Operating
Policy Optimisation
Optimised
System Operating Policy
Interactive
Model Construction
Mimic
Diagram Display of Results
User
Friendly Data Input
| 1)
The Need For Optimisation and Mathematical Programming
Methods top |
(i) Optimisation
is necessary so as to ensure that alternative operating
policies or system development plans are directly comparable
in terms of meeting specified demands with the same
levels of supply security.
(ii) For systems or operating policies of any complexity, 'manual optimisation' based on repetitive simulation is unrealistic due to the number of interdependent variables involved; some form of mathematical programming algorithm is normally required in order to obtain the 'optimal' solution. (iii) The optimisation method employed should reflect the nature of the problem to be solved, rather than tailoring the problem to comply with the limitations imposed by a particular method. PWSC
has wide experience in the development
and application of linear (LP), non-linear
and dynamic programming (DP) techniques,
many of a proprietary nature.
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| 2)
Integrated Application of Simulation and Mathematical
Programming Algorithms top |
(i) When
evaluating the operation of existing or proposed water
resource based systems, it is essential that adequate
consideration must be given to the effects of the 'persistence'
exhibited by historic flow series, particularly low
flow sequences.
(ii) For systems of any complexity, the effects of spatial and temporal stream flow variations can only be adequately modelled by employing a simulation model with an appropriately short time step. (iii) Availability of a detailed simulation model is a pre-requisite for properly evaluating the performance of any 'optimised' operating policy or planned system configuration. (iv) Such simulation models must be capable of representing the behaviour of the system to the satisfaction of those with practical knowledge of its characteristics, and of simulating both existing and alternative operating policies. (v) While many practitioners have advocated the combined use of simulation and optimisation models, it is also necessary to specify how they should be linked together. Within
PWSC programs, the performance of 'optimised'
policies are evaluated by simulating
their performance using a detailed model
of the system, so as to ensure that specified
(supply) reliability criteria are satisfied
and costs accurately assessed. The emphasis
is thus on the derivation of practical
operating policies or development plans,
rather than producing a mathematically
'optimal' solution for an over-simplified
representation of the 'real' problem.
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| 3)
The Use of Appropriate Measures of Supply Reliability top |
(i) System
performance should be quantified in terms of criteria
that are comprehensible by lay people as well as 'experts'.
Thus, esoteric concepts such as '2% droughts' or 'firm
energy' should be avoided whenever possible.
(ii) Given the uncertainties associated with the economic derivation and justification of penalty functions applied to unsatisfied demands, it is preferable to measure supply security in terms of the imposition of supply restrictions of quantified severity and acceptable socio-economic effect. PWSC
programs offer the user a variety of
supply reliability criteria, as quantified
by the simulation model. These include
the incidence of quantified supply restrictions,
maximum permissible supply deficits and
minimum reservoir draw down levels.
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| 4)
The Choice of Simulation and Optimisation Model Time
Steps top |
(i) The
time step used in the Simulation Model should be short
enough to ensure that seasonal hydrological and cost
variations are adequately considered.
(ii) The Simulation Model should preferably enable the user to investigate the use of different time steps, so as to evaluate the relationship between increased accuracy and execution times. (iii) The time step used in the Optimisation Model should reflect the practicalities of operating policy implementation as well as hydrological and cost variations.
It
is unlikely that such requirements will
be met by having the same time steps
in both the Simulation and Optimisation
models, and PWSC programs reflect this
situation.
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| 5)
Program Transparency and Outputs top |
(i) The
workings of complex computer programs should be as
transparent as possible, so as to aid both user understanding
and future development.
(ii) Such transparency requires the detailed monitoring of program inputs, intermediate and final results in annotated file form for presentation and archiving purposes. (iii) Understanding of results and simulated system performance is greatly aided by the graphical display of results, either in plot or 'mimic' diagram form.
PWSC
has developed Windows based Run-Time
Environments (GUI's) for its DOS/FORTRAN
programs such as MOSPA (MOSES), SYSIM
(SYSIME) and EPSIM (EPSIME). For presentation
purposes these GUI's 'read' the annotated
files produced by the FORTRAN programs
and offer the user a comprehensive number
of display options. Programs such as
AQUARIUS and EXODUS, which are entirely
written in MS Visual Basic, store results
directly in an ACCESS database.
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| 6)
Program Generality top |
(i) While
programs should ideally be generalised, i.e. data driven,
rather than 'bespoke', i.e. system specific, the development
of generalised programs is more complex and time consuming.
(ii) Many systems exhibit 'unique' features which must be adequately modelled if an adequate representation of the system is to be obtained. It is thus inevitable that the capabilities of 'generalised' programs may, from time to time, need to be enhanced. PWSC
is committed to the continued future
maintenance and enhancement of its software.
This is facilitated by having direct
access to all source code employed, including
the optimisation algorithms, and by taking
such requirements into consideration
during initial program design.
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| 7)
User Friendliness top |
(i) While
a high-level of 'user friendliness' is desirable, it
should not take precedence over providing the flexibility
required to model 'real world' system characteristics
and complexities.
(ii) The true value of computer programs lies in the integrity of the underlying concepts and calculations, rather than its cosmetic appearance. PWSC's
development of user friendly interfaces
only follows exhaustive testing of the
core analytical code.
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| 8)
Program Development top |
(i) New
software developments should seek to fill perceived
needs in the market place and incorporate improved
methodologies in terms of accuracy and optimality.
(ii) Advances in programming languages should be exploited in the interests of improved execution performance and user friendliness. (iii) Program architecture and the choice of methodology incorporated should reflect advances in computer performance, in terms of both processor and data storage capabilities. While
PWSC's original computer programs were
written in FORTRAN to minimise execution
times, recent packages such as AQUARIUS
and EXODUS are written in MS Visual Basic
(VB). With the advent of 'native code'
VB compilers, execution speeds of compiled
programs are now similar to those obtainable
with FORTRAN. VB also facilitates 'seamless'
communication with standard database
structures for the storage and retrieval
of program results. Advances in computing
power have also made feasible the use
of Linear Programming in EPSIM, AQUARIUS
and EXODUS for optimising source/supply
allocations within each simulation time
step, rather than the less exact methods
used previously. For more information
on this area see 'An
integrated simulation & dynamic programming
approach for evaluating the performance
of complex water resource systems and
optimising operating policies: Methodology & Applications'.
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| 9)
Software Maintenance top |
(i) While
every effort should be made to eliminate program 'bugs'
during development it is recognised that, as with all
complex software, malfunctions may from time to time
become apparent.
(ii) Due to the nature of the systems being analysed, no guarantees can be given regarding 'fitness of purpose' or against consequential losses arising from program applications. PWSC
offers (annual) maintenance contracts
under which it provides prompt rectification
of any program malfunctions brought to
its attention, and supplies updated versions
of the software package without additional
charge. |
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