Publication
Supplying electricity, heat and cold is an essential part in the development of more sustainable city districts. Previous papers have illustrated the interest of using the exergy efficiency concept to decompose the problem and rank the technology combinations according to their overall efficiencies. The idea of this paper is to extend these considerations to technology alternatives with a focus on two innovative technologies. The first one is very low temperature District Heating and Cooling (DHC) supplying directly air-conditioning needs as well as heating needs via local heat pumps. It is part of what is called fifth generation DHCs or "anergy networks" and based on water or, better, on CO2 heat transfer fluid. The second one is hybrid SOFC-GT cogeneration units with or without CO2 separation to supply electricity to the various users in the network including the decentralized heat pumps, while optimizing the heat recovery. The former allows a full synergy between heat providers and users and allow to design districts without cooling towers or chimneys except at one energy balancing plant, ideally close to a surface water energy source or sink. Starting with the composite curves generated from a GIS survey of a given district, the analysis provides an estimate of the performance of standard 3 or 4th generation DH supplying heat at a temperature of 85°C with a CO2 based fifth generation DHC delivering an energy source or sink at about 14°C and having an efficient SOFC-GT cogeneration unit somewhere on the network. Such advanced district energy system illustrates the power of using exergy efficiency indicators to judge on the efficiency increase in the use of the energy resources. This is done with a decomposition of the system in subsystems, as was previously published in a similar context but with more conventional technologies. The possibility of separating CO2 from the effluents of the cogeneration system is also accounted for and this can only be coherently done by using exergy considerations.