( www.eia.org ) requiring water for cooling in thermoelectric 
 ( steam electric ) power plants , it is important to understand how much 
 water is used for thermoelectric generation to assess vulnerability 
 of power generation to water shortages and determine potential impacts 
 of generation on water resources .
life cycle analysis indicates that most 
 water in thermoelectric power plants is used for cooling steam rather 
 than in fuel extraction , atmospheric emissions controls , or other 
 purposes .
large - scale expansion of natural gas production 
 using hydraulic fracturing and increased use of natural gas in thermoelectric 
 generation may have important implications for water resources , . because of the long 
 lifespan of power plants ( 3050 years ) , the current fleet represents 
 a legacy of past fuel choices , generator technologies , and cooling 
 systems .
there seems to be a lack of generic understanding on 
 water use 
 for thermoelectric generation .
water use is a general term that can 
 refer to water withdrawals , defined by the u.s .
geological survey 
 ( usgs ) as water removed from a water source , most of which is returned 
 to the source , or water consumption , defined as the portion of water 
 removed that is lost to the system by evaporation and unavailable 
 to other users .
many studies emphasize 
 the large water withdrawals associated with thermoelectric generation 
 ( 41% of u.s .
water withdrawals in 2005 ) ; however , 98% of this water 
 is returned to the source . equating water 
 withdrawals for thermoelectric generation to
the number of cities 
 that could be supplied with water ( 6770 new york cities ) seems to ignore the fact that 98% of 
 the water is available to other users downstream of the return , although 
 the water is not available to others between points of withdrawal 
 and return .
some studies suggest that 
 thermoelectric generation is also a major consumer of freshwater resources ; however , u.s .- based estimates indicate that consumption for thermoelectric 
 generation was 3% of total freshwater consumption in the u.s .
while much emphasis has been placed upon reducing 
 water withdrawals by converting cooling systems from once - through 
 cooling to wet cooling towers , a recent report highlights the increased 
 water consumption rates of cooling towers .
there are many concerns about data availability and quality 
 for 
 quantifying cooling water requirements for thermoelectric generation . 

the government accountability office ( gao ) recommended that the department 
 of energy - energy information administration ( doe - eia ) and usgs collaborate 
 to improve water use estimates .
the eia 
 relies on self - reporting from power plant operators on electricity 
 generation and water use and has revised its forms twice since 2009 
 to improve the data .
the usgs is using various approaches to estimate 
 consumption and withdrawals , including analysis of eia data and a 
 new linked water and heat balance .
however , usgs 
 suggests that accurate measurement and reporting of water use is required 
 to develop reliable estimates of water use at local scales .
if we want to reduce water use 
 for thermoelectric generation , we need to understand the primary drivers 
 or controls on water consumption and withdrawal . because of long lifespans of power plants , current power 
 plant choices may impact water resources for decades into the future . 

many studies show that different aspects of power plants , such as 
 energy source , generator technology , or cooling system have varying 
 water requirements .
for example , switching fuel sources from coal 
 to natural gas saves water and switching 
 from once - through cooling to cooling towers increases water consumption .
therefore , it is important to understand the 
 basic components of thermoelectric power plants and how they relate 
 to water consumption and withdrawal .
major components of thermoelectric 
 power plants include the ( a ) energy source , ( b ) generator technology 
 or prime mover to generate electricity , and ( c ) cooling system ( figure 1 ) .
thermoelectric 
 power plants primarily generate electricity by either burning fossil 
 fuels ( coal or natural gas ) or using nuclear fission to boil pure 
 water , creating steam that drives turbines that generate electricity 
 ( figure 2 ) . thermoelectric plants generated 
 86% of net electricity in texas and 89% in the u.s . in 2010
primary energy sources in thermoelectric 
 power plants in texas include nuclear , coal , and natural gas sources 
 ( www.eia.org ) ( figure 1).figure 1basic components 
 of power plant systems based on net electricity 
 generation for texas in 2010 ( 411 million mwh or twh ) , including thermoelectric 
 energy sources : nuclear ( 10% ) and fossil fuels [ coal ( 37% ) , natural 
 gas ( 46% ) ] , and nonthermoelectric energy sources , mostly wind ( 6% ) , 
 and other minor fuel sources ( total 1% , not shown ) , including 
 hydroelectric , petroleum , and biomass ; generator technologies or prime 
 movers ( pm ) ( steam turbines , st , 53% ) , combined cycle ( cc , 35% ) , gas , 
 or combustion turbines ( ct , 5% ) , wind turbines ( wt , 6% ) , and other 
 minor generator types ( total 1% , not shown ) , including hydroelectric 
 turbines and internal combustion engines ; and cooling systems , once - through 
 ( ot , 42% ) , wet cooling towers ( 45% ) , dry cooling towers ( 1% ) , and 
 no cooling ( 12% ) .
steam turbines use rankine cycle to generate electricity 
 and are not fuel specific : they are used with nuclear , coal , and natural 
 gas energy sources .
combined cycle plants use combustion turbines ( ct ) and steam 
 turbines ( st ) and are fuel specific , used with natural gas . integrated 
 gasification combined cycle ( igcc ) can be used with coal , but these 
 plants are very rare in the u.s . and globally .
combustion turbines 
 are also referred to as gas turbines and are similar to jet engines . 

