Evaluation of thermal performance and efficiency enhancement potential of low-temperature operation strategies in solar district heating systems

A defining innovation in fourth- and fifth-generation district heating systems is the emphasis on low-temperature operation, which is crucial for advancing the efficiency of modern systems. While considerable attention has been given to low-temperature operation in traditional district heating systems, its application and quantification in solar district heating systems remain underexplored. In the context of solar district heating systems, adopting low-temperature operation is pivotal for unlocking substantial improvements in overall energy efficiency. This study addresses this gap by systematically evaluating and comparing several low-temperature operation strategies, including reductions in supply and return temperatures, integration of water-to-water heat pumps, and dynamic regulation of solar collector field outlet temperatures—strategies that have not been comprehensively studied together within solar heating systems. A comprehensive thermodynamic model was developed in Matlab, with the Langkazi solar district heating system serving as a case study to validate and quantify the effectiveness of these proposed strategies. Results demonstrate that reducing return water temperature and integrating water-to-water heat pumps significantly lower solar collector field inlet temperatures, with the integration of the water-to-water heat pump yielding the most pronounced improvements in system performance. This led to a notable increase in solar fraction and heat collection efficiency, from 66.4 % and 35.2 % to 78.7 % and 40.3 %, respectively. Importantly, the dynamic regulation of solar collector field outlet temperature emerges as an especially effective and innovative approach that significantly reduces operating temperatures and further boosting system thermal efficiency. Notably, in non-direct supply mode with integrated water-to-water heat pumps, dynamic regulation based on heat storage temperature provided an additional improvement, increasing annual heat collection efficiency and solar fraction by over 2.0 % and 4.0 %, respectively, compared to the direct supply mode with WWHP integration. When all low-temperature strategies were applied, the system’s solar fraction and heat collection efficiency increased by an average of 16.51 % and 7.15 %, with maximum gains of 18.56 % and 8.06 %. Additionally, the potential for efficiency improvements is greater in regions with weaker solar radiation, and under low-temperature operation strategies, the system shows improved economic performance and carbon emission reductions. These findings offer valuable insights for optimizing low-temperature solar district heating systems.