This document provides an analysis of the functions of a multi-stage flash desalination system integrated with a solar energy system using parabolic trough collectors. It establishes the system context and boundary, models the key functions using object-process methodology and activity diagrams, and analyzes the process architecture using a design structure matrix. The aim is to decompose the processes required to produce fresh water from seawater and propose connecting the desalination system to the parabolic trough collectors as a renewable heat source for water heating.

1. Masdar Institute of Science and Technology
ESM501 . Systems Architecture . Fall 2014
Multi-Stage Flash Distillation System Integrated with Solar Energy
Reverse Engineering of System Function
Mouza M. Al Kaabi
mmalkaabi@masdar.ac.ae

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TABLE OF CONTENTS
1. Introduction ..........................................................................................................................................3
2. System Context and boundry................................................................................................................4
3. Function modeling of MSF and PTC systems ........................................................................................4
3.1. Object-Process Methodology .......................................................................................................5
3.2. Activity Diagrams ..........................................................................................................................6
3.3. Design Structure Matrix................................................................................................................8
4. Conclusion.............................................................................................................................................9
5. Refrences ............................................................................................................................................10

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1. INTRODUCTION
The importance of desalination systems in the UAE was established in the previous paper
regarding system form [1], in which it was noted that the total consumption of water resources
in the Emirates today exceeds 24 times its natural recharge capacity. The water planning and
management authority is planning to cover the shortage in water supply through constructing
desalination plants. There are eight seawater desalination plants in Abu Dhabi alone, and more
are to be built since the demand for water desalination is expected to almost double in the UAE
by 2030 [2, 3, 4, 5].
However, it is necessary to improve the performance of the existing systems and provide an
enhanced design for the desalination plants that are yet to be built. The physical representation
of the form that was produced in the previous paper, illustrated the most important components
and the interfaces of the system, and presented an opportunity of retrofitting the legacy system.
This can be achieved by connecting the main heat source to the brine heater that acts as an
interface between the Multi-Stage Flash (MSF) and the renewable source of heat; the Parabolic
Trough Collectors (PTC) [1].
The aim of this paper is to:
Analyze the function of the multi-stage flash desalination system, by decomposing the processes
required to produce fresh water, while proposing a connection between the system and the
parabolic trough collectors as a renewable source for the process of water heating.
The paper therefore proceeds by presenting the details of the system context and boundary, the
function modeling of MSF systems and finally a conclusion is drawn.

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2. SYSTEM CONTEXT AND BOUNDRY
The recent economic development and rapid
population growth has caused an increase in
the demand of production of water. The
production of water and the production of
energy are coupled in the energy-water
nexus. Lubega and Farid [6] have defined the
energy-water nexus as “system-of-systems
composed of one infrastructure system with
the artifacts necessary to describe a full
energy value chain and another infrastructure system with the artifacts necessary to describe a
full water value chain.” The multi-stage flash system is extant in cogeneration plants, where
power and water are produced simultaneously. The MSF process therefore requires low pressure
heating steam which can be extracted from the power plants at low cost [7].
Figure 1 illustrates the energy-water nexus system in context of the desalination system, for
which the system boundary line is drawn around the MSF and the PTC. This paper will provide a
bottom up approach for decomposing the functions and processes in the MSF system, and
further details the process of water heating. Additionally, it traces the inputs and outputs
required to propose an enhanced system architecture that could be integrated with PTC as a
renewable heat source.
3. FUNCTION MODELING OF MSF AND PTC SYSTEMS
Representing system architecture is a dynamic procedure, where the system engineer should
consider an exchange of information between function and form to be able to develop and
improve alternative solutions [8]. Since the MSF system is an existing technology, the analysis of
this paper follows a bottom up approach, in order to simplify the functionality of the system and
decompose the main process, while tracing the inputs and outputs. The following representation
of function will be used to verify the validity of the system form previously proposed, and test if
the proposed integration with PTC is conceivable [1].
Figure 1 : System context and boundary

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3.1. Object-Process Methodology
The object-process methodology is considered a descriptive method, where the system is
represented through visual diagrams and textual descriptions. OPM can express complex and
non-linear relationships, while integrating function, structure and behavior in the same diagram.
OPM consists of three types of elements: Processes, objects and states. Directional arrows are
connecting between processes and objects to indicate procedural links. [9]
The OPM diagram shown in Figure 2 demonstrates the main process and objects constituting the
MSF and the PTC system. The process (represented by ellipse) can be divided into two main
categories; MSF related process (indicated by blue), and PTC related process (indicated by
orange). The PTC set of processes starts with the solar radiation, concentrated to heat the oil,
which subsequently acts as an instrument of heating required for MSF processes. The main input
required for the desalination process is the seawater. The seawater is transferred between
different states through the procedural links with the MSF processes, to produce the desired
output which is the fresh water.
The OPM is a useful method to illustrate the general operational view of any system, including
the entities and links in between. However, it does not provide information on the sequence of
processes, which will be demonstrated in the following sections.
Figure 2 : OPM representation of MSF and PTC processes .

