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.

中文翻译：

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区域供冷变电站设计和控制以实现高回水温度

低回水温度是区域供冷系统中的一个普遍问题，会对运营成本和能源效率产生负面影响。在本研究中，研究了区域供冷变电站设计和控制的三个方面，旨在提高回水温度：1) 二次供水温度设定值，2) 一次流量，3) 一次流量和二次流量之间的流量关系。在由区域供冷的四栋建筑中测试了限制二次设定值和一次流量的两种不同控制策略。此外，还测量了二次流量以及 NTU 分析和热平衡和支持向量回归模型的预测。结果表明，控制策略成功地将一次回水温度提高了 0.6–1.6 °C，并消除了饱和区的流动。 16 个变电站中有 14 个的一次流和二次流不平衡，导致热交换器温度效率低。预测二次流的首选方法是支持向量回归。本文的新颖之处在于进行了现场测试和测量以及相关分析，有助于了解区域供冷变电站的实际运行情况以及实施提高一次回水温度的改进措施时的结果。