Reduction of Conveyance Power Consumption of District Cooling and Heating Systems Using Demand-Supply Coordinated Control

<p>This study discusses the effectiveness of “demand-supply coordinated control” in reducing conveyance power consumption of District Cooling and Heating (DHC) systems. Demand-supply coordinated control is introduced to address the problem of increased power conveyance, which is required by the DHC systems while providing heating to consumers. However, the effectiveness and limitations of this control have not yet been clarified. In our previous reports, therefore, we have numerically examined the operation of DHC systems with demand-supply coordinated control, and discussed its effectiveness and limitations for systems with simple configuration and a short piping length. Following the previous reports, this study deals with the effects of piping length on the supply temperature and conveyance power consumption, focusing on the fact that the longer the piping length, the further the occurrence of the disturbance in temperature mixing in the heat medium, which is caused by the unstable operation of the heat source machine. First, we determined the piping length at which the supply temperature does not deviate from the target temperature this is observed to become shorter as the thermal load following speed of the heat source machine and the thermal load fluctuation rate increase. We also showed that the relationship between the nondimensionalized factor of the supply temperature fluctuation range and the piping length is not unique even when the relative value of the thermal load fluctuation rate to the thermal load following speed of the heat source machine is constant. This is because the influence of the thermal load fluctuation period on the mixing is greater than that of the thermal load following speed of the heat source machine. Next, the effects of the piping length on the conveyance power consumption were discussed. The numerical results showed that when the relative value of the thermal load fluctuation rate to the thermal load following speed of the heat source machine is approximately 1.6 (or more), the conveyance power is approximately 1.25 times (or more) higher than when that value is less than approximately 1.6, regardless of the piping length. This result can be explained by the following facts: When the piping length is long, the inlet temperature of the heat source machine and the heat exchanger is not disturbed and therefore is stabilized due to a mixing effect, resulting in less-distorted shapes of the pump flow rate and the pump head. There is an increase in the pump flow rate and pump head due to a decrease in the outlet temperature of the heat source machine, which in turn is caused by control delay. However, this increase does not substantially differ from that when the piping is short. Terefore, under this condition, the conveyance power increases regardless of the piping length.</p>