Thermodynamic evaluation of a combined-cycle power plant with MSF and MED desalination

Rising water scarcity and abundant brine water resources, especially in desert locations, call for the wider adaptation of desalination techniques. Furthermore, the interdependency of water and energy has gained more attention in recent years and it is expected to play an important role in the near future. The present study deals with both topics in that it presents the coupling of a power plant with desalination units for the simultaneous generation of energy and water in Iran. The power plant used in the analysis is the Qom combined-cycle power plant. The plant is integrated, first, with a multi-stage flash (MSF) unit and, then, with a multi-effect desalination (MED) unit, and it is evaluated using energy and exergy analyses. We find that the generated power of the integrated systems is decreased by 9.7% and 8.5% with the MED and the MSF units, respectively. Lastly, the freshwater production in the plant using MED is significantly higher than in the plant with MSF (1,000 versus 1,521 kg/s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2020.025 om http://iwaponline.com/jwrd/article-pdf/10/2/146/701326/jwrd0100146.pdf er 2021 M. H. Khoshgoftar Manesh (corresponding author) S. Kabiri M. Yazdi Division of Thermal Sciences and Energy Systems, Department of Mechanical Engineering, Faculty of Technology and Engineering, University of Qom, Qom, Iran E-mail: m.khoshgoftar@qom.ac.ir F. Petrakopoulou Department of Thermal and Fluid Engineering, University Carlos III of Madrid, Madrid, Spain


CASE STUDY
The reference power plant The reference plant examined in this paper is the Qom com-

MSF desalination unit
The Qom reference plant is coupled, first, with an MSF desalination unit. MSF water desalination is a multi-stage evaporation process and it is a very common technology in the Middle East. Heated water is directed to tanks with decreasing pressure, where the water reaches a point of low pressure that causes its sudden evaporation. Figure 2 shows the combined-cycle plant coupled with the MSF desalination unit.
The energy consumption of this desalination method is high and the high operating temperature leads to an increase in sedimentation issues in these devices. Nevertheless, because of its relatively high-capacity operation, this method has a relatively high-water production, when compared to other methods.
Since this process operates at low temperature and pressure, it

MED desalination unit
At present, 5% of the world's water production capacity uses this technology. A simple flow diagram of the coupled reference Qom plant with a MED desalination unit is shown in Figure 3.
The process of MED is based on a multi-effect distilla-

METHODOLOGY Energy analysis
In this paper, we study the integration of a reference com-  Compressor: Combustion chamber: Gas and steam turbine: HRSG: Pump: Condenser: Combined-cycle calculations: In these thermodynamic equations S stands for entropy,

Energy analysis of the desalination units
The temperature difference in each stage is: We can introduce N as the number of stages, T N as the brine water temperature, and T BT as the brine water temperature at the top.
With the above equation, we can calculate the temperature at each stage: The mass flow rate of desalinated water at each stage can be obtained with the equation below: where, _ m r is the recoverable brine water that comes back to the MSF unit, and the y parameter is the amount of specific sensible and latent heat and can be obtained with the equation: where, c p is the specific heat and λ av can be calculated with the equation: The salt concentration of the recoverable salt steam (x r ) is calculated as follows: The mass flow rates of the required motive steam ( _ m s ) is obtained as follows: The performance ratio of desalination units is very important for comparing different technologies and it is defined as follows: The next equation is used to calculate the mass flow rate of desalinated water: where, _ m f is mass flow rate of supplied seawater, _ m b is mass flow rate of brine discharge and _ m d is mass flow rate of desalinated water.
The mass flow rate of brine discharge is then calculated as: where, X f shows the water salinity of the source (X f ¼ 0:035) and X b shows the salinity of the brine.
The heat transfer and the reversible work in each stage are obtained as follows: where, i is the number of the stage.

Exergy analysis
Exergy analysis is based on the second law of thermodynamics and enables us to estimate the efficiency of different processes and quantify the exergy destruction within individual plant components. Exergy analysis reveals the location and type of real thermodynamic inefficiencies, an insight that is very useful, especially for the design of new systems.
In all power equipment, all material flows can be divided into two categories: flows that carry energy to the component and can be considered as the fuel of the equipment (exergy of the fuel, _ E F ), and flows related to the desired output of the process or component, i.e., the exergy of the product ( _ E P ). The difference between these two values corresponds to the exergy destruction in a component k: The ratio between the exergy of the product and the fuel of component k is its exergetic efficiency:  Table 2.
Due to the use of steam from the power plant to generate freshwater in the desalination systems, the power production of the integrated systems is decreased relative to standalone operation. This reduction is found to be 9.7% (69.36 MW) in the simulation with the MED system and 8.5% (60.64 MW) in the simulation with the MSF.

Exergy evaluation
The stream exergies of the combined-cycle plant and the coupled simulations (combined cycle with MED and combined cycle with MSF) are shown in Table 3. By calculating the exergy of each material stream and defining exergy balances for each plant component, the exergy destruction for each component is obtained (Table 4).
From the results shown in Tables 3 and 4, we can conclude that the differences between using MED or MSF are rather small. The exergy destruction in the plant with the MED unit is found to be 5.77 MW lower than in the plant with the MSF unit. We further find that the plant with MSF results in a reduced exergy destruction in the steam turbine and the condenser. The exergy destruction in the compressors, combustion chamber, and expanders is similar

CONCLUSION
Desalination of seawater is aimed at supplying fresh, potable water for domestic, industrial, and agricultural uses.
The process of seawater salt separation requires energy that can be supplied by thermal, mechanical, or electrical energy. The addition of products in power plants can lead to more efficient, reliable, and economical solutions.
The cogeneration of energy and water generation in the same facility can play a very important role in the mitigation of water crisis, especially in arid regions. In this work, we simulate and evaluate the coupling of an existing combined-cycle power plant with two desalination units: first, with a multi-stage flash desalination and second, with a multi-effect desalination unit. In order to select the most viable desalination method for the Qom combined-cycle power plant, the system is evaluated using exergy analysis. We find that the generated power of the integrated systems is decreased by 9.7% and 8.5% with the MED and the MSF units, respectively. The results show that the exergy destruction of the multi-effect and multi-stage flash desalination units are 30.75 MW and 36.52 MW, respectively. The total exergy destruction of the combined-cycle power plant integrated with the multistage flash desalination is found to be higher than that of the combined cycle with the multi-effect desalination unit.