Efficiency improvement and exergy destruction reduction by combining a power and a multi-effect boiling desalination plant

Electric power and desalted seawater demand is increasing in Kuwait mainly due to residential and industrial growth, especially in summer season. In the past six years, Kuwait citizens have been facing the problem of automated power and water disconnection because of the electricity and water production is lower than the consumption. A common idea for resolving such a problem is to build new power plants but this solution is not practical due to environmental issues. Another choice but more engineer challenging approach for resolving this problem is to improve the efficiency and performance of the already existing power plants. Currently, there are six power plants in Kuwait; four of them have both stand-alone gas-turbine and steam-turbine power plants, one is steam power plant and one is gas turbine power plant. Combined power and desalination plant are more attractive in Kuwait since they have higher thermal efficiency than traditional ones and both electric power and process heat (e.g., desalting) can be produced simultaneously. The relatively low temperature multi-effect desalination (MED) process (around 75oC saturated temperature as the heat source) is thermodynamically the most efficient of all thermal distillation processes (source, and consumes about 2 kWh/m3 pumping energy). In this study, factors affecting the performance of a combined power and MED-desalination plant have been studied. This includes the atmospheric humidity, compressor inlet air temperature, top brine temperature, desalination unit capacity, cooling water temperature, and the number of evaporation stages of the MED unit. A first- and second-law analysis of the proposed system was carried out under several operating conditions. As an example, a 125 MW Siemens V94.2 gas turbine of Al-Zour gas turbine power plant in Kuwait has been selected. It is found that the overall thermal efficiency of the proposed system increases significantly as the desalination unit capacity increases and this increase can reach 25% (from = 55.1% to 69.9% at T1 = 30oC and = 0.30) as the capacity increases from 1 to 5 MIGD. The total work generated decreases insignificantly, i.e. the low pressure steam turbine power decreases from 27 MW to 23 MW while the total power output decreases from 208 MW to 204 MW. In addition, as the desalination unit gain ratio increases, the total exergy destruction is reduced, i.e. the desalination unit exergy destruction increases from 12 MW to 15 MW but the total exergy destruction of the cycle decreases from 352 MW to 349 MW. It can be concluded that the low-temperature MED process offers a potential efficient solution to the current Al-Zour power generation plant