The water-energy-food nexus and process systems engineering: A new focus
Introduction
Almost every conceivable human activity and technology requires either water, energy, food, or some combination of the three. We use water to grow our food and turn the turbines that produce our electricity. Energy is required to convey and purify water and is also needed to produce fertilizer, harvest crops, and cook our food. In turn, energy can be produced from crops like corn and sugarcane in the form of biofuels. Not to be overlooked, all of us individually consume water, energy, and food every day. Clearly, all three sectors are dependent on each other and all are highly interconnected; a phenomenon that is termed the Water-Energy-Food Nexus (WEFN). In numbers, approximately 15% of global water withdrawals are used for energy purposes (Birol, 2010), and 70% are used in agriculture – leaving only 15% for all other applications (Aquastat, 2011). Energy is required for transporting, treating, and pumping water (among other requirements), totaling 8% of global energy use (United Nations, 2014). However, food production and supply chains are even more energy-dependent, using approximately 30% of global energy (Food and Agriculture Organization, 2011). Conversely, approximately 1% of all food produced is directed to the energy sector with the rest dedicated to human (or livestock) consumption and waste. Food cultivation, processing, transportation, and distribution thus require more water than energy systems/processes do and require more energy than water processes/systems on a global basis. Clearly, the addition of food to the more familiar Water-Energy Nexus concept is both justified and necessary. A summary of the WEFN is given in Fig. 1.
Despite the critical importance of the WEFN for sustaining and improving human life around the world, not everyone has access to safe water, food, and energy. According to the United Nations, approximately 1.2 billion people do not have access to safe water (Watkins, 2006). With the global population expected to grow to 9.6 billion by 2050 (United Nations Environment Programme, 2012), WEFN systems must be resilient, sustainable, and well-managed to satiate the planet's growing thirst for water, hunger for food, and demand for energy. Energy consumption is expected to increase by one-third relative to 2011 levels by 2035. Water consumption is also expected to increase for irrigation by 10% by 2050 (Food and Agriculture Organization, 2011), and it is likely that food consumption – particularly meat – will also increase (United Nations Environment Programme, 2011). This is a daunting problem from a WEFN perspective, as it is estimated that each metric ton of beef requires 15,000,000 l of water (Mekonnen and Hoekstra, 2012) compared to other foods, such as soybeans which require approximately 2,000,000 l of water per ton (Hoekstra and Mekonnen, 2012). This is to mention nothing of livestock-based greenhouse gas emissions. These emissions equate to 18–25% of total global greenhouse gas emissions, based on studies by the FAO (Food and Agriculture Organization, 2006), the United Nations (United Nations Environment Programme, 2009), and academia (Fiala, 2008).
Even in the face of anticipated increases in demand of water, energy, and food, management systems can fail and have failed even in developed countries. Indeed, a prolonged drought in California, coupled with water laws dating from the early 20th century, has placed excessive stress on the state's water supply. Farmers have been forced to fallow their fields, reducing the food supply, and less water is readily available for energy production (Carrol, 2015). High-profile issues such as the California drought have prompted policymakers, researchers, and the general public to recognize the vital importance of sustainability of the WEFN. Sustainability problems have been described as “wicked,” (Azapagic and Perdan, 2014) that is, they are extremely difficult to model properly and solve. If the ultimate goal is to simultaneously ensure sustainability of water, energy, and food systems, then this “wicked” problem will take a variety of innovative, creative, and dedicated approaches from the process systems engineering (PSE) community. Gong and You (2015a) summarize some key challenges for the design of sustainable energy systems, but water, food, and water-energy-food systems require their own treatment as well.
The environmental science and policy literature has long recognized the presence and importance of the WEFN with both theoretical approaches and quantitative, regional analyses. Numerous studies characterize the WEFN, identify critical factors and links between the resources, and establish frameworks for future research, analysis, planning, and policymaking. Many of their discussions are focused on resource security and the necessary requirement for nations to cooperate on this issue, highlighted by Hussey and Pittock (2012) as one of the key outcomes of the COST (European Cooperation in Science and Technology) Climate-Energy-Water Links initiative. Indeed, Bazilian et al. (2011) note that security-related issues of the WEFN arising from inequalities between nations might galvanize more international cooperation than environmental impacts of the WEFN. Ringler et al. (2013) stress that environmental impacts of the WEFN as well as resource security must be integrated into cooperative political and social planning for sustainable and secure distribution of water, energy, and food. The literature then focuses on how to define “security” for further analysis and research. To that end, Beck and Walker (2013) later define WEFN security as a diversity-resilience problem, allowing for more targeted analysis and application of the WEFN concept. Biggs et al. (2015) develop a slightly alternative idea of WEFN security based on a sustainable livelihood standpoint, developing an “environmental livelihood security” concept. They then integrate this concept into an Environmental Livelihood Security (ELS) analysis framework.
