Laws in several U.S. states mandate zero-carbon electricity systems based primarily on renewable technologies, such as wind and solar. Long-term, largecapacity energy storage, such as those that might be provided by power-to-gasto-power systems, may improve reliability and affordability of systems based on variable non-dispatchable generation. Long-term storage can reduce costs of wind-solar-battery electricity systems at current technology costs by filling seasonal and multi-year storage functional roles. Innovation in long-term storage technology could further improve the affordability of reliable renewable electricity.

Negative emissions technologies will play an important role in preventing 2 °C warming by 2100. The next decade is critical for technological innovation and deployment to meet mid-century carbon removal goals of 10−20 GtCO 2 /yr. Direct air capture (DAC) is positioned to play a critical role in carbon removal, yet remains under paced in deployment efforts, mainly because of high costs. This study outlines a roadmap for DAC cost reductions through the exploitation of low-temperature heat, recent U.S. policy drivers, and logical, regional end-use opportunities in the United States. Specifically, two scenarios are identified that allow for the production of compressed high-purity CO 2 for costs ≤$300/tCO 2 , net delivered with an opportunity to scale to 19 MtCO 2 /yr. These scenarios use thermal energy from geothermal and nuclear power plants to produce steam and transport the purified CO 2 via trucks to the nearest opportunity for direct use or subsurface permanent storage. Although some utilization pathways result in the re-emission of CO 2 and cannot be considered true carbon removal, they would provide economic incentive to deploying DAC plants at scale by mid-century. In addition, the federal tax credit 45Q was applied for qualifying facilities (i.e., producing ≥100 ktCO 2 /yr).

Summary We use 36 years (1980–2015) of hourly weather data over the contiguous United States (CONUS) to assess the impact of low-cost energy storage on highly reliable electricity systems that use only variable renewable energy (VRE; wind and solar photovoltaics). Even assuming perfect transmission of wind and solar generation aggregated over CONUS, energy storage costs would need to decrease several hundred-fold from current costs (to ∼$1/kWh) in fully VRE electricity systems to yield highly reliable electricity without extensive curtailment of VRE generation. The role of energy storage changes from high-cost storage competing with curtailment to fill short-term gaps between VRE generation and hourly demand to near-free storage serving as seasonal storage for VRE resources. Energy storage faces “double penalties” in VRE/storage systems: with increasing capacity, (1) the additional storage is used less frequently and (2) hourly electricity costs would become less volatile, thus reducing price arbitrage opportunities for the additional storage.

Hydrogen solubility in ten transition metals (V, Nb, Ta, W, Ni, Pd, Pt, Cu, Ag, and Au) has been predicted by first-principles based on density functional theory (DFT) combined with chemical potential equilibrium between hydrogen in the gas and solid-solution phases. Binding energies and vibrational frequencies of dissolved hydrogen in metals are obtained from DFT calculations, and the sensitivity of solubility predictions with respect to the DFT-calculated variables has been analyzed. In general, the solubility increases with increasing binding strength and decreasing vibrational frequencies of hydrogen. The solubility predictions match experimental data within a factor of 2 in the cases of V, Nb, Ta, and W and within a factor of 3 in the cases of Ni, Cu, and Ag. In Pd, the deviation in solubility predictions is mainly attributed to the errors involved in the calculated vibrational frequencies of dissolved hydrogen. In Pt and Au, hydrogen in the octahedral interstitial site is less stable than in the tetrahedral site, contradicting the predictions based on the hard-sphere model. Potential energy surface analysis reveals a slightly downward concavity near the center of the octahedral sites in Pt and Au, which may explain the calculated imaginary vibrational frequencies in these sites and lead to unreliable solubility predictions.

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