite the strong connection between energy and desalination, it is very common that these two aspects were analyzed separately without taking into account that inefficiencies occurring in the devices of the power plant can affect the behavior of the desalination unit, and vice versa. When the efficiency of a component of either the power or desalination plant is degraded, then the efficiency of the whole system is affected, and as a consequence, the production costs of electricity and water are modified. Hence, the importance of a tool providing the analysis of combined power and desalination plants as a whole system, instead of two separated units, which allows analyzing and studying their interactions. In the example below, a very common operation mode in dual plants producing water and electricity is presented. A frequent way of reducing the power production while keeping constant the water production consists on throttling part of the steam produced in the boiler, avoiding its expansion in the steam turbines. This situation is quite often in the Gulf countries in winter, when the demand of electricity decreases but the water consumption remains almost constant with respect to the summer. Particularly, it is analyzed with the presented software the effect of throttling the steam. It will be shown that although the throttling process occurs in the power plant, it affects the behavior of both power and
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Fig. 1. Flow-sheet diagram of the analyzed dual-purpose power and desalination plant depicted with the graphical user interface of the Building Blocks Software for Water and Energy Systems.
desalination plants and, as a consequence, the costs of water and electricity are modified. 3.1. Dual plant description The dual plant (see Fig. 1) is a natural gas fueled power co-generation plant combined with a multistage flash (MSF) desalination plant. The plant modelled in this example is based on the Al Taweelah B plant, located in the United Arab Emirates [12]. The power plant consists of extraction/ condensing turbines containing two sections, a single flow high-pressure (HP) section with four stages and a single flow low-pressure (LP) section with two stages. Steam extraction outlets for the seawater desalination plant and extraction points for the feedwater heaters are available on both turbine sections. If there is no steam supply for the MSF plant, the steam flows through the
LP section and is returned (via a damper and bypass line). The power generation plant also contains a live steam reduction pressure station used to extract the steam flow to desalination in case there is a failure in the turbine system or in case it is necessary to reduce the produced power keeping the water production constant. The MSF plant is a brine recirculation flow, high-temperature (HT) anti-scale treatment, and cross tube configuration, the most typical of the MSF plant types [ 131.
3.2. Case study For illustrative purposes, in order to show the scope of the software being developed, and the type of analysis that can be conducted with it, two different operating conditions of the dual plant have been compared.
J. Uche et al. /Desalination 156 (2003) 367-3 78 ??
??
373
State 0: The power plant, at the “cogeneration base case”, produces 117 MW of electricity and 2,744 m3/h of drinking water. In this operation point there is no steam passing through the reduction pressure station. The total amount of live steam produced in the boiler is about 156 kg/s at 93 bar and 535°C. State 1: In this case some live steam produced in the boiler is throttled (50 kg/s), which represents 32% of the total amount of steam produced (156 kg/s) at 93 bar and 535OC.The net power produced is 78 MW maintaining the same production of drinking water (2,744 m3/h) than in state 0.
3.3. Thermodynamic
simulation
Once the plant layout has been depicted with the GUI facility, the user should provide the required input data for the thermodynamic simulation of the system. There are two types of input data: ?? those determining the behavior and simulation of each device. The user can input these data into the program through a dialog window for each device (Fig. 2). ?? Properties of selected mass and energy flow streams. A complete HMB corresponding to the mass and energy flow streams is provided in different windows. Flow stream window provides mass flow rate, pressure, temperature, internal energy, enthalpy, entropy, exergy, etc., for each one of the considered flow streams depicted in the plant layout. Process window informs about the energy involved in the processes occurring in the different plant devices. A third user interactive window in which are plotted the thermodynamic processes that occur in the plant. As explained above, six different thermodynamic charts are available
“.**
,,
^
^,/
*..
.,I
,-^
I
/fl.
Fig. 2. Typical dialog window for a device.
where the user can analyze and quantify the influence of different operating parameters on the global efficiency and production of the whole system. Fig. 3 shows the three previously mentioned windows containing the results corresponding to the thermodynamic simulation of the dual plant for state 0 conditions. Different plant states can be analyzed and compared conducting a set of simulations in which one or several variables are modified yielding new HMBs. For each simulation a set of windows is generated. The user can also have the more relevant information available
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Process
window
Fig. 3. Thermodynamic analysis windows displaying the results corresponding to the base case operating conditions of the analyzed dual plant.
related to each flowstream in the flowsheet diagram by selecting a given stream. 3.4. Thermoeconomic analysis From the information obtained in the thermodynamic simulation, a thermoeconomic analysis can also be performed, providing: ?? Exergy costs (kJ/kJ) of each mass and energy flow of the plant, that is the amount of fuel, measured in terms of exergy, consumed for producing a given product or flowstream. ?? Thermoeconomic cost (e/kJ) of each mass and energy flow of the plant, including, if desired, the capital cost of the plant components. The Thermoeconomic cost combines the exergy cost with economical information related to the capital cost of the plant devices and the monetary cost of the fuel plant.