Other environmental researchers quantified flows and interdependencies of water, energy, and food in many areas of the world (a summary of studies on the U.S. WEFN are given in the next paragraph). By and large, all studies agree that environmental conservation goes hand in hand with resource management and sustainable development of the WEFN. The studies often focus on resource scarcity, such as (Rasul, 2014) that studies the WEFN from the Hindu Kush Himalayan region. Wang et al. (2012) further promote the concept of water and energy conservation in China in order to meet ever growing demand of food, water, and energy in the country. The Middle East and North Africa is a growing region becoming more reliant on alternative sources of water. Siddiqi and Anadon (2011), in their analysis of that region's WEFN, state that strategies of water reuse and in some cases changes in the agricultural and energy sectors should be considered instead of energy-intensive desalination facilities. In addition to certain regions, cities’ impacts on the WEFN and vice versa have also been recently analyzed. Perrone et al. (2011) develop a geographical WEFN analysis framework for cities after defining them as “urban resource islands” that control the follow of energy and water resources in their surrounding region. Walker et al. (2014) follow a techno-economic approach for improved decision-making in the WEFN of large cities based on the interactions of each resource in an urban setting.
Thus, at this point, most research efforts focus on characterizing the WEFN, with a majority of these efforts focused on water-energy interactions. A host of different disciplines, research groups, and governmental agencies released lists of challenges and opportunities in the water-energy nexus, such as the United States Department of Energy (DOE) (U.S. Department of Energy, 2014). The National Renewable Energy Laboratory (NREL) summarized implications of energy decisions in the energy-water-land nexus (Newmark et al., 2012). The Congressional Research Service provided an overview of energy's water demand in 2010 (Carter, 2010). Finally, The U.S. Geological Survey penned an excellent, thorough review of the water-energy nexus from an earth sciences perspective (Healy et al., 2015). This report also touches upon key connections of the water-energy nexus to food, e.g. groundwater and surface water for irrigation. All reports emphasize that data and analysis of the WEFN is sorely needed to inform impactful planning, policy, and other decisions on both industrial and governmental levels. Important reviews from the PSE community focus on the WEFN as well. For example, Gabriel et al., 2014a, Gabriel et al., 2014b review key design decisions for gas-to-liquid technologies based on the water-energy nexus. Varbanov (2014) further emphasizes how important the water-energy nexus is to industrial concerns, while Yang and Goodrich (2014) elaborate on challenges in quantifying and analyzing the water-energy-climate-urban nexus. The PSE community can leverage their fundamental expertise in synthesizing data in order to model real-world processes and systems to help guide decision makers.
In this article, we first highlight relevant work on modeling the WEFN. As the WEFN is a topic with substantial breadth, we restrict our attention to largely recent studies in the PSE field. After a review of the literature, we present some major WEFN research challenges and knowledge gaps in the PSE community regarding modeling the WEFN. Specifically, we see three major challenges: multi-scale challenges, challenges in appropriate system boundary definition, and challenges in modeling all WEFN stakeholders and their objectives. Throughout the work, we discuss two motivating cases where novel modeling frameworks and algorithms will be needed: a water-producing coal mine-power plant-farm supply chain network and a regulated/subsidized water-energy-food production and distribution network subject to uncertainties.
Section snippets
Literature review
The water-energy-food nexus is an enormously broad topic. Indeed, this article could entirely focus on highlighting the myriad interactions within the nexus and all relevant research. Instead, the goal of this work is not only to briefly highlight the relevant literature, but also to identify research gaps and identify possible avenues forward for future PSE research. To that end, this section is divided into four subsections. The first section details PSE research centered on water-energy
Motivating examples
In this section, we discuss two motivating examples of interest that, if performed, would involve optimization of the WEFN as a whole. There are many other potential studies that deserve future investigation, but the following two are given as demonstrative cases to guide thought toward the types of studies that (1) could result in foundational work to guide future studies, (2) could result in significant water and energy savings when considering food, (3) could be based on readily available
Major water-energy-food nexus research challenges
We have identified three major challenge areas for modeling the WEFN that arise in the previous examples and many other WEFN processes/systems. There are multi-scale challenges, challenges with appropriate system boundary definitions, and challenges in modeling the importance of multiple stakeholders and their objectives. Each challenge has modeling and computational challenges. There are certainly other challenges involved in modeling the WEFN, such as data collection and data quality
Conclusion
The Water-Energy-Food Nexus (WEFN) is both an important and broad topic. Virtually all human activity requires water, energy, or food. With rising populations and appetites for these limited resources, the WEFN has taken center stage in the minds of many policymakers and nongovernmental organizations, including the United Nations. However, the WEFN is so expansive and its connections so important, that detailed process systems models for it do not yet exist at a level satisfactory for important
Acknowledgements
We gratefully acknowledge the financial support from the Institute for Sustainability and Energy at Northwestern University (ISEN) and Argonne National Laboratory via a Northwestern-Argonne Early Career Investigator Award for Energy Research.
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