Outline Description of Program AQUARIUS

Introduction top

The prospects of increased rainfall variability resulting from global warming and of rising fuel prices will tend to heighten appreciation of water as a valuable and limited resource. It is therefore becoming imperative, from both political as well as economic standpoints, to demonstrably operate water and power supply systems as efficiently as possible. At the same time, closer regulation of such systems increases the need for transparency in the methods employed to optimise their operation.

Recognition of the benefits associated with the conjunctive use of water sources and electricity generation plant has led to the development of computer programs for simulating and optimising the operation of increasingly complex systems over both long and short time scales. The scope of such programs range from the performance of simulations based on heuristic operating rules, to those incorporating mathematical programming algorithms for operating policy optimisation.

Until recently, computational constraints have meant that significant simplifications have had to be made when 'optimising' long term operating policies. For example, the use of individual rule curves to specify reservoir releases, non-consideration of transmission network constraints and the use of weekly or even monthly rather than daily time steps. Similarly, many programs have been system specific in nature, are difficult to modify and lack user friendliness and transparency.

Advances in computer performance have now made it feasible to develop a generalised, flexible and user-friendly computer program which does away with the need for many such simplifications, while providing high levels of transparency and mathematical robustness when used to optimise the operation of water supply and power generation systems.

Objectives top

Program AQUARIUS has been developed by PWSC to optimise and simulate the short and long term operation of water resource, water and power supply systems, and to specifically provide :

Development Philosophy top

AQUARIUS builds on experience gained from the successful application of earlier programs developed by PWSC for simulating water resource and hydro-thermal generation systems (Program SYSIM), and for simulating and optimising the operation of large scale water resource/water supply systems (Program MOSPA). Initially constructed as FORTRAN 'batch' programs, SYSIM and MOSPA incorporated logical rules to allocate resources in each simulation time step, rather than formal optimisation algorithms. In this way computation times were contained when simulating complex systems over long hydrological sequences.

Subsequently, PWSC developed Windows based GUI's for both programs in order to simplify the entry and modification of input data, and to provide comprehensive facilities for the screen and hard copy display of results in tabular, graphical and 'mimic' diagram form.

The impetus for developing AQUARIUS arose from a number of considerations and observations associated with the evolution of water supply and power systems, including :

The scope and design of AQUARIUS has also been influenced by continuing advances in computing power and disk storage, which now enable the incorporation of analytical methods not previously compatible with detailed and lengthy simulations of complex system operation.

At the same time, the development of programming languages such as Microsoft's Visual Basic now enable 'engineering' type programs to be written directly within a Windows compatible environment with execution times comparable with versions written in 'scientific' languages such as FORTRAN. An additional advantage of Visual Basic is that it allows direct read and write access to common database structures.

Such considerations led to the following key features being incorporated within AQUARIUS:

Simulations can be carried out at with a level of detail appropriate to the available data and application objectives, while the long-term optimisation methodology ensures that the resulting Stored Water Values are 'practical' in terms of complying with system operation constraints.

Definition of System Components and Configuration top

AQUARIUS enables the 'on-screen' construction of a simulation model for a given water resource and electricity supply system. The user selects, from a menu, the component type to be added to the current diagram and positions the component by 'clicking' the mouse. AQUARIUS currently allows the user to select from the following component types :

Water Resource and Supply System Components

Power System Components

The default colours and the relative symbol size of each component type can be modified by the user. Each component is initially assigned a default Component Name which can then be changed by the user via the component type data entry templates provided.

Transmission Lines, Transfer Aqueducts, Supply Aqueducts, Sewers and River Reaches ('lines') are 'connected' to appropriate component types by firstly clicking the mouse on the 'from' component, and then dragging the cursor to the 'to' component and clicking again. In-built logic restricts the component types that can be connected by 'lines'.

The position of individual symbols can be adjusted using 'drag and drop' to make the diagram visually acceptable and reflect actual topography. Any 'lines' entering or leaving the symbol will move with it, and Component Names can be moved independently of the corresponding symbol. It can be noted that, while moving symbols only alters the appearance of the diagram, deleting components or changing 'line' connections will alter the logic of the model. A specimen model diagram, created using AQUARIUS for a demonstration system, is reproduced on the front page of this description.

