A Comprehensive Exergy-Based Outlook of Renewable Technology Integration for the Fifth Generation District Energy Systems

Abstract

This chapter responds to the EU goals of decarbonization with heat pumps powered by renewable energy sources leading to fifth-generation district energy systems (5DE). The dilemma of low-exergy (low-temperature) district heating systems with renewable and waste thermal sources and the temperature-incompatibility of the current comfort heating equipment demanding higher supply temperatures is addressed. Until innovative, low-exergy equipment are developed, various methods of optimum temperature peaking and existing equipment oversizing are presented. Central temperature peaking at the plant site and the individual prosumer buildings are compared. Four different temperature peaking methods are discussed concerning their total CO2 emissions responsibility, including embodied emissions. These are; 1-temperature peaking with heat pumps with or without cascading (tandem heat pumps) powered with renewable energy and backed up with grid power, 2-photo-voltaic-thermal solar panels (PVT) in tandem, 3-deep geothermal wells, wherever available, and 4-on-site wind turbines coupled to heat pumps. Four main CO2 emission components are recognized. The first one is the direct CO2 emissions due to backup power demand from the grid by the heat pumps and their ancillaries. The second component is the emissions responsibility due to exergy destructions from exergy mismatches. The remaining two components are embodied emissions of the peaking equipment and the greenhouse emissions equivalence of refrigerant leakages from heat pumps, respectively. A case study is presented, which involves an individually optimized solar prosumer building with an optimum mix of heat pump oversizing and commercial radiator oversizing. Results show that CO2 emissions responsibility-based optimum mix compared to economy-based optimum mix reduces the responsibility by 30%. This analysis was repeated by cascading two smaller heat pumps instead of a single larger one for increasing the overall COP. This change further improved the solution by 4% points. Results have also been compared to a modified case where cascaded heat pumps are coupled with low-exergy heat pipe type of radiators. This coupling resulted in much more improvement by an additional 52% points. The paper concludes that the key is low-exergy heating and cooling equipment. Then the fifth-generation district energy systems with supply temperatures as low as 320 K (47 °C) and return temperatures as low as 300 K (27 °C) will be possible with renewable and waste energy sources if the one-way distance between the plant and the district is not more than 1.6 × 10–5 km/kW times the thermal capacity of the district raised to a power of 1.5.