Universal design framework for optimal application of chemical monolayer to open water surfaces

Abstract

Annual evaporation losses from farm water storages in Australia typically exceed 40% of their storage volume. Potentially chemical films such as monolayers are an economical low-impact means of reducing evaporative loss, however, their performance has been shown to be highly variable. They are affected by a number of climatic and environmental factors, principally wind-induced drift, deposition on the lee shore, submergence by waves, volatilisation and bio-degradation. Although these limitations must be accommodated for in the management of the applied monolayer by means of appropriate and timely autonomous application, these limitation will vary for every location. Every given site will have its own unique characteristic climatic and environmental factors. It is this variability that presents major difficulties to the general one-size-fits-all design approach. Hence, to achieve optimal evaporation mitigation performance the development of a methodology to inform the design, installation and operation of a tailored monolayer application system for any given site was seen as essential. This thesis reports the conception, development and desktop evaluation of a Universal Design Framework (UDF) to optimise the use of monolayer materials for evaporation mitigation. The UDF is designed to inform: (i) the most appropriate choice of monolayer material; (ii) the optimal type of application system and the site-specific cong- uration required; (iii) the amount and re-application rate of monolayer to be applied; and hence (iv) the expected performance of the application system. The UDF incor- porates all the necessary information with respect to water storage geometry, monthly climate data (in particular, detailed wind statistics), water quality and biological fac- tors plus user performance criteria (the desired extent and duration of coverage). This information is then used in four key analysis stages: 1. Monolayer material is selected via a decision table, which allows the user to make comparisons between three previously benchmarked South East Queensland (SEQ) reservoirs and their own, to determine a best match monolayer material. 2. Application system design is determined using a simulation platform, which allows the user to predict surface coverage and application rate according to wind conditions via an iterative process in which the number and/or location of applicators may be varied until user performance criteria are met. 3. Likewise application strategies, namely which applicators to apply from and their respective application rate for each wind condition, are also determined with the simulation platform for detailed wind conditions (both strength and direction) to create a decision table. This table forms the basis for real-time (hour-by-hour) decision and control when the system is installed on-site, and 4. system performance is calculated for monthly site-specic wind statistical data (using the simulation platform), and compared with user performance criteria to determine which months are suitable for application and monthly monolayer cost. The simulation platform and the algorithms used to calculate firstly, the spreading rate and spreading pattern of monolayer (without wind stress), and secondly the drift rate, spreading rate and spreading pattern of monolayer (with wind stress), are described. In order to calibrate the algorithms, and to research the requirements for (both current and future) monolayer material characterisation, an empirical study for the commonly used evaporation-retarding monolayer stearyl alcohol (`C18OH' as a water-emulsion) was undertaken. This involved the analysis of its observed spreading performance under different application and windspeed conditions on an indoor 6 m diameter tank with controlled airflow. Finally the scope of the UDF is discussed with regard to design, planning, installation and also daily, hour-by-hour management of monolayer application. This was informed by a demonstration of the UDF for a theoretical installation on a typical SEQ storage dam.