1、1 4677 Consumptive Water Use for U.S. Power Production Paul A. Torcellini, Ph.D., P.E. Member ASHRAE Associate Member ASHRAE Nicholas Long Ronald D. Judkoff Member ASHRAE ABSTRACT Evaporative cooling systems have been criticized for their water use and acclaimed for their low energy consumption, esp
2、ecially when compared to typical cooling systems. In order to determine the overall eflectiveness of cooling systems, both water and energy need to be considered; however, data are needed to compare the amount of energy used at the site to the amount of water used at the power plant. A study of powe
3、r plants and their water consumption was completed to efec- tively analyze evaporative cooling systems. In thermoelectric plants, 0.47 gallons (1.8 L) offiesh water is evaporated per kilowatt-hour Wh) of electricity consumed at the point of end-use. Hydroelectric plants evap- orate an average of 18
4、gallons (68 L) of fresh waterper kWh used by the consumer. The national weighted average is 2.0 gallons (7.6 L) of evaporated water per kWh of electricity consumed at the end-use. From this information, different types of building cooling systems can be comparedfor relative water consumption. INTROD
5、UCTION Eighty-nine percent of electricity production in the U.S. is created by thermoelectric power plants that use thermally driven, water-cooled energy conversion cycles. The evapora- tive or consumptive use is approximately 2.5% of the total water withdrawals or 3,310 million gallons per day (12,
6、530 x 1 O6 Wday). Moreover, hydroelectric plants produce approxi- mately nine percent of the nations electricity. Evaporative water loss from the reservoir surfaces also results in water being evaporated for electrical production. For large HVAC applications, evaporative cooling systems are normally
7、 used in most regions of the U.S. due to their lower capital and operating costs compared to non-evap- orative systems. In addition, direct and indirect evaporative systems are used for directly cooling buildings. However, in hot, dry desert regions, non-evaporative systems are some- times used in o
8、rder to preserve the finite supply of available water. There is a trade-off between water consumption and power consumption used at the site. Direct expansion (DX) systems deceptively consume no water to produce cooling, but they use more electricity than evaporative cooling systems. In many chiller
9、 systems, cooling towers are added to increase the efficiency of heat removal from the condenser, thereby increasing energy efficiency. The water consumption at the power plant and the building must be studied and documented to evaluate the overall water efficiency of different types of building coo
10、ling systems. This paper focuses on water consumption at power plants to provide the data needed to make accurate comparisons between water uses of building cooling systems. This paper does not answer the question of which system consumes more water but merely provides the metric for determining the
11、 amount of water used at the power plant when the amount of energy consumed at the site is known. Subsequent analysis will be completed to determine the water effectiveness of cooling systems. All values reported are for fresh water, which includes lakes, rivers, ponds, and domestic water. A search
12、of water use for thermal and hydroelectric systems was performed. The end result is an aggregated U.S. total of water evaporated by power plants per kWh of energy consumed by the end user (site energy). The analysis accounts for water evaporation at the power plant and is adjusted to incorporate tra
13、nsmission and distribution losses. Hydroelec- tric systems were also evaluated based on evaporative losses Paul Torcellini is a senior engineer, Nicholas Long is a staff engineer, and Ron Judkoff is director at the Center for Buildings and Thermal Systems, National Renewable Energy Laboratory, Golde
14、n, Colo. 96 02004 ASHRAE. from the reservoir per kWh of energy consumed by the end user. The results apply to the location of the production of the electricity, not the location of use. Only aggregated totals are presented because it is currently not possible to “tag” electrons from production to co
15、nsumption, due to the nature of power distribution. The total amount of water evaporated at first seems insignificant compared to the total amount of water passing through the power plant. These values become much more significant when compared to the amount of energy and water consumed in a typical
16、 commercial building or residen- tial home. It is then important to consider the energy-water interface when designing for building cooling systems. WATER CONSUMPTION FROM POWER GENERATION Approximately 89% of the energy produced in the U.S. is generated by thermoelectric systems that evaporate wate
