TES performance metrics in terms of first-law, second-law efficiencies, and storage effectiveness (or storage
fraction) provide general measurement for TES design and analysis. The first-law efficiency reflects heat losses by
the storage tank. The second-law efficiency indicates potential-energy conversion degradation through TES
charging/discharging processes. Storage effectiveness gives the fraction of usable storage with respect to the total
storage volume. Three efficiencies provide adequate performance metrics for preliminary TES design evaluations.
We applied them for the new concept of fluidized-bed thermal energy storage.