Food, Energy and Water Nexus: A Brief Review of Deﬁnitions, Research, and Challenges

: Vast expansion in consumption is leading to natural resource scarcity and global warming. The integrated management of natural resources, such as food, energy, food (FEW) as one of the most important aspects has been proposed as a solution to meet these challenges. The FEW nexus is a world-wide solution for simultaneously assessing the development and implementation of various approaches focusing on energy, water and food security, su ﬃ ciency. This approach is intended to foster sustainable development and improve the quality of life of communities while preserving the natural, human and social capital, address the long-term sustainability challenges and protecting all-natural resources. This paper tries to review some recent research on this topic. For this purpose, ﬁrst, we describe some facts about demand growth and exponential consumption in these three areas, with emphasis on presented statistics. Then, the most critical research published in this ﬁeld is reviewed, considering that it took a decade or so before that the original idea was introduced. The most important policymakers of this emerging concept, including committees and conferences, and ﬁnally signiﬁcant challenges and opportunities to the implementation along with future insights, are addressed.


Introduction
The food-energy-water (FEW) nexus is known as the significant interconnection between three essential resources for human societies. FEW is an extended concept that is introduced to overcome and manage resource scarceness challenges. The nexus target is to establish effective tradeoffs and synergies between energy, water and food, considering cross-sectoral policies, environmental and social impacts [1]. The FEW nexus implies that the availability of the others may limit constraints or changes of one edge, and implemented solutions in one sector can mainly affect other areas [2,3]. For example, food or energy production is limited by the availability of the water supply. Energy-saving can decrease water consumption, whereas water efficiency growth leads to a decrease in energy, which is used for transition and purification.
As an important note, and based on what is addressed in [4], the perspective of the policymaker determines the method to the FEW nexus. From the perspective of water, energy and food systems are mentioned as the users of the resource, and in the perspective of food and energy, energy and water; water and bio-resources are the outputs, respectively. In addition, each of the three resources affects the others, and ignoring the effects in one resource can have significant impacts on other sources. The main important types of FEW nexus are detailed as energy access and deforestation, by the prevalent fossil fuel usage in both transportation and power generation, is widely accepted as a real-world menace that has potentially severe effects on human health. Hence, one of the critical concerns is moving from fossil fuels to other nonconventional renewable sources of energy, such as wind and solar [15]. As shown in Figure 1, a major contributor to GHG emissions is heat and electricity extraction from oil, coal and natural gas [16]. Even though the nexus concept has been introduced in recent decades, it has evoked the eagerness of researchers all over the world [17].  [16].  [16].
Inventions 2020, 5, x FOR PEER REVIEW 3 of 14 Preserving the natural resources and preventing their destruction in the three fields of food, energy and water requires a comprehensive framework that considers not only the individual security of the three systems but also their interactions and interdependencies [18]. Figure 2 shows a conceptual model in this regard .As an important note, environmental concerns have become an important security issue in international relations and have been addressed by governments and policymakers. Due to the seriousness of environmental issues and related climate change, this issue can be considered as an effective variable in various conflicts. Climate changes have affected not only the natural habitats but also the social structures of communities and increased the likelihood of violent conflicts [19]. In addition, they endanger international peace and security and affect environmental justice. Some of the most critical connections between these three systems are as follows: -Water is needed for energy generation, primarily hydroelectric power plants, biofuels, etc.; -Water is needed for food production, various nutrients, agricultural irrigation, livestock systems, etc.; - The energy required for food production, all stages of food preparation including harvesting, transportation, preparation, packaging systems, etc.; - The energy required in the water sector: water and wastewater purification and desalination, water distribution systems, agricultural irrigation, electricity generation, etc.; -Nutrition for electricity generation: providing healthy food for personnel and operators in the industrial, economic, etc. Table 1 provides an example of the energy, food and water security nexus.  Some of the most critical connections between these three systems are as follows: -Water is needed for energy generation, primarily hydroelectric power plants, biofuels, etc.; -Water is needed for food production, various nutrients, agricultural irrigation, livestock systems, etc.; - The energy required for food production, all stages of food preparation including harvesting, transportation, preparation, packaging systems, etc.; - The energy required in the water sector: water and wastewater purification and desalination, water distribution systems, agricultural irrigation, electricity generation, etc.; -Nutrition for electricity generation: providing healthy food for personnel and operators in the industrial, economic, etc. Table 1 provides an example of the energy, food and water security nexus. Table 1. Energy, food and water nexus [18].

