A series hybrid “real inertia” energy storage system

Abstract

The wide scale market penetration of numerous renewable energy technologies is dependent, at least in part, on developing reliable energy storage methods that can alleviate concerns over potentially interrupted and uncertain supplies. Many challenges need to be overcome, not least among them is allowing capacity for the wide range of time scales required to ensure grid stability. In thermal power plant, high frequency/short duration demand fluctuations, acting at the milliseconds to several seconds time scale, are addressed passively by the inertia of the grid. Here, grid inertia can be thought of as the mechanical inertia of spinning steel in steam and gas turbines. This allows time for active control measures to take effect at the tens of second to hours time scale and for the system to recover without a supply frequency deviation that is noticeable to the customer. It is of paramount importance that, as thermal plant is retired, renewable energy generation and storage systems account for the loss of this inertia. In the literature, strategies to address the loss of "real" inertia have often relied on emulation rather than actual replacement. The present work focuses on the preliminary development of a novel energy storage system that makes use of real inertia to address short term supply/demand imbalances while simultaneously allowing for extended depths of discharge. The concept looks to combine flywheel and compressed fluid energy stores in order to power a synchronous generator. By combining these energy storage technologies through a differential drive unit, DDU, it is anticipated that the benefits of high system inertia can be exploited in the short term while allowing energy to be continually extracted from the flywheel in the long term during storage discharge. The use of a DDU makes the present design particularly novel and distinct from other hybrid systems. In essence, this inclusion allows energy to be extracted entirely from the flywheel, inducing "real" inertia, or entirely from the secondary store, inducing "synthetic" inertia, or some combination of the two. Fundamental sizing calculations for a 50MW system with 20MWh of storage capacity are presented and used to design a suitable control system that allows for the operation of both primary flywheel and secondary compressed fluid energy stores. The transient behaviour of the system is simulated for several charge/discharge time profiles to demonstrate response stability for the system. Comments on system turnaround efficiency, which is dependent upon loading history but for the intended applications can be considered to be greater than 90% are also made here, along with a case study application to an isolated Californian solar powered grid.

A case study investigation into the risk of fatigue in synchronous flywheel energy stores and ramifications for the design of inertia replacement systems

“…Clearly, such an application relies on robust control methods and power electronics are well known to be power limited, meaning the potential for overloading (during fault conditions, for example) is limited. Interested readers are directed to the author's previous work for a more comprehensive description of SHyKESS [34]. The relevance of these conditions will be made clear later in this section, as some loading profiles adopted in the present work exceed the bounds of what would normally be expected for a synchronous flywheel store.…”

“…The distinction between real and synthetic inertia is emphasised in the authors' previous work, wherein a "series hybrid kinetic energy store" (hereafter refereed to as SHyKESS) was developed [34] (see figure 2 b). ).…”

“…The relevance of these conditions will be made clear later in this section, as some loading profiles adopted in the present work exceed the bounds of what would normally be expected for a synchronous flywheel store. Note that, while references to flywheel axial lengths are omitted from the present work (the laminar flywheel design allows for modularity, after all), the authors' previous work sets out a case for square aspect ratios, wherein flywheel diameters and axial lengths are approximately equal [34]. The use of large flywheel rotors promotes the use of vacuum containment in order to minimise aerodynamic losses.…”

“…In such a system, magnetic bearings provide much of the load carrying capability, while relatively small rotating element bearings ensure an acceptable dynamic stiffness. A case study application of SHyKESS which incorporates standing loss estimates may be found in the authors' previous work [34]. Figure 2: A potential classification of inertia sources, showing a).…”

Promising Grid-Forming VSC Control Schemes Toward Sustainable Power Systems: Comprehensive Review and Perspectives

“…Regarding the energy storage systems with their V2G capabilities, they are relatively expensive and require intensive integration studies considering the various types (e.g. supercapacitors, Li-ion batteries, and flywheels) [134], [135]. Other general planning and control issues to be considered for all energy source types are the cost assessment, sizing and locating these energy sources, grid codes and market features, and the efficiency of the control scheme with respect to the operational requirements [136], [137].…”