Fourth Generation District Heating, the prospects of Gothenburg: An investigation of the 4GDH concept and the motivations to implement it in Gothenburg
Examensarbete för masterexamen
Sustainable energy systems (MPSES), MSc
Fourth Generation District Heating (4GDH) enables the District Heating (DH) system to be an integrated part of a future sustainable energy system and has the potential to make new markets accessible where current DH technology is not a viable option. The combination of smaller heating demands from low-energy buildings and more renewable energy sources beingutilized in the energy system requires current DH systems to evolve. Previous generations has been identiﬁed by higher supply and return temperatures, compared to the suggested temperature levels of the fourth generation of (50/20) ◦C. One aim with 4GDH is to secure the role of DH in a future sustainable energy system. Cost and resource eﬃcient heat production and distribution is essential to remain competitive as a DH company. Lower temperatures could enable reduced heat losses from the distribution system along with improved performance of several production technologies, namely in low-grade heat utilization. This thesis investigates the 4GDH concept in a literature study followed by the impact of reducing the temperature levels in the DH system of Gothenburg. An estimationofthepotentialoperatingcostsavingsofthepresentsystemispreformed through a simulation study by calculating the annual variable operating cost for decreasing system temperature levels. The starting point is a system temperature of (90/45)◦C, close to the annual average system temperature in Gothenburg 2017 of (90/43)◦C. The supply and return temperature are reduced simultaneously by steps of 1 ◦C to (75/30)◦C. At present temperature levels the saving potential is found to be about 2.3 SEK/MWh and decreases linearly towards 1.7 SEK/MWh at (75/30)◦C. The saving potential throughout the year is found to correlate with the outside temperature, being negligible oﬀ heating season. No claim is made that this represents the exact savings potential for the DH system of Gothenburg, however the general trend and the saving potential diversiﬁed throughout the year are of greater interest, supporting further analysis of the value in transitioning towards 4GDH in Gothenburg. Technologies supporting the lower DH temperatures includes larger heat transferring surfaces in substations and internal heating systems along with individual substations (one per apartment). A third distribution pipe is also introduced, dedicated to recirculating supply water in times of low demand, thus enabling lower return temperatures. The third pipe is found to potentially increase the total distribution losses whilst mainly reducing the return temperature summertime when the operating cost of DH is low and the saving potential is small. Using the results acquired earlier, the operating cost savings related to introducing a third pipe (not considering the investment or impact on distribution losses) for a contemporary case and a future case (with a high penetration of low-energy buildings) is found to be 0.5 % and 8 % respectively. The impact of reduced temperature diﬀerences in terms of ﬂows and distribution losses for varying system temperatures is also investigated. A trade of in distributionlosses,consideringheatlossesandpump-work,whentransitioningtowardslower temperatures and implicitly smaller temperature diﬀerences is found. In Gothenburg, along with similar mature DH systems, a smooth transition between present DH system design and 4GDH could be enabled by maintaining a high temperature in well established parts of the DH systems, whereas individual subsystems with strategic locations and well suited customers can incorporate at least parts of the 4GDH concept. In addition, this partial approach can lead to better utilization of present distribution systems and provide aid in congested parts of the system.