they use natural gas as a fuel source and the brayton cycle to generate 
 electricity .
basic components 
 of power plant systems based on net electricity 
 generation for texas in 2010 ( 411 million mwh or twh ) , including thermoelectric 
 energy sources : nuclear ( 10% ) and fossil fuels [ coal ( 37% ) , natural 
 gas ( 46% ) ] , and nonthermoelectric energy sources , mostly wind ( 6% ) , 
 and other minor fuel sources ( total 1% , not shown ) , including 
 hydroelectric , petroleum , and biomass ; generator technologies or prime 
 movers ( pm ) ( steam turbines , st , 53% ) , combined cycle ( cc , 35% ) , gas , 
 or combustion turbines ( ct , 5% ) , wind turbines ( wt , 6% ) , and other 
 minor generator types ( total 1% , not shown ) , including hydroelectric 
 turbines and internal combustion engines ; and cooling systems , once - through 
 ( ot , 42% ) , wet cooling towers ( 45% ) , dry cooling towers ( 1% ) , and 
 no cooling ( 12% ) .
steam turbines use rankine cycle to generate electricity 
 and are not fuel specific : they are used with nuclear , coal , and natural 
 gas energy sources .
combined cycle plants use combustion turbines ( ct ) and steam 
 turbines ( st ) and are fuel specific , used with natural gas . integrated 
 gasification combined cycle ( igcc ) can be used with coal , but these 
 plants are very rare in the u.s . and globally .
combustion turbines 
 are also referred to as gas turbines and are similar to jet engines . 

they use natural gas as a fuel source and the brayton cycle to generate 
 electricity .
generator technologies , or prime 
 movers , for thermoelectric power plants for texas include steam turbines 
 ( st ) , combustion turbines ( ct ) , sometimes referred to as gas turbines , 
 and combined cycle ( cc ) .
traditional cts have no cooling water 
 requirements ; however , newer models use a relatively minor amount 
 of water to prechill inlet air .
a combined cycle plant combines cts 
 with sts in sequence , generally in a ratio of 2 cts to 1 st .
because 
 cooling water is not required for traditional cts , the rule - of - thumb 
 is that cc plants require about a third of the water that st plants 
 require because of the general ratio of 2 cts/1 st . cooling systems in thermoelectric 
 power 
 plants are required to condense steam .
these cooling systems can be 
 based on water , air , or both ( hybrid system ) . cooling water forms 
 a separate system from the closed - loop steam cycle .
water - based cooling 
 systems are broadly classified as ( 1 ) once - through or open - loop systems 
 ( e.g. , rivers , lakes , reservoirs , ponds ) ( figure 2a ) ; and ( 2 ) recirculating or closed - loop systems ( wet cooling 
 towers ) ( figure 2b ) .
schematic diagrams of 
 power plant steam turbine generation and 
 cooling systems : ( a ) steam turbine ( coal , nuclear , or natural gas ) 
 with once - through recirculating pond cooling system .
the internal 
 closed - loop steam cycle is shown , including boiler , turbine , and condenser , 
 and the external cooling cycle is also shown from pond to condenser , 
 returning to pond .
the footprint could be defined for the power plant 
 with water circulation / withdrawal for cooling based on the cycle labeled 
 2 ; alternatively , a larger footprint could be defined including the 
 pond with water withdrawal from the river to the pond ( labeled 1 ) 
 and representing the amount of water required to replenish the pond 
 water ( termed makeup water ) .
( b ) wet cooling tower 
 with dominant circulation between the power plant and the tower and 
 low withdrawal from the pond or other water source to the tower .
water 
 can be returned to the pond ( discharge ) or the system may have zero 
 discharge . for classification of cooling systems by eia ,
the term once - through technically 
 refers to water 
 moving once through the condenser . once - through systems in the eastern 
 u.s .
generally consist of power plants that withdraw water directly 
 from a river and then discharge that water back to the river farther 
 downstream at a higher temperature .
dissipation of waste heat from 
 the power plant in water bodies ( rivers or ponds ) occurs by evaporation , 
 long wave radiation , conduction ( as sensible heat ) , and convection .
it is difficult to apply the 
 once - through concept to ponds where water is continuously recirculated 
 between the pond and the power plant ( labeled 2a in figure 2a ) . in many analyses ,
if the plant boundary is 
 redrawn to include the pond , withdrawals might instead refer to water 
 transfers from another river / lake / reservoir to the cooling pond ( typically 
 termed diversion , labeled 1 in figure 2a ) to replace evaporative losses in the cooling pond , in which 
 case circulation between the pond and the plant would not be accounted 
 for separately .
classifications of cooling systems according to eia 
 are shown in supporting information , si , 
 figure s1 . in wet cooling towers ,
water moves through the condenser 
 and then to the tower , where it evaporates ( figure 2b ) .
evaporation is the only significant heat - dissipation process 
 in wet cooling towers ; therefore , consumption is up to 50% greater 
 for cooling towers relative to once - through systems that can also 
 dissipate heat using radiation , conduction , and convection .
several recent studies provide reviews or meta - analysis 
 of water use estimates from the literature . however , many 
 estimates in the literature are based on dated information of uncertain 
 quality .
multiple regression was used to identify 
 important determinants of water use for power generation based on 
 19962000 eia data . however , reliable 
 estimates of water consumption were particularly difficult to obtain 
 from earlier data .
many of the values 
 provide representative water use estimates for common thermal power 
 plants and cooling system types rather than an assessment of original 
 data from eia or other sources .
some recent studies are based on analysis 
 of original data from eia , for example , 2006 eia data for texas and 2008 eia data for the u.s .
many of the studies focus on future projections of water 
 use for various environmental regulations , climate change scenarios , 
 and carbon - based policies . the primary objectives of this study 
 were to quantify water consumption and withdrawal for thermoelectric 
 generation and to better understand controls on water use .
texas provides 
 an excellent case study because of data availability on water use 
 from multiple sources ( eia , texas commission on environmental quality , 
 tceq , and texas water development board , twdb ) .
the detailed analysis 
 and cross - checking and verification of water - use data from multiple 
 sources conducted in this study and meetings with power plant operators 
 to assess data quality represent one of the most intensive examinations 
 of raw data to date . additionally , texas climatic conditions 
 range from humid in the east to arid in the west , providing an analog 
 for u.s . electricity generation .
heavy reliance on surface water for 
 cooling in texas is also typical of that in the u.s .
the range of 
 fuel sources ( coal , natural gas , and nuclear ) and variety of generator 
 technologies and cooling systems are typical of those in the u.s . 

