Staging of investment in desalination facilities and associated storage facilities.

Abstract

The principal objective of water resources planning is generally recognized to be the satisfaction of the continually growing desires and requirements of a population for usable water. In long term planning of such resources, fulfillment of these needs at a minimum cost can be defined as the objective. The time-capacity relationship that describes the decision process for the arrival at an optimum and feasible construction schedule hold the answer to the decision question: how much to build and when. The application of the time-capacity approach is used in the problem of staging of investment in desalination capacity and associated storage facilities. The forward dynamic programming technique is utilized in the solution process. A preliminary analysis is performed, with artificial data, in the initial development of a decision rule governing , an idealistic model of an arid region. The area of study is assumed to depend solely on desalination of sea water for its supply of potable water, with no appreciable groundwater source available in the region. Desire for water is assumed to follow a linearly rising trend for a finite period into the future taken as the duration of the project. A more realistic set of data is later considered in the development of an optimal incrementation rule for the augmentation of desalination production. The State of Kuwait is considered as the area of study, and pertinent data were collected from that region. Rate of demand growth for water use is described here to follow an exponential trend resembling that of the projected population growth at an assumed rate of growth. Capital costs only are considered in the minimizing functional equation of the decision rule, and an appropriate discount rate is assumed in the obtainment of the present value of incurred costs. A spatial construction schedule is described by the solution algorithm which specifies the sizes of the required increments to production and their optimal time of erection. An economic analysis of the state of the art in storage facilities resulted in the elimination of storage capacity as a state variable in the dynamic program. The operational problems of desalination units in production are dealt with, all within the total supply system requirements of meeting the desired demand for fresh water. The capacities of the incrementation schedule of desalination plants are modified to accommodate the expected shortages due to the annual scheduled maintenance, forced outages and peaking of the water use curve due to seasonal variations. Technical data of actual plants in operation in Kuwait are analyzed to obtain the restrictions on the operational requirements of the production plants. The plants considered are of the multi-stage flash type (MSF) currently in use in Kuwait. Simulation of the production operation of the required units at every stage of incrementation is performed. The final costs of the modified supply system components are obtained in accordance with the assumed probability of meeting demand within the total number of simulations. The general solution algorithm is viewed in two interrelated parts. The first part produces the schedule of incrementation and construction of the necessary desalination units. The second part modifies these capacities to account for the operational and seasonal requirements of the project. The ultimate result is a schedule of modified capacities of production and a maintenance program for every unit in operation, with the effects of forced outages and peaking of the demand curve applied on each plant.