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A Reference System Architecturefor the Energy-Water NexusWilliam Naggaga Lubega and Amro M. Farid, Member, IEEE

Abstract—The energy-water nexus has been studied predomi-nantly through discussions of policy options supported by datasurveys and technology considerations. As the degree of couplingbetween the energy and water systems is affected by the designand operation of various engineered system components, ourunderstanding of the nexus and ability to tackle its associatedchallenges would be enhanced by a system of systems engineeringmodel. Such a model however requires, first, the developmentof an appropriate system architecture that clearly identifies therelevant flows of matter and energy and the defining systemparameters. This paper presents a reference system architecturefor this purpose developed and presented with the SystemsModeling Language (SysML). Once instantiated, this architecturecan serve three purposes. First, the presented graphical modelscan serve qualitative discussions on where and how the supplyand demand of water and energy are interdependent. Second,within the operations time scale, the SysML models can supportthe development of automated IT and control solutions thatintegrate energy and water management. Finally, at a planningtime scale, the models can inform quantitative decisions on howto best grow and reconfigure the water, wastewater and energyinfrastructure.

Index Terms—Architecture, Power Systems, Water Resources,Wastewater, Sustainable Development

I. INTRODUCTION

The supply and demand of water and electricity are inextri-cably linked and consequently should be addressed together.Extraction, treatment and conveyance of municipal water andtreatment of wastewater are both dependent on significantamounts of electrical energy. It has been estimated [1] thatcurrently between 1% and 18% of electrical energy used inurban areas worldwide is for treatment and transportation ofwater and wastewater —the spread being due to differencesin topography, conveyance distances and water treatment pro-cesses. Simultaneously, large volumes of water are withdrawnand consumed from water sources everyday for electricitygeneration processes. In the United States, withdrawals forthermal power plant cooling account for 45% of all freshwater withdrawals [2], more than any other sector. Hydro-electric generation, the second most prominent electricitygeneration technology (16% of global generation, second to80.3% thermal generation [3]) incurs significant evaporativelosses as more water evaporates from dam reservoirs thanfrom free flowing rivers due to the increased surface areaand stillness; it has been estimated [4] that in the UnitedStates, evaporation of 14×106m3 per day can be attributed to

W.N. Lubega is with the Department of Engineering Systems and Manage-ment, Masdar Institute of Science and Technology

A.M. Farid is with the Department of Engineering Systems and Man-agement, Masdar Institute of Science and Technology and the TechnologyDevelopment Program, Massachusetts Institute of Technology

hydroelectric reservoirs, which is about three times the dailywater consumption of a large US city like New York.

This energy-water nexus, which couples the critical systemsupon which human civilization depends, has existed since thefirst implementations of the electricity, water and wastewatersystems. The coupling, however, is becoming increasinglystrained due to a number of global mega-trends [5]: (i) growthin total demand for both electricity and water driven bypopulation growth (ii) growth in per capita demand for bothelectricity and water driven by economic growth (iii) distortionof availability of fresh water due to climate change (iv) mul-tiple drivers for more electricity-intensive water and morewater-intensive electricity such as enhanced water treatmentstandards,water consuming flue gas management processes atthermal power plants and aging infrastructure which incursgreater losses. These trends raise concerns over the robustnessof the electricity and water systems today and their sustain-ability over the coming decades. There is a risk, if the nexus isnot optimally managed, that scarcity in either water or energywill create aggravated shortages in both.

There are however several opportunities to improve theresilience and sustainability of these critical systems throughintegrated management. Losses in the water and electricitysystems (leaked water, electrical losses and thermal losses),for example, have embedded electrical energy and waterconsumption respectively and thus losses in one system areessentially, and possibly more importantly, losses in the other.This creates the opportunity for rethinking financing for elim-ination of losses in either system which is typically a keyconstraint. Water supply portfolio management provides an-other illustration; use of energy-intensive water supply optionscan be deferred to seasons of low electrical demand. On adiurnal timescale, water storage can be used as a relativelycentralised demand smoothing lever, reducing the requirementfor electricity storage which is less technologically developedand incurs greater losses. Furthermore, an effort to reducea deleterious environmental effect such as carbon emissionsor excessive water withdrawals can be achieved by means ofchanges in either system, with one system perhaps providinga cheaper alternative.

