International Journal of Sustainable Energy Planning and Management Vol. 20 2019 1 International Journal of Sustainable Energy Planning and Management Vol. 20 2019 01–04 1Corresponding author - e-mail: poul@plan.aau.dk ABSTRACT This editorial introduces the 20th volume of the International Journal of Sustainable Energy Planning and Management. Papers included are all from the conference on 4th generation District Heating and Smart Energy Systems, Aalborg, 2017, treating the control of heat systems, technical and non-technical barriers to low-temperature district heating. Temperature levels are also key- parameters in an investigation of heat pumps and heat sources for heat pumps in Tallinn. In parallel to apps focusing on electricity systems, an app focusing on heat systems is presented. 1. Introduction The 4th generation district heating [1,2] and smart energy system [3–5] approaches are becoming well- established terms within the energy research community and is also the cornerstone of the annual conference on 4th generation District heating and Smart Energy Systems that is alternating between Aalborg and Copenhagen, Denmark. The 2018 conference in Aalborg follows successful events from previous years documented in this journal with special issues from 2015 ([6–9]), 2016 ([10–18]) and 2017 ([19–23]) – as well as special issues in Energy [24–26]. 2. Contents In this Special Issue, Roberto et al. [27] take a starting point in how district heating systems are developing in the same direction as electricity systems with more generators in the grid and how this leads to on the one hand better energy efficiency but also in a more complex energy system to control. The authors model the district heating system of Turin finding that a system with distributed heat generation also benefits from a system of distributed heat storage. Volkova et al. [28] develop a mobile app to increase district heating users’ awareness of the technology’s benefits compared to other heating solutions with the aim of strengthening district heating’s position. The app is intended to enable users to see the benefits of district heating, see how 4th generation district heating can impact fuel consumption, emissions and more as well as showing the operation of district heating systems. Brange et al.[29] treat how lower temperatures while positive for losses and efficiencies in the district heating systems also may decrease transmission capacity of the same with bottlenecks as a result. In bottleneck situations, a variety of solutions exist including increased supply tem- perature, larger pipes, increased pumping power and/or new pumps, better cooling, local heat supply and demand side management. These are explored and a method for choosing among them is elaborated. This is a continuation of work by the same authors presented in [30]. Pellegrini [31] stress the importance of heading for lower district heating temperatures, but also point to some issues encountered when performing the transition. Developments in 4th generation district heating Poul Alberg Østergaarda,*, Henrik Lunda and Brian Vad Mathiesenb a Department of Planning, Aalborg University, Rendsburggade 14, 9000 Aalborg, Denmark b Department of Planning, Aalborg University, A.C. Meyers Vænge 15, 2450 Copenhagen SV, Denmark Keywords: District heating; Control systems; Heat pumps; URL http://dx.doi.org/10.5278/ijsepm.2019.20.1 mailto:poul%40plan.aau.dk?subject= http://dx.doi.org/10.5278/ijsepm.2019.20.1 2 International Journal of Sustainable Energy Planning and Management Vol. 20 2019 Developments in 4th generation district heating The transition is meet by barriers of both a technical and non-technical nature to which the authors present advice for decision makers. Pieper et al.[32] investigate heat pumps in district heating systems and the use of optimal low-temperature sources. Analyses performed over the year probe into optimal heat sources over the year and optimal heat pump investment strategies with a view to minimising consumer costs. Based on a case study of Tallinn, they show how large-scale heat pumps based on sewage water, river water, ambient air, sea- and ground water can play an important role in the local district heating system. 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