This report is made as a literature review, focusing on the work done to increase understanding of efficiency in the categories of substations and secondary heating systems, with respect to the deviation from the theoretically possible cooling off in the distribution network as well as the economic benefit that appear. The main purpose of a considerable part of the literature used in this report addresses the issue of identifying individual causes of reduced cooling in district heating systems. These literature resources have been compiled and summarized as part of the report. The technology of district heating is associated with benefits such as better use of the energy in a fuel. This is the case of cogeneration plants where serial generation of electricity and thermal energy increases efficiency compared with the parallel generation where heat is generated locally and electricity is generated centrally. Serial generation thus allows for lower primary energy demand. Another benefit from combustion in units with higher capacity installed is that a higher control of emission with environmental impact is permitted. Additionally local environment change drastically when a few large supply units replace a large number of local supply units. It has also been shown that district heating can reduce greenhouse gas emissions in a cost efficient way. Thus being a part of the energy system to achieve the EU climate goals In Sweden, district heating is developed to a high degree. In connection with decreasing focus on expansion, the focus on maintenance and optimization and how district heating should look like in the future increases. In conjunction with lower heat demand from new and renovated buildings distribution cost will increase. For district heating to maintain competitiveness a development in distribution technology that move toward the next generation of distribution technology is necessary. Average temperatures today in Swedish district heating systems are for supply water 86 ° C and for return water 47 ° C. In the future temperature levels could decrease to current with temperatures down against 55 ° C supply temperature and 25-20 ° C return temperature. The latter system temperature levels moves towards the ideal possible. It is possible to distinguish four generations of district heating distribution technology. The differences between generations are essentially depending on temperature levels but also depend on state of matter. The first generation district heating used high-temperature steam for heat transfer and then the newer distribution technologies resulted in lower temperatures and change of phase, from gas to liquid. The third generation of district heating distribution technology meant lower temperature than the second generation, and likewise the fourth generation will have a lower temperature level than the third-generation distribution technology for district heating. The development is driven by the benefits of lower temperature levels. One of the more appealing benefits of lower supply temperature is the possibility to use low exergy heat, resulting in reduced need of primary energy. The potential heat sources where increased heat supply with lower system temperatures becomes available can be seen in the four next bullets. Waste heat Geothermal heat Solar heat Heat pump Other advantages obtained with lower temperature levels in heat distribution are. Lower distribution losses Higher electrical power efficiency in CHP Increased efficiency in flue gas condensation Increased capacity in the distribution network Reduced need for pump power in the distribution network Lower risk of serious scalding Increased capacity in heat storage Ability to use other materials for distribution at lower cost There seem to be a consensus in the literature that lower temperature levels in district heating systems are a desirable change. The reason for this is likely that there are mostly advantages of lower temperature levels. The drawbacks of lower temperature levels are negligible which make the risk of investment low.

Sveriges regering har antagit tydliga mål för minskad energianvändning i bostadssektorn. En energikartläggning av bostadsområdet kvarteret Körsbäret i Växjö utförts. Området är byggt år 1962, hus från en tid som idag har stor potential för energieffektivisering. I nuvarande värmesystem är en fjärrvärmecentral placerad i ett av husen och betjänar även via markkulvertledningar de tre övriga huskropparna med värme och tappvarmvatten. En simulering om det skulle bli energi- och kostnadseffektivt att byta ut kulvertledningarna mot nya alternativt att förlägga en fjärrvärmecentral i varje huskropp har utförts. Energikartläggningen visade på stor variation av den specifika energianvändningen för området och att inget av husen uppfyllde dagens krav för nybyggnation i BBR på 80 kWh/m2Atemp och år. Dock låg energianvändningen klart under genomsnittet för flerbostadshus byggda under 1960-talet. Simuleringen visade att ett byte av kulvertledningar skulle medföra stora energibesparingar, vilket också resulterade i att detta alternativ hade den lägsta livscykelkostnaden.