Energy—Water Nexus: Integration, Monitoring, KPIs Tools and Research Vision

The relationship between water and energy is a strong one characterized as having integration and coupling as two important features. While energy is responsible for delivering water to the end-users, it needs energy in order to be generated, and water. In this paper, a thorough review is presented regarding the different relationships between water and energy in terms of (i) the significance of the close relationship between water and energy by means of water/energy generation and consumption. Water consumption, water cooling and heating must be taken into account in order to avoid the obstacles related to future use of water for energy generation; (ii) the measuring and monitoring technologies for the energy-water nexus, focusing attention on the variables that are interrelated in the water and energy sectors. In addition, the consequences of finding several parameters, unknown variables and unclear dependencies in measuring of energy usage in the applications of water usage should also be taken into account. Innovative developments including nanotechnology, biotechnology, and wireless networks, as sensor technologies, may resolve the challenges of sensing; (iii) the different key performance indication tools for assessing and quantifying this nexus by analyzing and categorizing recent case studies of the water energy nexus and applicable evaluation methods; and (iv) the different research dimensions conducted on this nexus. Hopefully, this review will contribute to the development of this nexus adding value to the field while reducing duplication efforts.


Introduction
Being essential for human life, energy and water are interrelated resources. Not only is water essential for drinking, crops cultivation, extracting fuels, generating power, and producing goods, but the life and health of ecosystems depend on water as well. At the same time, energy is found by humans by means of the conversion of (i) fossil fuel resources, (e.g., natural gas, coal, oil) and (ii) renewable resources such as water, wind, and the sun. Thanks to the energy generated, humans have created a vibrant civilization that reinforces food production, agriculture, industry, transportation, science, a comfortable life, and so on. Interrelated in various ways, energy and water are mutual in the sense that energy is necessary to secure water, to treat and desalinate water for the usage of humans, and to transfer water. Meanwhile, water is required to generate energy by extracting and operating fossil fuels, growing biofuels, and for cooling thermal power stations. Energy and water fulfil reciprocal functions in which working on one of them relies on the status of the other in terms of cost and availability [1,2].
A boom in studies on the topic has contributed to the interest that has been paid to the water-energy nexus (WEN). A very large spectrum of challenges, sizes, and the creation of several models and tools are addressed by the research undertaken. Themes range from conceptualization to case studies, but all are related to resource depletion and continuous condensation. The challenges include a very wide range of scales starting from micro-level to macro-level scales. Also, the studies' scope includes city, regional, and international [3]. Different WEN studies use or expand a particular method and adopt this method to their characteristics. For example, many countries such as China and the United Kingdom use the developed models using the Foreseer online tool to meet their research demands with regard to the nexus. Other developed models are used to understand the nature of the WEN nexus in countries such as the United States and Australia [4].
Experiences and perceived benefits of replicating or changing these models and procedures in different situations can hardly be measured and compared. Until now, few studies have been conducted on WEN. Many of them concentrate on classifying types of approach such as physical model, saving analysis, interconnected indexes, and management model of optimization. A lot of these studies rely specifically on a specific sector or industry feature, and almost all the models have a water footprint [5].
The main contributions of this work are the introduction of a complete review about the different relations between water and energy that can be summarized as: (i) The importance of the relationship between water and energy through water/energy generation and consumption. (ii) The different measuring and monitoring technologies used for the energy-water nexus. (iii) The different key performance indication tools that are used to evaluate and quantify the water and energy nexus. (iv) The research dimensions and trends of water-energy nexus and outcomes that should be taken into account.

Water as a Prerequisite for Generating Energy
The idea behind the water use concept basically refers withdrawal and consumption. While withdrawing water refers to the notion that when a total amount of water is obtained from the water source, a portion of which is returned to the source and available for further use, the consumption of water is part of the withdrawn water that is not returned to its source.

Water Use for Current Energy Generation
Previous studies show that the largest energy-consuming and -producing country in the world is China, so the process of generating energy in China requires a large amount of water and more attention [6]. Several studies have been carried out to calculate the quantities of water that are required to generate and distribute energy from conventional and renewable energy sources. The studies have shown the total blue water footprint of 35 selected hydroelectric power plants worldwide. When comparing the hydroelectric blue footprint of hydro energy to that of other natural sources, such as wind, solar, and biomass energy, hydroelectric power generation is an important water consumer in most cases and it reaches from 12% to 60% of the gross water consumption [7]. In biomass power generation, the basis for water consumption is in the cultivation of biofuel crops. Therefore, rainwater or soil moisture evaporates through crops [8].
The methods applied in the estimation of water required for energy generation include life cycle analysis (LCA), material flow analysis (MFA), input-output (I-O), process-based analysis, and a combination of various methods (such as, I-O-LCA) [9][10][11][12]. The LCA method shows the amount of water used in entire energy generation processes e.g., raw material extraction, transportation, and material processing. It can identify key water-intensive processes among the life-cycle processes. For example, the LCA water consumption for wind power in China is 0.6 L/kWh, of which less than 50% is owing to wind power generation [13]. The I-O method applied in water-energy nexus studies can be used to identify the sources of energy (energy)/water consumed within a given region. Generally, the water withdrawal is higher than the water consumption, particularly for open-loop cooling, the water withdrawal of which would be 100 times greater than the water consumption. However, the water consumption for open-loop cooling is generally less than that for closed-loop cooling. Great differences exist in the water use of various energy generation types. Generally, the water use in thermal power generation can be as much as 43-66% of the society's total water use [14]. After comparing various renewable energy types, the solar photovoltaic and wind energy are the energy-generation types that could dramatically decrease the water footprint and carbon footprint. For a thermal energy generation, it is estimated that water use for energy generation in July (summer) is 15-28% higher than the annual average, while it is 12-24% lower in January (winter) [15].

