## 6 Conclusions  
In this paper, the operating performance and energy efficiency of a novel DVFP and WS system that is applied to the cooling water operations in a DCS was analyzed. The basic principles and a schematic diagram of the proposed system have been presented, along with an analysis of the energy-saving potential. A DCS located in Beijing, China was selected for a case study; this included an on-site investigation of the system. Using this DCS as a reference, a series of simulations were conducted and effective operating data for two different systems exposed to various weather conditions were calculated. Through comparison with field test results and a simulated CCCP system, the proposed system demonstrates a 10% saving for both energy and cost part for the whole system.  
The throttling valves present in the CCCP system were replaced with variable-frequency pumps to ensure appropriate water flow regulation; additionally, in contrast to the CCCP system, water pressure loss via valves was prevented. This resulted in increased pump efficiency, the prevention of excessive water flow, and a reduction of transportation energy consumption. In addition, the water storage tanks enabled the resourceful exploitation of the electric tariff and pump control flexibility. The cold water was stored at night and was released during the period when the cost of electricity was highest. Via the proposed system, not only were the operating costs of pumps and cooling towers reduced, but also the chiller COP was increased because of the low cooling water inlet temperature of the condenser.  
It can be concluded that the size of the transportation system and the cooling load profile play an important role in the applicability of this new system. In a large district energy system with long pipes and various end uses, the transportation system consumes a majority of the energy; under these conditions, the advantages of the proposed system are more notable. As the concept of distributed variable-frequency pumps is currently being applied in large city-scale district heating systems, it would not be exceedingly difficult to begin implementing the proposed cooling system on an equivalent scale. Moreover, the DVFP and WS system is also applicable in systems with changing end-use cooling loads, as it can promptly initiate the changes that are necessary to maintain its level of efficiency by quickly adjusting flow as required. Thus, from what has been presented in this paper, it can be ascertained that there is significant potential of the DVFP and WS system application in many cases to save energy and reduce operating costs. However, some defects still exist in this study, in that all of the research works are based on a field test of the current system and simulation of the proposed system. There is a lack of the applications of DVFP and WS system in real operating projects and the real performance of this new system, which would be our future focus.  
This work was supported by the Natural Science Foundation of China (Grant No. 51521005), the 13 th Five-Year National Key Technology R & D Program of China (Grant No. 2016 YFC0700704).  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] and [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data; [PERSON] contributed analysis tools; [PERSON] and [PERSON] wrote the paper.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cooling Loads** & **Systems** & **Energy Consumption (kWh)** & **Cost (RMB)** \\ \hline \multirow{2}{*}{Full Load} & CCCP & 26,151 & 30,030 \\  & DVFP and WS & 23,455 & 27,191 \\ \hline \multirow{2}{*}{80\% Load} & CCCP & 21,651 & 25,969 \\  & DVFP and WS & 18,843 & 23,736 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of systems on typical days with different cooling loads.