Comparison of Energy Storage Devices

Not all ESS technologies are ready for large-scale and widespread deployment. As of 2009, only four energy storage technologies (sodium-sulfur batteries, pumped hydro, CAES, and thermal storage) have a total worldwide installed capacity that exceeds 100 MW [21]. The main reason is that, in spite of the large variety of ESS technologies, no technology offers sufficient performance in respect of key figures of merit needed of an electrical energy storage medium. The performance matrix needed for ESS to handle HRES is described in detail in section 4.4. For instance, a high-performance ESS should exhibit high cycle efficiency, high power and energy storage capacity, low cost, high volumetric and/or gravimetric density, efficiency of charge/discharge cycle, and long cycle life. The breakdown of various storage technologies based on its function as energy management or power quality and reliability is illustrated in Figure 4.2 [2]. The ESS technology of choice for many applications (especially those requiring high volumetric and/or gravimetric density) is battery storage. However, as shown in Table 4.2, different battery technologies have different storage characteristics. Again, no single storage technology can simultaneously achieve all the desired characteristics of a high-performance ESS mentioned above

FIGURE 4.2 Energy storage classification with respect to function [2]

(Continued)

FIGURE 4.3 Technical maturity of EES systems [2].

(see Table 4.2). Furthermore, as shown in Figure 4.3, different storage devices have different degrees of readiness for commercialization. When considering a particular energy storage device, as mentioned above, a number of property requirements need to be considered, which include the technical and commercial maturity of the storage technology. While individual storage system may satisfy some of these elements, just like sources for power generation, each storage device possesses some drawbacks. Despite numerous research efforts in order to improve ESS capabilities over the past decade, a perfect ESS technology that copes the drawbacks in terms of all aspects is not to be expected to develop in the near future.

As mentioned in the earlier chapters, hybrid energy generation has become important because no single energy source satisfies all the criteria needed for the sustainable energy generation. The same principle applies for energy storage. As shown in Table 4.2, no single energy storage device has all the favorable properties for the storage. This has led to the concept of hybrid energy storage. A hybrid energy storage system comprised of heterogeneous types of energy storage elements organized in a hierarchical manner so as to hide the weaknesses of each storage element while eliciting their strengths has become important. The hybrid storage system also offers challenges that one faces when dealing with the optimal design and runtime management of a hybrid energy storage system, targeting some specific application scenario, for example, grid-scale energy management, household peak power shaving, mobile platform power saving, and more. In order to increase the range of advantages that a single ESS technology can offer, and at the same time, enhance its capabilities without fundamental development of the storage mechanism and only via complementary use of the existing ESS technologies, more than one ESS technologies can be hybridized. A hybrid energy storage system (HESS) is composed of two or more heterogeneous ESS technologies, with matching characteristics, and combines the power outputs of them in order to take advantage of each individual technology.