Prediction of Energy Generation and Associated Water Use
More studies has been conducted on future energy generation profiles and the associated water use for a wide temporal (e.g., 2020, 2030, 2050, 2095, and 2100) and spatial span (e.g., worldwide, US, China, and Europe). The energy-generation profile is the key input for prediction, followed by the cooling technologies. The key findings of some relevant publications are: (1) Water-saving measures (e.g., less water use-intensive cooling technologies or energy types) will be the trend in the near future [16]. (2) The requirement for both the availability of water resources and reductions in the greenhouse gas (GHG) production will limit the energy generation [17]. (3) Simultaneously reducing the water use and GHG production in energy generation in the majority of the regions could be challenging [18].
Socioeconomic-related approaches, such as the TIMES model and long-range energy alternative planning (LEAP) model, have been developed to provide information regarding energy generation as well as consumption changes, these models could shed light on future energy planning. The TIMES model and its extensions have been successfully implemented in Spain, India, Ethiopia, and South Africa. The LEAP model can be integrated with the water evaluation and planning (WEAP) model to simulate water and energy systems from both the supply and consumption perspectives simultaneously [18].

Energy-Related Water Shortage Issues
In this section, an overview is given concerning the mismatch between the water resources and energy generation, and the water saving potential is then identified as a method for mitigating the water shortage caused by energy generation and consumption.

Water Shortages Worsened by Energy Generation and Transmission
The heterogeneity of water endowment and energy generation is a worldwide issue. Owing to the spatial disparity of water resources and energy generation, relevant research has been conducted at: (1) the administrative unit level, (2) the watershed unit level, and (3) the grid units level to identify and quantify the water use for energy generation that is obtained from water-stressed regions. This spatial conflict between water and energy has significant seasonal variation characteristics. For instance, only 23% of the water is used to generate energy from water-stressed basins in May in the eastern United States, and this number increases to 47% in November [19]. The water utilized to generate energy is about 44% in Europe [20], 49% in the Asia-Pacific economic cooperation zone [21] and 97% in Delhi, India [22]. The largest value is in China and equals 98% where its energy is generated using coal-fired processes [23]. This seasonal variation results from a combination of different factors: There is a seasonal difference in precipitation, and the problem of water shortage may be more severe in winter as freezing in cold temperatures reduces the amount of water available, and more energy may be required during the cold season. The heterogeneity of water resources and energy generation can be more important as energy can be transferred easily. Hence, the lack of spatial and temporal analysis and the optimal distribution of the water-energy correlation may have a detrimental effect on the future development of the energy sector due to the increase of water shortages.

The Possibility of Providing Water to Generate Energy
Given the high demand of water for energy generation, it is necessary to make an additional estimate of the water-saving potential. Water savings can be achieved through changes to the energy generation structure, and energy-saving measures. Water-saving cost evaluation can also be made for different types of energy generation. On the supply side, the switch to a lower water-intensive energy profile has been recognized worldwide as an effective water saving measure. On the consumption side, energy-saving measures can have a synergistic effect of water saving [23].

The Relationship between Water and Energy for Home End-Use Activities
Recent studies have shown that approximately 45% of the energy consumed by households in the United States is closely related to residential water consumption. Also, almost 70% of household electricity consumption in Delhi, India, is spent on resolving water shortages [24]. In Australia, household water consumption is projected to account for around 30% of the resources used in the urban water cycle [25]. Due to the difference in individual behaviors and technology, there is a heterogeneous use of water and energy at the household use level.

Energy-Water Nexus in Polygeneration Systems
Polygeneration means the generation of different energy products in one process. Combined heat and power (CHP) is an example of a polygeneration system where the main energy product is the electric power and usable heat is generated at the same time by utilizing the additional heat that is lost by conventional condensing power. The traditional steam turbine has an efficiency of 20-38% but by using the CHP system the efficiency is increased to 80-90%. Also, the CHP saves fuel and pollution emissions by 10-40% depending on the replacement methods by manufacturers. The polygeneration system capacity is not completely utilized and this is for two reasons. The first reason is due to many industries using polygeneration stations (e.g., CHP stations) depending on a power-to-heat ratio where changing the power outputs of the heat generation unit is not fixable. The second reason is due to the high penetration of renewable energy to generate power and less availability of generating heat.
A polygeneration system comprises generation installations and non-generation aspects. The generation installations include different facilities such as traditional condensing, heat stations (boilers), and cooling stations (chillers). The non-generation aspects include different bilateral contracts and demand side management for many energy products.

Integrated Management of the Connection between Water and Energy
Current silo management may be ineffective due to bilateral relations in the water-energy relationship. Legislation and laws serve as rules for formulating an integrated management of the water-energy relationship. There are gaps between theoretical pedagogy and practical applications regarding the water and energy connection in the form of (1) challenges in coordination and cooperation between different sectors, (2) conflicts of interest between different stakeholders, and (3) the interruption of political and cognitive factors [26].

Energy-Water Nexus: Monitoring
This section discusses measuring and monitoring the technology of the energy-water nexus through an overview of the latest technologies, research gaps, and future research directions.

Monitoring Functions in Energy-Water Nexus
The relationship between water and energy is divided into several categories. There are four sectors related to energy applications (for example, power generation, thermal power plants, oil drilling, hydraulic fracturing, and biofuels). One sector is related to the agricultural sector. Five sectors are related to water applications (for example, freshwater protection, reservoirs, air and water quality measurement, water treatment, distribution and recycling, refrigeration and recycling systems).