Impact on fuel analysis [ 143. Also called thermoeconomic diagnosis, it informs of the cost of inefficiencies and about their effect on the rest of the devices. When performing a thermoeconomic analysis, it is necessary to define for each plant component a productive purpose and the resources required for achieving it (fuel-product definition). For example, the productive purpose of the MSF unit consists of producing desalted water, and the resources consumed for producing the water are the steam extracted from the power plant -heat released by the steam in the brine heater. Fig. 4 shows the fuel-product window and the unit exergy costs window for all mass and energy flowstreams of our example in the two different states. One of the most outstanding results is the increase (approximately 50%) of the cost of
??
J. &he et al. /Desalination 156 (2003) 367-378
375
Fig. 4. Thermoeconomic analysis windows for the two different plant states of the dual plant analyzed.
desalted water when throttling the steam in the reduction pressure station. This means that the production of 1 kg of desalted water when the reduction pressure station is in operation requires 307.4 kJ of natural gas instead of the 2 14.7 kJ of natural gas consumed in state 0. This amount represents an additional natural gas consumption when operating with the reduction pressure station at 92.7 MJ/m3, that is, 254.4 GJ/h. Taking into account that the natural gas market price is about 2.84 e/GJ, the additional cost of water production is around 722 e/h (5.77 million e/y, considering 8,000 h of operation), just considering the natural gas consumption (Table 3). Furthermore, the unit cost of electricity pro-
Table 3 Unit cost of relevant mass and energy flow streams of the dual plant
Net power (kJ,#,,) Distilled water (kJ,,_!kg_) Live steam (kJ&) Throttled steam (kJ,,_/kJ) Steam to MSF (kJ,,&kJ)
State 0
State 1
2.48 214.7 2.30 2.30
2.51 307.4 2.32 3.45 3.20
duced also increases 0.03 kJ of natural gas per kJ of electricity produced, that is 108 kJ of natural gas per kWh of electricity produced. In the case of the plant analyzed, this represents an
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Fig. 5. Additional natural gas consumption provoked by some inefficiencies in the 4th and 6th turbine sections and in the
feed pump. additional consumption of 202 GJ of natural gas per day (approximately 574 E/d or more than E 190,000/y - 8,000 h of operation). Considering both factors, the additional cost of state 1 (partial load with throttled steam) with respect to state 0 (base case) is, in terms of natural gas, 2.1 x 1O6GJ/y, which represented in monetary units, is almost E6 million/y. In the example presented here, the capital cost of the devices has not been taken into account because it is the same in both states and remains constant. However, the software also provides a facility for performing cost analysis considering also capital and maintenance costs of the different plant units.
As previously explained, the thermoeconomic analysis facility contains the so-called impact on fuel analysis or thermoeconomic diagnosis (see Fig. 5). This facility evaluates the cost of the inefficiencies in terms of additional natural gas consumption and also the effects on the rest of the plant components. The example shown in Fig. 5 corresponds to the analysis of decreasing the efficiency of the fourth and sixth sections of the turbine (- 5%) and of the feed pump (- 10%) in a dual plant, similar to the one analyzed in the previous example. The results are very interesting and reveal that when inefficiencies in the power plant appear, the cost of water is also affected, even though the operating conditions of the
J. Uche et al. /Desalination 156 (2003) 367-378
desalination unit have not been changed. In the case analyzed, it can be seen that additional fuel consumption of the whole plant is due to the inefficiencies of the components. Note that some induced inefficiencies [l 1, 141, i.e., inefticienties provoked in other plant units other than those suffering the behavior degradation, have occurred in the desalination unit; as a consequence, the increase of natural gas consumption provoked by the MSF plant is more than 10% of the total fuel impact. The additional natural gas consumption provoked by these inefficiencies is about 144x 1O3GJ/y (considering 8000 h of operation in a year), representing an additional cost of e408,96O/y (natural gas market price, 2.84 ~/GJ).
377
A more advanced version of the software is being developed, including more complex and realistic simulation models [15] and environmental information, in order to perform a more complex simulation oriented to the synthesis and design of water and energy systems.
Acknowledgments We thank the ICWES (Abu Dhabi, UAE) particularly to its president, Dr. D.M.K. AlGobaisi, and Prof. Husain for their helpful, technical support and availability for developing this project. We also are thankful for highly valuable professional advice and participation of Prof. El Sayed (AESA).
