**5. Development of EMS**

To reduce the peak current in LIB pack, a physical battery is employed in LIB effectively, but range extension is still limited in the case study [33]. The effect of life cycle extension is discussed [34] by the transient supply of physical battery. Given the traditional large DC/DC converter in EMS, a small prototype of DC-DC and simple circuit may be proposed to isolate the battery pack and not harvest energy from random peak power [35, 36]. The scenarios of usage cover the regenerative power supply and charging/discharging between individual and physical batteries. Some studies have focused on the design of levering DC-DC converter [37, 38], but several researchers have introduced a converterless circuit in EVs based on a DC inverter [39, 40]. The literature implies the possibility of EMS with high efficiency and low cost. Specific control strategies including neutral networks are illustrated in [40–42]. Economic analysis shows that the high price of LIB leads to superior benefits in elongating life cycle. Real-time simulators are a powerful platform before on-board tests [42]. In [43], a simple circuit of elongating life cycle life was reported. Without a complex DC-DC converter, only duty control using a suitable physical battery can narrow DoD of LIB and elongate the life cycle of batteries [44–48]. **Figure 6** shows the relationship between DoD and life cycle. None of the lines in **Figure 8** are linear, thereby indicating that DoD plays a major role in gaining life cycle.

**Figure 7** shows a simple, converterless parallel circuit. EMS can achieve active control by switching the discharging ratio between LIB and auxiliary physical battery at unit time. The architecture of EMS is shown in **Figure 8**. It is modified from battery management system. EMS is disposed as an interface among RLIB, auxiliary physical battery (ultracapacitor, UC), and systematic grid. The control strategy aims to keep the switch periodically close and open by a predetermined duty cycle, namely, the sharing ratio of RLIB's loading controlled by EMS. In detail, EMS generates a PWM (pulse width modulation) signal to control the on/off time of the lower arm of the switch module.

**Figure 6.** Relationship between DoD and charge/discharge cycles (life cycles) modified from [49].

**Table 2.** Benefit assessment of 5 WWTPs applying RLIB (NTD; 30NTD = 1USD).

(**Figure 2**). Furthermore, the cost of RLIB is roughly cheaper than 1/3 of a new battery. This merit enhances strong competition compared with other cheap flow batteries or NAS batteries.

For instance, two packs of RLIBs are shown in **Figure 3**. Both of them are originally applied in pure EVs. After working for several years, they are used as experimental targets before cycling and recuperation by chemical method. In this study, two different types of packs are selected. The flowchart in **Figure 4** shows that suitable cells are activated and selected based on log file and DC internal resistance, and each new module is assembled with EMS. Subsequently, the module is installed in a test bench to update voltage of open circuit (VOC). In addition to establishing water, energy, and reusing nexus in urban areas, the Dihua WWTP is chosen for its large area of 8 hectares. Thus, an extensive enclosed space is available for placing RLIB

Reducing variability in renewable energy is crucial in managing the peaks in WWTP.As a result, this strategy is dispensable for employing energy storage systems charging during off peak times and injecting energy into smart grids during peak times. Benefits can be estimated from the low price at night, cost of basic contract fee of electricity, and effect of frequency regulation. Results of the economic benefit assessment are shown in **Table 2**. We assume that renewable energy's purchase price is 0.143 USD. Renewable energy is assumed to be fully fed back to the grid. About 80% of the total RLIB is used as night storage, and the cost of RLIB is 133USD unit kWh. In the case of Dihua plant, the calculation of RLIB demand is 32,106 kWh, which is roughly equivalent to 3200 pure EV battery pack. This value is also about 1/20 of the total number of domestic sales of EVs from 2011 to 2016 in Taiwan. The initial cost of RLIB packs is 4.3 million USD. However, only the sales of renewable energy power into the grid based on feed-in tariff (FIT) are 1.68 million USD. The annual electricity rate difference at noon and night is 140.6 million NTD, and the annual income at noon and night is 4.69 million USD. Therefore, the plant can break even in 2 years and continue to profit each year without considering the installation fee. Other plants also show similar profitable results such as

between the aeration tank and green park in the ground (**Figure 5**).

**4. Benefit of energy management in WWTP**

Dihua plant in **Table 2**.

168 Energy Systems and Environment

**Figure 7.** RLIB in parallel connection with auxiliary physical battery (ultracapacitor) controlled by EMS.

**Figure 8.** Architecture of EMS (symbol B is a safety device for estimating RLIB pack's insulation resistance).
