Exergy-rational utilization of solar energy with advanced PVT systems and heat pipe technology in 100% renewable cities

Abstract

This paper responds to the EU goals of decarbonization with solar energy 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 and higher-exergy solar photo-voltaic-heat systems 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. 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 larger one to increase 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 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 1.6x10-5 km/kW times the thermal capacity of the district raised to a power of 1.5.