Transition to low temperature distribution in existing systems

One key aspect of the 4th generation district heating systems 1 is a low temperature solution for distribution – i.e. the supply temperature in the distribution network is set to a lower level (30-70 °C, depending on design) than traditionally (80-115 °C). The temperature level has a defining impact on the efficiency of the distribution through heat losses and it improves the potential for integrating new sources of heat into the system. These new heat sources include heat pumps, renewable energy sources and excess heat from communities and industry. Combined heat and power (CHP) production and boiler based heat supply benefit from a lower temperature level as well. At the same time the transition to lower temperature level imposes challenges on pipe capacities within the distribution network, heat exchanger and secondary side design and heat distribution systems within buildings. While low temperature distribution is simply a design choice for new systems, refurbishing existing systems is much less straightforward and a current challenge in countries with developed district heating systems. Although technically feasible, the changes needed should be studied on a system level for a throughout evaluation of the cost-efficiency in terms of both energy savings and emissions. The needed investment and the benefits can end up to be unevenly distributed among the involved parties, e.g. the local utility and customers. This can represent a practical barrier impeding or blocking the transition process. This paper studies the effects and impacts of the transition to low temperature distribution in existing district heating systems, the needed technical solutions and the improved potential of utilising new, enabled heat sources. A systematic method and tools for evaluating the system level benefits of the transition are described. This study lays the groundwork for a system specific case study of a major refurbishment of an existing district heating system.