the understanding of controls on water use developed in this study 
 should be applicable to all thermoelectric power generation .
recent 
 trends in water use related to expansion of natural gas ( ng ) plants 
 in texas is much more advanced that what is occurring in the u.s . 
 but may provide some ideas of what is projected to occur throughout 
 the u.s . in the near future , with large - scale expansion of ng production 
 through hydraulic fracturing .
data from 2010 were used because this 
 year represents near normal climatic conditions ( precipitation 35.4 
 in .
[ 899 mm ] , 26% higher than long - term mean of 28.1 in .
this work builds on a previous study quantifying 
 water consumption for thermoelectricity in texas in 2006 by taking advantage of improvements in eia data 
 since 2006 and focusing on controls rather than on future projections . 

quantitative information on water use and understanding of controls 
 provided by this study should be very valuable for future management 
 of water and energy to ensure reliable electric generation .
water use for thermoelectric cooling 
 was obtained from the eia , 
 tceq , and twdb databases .
power generation is organized by generator 
 technology and water use is associated with cooling systems in eia 
 form 860 , but direct links between generators and cooling systems 
 are not provided .
generators are associated with boilers and boilers 
 are , in turn , associated with cooling systems .
each connection may 
 represent a one - to - one , one - to - many , or many - to - one association .
operational information , 
 including monthly power generation and water use , are reported in 
 eia form 923 .
water use is categorized as rates of diversion ( labeled 
 1 in figure 2a ) , withdrawal ( 2a ) , discharge 
 ( 2b ) , and consumption ( si , figure s1 ) .
the tceq water rights database includes reported monthly total 
 diversion ( assumed to represent eia withdrawal ) , return , and consumption 
 volumes from surface water .
the twdb database includes reported annual 
 total power plant water intakes ( purchased or self - produced ) by source 
 ( surface water , groundwater , salt water , or reuse ) and reported as 
 water withdrawal and water use .
for comparison with eia data , twdb 
 water use was assumed to represent consumption ; however , it may reflect 
 diversion .
data from tceq and twdb , unlike that from eia , represent 
 total plant values for all uses and may not be cooling system specific 
 because information on cooling systems is not included in these databases . 

water consumption or withdrawal intensities or rates ( gal / kwh or l / kwh ) 
 were calculated by dividing water consumption or withdrawal by net 
 generation .
power plant capacity factors were calculated by 
 dividing actual 
 power plant output ( net generation ) by theoretical output if operated 
 at full nameplate capacity .
thermal efficiencies were calculated according 
 to fuel source and generator technology by dividing electricity output 
 ( net generation mwh ) by heat input of fuel source ( gigajoule , gj ) , 
 using the conversion factor ( 1 mwh = 3.6 gj ) .
cooling 
 water was required for 264 generators at 143 operational power plants 
 in texas in 2010 , representing 354 twh of cooled net generation , and 
 accounting for 86% of total net generation ( 411 twh ) .
data on water 
 consumption was reported by eia for 71 plants ( 65% of net generation ) , 
 by tceq for 43 plants ( 56% of net generation ) , and by twdb for 62 
 plants ( 58% of net generation ) ( si table 
 s1 ) .
data on water withdrawal was reported by eia for 82 plants ( 73% 
 of net generation ) , tceq for 45 plants ( 55% of net generation ) , and 
 twdb for 64 plants ( 61% of net generation ) .
there was no water consumption 
 or withdrawal data for 39 plants ( 10% of net generation ) .
eia 
 data on water consumption / withdrawal were used as the default data 
 and represent the only data source for most regions in the u.s .
comparison 
 of water use from eia and tceq , where both are available , indicates 
 that water use values agree to within 20% for 80% of overlapping consumption 
 rates and 57% of overlapping withdrawal rates for once - through systems 
 ( figure 3 ) .
many of the consumption and withdrawal 
 rates for power plants with cooling towers reported to tceq , though 
 reasonable , tend to be somewhat greater than those reported to eia , 
 possibly reflecting additional consumption for purposes other than 
 cooling .
some of the outliers are up to an order of magnitude different , 
 suggesting reporting of consumption for withdrawal or vice versa . 