It is clear that the energy-water nexus is a multifacetedchallenge that calls for a holistic system of systems approachfor its effective study and management. To this end it isnecessary to develop a descriptive system architecture toclarify the flows of matter and energy between the electricity,water and wastewater systems and between these systems andthe environment, and furthermore to identify the parametersof the elements of these three systems that are germane to

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this challenge. This paper presents an instantiable referencesystem architecture for this purpose. Section II presents a briefreview of issues covered in various existing publications onor related to the energy-water nexus. In Section III, internalblock, activity and block definition diagrams are presented foran architectural view of the electricity, water, and wastewaterengineering systems. Section IV describes the major researchthrusts that can be taken to build on the presented architecture.Section V concludes the work.

II. BACKGROUND

Olsson’s 2012 book [1] is perhaps the first book dedicatedentirely to the energy-water nexus. It covers the major couplingpoints between the energy and water systems as well as theinteractions of the nexus with population growth, climatechange and food supply. Governments and major internationalorganizations have recognized the importance of the energy-water nexus and produced reports [4]–[6] discussing potentialfuture challenges and technology options. In keeping withthe typically local nature of energy-water nexus issues, pub-lications have evaluated policy options for specific locationssuch as Texas [7] and California [8] in the United States, andthe water-scarce, energy-rich Middle East and North Africa(MENA) region [9].

Estimates of unit electricity requirements (kWh/m3) forsurface water treatment, groundwater treatment and represen-tative wastewater treatment processes have been provided involume 4 of the Electric Power Research Institute’s (EPRI)Water and Sustainability series [10]. Unit water requirementsfor different thermal power plants depending on coolingtechnologies and fuel types can be found in [11] as well asin volume 3 of the aforementioned EPRI series [12]. Theunit requirement estimates provided in all these studies arebased on data collected from representative surveys of watertreatment plants and thermal power plants.

The studies mentioned above highlight the two approachesthat have predominantly been taken in the literature to com-ment on and study the energy water nexus: (i) discussionsof challenges, technologies and policy options and (ii) pri-marily empirical evaluations of the electricity-intensity ofwater technologies and the water-intensity of electricity tech-nologies.Empirically determined constants, however, are blackbox models that do not provide an indication of underlyingfactors. They are also dependent on data that must be collectedin extensive and expensive surveys. High-level qualitativediscussions provide a good overview of possible solutions butit remains for engineering models to realise them. There is thusa need and an opportunity to augment the study of the energy-water nexus with hybrid physics-based system of systemsmodels which would address both these deficiencies. The useof phyics-based models of the engineered components of thesethree systems will provide greater insight than the black boxempirical unit requirements thus enabling integrative systemdesign. Modelling the three systems together as opposed toin isolation as is traditionally done enables the elucidation ofeffects of one system on another and on the environment thatwould not be immediately apparent in traditional models ofelectricity, water and wastewater systems. Efforts have been

made towards physics-based models in [13], [14] in whichformulations for estimating water use by thermal power plantsbased on the heat balance of the plant have been derived; andtowards an integrated operational view of the water and powernetworks in [15]–[17] in which simultaneous co-optimizationof the economic dispatch for power and water is presented. Tounlock the full potential for integrated analysis, planning andcontrol of the electricity, water and wastewater systems, a fullydescriptive engineering system of systems model is required;such a model has to be built on an appropriate descriptivesystem architecture as presented in Section III.

It must be mentioned that the energy-water nexus is part ofa larger energy-water-food nexus [18]. Both energy and waterare required for food production and increasing use of biofuelshas introduced, in some parts of the world, competition be-tween food and biofuel production for land use and irrigationwater. This work focusses only on energy (specifically elec-tricity) and water as the objective is to support development ofengineering models of the nexus. Any models and quantitativemeasures thus developed however have applicability in waterresource planning for agriculture [19] and can be integratedwith scientific food production models for a complete scientificview of the energy-water-food nexus.

III. MODELING

This section presents the models of the energy-water nexusas a reference system architecture. First, the system boundaryand internal block diagram are described in Section III-A.Next, attention is given to modeling the system function,concept and form of the electricity, water and wastewatersystems in subsections III-B, III-C and III-D respectively. Thesystem function presented and discussed in subsection III-Bbuilds on work hitherto presented in [19], [20].

