District Cooling – Towards Improved Substation Performance

Doctoral thesis, 2022

The global cooling demand in buildings is rapidly increasing and to supply this demand, district cooling is one solution. However, many district cooling systems suffer from low delta-Ts (receiving low return temperatures from the buildings connected to the system). The low delta-Ts cause an increased water flow rate, congested distribution networks, require more chiller to operate and reduce the amount of free cooling possible to use. Ultimately, the costs increase, and energy is being wasted. The problem with low delta-Ts has previously mainly been investigated in district cooling systems without heat exchangers in the substation separating the system and the buildings. The purpose of this thesis is therefore to contribute with such knowledge with the aim to develop a systematic approach on how to evaluate and improve the performance of district cooling substations with heat exchangers, to achieve high delta-Ts.

The work was done by analyzing data from both sides of the heat exchanger of approximately 40 substations of buildings connected to an actual district cooling system. The results show a majority of the investigated substations perform poorly. The reasons to this are many but some examples include incompatibility with the district cooling system and limited or lacking follow-up and optimizations of the buildings’ systems and substations. Low delta-Ts, high overflows and flow in the saturation zone are primary side performance indicators, developed and tested to evaluate the substation’s performance. The performance indicators showed improvements can be done to almost all investigated substations. Three additional performance indicators were developed including data from the secondary side. These showed further reasons to poor substation performance can be explained by the heat exchanger’s temperature approaches. Moreover, high primary flow rates in relation to the secondary flow rates were also shown to cause low delta-Ts. Two control strategies were identified as potential solutions to resolve some low delta-Ts. These control strategies were field tested in four buildings and shown to successfully increase delta-T and eliminate the flow in the saturation zone.

To achieve improved performance, it is recommended to employ a systematic method to follow up the substations on a regular basis. This can be done by using the developed performance indicators together with a ranking system as support for decision-making on which substations to address first. Moreover, a systematic follow-up method can be used for collaboration between the utility company and the customers, or it can be used by the property owners as part of their optimization work. Lastly, incentives for improved substation efficiency can be strengthened by adding a temperature component to the price model.