ONE SIZE DOES NOT FIT ALL..
As lovely as it would be, one size will not fit all energy storage requirements. Fortunately the industry is developing an array of storage solutions which can be fit together to give us the storage solutions we need. The end result will be Hybrid Energy Storage Systems (HESS) with different storage solutions for each time scale desired.
In the limit case the storage system must function across all time scales, from milliseconds to hours, ultimately to days and to months. All the pieces necessary to do this are going to be available in the near term, and most are available now.
For extremely short periods of time, fractions of an AC cycle to a few seconds, capacitors and super capacitors fit the bill. These capacitors maybe mounted and containerized separately or may already be part of a bidirectional inverter for any of the other components. For longer periods of time the high power super capacitors are too expensive and don’t have enough energy. The HESS then uses the most advanced flywheel systems, which can have as much power as a super capacitor, but contain far more energy. Once the storage needs, either supply or demand, exceed tens of minutes battery energy storage systems become economical. When the demand on the system stretches to multiple hours a HESS will switch to extremely high energy storage systems such as flow batteries, and over the scale of weeks and months to hydrogen storage. Only with a combination of systems can a HESS be both energy and economically efficient.
A good comparison of the various storage techniques is shown by the Ragone chart of existing power sources (Ghoniem 2011) published by Gabriel Ertz*, Jens Twiefel and Malte Krack, in Energy Harvesting and Systems2014; 1(3-4): 233–241, as their Figure 1.
Since the horizontal axis is the specific energy contained in the storage system it is also a good proxy for the time scale over which the storage system is useful. Super capacitors, having very large power capacities work for short times, but don’t have enough stored energy for longer periods. Although conventional flywheels have long been surpassed by super capacitors advanced flywheels, of which QuinteQ’s is the class leader, can deliver the same power as a super capacitor, but over a much longer period because of their larger energy storage. As the requirements on the storage system lengthen the HESS must switch to lower power density systems, but higher energy, such as batteries, and eventually stored hydrogen and fuel cells.
