An exergy-based minimum carbon footprint model for optimum equipment oversizing and temperature peaking in low-temperature district heating systems

Highlights: • Low-temperature district heating and current building equipment are incompatible. • Exergy-based metrics guide for optimally to minimize CO₂ emissions responsibilities. • These metrics may reduce CO₂ responsibility of district heating systems by 90%. • Their exergy destructions make renewable energy systems also emission-accountable. • Two cases are presented about wind power-to-heat options. Total decarbonization strategies are facing technical and environmental challenges according to the 2nd Law of Thermodynamics, such as equipment oversizing versus temperature peaking by heat pumps to accommodate low-temperature renewable and waste energy sources. As a method of this paper, the Rational Exergy Management Model (REMM) derived fourteen metrics, aiming to minimize the CO₂ emissions responsibility. Two case studies are presented. One of them is a fifth-generation district energy system concept with a 250 MW design heating load of 20,000 residence-equivalent apartments at a supply temperature of 35 °C. Results showed that two heat pumps in a cascade achieved a 23% higher exergy utilization rate with an optimum temperature peaking to 45 °C and 25% radiator oversizing. The second case study is concerned with the strategy of the Chinese government to substitute the domestic use of coal and wood with local wind turbines for electric heating to combat global warming. Five alternatives were considered; 1-Wind electricity to resistance heating, 2-Wind electricity to heat pumps, 3-Wind electricity to mini hydrogen fuel cells, 4-Wind electricity to hydrogen and micro-cogeneration, and 5-Hydrogen district systems with biogas, geothermal, and solar energy. Results showed that Case 1 has the maximum carbon footprint, whereas a custom-designed hydrogen house has the least footprint leading to about 90% CO₂ emissions responsibility emanating from exergy destructions. The paper concludes that low-temperature heating either in district energy systems, in private buildings, or prosumers is both environmentally, economically, and technically feasible.