Developments in 4^th generation district heating

This editorial introduces the 20^th volume of the International Journal of Sustainable Energy Planning and Management. Papers included are all from the conference on 4^th 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.