Food Security Energy Security Water Security
Food availability Supply of energy on demand Water availability Equal access to food Physical accessibility of supply Water health Optimal water utilization Supply to satisfy demand at a stable rate Cost-effectiveness of water

Literature Review and Real Case Studies
Since the Bonn conference (2011) with "The Water, Energy and Food Security Nexus-Solutions for the Green Economy" title was held, several numbers of FEW nexus have been published, whereas almost  [17]. The related search keywords were: two-pronged nexus approach "energy-food" (EF), "energy-water" (FW), three-pronged nexus approach "energy-food-water" (EFW) or multipronged such as "energy-food-water-land" (EFWL). In ref. [17], by investigating 37 projects, it was described that 6, 11, 12, and 8 projects (16%, 30%, 32% and 22%) of them had a close linkage with FW, FEW, WE and FEWL nexus, respectively, which is demonstrated in Figure 3.

Literature Review and Real Case Studies
Since the Bonn conference (2011) with "The Water, Energy and Food Security Nexus-Solutions for the Green Economy" title was held, several numbers of FEW nexus have been published, whereas almost 300 organizations have formed across the world from 2011 to 2015 [17]. The related search keywords were: two-pronged nexus approach "energy-food" (EF), "energy-water" (FW), threepronged nexus approach "energy-food-water" (EFW) or multipronged such as "energy-foodwater-land" (EFWL). In ref. [17], by investigating 37 projects, it was described that 6, 11, 12, and 8 projects (16%, 30%, 32% and 22%) of them had a close linkage with FW, FEW, WE and FEWL nexus, respectively, which is demonstrated in Figure 3.  [17].
In another viewpoint, it is claimed every element in this nexus may be a user or source, whereas the taken perspective will affect the policy design. For example, from a water view, energy and food systems are inputs [20], while water and energy are inputs from a food perspective studies [21,22]. Water consumption control considering water supply efficiencies is proposed in food production simultaneously. Projects on energy-water nexus ranged from water for energy to energy for water. Biofuel using water and hydropower generation are examples of water demand for energy production. Energy consumption examples included water pumping in food generation and electricity consumption in water purification.
In [23], a comprehensive survey on modeling and optimization techniques for energy hubs, as one of the basic concepts of future energy systems, is presented. The authors tried to investigate the impacts of distributed energy resources (DERs) in the presence of a smart grid by addressing the economic and environmental considerations. In addition, they mentioned plug-in hybrid electric vehicles (PHEVs), multiple energy flow carriers with storage and the hydrogen economy as a simple EW nexus. Different technologies, including combined heat and power (CHP), solar, wind, hydrogen and different energy vectors, including electrical, natural gas, bio and hydrogen, are addressed in this study.
A sample multi-energy nanogrid (MEN) considering both economic and low-carbon objectives in islanded and grid-connected modes was optimized in [24] by applying linear programming. The case study of a single-family house of 200 m 2 located in Italy was mentioned, and the TRNSYS as the dynamic simulation software is used. The proposed MEN consisted of micro CHP, a boiler and a PV in the generation section, a heat pump and a chiller in the conversion part and some batteries and thermal energy storage (TES) devices for heat and cool in the storage section. This system delivers the electricity, heat and cooling to the end-user.
In [25], a literature review on using nonconventional resources for water and the other alternatives that could be applied to increase food security in water-scarce countries is presented. For this goal, some selected solutions, including the rainfall-runoff water, the desalinated seawater and highly brackish groundwater, marginal-quality water resources and captured by water harvesting, In another viewpoint, it is claimed every element in this nexus may be a user or source, whereas the taken perspective will affect the policy design. For example, from a water view, energy and food systems are inputs [20], while water and energy are inputs from a food perspective studies [21,22]. Water consumption control considering water supply efficiencies is proposed in food production simultaneously. Projects on energy-water nexus ranged from water for energy to energy for water. Biofuel using water and hydropower generation are examples of water demand for energy production.