4. Conclusions Power and desalination plants are very complex systems. Interactions occurring among their plant components have a great impact. As shown in this example, in a dual plant the behavior of the desalination plant is affected when inefficiencies occur in the power plant and vice versa. Thus, a more appropriate approach consists ofthe integrated analysis of both plants as a whole system instead of two separated subsystems. The complexity of such plants makes the integrated analysis of both systems very difficult. A systemic approach such as the one implemented in the BBSWES software, is required in order to detect the key aspects of the optimum operation of such complex systems and for a proper and efficient synthesis and design. The BBSWES program is intended to fill the gap between water and energy issues research in having user-friendly software, available for students, researchers and technicians, in the form of building blocks as an appropriate framework for the analysis, synthesis and design of either individual or integrated systems for both energy and water.
References VI D.M.K. Al-Gobaisi, Sustainable augmentation of
VI [31
[41
[51
VI
171
fresh water resources through appropriate energy desalination technologies, in: Proc. IDA World Congress on Desalination and Water Reuse, Madrid, 1997. J. Uche, Thermoeconomic analysis and simulation of a combined power and desalination plant, PhD Thesis, University of Zaragoza, Spain, 2000. K. Wangnick, Regional review for Europe - The region leads in seawater desalination, in: Proc. IDA World Congress on Desalination and Water Reuse, Manama, Bahrain, 2002. Y.M. El-Sayed, A second law based optimization, Parts I & II. J. Engineer. Gas Turbine Power, 118 (1996) 693-703. Y.M. El-Sayed, The thermoeconomics of seawater desalination systems, in: Proc. IDA World Congress on Desalination and Water Reuse, Madrid, 4 (1997) 149-166. Y.M. El-Sayed, Revealing the cost efficiency trends of the design concepts of energy intensive systems, Energy Conv. Mgmt., 40 (1999) 1599-1615. L. Serra, A. Valero, C. Torres and J. Uche, Thermoeconomic analysis: fundamentals, in: A. Husain, ed.,
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Integrated Power and Desalination Slants. EOLSS Publishers, UK, 2003. [8] A. Valero, L. Serra and M.A. Lozano, Structural theory of thermoeconomics, in: International Symposiumon Thermodynamicsand the Design,Analysis and Improvement of Energy Systems, ASME Book No. H00874, H.J. Richter, ed., New Orleans, USA, 1993, pp. 189-198. [9] L. Serra and C. Torres, Structural theory of thermoeconomics, in: Encyclopedia of Life Support Systems, http://www.eolss.net,2003. [lo] J. Uche, L. Serra and A. Valero, Cost analysis of a dual purpose power and desalination plant, in: A. Husain, ed., Integrated Power and Desalination Plants, EOLSS Publishers, UK, 2003. [I 1] J. Uche A. Valero and L. Serra, Thermoeconomic diagnosis of a dual purpose power and desalination
plant, in: A. Husain, ed., Integrated Power and Desalination Plants, EOLSS Publishers, UK, 2003. [12] B. Al Taweelah, Modem Power Systems, Special Issue, Wilmington Publishing, 1995. [ 131 A. Husain, A. Woldai, A. Al-Radif, A. Kesou, R. Borsani, H. Sultan and P.B. Deshpandey, Modelling and simulation of a multistage flash (MSF) desalination plant, Desalination, 97 (1994) 555-586. [ 141 A. Valero, C. Torres, F. Lerch, J. Royo and L. Serra, Structural theory and thermoeconomic diagnosis, Parts I and II, Energy Conv. Mgmt., 43 (2002) 15031535. [ 151 J. Uche, J.A. Turegano, L. Serra, A. Vafero, A. Husain, D.M.K. AI Gobaisi and Y.M. El Sayed, Building Blocks Software for Water and Energy Systems, Desalination Water Reuse, 11 (2001) 2430.
Desalination 156 (2003) 367-378 www.elsevier.com/locate/desal
Software for the analysis of water and energy systems Javier Uche* , Luis Serra, Luis Albert0 Herrero, Antonio Valero, JosCAntonio Turdgano, CCsarTorres International Study Group for Water and Energy Systems (ISG WES), Centre of Research for Energy Resources and Consumption (CIRCE), University of Zuragoza, Zuragoza, Spain Tel. i-34 (976) 76 18 63; Fax +34 (976) 73 20 78; email: [email protected] Received 10 February 2003; accepted 15 February 2003
Abstract
Waterand energy are crucial aspects strongly connected when dealing with the problem of augmenting f?esh water resources. Surprisingly, inthe scientific and technological community there is a marked lack of attention to the combined research of water and energy issues. The present paper outlines a development-oriented object program in the said direction, oriented to the integrated analysis of power and desalination plants, which would hopefully fill the gap in having user-friendly software in the form of building blocks as an appropriate Ii-ameworkfor the analysis, synthesis and design of either individual or integrated systems for energy and water. Such an approach would transcend other traditional approaches for process integration and obviously prove of great help not only to designers but also to researchers, educators and students.For iliustraGvepurposes,in orderto show the scopeofthe softwarebeing developed, an exampleis presentedin whichtwo differentoperationconditionsof a dual-purposepower and desalinationplant are compared.In particular,the effectof the throttlingpart of the steamextractedto the desalinationunit is analyzed,which significantlyincreasesthe cost of water and electricityproduced. Keywordr: Desalination; Simulation; Optimization; Combined production of water and energy; Power generation
1. Introduction
Although the total amount of water on the planet remains constant, fresh water supply is becoming more and more scarce. Hence, augmenting this supply by desalting seawater on a large scale has become inevitable [l]. On the *Correspondingauthor.