Power Generation
Most power-generation processes consume water directly or indirectly. The energy loss in the process of power generation, transmission and distribution accounts for about 58% of the total primary capacity [27]. Sensing functions currently in use include the engine pressure sensor, which has a permanent monitoring function that can control emissions and engine balance [28]. State-of-the-art sensor technology includes piezoelectric pressure sensors, which can monitor and detect dynamic pressure peaks, peaks and pulses in liquid or gaseous media used for power generation. Remote sensing includes micro-electromechanical systems (MEMS) based on wireless vibration sensing [29]. Variables of interest include pressure, stroke of linear variable differential transformers (LVDT), and temperature [27]. The power consumption of the sensor is about 140.4 MW [29], and the prices of all types of sensor range from 100 USD to 1000 USD.

Thermal Power Plants
Thermal power plants are the largest source of targeted water for cooling [30]. In an electrothermal facility, approximately 25 gallons of water is required per kilowatt-hour of energy generation, but they actually consume less energy. The increase in consumption is due to evaporation [31]. Types of sensor in thermal power plants include optical sensors [32], chemical-resistant gas sensors [33], electrochemical sensors [34], and surface acoustic wave sensors [35,36]. The remote-sensing system includes remote optical detectors, wireless gas detectors and screens [37]. Variables of interest include film thickness, gas concentration, agglomeration, porosity, surface geometry, grain size, grain network, face, and film texture [33]. An optical sensor costs $100, but a high-temperature gas sensor costs about $250 [38]. The power consumption of the electronic gas sensor is 20 µW [39].

Oil Drilling and Hydraulic Fracturing
In addition to oil drilling, groundwater is also brought to the surface, and this water is called produced water. The sensing challenge here is related to the diversity of the water produced and the proportion of oil produced [40]. Conventional sensors include drill monitor tracking sensors, flow tracking sensors, conduction sensors, depth-tracking sensors, pressure-tracking sensors, and high-temperature accelerometers [41][42][43]. There are new types of sensor that are augmented by optical fibers [44,45]. New remote-sensing technologies include radar-based remote depth sensors [46]. The variables of interest relate to the depth of the drill bit, levels of inflow and outflow rate, fluid pressure and angular movement. The cost of the pressure sensor is around US $100 to US $1000, while the cost of the torque sensor is around US $3000. The power consumption range of conductivity sensors, density meters, and flowmeters is 120 watts [47].

Biofuels
Biofuel production requires additional water consumption. Types of sensor include humidity sensors and thermostats as well as biosensors. The purposes of remote sensing include airborne imagery and images gathered from satellites and radar-based sensors. Variables of interest include the ratio of biomass to leaves, and the ratio of mixing impurities; the content of volatile organic components (VOC) in soil; salinity; and nutrient concentration [48]. The price of the moisture sensor is $100, the price of the thermostat is $250. The power consumption value is 1000 watts.

Farming
Types of agricultural sensor include flow/precipitation velocity sensors, optical sensors and dielectric constant sensors [49]. Aircraft or satellites are used for spectrum-sensing remote sensing applications [50]. Smoke and fire sensors are shown on the map for fire management to provide the energy and water needed to fight the fires [51]. These variables include air, water, biofuel, soil, heavy metals in water, and the concentration of pollutants in pesticides [48]. The cost of the high-speed image sensor is $1000 [52]. The power consumption of the optical sensor is 900 MW.

Fresh Water Protection
Types of sensor in this area include spectrum-based sensors, optical sensors, and ultraviolet (UV) light meters [53]. Remote-sensing capabilities in rivers and lakes include optical sensors of appropriate lengths or other anti-fouling technology. The variables of interest relate to the amount of oxygen or dissolved organic matter (DOM) [54], rainfall intensity, seasonal runoff variations, disturbance frequency, storage, production and transport of carbon and nitrogen in watersheds [53]. The cost of the DOM fluorescent optical sensor is approximately $2000 to $5000; the cost of the optical nitrate UV sensor is approximately $15,000 to $25,000 [53]; and their power consumption is 900 milliwatts.

Water Reservoirs
Types of sensor in this field include smart tracking devices, continuous level measurements, and capacitive level sensors [55]. Remote sensing used for tank monitoring is based on wireless measurement [56]. The variables of interest include the concentration of dissolved components [53]. Capacitive level sensors cost about $200 and use around 600 watts of power.

Measurement of Air and Water Quality
Water is transported from remote areas via pipelines and bottled in containers. This requires sensing in pipelines and underground pipelines. Types of sensor used in these systems include oxygen, pulse polarization imaging, dissolved optical contaminants, and current transformer (CT) and film sensors [57]. Remote sensing includes surface reflection and satellite-based technology [58]. These variables relate to the radiance, absorption or scattering of suspended or dissolved substances in water. The cost of the water and air-quality sensors for the electrostatic membrane and dissolved oxygen sensors is approximately $1000 [59]. The power consumption value is from 100 VAC to 240 VAC at 60 mA [57].

Water Treatment, Distribution and Recycling
In water treatment and circulation, the types of sensor used in these systems include water pressure sensors, flow sensors, micro tensiometers [60] and acoustic leakage sensors [61]. The purpose of remote sensing involves remote sensing satellite imagery and field spectroscopy [62]. Variables of interest include water pressure, seepage, and pollutant concentration. The cost of the acoustic leakage sensor is about $2500 and the power consumption is about 10 W [63]

Cooling and Recycling Systems
In the refrigeration and recirculation system, the types of sensor used include thermostats, light sensors, and moisture and humidity sensors [64]. For remote sensing, infrared detectors and hyperspectral imaging are used. The variables of interest relate to ambient room temperature, operating chamber, and cycle pressure. The cost of the thermostat sensor is about $250, the cost of the humidity sensor is about $350, and the cost of the light sensor is about $100. The power consumption of the thermostat may be around 2000 watts [65], but the power consumption of the humidity sensor may be in the milliwatt range [65].