A. System Boundary and Internal Block Diagram

The energy-water nexus has developed to be a major sus-tainable development challenge in part because the engineeringof an industrial facility gives limited attention to the otherindustrial facilities upon which it depends. The required inputand subsequent output flows are specified during the facility’sdesign without the awareness that such flows cause subop-timal performance of the multi-facility system as a whole.Furthermore, given that cost/benefit and ROI analyses areoften conducted purely within the scope of the facility designas a project, it is not clear that any design changes wouldoccur even with greater awareness of the holistic systemperformance. For this reason, an appropriate system boundaryfor consideration of the energy-water nexus must be chosenjudiciously.

Fig. 1 chooses the system boundary around the three en-gineering systems of electricity, water and wastewater. It alsodepicts the high level flows of matter and energy betweenthem and the natural environment. The valued products ofelectricity, potable water, and wastewater are all stationarywithin the region’s infrastructure; in contrast, the traditionalfuels of natural gas, oil, and coal are open to trade andconsumption by another sector if not consumed by the localthermal power generation. Consequently, the fuel processing

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Fig. 1: System Internal Block Diagram for Combined Electricity, Water & Wastewater Systems

function is left outside of the system boundary. Agriculture,mining and decentralised water and wastewater municipaltechnologies [21], [22] have also been excluded such thatthe selected system boundary only encompasses functions andobjects that fall under the purview of grid operators and thatthus lend themselves to both engineering analysis and cen-tralised planning or control. The system internal block diagramshown in Fig. 1 makes it possible to relate a region’s energyconsumption to the required water withdrawals in a complexinput-output model. A clearly drawn system boundary furtherraises awareness of the importance of cumulative water andenergy losses that tax a region’s natural water resources withno added benefit.

B. System Function

To provide further insight into the processes that realize theoverrall functionality of these three systems of interest andthat thus create the energy-water nexus, the system function ofeach shall now be discussed individually. From an engineeringstandpoint, all three systems (and the coupling points betweenthem) involve only the flow of matter and/or energy, thereforethe SysML activitiy diagram has been selected to model thisflow-based behaviour [23]. The functions are defined with anaction-object convention thus each identified function depictsan activity that transforms an object (matter or energy, in thiscase) from one state to another [24].

1) Electricity System Function: Fig. 2 is an activity diagramfor the electricity supply system. The water inputs for the ther-mal and hydro power generation functions on the left [11], [12]are to be interpreted as withdrawals rather than consumption.

Unlike thermal and hydroelectric power, wind and photovoltaicgeneration, the dominant non-hydro renewable generationtechnologies, do not directly consume any water in generation[25]. Wind and solar power generation are often called variableenergy sources because they are characterized by intermittencyand stochasticity [26], [27]. To support their adoption, utility-scale electricity storage is often discussed as a key enabler.Pumped hydro storage —for which water is required —is themost developed storage technology, accounting for 95% ofglobal grid storage capacity [28]. Other grid storage optionssuch as various battery technologies and compressed air energystorage (CAES) have shown promise but are characterized bylower efficiencies, capacities, and lifetimes [29]. Therefore,although substitution of thermal generation with renewableswill certainly have a positive impact on the water footprint ofelectricity generation, it cannot be said that it will completelyuncouple the energy-water nexus as there will be a resultantincrease in the water footprint of electricity storage.

2) Water System Function: An activity diagram for theengineered water supply system is shown in Fig. 3. Of thefive indicated water supply options, desalinated sea water isthe most energetically expensive to produce. The dominantdesalination technologies are Reverse Osmosis (RO) whichaccounts for 60% of global desalination capacity followed byMulti-stage Flash (MSF) Distillation which accounts for 27%[30]. RO is a process in which semipermeable membranesact as filters allowing fresh water to pass while holding backdissolved salts. Electrical energy is utilized in RO plants forpumping to generate the significant hydraulic pressure requiredto overcome the natural osmotic pressure which would cause

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Fig. 2: Activity Diagram of Electricity Supply System Functions