other hand, desalination, particularly using thermal methods, requires a great amount of energy [2] (see Table 1). However, desalination is already a means of augmenting fresh water resources in many parts of the world, including the European Union and particularly in Spain and in the Mediterranean countries; it is of continuously increasing
Presented at the European Conference on Desalination and the Environment: Fresh Waterfor All, Malta, 4-8 May 2003. European Desalination Society, International Water Association. 001 l-9164/03/$-
See front matter 0 2003 Elsevier Science B.V. All rights reserved
PII: SO01 l-9164(03)00370-9
J. Uche et al. /Desalination 156 (2003) 367-378
368
Table 1 Specific consumption of more widespread desalination technologies [2] Thermal processes
Specific consumption Qil&&V*,
Mechanical processes
MSF
MED
VC”
RO”
400-50 200-300b
350-400 200-250b
100-200
70-90 30-50’
-
“Electrical energy produced in a conventionalpower plant with 30% effkiency. bDesalinationprocess in cogeneration plant. ‘Including energy recovery system in the RO process.
importance. In 2002 just over 30 Mm3/d of fresh
water were being produced all over the world, with 18 Mm3 desalted from seawater [3]. That represents a 30-fold increase in global capacity over three decades. However, desalinated seawater happens to be yet less than 0.2% of the fresh water used worldwide. Desalination, as currently practiced, is driven almost entirely by the combustion of fossil fuels. It is an energy-intensive process. Taking into account that more or less 60% of desalinated water is produced with thermal processes and 40% with mechanical processes [3], then the required primary energy on average is about 150 kJ/kg of desalted water. That is, the value of specific oil consumption is about 3.5 L of oil per m3 of fresh water produced. Correspondingly, producing the total amount of desalted water of all facilities in the world requires a huge amount of energy, which is approximately equivalent to the 0.3% of primary energy in terms of fossil fuels consumed worldwide. Thus, the present situation demands attention to be diverted to the combined research, design and development of efficient water and energy systems. Moreover, power generation and fresh water production systems represent complex networks of mass and energy flows. For the synthesis and design of such systems, it is highly desirable and much more flexible to leave the choice of any
system configuration entirely up to the designer. This, in turn, requires availability of suitable software which should be totally modular in nature. At the same time, it should provide needed computational facilities for modeling, simulation, optimization and control, as well as assessing capability ofthermoeconomic analysis, environmental impact and eco-efflciency. The computational tools incorporated in the software should go down to the level of basic elements, for example, heat exchange devices, expanders, compressors, combustors, reactors, mixers, etc. This paper outlines a development-oriented object program in the said direction, which hopefully will fill the gap in having user-friendly software in the form of building blocks as an appropriate framework for the synthesis and design of either individual or integrated systems for energy and water. Examples of development in the same direction are given by El-Sayed [461. Such an approach would transcend other traditional approaches for process integration and obviously prove of great help not only to designers but also to researchers, educators and students. The subsequent sections outline the features of the program and an example, performed with the software at its present level of development, of an integrated analysis of a combined power and desalination plant.
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2. Software overview The Building Blocks Software for Water and Energy Systems (BBSWES) being developed is a stand-alone application for scientific and educational use containing, among others, the following features: It is amulti-platform (Java language) oriented object program, compatible with Windows OS, Linux and MacOS. It allows for building a system for a given energy and desalination plant using the subsystems representing the various parts of the plant as building blocks. The program automatically simulates the mass and energy balances in the plant, thereby providing a simulator (in its present level of development based on simple models) for any given structure. The program performs a thermoeconomic analysis evaluating the exergy and monetary costs of all significant flows in a given plant. It also allows the simulation of inefficiencies and generates new data for comparison with the design or reference conditions. As a result, the program performs a causal diagnosis of the deviations of the plant behavior with respect to the design conditions. The user has a graphic user interface (GUI) available, which follows the standard windows appearance containing menus, toolbars and shortcuts to access the most important commands; assisted set-up and uninstall; on-line help and customization and preferences. Concerning hardware interfaces, the software should run correctly on Intel Pentium platforms with VGA displays with a minimum resolution of 800x640 a-byte color definition. 2. I. Sofiare
functions
Conceptually, the functions to be performed are the following:
??