17、r during the cooling of the condenser water (EIA 1999). Hydro- electric plants evaporate water off the surface of the reservoirs and represent approximately 9% of the total power generated in the U.S. Thermoelectric Power Plants In a typical thermoelectric power plant (powered by fossil fuel, nuclea
18、r, or geothermal energy), cooling water removes heat from the cycle with a condenser. Energy from the cooling water is discharged either to a water body or through cooling towers to the atmosphere. The total power plant water with- drawals data used for the analysis described in this paper were foun
19、d through the United States Geological Survey (USGS) (Solley et al. 1998). The USGS calculated the consumptive use of water (amount of water evaporated, transpired, or incorporated into products) for thermoelectric power plants. According to the USGS, these values were calculated by multiplying the
20、water withdrawals by a coefficient of water loss, approximated for each cooling design. If the cooling water was recycled through cooling towers or cooling ponds, the consumptive use was high. Conversely, if the water was used once from a nearby river, then returned to the river, the evaporation at
21、the site was low, but the added heat to the river increased the evaporation rate of the river, thus increasing the overall evaporation. According to the USGS, the total amount of fresh water used at US. thermoelectric power plants in 1995 was 132,000 million gallons (500 x 1 O9 liters) per day (Mgal
22、/day), of which 2.5%, or 3,310 Mgal/day (12.5 x lo9 literdday), was evapo- rated. Mining Water The amount of water that is used for the mining and processing of the fossil fuels that are sent to the power plants also needs to be considered for an accurate analysis. Unfortu- nately, the data availabl
23、e for mining water use are for all types of mining, including coal and ore. This analysis did not attempt to break down the percentages of water that each mining process used. Hydroelectric Power Plants Reservoirs and dams are built for many different reasons, including electric power production, fl
24、ood control, water stor- age, and recreation. Most dams currently provide more than one of the mentioned functions. The discussion of hydroelec- tric dams brings up many difficult issues related to the value of the dam and the values of different individuals. This paper does not make statements or j
25、udgments regarding the ecolog- ical impacts or the human value of the dams but merely provides the amount of water evaporated off the reservoirs as a function of the amount of energy produced. There is no easy way to disaggregate on a national level the end uses for hydro- electric dam water into ir
26、rigation, flood control, municipal water, and thermoelectric power plant cooling. Development of hydroelectric facilities was integral to providing reliable power in the US. and reliable water supplies over the last century. Because of the reliable water supplies, thermoelectric power plant developm
27、ent could occur. These plants not only consume water but also need the consistent flow of cooling water. The analysis will assume that consumptive use of water in hydroelectric facilities should be considered, but the values reported contain aggregate totals with and without hydroelec- tric water us
28、e to allow for individual interpretations. Also, the data are broken up into the different geographic regions to allow for analysis and interpretation of hydroelectric power water use on a regional basis. Water flowing through the turbines and into the river is not considered consumptive because it
29、is still immediately available for other uses. However, the increased surface areas of the reservoir, when compared to the free flowing stream, result in additional water evaporation from the surface. A free water surface evaporation map was used to calculate the amount of water evaporated off the r
30、eservoirs (Farnsworth et al. 1982). The map was used to approximate the average evap- oration per year by location in the U.S. Based on the latitude and longitude of the dam given by the Army Corp of Engineers (ACE), it was possible to approximate the amount of water evaporated by estimating the ave
31、rage value of the isopleths covering the reservoir (ACE 2001). Isopleths are lines of constant yearly evaporation rates that are drawn on maps to represent the third dimension. The surface areas of the reser- voirs were measured in acres at a normal height, as defined by the National Inventory of Da
32、ms (ACE 2001). With this infor- mation, the volume of water evaporated can be calculated from each reservoir. This analysis was completed on a collection of hydroelec- tric dams, most of which produced more than 1 TWhyr (10l2 Whyr) or the 120 largest hydroelectric facilities in the U.S. These hydroe
33、lectric facilities represent approximately 65% of the total electricity produced by hydroelectricity in 1999. There are approximately 2,300 hydroelectric dams currently in the U.S. (Corso 1998). Using the analysis above, it was esti- ASHRAE Transactions: Research 97 mated that the analyzed reservoir