Energy consumption examples included water pumping in food generation and electricity consumption in water purification.
In [23], a comprehensive survey on modeling and optimization techniques for energy hubs, as one of the basic concepts of future energy systems, is presented. The authors tried to investigate the impacts of distributed energy resources (DERs) in the presence of a smart grid by addressing the economic and environmental considerations. In addition, they mentioned plug-in hybrid electric vehicles (PHEVs), multiple energy flow carriers with storage and the hydrogen economy as a simple EW nexus. Different technologies, including combined heat and power (CHP), solar, wind, hydrogen and different energy vectors, including electrical, natural gas, bio and hydrogen, are addressed in this study.
A sample multi-energy nanogrid (MEN) considering both economic and low-carbon objectives in islanded and grid-connected modes was optimized in [24] by applying linear programming. The case study of a single-family house of 200 m 2 located in Italy was mentioned, and the TRNSYS as the dynamic simulation software is used. The proposed MEN consisted of micro CHP, a boiler and a PV in the generation section, a heat pump and a chiller in the conversion part and some batteries and thermal energy storage (TES) devices for heat and cool in the storage section. This system delivers the electricity, heat and cooling to the end-user.
In [25], a literature review on using nonconventional resources for water and the other alternatives that could be applied to increase food security in water-scarce countries is presented. For this goal, some selected solutions, including the rainfall-runoff water, the desalinated seawater and highly brackish groundwater, marginal-quality water resources and captured by water harvesting, are addressed. In addition, the opportunities contributing to food security in water-scarce countries are detailed among the physical transportation of fresh water and food imports and the "'virtual water" nexus. As a result, this reference claims that there is no choice for water-scarce countries to supply the needed food by applying either the nonconventional or conventional water resources available within their boundaries. In addition, this ref. forecasted that many water-scarce countries should continue to import food. However, they can rely on producing a proportion of their requirements domestically.
Reference [26] classified the main factors to drive food production and agricultural growth in India as access to utilizable water resources and arable land. This reference investigated the water management and food security challenges that lie in the mismatch between agricultural water demand and water availability.
In [27], the training needs and limitations of the oil palm fruit processor in Nigeria by applying a two-stage sampling method to select 160 households included in palm oil extraction activities among the study case are investigated. The data were collected by using an interview and were investigated using both training needs and descriptive analyses.
Both the water needed to run the energy sector and energy applied to the water sector in Spain are investigated in the WE nexus [28]. It considers different policy objectives for expected renewable energy and biofuels resources.
In [29], the role of nonconventional waters (e.g., desalinated and recycled waters), in arid areas, like the city of Kashan, Iran, was focused as the alternative resource of water. This research investigated the costs and environmental impacts of the energy by using these alternative resources. It was claimed that the maximum capacity of the nonconventional water is not necessarily the optimal point. Different methods of using solar energy for getting usable water from seawater and wastewater were reviewed in [30]. For that goal, the usage of sunlight for water treatment is divided into desalination, or solar distillation, solar detoxification, solar disinfection, and a brief overview of these topics are discussed.
Scott et al. [31] demonstrated the impacts of the coupling of WE at multiple scales. They investigated three WE nexus cases in the US, located in Central, Eastern, and Western regional stakeholder priorities. They stated that localized problems and challenges could be diminished when considered from broader perspectives; meanwhile, regionally significant challenges are not prioritized locally.