. ?? ?? ?? ??
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graphical design of plant layouts calculation of heat and mass balances (simulator) thermoeconomic analysis parametric calculation report generator ancillary functions
An overview of each is presented in the next paragraphs. 2.1.1. Graphical design ofjlow sheets The GUI allows the user to build up a plant layout by dragging and dropping elements onto a blank sheet from a collection (Fig. 1). These elements represent processes or equipment (e.g., turbine, heat exchanger, gas turbine or different types of desalination units) that are parametrically described by means of some properties (e.g., isentropic efficiencies, GOB, specific energy consumption) that are supposed to behave physically in a particular manner for each type (see Fig. 2). The flow sheet will be complete with the coherent interconnection of the elements and with the definition of control restrictions and parameters. Note that interconnections as well as control restrictions could include properties and methods as well. The GUI contains the normal features in 2-D design. The elements and interconnections include drag and drop, rotation, sizing, move, copy, automatic reposition or collating, doubleclick to modify properties, etc. Numerical results from the heat and mass balances are accessible or visible directly on the screen: mass flow, temperature, pressure, enthalpy, internal energy, entropy, specific volume, exergy, etc. (see Fig. 3). Further, the software has a tool available for the graphical representation of the processes being analyzed in six different thermodynamic charts (see Fig. 3). Once the installation has been depicted and solved, this facility represents the set of thermodynamic processes involved in the
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flow sheet, allowing the user, in an interactive way, to analyze possible design modifications in the proposed system. Extensibility features have been taken into account so as to include new collections of elements through configuration. The basic elements (some of them are not implemented yet in the present version) proposed are shown in Table 2. 2.1.2. Simulator (calculation of heat and mass balances) A coherently defined flow sheet must agree with a set of validity rules, as for instance determination of the equations system: an equal number of variables and equations. Table 2 Simulation units of the Building Blocks Software for Water and Energy Systems . . . . . . . . . . . . . . . . . . . .
General gas turbine Steam turbine: condensing, non-condensing Heat recovery steam generator: tired or unfired (tired not included yet in the present version) Open heater Closed heater General heat exchanger Furnace/combustion chamber (not included yet in the present version) Absorption unit, general (not included yet in the present version) Furnace/combustion chamber (not included yet in the present version) Absorption unit, general (not included yet in the present version) Compressor Expander Valve Pump Distributor AC generator (not included yet in the present version) MSF unit/element MED unit/element RO distiliation unit VC distillation unit
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Once the flow sheet is checked against these rules (compilation), the resolution of the complete equations system can be conducted (simulation). As a result, mass flows, composition, temperature and pressure of every interconnection are calculated, which is called thermodynamic analysis or heat and mass balance (I-IhW. Flow-sheet-defined variables are also calculated. These correspond to an algebraic formula where the parameters are taken from the flow sheet, for instance, the definition of net or raw efficiency as a function of the input fuels and the output products. 2.1.3. Thermoeconomic analysis Thermoeconomic analysis is a powerful energy analysis tool combining thermodynamics with economic aspects (71.Thermoeconomic cost and profitability calculations based on the Structural Theory of Thermoeconomics [8,9] are proposed. The proposed method will allow using the software to analyze any water and energy system. 1. Calculate the costs [7, lo] of the flows and products of a plant based on physical criteria (Second Law of Thermodynamics), i.e, the amount of fuel consumed for producing a given internal flow or a plant product. 2. Assessment of alternatives for energy savings. 3. Energy audits and assessment of fuel impact of malfunctions (operation diagnosis [7,11]). The operation diagnosis gives information about the cost of the inefficiencies occurring in the different plant units, and also how the inefficiency of a given unit affects the behavior of the other devices, quantified in monetary units or in terms of additional fuel consumption. 2.1.4. Parametric caIcuIations Following a successful simulation and defining a free variable (control parameter or element
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property), a set of simulations can be conducted varying stepwise the free variable in a range, yielding new HMBs, one for each simulation. The parametric calculation can be coupled with the Structural Theory of Thermoeconomics cost calculation method to obtain the impact on fuel from the comparison of the base HMB with the set of HMBs generated through the parametric calculations. This facility can be used also for optimization purposes; however, it is planned in near future to implement optimization facilities, based on the definition of an objective function from a predefined set (maximizing outputs, profitability, revenue, minimizing global or specific cost/ greenhouse emissions) allowing the optimization of the original HMB, yielding another HMB. 2.1.5. Report generator The complete set of results for a HMB are accessible in an organized way: 1. Boundary conditions: control parameters and element properties. 2. Interconnection properties: mass flows, composition, temperature, pressure by default. Other thermodynamic properties on user request: enthalpy, entropy, specific volume, exergy. 3. List of flow sheet defined variables. 4. Results of the cost calculation performed (if any) and comparison between different HMB. 2.1.6. Ancillary
finctions
Among others, the following functions are also provided: 1. New/open flow sheet and Quit, with option to save the current work. 2. Generation of different plant states. 3. Different calculation options. 4. Direct accesses to the last flow sheet. 5. Printing current form. 6. On-line help 7. Customization and preferences command
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2. I. 7. User characteristics The users may range between technical, scientific or academic personnel, who should have a relatively good knowledge of thermodynamics, economics, power plant technology and desalination processes.
