TODAY’S STUDY: TEXAS SHOWS WHAT DROUGHT DOES TO WATER FOR ENERGY

Impact of Future Climate Variability on ERCOT Thermoelectric Power Generation; In Support of Interconnection-Wide Transmission Planning

Y. Eugene Yan, Vince C. Tidwell, Carey W. King, and Margaret A. Cook, January 2013 (Argonne National Laboratory and the University of Texas at Austin)

Executive Summary

This report summarizes a study to determine the medium‐term (through the year 2030) impacts of future climate and drought scenarios on electricity generation by the Electric Reliability Council of Texas (ERCOT). Because water in reservoirs is used to cool many steam cycle‐based power plants, significantly low water levels can reduce the ability to cool power plants. This reduced cooling ability can come from physical supply limitations or environmental constraints (power plant effluent temperatures exceeding permitted limits.).

The objective of this report is to inform ERCOT as to the potential water‐related risks for power plant operations and possible future water supply (other than historical supply) for electricity generation. The approach projects future climate and water demands to determine stream flows, water storage in reservoirs, and power plant effluent temperatures. The results for historical and future water availability, demand, its cost, reservoir storage, and stream flow are reported for U.S. Geological Survey 8‐digit hydrologic unit code (HUC8) water basins. The water and climate data are compared to power plant characteristics and past performance data to infer the likelihood that future summer power generation could be curtailed at a power plant. Beyond impacts on the existing fleet of power plants, this study also considers siting of future power plants to avoid regions of limited water availability.

The main findings from the study relate to four categories, as outlined below:

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Water Availability

Water is projected to be available for ERCOT thermoelectric power plant operations until 2030. However, water for new development will likely need to come from sources other than unappropriated surface water. This conclusion largely means that future water supplies for thermoelectric power will be more expensive than historical supplies. Specifics are as follows:

•In general, very little unappropriated surface water is available for any use, including thermoelectric power.

• Water availability from appropriated surface water supplies, assumed as “low‐value” agriculture, is limited. This appropriated water is present in quantities > 5,000 ac‐ft/yr in only a few HUC8 basins.

•Several HUC8 basins have wastewater, potable groundwater, and brackish groundwater vailability at greater than 10,000 ac‐ft/yr (enough for a large power plant).

•A number of basins with severely limited water supplies are targeted for siting of new electric power production.

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Water Supply Costs

•The cheapest water supply (at $18/ac‐ft) that has enough water (roughly > 5,000 ac‐ft/yr) to supply wet cooling at a medium to large‐sized thermal power plant is from low‐value agriculture.

•Estimated costs for brackish water availability per HUC8 basin vary widely, from tens to thousands of dollars per acre‐foot, with most in the range of $500‐900/ac‐ft (or about $1.7‐2.7/1,000 gal). This price for water is close to but below some estimates for the cost of water needed to incentivize the use of dry cooling systems at > $3/1,000 gal.

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Potential Derating of Thermoelectric Cooling during Drought due to Lack of Water Supply

The project team constructed a model of the Texas‐Gulf river basin by using the Soil and Water Assessment Tool (SWAT).

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This hydrologic model uses input of meteorological data (e.g., temperature, precipitation, together with water demands (e.g., municipal, agriculture, power plant operation), to estimate evapotranspiration, stream flow, and water storage in soil and reservoirs. The team used the reservoir storage information and two matrices based on water use versus water availability to assess the risk that power plants would not be able to take water into cooling systems. Specifics are as follows:

•Three drought scenarios were evaluated: (1) the recent drought in 2011, with the current level of water use; (2) a single‐year drought in 2022, with the assumed water use level projected for 2030; and (3) a multiyear drought under 1950‐1957 climate condition, with the projected 2030 water use.

• The projected drought scenario in 2022 and the historical droughts in 2011 and 1954‐1956 represent two different precipitation patterns in Texas‐Gulf river basin. The projected 2022 drought is characterized by low precipitation (< 25 in.) in the eastern basin and moderate precipitation (25‐30 in.) in the western basin, while the historical 2011 scenario shows extremely low precipitation (< 20 in.) in western basin and high precipitation (> 30 in.) in only the southeastern basin.

•Hydrologic modeling results indicate significant impact on water availability (water yield, stream flow, and reservoir storage) for single‐year drought (2011 and 2022) and multiple‐year drought (1950‐1957).