Energy Generation
The impact of changes in operating temperature and pressure on power generation must be addressed in order to understand the subtle dependence on energy and water. Hydroelectric power plants, geothermal power stations and solar power plants consume more water per kilowatt-hour than non-renewable resources such as natural gas and oil. The latter's non-renewable energy generation combustion platform requires less water per kilowatt-hour [66]. Stability, temperature, pressure, and vibration sensors require increased sensitivity and efficiency. Intelligent humidity sensors, automatic weather forecast calibration and liquid level sensing technology enable the water and energy consumption demand between power and water to be understood.

Thermal Power Plants
Thermal power plants convert less than half of the primary mechanical energy into electrical energy. Cheaper and more efficient temperature sensors have a big impact on water consumption and water withdrawal. The new sensor materials must be stable at elevated temperatures, and the sensors must be highly responsive to common oxidation and gas reduction at elevated temperatures [67].

Oil Drilling and Hydraulic Fracturing
The environmental costs of development and drilling are not always well understood. Seismic sensing may measure the ultimate movement of the earth based on the environmental impact of drilling and fracturing activities, which may expose the true cost of the development and drilling operations. Humidity sensing may be useful to see if any oil or natural gas has penetrated the water body during the cracking process. Downhole sensors, pressure and temperature sensors can enhance understanding of the effects of fracturing and drilling conditions [68].

Biofuels
In industrial processes, there are opportunities to obtain better water efficiency, including the efficiency of biofineries and raw materials for permanent bioenergy [30]. The electrical sensor can detect the clean water and energy needs for biofuel production.

Farming
According to the energy and water communication, nanosensing of harmful and toxic organisms and fertilizers has future applications in agriculture [69]. By mixing seeds with the sensors, future biosensors can be placed in the soil through the seeding process so that they can gradually degrade without leaving toxins in the soil [70].

Fresh Water Protection
Optical nitrate sensors can be used in oceans of low color and turbidity, in addition to treating wastewater with high color and turbidity. Wet chemical sensors can be used in freshwater systems to demonstrate in situ observations of ammonium and soluble reactive phosphorous [53].

Water Reservoirs
Water-quality issues need to monitor sensor performance through sensor communication and real-time data transmission for measurement and sampling. In the future, remote-sensing technology will be used for spectrometer imaging to control the storage of large tanks [71], as well as to enhance the efficiency of smart tracking devices and wireless telemetry systems.

Measurement of Air and Water Quality
Electro induction technology is used to detect the carbon content as a variable to determine the improvement of water and air quality. Optical spectroscopy technology is also used as a sensor function to measure variables related to biology, chemistry, air and water for quality control. Sensors also need concepts in real-time, data storage and retrieval.

Water Treatment, Distribution and Recycling
Remote sensing of pollutants is undertaken through field spectroscopy and satellite image technology [62]. A specially designed fluorometer can be used to detect wastewater and various contaminants [53]. Nano bubbles and micro-bubbles are studied for water treatment [72,73]. Nanometers and precise tensiometers can provide lower energy consumption and higher efficiency in these systems.

Cooling and Recycling Systems
Wireless sensor networks are studied to improve the sensing of cooling systems [74] and are also used to improve the performance and efficiency of circulatory systems for fluid leak detection [75], as shown in Figure 1.

Measurement of Air and Water Quality
Electro induction technology is used to detect the carbon content as a variable to determine the improvement of water and air quality. Optical spectroscopy technology is also used as a sensor function to measure variables related to biology, chemistry, air and water for quality control. Sensors also need concepts in real-time, data storage and retrieval.

Water Treatment, Distribution and Recycling
Remote sensing of pollutants is undertaken through field spectroscopy and satellite image technology [62]. A specially designed fluorometer can be used to detect wastewater and various contaminants [53]. Nano bubbles and micro-bubbles are studied for water treatment [72,73]. Nanometers and precise tensiometers can provide lower energy consumption and higher efficiency in these systems.

Cooling and Recycling Systems
Wireless sensor networks are studied to improve the sensing of cooling systems [74] and are also used to improve the performance and efficiency of circulatory systems for fluid leak detection [75], as shown in Figure 1.

Sensor Research Directions with Merit
Three immediate research projects with merit are identified as: (a) sensors for sectors where water is used for energy applications, (b) sensors for the agricultural sector where both water and energy are used, and (c) sensors for sectors where energy is used for water applications.

Water for Energy
Research projects with immediate research merit include sensor improvements in energy generation, in thermoelectric and biofuel plants, and in hydraulic fracturing and oil exploration. Sensors are needed to generate energy to analyze the stability of the system. In hydraulic fracturing and oil drilling, pressure sensors for remote sensors are needed. In thermoelectric plants, new sensors are required that are stable in reducing high temperature, and high oxidation environments [67]. In

Sensor Research Directions with Merit
Three immediate research projects with merit are identified as: (a) sensors for sectors where water is used for energy applications, (b) sensors for the agricultural sector where both water and energy are used, and (c) sensors for sectors where energy is used for water applications.