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3.2. Activity Diagrams
The activity diagram, through comparisons with the OPM, provides a finer level of detail by
demonstrating the order in which the processes follow in the system. The activity diagram is used
to model the general behavior of the system, using a controlled sequence of actions to transform
inputs to outputs, and it can support continuous flow modeling, which makes it suitable to
represent the desalination system [10].
Figure 3 illustrates the flow of inputs into the two main systems; MSF and PTC. The processes
start with an initial node, while the flow takes place between inputs and outputs. The flow of sea
water can be considered an open system, where the desalination process are defined by the
required output; the fresh water. On the other hand, the PTC is a closed loop that continuously
transfers between the states of heated oil composite and cooled oil composite. The heated oil
and water exchange thermal energy in the brine heater. Since the system mainly consists of
physical process of heat and fluid transfer, these processes obey the rules of thermodynamics,
hence it is expected to have some loses in the heat energy, these loses are reduced through
added insulation to the tubes and the brine heater tank. Furthermore, the system is constructed
to produce a recovery ratio, by recycling the saline water until it reaches a high level of salinity
(>5%) where it turns to brine, it returns back to be resolved in the sea. [11]
The diagram indicates the importance of the heating process. Analyzing the function of heating
will provide further validation to the possibility of integrating a renewable heat source with the
existing MSF system, and in a cost efficient manner. Figure 4 represents an activity diagram that
provides a magnified view of the water heating process.

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Figure 3 : Activity Diagram of MSF Heated with PTC
Figure 4 : Activity Diagram of MSF Heated with PTC

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3.3.Design Structure Matrix
Design Structure matrix (DSM) is a network modeling tool, which was applied in this section to
represent and analyze the process architecture of MSF and PTC as shown in Figure 5. Some of
the advantages of representing the system in DSM are highlighting the relationships patterns
between system functions, in addition to providing a system-view that supports an optimal
decision making [12].
The connections between the processes in the matrix are indicated by different colors and each
indicates the type of interaction. Most of the connections are based on transporting the basic
fluids in the system, while the rest are based on transforming the seawater intake and the oil into
the different states, through heat exchange.
The processes of creating and transforming heat source (indicated in Figure 5 by an orange line)
is the key set of processes required to integrate the renewable energy. The MSF system will only
require retrofitting of the brine heater to adapt to the change in the heat source.
Figure 5 : DSM of MSF integrating PTC

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4. CONCLUSION
This paper conveyed a representation of the function of multi-stage flash system, integrated with
parabolic solar collector as a low-cost, renewable source of heat used for the desalination
process. The methods used to represent the function include OPM which was used to present
the operational view of the MSF and PTC systems. These MSF and PTC systems were shown to
consist of system entities, processes and states. The activity diagram of the systems presented
the same processes with a sequence, by tracing inputs to produce the required outputs. Another
activity diagram was used to take a closer look at the heating processes that integrate the PTC
with the MSF and thus demonstrating the possibility of introducing a renewable heat source,
while maintaining the same form of the existing legacy MSF system. Finally, a DSM was used to
represent and analyze the process architecture of MSF and PTC, while indicating the types of
interactions between the functions.
It was concluded that the main set of processes that provide the potential of renewable energy
integration, is the heating process. The analysis of the functions demonstrate the possibility of
retrofitting of the form, by modifying the brine heater to receive the heat source.
The previous representations of architecture function for the MSF and PTC provide the initial
validation for integration of PTC as a heat source for the system of MSF, yet further analysis needs
to be conducted to examine the production efficiency, and the feasibility of physically connecting
the PTC to the desalination plant.

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5. REFRENCES
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Engineering of System Form," Abu Dhabi, 2014.
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water nexus," Applied Energy, vol. 135, p. 142–157, 2014.
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Processes, Palermo: Springer, 2009.
[8] Y. Grobshtein, V. Perelman, . E. Safra and D. Dori, "Systems Modeling Languages: OPM Versus
SysML," Israel institution of Technology, Haifa, 2011.
[9] N. Soderborg, "Representing systems through object-process methodology and axiomatic design,"
11 1 2011. [Online]. Available:
http://dspace.mit.edu/bitstream/handle/1721.1/34725/50944745.pdf?sequence=1. [Accessed 18
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[10] A. Farid, "System Architectuing: Lecture 17," Masdar Institute, Abu Dhabi, 2014.
[11] C. Sommariva, "MIT OpenCourseWare," 2 2009. [Online]. Available:
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