A System-Level Approach to Evaluating Grid Storage Technologies: Net Energy Analysis of Hydrogen and Batteries for Smoothing Intermittent Renewables
Authors: Pellow Matthew , Emmott Christopher , Barnhart Charles J. , Benson Sally 
Published: February 2020

Integrating variable, low-emissions electricity generation into the electric grid is one of the outstanding grand challenges in developing a sustainable energy infrastructure. Energy storage has an important role in integrating variable generation into the grid at large scale.  Building this energy storage capacity has an energy cost, as with any manufacturing process. However, different energy storage technologies have different energy costs for manufacturing the same kilowatt-hour of storage capacity. These life cycle energy costs are quantified in net energy analysis, a systematic life cycle approach. Using energy more efficiently to provide the services we need — including energy services such as grid storage — is another grand challenge . Net energy analysis of storage technologies therefore sits at the nexus of these two grand challenges in energy.

In this work, we first present a framework to determine the energy cost of a regenerative hydrogen fuel cell (RHFC) for storing electrical energy in hydrogen. We then evaluate the overall energy efficiency of using a RHFC to store intermittent (wind and solar) electricity, and compare to earlier results for various battery technologies. Although the round-trip efficiency of RHFC electricity storage (30%) that is less than half that of batteries, less energy is required to manufacture the RHFC system than a battery. The RHFC's lower manufacturing energy requirement offsets its larger efficiency losses as compared to batteries.

As a result, the RHFC system has a life-cycle energy efficiency as high as the most efficient batteries when storing wind power -- and considerably better than lower cost lead-acid batteries -- when considering both manufacturing and round-trip efficiency.  The overall energy return on investment (EROI) of a storage-equipped wind farm using hydrogen storage would be significantly higher than one that uses lead-acid batteries (widely deployed for grid storage), and similar to lithium-ion batteries. However, the RHFC system is, overall, less efficienct than even the least efficient batteries when storing solar power.  Its low round-trip efficiency significantly degrades the overall energy performance of the facility.  In this application, battery technologies with higher round-trip efficiencies, such as lithium-ion batteries, are much more attractive.  

To compare RHFC's to other storage technologies, we use the energy stored on invested (ESOIe) ratio: the ratio of energy stored in the device over its lifetime to the energy required to build and operate the device.  A storage system with a higher ESOIe ratio provides at a lower cost of manufacturing energy, leaving the difference available for other productive uses. We considered a RHFC system that pairs an alkaline water electrolyzer with a PEM fuel cell. This hypothetical RHFC system has a more favorable ESOIe than the best battery technology (Li-ion, ESOIe = 35; Energy Env. Sci., 2015, 8, 1938), though still lower than those of pumped hydro (ESOIe = 1100) and compressed air (ESOIe = 830). We also present preliminary data on net energy values for RHFC systems based on solid oxide cells.