water Review Water Footprints and Virtual Water Flows Embodied in the Power Supply Chain Like Wang 1, Yee Van Fan 1, Petar Sabev Varbanov 1,* , Sharifah Rafidah Wan Alwi 2 and Jiˇrí Jaromír Klemeš 1 1 Sustainable Process Integration Laboratory—SPIL, NETME Centre, Faculty of Mechanical Engineering, Brno University of Technology—VUT Brno, Technická 2896/2, 616 69 Brno, Czech Republic; [email protected] (L.W.); [email protected] (Y.V.F.); [email protected] (J.J.K.) 2 Process Systems Engineering Centre (PROSPECT), Research Institute for Sustainable Environment, School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), UTM Johor Bahru 81310, Johor, Malaysia; [email protected] * Correspondence: [email protected] Received: 10 September 2020; Accepted: 23 October 2020; Published: 26 October 2020 Abstract: Water use within power supply chains has been frequently investigated. A unified framework to quantify the water use of power supply chains deserves more development. This article provides an overview of the water footprint and virtual water incorporated into power supply chains. A water-use mapping model of the power supply chain is proposed in order to map the analysed research works according to the considered aspects. The distribution of water footprint per power generation technology per region is illustrated, in which Asia is characterised by the largest variation of the water footprint in hydro-, solar, and wind power. A broader consensus on the system boundary for the water footprint evaluation is needed. The review also concludes that the water footprint of power estimated by a top-down approach is usually higher and more accurate. A consistent virtual water accounting framework for power supply chains is still lacking. Water scarcity risks could increase through domestic and global power trade. This review provides policymakers with insights on integrating water and energy resources in order to achieve sustainable development for power supply chains. For future work, it is essential to identify the responsibilities of both the supply and demand sides to alleviate the water stress. Keywords: power-water nexus; water footprint; virtual water; power supply chain; power trade 1. Introduction Water and energy security have become two major challenges in the world due to the growing gap between demand and supply [1], with constantly emerging new water pollution threats [2] that have to be minimised and prevented. Water use for power production is reported to contribute 25% (51.9 km3) of the total global water consumption in energy production, of which 75% is contributed by fuel production [3]. It is reported that up to 93% of the water for energy production is contributed by the power supply in some Eastern countries [4]. In 2050, the total power generation is estimated to be double that in 2020 (see Figure1a), while the share of energy sources is projected to be dominated by renewables in 2050 [5]. Figure1b shows the world power demand for 2020 and the projection for 2050 broken down by regions, whose overall trend is in line with the projection of the power supply. It is essential to understand the water use of the power supply chain to ensure water sustainability in terms of availability, minimising ecosystem pollution and maximising power supply security. Water 2020, 12, 3006; doi:10.3390/w12113006 www.mdpi.com/journal/water Water 2020, 12, 3006 2 of 21 Water 2020, 12, x FOR PEER REVIEW 2 of 21 Figure 1. 1. GlobalGlobal power power supply supply and demand. and demand. (a) Global power (a) Global generation power from generation 2000–2050; from(b) Comparison 2000–2050; (ofb )power Comparison demand ofin different power demand regions worldwide in different in regions2020 and worldwide 2050. (Data insource: 2020 The and International 2050. (Data Energy source: TheAgency International during the Energy period Agency2000–2017) during [6]; the the predicted period 2000–2017) data from [2020–20506]; the predicted [7]. data from 2020–2050 [7]. The water footprint [8] [8] is is a a well-established metric metric,, defined defined as as the the volume volume of of consumed consumed or or polluted polluted freshwaterfreshwater required to produce a a certain amount amount of of goods goods or or services services [9]. [9]. The The water water footprint footprint includes includes greengreen water, referring referring to to the the rainwater rainwater stored stored in insoil soils,s, blue blue water water (freshwater (freshwater from fromrivers rivers and wells and and wells andgreywater), greywater), and andgreywater greywater (polluted (polluted water) water) as has as be hasen beenoverviewed overviewed [10]. [It10 has]. It been has beenwidely widely used usedto toquantify quantify water water