•The model predictions for average and minimum monthly reservoir storage during the 2011 drought year were statistically validated, with coefficient of determination R2 = 0.81 and 0.72, respectively, for 22 of 37 reservoirs that provide water supply to 47 power plants.

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•Using criteria based on observed (< 50% storage) and predicted (< 55% storage) reservoir data, we identified 15 low‐storage reservoirs in 2011, 10 in 2022, and 20 in 1956 (the last year for the multiple‐year drought). Among them, 4 reservoirs (Addicks Reservoir, Texana Lake, Martin Lake, and Smithers Lake) are under low‐storage conditions in all three drought scenarios. The affected reservoirs, as predicted by the model, are located near Austin, Houston, and San Antonio, as well as south of Lubbock.

• Reservoir water storage declines gradually over the duration of the multiple‐year drought, suggesting that the reservoirs can mitigate effects of water shortage during short‐term drought but would be less effective in long‐term drought.

•Analysis of available water intake levels for 9 power plants found that all would be able to take in cooling water, in spite of low reservoir water levels, in the three drought scenarios. Such an analysis is recommended for all reservoirs, especially low‐storage reservoirs predicted for the drought scenarios, when water intake level data for other reservoirs with power plants become available.

•The different drought scenarios (2011, 2022, and 1950‐1957) show different drought effects in terms of spatial distribution of water availability and reservoir storage reduction because of variations in the climate pattern.

•Vulnerable HUC8 basins, identified by two matrices on the basis of water use versus water availability in three drought scenarios, need to be evaluated carefully for future power plant siting to avoid the basins with high water demand and limited water availability. The predictions for the 1956 scenario (reflecting cumulative effects of long‐term drought and increased water use for 2030) suggest more vulnerable HUC8 basins near Dallas, Houston, Austin, San Antonio, Brownsville, and Lubbock than do the predictions for the other scenarios.

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Potential Derating of Thermoelectric Cooling during Drought due to Effluent Discharge Temperature Limits

The assessment of ERCOT thermal power plant operations above Environmental Protection Agency (EPA) limits for effluent discharge temperatures indicated that a few power plants and a significant quantity of generation capacity have operated at or near these temperature limits in the past. In addition, due to anticipated warming from climate change (the major factor affecting effluent temperatures), ambient temperature can be expected to result in future derating potential (near 1,000,000 MW‐h per summer month) being limited by cooling water effluent temperatures. However, although some power plants are projected to be exposed to curtailment because of these EPA temperature limits, we estimate that six times more electricity generation potential (roughly 6,000,000 MW‐h per summer month) is available from other existing generators where power plants will not reach thermal effluent temperature limits. Specifics are as follows:

•The regression models derived for this study reasonably model average monthly effluent temperatures for most of the open‐loop and recirculating cooling pond systems in ERCOT.

•The data on effluent water thermal discharges from power plants reveal that at least 2 power plants (Martin Lake, Coleto Creek) operated above their average temperature effluent discharge limits in 2007‐2011.

•By 2030, up to 6 power plants could have effluent discharge thermally limiting their generation at roughly 20,000‐200,000 MW‐h per month, if they attempt to operate at 2011 capacity factors.

•By 2030, up to 13 power plants could have effluent discharge thermally limiting their generation at about 1,000,000 MW‐h per month, if they attempt to operate at 100% summer capacity factors.

•Approximately 6,000,000 MW‐h of electricity is available (up to 100% capacity factor in summer months) from thermal generators that would not be limited by effluent temperature limits.

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Conclusions

Water Availability

Water is projected to be available for ERCOT thermoelectric power plant operations out to 2030. However, water for new development will probably need to come from sources other than unappropriated surface water. This conclusion largely means that future water supplies for thermoelectric power will be more expensive than historical supplies, for the following reasons:

•In general, very little unappropriated surface water is available for any use, including thermoelectric power.

• Water availability from appropriated water supplies, specifically “low‐value” agriculture, is present at >5,000 ac‐ft/yr, but only in a few HUC8 basins.

• Several HUC8 basins have wastewater, potable groundwater, and brackish groundwater availability at over 10,000 ac‐ft/yr (enough for a large power plant).

•A number of basins (14) with severely limited water supplies are targeted for siting of new electric power production.

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Water Supply Costs

•The cheapest water supply (at $18/ac‐ft) that has enough water (roughly >5,000 ac‐ft/yr) to supply wet cooling at a medium to large‐sized thermal power plant is from low‐value agriculture.