34、s used for hydroelectric evap- orate an average of 9,063 Mgallday (34.3 x lo9 Llday). Compared with the river without the reservoir, the increase in evaporation is significant. To compute evaporation from a river, the length of the river was approximated as the present length of the reservoir. The a
35、verage width of the river was estimated, as well as the winding of the river. The evap- oration rate was assumed to be the same as the free-water surface evaporation rate. The analysis was done for Glen Canyon Dam (Lake Powell) in UtaWArizona and Hoover Dam (Lake Mead) in NevadaArizona, both located
36、 in high evapo- ration areas. For the two dams, the evaporation from “the river” was only 3.2% of the reservoir that replaced it. This value was considered negligible compared to the total amount consumed and was not included in the overall numbers or calculated for other dams. Power Provider Wester
37、n Interconnect Eastern Interconnect NET POWER PRODUCTION The metric being determined is the amount of water consumed per energy unit at the point of end-use. The Energy Information Administration (EIA) tabulates the amount of power generated in the U.S. (EIA 1996). Thermoelectric power plants typica
38、lly use approximately 5% of their gross generation to power equipment. This power is used to crush coal, transport coal, and as excitation for generators and other machinery within the plant. The EIA estimates the transmis- sion and distribution losses for the U.S. as 9% of the gross generation. Fig
39、ure 1 details the power flow from the thermo- electric power plant to the site. The transmission and distribution losses for hydroelectric power plants must also be considered. The calculation was slightly different from the thermoelectric power plants Gallons Evaporated per kWh at Gallons Evaporate
40、d per kWh at Thermoelectric Plants Hydroelectric Plants per kWh of Site Energy 0.38 (1.4 L) 12.4 (47.0 L) 4.42 (16.7 L) 0.49 (1.9 L) 55.1 (208.5 L) 2.33 (8.8 L) Weighted Gallons Evaporated Power Plant Use Transmission/Distribution -5% of Gross Losses, -9% of Gross f, because hydrofacilities use litt
41、le internal energy to power their machinery. As a result, another assumption was made, stating that the gross generation was approximately equal to the net generation in a hydroelectric power plant. WATER CONSUMPTION AND POWER GENERATION Using the information above, it was possible to calculate the
42、amount of water consumed by electricity production for each kWh of end-use energy for the entire U.S. The metric was calculated by taking the total consumptive water use divided by the total energy output. The values were broken down into three categories: thermoelectric, hydroelectric, and a combin
43、ed aggregate (Table 1). Also, the values were broken down into three regions in the U.S. based on the three main electrical grid interconnects: western, eastern, and Texas. The assumption was made that the regions did not import or export power. The initial interest was a U.S. aggregated average; ho
44、wever, it was possible to break down the values per state, with the following assumption that states did not import or export power-a poor assumption, but one typically used when reporting other power generation numbers. The state values were calculated and reported, as listed in Table 2. Note that
45、the hydroelectric power production reported in the table is not the net production for the state over the year. The values reported are only for the analyzed hydroelectric dams. SUMMARY AND DISCUSSION The U.S. uses several different methods to produce power, many of which evaporate water. The number
46、 of evap- orative power plants significantly outweighs the number of non-evaporative power plants; therefore, it is important to consider water use at power plants when concerned about water conservation. Nonetheless, a detailed search of consumptive water use for thermal and hydroelectric systems w
47、as performed and evaluated. For thermoelectric plants, the analysis accounts for water evaporation at the power plant. All power numbers were adjusted to incorporate transmission and distribution losses so the values related to the end use. The final result for typical thermoelectric power plants wa
48、s 0.47 gal (1.8 L) of fresh water Figure 1 Thermoelectric power flow diagram detailing where power was consumed and lost before reaching the consumer Table 1. Total Consumptive Use of Water for United States Power Plants 98 ASHRAE Transactions: Research Table 2. State Water Consumption per kWh of En
49、ergy Consumed for the U.S. Thermoelectric Hydroelectric* Thermoelectric Hydroelectric Weighted Total Site Power Site Power Site Water Site Water Site Water State million kWh/Year million kWh/Year GailonslkWh GalionskWh GallonsIkWh Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware D.C. Florida Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma 81,708 3,611 62,55 1 35,8
