Energy Storage: Options for Fossil Power

The large adoption of variable output wind and solar energy into the electricity market in recent years has made energy storage increasingly important. Storage is, in fact, often viewed as the direct complement of wind and solar power, but this perspective misses the central role storage can play in supporting fossil capacity. The linkages between thermal fossil power and storage are important because although the grid will become increasingly renewable-centric, for the foreseeable future, dispatchable fossil power will continue to play an important role in meeting demands for peaking, cycling, and regulation services, as well as baseload power. 

OnLocation evaluated energy storage options for the Department of Energy’s (DOE) Office of Fossil Energy and Carbon Management (FECM). The objective of the report, titled “Electricity Storage Technology Review,”  is to identify and characterize commercial and developing storage technologies that could benefit fossil plant operations. The technologies evaluated by OnLocation include stationary batteries and mechanical, thermal, and chemical energy storage technologies, and each was evaluated in terms of costs, operating characteristics, development status, and potential for supporting fossil thermal generation (see matrix below). 

Lithium-ion batteries are currently the major source of new storage capacity in the United States by far, but other technologies have promise and could provide better integration with fossil thermal units for some purposes. OnLocation identified several storage technologies which appear to have high potential, including: 

• Liquid Air Energy Storage (LAES): A form of thermal storage, LAES uses excess power and steam from a fossil host to compress and liquefy air. When electricity is needed, the air is heated to its boiling point and expanded through a generator. Efficiency is increased by capturing and storing heat from compression and cold from expansion, which aids the ability to cycle on a daily basis.  

The compression equipment and power generators come from established supply chains in mature industries. The technological innovation here is using them for grid storage. A concern is that the equipment is expensive, and the opportunities for further efficiency gains may be limited. Currently, a commercial-scale LAES plant is operating in the United Kingdom, and a second and larger unit is under construction. 

• Hydrogen and Ammonia storage link fossil energy to the prospective hydrogen energy economy, as noted in our recent blog. The simplest applications are analogous to mechanical storage; that is, during off-peak periods, the surplus power from a fossil plant can be used to produce hydrogen via electrolysis, which is then stored for future use. However, there are opportunities for closer fossil-hydrogen integration. The waste heat from fossil plants can be used to increase the efficiency and lower the cost of hydrogen extraction via electrolysis. The hydrogen can be used directly for power generation or converted to ammonia for storage at ambient conditions. The ammonia in turn can be consumed directly in a combustion turbine when needed for power generation; used as the energy source for a fuel cell; or stored and transported to other locations where the waste heat from another fossil facility can be used in an ammonia cracking process to release the hydrogen for direct use.  
• Methanol storage may be most attractive for fossil plants equipped with carbon capture, utilization, and storage (CCUS) equipment. In the methanol process, the pure CO₂ stream from a CCUS system is reacted with hydrogen produced from electrolysis (which, as noted above, benefits from the fossil plant’s waste heat). The resulting aqueous methanol can be stored at ambient conditions, transported, or used on-site. Applications include methanol fuel cells, direct use in combustion turbines, and as a supplemental or primary transportation fuel.  

Hydrogen, ammonia, and methanol are all forms of chemical storage. These options have great potential, but more work is needed to achieve commercial deployment. This includes additional research, development, and demonstration (RD&D) and the buildout of hydrogen infrastructure, including production, storage, and transportation facilities. 

• Supercritical CO₂ Energy Storage (SC-CCES) is an advanced system that allows the use of coal-fired power in a cycle that includes storage and low carbon emissions. It involves the combination of three technologies:
  • A novel pure oxygen-fired, coal-burning pressurized fluidized-bed combustor (Oxy-PFBC) providing heat and CO₂;
  • An indirect-fired supercritical (i.e., very high pressure and temperature) CO₂ power circuit; and
  • A thermal energy storage system (such as molten salt) to balance electrical grid demand.

The SC-CCES technology offers several potential benefits: high energy efficiency and low carbon footprint; the thermal storage system would allow the unit to run at a steady-state level even during low demand periods; and a portion of the CO₂ can be sold for enhanced oil recovery or paired with methanol production. The technology is new and will require substantial RD&D.  A natural gas-fired pilot plant that uses a CO₂ power cycle, albeit without thermal storage, recently began operating in Texas. 

Additional detail, covering the full range of technologies shown in the matrix above, is available in our report to FECM. 

OnLocation is also assisting DOE, FECM, and the Energy Information Administration incorporate other clean energy technologies into their analyses and models, particularly the National Energy Modeling System (NEMS) for which OnLocation has been the prime contractor for decades. This work includes modeling the hydrogen energy economy and an enhanced representation of industrial CCUS, both of which are expected to play important roles in achieving a net-zero carbon solution.