**4.1 Advantages and disadvantages of energy storage**

Without storage, the production and use of electricity must still be balanced. And since generation and consumption are physically different, transport capability is also a limiting factor. Owing to these limitations, the value of energy on wholesale markets will change rapidly and considerably over time, often even adversely. Electricity spot market rates reflect this abrupt shift in value [9].

Various storage devices are found in energy systems. They can be cataloged as: chemical or electrochemical, mechanical, electromagnetic, or thermal storage. Generally, the energy storage plant consists of a storage medium, a power conversion device, and a balance of plant [10]. Obviously, these forms of energy storage have many promising features, but on the other hand, certain important elements are still lacking.

Electricity storage systems have major beneficial energy, economic, and environmental impacts:


Despite the apparent benefits for the use of storage systems, it is evident that more measures are needed to address the major obstacles. The key one is the initial capital cost of storage per kW of all storage technology. It should be remembered that the cost of using Pumped Heat Electrical Storage (PHES) and Compressed air energy storage (CAES) systems is comparatively low. Battery technology is less efficient than PHES technology but can be found to be comparatively cost-effective.

While there have been significant technological improvements in energy storage systems, in many countries, it is still not cost-competitive for electricity consumers (whether at a residential, commercial, or utility scale) to store their energy. It means that when a consumer demands electricity, supply across the transmission and distribution networks must be carried out in real-time. One of the biggest disadvantages of energy storage is the fact that energy storage usually uses electricity and stores it but afterward distributes it back to the grid, which is called "round-trip" as a proportion of energy put in to energy returned, measured in %. This is inefficient, because the energy lost in the process of this round trip could be stored better, and no wastage of energy would be possible in this cycle. It depends on the use of storage technology; the higher the round-effectiveness, the lesser will be the amount of energy lost [11]. The last disadvantage is certainly the necessity of a significant amount of additional energy, which represents a reserve used for energy storage [12]. Reserve or in other words "back-up" can have two types, i.e. capacity and operational backup. The optimum proportion of variable renewable energy (VRE) sources in the composition of energy sources depends on different factors [13]. Backup power, network flexibility, transmission-system quality and capacity [14], as well as load efficiency characteristics [15] and real local weather models will determine the amount and variety of

VREs required to backup and that can be safely fed into the system. By adding storage space to the energy grid, it is possible to gain greater resilience of VRE by supplying contingency ability for peak load shaving or valley loading [16]. There is considerable variation in national markets because of the different endowments of indigenous fossil fuels and renewable energy potential, levels of technological development, and environmental and energy security risks. As a result, the relative importance of each of the above characteristics defining electricity as a 'mixed good' varies between countries [17].
