Conventional and advanced district energy systems
District energy systems have the potential to decrease the CO2 emissions linked to energy services (heating, hot water, cooling and electricity), thanks to the implementation of large polygeneration energy conversion technologies, connected to a group of buildings over a network. To transfer the energy from the large polygeneration energy conversion technologies to the users, conventional district energy systems use water as energy transfer medium with often two independent supply and return piping systems for heat and cold. However, sharing energy or interacting with decentralized heat pump units often results in relatively large heat transfer exergy losses due to the large temperature differences that are economically required from the water network. Using refrigerants as a district heating or cooling fluid at an intermediate temperature could alleviate some of these drawbacks. Because of the environmental concerns about conventional refrigerants, CO2, which is a natural refrigerant, used under its critical point, could be an interesting candidate. Pipe sizing of a multiservices superstructure, based on a two pipe CO2 network at 18°C is compared with a standard 4 pipes water network for heating and cooling.