3. Simulation and analysis of a dual plant In this section an example of a possible analysis that can be performed with the software presented in this paper is presented for. Despite the strong connection between energy and desalination, it is very common that these two aspects were analyzed separately without taking into account that inefficiencies occurring in the devices of the power plant can affect the behavior of the desalination unit, and vice versa. When the efficiency of a component of either the power or desalination plant is degraded, then the efficiency of the whole system is affected, and as a consequence, the production costs of electricity and water are modified. Hence, the importance of a tool providing the analysis of combined power and desalination plants as a whole system, instead of two separated units, which allows analyzing and studying their interactions. In the example below, a very common operation mode in dual plants producing water and electricity is presented. A frequent way of reducing the power production while keeping constant the water production consists on throttling part of the steam produced in the boiler, avoiding its expansion in the steam turbines. This situation is quite often in the Gulf countries in winter, when the demand of electricity decreases but the water consumption remains almost constant with respect to the summer. Particularly, it is analyzed with the presented software the effect of throttling the steam. It will be shown that although the throttling process occurs in the power plant, it affects the behavior of both power and
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Fig. 1. Flow-sheet diagram of the analyzed dual-purpose power and desalination plant depicted with the graphical user interface of the Building Blocks Software for Water and Energy Systems.
desalination plants and, as a consequence, the costs of water and electricity are modified. 3.1. Dual plant description The dual plant (see Fig. 1) is a natural gas fueled power co-generation plant combined with a multistage flash (MSF) desalination plant. The plant modelled in this example is based on the Al Taweelah B plant, located in the United Arab Emirates [12]. The power plant consists of extraction/ condensing turbines containing two sections, a single flow high-pressure (HP) section with four stages and a single flow low-pressure (LP) section with two stages. Steam extraction outlets for the seawater desalination plant and extraction points for the feedwater heaters are available on both turbine sections. If there is no steam supply for the MSF plant, the steam flows through the
LP section and is returned (via a damper and bypass line). The power generation plant also contains a live steam reduction pressure station used to extract the steam flow to desalination in case there is a failure in the turbine system or in case it is necessary to reduce the produced power keeping the water production constant. The MSF plant is a brine recirculation flow, high-temperature (HT) anti-scale treatment, and cross tube configuration, the most typical of the MSF plant types [ 131.
3.2. Case study For illustrative purposes, in order to show the scope of the software being developed, and the type of analysis that can be conducted with it, two different operating conditions of the dual plant have been compared.
J. Uche et al. /Desalination 156 (2003) 367-3 78 ??
??
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State 0: The power plant, at the “cogeneration base case”, produces 117 MW of electricity and 2,744 m3/h of drinking water. In this operation point there is no steam passing through the reduction pressure station. The total amount of live steam produced in the boiler is about 156 kg/s at 93 bar and 535°C. State 1: In this case some live steam produced in the boiler is throttled (50 kg/s), which represents 32% of the total amount of steam produced (156 kg/s) at 93 bar and 535OC.The net power produced is 78 MW maintaining the same production of drinking water (2,744 m3/h) than in state 0.
3.3. Thermodynamic
simulation
Once the plant layout has been depicted with the GUI facility, the user should provide the required input data for the thermodynamic simulation of the system. There are two types of input data: ?? those determining the behavior and simulation of each device. The user can input these data into the program through a dialog window for each device (Fig. 2). ?? Properties of selected mass and energy flow streams. A complete HMB corresponding to the mass and energy flow streams is provided in different windows. Flow stream window provides mass flow rate, pressure, temperature, internal energy, enthalpy, entropy, exergy, etc., for each one of the considered flow streams depicted in the plant layout. Process window informs about the energy involved in the processes occurring in the different plant devices. A third user interactive window in which are plotted the thermodynamic processes that occur in the plant. As explained above, six different thermodynamic charts are available
“.**
,,
^
^,/
*..
