Utilities across the nation are worried about revenue losses to distributed generation and energy efficiency, but a new study suggests they might want to look harder at how climate change will affect their grids as well. 

Summertime output losses in the American West from severe weather are now predicted to be 3% at worst, but they could reach as high 8.8% by mid-century because of the effects of climate change, researchers from Arizona State University report. Other factors could magnify those losses, meaning that utilities and grid operators may be "significantly overestimating" their preparedness for increasing electric demand in a warming world.

“Generation capacity reductions, electricity demand spikes, and transmission bottlenecks present a threefold challenge for electricity providers,” explained Arizona State University Researcher Matthew D. Bartos, co-author of the study "Impacts of climate change on electric power supply in the Western United States."

“As extreme heat events occur with greater frequency and duration, these three factors may interact in ways that electric utilities are not currently prepared to deal with,” he said.

The study looks at power delivery over the next 50 years for the 14 Western states served by the Western Electricity Coordinating Council (WECC). The WECC presently supplies some 200 GW of summertime generation.

The study focuses on five types of generation: steam turbine, combustion turbine, hydroelectric, wind turbine and photovoltaic (PV). It estimates how changes in six climate factors would impact the 978 most vulnerable power stations in the system. Those stations represent 46% of existing WECC generation capacity.

The six factors are streamflow — the amount of water flowing through streams for plant cooling and hydropower — water temperature, air temperature, vapor pressure, wind speed, and air density. The study applies them in three climate future scenarios chosen from those studied by the Intergovernmental Panel on Climate Change.

Planning reserve margin

Power providers use planning reserve margin (PRM) to quantify the percentage of electricity supply “left over” after meeting demand. It is considered a good “first-order estimate of electricity supply adequacy,” the study reports. “WECC anticipates a PRM of 18% for the year 2023.”

But because PRM does not account for climate change impacts, that forecast could be “overly optimistic,” the researchers explain. With climate change taken into account, “PRM could be reduced from 18% to 14% during a future ten-year drought event.”

That is not the full extent of the possible under-estimation of capacity. “Generating capacity is expected to reach 273 GW by 2040, with combustion turbines and renewables accounting for the majority of planned additions—41% and 53%, respectively,” the study reports.

If impacts to planned capacity are similar to impacts on existing capacity, the capacity reduction from a ten-year drought event could be 1.8 GW to 2.5 GW.

That means “power providers could be overestimating PRM by as much as 20% to 25%,” the study reports.

“The biggest surprise was that the results are as consistent as they are,” Bartos explained. “Under all the climate models and carbon emissions scenarios we considered, we detected reduced generation capacity.”

Impacts on base-load generation

To model impacts at mid-century, the researchers assumed they would be comparable to impacts to existing facilities.

“Base-load coal, nuclear and gas facilities are expected to retain 85% of their capacity by 2040, and no cumulative retirements are expected for combustion turbine or renewable generation sources,” the study reads.

The capacity calculations were based on peak load conditions “when power systems are likely to experience the greatest strain.”

Steam turbine facilities, which are base-load coal and nuclear plants, were found to be most severely constrained by the streamflow of cooling water, but the ambient air and water temperatures were also important in how effective the cooling water could be.  

Bartos does not foresee the impacts becoming severe enough to make coal and nuclear plants inoperable.

“The effects of climate change on base-load power generation are gradual and incremental,” he explained. Compromised capacity would primarily come during the heat of summer. The rest of the year they would likely be unimpaired.

There have already been instances of base-load power capacity losses because of “extreme heat and drought events,” Bartos noted.

In an August 2006 heat wave, nuclear plants in Illinois and Minnesota had to shut down units because the water to cool them was too hot, he pointed out, and similar curtailment was necessary at a Tennessee nuclear plant in August 2007.

There were also curtailments at nuclear facilities in Europe in the summer of 2003, when 4,000 MW of capacity was shut down for several days. Such events, Bartos added, are likely to become more frequent and severe.

Natural gas, PV, and wind

Both combustion turbines, which typically burn natural gas or biogas, and PV cells lose output as the air temperature rises, according to the study. Hydroelectric power diminishes when streamflow is compromised. Wind turbines’ output is dependent on wind speed and air density.

