Obtaining direct access to all

Obtaining direct access to all three types of actual projects is difficult; therefore, we seek assistance from other researchers. Case 1 (Li, 2012) and 3 (Sun, 2006) have been analyzed in previous studies, and the details of energy data are sufficient for our analysis. For Case 2, we conducted field measurements in the orexin receptor antagonist to obtain the primary data for our study. The data collection procedure for each case study is as follows:
Survey and measurement
Analysis and discussion The composition of the electrical and cooling consumption in the three cases is listed in Tables 4 and 5 lists the energy efficiency of each segment. The calculation method of energy efficiency is based on the requirement in the Chinese standard for “economic operation of AC control system”. The COP is equal to the cooling consumption divided by the electricity consumption of the heat pumps; the transport coefficient of chilled water (TCchw) is equal to the cooling consumption divided by the electricity consumption of chilled-water pumps, while the transport coefficient of cooling water (TCcdp) is equal to the cooling consumption divided by the electricity consumption of cooling water pumps; the system efficiency is equal to the cooling consumption divided by the sum of the electricity consumption of heat pumps and water pumps. The electricity consumption is the sum of the consumptions of all components (cooling water pumps, chilled-water pumps, and heat pumps). Through the comparison and analysis, the main conclusions from the three study cases are as follows: In Cases 2 and 3, the control of AC terminals possesses the feature of that in split AC systems; however, the refrigerating machines and distribution systems are conventional types in centralized AC systems. In Cases 2 and 3, the energy consumption of the AC system is approximately 8kWh/m2, which is approximately 1.5 to 3 times those in decentralized AC systems. This difference in energy consumption is mainly caused by the distribution energy consumption in Cases 2 and 3. Table 5 indicates that the transport coefficient of chilled water and cooling water cannot exceed 10. In Case 3, the transport coefficient of chilled water is only 3.7. According to the stipulation in the Chinese standard for “economic operation of air-conditioning control system,” when the annual working condition is evaluated, the lower limit value for TCchw and TCcdp is 30 and 25, respectively. Therefore, in Cases 2 and 3, the transport coefficients of the pumps are extremely low. As mentioned above, in Cases 2 and 3, the cooling consumptions do not differ considerably from those in decentralized AC systems. However, the circulating pumps in the distribution systems operate continuously for 24h; thus, most of the time, the efficiency of distributing the cooling consumption is low. For example, in Case 2, during the cooling season, the circulating pump electricity consumption alone is 3.2kWh/m2, and this value is equal to the average electricity consumption of the split AC system in summer in that area. Case 3 can explain the phenomenon further. In Case 3, only the cooling water is circulating in the system. With the ability to control the AC terminals, residents will take the control mode of part time and part space; thus, the operating time of the heat pumps will be reduced considerably. From the results in Case 3, the electricity consumption of heat pumps and terminal equipment is only 48%, and the electricity consumption of cooling water pumps is more than half of the total consumption. If electric on/off valves are installed in the users’ side, and the water pumps use frequency conversion control in Cases 2 and 3, then the water temperature differences of the chilled-water system can be increased, thereby reducing the distribution system׳s energy consumption in the centralized system to a certain extent. However, with desynchrony and low load ratio feature of the cooling load in residential buildings, the energy consumption of the distribution system will still be a major component of the total consumption. Taking Case 2 as an example, Fig. 12 shows the statistical result of each household׳s AC terminal operating time frequency on July 4. The majority of users’ operation ratio of FCU terminals is within 10%, and only a few users’ operation ratio reaches 60%. Under this situation, the most ideal control mode of the water system is that many circulating pumps are installed using parallel operation control, and the operation number and frequency of the water pumps are adjusted according to the water temperature differences. However, in actual engineering cases, in general, the number of pumps would be two to three. Therefore, when the AC terminal operation ratio is low, the water pump operating point would have a serious deviation that will result in high consumption and low efficiency of the distribution system. Meanwhile, the chillers also limit the reduction of the energy consumption of the distribution systems. Centralized AC systems have two to three chillers at most, and each chiller has a minimum flow restriction. Therefore, the flow rate of the distribution system cannot be reduced considerably under low load ratio. Consequently, the system will inevitably operate under a situation with large water flow rate and small differences between the supply and return water temperatures.