Simulation Experiments

In the power and water production simulation experiments, a quasi-experimental design was used [10]. This approach focuses on comparing the results of the cur­rent situation (control experiment) with “what-if ’ scenarios. In the case study plant, power generation is driven by gas turbines and thermal energy from their exhaust is used to generate low-to-medium pressure steam through the waste heat recovery boilers. In this set-up, steam is routed directly to the brine heater of the distillation plant. Auxiliary boilers are usually used for higher yields. Although it is flexible (within the plant design parameters), the power to water ratio produced depends on operational practice. However, fuel savings increase with decrease in the power to water ratio. Therefore, an optimal power to water ratio is often sought in practice.

Since the case study plant will always produce both water and electricity, op­erational differences have an influence on the power to water ratio. Alternatively, a desired power to water ratio can be achieved by optimizing the feasible production operation strategies implied by the plant design. Such operational strategies have a bearing on the unit cost of either water or electricity as well as primary energy con­sumption. An important observation in such plants is that the influence of energy
efficiency improvements of installed capacity is less compared to the influence based on plant design changes or retrofit/re-design issues.

Water desalination processes are energy intensive and their coupling with power plants should result in appreciable cost saving when compared with separate single purpose power generation and desalination installations. The average specific fuel consumption for the case study plant was calculated to be 306 NM3/MWH. The specific energy consumption of the water desalination process can be reduced either by use of innovative modifications in conventional power cycles and/or by reducing the irreversibility of the multi-stage flash (MSF) process through improvements in the design and operating features of the distillers.

In experimenting with the power/water production ratios, several configurations of the installed capacity can be simulated. For example, the case study plant usually operates with a duplicate “standby” line. This arrangement enables smooth produc­tion of the required power and water capacities in a bid to meet the fluctuating de­mands for water and electricity. Simulation experiments can then be used to mimic different couplings of the available gas turbines, compressors and heat recovery boilers as well as auxiliary boilers. These different couplings were assumed to rep­resent different operational states in the dual purpose power and water production plant.

The focus of this investigation was on operational issues assuming that the de­sign issues are satisfied, in the plant design, based on thermodynamic analysis. More specifically, this investigation focused on identifying the optimal production operation strategies within the limits of the general plant design that in turn can re­duce energy consumption in the production of water and power. Further, simulation modeling was used to experiment with a number of feasible plant configurations (assuming retrofits in plant designs) for the sake of comparison. Since the “enti­ties” for simulation of power and water production strategies are different, water production and power production units were treated separately in simulation models created in AREANA simulation software. Figures 46.3 and 46.4 depict the simula­tion models that were used for “what-if analysis experiments for water and power production. Data was collected from runs of the simulation models based on the simulation models presented in Figs. 46.3 and 46.4.

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