Energy Storage Metrics: Cost Of Energy Storage

Financial costs are important metrics for evaluating any new technology. Unit purchase costs are commonly expressed relative to the power rating ($/kW) and the amount of energy ($/kWh) that can be produced by the storage technology. This is common because many energy storage technologies tend to be scalable. The power rating can be scaled through equipment upgrades while energy rating can be raised by increasing the size of the reservoir of the storage media (Baxter, 2006).  Though there are numerous costs of ownership, such as maintenance costs, the replacement cost of subsystems of the energy storage system often merit special attention. This is especially true for many electrochemical storage devices where the electrochemical storage media is often predicted to be replaced several times throughout the life of the storage system. The disposal cost for spent media and the decommissioning costs for the entire storage system are also important costs to consider (Baxter, 2006).

Another important cost is the cost of energy to charge the energy storage system. In some markets a significant return on investment can be seen by using lower cost off-peak electric energy to charge the energy storage system. Profits are realized by discharging during high cost, peak periods (Denholm, Ela, Kirby, & Milligan, 2010). This usage is profitable if another important energy storage system metric, the “Round-Trip Efficiency”, is high enough to allow for sufficient discharge during peak periods. Round-trip efficiency is simply the ratio of the output discharge energy to the input charge energy. There can be a significant range of values across technologies with some round-trip efficiencies reaching as high as 95% (Baxter, 2006).

Beyond these rather standard metrics it is important to consider the following when evaluating energy storage technologies:

• ·        The physical footprint of the energy storage device. Commonly this footprint is divided into the space required for the storage medium, the power conversion system and the balance of the plant (Baxter, 2006). The ratio of the size of the energy storage device to its energy output is called the “Energy Density” of the storage device.
• ·        The environmental footprint associated with building, installing, decommissioning and in some cases operating the energy storage device (EPRI, 2003).
• ·        The modularity and scalability of the system, which would provide the ability to match variable demand.
• ·        In some instances energy storage devices are prized for their transportability allowing the rapid deployment of storage services to different locations (Baxter, 2012).

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Works Cited

Baxter, R. (2006). Energy Storage; A Nontechnical Guide. Tulsa, Oklahoma: PennWell Corporation.

Baxter, R. (2012, November 28). Author, Energy Storage; a Nontechnical Guide. (M. Banta, Interviewer)

Denholm, P., Ela, E., Kirby, B., & Milligan, M. (2010). The Role of Energy Storage with Renewable Electricity Generation. Las Vegas: National Renewable Energy Laboratory.

EPRI. (2003). EPRI-DOE Handbook of Energy Storage for Transmission & Distribution Applications. Washington DC: EPRI, Palo Alto, CA, and the U.S. Department of Energy.