Charles J. Barnhart

On the importance of reducing the energetic and material demands of electrical energy storage

Two prominent low-carbon energy resources, wind and sunlight, depend on weather. As the percentage of electricity supply from these sources increases, grid operators will need to employ strategies and technologies, including energy storage, to balance supply with demand. We quantify energy and material resource requirements for currently available energy storage technologies: lithium ion (Li-ion), sodium sulfur (NaS) and lead-acid (PbA) batteries; vanadium redox (VRB) and zinc-bromine (ZnBr) flow batteries; and geologic pumped hydroelectric storage (PHS) and compressed air energy storage (CAES). By introducing new concepts, including energy stored on invested (ESOI), we map research avenues that could expedite the development and deployment of grid-scale energy storage. ESOI incorporates several storage attributes instead of isolated properties, like efficiency or energy density. Calculations indicate that electrochemical storage technologies will impinge on global energy supplies for scale up — PHS and CAES are less energy intensive by 100 fold. Using ESOI we show that an increase in electrochemical storage cycle life by tenfold would greatly relax energetic constraints for grid-storage and improve cost competitiveness. We find that annual material resource production places tight limits on Li-ion, VRB and PHS development and loose limits on NaS and CAES. This analysis indicates that energy storage could provide some grid flexibility but its build up will require decades. Reducing financial cost is not sufficient for creating a scalable energy storage infrastructure. Most importantly, for grid integrated storage, cycle life must be improved to improve the scalability of battery technologies. As a result of the constraints on energy storage described here, increasing grid flexibility as the penetration of renewable power generation increases will require employing several additional techniques including demand-side management, flexible generation from base-load facilities and natural gas firming.

The energetic implications of curtailing versus storing solar- and wind-generated electricity

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 (ESOIe) 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 ESOIe > 80. We identify improvements in cycle life as the most feasible way to increase battery ESOIe. Depending upon the batterys 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.

Hydrogen or batteries for grid storage? A net energy analysis

Energy storage is a promising approach to address the challenge of intermittent generation from renewables on the electric grid. In this work, we evaluate energy storage with a regenerative hydrogen fuel cell (RHFC) using net energy analysis. We examine the most widely installed RHFC configuration, containing an alkaline water electrolyzer and a PEM fuel cell. To compare RHFCs to other storage technologies, we use two energy return ratios: the electrical energy stored on invested (ESOIe) ratio (the ratio of electrical energy returned by the device over its lifetime to the electrical-equivalent energy required to build the device) and the overall energy efficiency (the ratio of electrical energy returned by the device over its lifetime to total lifetime electrical-equivalent energy input into the system). In our reference scenario, the RHFC system has an ESOIe ratio of 59, more favorable than the best battery technology available today (Li-ion, ESOIe = 35). (In the reference scenario RHFC, the alkaline electrolyzer is 70% efficient and has a stack lifetime of 100 000 h; the PEM fuel cell is 47% efficient and has a stack lifetime of 10 000 h; and the round-trip efficiency is 30%.) The ESOIe ratio of storage in hydrogen exceeds that of batteries because of the low energy cost of the materials required to store compressed hydrogen, and the high energy cost of the materials required to store electric charge in a battery. However, the low round-trip efficiency of a RHFC energy storage system results in very high energy costs during operation, and a much lower overall energy efficiency than lithium ion batteries (0.30 for RHFC, vs. 0.83 for lithium ion batteries). RHFCs represent an attractive investment of manufacturing energy to provide storage. On the other hand, their round-trip efficiency must improve dramatically before they can offer the same overall energy efficiency as batteries, which have round-trip efficiencies of 75–90%. One application of energy storage that illustrates the tradeoff between these different aspects of energy performance is capturing overgeneration (spilled power) for later use during times of peak output from renewables. We quantify the relative energetic benefit of adding different types of energy storage to a renewable generating facility using [EROI]grid. Even with 30% round-trip efficiency, RHFC storage achieves the same [EROI]grid as batteries when storing overgeneration from wind turbines, because its high ESOIe ratio and the high EROI of wind generation offset the low round-trip efficiency.

Can we afford storage? A dynamic net energy analysis of renewable electricity generation supported by energy storage

Global wind power and photovoltaic (PV) installed capacities are growing at very high rates (20% per year and 60% per year, respectively). These technologies require large, ‘up-front’ energetic investments. Conceptually, as these industries grow, some proportion of their electrical output is ‘re-invested’ to support manufacture and deployment of new generation capacity. As variable and intermittent, renewable generation capacity increases grid penetration, electrical energy storage will become an ever more important load-balancing technology. These storage technologies are currently expensive and energy intensive to deploy. We explore the impact on net energy production when wind and PV must ‘pay’ the energetic cost of storage deployment. We present the net energy trajectory of these two industries (wind and PV), disaggregated into eight distinct technologies—wind: on-shore and off-shore; PV: single-crystal (sc-), multi-crystalline (mc-), amorphous (a-) and ribbon silicon (Si), cadmium telluride (CdTe), and copper indium gallium (di)selenide (CIGS). The results show that both on-shore and off-shore wind can support the deployment of a very large amount of storage, over 300 hours of geologic storage in the case of on-shore wind. On the other hand, solar PV, which is already energetically expensive compared to wind power, can only ‘afford’ about 24 hours of storage before the industry operates at an energy deficit. The analysis highlights the societal benefits of electricity generation–storage combinations with low energetic costs.

Energy Return on Investment (EROI) of Solar PV: An Attempt at Reconciliation [Point of View]

Examines the importance of energy return on investment (EROI) as a useful metric for assessing long-term viability of energy-dependent systems. Here, focuses on the methods, applications, and analyses for determining EROI for solar power and solar energy technologies.

Energy and Carbon Intensities of Stored Solar Photovoltaic Energy

Abstract This chapter shows how storage affects the energy performance and carbon intensity of solar photovoltaic (PV) generated electricity paired with electrical energy storage technologies. These results show that it is more energetically favorable to store solar PV energy than it is to curtail electricity production. Electrochemically based storage technologies, while higher in energy density, result in lower (worse) energy return ratios than large-scale geologically based storage technologies such as compressed air energy storage and pumped hydroelectric storage. Carbon performance of all PV-storage pairings considered here is better than the average US power grid. The lowest carbon storage technologies are pumped hydro, vanadium redox, and lithium-ion. On an energetic basis, stored-PV electricity is more intensive than the average US power grid. Reducing embodied energy costs, increasing efficiency, and increasing depth of discharge will improve the energetic and carbon performance of batteries. On an energetic and carbon performance basis, solar PV energy paired with storage performs trades greatly reduced carbon intensity for increased energy intensity when compared with the US power grid average.

Energy and Carbon Intensities of Stored Wind Energy

This chapter shows how storage affects the energy performance and carbon intensity of wind generated electricity pair with electrical energy storage (EES) technologies. These results identify conditions under which it is more energetically favorable to store wind energy than it is to simply curtail electricity production. Electrochemically based storage technologies results in much lower (worse) energy return ratios than large-scale geologically based storage technologies like compressed air energy storage (CAES) and pumped hydroelectric storage (PHS). Due to their low energy throughput on energy invested ratios, conventional battery technologies decrease the energy return ratios of wind generation below curtailment ratios. Carbon performance of all wind storage pairings considered here is better than the average US power grid. The lowest carbon storage technologies are pumped hydro, vanadium redox, and lithium-ion. Reducing embodied energy costs, increasing efficiency and increasing depth of discharge will improve the energetic and carbon performance of batteries. On an energetic and carbon performance basis, wind energy paired with storage performs better than the US power grid average.