Exergoeconomic modeling and evaluation of a combined-cycle plant with MSF and MED desalination

In the coming years, numerous regions are expected to suffer from water scarcity. One of the technologies of great interest in facing this challenge has been the generation of freshwater through water desalination, a process that reduces the amount of salt and minerals to a standard level, making the water suitable for drinking or agricultural/industrial use. The efficiency of each desalination process depends on the concentration of salts in the raw water and the end-use of the produced water. The present study presents the exergetic and exergoeconomic analyses of the coupling of a power plant with desalination units for the simultaneous generation of energy and water in Iran. The plant is integrated, first, with a multi-stage flash (MSF) unit and, then, with a multieffect desalination (MED) unit. We find that the cost of exergy destruction of the MED and MSF integrated plants is lower when compared to the standalone power plant by about 0.1% and 9.2%, respectively. Lastly, the freshwater production in the plant using MED is significantly higher than that 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.2020.074 om http://iwaponline.com/jwrd/article-pdf/10/2/158/701394/jwrd0100158.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
One of the most common methods of water desalination is heat distillation. In this process, the water is first boiled using heat and then evaporated. Next, pure water can be obtained by cooling and condensing the water.
The process can be done in two ways: first, by heating the hot water to a boiling temperature, and second, by using thermal energy of the steam in the Rankine cycle.
In most cases, distillation is more efficient than other membrane processes and the quality of water produced by this method is higher.
One of the most important challenges of using thermal methods is the amount of thermal energy consumed. It is thus an advantageous method when thermal energy is available and is more widely used in countries where it is possible to build a water desalination station next to a thermal power plant. In this paper, the MED and MSF desalination units are connected to a combined-cycle block. Schematics of these cycles are presented in Figures 2 and 3.

METHODOLOGY
The energy and exergy equations used are presented in the Appendix of the paper (Manesh et al. ).

Exergoeconomic analysis
An exergoeconomic analysis combines an economic analysis with the results of the exergy analysis. With  The present worth (PW) of a plant's equipment is calculated as: with C i the cost of each stream, S n the salvage value, PWF the present worth factor for each piece of equipment, and CRF the capital recovery factor. The PW is converted to annualized costs using the CRF. By calculating the purchase equipment costs (PECs), we can obtain the investment cost rate _ Z k for each component (  find the cost rate for each component k as follows (Kwak et al. ): The cost rate _ Z k includes the cost rate of capital investment (Z CI ) and the cost rate of operating and maintenance costs (Z OM ) (Bejan et al. ). The maintenance cost has been defined through the factor φ k ¼ 1:06 for each component of the plant, for which the expected economic life has been assumed to be 30 years (Kwak et al. ).
A cost balance is written for each plant component as (Ahmadi et al. ): The defined equations and auxiliary equations for each component are shown in Table 2.
Lastly, the exergy destruction level (EDL) and the exergy cost destruction level (ECDL) are also used to better understand the cost of destruction and overall plant Gas turbine   performance.

Energy and exergy evaluation
The thermodynamic properties of the combined cycle coupled with the MED and MSF units are shown in Table 3

Exergoeconomic evaluation
When looking at the economic performance of the plants, we find that despite the relatively high capital cost of the          In the study by Hafdhi et al. (), energy, exergy, and economic analyses were carried out for a steam cycle along with an MSF water desalination unit. The results included appropriate parameters such as overall efficiency and the level of heat exchange and the design parameters of the cycle. In our study, we examined the detailed thermodynamic and exergy performance of the combined-cycle power plant, calculating the exergy of each stream and the exergy destruction associated with each piece of equipment.
Lastly, the economic analysis includes new parameters, like the EDL and ECDL.

CONCLUSION
Desalination of seawater is aimed at supplying fresh and potable water for domestic, industrial, or agricultural uses.
The process requires energy that can be supplied by thermal, mechanical, or electrical energy. In this work, we evaluated the coupling of a combined-cycle power plant with desalination units: first with an MSF and, second, with an MED unit.
The starting combined-cycle power plant was based on the existing Qom combined-cycle power plant. In order to select the most viable desalination method for the power plant, the system was evaluated using exergoeconomic analysis.
The investment cost of the integrated combined cycle with the MSF desalination unit is higher than that of the integrated plant with the MED unit. This leads to a somewhat more expensive product, when compared to that of the plant with MED. It is seen, thus, that the use of MSF can lead to a higher profit due to the increased production of freshwater. Coupling the plant with an MED unit, on the other hand, can provide a cheaper alternative, when it comes to investment costs.