Gothenburg District Cooling System - An evaluation of the system performance based on operational data

Licentiate thesis, 2020

The global energy demand for providing cooling in buildings is expected to increase the next decades, along with a rapid growth in the number of air conditioners and chillers. A more energy efficient, economical and environmentally viable solution to this increased cooling demand, is district cooling. In Sweden, this technology has been developed since the mid-1990’s and currently delivers about 1 TWh of cooling annually, to 40 cities.

Common issues with district cooling are mainly related to the temperatures. First, a low temperature difference between the supply and return water, called low delta-T, persist despite extensive efforts by previous research to provide solutions. Second, low conventional supply and return temperatures remain, potentially as a result of limited knowledge about the temperatures used in the connected buildings. Previous research on the low delta-T has primarily focused on district cooling systems without heat exchangers separating the connected buildings from the distribution system.

The purpose of this thesis is therefore to investigate issues with low delta-T in a district cooling system with heat exchanger separation and exploring the potentials of using higher temperatures, by increasing the knowledge about the connected buildings. The investigation is based on analyses of operational data from both primary and secondary sides of the heat exchangers in 37 of the connected buildings in Gothenburg district cooling system. This system is designed for a delta-T of 10 °C and chilled water supply temperatures of 8 °C in the connected buildings.

The delta-T in Gothenburg district cooling system varies between 6-8 °C and the results showed that the main causes to this low delta-T are the following: a low temperature approach between the supply streams of the heat exchanger; operation in the saturation zone on the primary side of the heat exchanger; and low return temperatures from cooling coils and fan coil units in connected building chilled water systems. The results also demonstrated that 75% of the recorded chilled water supply temperatures are higher than 8 °C, when the outdoor temperature was 28 °C. If high temperature district cooling was used, more than 50% of the annual district cooling generation would be supplied by free cooling from the river.