although tceq reports diversion , the values are often close to withdrawals 
 reported by eia but sometimes may reflect true diversion .
for example , 
 the south texas project nuclear plant reports diversion , which is 
 close to consumption for that plant , whereas the comanche peak nuclear 
 plant reports withdrawal values . where eia data differ markedly from 
 tceq data and were judged less reliable , that is , highly inconsistent 
 with other plants within the same generator / cooling category , then 
 tceq estimates were used . in a limited number of cases ,
twdb data 
 were used where data were not available from eia or tceq , or twdb 
 data were most consistent with other power plants in similar categories . 

more detailed information on data comparisons is provided in si , section 2 . in the absence of any water use 
 data or for systems with questionable data ,
the category weighted 
 average values of similarly configured plants were assigned ( representing 
 18% of consumption and 24% of withdrawal total volume ) .
comparison 
 of ( a ) water consumption and ( b ) water withdrawal intensities 
 for power plants reported by eia and by tceq .
solid symbols represent 
 eia data that were selected , while open symbols indicate tceq data 
 that were selected .
statewide withdrawal required for cooling was calculated 
 separately 
 for once - through systems by multiplying hours of plant operation required 
 to generate reported electricity output at full nameplate capacity 
 by cooling system pump capacity rating specification and represented 
 66% of reported actual withdrawal derived from the various databases . 

