District cooling substation design and control to achieve high return temperatures

Artikel i vetenskaplig tidskrift, 2022

Low return temperatures are a prevailing issue in district cooling systems negatively affecting operating costs and energy efficiency. In this study, three aspects of district cooling substation design and control were investigated with the aim to increase the return temperatures: 1) secondary supply temperature setpoint, 2) primary flow rate and 3) the flow rate relation between the primary and secondary flows. Two different control strategies limiting the secondary setpoint and the primary flow were tested in four buildings supplied by district cooling. Also, the secondary flow was measured along with an NTU analysis and predictions with a heat balance and a support vector regression model. The results showed the control strategies successfully increased the primary return temperature with 0.6–1.6 °C and eliminated flow in the saturation zone. The primary and secondary flows were shown to be unbalanced in fourteen of sixteen substations causing a low heat exchanger temperature effectiveness. The preferred method for predicting the secondary flow was support vector regression. The novelties of this paper are the conducted field tests and measurements with associated analyses, contributing with knowledge about the actual operation of district cooling substations and outcomes when implementing improvement measures to increase the primary return temperature.