Water for Energy
Research projects with immediate research merit include sensor improvements in energy generation, in thermoelectric and biofuel plants, and in hydraulic fracturing and oil exploration. Sensors are needed to generate energy to analyze the stability of the system. In hydraulic fracturing and oil drilling, pressure sensors for remote sensors are needed. In thermoelectric plants, new sensors are required that are stable in reducing high temperature, and high oxidation environments [67]. In biofuels, genetically modified (GM) microorganisms are needed for biosensing with desirable signal outputs, sensitivity and selectivity [77].

Farming
Research projects in agriculture include nano-sensors and biosensors for fertilizer use, pests, and toxicity detection. Nanosensors provide cost-effective and robust ways to monitor variables, and are more accurate, sensitive, expensive and selective than conventional sensors.

Energy for Water
Research projects include developing sensors in water tanks, freshwater protection, water and air-quality measurement, water treatment and recycling, and cooling systems. In freshwater protection, wet chemical sensors are needed to monitor soluble reactive ammonium and phosphorous [53]. In water tanks, remote sensing is required to control storage in the large tank from the satellite. In measuring water and air quality, radiometers and spectrometers are essential to detect pollutants. In water treatment and recycling, meters can measure water potentials under −10 MPa [74]. In cooling systems, wireless sensor networks can notice leaks or unwanted changes in temperature [75].

Biosensors and Nanosensors
Nanosensors have great selectivity, sensitivity, small sizes and compact designs. They can be used to detect toxins and pathogens in food and water, reduce the energy consumed in food production, and for water purification. They can also be used to save energy and reduce waste [78] and to protect animal health in agriculture, as well as in water treatment and disinfection [79][80][81], in the detection of pathogens [82][83][84], in the detection of pheromones, pH and moisture in water [85], soil, plants, air [86,87] and food [88,89], and in agricultural sustainability projects [90][91][92][93]. This can help improve water-use efficiency. They can be linked to wireless communication to calculate related energy cost. Various types of method are used biosensors including the use of normal tissues, antibodies, enzymes, cells, bacteria, and DNA to generate an electrical signal as shown in Figure 2. New research includes organic microbes used in biomedical fields and mineral-binding proteins (mineral proteins) such as phytochelatins and metallothioneins (MTs) [94,95].
Energies 2020, 13, x FOR PEER REVIEW 9 of 22 biofuels, genetically modified (GM) microorganisms are needed for biosensing with desirable signal outputs, sensitivity and selectivity [77].

Farming
Research projects in agriculture include nano-sensors and biosensors for fertilizer use, pests, and toxicity detection. Nanosensors provide cost-effective and robust ways to monitor variables, and are more accurate, sensitive, expensive and selective than conventional sensors.

Energy for Water
Research projects include developing sensors in water tanks, freshwater protection, water and air-quality measurement, water treatment and recycling, and cooling systems. In freshwater protection, wet chemical sensors are needed to monitor soluble reactive ammonium and phosphorous [53]. In water tanks, remote sensing is required to control storage in the large tank from the satellite. In measuring water and air quality, radiometers and spectrometers are essential to detect pollutants. In water treatment and recycling, meters can measure water potentials under −10 MPa [74]. In cooling systems, wireless sensor networks can notice leaks or unwanted changes in temperature [75].

Biosensors and Nanosensors
Nanosensors have great selectivity, sensitivity, small sizes and compact designs. They can be used to detect toxins and pathogens in food and water, reduce the energy consumed in food production, and for water purification. They can also be used to save energy and reduce waste [78] and to protect animal health in agriculture, as well as in water treatment and disinfection [79][80][81], in the detection of pathogens [82][83][84], in the detection of pheromones, pH and moisture in water [85], soil, plants, air [86,87] and food [88,89], and in agricultural sustainability projects [90][91][92][93]. This can help improve water-use efficiency. They can be linked to wireless communication to calculate related energy cost. Various types of method are used biosensors including the use of normal tissues, antibodies, enzymes, cells, bacteria, and DNA to generate an electrical signal as shown in Figure 2. New research includes organic microbes used in biomedical fields and mineral-binding proteins (mineral proteins) such as phytochelatins and metallothioneins (MTs) [94,95].

Microbes as Sensors
Microbial-based electrochemical systems (MES) can be used in electrolysis to reduce heat, product waste, and biofuel processing [97]. Microbial electrochemical sensors can have many applications in the energy-water relationship, and they can sense the presence of biogas, hydrogen, ammonia, ethanol or water in biofilters.

Microbes as Sensors
Microbial-based electrochemical systems (MES) can be used in electrolysis to reduce heat, product waste, and biofuel processing [97]. Microbial electrochemical sensors can have many applications in the energy-water relationship, and they can sense the presence of biogas, hydrogen, ammonia, ethanol or water in biofilters.

The Effect of Nanomaterials/Microelectromechanical Systems on the Environment or People
Nanomaterials have certain physical and chemical properties that make them beneficial and environmentally friendly products in the relationship between energy and water [98]. These properties are higher material durability, longer product life, corrosion, dirt and waterproof coatings, better insulation, lower weight and energy consumption. Nanogenic sensors may have a problem because they contain toxicity far from that which the nanomaterial used enters. Figure 3 shows the different types of sensor used in recent projects being implemented in the hydropower field as well as the sensor technologies needed to participate in several sectors.

The Effect of Nanomaterials/Microelectromechanical Systems on the Environment or People
Nanomaterials have certain physical and chemical properties that make them beneficial and environmentally friendly products in the relationship between energy and water [98]. These properties are higher material durability, longer product life, corrosion, dirt and waterproof coatings, better insulation, lower weight and energy consumption. Nanogenic sensors may have a problem because they contain toxicity far from that which the nanomaterial used enters. Figure 3 shows the different types of sensor used in recent projects being implemented in the hydropower field as well as the sensor technologies needed to participate in several sectors.