Input of Technical and Cost Data top

With the exception of stream flow and demand time series, all input data is entered into AQUARIUS via a collection of 'on-screen' templates which vary with Component Type. These allow both initial data entry as well as the modification of existing values. The appropriate template is accessed from either a menu, or by 'right-clicking' a component on the model diagram. Wherever appropriate, the user is offered a 'pick list' to select from, so as to minimise data entry errors and, where possible, data validation is performed before new entries are accepted.

System level data input to AQUARIUS includes :

Input operating cost and penalty cost data used by AQUARIUS to calculate the least-cost of meeting specified electricity and water demands within each simulation time step include :

Technical and demand data that can be considered during each time step of the simulation include :

Daily time series of electricity demands, water demands and stream flows can be input to the simulation from individual disk files, the names of which are user defined as part of the model data.

Model and Data Storage top

With the exception of individual time series data files, all information relating to a particular AQUARIUS Model is stored in a single comma delimited disk file, and automatically given the attribute *.mdl. Thus any changes made to an existing Model, including those only data related, can be saved under a different file name for later retrieval Such files may be archived, so as provide a permanent record of particular program run inputs as may be required for, say, regulatory auditing.

Simulation Module top

The steps performed during an AQUARIUS simulation are shown in flowchart form as Figure 1.

The start and end of the simulation period are set by the user, with the duration only limited by the coincident period for which any input time series are available. The minimum simulation time step is one day, but simulations can also be undertaken over calendar weekly or monthly time intervals.

In all cases, the outputs ('dispatch') of any power generation plants and transmission line flows are optimised across a number of user defined load blocks, corresponding to the definition of imposed electricity demands. The duration of each load block is defined on a system basis with the total duration summing to 24 hours. In this way setting a load block duration to 0.25 hours will, for example, enable the modelling of 15 minute 'instantaneous' loads.

Within each simulation time step, AQUARIUS minimises the total cost of satisfying the imposed electricity and water related demands using PWSC's proprietary Linear Programming (LP) algorithm.

The LP input matrix is automatically constructed on the basis of the specified model components and topography (linkages), and the associated physical and economic data, including imposed constraints. To avoid the occurrence of 'unfeasible solutions', electricity demands, water demands and minimum river flows are subject to (user specified) penalty costs which are applied in the case that a such demands or requirements cannot be satisfied.

In order to reduce execution times, the basic input matrix is created for the first simulation time step and only those elements (constants) which change between time steps are modified. For similar reasons, the maximum dimensions used by AQUARIUS e.g. the maximum number of reservoirs or transmission lines etc., can be adjusted by the user so as to minimise computer memory requirements when running a specific model.

Within the LP formulation, reservoir releases are assigned costs in accordance with input (long-term) Stored Water Values as a function of calendar week or month and reservoir content. Such releases are optimised taking account of all (downstream) incremental inflows within the river system. Additionally, target (minimum) reservoir releases can be specified by Control or Multiple Regime operating rules. Total outputs from a reservoir are divided into 'compensation', 'release' and 'spill', each of which can be associated with a specific River Reach. In the case of 'release' this can also be assigned to one or more Supply and Transfer Aqueducts, and subject to a specified maximum value and target flood release quantity.

Hydro plant capacities are calculated as a function of the available flow and given MW per m3/s conversion factors. This conversion factor can be fixed or given as a function of flow and available generating head, while the latter may in turn be dependent on an upstream reservoir level.

Wind plant generation is based on a daily wind speed from a time series or as a randomly selected variable from a Normal distribution. This distribution is defined in accordance with input calendar monthly Means and Standard Deviations. The average wind speeds in each load dispatch block are then determined by random sampling from the Normal distribution so as to maintain the average daily wind energy value. The load block wind speeds are converted to power outputs using a given wind speed (m/s) to output (MW) conversion function, which allows zero output if the wind speed falls outside the function limits. An option allows the user to fix the 'seed' of the random number generator so that the same series of random numbers can be used in consecutive simulations.