•Estimated costs for brackish water availability per HUC8 basin vary widely, from tens to more than thousands of dollars per acre‐foot, with most in the range of $500‐900/ac‐ft (or about $1.7‐2.7/1,000 gal). This price for water is close to but below some estimates for the cost of water needed to incentivize the use of dry cooling systems at > $3/1,000 gal.

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Potential Derating of Thermoelectric Cooling during Drought due to Lack of Water Supply

•Hydrologic modeling results indicate significant impact on water availability (water yield, stream flow, and reservoir storage) in single‐year drought (2011 and 2022) and multiple‐year drought (1950‐1957) scenarios.

•The model predictions for average and minimum monthly reservoir storage during the 2011 drought year were validated with R2 = 0.81 and 0.72, respectively, for 22 reservoirs out of 37 that provide water supply to 47 power plants.

•With a criterion based on observed and predicted reservoir data, we identified 15 low‐storage reservoirs in 2011, 10 in 2022, and 20 in 1956 (the last year for the multiple‐year drought scenario). Among them, 4 reservoirs (Addicks Reservoir, Texana Lake, Martin Lake, and Smithers Lake) would be under the low‐storage condition in all three drought scenarios.

•Reservoir water storage declined gradually over the period of the multiple‐year drought duration, suggesting that the reservoirs would be more vulnerable in a long‐term drought than in a single‐year drought.

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•Analysis of available water intake levels found that none of the 9 power plants considered would be unable to take in cooling water because of low reservoir water levels in the three drought scenarios. Such an analysis is recommended for all reservoirs, especially low‐storage reservoirs as predicted for the drought scenarios, if water intake level data for other reservoirs with power plants become available.

•The predicted low‐storage reservoirs in 2011, 2022, and 1956 would potentially affect 18, 11, and 26 power plants, respectively. The total generation capacity for those power plants is 16,898 MW in 2011, 10,169 MW in 2022, and 21,734 MW in 1956.

•The different drought scenarios (such as 2011, 2022, and 1950‐1957) have different drought effects in terms of spatial distribution of water availability and reservoir storage reduction, because of variations in the climate pattern.

•Vulnerable HUC8 basins, identified by two matrices based on water use versus water availability in three drought scenarios, need to be evaluated carefully for future power plant siting to avoid basins with high water demand and limited water availability. The predictions for the 1956 scenario suggest more vulnerable HUC8 basins near Dallas, Houston, Austin, San Antonio, Brownsville, and Lubbock than do other scenarios.

•Future work on potential derating due to low reservoir storage should involve analysis for the individual reservoirs with low storage, as predicted in this study for the three drought scenarios. A full analysis of water intake level at each reservoir should be performed for all power plants if the intake level data become available. This information can be used to quantify the direct impact of drought on electricity generation.

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Potential Derating of Thermoelectric Cooling during Drought due to Effluent Discharge Temperature Limits

The assessment of ERCOT thermal power plants potentially operating above EPA limits for temperature of effluent discharges indicated that a few power plants and a significant quantity of generation capacity have operated at or near these temperature limits in the past. In addition, because of expected warming from climate change, the major factor affecting effluent temperatures — ambient temperature — can be expected to result in even greater future derating potential (near 1,000,000 MW‐h per summer month) under limitations by cooling water effluent temperatures. However, while some power plants are projected to be exposed to curtailment because of these EPA temperature limits, we estimate that six times more electricity generation potential (~6,000,000 MW‐h per summer month) can occur from other existing generators where power plants will not reach thermal effluent temperature limits. The specific findings are as follows:

•The regression models derived for this study reasonably simulated average monthly effluent temperatures for most of the open‐loop and recirculating cooling pond systems in ERCOT.

•The data on effluent water thermal discharges from power plants revealed that at least two power plants (Martin Lake, Coleto Creek) operated above their average temperature effluent discharge limits in 2007‐2011.

•By 2030, up to 6 power plants could have effluent discharge thermally limiting their generation at ~20,000‐200,000 MW‐h per month if they attempt to operate at 2011 capacity factors.

•By 2030, up to 13 power plants could have effluent discharge thermally limiting their generation at ~1,000,000 MW‐h per month if they attempt to operate at 100% summer capacity factors.

•Approximately 6,000,000 MW‐h of electricity is available (up to 100% capacity factor in summer months) from thermal generators that would not be limited by effluent temperature limits.

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