Present and Future Electricity Storage for Intermittent Renewables

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Energy Storage Technologies
Electricity is not usually stored per se. Energy storage technologies instead convert electricity to other energy forms (gravitational, pneumatic, kinetic, chemical), with a characteristic turnaround efficiency usually driven by the simplicity or complexity of conversion and reconversion between electricity and the stored energy form. For example, it can be 90-95% efficient to convert electricity to kinetic energy and back again by speeding up or slowing down a spinning flywheel. Storing electricity by compressing and later re-expanding air is usually less efficient (75%), since rapid compression heats up a gas, increasing its pressure, making further compression difficult. The electric energy lost in energy storage drives up the overall cost of generating reliable electricity from wind or solar power. Another cost of energy storage is the capital investment required for the energy storage system. These costs are driven by the weight of material or volume of containment vessels needed to store a given amount of energy, termed energy density (kWh/kg or kWh/liter), again characteristic of each energy storage form.



Energy Storage within a Hydrogen Transportation Fuel Infrastructure
A predominantly renewable electricity supply could be combined synergistically with a future carbon-free electrolytic H2 transportation sector. Co-production of electricity and H2 fuel would enable massive deployment of intermittent electric generation by making efficient use of otherwise almost unavoidable excess generation during some time periods. The reliability of solar and wind power could also be improved through intentional “oversizing” of generation capacity relative to demand, since the additional excess electricity could produce H2 fuel. The energy stored in the H2 infrastructure and/or onboard H2 vehicles would be large enough to buffer H2 demand on the time scale of days. When wind or sunshine were low, higher H2 prices would temporarily reduce H2 demand from vehicles. Later, when solar and wind electricity supplies returned to higher levels, accumulated demand for H2 fuel could be easily met and H2 prices could drop. Furthermore, if travel patterns and/or vehicle use were flexible over short periods (i.e. days), then the transportation sector could also vary transportation use in response to H2 supply and cost levels.
Further integration of the electricity and H2 transportation sectors could, in principle, include the very large latent energy storage capacity of H2 vehicles as back-up power sources or even routine energy storage. A future H2 hybrid-electric or fuel cell automobile with 5-10 kg of H2 onboard (energy equivalent to 5-10 gallons of gasoline) could provide 75-150 kWh of electricity, enough to power a typical home for up to a week. The amount and efficiency of back-up power available from a H2 automobile would actually grow with consumer demands for greater size, power, and driving range. This would turn what is otherwise an engineering challenge (designing a cost-competitive H2 vehicle with range and performance comparable to or better than conventional gasoline vehicles), into a value-added benefit.