.,I
,-^
I
/fl.
Fig. 2. Typical dialog window for a device.
where the user can analyze and quantify the influence of different operating parameters on the global efficiency and production of the whole system. Fig. 3 shows the three previously mentioned windows containing the results corresponding to the thermodynamic simulation of the dual plant for state 0 conditions. Different plant states can be analyzed and compared conducting a set of simulations in which one or several variables are modified yielding new HMBs. For each simulation a set of windows is generated. The user can also have the more relevant information available
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Process
window
Fig. 3. Thermodynamic analysis windows displaying the results corresponding to the base case operating conditions of the analyzed dual plant.
related to each flowstream in the flowsheet diagram by selecting a given stream. 3.4. Thermoeconomic analysis From the information obtained in the thermodynamic simulation, a thermoeconomic analysis can also be performed, providing: ?? Exergy costs (kJ/kJ) of each mass and energy flow of the plant, that is the amount of fuel, measured in terms of exergy, consumed for producing a given product or flowstream. ?? Thermoeconomic cost (e/kJ) of each mass and energy flow of the plant, including, if desired, the capital cost of the plant components. The Thermoeconomic cost combines the exergy cost with economical information related to the capital cost of the plant devices and the monetary cost of the fuel plant.
Impact on fuel analysis [ 143. Also called thermoeconomic diagnosis, it informs of the cost of inefficiencies and about their effect on the rest of the devices. When performing a thermoeconomic analysis, it is necessary to define for each plant component a productive purpose and the resources required for achieving it (fuel-product definition). For example, the productive purpose of the MSF unit consists of producing desalted water, and the resources consumed for producing the water are the steam extracted from the power plant -heat released by the steam in the brine heater. Fig. 4 shows the fuel-product window and the unit exergy costs window for all mass and energy flowstreams of our example in the two different states. One of the most outstanding results is the increase (approximately 50%) of the cost of
??
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Fig. 4. Thermoeconomic analysis windows for the two different plant states of the dual plant analyzed.
desalted water when throttling the steam in the reduction pressure station. This means that the production of 1 kg of desalted water when the reduction pressure station is in operation requires 307.4 kJ of natural gas instead of the 2 14.7 kJ of natural gas consumed in state 0. This amount represents an additional natural gas consumption when operating with the reduction pressure station at 92.7 MJ/m3, that is, 254.4 GJ/h. Taking into account that the natural gas market price is about 2.84 e/GJ, the additional cost of water production is around 722 e/h (5.77 million e/y, considering 8,000 h of operation), just considering the natural gas consumption (Table 3). Furthermore, the unit cost of electricity pro-
Table 3 Unit cost of relevant mass and energy flow streams of the dual plant
Net power (kJ,#,,) Distilled water (kJ,,_!kg_) Live steam (kJ&) Throttled steam (kJ,,_/kJ) Steam to MSF (kJ,,&kJ)
State 0
State 1
2.48 214.7 2.30 2.30
2.51 307.4 2.32 3.45 3.20
duced also increases 0.03 kJ of natural gas per kJ of electricity produced, that is 108 kJ of natural gas per kWh of electricity produced. In the case of the plant analyzed, this represents an
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Fig. 5. Additional natural gas consumption provoked by some inefficiencies in the 4th and 6th turbine sections and in the
feed pump. additional consumption of 202 GJ of natural gas per day (approximately 574 E/d or more than E 190,000/y - 8,000 h of operation). Considering both factors, the additional cost of state 1 (partial load with throttled steam) with respect to state 0 (base case) is, in terms of natural gas, 2.1 x 1O6GJ/y, which represented in monetary units, is almost E6 million/y. In the example presented here, the capital cost of the devices has not been taken into account because it is the same in both states and remains constant. However, the software also provides a facility for performing cost analysis considering also capital and maintenance costs of the different plant units.
As previously explained, the thermoeconomic analysis facility contains the so-called impact on fuel analysis or thermoeconomic diagnosis (see Fig. 5). This facility evaluates the cost of the inefficiencies in terms of additional natural gas consumption and also the effects on the rest of the plant components. The example shown in Fig. 5 corresponds to the analysis of decreasing the efficiency of the fourth and sixth sections of the turbine (- 5%) and of the feed pump (- 10%) in a dual plant, similar to the one analyzed in the previous example. The results are very interesting and reveal that when inefficiencies in the power plant appear, the cost of water is also affected, even though the operating conditions of the
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desalination unit have not been changed. In the case analyzed, it can be seen that additional fuel consumption of the whole plant is due to the inefficiencies of the components. Note that some induced inefficiencies [l 1, 141, i.e., inefticienties provoked in other plant units other than those suffering the behavior degradation, have occurred in the desalination unit; as a consequence, the increase of natural gas consumption provoked by the MSF plant is more than 10% of the total fuel impact. The additional natural gas consumption provoked by these inefficiencies is about 144x 1O3GJ/y (considering 8000 h of operation in a year), representing an additional cost of e408,96O/y (natural gas market price, 2.84 ~/GJ).
