NZEB, low temperature DH, optimization, building refurbishment
The European directive 2010/31/EU states that within the end of 2020, all new buildings should be nearly zero-energy buildings (NZEB), and in the meanwhile, new and renovated buildings performance should comply with new requisites established through a cost-optimal approach. For new buildings, Member States shall ensure that, before construction starts, the technical, environmental and economic feasibility of high-efficiency alternative systems are considered and taken into account, such as, among others, decentralised energy supply systems based on energy from renewable sources, cogeneration, district or block heating or cooling. However, the reduction of the heat needs of the buildings causes a partial utilization of the district heating (DH) capacity with a consequent possible reduction of the distribution efficiency. How to combine the reduction of energy needs and their duration with the technical and economical sustainability of DH and cogeneration systems is an open question. This paper aims to define the cost-optimal solutions of refurbishment for buildings connected to a DH considering the impact on both the heat demand reduction and network distribution losses. The possibility to shift to a low-temperature DH is considered a feasible solution to reduce thermal losses and hence increasing the network efficiency that is connected with a systematic approach to the refurbishment of the connected buildings. For this purpose, an integrated model has been developed and calibrated on real data of a DH system based on biomass and located in northern Italy. The energy performance of the coupled DH - buildings system has been assessed taking into account different measures for the improvement of the existing buildings and different numbers of refurbished buildings. The improvements involve building envelope, heating system and strategies of heating management. A multi-objective optimization has been carried out considering the minimization of the energy needs, the minimization of the net present value (NPV) for the final user and the maximization of the distribution efficiency of the DH, in terms of distribution temperature. The optimization has been also carried out considering different prices policies for heat sales according to its temperature level to assess how it can promote the efficiency of the DH. The results highlight the optimal measures that allow the minimum NPV of the building refurbishment and the highest efficiency of the considered DH system.