Fig. 3: Activity Diagram of Engineered Water Supply System Functions

filtered fresh water to flow back across the membranes. Thespecific electric energy requirement for RO varies with thesalinity of the seawater but is typcially in the range of 3to 5 kWh/m3 [30], [31]. MSF, a thermal process, is moreenergy intense than RO, however large scale MSF distillationis typically integrated with thermal generation in cogenerationplants with the desalination process deriving its requisitethermal energy from steam extracted from along the powerplant turbine at the appropriate pressure and temperature[31]–[33]. Determination of the specific energy requirementin this case is complicated by the need to apportion theprimary energy consumed between the electricity and water

generation processes. In addition, for comparison purposes, itis impossible to compare heat, which is of low energy grade,with electric power. The solution commonly employed [31],[33] is to express the energy associated with the steam inputto the desalination plant in terms of an equivalent loss ofelectric power that would otherwise have been generated bythe steam. With this approach the specific energy requirementof MSF desalination has been estimated to be between 10and 20 kWh/m3 [31]. In contrast to these two desalinationtechniques the specific energy requirements for groundwaterand surfacewater treatment have been estimated [5] to be, onaverage, 0.16 and 0.06 kWh/m3 respectively.

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Treated and desalinated water is then distributed to endusers with the aid of pumps within the distribution system.The amount of electrical energy required depends on theconveyance distances, the topography and the volume of waterbeing transported. The additional pumping energy required forwater storage either to serve as a security buffer or to minimizeoperational costs where time-of-use elctricity tariff structuresexist, is explicitly shown in Fig. 3, as time displacement ofthis energy presents a demand side management opportunityas discussed in Section I.The activity diagram clarifies thesources of water loss and alteration. An issue of concernin public water distribution systems is pipe leakages; oftenestimated in double-digit percent losses. In absolute terms,this accounts for approximately 32 billion cubic meters oftreated water per year [34]. As shown in Fig. 1, leaked watereventually finds its way to the water table and is thus not trulylost. However, that it is displaced from its original sourcesignifies that it is not necessarily easily recoverable and, inaddition, the embedded energy in the water up to the point ofleakage is lost. Furthermore, as mentioned in the electricitysystem discussion, stored water in man-made lakes and openair storage tanks suffer from evaporative losses that may bemeasured in water or embedded energy terms. The output ofbrine from desalination facilities also provides a significantsource of altered water with potential environmental impactson the water bodies to which it is returned. Finally it must bementioned, that in addition to supply and conveyance, energyis utilized in conditioning water for end use applicationssuch as heating, cooling, pressurizing or purifying. In somecases, this energy consumption is greater than in supply anddistribution functions. In California, for instance, it has beenestimated that 5% of all consumed electrical energy is usedfor water supply while 14% is used for activities involvingor related to domestic water use such as water heating andclothes washing [4]. Energy intensities for various categoriesof domestic and commercial water uses have been estimated[35] and range from zero to as much as 50 kWh/m3. End-use devices and processes that minimize water consumptioncan therefore conserve energy both upstream in supply andconveyance, and downstream at the point-of-use.

Fig. 4: Activity Diagram of Wastewater system functions

3) Wastewater System Function: Wastewater collection, asshown in Fig. 4, typically does not require electric power input.Wastewater is typically conveyed by gravity-flow sewers [1] aswastewater treatment plants are built at low elevations, closeto the water bodies into which effluent is to be discharged.The wastewater system, however, does require electric powerfor treatment: various types of electric motor-driven equip-ment including pumps, blowers and centrifuges are used inwastewater treatment operations. In addition to the standard

