Table 3 The baseline scenario and 38 alternate scenarios grouped in 5 sets

Baseline model of the Western Electricity Coordinating Council (WECC)

The baseline scenario models a 2050 zero-emissions future. It uses NREL’s 2020 Annual Technology Baseline (ATB)⁵⁶ cost projections for 2050 and a high-electrification high-energy-efficiency demand scenario. Storage power and energy capacity costs are 19.58 $/kW and 22.43 $/kWh, respectively, with O&M costs of 6.10 $/kW-year. These costs represent a scenario where NREL’s lithium-ion 2020 storage costs drop by 90% by 2030 per the U.S. Department of Energy’s (DOE) Energy Storage Grand Challenge, and then drop further between 2030 and 2050 at the same rate as the moderate projections in NREL’s 2020 ATB.

Set A: Varying wind-vs-solar capacity shares

Set A compares the baseline to 8 scenarios where an extra constraint fixes the ratio of total wind capacity to total solar capacity in the WECC. The ratio constraint has a capacity expansion perspective, it does not force early retirements to achieve the desired ratio. This enables the study of a WECC with wind-vs-solar shares varying from 91% solar and 9% wind to 40% solar and 60% wind. For reference, the baseline scenario’s wind-vs-solar share is 81% solar and 19% wind. This set is of interest since wind and solar are projected to be dominant technologies in a zero-emissions WECC²⁷ but their expected relative share of the generation mix might change as technology costs and system characteristics evolve.

Set B: Reduced hydropower generation

Set B compares the baseline to 5 scenarios where hydropower generation is limited in the WECC by derating monthly average water flows at all hydropower plants equally. Water flows are derated by anywhere from 15% to 100% depending on the scenario. This set is of interest since hydropower generation patterns are changing under the effects of climate change^33,34,35 and we wish to understand how these changes, and more extreme ones, might impact long-duration energy storage (LDES). This set with decreases in hydropower generation can also be considered more generally representative of a future where additional LDES may need to be built to compensate for a loss of flexible, zero-emissions resources.

Set C: Varying transmission expansion costs

Set C compares the baseline to two different transmission expansion scenarios. The first scenario represents a grid with limited transmission expansion (by increasing the cost of expanding transmission lines tenfold). The second scenario represents a grid without transmission congestion due to unlimited transmission capacity (by setting the cost of expanding transmission lines to zero). This second scenario is not a “copperplate” scenario since transmission losses still occur. Both scenarios represent opposite extremes and therefore provide bounds on the behavior of future grids. These bounds are particularly useful since transmission expansion is difficult to model due to its dependence on political and social factors that are not captured in a purely cost-based model.

Set D: Varying storage energy capacity costs

Set D compares the baseline where storage energy capacity costs are 22.43 $/kWh to 10 scenarios where storage energy capacity costs range from 0.5 to 102 $/kWh. The upper bound of 102 $/kWh corresponds to NREL’s 2021 ATB moderate scenario cost projections for utility-scale battery in 2050⁴. This set is of interest since storage energy capacity costs are one of the greatest determinants of LDES deployment²⁵ and may vary significantly depending on the development of various LDES technologies.

Set E: WECC under different LDES mandates

Set E compares the baseline that has a total of 1.94 TWh of storage energy capacity in the WECC to 13 scenarios where an extra constraint increases the total WECC storage energy capacity to anywhere from 2 to 64 TWh. This constraint represents a WECC-wide energy storage mandate. Using this region-agnostic approach to study storage mandates, policy makers can evaluate the impact of mandates on electricity pricing and grid behavior.