Round-Trip Efficiency

Round-trip efficiency takes into consideration energy losses from power conversions and parasitic loads (e.g., electronics, heating and cooling, and pumping) associated with operating the energy storage system. This metric is a key determinant of the cost-effectiveness of energy storage technologies. Among energy storage options, compressed-air energy storage (CAES) has the lowest reported efficiency (40%–55%), and Li-ion batteries have the highest (87%–94%).

For energy storage coupled with photovoltaics, efficiencies of less than 75% are unlikely to be costeffective.

Response Time

The need for fast response times is expected to be more important for variability-damping than for load-shifting applications, and hence more relevant to utility-scale photovoltaic generation in this evaluation. Passing clouds are the primary source of rapid changes in photovoltaic power output. Solar insolation at a single point can change by more than 60% in seconds.

Changes of this magnitude in power output from utility-scale photovoltaic systems are expected to occur within minutes.

Photovoltaic power output ramp rates were measured over a year at a photovoltaic system in Hawaii that operated at 50% capacity. In that study, only 0.07% of the one-minute ramps were operating at greater than 60% capacity and only 5%, at greater than 10% of that capacity.

System operatorexperience suggests that a response time of seconds would be adequate to dampen short-term variability events of significant magnitude.

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