Constraints are automatically included in the LP problem formulation to model limits imposed by Time and Flow Dependent licences, maximum and minimum component capacities and throughputs, and water and electricity mass balances. In addition, a facility is included which allows the user to specify additional constraints by assigning coefficients to combinations of the defined objective function variables. This enables, for example, the modelling of water blending constraints.

Figure 1 : AQUARIUS - Outline Flowchart of Simulation Module

Program Outputs top

AQUARIUS provides the user with a variety of outputs as described below.

a) 'Real Time' graphs top

As the simulation proceeds, AQUARIUS can simultaneously display screen graphs to show the behaviour of up to 15 user selected system components. The values shown are a function of the component type, as indicated below :

The time span for each graph can be set separately so as to cover 3, 12 or 36 months at a time, and the user can also set the maximum ordinate for each graph independently. A separate 'window' gives the simulation results 'to date', including operating costs and penalties. An example of the Real-Time graphs produced is reproduced as Figure 2 below.

Figure 2 : AQUARIUS - Example of 'Real-Time' Screen Graphics Output

b) Database Outputs top

For each time step of the simulation, information on the inputs, outputs and performance characteristics of each component, and for the overall system, are automatically stored in a model specific Microsoft ACCESS© format database. A new database is created at the start of a simulation and assigned the same name as the AQUARIUS Model file, but with the standard Microsoft ACCESS© attribute i.e. *_SIM.mdb. The contents of each database table vary by Component Type, but all have the following common fields :

The contents of the database can be viewed directly from Program AQUARIUS using the Microsoft VISDATA facility provided.

c) Plotting and Tabulation of Time Series Values Stored in Database top

AQUARIUS allows the user to plot or tabulate any combination of time series values stored in the database, and such plots can be viewed on screen or printed with a high level of definition.

For a daily simulation time step, values can be automatically summed to provide weekly, monthly or annual time series, or to give average calendar weekly, calendar monthly or period averages based on the total duration of the simulation.

Depending upon the summation or averaging option used, the following graph types can be employed :

The user is able to select the number of values shown on the screen and, if this is less than the number of available values, it is possible to 'scroll through' the simulated period. A 'hot hit' facility is also included so that any plotted value can be identified in terms of the component name and value simply by clicking the mouse.

The Graphics Server© facility employed within AQUARIUS gives the user considerable control over the content and appearance of any graph, so that they can be tailored for inclusion in reports. It is also possible to output a graph to disk file in a number of standard formats e.g. *.bmp, *.wmf etc. Examples of the type of graphs that can be produced are shown in Figures 3 & 4.

The selected time series can also be viewed in tabulated form and subsequently output as a 'comma delimitated' (*.prn) file for direct input into spreadsheet programs such as Microsoft EXCEL for further analysis by the user.

Figure 3 : AQUARIUS - Example of Line Graph Using Stored Database Values

Figure 4 : AQUARIUS - Example of Stacked Area Graph Using Stored Database Values

d) 'Mimic' Diagram Display of Time Step or Average Simulation Period Values top

AQUARIUS enables the display of values stored in the simulation database in 'mimic' diagram form. Such values may relate to a particular time step or to averages over the simulated period. Mimic diagrams can be constructed interactively based on the components included in an AQUARIUS Model file. Multiple mimic diagram files can be built thus enabling, for large or complex systems, the display of results for different parts of the system in varying levels of detail.

The type of value to be displayed for each component type can be selected from a comprehensive menu. Thus, for Reservoirs, it is possible to display the contents at the start or end of the time step either as volumes or percentage fullness, releases, spills, water values or stored water costs. For power system components it is also possible to display energy based quantities over the time step, or power based quantities in each load block. The mimic diagrams also display total or average system values including, for example, total hydro, thermal and wind generation, transmission losses, operating costs, penalty costs and supply benefits. A facility is also provided to automatically step through the simulated period.

Such displays provide a high level of transparency to AQUARIUS results, and enable the user to rapidly analyse system behaviour. An example of a mimic diagram display is illustrated in Figure 5 below, and such displays can be printed in high definition for inclusion in reports.

