*Article* **Challenges and Barriers for Net-Zero/Positive Energy Buildings and Districts—Empirical Evidence from the Smart City Project SPARCS**

**Daria Uspenskaia \*, Karl Specht , Hendrik Kondziella and Thomas Bruckner**

Institute for Infrastructure and Resources Management, Leipzig University, 04109 Leipzig, Germany; specht@wifa.uni-leipzig.de (K.S.); kondziella@wifa.uni-leipzig.de (H.K.); bruckner@wifa.uni-leipzig.de (T.B.) **\*** Correspondence: uspenskaia@wifa.uni-leipzig.de

**Abstract:** Without decarbonizing cities energy and climate objectives cannot be achieved as cities account for approximately two thirds of energy consumption and emissions. This goal of decarbonizing cities has to be facilitated by promoting net-zero/positive energy buildings and districts and replicating them, driving cities towards sustainability goals. Many projects in smart cities demonstrate novel and groundbreaking low-carbon solutions in demonstration and lighthouse projects. However, as the historical, geographic, political, social and economic context of urban areas vary greatly, it is not always easy to repeat the solution in another city or even district. It is therefore important to look for the opportunities to scale up or repeat successful pilots. The purpose of this paper is to explore common trends in technologies and replication strategies for positive energy buildings or districts in smart city projects, based on the practical experience from a case study in Leipzig—one of the lighthouse cities in the project SPARCS. One of the key findings the paper has proven is the necessity of a profound replication modelling to deepen the understanding of upscaling processes. Three models analyzed in this article are able to provide a multidimensional representation of the solution to be replicated.

**Keywords:** smart city; net- and nearly-zero-energy buildings; positive energy communities and districts; renewable energy integration; energy flexibility in buildings and communities; simulation and optimization methods; practical experience from demo sites