Combined-cycle natural gas facilities, where combustion turbines operate with steam turbines, are vulnerable to several factors, according to Bartos. A lack of adequate cooling water, elevated cooling water temperatures, or incremental efficiency reductions due to high ambient air temperature and humidity would compromise the steam turbines while a hotter ambient air temperature slows the flow of air through the combustion turbine, diminishing output.

Solar PV power plants would also be impacted by hotter air temperatures that decrease “the open-circuit voltage of the solar cell,” Bartos explained, though “temperature-attributable capacity reductions to solar cells are relatively small.”

Because of this limited impact on PV output, he added, “rooftop solar may help to manage load during hot summer days when electricity demand is high.”

Though wind turbines could lose output to diminished wind speed and air density, Bartos explained, existing climate models do not show those changes becoming severe enough to alter wind speeds significantly. And whatever changes there may be to ambient air temperature and humidity, he added, “the effect on wind turbine generation is negligible.”

Streamflow limits hydroelectric generation and that can be expected to change significantly by mid-century. But it will likely change in both directions. “Historical records show a strong correlation between available streamflow and net power generation at hydroelectric facilities,” Bartos noted. “Hydroelectric power will likely be impaired in the Southwestern US. However, in the Pacific Northwest, hydroelectric potential may actually increase due to predicted increases in precipitation.”

Output losses by the numbers

By mid-century, between 2040 and 2060, climate change could impede “average summertime generating capacity by 1.0 GW to 2.7 GW, the study finds. While there could be as much as a 4% increase in output at some facilities, there could be losses in generation as high as 14%."

California and the desert Southwest would begin to see disruption in electricity delivery, the study adds.

The generation portfolio is the dominant factor in what a state or region can expect in capacity loss, the researchers conclude from a comparison of projected 50-year impacts with two conventional resources and two renewable resources.

Steam turbine and combustion turbine output gets hit worst by climate change, with vulnerable plants losing 1.6% to 3.0% by mid-century.

Combustion turbines, which are likely to be impacted by the gradual and limited change in air temperature, show the most consistent capacity reductions on a year-to-year basis. Their average summertime losses are projected at “1.4% to 3.5% relative to the historical period,” the study reports.

Because streamflow is likely to be seriously impacted by climate change, “base-load steam turbines are more likely to suffer extreme capacity reductions as a result of drought events,” according to the study. Steam turbine plants’ average ten-year summertime capacity reduction “is expected to increase from 2.5% under the historical period to 7.4% to 9.5% by midcentury.”

Production by renewables will be, in comparison, more resilient but more uncertain in the face of climate change. Increased air temperature will reduce PV solar power plant output only 0.7% to 1.7% but it is not possible to reliably quantify changes in solar irradiation.

Decreased humidity from the increased air temperature will likely allow more output from wind turbines but the researchers were also unable to make that estimate reliably.

Because Pacific Northwest increases in hydropower will balance losses in the Southwest, the WECC region’s hydro production is projected to be unchanged. Uncertainties regarding water demand and reservoirs limit that conclusion.

The bottom line for utilities and grid operators

Because power providers do not presently account for climate change impacts, they may be “significantly overestimating” their preparedness for the West’s expected increases in electricity demand from a rapidly growing population seeking relief from severely worsening heat.

As result, the study projects, “the WECC grid will probably be operating closer to the margin for longer periods of time.”

“Over-reliance on traditional thermoelectric generation may result in unforeseen constraints to generating capacity,” the researchers conclude, while “renewables are generally less susceptible to the effects of climate change, meaning that increased adoption of renewables may not only help reduce greenhouse gas emissions—it may also contribute to a more climate-resistant power infrastructure.”

Things that would “climate-proof” the grid include increased transmission, more conservation, more renewables, and climate change-savvy siting of new generation.

If he was a Western state PUC Chair right now, Bartos told Utility Dive, “I would encourage utilities to invest in technologies that help mitigate the effects of extreme heat and drought.”