Storing electrical energy in hydrogen requires less manufacturing energy than batteries, per unit of energy dispatched over the system's lifetime.

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[1] We use a computer landform evolution model to show that Noachian-Hesperian-aged, late-stage valley network formation required numerous and repeated moderate flood events rather than one or a few continuous, multiyear, deluge-style flows. We introduce a technique that generates an estimated ''initial conditions'' digital elevation model (DEM) of the Parana Valles drainage catchment (PDC) prior to valley network incision. We then explored how variations in three classes of environmental parameters related to fluvial processes, and surface material properties evolve the initial conditions DEM. Specifically, we parameterized discharge scaling, evaporation from ponded water, and the effects of an indurated surface crust. Each simulation run produced a model output DEM that was qualitatively and statistically compared to the actual surface DEM. Simulations with an arid to semiarid climate, moderate evaporation rates, and an indurated surface crust provide the best match to the actual surface. Simulated valley network formation requires periods of fluvial activity that last a minimum of 10 3 -10 4 years under constant delugestyle conditions. However, craters within the PDC in deluge-style simulations overflow and generate exit breaches that cut through all crater walls. Longer simulations (10 5 -10 6 years) that modeled repeated, episodic flows with interim evaporation avoid universal crater breaching. The paucity of crater rim exit breaches in the PDC and the southern highlands in general implies both that the precipitation was not continuous and that formation conditions were inconsistent with a few short-lived extreme climate excursions such as might be induced by large-scale impacts or other cataclysmic events.

We present a theoretical framework to calculate how storage affects the energy return on energy investment (EROI) ratios of wind and solar resources. Our methods identify conditions under which it is more energetically favorable to store energy than it is to simply curtail electricity production. Electrochemically based storage technologies result in much smaller EROI ratios than large-scale geologically based storage technologies like compressed air energy storage (CAES) and pumped hydroelectric storage (PHS). All storage technologies paired with solar photovoltaic (PV) generation yield EROI ratios that are greater than curtailment. Due to their low energy stored on electrical energy invested (ESOI e) ratios, conventional battery technologies reduce the EROI ratios of wind generation below curtailment EROI ratios. To yield a greater net energy return than curtailment, battery storage technologies paired with wind generation need an ESOI e > 80. We identify improvements in cycle life as the most feasible way to increase battery ESOI e. Depending upon the battery's embodied energy requirement, an increase of cycle life to 10 000-18 000 (2-20 times present values) is required for pairing with wind (assuming liberal round-trip efficiency [90%] and liberal depth-of-discharge [80%] values). Reducing embodied energy costs, increasing efficiency and increasing depth of discharge will also further improve the energetic performance of batteries. While this paper focuses on only one benefit of energy storage, the value of not curtailing electricity generation during periods of excess production, similar analyses could be used to draw conclusions about other benefits as well. Broader context Rapid deployment of power generation technologies harnessing wind and solar resources continues to reduce the carbon intensity of the power grid. But as these technologies comprise a larger fraction of power supply, their variable nature poses challenges to power grid operation. Today, during times of power oversupply or unfavorable market conditions, power grid operators curtail these resources. Rates of curtailment are expected to increase with increased renewable electricity production. That is unless technologies are implemented that can provide grid exibility to balance power supply with power demand. Curtailment is an obvious forfeiture of energy and it increases the lifetime cost of electricity from curtailed generators. What are less obvious are the energetic costs for technologies that provide grid exibility. In this study we employ net energy analysis to compare the energetic cost of wind and solar generation curtailed at various rates to the energetic cost of those generators paired with storage. We nd that energetic cost depends on the generation technology, the storage technology, and the rate of curtailment. In some cases it is energetically favorable to store excess electricity. In other cases, it is favorable to curtail these resources. Our goal is to stimulate the identication of new and optimum uses for excess rene...

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