In [32], the governing formulation for WE on catchment scales and long-term time are presented in the form of partial differential equations in a three-dimensional space. As the authors claimed, the presented solution is a useful technique to analyze the impacts of land-use and climate changes on the hydrologic cycle.
Hang et al. [33] developed some process systems and engineering tools combined with the nexus concept by using exergy for the design of local generation systems. The suggested framework comprised an optional preliminary design stage combined with a simultaneous design stage based on mathematical optimization. Furthermore, potential interactions between different subsystems were modeled. The suggested method was simulated on a sample WEF at an eco-town in the UK.
The food waste within the FEW nexus is mentioned in [34], and the differential FEW impacts of producing uneaten food and managing food loss and waste were parsed. In addition, different food waste management techniques are classified as landfilling, waste prevention, anaerobic digestion, composting, and incineration. Furthermore, the concept of the "food-waste-systems" technique to optimize resources within the FEW nexus was addressed, and relevant definitions and opportunities were detailed.
The current research needs for understanding the FEW nexus and the relevant implementing solutions in terms of technologies, infrastructures, and policies were discussed in [35]. The main goal of this work is to achieve sustainable development by presenting some guidelines for hydrologists, water resources engineers, economists and policy analysts.
In [36], by focusing on the global value chains (GVCs) concept, the transnational inter-regional input-output approach is used in a tele-connected WEF nexus of East Asia was assessed. For this purpose, China, South Korea and Japan countries were selected as case studies, and different strategies were investigated.
Authors of [37] presented a new framework for optimal short-term scheduling of WE nexus to minimize total electricity cost and seawater desalination considering different constraints. For this purpose, the proposed model was solved by the mixed-integer nonlinear programming (MINLP) using a general algebraic mathematical system (GAMS) software to minimize differed objective functions.
In addition, an interconnected WE nexus, composed of thermal generation units, combined potable water and power (CWP) units, and desalination only processes, is investigated in [38] by using the GAMS software to schedule the water-power hub networks in the presence of the hydro units.
A new technique for the day-ahead optimization of integrated heat-water-electricity systems to optimize different objective functions is proposed in [39], considering a real-time demand-side management strategy. In addition, the impacts of electric vehicle participation are mentioned. The suggested mathematical formulation was solved in the GAMS optimizer using the branch-andreduce optimization navigator (BARON) tool.
Authors of [40] presented a useful review of demand response strategies applied to the industrial section as well as in the wastewater treatment plant operation.
Peri et al. [41] have investigated the financial impacts of WEF nexus among the volatility spillovers by applying a multivariate generalized autoregressive conditional heteroskedasticity (GARCH) technique. The authors have used the daily data and have applied two indexes of the S&P Goldman Sachs (GS) commodity index for modeling the food and energy variable. In addition, the equity index is used to model the water variable.
In [42], considering an EW nexus, the impact of agriculture and energy prices on the stock performance of the water industry, by applying a multifactor market model, is investigated in a case study that considers the economic and financial crisis during 2008. The results were confirmed in three steps, including a GARCH approach, a rolling OLS and the state space representation estimating Kalman filter.
In addition, modeling the possible interactions between the future energy prices in European Union Allowances (EUA) is addressed in [43] by applying a nonlinear co-integration dynamic method through CO 2 futures and Brent. Table 2 reviews some research information on this context.  In addition, due to the great importance of this novel subject, different techniques have been adapted around the world on real case studies or projects to prove the effectiveness of this subject. Some of the significant studies are highlighted in Table 3.