this comparison indicates that about one - third of water withdrawn 
 for thermoelectric generation in the state is not actually required 
 for cooling .
plants with high capacity factors ( nuclear , 92% ; coal , 
 73% ) are considered baseload plants and reported withdrawals are equal 
 to ( for nuclear st ) or slightly higher than ( 12% for coal st ) cooling 
 water requirements .
however , plants with low capacity factors that 
 operate as peaking plants often have low electrical output levels , 
 that is , are idling in anticipation of meeting a near - term peak demand ; 
 however , their cooling circulation pumps can only operate at full 
 capacity , resulting in overestimation of water withdrawal requirements 
 for cooling , particularly for ngst ( 24% capacity factor ) and ngcc 
 ( 40% capacity factor ) plants with once - through cooling systems . estimated 
 withdrawals required for cooling in these plants were only 16% ( ngst ) 
 to 33% ( st part of cc plants ) of reported withdrawals .
water 
 consumption totaled 0.43 maf ( 0.53 km ) for thermoelectric 
 generation in texas in 2010 , representing 4.2% of the states 
 water consumption ( 10.1 maf , 12.5 km ; 2010 ) ( tables 1 and 2 , figure 4 ) .
this water consumption is much lower than consumption for 
 irrigation ( 65% ) , municipal use ( 17% ) , and manufacturing ( 9.2% ) . indirectly
, 
 water consumption for thermoelectric cooling represents 5% 
 of typical household water withdrawals ( si , section 3 , figure s3 ) . although water consumption for thermoelectric 
 cooling represented only 4.2% of total statewide water consumption , 
 locally it can be important if water availability is low .
water consumption 
 for thermoelectric generation ( te ) in texas relative 
 to water consumption for other sectors based on 2010 data from the 
 texas water development board ( www.twdb.state.tx.us ) .
water consumption 
 was estimated from water withdrawal reported by twdb by assuming 85% 
 of irrigation water and 85% of manufacturing water was consumed .
these 
 percentages are based on comparison of water consumption and withdrawal 
 data for texas for 1995 from the usgs data .
the only sector with low water consumption is 
 municipal , with an estimated 30% consumed for residential irrigation 
 and 10% lost to leakage in texas .
values in bold indicate dominant 
 control of generator technology on water consumption , with combined 
 cycle plants having 63% lower consumption intensities than steam turbines 
 and cooling technology on water withdrawal with once - through cooling 
 ponds having about two orders of magnitude higher withdrawal than 
 wet cooling towers .
the statewide estimates are represented by three 
 different approaches : ( 1 ) fuel source , ( 2 ) generator technology , and 
 ( 3 ) cooling system .
netg : 
 net generation in twh = million mwh ; ng : natural gas ; nuc : nuclear ; 
 oth : other ; gen tech : generator technology or prime mover ; cool : 
 cooling system type ; st : steam turbine .
total cooled net generation of 354 twh represents 
 86% of 411 twh total net generation .
consumption of 427 thousand ac - ft 
 ( kaf ) is equivalent to 0.526 km .
total water withdrawal 
 of 26  184 kaf is equivalent to 32.3 km . because 
 water consumption is affected by generator technology and cooling 
 systems , totals and rates do not only reflect the reported category 
 but are also impacted by other categories .
for example , sts are predominantly 
 once - through systems and ccs are mostly cooling towers , both of which 
 affect water consumption . note water consumption intensity or rate 
 ( gal / kwh ) is controlled primarily by generator technology ( cc 63% 
 lower than st , shown in bold ) , and withdrawal intensity or rate ( gal / kwh ) 
 is controlled primarily by cooling system ( towers 2 orders 
 of magnitude lower than ponds , shown in bold ) .
number of generators ( n ) , fuel 
 sources include nuclear ( nuc ) , coal , natural gas 
 ( ng ) , other ( oth ) .
generator technologies ( gen tech ) include steam 
 turbines ( st ) and combined cycle ( cc ) .
cooling systems ( cool ) include 
 once - through ( ot ) , wet cooling towers ( t ) , and cogeneration ( combined 
 heat and power , c ) .
assessing impacts of generator technologies on water consumption rates with similar cooling systems : once - through 
 ponds , cc 73% lower consumption intensity than st ( 0.12 vs 0.44 gal / kwh ) ; 
 wet cooling towers , cc 66% lower than st ( 0.23 vs 0.68 gal / kwh ) . assessing 
 impacts of cooling systems on water consumption rates 
 with similar generator technologies : coal st , towers 8% higher than 
 once - through ponds ( 0.56 vs 0.52 gal / kwh ) ; ngst , towers 55% higher 
 than once - through ponds ( 0.68 vs 0.44 gal / kwh ) ; ngcc , towers 92% higher 
 than once - through ponds ( 0.23 vs 0.11 gal / kwh ) .
note : there are not 
 many examples of ngcc with once - through ponds , accounting for only 
 2.4% of all net generation and the category represents only 0.8% of 
 all water consumption and 7% of all water withdrawals . assessing the 
 impacts of generator technology on water withdrawal rates with similar 
 cooling systems : once - through ponds , water withdrawal intensities 
 are 58% lower for ngcc plants relative to ngst plants ( 59 vs 141 gal / kwh ) ; 
 cooling towers , water withdrawal intensities are 63% lower for ngcc 
 plants relative to ngst plants ( 0.26 vs 0.71 gal / kwh ) .
all fuel , all 
 generator technologies , and all cooling systems refer to both thermoelectric 
 and nonthermoelectric generation .
total water consumption of 427 thousand 
 acre feet ( kaf ) = 0.43 million acre feet = 0.53 km = 526 
 mm
water consumption intensity 
 of 0.34 gal / kwh = 1.3 l / kwh statewide .
water 
 withdrawals for thermoelectric generation vary depending on where 
 the system boundary is defined .