processes of filtration and biological decomposition, a widerange of processes with different energy requirements suchas chemical precipitation, ion exchange, reverse osmosis anddistillation [36] are variously employed in different wastewatertreatment plants to eliminate specific residual constituents asrequired by local environmental discharge regulations andreuse quality requirements. Attempts to quantify the per-unitenergy requirements for wastewater treatment have typicallyclassified treatment plants in four representative categories.The per-unit energy requirements for these categories havebeen estimated by survey [5], [10], [36] as given in TableI. While treated wastewater, has in the past, predominantlybeen returned directly to surface water bodies as disposableeffluent as shown in Fig. 4, wastewater reuse offers sizablepotential energy benefits. The most prominent wastewaterreuse categories are agricultural irrigation, landscape irriga-tion, groundwater recharge and industrial processes [36], [37].Groundwater recharge has an energy benefit, in that it preventsthe depletion of acquifers close to the surface and thus the needto extract water from deeper ones. The integration of recycledwastewater into the industrial water supply system has beenimplemented in Singapore under the NEWater scheme [5],[38] which supplies water, that in addition to conventionalbiological treatment and filtration processes, has been purifiedwith ultraviolet, microfiltration and reverse osmosis technolo-gies making it suitable for industrial applications requiringwater of high purity. It has been shown [9] that, in severalMENA countries, recycled wastewater has the potential tomeet nearly all industrial water demand. Direct water reusethrough blending of recycled wastewater into the potable watersupply is far less common but has been succesfully employedin water scarce Namibia since 1968 [38]. However, pathogentransmission concerns and public sentiment [36] have thusfar prevented widespread adoption of direct water reuse inmost places. In regions where water is reused for irrigationor industrial purposes but not blended into the potable watersupply, the recycled wastewater must be distributed —with anenergy cost —to users through a seperate water distributionsystem as shown in Fig. 4. For completeness, the less commoncase of direct water reuse is also shown.

TABLE I: Wastewater treatment energy requirements

Treatment type kWh/m3

Trickling filter 0.25Activated sludge 0.34Advanced 0.4Advanced with nitrification 0.5

C. System Concept

Once the system function has been well understood, thedevelopment of an energy-water nexus reference system ar-chitecture turns to the allocation of system function to sys-tem form in the system concept. The electricity, water andwastewater systems may all be characterized as large flexiblesystems as defined by Suh’s Axiomatic Design theory [39].In other words, system functions can be realized by oneor more modules which may evolve over a planning time

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scale. This allocation of system function to form can begraphically represented using the SysML language [23] orequivalently captured in a binary axiomatic design knowledgebase with the functions along the row and the componentmodules along the columns [39]. At the meta-level, Table IIshows a nearly diagonal knowledge base where each systemfunction corresponds to its intuitive type of facility. Thermaldesalination plants and pumped-hydro storage facilities are theexception in that they realize two functions. Additionally, therealization of the water storage function is classified by theneed for water storage tanks, man-made lakes, and pumped-hydro facilities. These facilities serve as the basis of the classdiagram description of form in the next section.

D. System Form

In order to translate the functions identified in SectionIII-B into an engineering system model, parameterized modelsof the form elements identified in Section III-C must bedeveloped. This section describes the parameters of thesesystem elements that any such models should characterize.As with Section III-B each of the three systems willl bediscussed in turn. To model these object characteristics, theSysML block definition diagram [23] has been selected. Withthis diagramming tool, each modelled object is characterizedby value properties which are quantifiable characteristics ofthe object and reference properties which are parts that arenot owned by the object.

1) Electricity System Form: From an energy-water nexusperspective, the key parameter of interest for thermal gen-eration plants is the unit water requirement (m3/kWh) forwithdrawal, consumption or both depending on the objectiveof the modelling effort. This requirement is dependent on theheat load placed on the cooling system and thus the heat rateof the plant. The heat rate and fuel heating value (kJ/kg)ultimately determined the required fuel flow rate. Thermalcooling systems are of three types: once-through, recirculatingand less commonly dry [5]. For an open loop system, aneffluent temperature limit(K) to protect the aquatic ecosystemdetermines the volume of water that must be withdrawn tocarry away a given heating load. The water consumptioncaused by the evaporation of the water that is released backinto the environment at an elevated temperature can thenbe estimated using models of evaporation from reservoirs.There are several such models and a discussion on theseis beyond the scope of this work; the reader is referred to[40] for such a discussion. For recirculating cooling systems,the water withdrawal consists of two components [41], [42]:make-up water for evaporation and blowdown. Blowdownis required to keep the concentration of impurities in thecooling tower within acceptable limits to prevent fouling andscaling and thus ensure effective heat transfer. The ratio of theacceptable concentration of impurities in the cooling tower tothe concentration of these impurities in the make-up water,referred to as the number of cycles of concentration [41], [42],controls the rate of blowdown discharge and thus contributesto the rate of water withdrawal. In areas where the make-upwater is of high purity, the number of cycles of concentrationwill be higher and thus water withdrawals will be lower.

