# Considering the Life-Cycle Cost of Distributed Energy-Storage Planning in Distribution Grids
## 2 ESS Type Selection  
ESS has brought multiple attractive functions to a distribution network operation, such as alleviating the intermittence of DERs, improving the quality and reliability of the grid, meeting peak demand at extreme circumstances, providing time-varying energy management, and meeting the needs of charging and discharging of electric vehicles, etc. [13]. Reasonable allocation of ESSs can provide an effective replenishment and guarantee for multi-energy complementary and distributed energy comprehensive utilization. ESS can be categorized into mechanical storage systems, including Pumped Hydroelectric Storage (PHS), Compressed Air Energy Storage (CAES), etc. ; electrochemical storage systems, including lead-acid batteries and Lithium-ion (Li-ion) batteries etc. ; electrical storage systems, including the supercapacitor and the Superconducting Magnetic Energy Storage (SMES) etc. ; thermochemical storage systems, including solar fuels etc. ; chemical storage systems including hydrogen storage, etc. ; and thermal energy storage systems, including sensible heat storage, latent heat storage, etc. [14]. According to the characteristics of the charging and discharging time response, ESS can be categorized into power-type ESS and energy-type ESS. The former, such as a supercapacitor and flywheel energy storage, is suitable for short-term power fluctuation compensation, due to their fast response, long lifetime, and small capacity. Compared to the power-type ESSs, the energy-type storages, i.e., batteries, have large capacities which can be used for long-term power fluctuation compensation [15]. There are commonly used ESSs, such as PHS, flywheel energy storage, supercapacitor, and batteries. According to the authors of Reference [14], the rated power of various PHS devices ranges from 1 MW to 3003 MW, with a cycle efficiency of about 70-85%, and a service life of more than 40 years. However, due to the limitation of site selection, the construction time of PHS power plants is long and the investment of funds is high. Flywheel energy storage has some advantages of high power density, high cycle efficiency, no depth-of-discharge effects, and easy maintenance, but its main disadvantage is a relatively high self-discharging rate, up to 20% of stored capacity per hour. Batteries can be widely used in different applications, such as power quality, energy management, and through power and transportation systems. The construction of battery system requires a relatively short time-period (roughly 12 months). The location of the installation can be very flexible, which can accommodate the building or be close to the required facilities. At present, a relatively low cycle-time and high maintenance costs are considered to be the main obstacles to the implementation of large-scale facilities. If toxic chemicals are used, disposal or recycling of waste batteries must be considered. Compared with batteries, supercapacitors have a higher power density and a shorter charging time. However, due to the high self-discharging losses, the capacity is limited, energy density is relatively low, and energy dissipation is fairly high. Currently, lead-acid batteries have been recognized as the most economical and mature storage technology among the various types of ESSs. However, the environmental concern during manufacturing and recycling have impacts on the technology development. Li-ion batteries are gradually receiving attention from the industry because of its small size, high energy density, long life cycle, no memory effect and no pollution, low self-discharging, and a high comprehensive efficiency. Additionally, the cost of Li-ion batteries continues to drop due to technology development and material innovation [16]. Figure 1 shows the characteristics of different types of batteries, the Li-ion batteries, therefore, have been selected in this study from the perspective of economy, efficiency, and technology maturity. Figure 1: Characteristics of different batteries.