eia classifies cooling systems as 
 once - through , recirculating ponds , and cooling towers .
only two power 
 plants with once - through cooling systems withdraw water directly from 
 a river in texas ( the guadalupe river , si , section 4 , figure s4 ) , similar to typical once - through systems 
 in the eastern u.s .
( si figure s5 ) . the 
 remaining once - through power plants in texas withdraw water from ponds / reservoirs , 
 which are also found throughout the eastern u.s .
there is no systematic variation in terms of 
 storage volume and recirculation estimates ( pond storage / withdrawal 
 rate ) for systems classified by eia as once - through systems versus 
 recirculating ponds in texas ( si figure 
 s6 ) , suggesting that the classification is somewhat arbitrary and 
 generally lacks physical meaning . examining google earth maps of most 
 cooling ponds / reservoirs
shows structures designed to increase recirculation 
 efficiency ( si figure s7 ) ; therefore , we 
 classified all these systems as recirculating ponds / reservoirs for 
 simplicity .
withdrawals are generally calculated assuming the 
 system boundary is at the power plant , with withdrawals representing 
 water circulation between the cooling pond and power plant ( figure 2a ) .
the withdrawal value from analysis of the eia / twdb / tceq 
 databases for 2010 totaled 26.2 maf ( 32.3 km ) .
withdrawals 
 for thermoelectric generation represent 66% of the state total ( 26.2 
 maf + 13.4 maf [ nonthermoelectric withdrawals ] = 39.6 maf , 48.8 km ) ( table 1 ) .
however , only about 2% 
 ( 0.43/26.2 maf ) of this water is lost to evaporation from cooling 
 towers or forced evaporation from ponds / reservoirs due to increased 
 temperatures .
the system boundary could also be defined to include 
 the cooling 
 pond ( figure 2a ) , which may be appropriate 
 for recirculating ponds and in this case withdrawal from the river / lake 
 to the recirculating cooling pond , sometimes referred to as diversion 
 ( si figure s1 ) , would be much lower , reflecting 
 the net watershed balance for the pond ( rainfall + runoff into pond 
 minus natural + forced evaporation ) . these withdrawals to replenish 
 the pond , termed makeup water , would be closer to water consumption . 
 with the larger footprint ( including the pond ) , water consumption 
 should increase because it would include natural evaporation from 
 the pond .
although initial reservoir impoundment removes a significant 
 amount of water from the river circulation , withdrawals from rivers / reservoirs 
 afterward are only required to replenish the cooling pond .
a variety 
 of factors can impact water consumption / withdrawal , including fuel 
 type , generator technology , cooling system , thermal efficiency , capacity 
 factor , and age of plant .
water consumption is 
 controlled primarily by ( a ) generator technology , which is related 
 to thermal efficiency , and by ( b ) cooling system design with up to 
 50% higher consumption in cooling towers relative to ponds ( figure 5 ) .
( a ) generator technologies impact water consumption 
 through their thermal efficiency , which is defined as the fraction 
 of heat from fossil fuels or nuclear fission that is converted to 
 electricity ( electricity output / heat or energy input ) ( si , section 5 ) .
increasing 
 the thermal efficiency of a system means that more of the energy from 
 the fuel source is converted to electricity , reducing residual or 
 waste heat requiring cooling .
combined cycle plants have the highest 
 thermal efficiency ( ngcc , 44% ) because residual heat from the ct part 
 is used by the st part of cc plants through a heat recovery steam 
 generator ( hrsg ) system ( figure s8 , table s2 ) .
in contrast , thermal efficiencies of coal and nuclear st plants 
 are lower ( both 36% ) than that of ngcc plants .
increasing temperatures 
 of cooling ponds would also reduce thermal efficiency ; however , this 
 effect is much lower than temperature differentials between different 
 generator technologies ( st , ct , cc ) .
relationship between water consumption 
 and water withdrawal for 
 different power plants according to fuel type  generator
cc : combined cycle . note the log scales for consumption 
 and withdrawal intensities or rates .
withdrawal rates are controlled 
 primarily by the cooling systems as shown by the large difference 
 between once - through cooling ( mostly recirculating ponds ) on the left 
 and cooling towers on the right .
note the trade - off between water 
 withdrawal and consumption rates , with higher withdrawal and lower 
 consumption for once - through systems and lower withdrawal and higher 
 consumption for wet cooling towers .
the low withdrawal rates for some 
 ngcc plants with once - through cooling reflect plants that mostly operated 
 the ct parts of cc plants with low capacity factors . within each cooling 
 system ,
total water consumption by generator 
 technology is 82% for sts 
 and 18% for ccs in texas ( table 1 ) .
all traditional 
 st generation requires cooling , whereas , statewide , generation from 
 cc plants is 71% ct , which does not require cooling , and 29% st , which 
 requires cooling .
therefore , cc plants should have about 70% lower 
 cooling water requirements than st plants on average statewide , similar 
 to the 63% found for the 2010 data ( cc 0.19 gal / kwh vs st 0.52 gal / kwh ) 
 ( table 1 ) . comparing similar cooling technologies 
 ( once - through and cooling towers ) results in water consumption intensities 
 for cc generation being 6673% lower than for st generation 
 ( table 2 , figure 5 ) , 
 consistent with that expected from the noted ct to st ratio in cc 
 plants .
the impact of cooling systems on water consumption was 
 evaluated 
 by comparing cooling systems for plants with similar generator technology . 