The Energy-Water Nexus: Key Performance Indicator (KPI) Tools
This section provides a potential framework for investigating and ranking late contextual analyzes of the WEN nexus and the evaluation strategies applied. This aims to add to the advancement of WEN technologies that increase field value while reducing the duplication of endeavors. The key performance indicator (KPI) tools' case studies are divided into four groups according to the scale and nature of their application: city level, regional level, national level and transboundary level.

City-Level Scale
Cities are the main hubs where water and energy flows mix. In cities, sources and sinks of water and energy are more complex and intensive than in rural regions. The city is considered as the core of WEN research. Globally, the selected studies include case cities around the world. For example, the study of the relationship between water, energy, food, and climate is conducted for an increasingly robust and maintainable urban framework in Africa [99]. On the island of Skiathos, Greece, water and energy are managed and the resources are sustained by comparing the various sectors that need energy-residential, industrial, commercial, agricultural, etc.-and by studying the correlation between them and the use of water [100]. In Beijing, China, this nexus is studied from both generation and usage perspectives, where input-output are used to analysis the consumption of water to generate energy [101]. 4.1.2. Regional-Level Scale

The Energy-Water Nexus: Key Performance Indicator (KPI) Tools
This section provides a potential framework for investigating and ranking late contextual analyzes of the WEN nexus and the evaluation strategies applied. This aims to add to the advancement of WEN technologies that increase field value while reducing the duplication of endeavors. The key performance indicator (KPI) tools' case studies are divided into four groups according to the scale and nature of their application: city level, regional level, national level and transboundary level.

City-Level Scale
Cities are the main hubs where water and energy flows mix. In cities, sources and sinks of water and energy are more complex and intensive than in rural regions. The city is considered as the core of WEN research. Globally, the selected studies include case cities around the world. For example, the study of the relationship between water, energy, food, and climate is conducted for an increasingly robust and maintainable urban framework in Africa [99]. On the island of Skiathos, Greece, water and energy are managed and the resources are sustained by comparing the various sectors that need energy-residential, industrial, commercial, agricultural, etc.-and by studying the correlation between them and the use of water [100]. In Beijing, China, this nexus is studied from both generation and usage perspectives, where input-output are used to analysis the consumption of water to generate energy [101].

Regional-Level Scale
Regularly covering a larger geographic area, large-scale inspections at the regional level generally rely on river basins, including urban areas as well as more extensive drainage areas. The water-energy relationship has developed over the past decade. The systematic structures of principles applied and established between those checks include, for example, water assessment and planning (WEAP), long-term energy alternatives planning (LEAP), and coordinated WEAP and LEAP for a better understanding of the global energy grid on a regional scale [102], and the integrated model system for New South Wales [103]. All works applied the systematic structures and their objectives are for policy requirements and helping the decision maker by focusing on articulating the synergies of integrated resource planning and the trade-offs between them.

National-Level Scale
Policy-making and management analysis in arranging resources are the main concerns of tests at the national level. In terms of policy needs and major difficulties, water need, and water system pressure appear in the bulk of the national audited levels considered by the WEN [104][105][106][107][108]. For example, the heavily petroleum-dominated WEN in three countries (Kuwait, Qatar and Saudi Arabia) of the Gulf Cooperation Council (GCC) uses the information obtained across local scales and along the time to assess the nexus and detect the stresses on the nexus [109]. The oil exports income in the GCC makes it possible for the country to compensate for the shortage in the supply of water.

Transboundary-Level Scale
Cross-border assets and arrangements in some major regions over a few states are more intertwined than those in a single city or state. Management authorities at the transboundary level need to adjust the needs of the proximal network with the needs of society and the more inclusive situation [110]. The incomparable Mekong River Basin is a model for delegates [110][111][112]. Part of the current investigations recommended that the nexus approach would likely be beneficial to partners to understand the links between resources and policies [110][111][112], and they proposed an interconnected system that joined some specific models to investigate water, energy and food security in the Mekong River Basin. Similar examinations, for example, in the Euphrates-Tigris waterway basin [113] and the Amu Darya Basin in Central Asia [114] have suggested their own interconnected model structures to investigate how benefits are shared between those states.
Among the cases, there is a wide range of components that fundamentally challenge and affect the reasonable use and arrangement of water and energy as illustrated in Figure 4. Regularly covering a larger geographic area, large-scale inspections at the regional level generally rely on river basins, including urban areas as well as more extensive drainage areas. The water-energy relationship has developed over the past decade. The systematic structures of principles applied and established between those checks include, for example, water assessment and planning (WEAP), long-term energy alternatives planning (LEAP), and coordinated WEAP and LEAP for a better understanding of the global energy grid on a regional scale [102], and the integrated model system for New South Wales [103]. All works applied the systematic structures and their objectives are for policy requirements and helping the decision maker by focusing on articulating the synergies of integrated resource planning and the trade-offs between them.

National-Level Scale
Policy-making and management analysis in arranging resources are the main concerns of tests at the national level. In terms of policy needs and major difficulties, water need, and water system pressure appear in the bulk of the national audited levels considered by the WEN [104][105][106][107][108]. For example, the heavily petroleum-dominated WEN in three countries (Kuwait, Qatar and Saudi Arabia) of the Gulf Cooperation Council (GCC) uses the information obtained across local scales and along the time to assess the nexus and detect the stresses on the nexus [109]. The oil exports income in the GCC makes it possible for the country to compensate for the shortage in the supply of water.

