## 1 Introduction
[PERSON] and [PERSON] developed a control strategy that implements an adaptive and derivative strategy to optimize the speed of pumps by resetting the pressure set point [12]. [PERSON] studied the components in pumping systems to provide insight into the assessment of VFPS efficiency and energy consumption [13]. [PERSON] studied the performance of several check valves that were connected to DVFPs and the influence of these valves on energy consumption [14]. Some studies also focused on the performance of DVFPs in actual projects and calculated the energy-saving and cost-saving potential; this includes the hydraulic performance of a district heating project in Kuerle, China [15], the minimum of capital cost and energy consumption in a district heating projects in Dalian, China [16], and a project involving a municipal water distribution system [17]. [PERSON] et al. simulated a multi-zone heating system and developed the Newton-Raphson method to solve various hydraulic problems [18]. [PERSON] analyzed the factors affecting energy saving rate in the DVFPs system [19]. Although these studies yielded significant contributions to the field, most current studies are focused on chilled water pumps and fail to adequately investigate cooling water pumps, the influence of cooling water temperature on the entire system, and, more significantly, the chiller performance. Furthermore, there is a lack of studies on timely effective system performance in response to cooling loads that fluctuate in real time. Alternatively, thermal storage systems including water storage, ice storage, and eutectic salt storage systems have also been widely used in DCSs. Under the conditions of time-of-use tariff of electricity, thermal storage systems produce and store hot/cold water overnight at relatively cheaper electricity costs and release the stored contents at the peak price time. As a result, the system has been proven as cost-saving and peak shaving, which is beneficial for electric power plants [20; 21]. Furthermore, the nominal cooling capacity can be reduced. As an example, many studies focusing on cold storage have aimed to optimize the control and performance. [PERSON] et al. presented an optimal control strategy based on artificial neural networks to predict cooling loads [22]. [PERSON] et al. conducted simulations to evaluate the performance of a combined DCS and ice storage system, and studied the influence of tariff structures on the combined system [23]. [PERSON] reviewed the research on thermal storage and compared three cold storage media and rated ice storage [24]. In contrast to the numerous studies on other types of storage systems, there is a lack of studies and applications on cooling water storage. Water temperature from cooling towers is low, due to the low environmental wet bulb temperature at night, plus the valley electrical price it is cost-efficient to use cooling towers to generate cooling water and store it overnight. Furthermore, cooling water with lower temperature supplied during daytime hours would increase the COP of the chiller, thereby further decreasing the electricity consumption. Additionally, with the assistance of water tanks, there is no need to keep pumps and cooling towers in constant operation; this provides a greater flexibility for the controlling system. Therefore, by exploiting the advantages of each of the related components, a novel DVFP system with water storage (DVFP and WS) designed for implementation in a DCS with variable end-user-required cooling loads has been developed and is proposed in this paper. In this new system, chillers and variable-frequency pumps are installed at each end-use building and independently controlled while sharing a single cooling water loop. Cooling water is generated at a central cooling tower and is stored in a water tank that is located near the cooling tower. Then, the cold water in the water tank is transported to the end users.