use useat various at various scales scales [11] using [11] usingthe Life the Cycle Life Assessment Cycle Assessment (LCA), and (LCA), it has and been it detailed has been detailedfurther [12]. further This [concept12]. This has concept been extended has been to extended be more general to be more so as general to describe so as embodied to describe water embodied when compared to the original definition of virtual water [13]. Virtual water is proposed as a concept to measure water when compared to the original definition of virtual water [13]. Virtual water is proposed as a the (direct and indirect) embodied water of products or services in international trade [14]. Recently, virtual concept to measure the (direct and indirect) embodied water of products or services in international water has been widely applied to analyse the water resources used in the food trade [15]. trade [14]. Recently, virtual water has been widely applied to analyse the water resources used in the The impact of the power supply chain on water sources has been assessed in terms of power supply, foodtrade, trade and demand. [15]. The scope of study on the power supply side mainly focuses on assessing the water footprintThe impactfor various of the fuel power types supply[16], different chain oncooling water technologies sources has (open-, been assessed closed-, hybrid, in terms and of powerdry cooling) supply, trade,[16], low-carbon and demand. technologies The scope of(solar, study wind, on the powerand carbon supply emissions side mainly captur focusese technology) on assessing [17], the waterand footprintenvironmental for various issues (climate fuel types change, [16], water different scarcity cooling, pollution) technologies contributed (open-, by closed-,the power hybrid, supply and chain dry cooling)[18]. Power [16 ],supply low-carbon and demand technologies are typically (solar, spatia wind,lly andnonuniform carbon emissions[19], which captureinduces technology)regional power [17 ], andtrade, environmental and regional differences issues (climate in power change, supply water and scarcity, water resources pollution) are contributed reallocated. byFigure the power2 shows supply the chainpower [ 18generation]. Power and supply net trade, and demandas well as are the typically global distribution spatially nonuniformof water scarcity [19 ],over which the world’s induces regions, regional powerfor the trade,year 2017. and The regional Water diStressfferences Index in (WSI) power ranges supply widely and from water 0.98 resources (Egypt) to are 0.1 reallocated. (Canada). The Figure total 2 showspower thegeneration power generationof the listed and coun nettries trade, (24 ascountries well as in the Figure global 2) distribution accounts for of over water 80% scarcity of the overglobal the amount. The top three power generation countries in 2017, China (6.2 PWh), the United States (4.0 PWh), world’s regions, for the year 2017. The Water Stress Index (WSI) ranges widely from 0.98 (Egypt) to and India (1.2 PWh), face a conflict between water resources and electricity generation, with the WSI being 0.1 (Canada). The total power generation of the listed countries (24 countries in Figure2) accounts for as high as 0.48, 0.50, and 0.97. The question of how to assess and evaluate the power trade effect on water over 80% of the global amount. The top three power generation countries in 2017, China (6.2 PWh), resource relocation becomes an issue. This is of increasing concern to virtual water resource transfers in thecross-regional United States power (4.0 trade. PWh), This and is India motivated (1.2 PWh), by the facesignificant a conflict footprints between that water stem resources from satisfying and electricity power generation,demands via withboth local the WSI generation being asand high regional as 0.48, power 0.50, trade and flows. 0.97. The question of how to assess and evaluateA network the power of transferring trade effect virtual on water water resource resource relocations was assessed becomes based an on issue. the inter-regional This is of increasing power concerntrade [20], to and virtual the spatial water resourcevirtual water transfers transfers in cross-regionalwere analysed. powerWater scarcity trade. Thishas been is motivated highlighted by as the significantanother essential footprints indicator that stemfor characterising from satisfying the regi poweronal demands water consumption via both local and generationavailability and[21], regionaland it powerallows for trade a better flows. understanding of what is causing water scarcity and which regions are suffering from it [22]. ATo network understand of transferring and compare virtual the spatial water distributi resourceson was of water assessed use based