Transboundary-Level Scale
Cross-border assets and arrangements in some major regions over a few states are more intertwined than those in a single city or state. Management authorities at the transboundary level need to adjust the needs of the proximal network with the needs of society and the more inclusive situation [110]. The incomparable Mekong River Basin is a model for delegates [110][111][112]. Part of the current investigations recommended that the nexus approach would likely be beneficial to partners to understand the links between resources and policies [110][111][112], and they proposed an interconnected system that joined some specific models to investigate water, energy and food security in the Mekong River Basin. Similar examinations, for example, in the Euphrates-Tigris waterway basin [113] and the Amu Darya Basin in Central Asia [114] have suggested their own interconnected model structures to investigate how benefits are shared between those states.
Among the cases, there is a wide range of components that fundamentally challenge and affect the reasonable use and arrangement of water and energy as illustrated in Figure 4.

City-level
• Water and energy flows • Climate changes and pollutions • Urban nexus conceptual framework

Water-Energy Nexus (WEN) KPI Tools
There are different KPI tools for assessing the water-energy relationship [115]. Until recently, the most important of these tools and techniques were used for sectoral assessment, whether in water or energy. For example, the MODISM model for simulating and evaluating water quality and quantity [116], and the LEAP software tool designed by the Stockholm Environment Institute for Energy Planning and Energy Policy Analysis [117]. There are six KPI tools used to assess the relationship between water and energy.

Energy Density (EI)
EI (energy density) is a top-down and bottom-up hybrid model used to quantify energy flows in a civilian water frame. The top-down model is planned to generate high-level calculations of energy intensity per month for the civil water framework. The bottom-up model is designed to estimate energy calculations in detail for a subset of the civil water framework. It is an estimation tool used in the case study of the Municipal Utilities District in East Bay in Northern California [118], but this tool has neither a strategic role nor a viable program.

Jordan Framework
This frame consists of three connected parts. The first is a quantitative test portion that designs the water and energy physical joints to assess the major subsections within the water and energy fields. The second part is the stakeholder testing of energy and water policy authorities and is used to uncover key actors and agencies. Part three used the findings from the previous section to uncover the controlled stakeholders who could be intermediaries for the transit of the primary water and energy leadership. This structure was effectively used for the case study of Jordan [119].

Correlation Analysis
Correlation analysis, drawn from the testing of inputs and outputs, is used to determine direct and indirect resource uses as well as the function of each financial branch (such as resource generation or resource uses) [120]. In the case of Beijing, correlation analysis was used to examine summarized water and energy capacity between urban finance departments [121]. For this case study, the correlation analysis has been modified to a hypothetical extraction method (HEM), which isolates the impacts of financial activities on resources into four divisions: internal impact (IE), mixed effect (ME), and net or external backlink. (NBL), and net or external front link (NFL).

Multi-Territory Nexus Network (MRNN)
MRNN (multi-territory nexus network), joined with the multi-regional input-output model (MRIO) and environmental network analysis model (ENA), is an integrated model created to fundamentally evaluate water and water energy for city energy and regional scales [122]. The MRIO model can assess water currents or indirect energy, to calculate the amount required to create products and projects based on sectoral cooperation and trade in a complex framework [123]. Instead of the MRIO model, the ENA model can use dynamic flows to evaluate not only direct and indirect resource flows in making change, but also the links between financial departments [124,125]. ENA can also track patterns of energy use or inversely use water to represent the overall use or use involved, and reveal features of frame structure and capacity [126]

A Dynamic Approach to the System
To understand the provincial water and energy resource arrangement in the long run, Zhuang [127] built a dynamic model of an integrated framework which was a four-step procedure model including the steps of the structure test, the structured behavior test of the structure, the implementation test and the attitude plan estimation.

Optional Urban Water Driving Tool (UWOT)
The UWOT (optional urban water driving tool) is also a tool that focuses on urban water frameworks [128][129][130]. It has four advantages: (1) evaluation of optional media to reduce consumer water demand; (2) energy assessments required by water devices; (3) assessment of beneficial uses of volumes of runoff and waste; and (4) evaluating the advantages of green areas on the urban heat island effect. This tool was used to create and quantify much of the "evidence" towards wastewater reuse for urban water arrangement in Athens [131]. Baki and Makropoulos [132] broaden UWOT to display the energy impression within urban water supply frameworks and discuss the use of this tool in the dynamic economic arrangement and effective management of water and energy resources in urban communities. Table 1 provides a comparison of the six KPI tools in terms of model type, developer, software, geographic scale, purpose, and association challenge level.

The Essence of the Relationship between Energy and Water
The relationship between energy and water is multidimensional: environmental, technological, political, economic and social. These dimensions influence each other, often paradoxically.

The Environmental Dimension
The environment is the source of all water and energy supplies, so it provides a background for many communications. There are two basic dimensions: climate change and drought. Burning fossil fuels to generate energy is the main cause of climate change. This has a negative impact on the availability of water sources, which is reflected in the generation of energy. Drought harms living ecosystems. The availability of water sources decreases and may decrease further due to climate change. At the same time, the demand for water and energy is increasing, along with carbon emissions. There are numerous attempts to mitigate these impacts, such as installing wastewater treatment technologies, constructing desalination plants, and supporting investment in technologies that reduce water consumption.
Even policies legislated to conserve the environment could lead to energy and water imbalances elsewhere. To illustrate, stricter environmental controls to promote the health of ecosystems need more high-quality remediation techniques and this is reflected in energy costs. Conversely, in the absence of policies or the implementation of policies, the environment may lose more.

The Technological Dimension
The technological dimensions are related to the physical connections between water and energy. Electrical technology options are increasing in the water industry such as groundwater extraction, desalination, water transportation and water recycling. Energy-generation technologies also require different amounts of water and emit different amounts of carbon.