one would expect up to 50% higher water consumption for wet cooling 
 towers relative to once - through ponds , if everything else is the same . 

results show that , for ngst generators alone , water consumption intensity 
 is 55% higher for wet cooling towers relative to once - through ponds 
 ( table 2 ) .
similarly , for ngcc generators alone , 
 water consumption intensity for towers is 92% higher than that for 
 ponds . for coal plants ,
the difference is only 8% higher . the large 
 range in water consumption intensities for different cooling systems 
 may reflect limited data for some categories ( table 2 ) , possible operational differences , ( for example , relative 
 use of ct [ no cooling ] versus st in cc plants with different cooling 
 systems ) , or reporting problems .
although wet cooling towers generally 
 consume more water than once - through ponds for a given generator technology , 
 the average statewide consumptive water intensity for all cooling 
 towers ( 0.31 gal / kwh ) is contrarily 35% lower than that of all cooling 
 ponds ( 0.48 gal / kwh ) because most cooling tower generation is associated 
 with cc generators ( table 1 ) , with much lower 
 cooling requirements per kwh than other generator technologies ( 71% 
 of cc net generation is from cts , requiring no cooling ) .
these statewide 
 comparisons of once - through systems and towers underscore the dominant 
 role of generator technology ( st , ct , cc ) over cooling system technology 
 in controlling water consumption .
water consumption rates from this 
 analysis according to fuel , generator , and cooling system ( table 2 ) are generally within the range of those reported 
 from the literature ( si table s3 ) .
the age of a plant is also a factor in 
 thermal efficiency and water 
 use .
cc plants were the only plants to 
 show a distinct increase in thermal efficiency based on analysis of 
 decadal data , with less efficient plants constructed before 1990 ( 36% 
 efficient ) compared to those that have come on - line since ( 46% 
 efficient ) .
traditionally 
 water withdrawals 
 are calculated from the water source ( pond / reservoir ) to the power 
 plant , resulting in statewide water withdrawal intensities for 2010 
 of 49 gal / kwh for once - through systems , more than 2 orders of magnitude 
 higher than withdrawal intensities for wet cooling towers ( 0.35 gal / kwh ) 
 ( table 1 ) .
water withdrawals are also affected 
 to a lesser extent by generator technology or thermal efficiency . 
 for similar cooling systems , water withdrawal intensities
are 5863% 
 lower for ngcc plants relative to ngst plants ( table 2 ) .
withdrawal intensities from this study are generally within 
 the range of reported withdrawals from the literature ( si table s3 ) .
water withdrawal 
 volumes are much higher in the eastern part of the state , where ponds 
 and reservoirs are concentrated ( si figure 
 s9 ) .
( 26 maf ) and 1% for wet cooling towers ( 0.2 maf ) for 2010 
 data ( table 1 ) .
there are several issues 
 associated with water withdrawals , including ( a ) water storage requirements 
 to support withdrawals ,
( b ) impacts of withdrawals on other water 
 users withdrawing water from the same pond / reservoir or on downstream 
 users , ( c ) discharge temperature issues ( clean water act 316a ) , and 
 ( d ) effects of cooling water intake structures on aquatic species 
 ( cwis , clean water act 316b ) .
the issues vary depending on whether 
 it is an industrial pond or multipurpose reservoir that is used for 
 power plants and water supply .
of the 43 ponds / reservoirs supplying 
 water directly to power plants in texas , 25 are industrial ponds and 
 18 are multipurpose , primarily water supply .
the industrial ponds 
 represent only 1 maf ( 1.2 km ) of the associated water 
 storage but account for 80% of the net electricity generation from 
 ponds , whereas the multipurpose ponds / reservoirs represent 3.4 maf 
 ( 4.2 km ) of water storage but only 20% of the net electricity 
 generation .
industrial ponds generally have smaller drainage areas 
 ( 300 mi , mean 42 mi ) than multipurpose 
 reservoirs / ponds ( > 150 mi , mean 3700 mi ) ( si figure s10 ) .
although reservoir 
 storage must be available to support the large 
 water withdrawals at power plants with once - through cooling systems , 
 less water is required to replenish industrial ponds supporting once - through 
 cooling systems than those supporting wet cooling towers , other factors 
 being equal , because of up to 50% to 100% higher water consumption 
 associated with towers relative to once - through cooling systems .
once - through 
 systems supported by multipurpose reservoirs may reduce water availability 
 for municipal water supplies abstracting water from the same reservoir 
 but can actually increase water available for downstream users because 
 of lower consumption relative to wet cooling towers .
many industrial 
 cooling ponds often need only to meet discharge temperature regulations 
 where the pond physically discharges back to the supplying river / reservoir 
 system ( figure 2a ) .
however , industrial ponds 
 only discharge such water during very high flow periods ; therefore , 
 discharge temperatures may not a concern for many industrial ponds
. 
 in fact , industrial ponds may protect larger reservoir / river systems 
 from high temperatures . cooling water intake structures ( cwis )
because 
 industrial ponds are man - made , they may or may not be stocked with 
 fish .
impacts on fish can be addressed using best technology available , 
 such as screens etc .
, where there are concerns about cwis . therefore , 
 there are trade - offs between once - through systems , with at least order 
 of magnitude higher water withdrawals and impacts on aquatic species 
 and thermal discharges , relative to wet cooling towers with much lower 
 water withdrawals but up to 50%- to 100%-higher water consumption
. 
 attributes of industrial ponds and multipurpose ponds / reservoirs should 
 be considered when promoting or mandating a particular cooling technology .
the long lifespan 
 of power plants ( 3050 years ) results in the current texas 
 power plant fleet representing a legacy of past variations in fuel 
 availability and costs , water availability and water rights , and advances 
 in technologies  for example , ngcc systems , which impact trends 
 in water consumption ( figure 6 ) .
fuel sources 
 varied over time , from predominantly natural gas in the 1960s and 
 early 1970s to coal and nuclear sources following the 1973 oil embargo 
 and more recently to large increases in natural gas generation ( 85% 
 increase 19982004 ) in response to hydraulic fracturing and 
 low natural gas prices .
the dominant generator 
 technology was sts until the early 1990s ; however , cc plants markedly 
 increased in the late 1990s ( 400% increase 19982004 ) .
proliferation 
 of ponds , mostly in the 1970s and 1980s ( 340% increase ) reflects availability 
 of large quantities of unappropriated surface water and increases 
 in water rights permitting during this time and lower cost and higher 
 cooling efficiency of ponds relative to wet cooling towers .
cumulative 
 generation capacity in 1000s of mw or gw of the existing 
 texas power plant fleet by ( a ) fuel type , ( b ) generator technology , 
 and ( c ) cooling system type .
hindcasts were developed from the current power 
 plant fleet based on information on when the systems came online ( www.eia.org ) . the original fleet consisted mostly of ngst with 
 increasing coal st and also increasing nuclear st in the late 1980s 
 in response to the 1973 oil embargo .
natural gas increased markedly 
 from 19982004 ( 85% increase ) and consists mostly of combined 
 cycle generators ( 400% increase 19982004 ) and cooling towers 
 ( 118% increase ) .
wind energy increased significantly in mid-2000s 
 related to the texas renewable portfolio standard and the federal 
 production tax credit for wind energy , renewed in 2013 .
if future epa regulations mandate conversion of 
 existing once - through 
 pond cooling systems to tower cooling systems , water consumption would 
 increase by up to 50100% if everything else ( e.g. , generator 
 technology etc ) remained the same . what we have seen in the past decade 
 and a half
is that new plants are mostly ngcc with cooling towers . 
 reduced cooling requirements associated with cc plants should more 
 than compensate for increased consumption intensity associated with 
 cooling towers because the latter only applies to the st part of cc 
 generation and should result in net reduction in water consumption 
 intensity .
future projections of water use are based on electric 
 reliability 
 council of texas ( ercot ) projections of future net generation for 
 texas .
historically , annual energy for 20032012 grew at an 
 average annual growth rate ( aagr ) of 1.5% .
future projections of water use presented here are based on extending 
 the current power plant fleet to 2030 with no retirements except for 
 ngst plants , for which the declining generation rate of 2 
 mwh / year that occurred between 2004 and 2010 is projected forward 
 to result in no ngst generation by 2023 . finally , applying the forecasted 
 aagrs results in 30% total growth between 2010 and 2030 , with all 
 new generation from ngcc plants with wet cooling towers .
based on 
 these assumptions , water consumption in terms of total volume is projected 
 to grow at an average annual rate of 0.2% , resulting in total water 
 consumption by power plants increasing 4.6% by 2030 relative 
 to current levels . however , water consumption intensity is projected 
 to decrease by 19% relative to the overall 2010 rate because 
 of the assumed new power plant type ( ngcc with cooling towers ) .
withdrawal 
 volumes are projected to decline slowly ( average 0.8% annually ) 
 until 2023 , reflecting the ngst retirements , and because the assumed 
 new systems use cooling towers that withdraw at least an order of 
 magnitude less water than ponds , and withdrawal intensity is projected 
 to decrease by 31% overall by 2030 .
these projections of water 
 consumption and withdrawal are considered upper bounds in terms of 
 ercot projections because retirements of current plants ( except ngst 
 plants ) were not considered and new generation was assumed to be ngcc , 
 whereas ercot ( 28 ) estimates 36% from ngct 
 ( no cooling requirement ) and 64% from ngcc in a business as usual 
 ( bau ) projection and much more power from renewables in other scenarios 
 ( bau with updated wind shapes , ngcc 10% , ngct , 19% , solar , 27% , and 
 wind , 45% ) .
however , ercot projections 
 might change markedly if prices for natural gas increase and generation 
 from coal and nuclear sources increase .
in addition , water use would 
 increase if carbon capture is required in the future .
while the analysis in this study 
 focused on texas , the results have broader implications .
the understanding 
 of controls on water consumption and withdrawal can apply to power 
 plants anywhere and should be considered as various regulatory and 
 economic forces shift emphasis to different technologies , allowing 
 impacts on water resources to be considered .
the importance of generator 
 technology controlling water consumption with up to 63% reduction 
 in consumption intensity for ngcc plants relative to traditional sts 
 ( 0.19 vs 0.52 gal / kwh ) is very important , with projected future expansion 
 of ngcc plants in texas and throughout the u.s .
the use of cooling 
 towers instead of once - through cooling ponds or rivers has been decreasing 
 water withdrawal rates and is expected to continue in the future .
this study emphasized the evolution of the power plant fleet in texas 
 and the legacy infrastructure that can not be modified in a short period . 

