SOLAR TODAY - May 2012

the challenge for energy developers is to reduce the capital cost of renewable technologies and improve the energy conversion processes.

TABlE 2. SToraGE MECHaNISMS aNd PoSSIBlE aPPlICaTIoNS Storage Energy discharge Potential Technology Category Time application Batteries and Power Quality, Flow Battery Distributed Gen Reversible Distributed Gen Fuel Cell Superconducting Electromagnetic Electromagnetic Seconds/ PowerQuality Magnetic Energy Field Minutes Storage (SMES) Fly wheels Power Quality

are suitable for storing large amounts of electricity, and their capacity may be limited only by the size of the storage space (cavern or reservoir) that is geographically available.

Thermal energy is stored as a form of internal energy that includes both sensible (associated with a change of temperature) and latent (associated with a change of phase) heat. It is generally easy and inexpensive to store, but it does not convert directly to electricity as with the electrochemical or electromagnetic methods. Instead, TES relies on thermal electric generation to convert the thermal energy back to electricity. Unlike other electric storage devices, the downstream conversion of electricity to thermal energy can be 100 percent by Joule heating (whereby passage of current through an electrical resistance releases heat). However, the conversion of thermal energy back to electricity is limited by the thermal energy quality (which depends on temperature) and the associated power conversion efficiency. Therefore, one key to enabling thermal energy for electricity generation is to improve the thermal cycle efficiency. Based on efficiency improvements over the past century, there are several prospects for achieving high-efficiency TES that promise reliable and relatively low-cost electricity.

Stored
Electron/Ion
Charge
Hydrogen
Electrochemical Hours
Chemical Days
Compressed-Air
Energy Storage
(CAES)
Pumped Hydro
Storage (PHS)
Thermal Energy
Storage ( TES)

Mechanical
Inertia
Compressed
Energy
Mechanical
Mechanical
Minutes/
Hours
Days

expected life of 30 to 40 years, which is more than three times longer than the lifetime of electrochemical methods, resulting in long-term benefits and lower life-cycle cost.

Figure 3 (facing page), shows storage performance in terms of roundtrip efficiency for typical storage technologies. Energy storage efficiency is a metric that accounts for the net electricity delivered from the storage system relative to the amount of electricity put into the storage system. Electric storage efficiency often excludes electricity generation and transmission efficien-cies from the original power sources. From the metrics of power rating and storage capacity in figure 2, utility-scale energy storage favors PHS, CAES and TES. This article will focus on comparing the performance of those three storage methods in renewable applications. PHS is a well-known, highly efficient, long-term and large-scale energy storage method. PHS stores potential energy in the form of water, pumped from a lower-elevation reservoir to a higher-elevation reservoir, The efficiency of CAES and PHS storage methods can be affected by the geological configuration and method of operation. In a conventional CAES arrangement, the air in a cavern or container is heated during compression

Bulk Energy
Storage
Water Potential
Energy
Internal Energy
Mechanical
Months
Thermal
Hours/
Days
Bulk Energy
Storage
Bulk Energy
Storage

quadrant — batteries, electrochemical capacitors, flywheels and SMES — serve short-term electricity storage needs for power quality and distributed generation. For long-term bulk energy storage and stationary generation, CAES and PHS are broadly accepted. TES can achieve the same power rating and storage capacity as CAES and PHS by expanding the plant size as needed. The power rating of existing and planned CSP plants ranges from 50 to 300 MWe. Similar to CAES and PHS, CSP plants with TES have an

InCREASE In EnERGY ABunDAnCE

24 May 2012 SOLAR TODAY solartoday.org
comparison of the storage
technologies and tes as
a Means for electric energy storage

In selecting storage technologies one must consider each storage capability at its power rating, storage capacity, performance in terms of total roundtrip efficiency, lifetime, reliability, ease of installation and cost. Power rating and storage capacity determine the scale of the storage application. The other factors affect economic benefits.