Environmental Engineering Reference
In-Depth Information
3.2 Energy-Water Nexus
Water and energy, two basic necessities of any civilization, are closely intertwined
(Gentleman 2011 ; Schnoor 2011 ). Most ancient civilizations were based on access
to water and its energy (the hydric civilization). The water-energy nexus involves
bi-direction consequences originating from coupled processes and factors govern-
ing use ef
ciency of resources involved. There are three types of water: blue, green
and grey. Plants can utilize only the green water (transpiration). Thus, conversion of
blue (runoff, stream
flow, groundwater) and grey (human waste) into green water
requires energy. It is needed for transformation of blue (uplift) and grey (purifi-
cation) water for increasing plant uptake and improving the NPP. Thus, increase in
global material consumption also increases the water demand and vice versa. About
20 gallons per megawatt-hour are consumed by evaporation of hot water from the
surface of the receiving body, and a power plant with cooling towers requires
500 gallons per megawatt-hour for evaporation (Hightower 2011 ). Indeed,
water use is expected to grow globally by 30
100 % for the energy sector, 20
40 %
for agriculture, and 20
40 % for domestic water supply. Yet, the supply of blue
water may decrease by 25 % because of reduction in surface water
flows in the mid-
latitude region due to projected climate change (Hightower 2011 ). Thus, enhancing
the use ef
ciency of water and energy for diverse uses and conversion of grey into
green water are critical strategies. Indeed, sewage,
flowing (blue) water and warm
wastewater are potentially important energy sources (Venkatesh and Dhakal 2012 ).
In the context of fossil fuel consumption, C footprint must be assessed through
life cycle analyses (LCA) at all stages of the production chain, and the baseline or
system boundaries must be carefully de
ned. Because of the increasing urbaniza-
tion, with more than 50 % of the world
s population already living in urban centres
and 80 % projected to be urbanized by 2050, the water-energy nexus is more
important than ever before for the cities of the future. Thus, there is a strong need of
achieving net zero C and pollution through reuse and recycling of water and
recovering the plant nutrients and other resources. Production of biofuel feedstocks,
through establishment of energy plantations is also water-intensive. Both C and
water footprints are sub-components of the overall environmental footprint
(Table 1 ). There are large differences in water required per unit quantity of biofuel
(ethanol) produced from different biofuel feedstocks, and for different management
systems. Thus, problems must be addressed rather than shifted, because the water-
energy nexus is a high priority at regional (CEC 2005 ), national (Hardy et al. 2012 )
and international levels (Venkatesh and Dhakal 2012 ). In terms of policy inter-
ventions, localized challenges are diminished when approached in the context of
broader perspectives. Similarly, regionally important challenges cannot be priori-
tized locally (Scott et al. 2011 ).
The water-energy nexus is also linked with the virtual water and the water
in relation to the production-consumption patterns. Virtual water is
ned as the amount of water needed to produce the goods and services to be
consumed by a country or individual. It is the amount of water needed to generate a
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