Active Condensation in a 3rd Generation District Heating System

Third-generation district heating (3GDH) systems, which represent the majority of existing networks, typically use pressurized water at temperatures often below 100°C. Utilizing the fuel's higher heating value (HHV) in such systems requires additional energy input for active condensation (AC). This study analyzes an AC system capable of operating in both active (with a heat pump (HP)) and passive modes, evaluating the performance of five HP variants. These include three compressor-based (low-lift, high-lift, industrial) and two absorption-based (single-stage H 2 O/LiBr, GAX) types, assessed across three design scenarios within a case-study DH system. The case study is a 37 MW system, representative of a 3GDH plant, designed for a rated thermal output with supply/return temperatures of 130/75°C and annual averages of 69.4/49.5°C. A simplified model, combining operational measurements with published and industry data, is developed to determine: (i) the optimal compression HP type, (ii) the operational strategy for peak economic and energy efficiency, (iii) when direct flue gas condensation should be prioritized, and (iv) the impact of ancillary electrical consumers. The findings indicate that a permanent operational strategy with the highest possible COP, achieved by minimizing temperature lift and allowing a variable flue gas outlet temperature, is economically optimal. Compressor HPs are best for limited HHV utilization and shorter payback periods, while absorption HPs maximize HHV utilization. Furthermore, compressor HPs have a distinct optimal operation point, whereas absorption HPs perform consistently across a wider range of flue gas outlet temperatures.