Steps towards Low temperature (4G) district heating
We are living in a period of crucial change (paradigm shift) in energetics, including district heating (DH) systems. Developed and built between 1930 – 1970, 2nd Generation (2G) DH (with supply temperature above 100°C and coal as main energy source) has been replaced later by 3G DH using coal, biomass and wastes as primary energy sources and lower supply temperature (80 – 100°C). The 4G DH [1, 2] is knocking at the door (with pilot installations in Denmark , England , Norway , Belgium , Finland  and Germany [2,7]). The 4G DH with a supply temperature below 70°C enables lower heat losses, integration of renewable heat (solar, geothermal, wastes and biomass sources) and compatibility with cooling networks and smart energy systems – see e.g. .The role of district heating (especially Low Temperature DH - LTDH) in decarbonization of energy systems is significant. The LTDH systems allow utilization of surplus heat from industry and waste-to-energy systems, use of geothermal and solar thermal heat. The technologies converting solid biomass into bio(syn)gas as well as liquid biofuels will also play an important role in future "smart energy systems". Such systems are characterized by a high degree of integration with district heating, cooling, electricity and transport fuel, leading to possible synergies among them.
Nowadays the burning issue is: how to transform existing heat supply systems into 4G systems? What roadmap to LTDH is possible and most effective (economically and ecologically)? In Albertslund , HojeTaastrup or Berlin Adlershof  LTDH grids constitute a subgrid of the main (3G DH) grids; in HojeTaastrup the grid is fed from DH return-flow.
In Kalundborg an independent ultra-LTDH grid is considered to utilize the low temperature surplus heat (around 20°C) as supply for nearby located municipalities. New, innovative ideas of integrated district heating and cooling networks are being developed, e.g. Ectogrid™ to be implemented by E.ON Sverige AB . In Gulbene a LT grid with local biomass boiler was successfully implemented . In Poland, a different approach was considered due to recent political, economic and technology changes. Widely implemented energy efficiency measures and better insulation technologies result in much lower heat demand and thus an oversized DH grid. Therefore, one can consider a temperature decline in DH feed flux. An example of such changes in the Lomża DH grid was analysed .
The application of low temperature district heating technology on a regional level requires a comprehensive review of all process steps: from heat generation over distribution to consumption within the built environment. The main issue is to assess which of the road towards LT (4G) district heating is feasible taking into account local conditions (see above). The approach includes taking primary, secondary, end and useful energy and exergy into account. This allows an overall optimization of energy and exergy performance of new district heating systems and the assessment of conversion measures (from high to low temperature DH) for existing DH systems.
In frame of LowTEMP project Riga University in cooperation with other partners developed methodology for local LTDH implementation . Any DH strategy should be developed in close cooperation with the municipality and relevant stakeholders (see Fig. 1) as there are steps (see Fig. 2) that cannot be implemented without the support of heat supplier or DH network operator. Besides, municipality has an important role in transformation process as it has necessary tools to ensure legislative framework, identify long term goals for particular area and ensure communication between heat supplier, developers, consumers and other interested parties.
The Methodology for strategies to implement LTDH is a cyclical process presented in several consequential steps. This methodology suggests the continuous reflection of particular steps to achieve the general transformation goals, including review, adaptation and concretization of the targets.
Figure 2 presents the conceptual scheme of the proposed methodology to implement transformation strategy to move towards LTDH at municipal level.
Useful links describing different tools for energy plans and strategies development:
-  Schmidt D and Kallert A2017Future low temperature district heating design guidebook: Final Report of IEA DHC Annex TS1. Low Temperature District Heating for Future Energy Systems. International Energy Agency.
-  Walnum HT and Fredriksen E2018 Thermal energy systems in zen. Review of technologies relevant for ZEN pilots.ZEN Report No. 3 (SINTEF Building and Infrastructure).
-  Olsen PK, Christiansen CH, Hofmeister M, Svendsen S, Rosa AD, Jan-Eric Thorsen J-E, Gudmundsson O, Brand M (Eds)2014 Guidelines for Low-Temperature District Heating A deliverable in the project financially supported by the Danish Energy Agency in the R&D programme EUDP; EUDP 2010-II project Journal No. 64010-0479 final report.
-  Wiltshire R 2011Low temperature district energy systems. 16th Building Services, Mechanical and Building Industry Days International Conference, 14-15 October 2011, Debrecen, http://www.aaltopro2.aalto.fi/projects/up-res/UPRES-DebrecenConf-paper-RWiltshire.pdf
-  Line T 2013 Ultra-lavtemperert nærvame. VVS-konferansen, Stavanger.
-  Rämä M and Sipilä K (Eds) 2016 ANNEX TS1: Low Temperature District Heating for Future Energy Systems – Subtask D: Case studies and demonstrations. Final subtask D report of the IEA DHC Annex TS1:Low Temperature District Heating for Future Energy Systems, VTT Technical Research Center of Finland, Espoo.
-  Reinholz A 2019 Wohnen am Campus: BTB´s low temperature district heating system with solar feed-in in Berlin-Adlershof, BTB, Berlin, 17.01.2019; https://www.btb-berlin.de
-  A. Cenian, M. Dzierzgowski, Bartosz Pietrzykowski, On the road to low temperature district heating, Journal of Physics: Conference Series 1398 (2019) 012002, doi:10.1088/1742-6596/1398/1/012002.
-  Methodology for strategies to implement LTDH”, made by RTU.
-  Oxenvad C 2017 Proc. Energy Efficient Cities, Gdynia.
-  Strömberg S 2018 Ectogrid™. Shared energy for a sustainable city, http://decarbcities.eu/wp-content/uploads/2018/05/10_Stromberg.pdf
-  F. Diaz, I. Pakere and F. Romagnoli, Life Cycle Assessment of Low Temperature District Heating in Gulbene Region, Procc. CONECT conference 2020, RIGA.