The evaporation from the cooling tower is dependent on thevaporization enthalpy (kJ/kg) of the water as the heat iscarried away by evaporating water at saturation temperature—this is in contrast to open-loop systems in which the heatis carried away by a temperature increment and for which thespecific heat capacity (kJ/kg ·K) is thus the water propertyof interest.

Wind and solar photovoltaic power plants, for purposesof an energy-water nexus model, need only be characterizedby their electrical characteristics as they do not consumewater in operation. Particularly of interest is their abilityto provide reactive power and their integration into reactivepower support [43], as users’ reactive power demand (V Ar)currently necessitates the operation of synchronous generatorsin thermal and hydro electric power plants which do havea water footprint. Hydroelectric power plants have a specificflow rate requirement (m3/kWh) given the dam reservoirheight(m) and turbine efficiency. In the event that this flowrate cannot be sustained for a desired power output level, dueto drought for example, the power output must be lowered.From a consumptive point of view, the quantification of thepotentially significant water evaporation from the reservoirrequires an evaporation model [40]. Alternatively evaporationisolines constructed from annual-average pan evaporation data[44], [45] can be used together with the coordinates andsurface area of the reservoir to determine evaporative losses.Either approach would yield an evaporation rate (m3/s). Ifthis rate is divided by the power output, it yields a specificevaporative loss (m3/kWh) for the hydroelectric plant; av-erage values of this specific evaporative loss for the UnitedStates obtained with the pan evaporation data approach areprovided in [44]. Dividing by the power output however,suggests that there is causality between the power generationand the water consumption which is not the case, as the rate ofevaporation from a particular reservoir will not be altered if thedam is generating below or at its full capacity. Furthermorethe water stored in the reservoir is often used not only forpower generation but for water supply and recreation andthus the evaporation cannot be fully ascribed to electricitygeneration. The water consumption is therefore a characteristicof a particular dam, not of the power generation process andthe evaporation rate is preferred as a parameter of a dam froman energy-water nexus perspective.

An electrical power system model must also, from anenergy-water nexus perspective, characterize transmission anddistribution losses by means of an impedance (Ω) as theselosses have embedded water withdrawals and consumption. Ifa dynamic model incorporating storage is desired, the capacity(kWh), rating (kW ) and efficiency of the storage is required;if the storage is pumped-hydro —which is effectively a hydro-electric power plant run in two directions —the evaporationrate, and specific flow rate requirement as obtained for thehydroelectric power plant, are also required.

2) Water System Form: The surface water bodies andaquifers that supply the surface and groundwater treatmentplants are characterized most importantly from an energypersepctive by their elevation (m) and depth (m) respectivelyas this determines the energy requirement for pumping the

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TABLE II: Electricity, Water and Wastewater Systems Knowledge Base

HE

PPl

ants

The

rmal

Pow

erPl

ants

Sola

rPV

Plan

ts

Win

dPo

wer

Plan

ts

Non

-Wat

erG

rid

Stor

age

T&

DIn

fras

truc

ture

TD

P

MD

P

SWE

TP

GW

ET

P

Pum

ped-

Hyd

roSt

orag

e

Man

-Mad

eL

akes

Wat

erTa

nks

Mun

icip

alW

ater

Use

rs

Sew

erN

etw

ork

WW

TP

Rec

ycle

dW

ater

Net

wor

k

Generate Electricity from Water xGenerate Electricity from Fuel x x

Generate Electricity from Solar Irradiation xGenerate Electricity from Wind x

Store Electricity x xTransmit Electricity x

Desalinate Seawater (cogeneration) xDesalinate Seawater (osmosis) xExtract & Treat Surface Water xExtract & Treat Ground Water x

Store Water x x xUse Water x

Collect Wastewater xTreat Wastewater x

Distribute Recycle Wastewater x(TDP=Thermal Desalination Plants, MDP= Membrane Desalination Plants,SWETP=Surface Water Extraction and TreatmentPlants, GWETP = Groundwater Extraction and Treatment Plants, WWTP = Wastewater Treatment Plants)

Fig. 5: Form of Electricity Supply System

untreated water to the treatment plant. Sea water is, in ad-dition, importantly characterised by its salinity (ppm) as thisdetermines the energy required for desalination: in the caseof reverse osmosis, the salinity of the sea water determinesthe osmotic pressure which has to be overcome by a pumpto drive the water across the membrane while in the case ofthermal desalination this salinity determines the elevation ofthe boiling point above that of pure water and thus contributesto the thermal energy required for evaporation [33].