use of once - through cooling ponds is unlikely to expand in texas in 
 the future as use of ponds essentially leveled off in the early 1990s . 

retirement of existing power plants with once - through cooling ponds 
 may be replaced by plants with similar cooling systems because the 
 power plant operators have the water rights and the ponds are being 
 maintained , as seen in the proposed new ferguson plant on lake lbj . 
 even when power plants are retired and not replaced , the ponds / reservoirs 
 are often maintained for recreational purposes ( e.g. , lake lady bird 
 johnson in austin ) . if replacement plants change generator technology 
 from st to cc along with cooling systems ( cooling towers replacing 
 once - through ponds ) , both water consumption and withdrawal rates will 
 decrease . projected increases in ngcc plants with cooling towers in 
 texas based on ercot forecasts should further reduce water consumption 
 and withdrawal rates in the state .
in general , with recent increases in ng generation ( 120% 
 increase 19962010 ) and increasing use of cooling towers reducing 
 water withdrawals in the u.s .
understanding 
 fundamental controls on water consumption and withdrawal 
 of thermoelectric generation within the context of power plant fleets 
 with long lifespans and legacy infrastructure is an essential prerequisite 
 to assessing impacts of water availability on thermoelectric generation 
 and impacts of thermoelectric generation on water resources .
 large - scale u.s . dependence on thermoelectric 
 ( steam electric ) 
 generation requiring water for cooling underscores the need to understand 
 controls on this water use . the study objective was to quantify water 
 consumption and withdrawal for thermoelectric generation , identifying 
 controls , using texas as a case study . 
 water consumption for thermoelectricity 
 in texas in 2010 totaled 0.43 million acre feet ( maf ; 0.53 
 km3 ) , accounting for 4% of total state water consumption . 
 
 high water withdrawals ( 26.2 maf , 32.3 km3 ) mostly reflect 
 circulation between ponds and power plants , with only two - thirds of 
 this water required for cooling . 
 controls on water consumption include 
 ( 1 ) generator technology / thermal efficiency and ( 2 ) cooling system , 
 resulting in statewide consumption intensity for natural gas combined 
 cycle generators with mostly cooling towers ( 0.19 gal / kwh ) being 63% 
 lower than that of traditional coal , nuclear , or natural gas steam 
 turbine generators with mostly cooling ponds ( 0.52 gal / kwh ) . 
 the primary 
 control on water withdrawals is cooling system , with 2 orders 
 of magnitude lower withdrawals for cooling towers relative to once - through 
 ponds statewide . 
 increases in natural gas combined cycle plants with 
 cooling towers in response to high production of low - cost natural 
 gas has greatly reduced water demand for thermoelectric cooling since 
 2000 . 