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A more advanced version of the software is being developed, including more complex and realistic simulation models [15] and environmental information, in order to perform a more complex simulation oriented to the synthesis and design of water and energy systems.
Acknowledgments We thank the ICWES (Abu Dhabi, UAE) particularly to its president, Dr. D.M.K. AlGobaisi, and Prof. Husain for their helpful, technical support and availability for developing this project. We also are thankful for highly valuable professional advice and participation of Prof. El Sayed (AESA).
4. Conclusions Power and desalination plants are very complex systems. Interactions occurring among their plant components have a great impact. As shown in this example, in a dual plant the behavior of the desalination plant is affected when inefficiencies occur in the power plant and vice versa. Thus, a more appropriate approach consists ofthe integrated analysis of both plants as a whole system instead of two separated subsystems. The complexity of such plants makes the integrated analysis of both systems very difficult. A systemic approach such as the one implemented in the BBSWES software, is required in order to detect the key aspects of the optimum operation of such complex systems and for a proper and efficient synthesis and design. The BBSWES program is intended to fill the gap between water and energy issues research in having user-friendly software, available for students, researchers and technicians, in the form of building blocks as an appropriate framework for the analysis, synthesis and design of either individual or integrated systems for both energy and water.
References VI D.M.K. Al-Gobaisi, Sustainable augmentation of
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fresh water resources through appropriate energy desalination technologies, in: Proc. IDA World Congress on Desalination and Water Reuse, Madrid, 1997. J. Uche, Thermoeconomic analysis and simulation of a combined power and desalination plant, PhD Thesis, University of Zaragoza, Spain, 2000. K. Wangnick, Regional review for Europe - The region leads in seawater desalination, in: Proc. IDA World Congress on Desalination and Water Reuse, Manama, Bahrain, 2002. Y.M. El-Sayed, A second law based optimization, Parts I & II. J. Engineer. Gas Turbine Power, 118 (1996) 693-703. Y.M. El-Sayed, The thermoeconomics of seawater desalination systems, in: Proc. IDA World Congress on Desalination and Water Reuse, Madrid, 4 (1997) 149-166. Y.M. El-Sayed, Revealing the cost efficiency trends of the design concepts of energy intensive systems, Energy Conv. Mgmt., 40 (1999) 1599-1615. L. Serra, A. Valero, C. Torres and J. Uche, Thermoeconomic analysis: fundamentals, in: A. Husain, ed.,
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Integrated Power and Desalination Slants. EOLSS Publishers, UK, 2003. [8] A. Valero, L. Serra and M.A. Lozano, Structural theory of thermoeconomics, in: International Symposiumon Thermodynamicsand the Design,Analysis and Improvement of Energy Systems, ASME Book No. H00874, H.J. Richter, ed., New Orleans, USA, 1993, pp. 189-198. [9] L. Serra and C. Torres, Structural theory of thermoeconomics, in: Encyclopedia of Life Support Systems, http://www.eolss.net,2003. [lo] J. Uche, L. Serra and A. Valero, Cost analysis of a dual purpose power and desalination plant, in: A. Husain, ed., Integrated Power and Desalination Plants, EOLSS Publishers, UK, 2003. [I 1] J. Uche A. Valero and L. Serra, Thermoeconomic diagnosis of a dual purpose power and desalination
plant, in: A. Husain, ed., Integrated Power and Desalination Plants, EOLSS Publishers, UK, 2003. [12] B. Al Taweelah, Modem Power Systems, Special Issue, Wilmington Publishing, 1995. [ 131 A. Husain, A. Woldai, A. Al-Radif, A. Kesou, R. Borsani, H. Sultan and P.B. Deshpandey, Modelling and simulation of a multistage flash (MSF) desalination plant, Desalination, 97 (1994) 555-586. [ 141 A. Valero, C. Torres, F. Lerch, J. Royo and L. Serra, Structural theory and thermoeconomic diagnosis, Parts I and II, Energy Conv. Mgmt., 43 (2002) 15031535. [ 151 J. Uche, J.A. Turegano, L. Serra, A. Vafero, A. Husain, D.M.K. AI Gobaisi and Y.M. El Sayed, Building Blocks Software for Water and Energy Systems, Desalination Water Reuse, 11 (2001) 2430.