Thermo-economic analysis for optimal selection of desalination techniques based on combined cycle power plant

Abstract

Scarcity of water has become a challenging problem for countries in arid and semi-arid regions. Seawater desalination is considered to be one of the main sources of potable water in areas such as the Middle East and North Africa, with dependency on desalination reaching 90% in some of the Gulf Cooperation Council (GCC) countries. Libya is one of those counties which suffers from a lack of natural water resources. This study focuses on the prospect of coupling desalination plants with existing Combined Cycle Power Plant (CCPP) to produce potable water. This study was based on seawater in the north of Libya and on a source of brackish water in the south of Libya at Waddan City. The study begins by considering the improvement that could be achieved in CCPP performance by cooling the inlet air using a waste heat driven Absorption Chiller (AC) and then continues to determine the optimal selection of the desalination technique suitable for the environmental conditions in Libya. The methodology depends on validated simulation models developed from IPSEpro software. The CCPP model was validated against vendor data and the AC was validated against manufacturer data. Different desalination units, Multi-Effect Desalination with Thermal Vapour Compression (MED-TVC), two-pass Reverse Osmosis (RO) and Single Effect Desalination (SED) desalination systems were modelled and validated against actual operation data. After validation the performance of each model was investigated from energy, exergy and economic standpoints for design and off-design conditions using real Libyan environmental data. This study has, for the first time, compared the exergy efficiency, power consumption and economic characteristics of different configurations of two-pass RO with energy recovery devices such as a Pressure Exchanger (PX) and an Energy Recovery Turbine (ERT). The results shows that when PX is used in the first and second stages of RO the exergy efficiency increases by 81% and the specific power consumption declines of more than 100%. MED-TVC Gain Output Ratio (GOR) and exergy efficiency are improved by adding a preheater on the distillate water stream to increase the feed water temperature, and the amount of steam extracted from the CCPP is reduced. A comparison between two-pass RO or MED-TVC coupled with a CCPP has been carried out using thermodynamic and economic analysis. The result shows that the N. M. Eshoul PhD Thesis ii Newcastle Uinversity power plant exergy efficiency deceases by about 4% when MED-TVC desalination plant is coupled with a CCPP but only 0.5% when RO is coupled with a CCPP. Also the net power output declines by about 22.5MW when MED-TVC desalination is used but only 5MW when the CCPP is coupled with a RO with PX desalination plant. In addition, economic analysis shows that RO desalination is a better process compared with MED-TVC, either standalone or coupled with CCPP. An exergy and economic analysis of two different desalination technologies to produce drinking water from brackish water was investigated. The analysis shows that the exergy efficiency of a single-pass RO unit is nearly double that of single effect desalination and the cost of water produced by a single effect desalination unit is higher than that of a single-pass reverse osmosis unit by about 60%. This confirms that reverse osmosis is the suitable desalination system in Libya.