Pumping consumes 98 percent of the power at a surfacewater treatment plant [10] and an even higher proportion ata groundwater treatment plant; these plants can therefore beviewed from an energy persepctive as consisting purely of aseries of pumps as shown in Fig. 6, for which the parameter of

interest with regards to energy consumption is the efficiency.These pumps must be appropriately sized for the capacity ofthe plant (m3/s) to raise the water to the plant’s storage tankelevation (m) for distribution.

Reverse Osmosis desalination is a pressure driven process asdiscussed in Section III-B and thus a reverse osmosis plant canalso be viewed as consisting of pumps. Thermal desalinationplants, since they operate by heating and evaporating sea waterhave a specific energy requirement(kWh/m3) —determinedas discussed in Section III-B —that is dependent on the salin-ity, specific heat capacity (kJ/kg ·K), vaporization enthalpy(kJ/kg), and temperature (K) of the sea water. In addition, thetransmission of heat from a heat source (usually steam froma power plant) to the sea water requires the maintenance of

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Fig. 6: Form of Engineered Water Supply System

a temperature gradient referred to as a terminal temperaturedifference (K) that is dependent on the design of the heatexchanger but is usually designed to be in the range of 2− 6Kelvin [33].

Piping infrastructure, from an energy persepctive, acts asa resistor, dissipating hydraulic energy provided by pumps.The pressure drop across a pipe is determined typically byone of two formulations: the Darcy-Weisbach equation andthe Hazen-Williams equation [46]. Both formulations providea pipe resistance (Ns/m) that is dependent on the pipediameter, length and material. Also of concern from an energyperspective is the proportion of water that leaks out of the pipethat can be modelled by a leakage coefficient, that is to say apipe leaks a particular fixed proportion of the water it conveys.For greater precision however, leakage can be modelled asbeing pressure dependent [47] which opens up opportunities toexplore reduction of leakage (and thus energy losses) throughthe management of pressure.

Users (or demand nodes) are of course characterized by theirlevel of demand (m3/s) (which may also be modelled as beingpressure dependent [47]) and their elevation (m) both of whichdetermine the energy required to service these users. There aretwo types of water storage both of which are characterized bya capacity(m3): tanks, which are covered and that therefore donot have evaporative losses (with their embedded energy) andartificial reservoirs that do have an evaporation rate (m3/s)that can be determined in the same manner as for hydroelectricreservoirs.

3) Wastewater System Form: The wastewater managementsystem’s form is represented in Fig. 7. The collection portionof the system consists of a largely passive sewer network,as mentioned above, that does not require parameterizationfrom an energy perspective. The wastewater treatment plants,however, have a unit energy requirement(kWh/m3) as pro-vided for representative processes in Table I. As discussed,in most cases, there is an independent recycled wastewaternetwork to serve specific users. This network is similar to the

conventional water distribution system consisting of pumps,pipes, and storage characterized in the same way.

E. Parametrics

The equations that relate the value properties of the blocksidentified in Section III-D can be defined using the SysMLConstraint Block type and thus depicted using a SysML Para-metric Diagram [23]. This would yield a reference architecturethat includes the physics-based models alluded to in SectionII and elaborated upon in Section IV. However as a verylarge number of such constraint blocks would be required tomodel the entirety of relationships for the energy-water nexus,this functionality has not been presented in this paper. Theseparametrics however can readily be implemented in severalmodel based systems engineering tools.

IV. DISCUSSION

While the presented descriptive architecture certainly aidsa qualitative discussion of the various aspects of the energy-water nexus as in the previous section, its chief benefit is inthe facilitation of the development of a hybrid engineeringsystem model of the energy-water nexus to support planningand control applications.