The Economic Dimension
The dimension of the economic relationship between energy and water is gaining prominence, in part due to reforms taking place in the two industries. In the energy industry, a national energy market has been established by eligible electric consumers (HT). In water, an urban water market (SEAL, ADE, and SEOR, and others) has been established to distribute water to water users and enhance water efficiency (law of 4 August 2005 on Water) [104]. Subsidy price and tariff structures that are not charged on a volume basis reduce the value of energy and water, making consumers believe that energy and water are cheap.

The Social Dimension
Water and energy are directly affected by interdependence and have an important social dimension. Indirectly, the entire community is sensitive to the connections between energy and water. For example, the use of air conditioners will increase in hot weather, which requires more energy directly and indirectly more water. In another example, the consumption of hydropower plants for irrigation is at risk of wasting water distribution for hydropower generation, especially during times of drought. The apparent general behavior in some areas is that the social notion of the value of energy and water may relate to pricing management, and in others it does not [133].

The Political Dimension
The political dimension is very vital as it may influence the scale of significance of the relationship between energy and water in different dimensions. In the environmental dimension, stringent environmental rules require more processes to address the energy conflict, and this conflicts with the goals of reducing carbon emissions. Conversely, lack of water and energy policies or poor enforcement of regulations may lead to increased energy use, groundwater overexploitation, and efficacy of water drainage without proper treatment.

Accurate Analysis
There is a difference in the policies of the energy and water industries. Both industries are working to strike a balance between ensuring short-term energy and water supplies, meeting immediate environmental needs, designing to increase demand and preparing for climate change. The relationship between energy and water is generally multidimensional, and these dimensions affect each other. For example, policies can be complementary between water and energy, and efforts in one industry may be reduced by efforts in another industry as well, and there is a relationship between water and energy outside their borders that exists in the agricultural sector.

Review of Studies
Further studies are being reviewed on the relationship between energy and water in terms of purpose, scope, basic research methods, and the main findings of the studies.

The Main Limitations of Studies
The main limitations of the studies are presented in the following terms: • Objectives: These studies were classified into three groups. The first group examines the impact of energy in providing water and wastewater services. In [134,135], the authors have studied future outlines in the water industry, to determine the energy or environmental impacts of each. Studies in [136,137] have been presented to understand the interactions of water and energy, in the energy crisis in California in 2001. In [138], the energy effect of wastewater treatment plants has been studied in the realization that the entry of carbon emissions into the water industry will need to reduce energy consumption.
The second group studies the hydro effect of generating energy [139][140][141]. The key to these studies is that generating energy-especially from hydropower plants-may lead to potential trade-offs for water users, such as the environment and irrigation.
The third group studies the effect of energy on water users [142][143][144]. These studies examine the interaction between water price and/or water and energy use. In addition, in [144] the authors took into account changes in policy.

•
Focus Studies: Studies [136][137][138] examined the technological dimension. Ref. [134][135][136]141] studied the link between the technological dimension and the environmental dimension. Ref. [143,144] studied the relationship between the technological dimension and the economic dimension. Only two studies examine more than two dimensions. A study [139] examined the economic and technological impact of reservoir water allocation, whereas the study [142] examined the effect of energy price on groundwater productivity and clarified the environmental, political and social consequences.

•
Research Methods: These studies use different research methods to measure the relationship between energy and water. These include life-cycle assessment (LCA), econometrics, numerical modeling, productivity analysis, and simulation or improvement models.

Key Findings:
These studies give some important outcomes that should be taken into account for the energy-water nexus: The energy impacts of transporting imported water and recycled water are greater than water treatment. The opposite is true only for desalinated water [134]. The energy deposited in agriculture is large, but the opportunity cost of giving up energy is greater [142]. Voluntary relief programs are solutions to manage competition uses and stem the energy crisis. The recycling and conservation are less energy-intensive. Recycling water and desalination are more reliable [137]. There is scope for substituting water system versions for improved irrigation in countries if these countries increase energy supply. The consumption of fresh water is minimal compared to the total consumption of water, with the exception of power plants. The development of specific reservoir models is useful for water allocation among competing users [141]. Transporting water to the environment saves energy. Transporting water to urban users needs to be handled and pumped out in large quantities [137].
Reallocating water to other lands increases consumption, but the scale depends on the types of crop. Energy prices can affect the use of groundwater. The effect is significant when the water is volumetrically allocated [142]. The demand for water depends on energy prices. Increases in energy prices and carbon taxes could have significant impacts on residential water demand. Supply-based pricing saves more energy and water than flat rates [143,144]. Poor maintenance and design lead to increased energy consumption [138]. Energy demand depends greatly on the nitrogen: chemical oxygen demand (N:COD) ratio in raw wastewater. When the ratio is lower, it indicates less energy consumption.

Conclusions
This paper provides a complete review of the different relationships between water and energy. The aim of this work is to provide readers with the scientific vision to know and understand: (1) the importance of the relationship between water and energy, taking into account water consumption, cooling and heating, in order to avoid impeding the future use of water to generate energy; (2) the techniques used to measure and monitor the relationship between energy and water, taking into account the interrelated variables in the water and energy sectors and the effects of creating many unknown variables and the unclear dependence in measuring energy use in water applications and water use in energy applications. Nanotechnology, biotechnology and wireless networks are innovative developments in sensor technologies that may solve measuring and sensing challenges; (3) various KPI tools to assess and evaluate this relationship by reviewing and categorizing recent case studies related to water energy; and (4) the different dimensions of the research being conducted on this association This review contributes to the development of these interconnected approaches that add value to the field while reducing the repetition of effort.