Nexus Committee, Conferences and Real Case Studies
Various conferences, committees, projects and research highlights were organized around the world, relying on nexus. Table 4 shows brief information about the committee and gathering.

Nexus Challenges
Given the broad scope of the FEW nexus, including its three large-scale systems, as well as its newness and capitalization of its wide-ranging discussions, there are significant challenges to its implementation. Different researchers have focused on some of these challenges, as well as some relevant solutions. Some of these items are addressed as follows: Authors of [61] reflected recent research in stakeholder engagement regarding the nexus field. The paper outlined four main emerging concepts in this regard, including using the trans-disciplinary approaches for assessing as well as visualizing nexus, the understanding capacity of building and governance, accounting for inter-scalar and multi-relationships and investigating the implications of future socioeconomic, climatic and technological changes. The authors claimed that "it seems likely that without new trans-disciplinary approaches, there will continue to be poor coordination in addressing challenges across the water food and energy domains".
The existence of some serious challenges in the issues of planning, operation and privatization in the current three separate subsystems of food, water, and energy and necessitate to present the practical, useful and economical solutions to these issues in the FEW nexus; needing some new laws and governance structures and mechanisms for implementing the nexus concept, needing to find some solutions to the consequences of implementing this integrated system, are the most challenging issues from the perspective of reference [62].
Zaidi et al. [63] addressed the water-energy nexus problems and alternatives focusing on machine learning contexts. They categorized various challenges as data challenges: missing data, spatiotemporal data, heterogeneity in data, data collection standards and data availability; and machine learning challenges: modeling spatiotemporal data, modeling in the presence of missing data, identifying outliers (including imperfect collection methods/sensors and extreme events). In addition, they categorized the machine learning techniques used in the energy-water nexus based on applying artificial neural networks (ANN), support vector machines (SVM), time-series analysis, regression, unsupervised learning (including Bayesian model averaging, random forests and hybrid models) and reinforcement learning in energy generation, energy use, water use, energy for water and water for energy. Furthermore, the authors detailed the machine learning alternatives for the energy-water nexus as mining patterns and relationships in data, addressing heterogeneity in data, predicting energy-water nexus variables, modeling unobserved variables, integration of models and deep learning.
Reference [64] outlined the directions for researchers, decision-makers or practitioners, as Identifying nexus methods as well as tools which are suitable for implementation in nexus approaches, creating a practical roadmap, distinguishing the targeted resources, structures and subsystems, deciding system constraints and boundaries, categorizing method types based on research objectives, determining the capabilities and drawbacks and limitations of existing techniques and tools, classifying bottom-up and top-down techniques, describing the uncertain data and uncertain modeling. Authors of [5] categorized the future studies in the nexus field as system boundary, data and modeling uncertainty, the essential mechanism of the nexus and the evaluation of coupled nexus systems even though internal and external impact analysis is of great importance in this regard.
In addition, introducing new concepts such as a smart city (SC), mainly in power systems, has forced the operators and planners to search for new secure and reliable solutions for implementing the Nexus missions. In this regard, the reliability and security of design, planning and operation concepts of different nexuses in SCs should be analyzed and investigated carefully [65].
Analyzing various references and studies related to the challenges of nexus systems, which some of them are mentioned above, shows that there are some significant challenges, as well as solutions in implementing the nexus systems. Table 5 summarizes some of these concepts. Table 5. Some of the challenges and solutions for implementing nexus systems.

Challenges Solutions
Lack of integrated policy and legislation for the system Integrated policy-making such as integrated pricing in water and energy fields, developing a model of agricultural complex and industry proper allocation Data uncertainty Implementing the appropriate uncertainty modeling such as stochastic programming, scenario generation, and so on.
Large numbers of data for subsystems Applying data-mining techniques System boundary Accurate detection of cases using precise and rapid identification of subsystems Lack of sufficient standards and laws Forming committees comprising subdiscipline specialists to address this gap Lack of efficient software platforms Presenting multi-domain software

Conclusions
In this paper, an overview of the processes, methods, policies and interconnections of several resources was reviewed. Recent researches show the trend of societies to establish a comprehensive plan for handling energy, food and water consumption effectively. This paper implies a different nexus method relationship with inherent water, energy and food resource interactions approach considering political and environmental aspects. We reviewed the current state of research on nexus approaches. Food, energy and water systems are interconnected in such a way that one action in a system often affects the others. Therefore, centralized techniques to investigate, planning and operation should be integrated to reduce the side-effects and increase collaboration and synergism. A significant interconnection between these three subsystems and their direct impacts on environmental concepts, climate changes, socioeconomic, policy-making, etc., needs stakeholder engagement so that integrated management overall subsystems is concerned. This is essential for achieving the nexus objectives and gaining sustainable development. Planning and policy-making between the departments and organizations implemented to get a common point to require the discourse between stakeholders and the structures of conflicting objectives to fully cooperate and reduce the interference. As it was concluded, the main challenges for implementing this concept are categorized as lack of integrated policy and legislation for the system, data uncertainty and large numbers data of subsystems, system boundary, lack of sufficient standards and laws and lack of efficient software platforms.