A. Engineering Systems Model Development

At the most basic level, the mass and energy flows de-picted in the activity diagrams could be used together withempirically-determined values of the water intensities of elec-tricity generation technologies and electricity intensities ofwater and wastewater system elements such as those providedin the studies mentioned in Section II to develop an input-output model for the electricity, water and wastewater systemsin unison. Such a model, readily implemented in a spreadsheet,could provide a guide as to whether environmental resourcesare adequate to meet societal needs with respect to thesethree systems. From an engineering perspective however, it isdesirable to replace the empirically determined intensities withphysics-based models. These models would provide insight

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Fig. 7: Form of Wastewater Management System

into the underlying factors determining the various intensitiesthus opening the door to exploration of how these intensitiescould be tuned for maximum system-wide benefit. Finally,since the engineered wastewater, electricity and water systemshave, at their cores, three connected grids, the topologies ofwhich contribute to their degrees of coupling, a hybrid system-of-systems model could be developed. This model, and anassociated numerical solution technique could, where the threegrids are sufficiently coupled, be beneficially used in place ofsimple mass and energy balances to provide greater control andplanning fidelity. Such a system-of-systems model, built onthis work, utilizing bond graph representations of the variouscoupling and boundary points, has been presented [48].

B. Integrated Planning and Control

A hybrid model of the three discussed systems, oncedeveloped, would yield a set of descriptive equations thatcould be used to support an integrated approach to planningin various ways: (i) as part of an optimization procedureto determine the best system design and upgrade choicesacross the three systems to achieve a desired output subjectto a set of environmental constraints (ii) to establish whichsystem choices have the greatest effect on a desired output orinput by means of differential calculus, and (iii) integrationwith models of population and climate dynamics to study thepotential evolution of the environmental effects of and cou-pling between the three systems. The developed model wouldalso find application in an integrated approach to operationscontrol of the three systems. A system-of-systems model, asdescribed, would facilitate utilization of the water and wastew-ater systems —which are significant and relatively centralizedelectrical loads —as demand response levers, particularly withthe greater demand flexibility requirement necessitated bylarge scale integration of renewable energy sources. Anotherpotential application would be optimal dispatch of water andpower from cogeneration plants subject to demands, topologiesand capacities of other source nodes, in both the water andpower networks.

V. CONCLUSIONS AND FUTURE WORK

This work has developed an architecture for the energy-water nexus in the engineered electricity, water and wastewatersystems that describes how these systems interact with eachother and with the environment. The presented graphical

models can aid qualitative discussions on the nature of thecoupling between these systems as well as opportunities forimproved holistic management as demonstrated through thediscussion presented here. Furthermore, the SysML modelssupport the development of engineering models of the nexusthat characterize the parameters described in Section III-D forintegrated control and planning of the electricity, water andwastewater systems as discussed in Section IV. Ongoing work[48] is on the development of such engineering models [48].In addition, as discussed, any quantitative measures obtainedfrom the engineering systems model have applicability in wa-ter resource planning for agriculture [19] while the referencearchitecture can itself be enhanced to include food productionsystems for a full system architecture of the energy-water-foodnexus.

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William Lubega received his bachelors degree inelectrical engineering from Makerere University,Uganda in 2009. He is currently working towardshis masters degree in the Engineering Systems andManagement program at Masdar Institute of Scienceand Technology, Abu Dhabi, U.A.E.

Amro M. Farid received his Sc.B and Sc.M degreesfrom MIT and went on to complete his Ph.D. degreeat the Institute for Manufacturing within the Univer-sity of Cambridge Engineering Department in 2007.He is currently an assistant professor of EngineeringSystems and Management program and leads theLaboratory for Intelligent Integrated Networks ofEngineering Systems (LI2NES) at Masdar Instituteof Science and Technology, Abu Dhabi, U.A.E. He isalso a research afliate at the Massachusetts Instituteof Technology Technology and Development Pro-

gram. He maintains active contributions in the MIT Future of the ElectricityGrid study, the IEEE Control Systems Society, and the MIT-MI initiativeon the large-scale penetration of renewable energy and electric vehicles. Hisresearch interests generally include the system architecture, dynamics andcontrol of power, water, transportation and manufacturing systems.