District heating with low-carbon heat sources and low distribution temperatures

Energy supply sector is the largest source of anthropogenic emissions with a share of more than a third in global greenhouse gas (GHG) emissions. In terms of energy demand, the heating and cooling sector corresponds to more than a half of the global final energy consumption. Urban areas have been evaluated to be responsible of 76 % of global energy use and 37-49 % of the GHG emissions. The relevance of cities in the context of climate change mitigation and the significance of the heating and cooling sector imply that solutions for urban areas are an essential part of the mitigation measures needed. This is where district heating (DH) and district cooling (DC) technologies can play an important role. Both DH and DC are energy solutions designed for densely populated areas. This thesis focuses on developing DH systems by studying means for improving their efficiency and integrating more low-carbon heat sources for heat supply. The potential impact of low temperature distribution is also assessed. The heat sources considered in the scope of the presented research are heat pumps (HPs), solar collectors (SCs) and nuclear district heating (NDH). Centralized SCs provide 4-5 times higher heat output than decentralised building-specific units with the same total investment. However, HPs outperform the centralized SCs in Finnish DH systems by producing 9-30 times more heat with the same investment. The feasibility of NDH is strongly linked to the DH demand and distance between the site and the DH system. Low temperature distribution reduces the heat losses and, more importantly, improves the efficiency of connected heat supply. A fair and transparent allocation of benefits and costs is a prerequisite for the transition due to stakeholders involved. The benefits are for heat supply and distribution (DH company) while costs mainly result from required building-level changes (customer). The use of low temperature distribution requires more careful management of the temperature levels, making network simulation tools both in planning and operational phase important. Both HPs and SCs benefit from low distribution temperatures.