Figure 5 : AQUARIUS - Mimic Diagram Display of Simulation Results

Optimisation of Long-Term Stored Water Values top

Within each AQUARIUS simulation time step, the optimum quantity of water to be released from each reservoir takes account of input (long-term) Water Values, which will normally vary by stored water volume and calendar week or month. Using AQUARIUS, such Water Values can be optimised with PWSC's proprietary Policy Iteration Stochastic Dynamic Programming (DP) algorithm1. Such capability is sometimes ascribed to a Water Value Model.

This algorithm requires two data streams for each alternative water value, namely .

This data is obtained by simulating system performance over the hydrological sequence, with the reservoir contents being re-set to a specific value at the start of each time step. Within the DP algorithm, costs are divided into two categories :

The system storage is equal to the total active storage provided by one or more reservoirs in the system, and is partitioned into a number of equal state intervals, The DP algorithm then identifies which water value should be adopted in each storage state and optimisation interval, so that the total future operating cost will be minimised. The process iterates over a number of complete years until convergence is obtained i.e. when the associated water value table remains unchanged between successive iterations.

Using the simulation model to provide DP input data ensures that :

The way in which simulation is integrated within the optimisation procedure is shown in Figure 6.

The algorithm settings and optimisation results are automatically stored in tables within a database file which is assigned the name *_OPT.mdb. The contents of these tables can be inspected using the Microsoft VISDATA facility provided.

Figure 6 : AQUARIUS - Integration of Simulation and Optimisation Modules

An example of optimised weekly water values obtained using AQUARIUS is given as Figure 7 below. For clarity three alternative water values were defined in this case, and it can be seen that the resultant curves displays the type of seasonality that can be normally expected.

Figure 7 : AQUARIUS - Optimised Calendar Weekly Water Values (3 Regimes)

Yield Determination top

A requirement frequently imposed by planners and regulators is the assessment of the maximum water or electricity supplies that can be met by a system based on given streamflow sequences and supply reliability criteria, as may be defined by the consecutive or total incidence of demand management measures, supply deficits or minimum reservoir levels. For water resource/supply systems such estimates are sometimes referred to as 'firm yields' or 'deployable outputs'. For hydro-thermal power generation systems they are analogous to 'firm energy' assessment if a single load block is employed in the simulation.

AQUARIUS incorporates a search procedure for identifying the demand multiplier consistent with satisfying such criteria and also permits the user to select the individual water and electricity demands to be subjected to the multiplier. A key attribute of the approach is that the assessment takes into account the constraints on system operation incorporated within the AQUARIUS simulation model.

Program Details and Usage top

Program AQUARIUS is written in Microsoft Visual Basic© (Version 6), and runs under the Microsoft Windows 95, 98, NT4, 2000 & XP operating systems. All code, including that associated with the proprietary Linear and Dynamic Programming algorithms, has been written by PWSC, thereby eliminating reliance on any third-party suppliers.

The time required to simulate behaviour of a water resource or power supply system depends on :

In practice the level of detail used for a particular simulation will vary with the application. Thus, for power system planning studies it may be sufficient to employ, say, 3 load blocks, whereas for optimising 'day ahead' system operation the use of 24 load blocks might be appropriate. AQUARIUS allows the use of daily, weekly or monthly time steps and, in this way, investigation of the sensitivity of simulation results to the time interval employed. When optimising long-term Water Values, a further determinant of execution time is the number of alternative water value 'regimes' used when producing the input data for the optimisation process.

Execution times required to solve Linear Programming problems increase with both the number of objective function variables and constraints. However, experience gained in modelling large and complex integrated water resource/supply systems suggests that, with powerful modern computers, simulation times are unlikely to be a deterrent to the application of AQUARIUS to large systems.

Applications and Availability top

AQUARIUS is equally applicable to planning and operation studies, as for use in optimising 'real time' operation. It is available for purchase by utilities, subject to standard software protection measures being installed. Usage by consultants and international agencies on a project-by-project basis is by negotiation. Alternative versions can be made available; for water resources/supply system and power system application, and with or without the water value optimisation module. For further details on Program AQUARIUS please contact us :

A 'demonstration' and latest news of AQUARIUS can be viewed here.