Jan Eric Thorsen, Director, Danfoss Heating Segment Application Centre, Oddgeir Gudmundsson, Director Projects, Andre Hansen, Director Applications & Technology, Danfoss GmbH
Introduction Transforming the energy system towards a sustainable system based on a high share of often fluctuating renewable sources is a challenge. At the same time, building energy consumption is being reduced by energy renovations, which puts up challenges for the sustainability of traditional district heating (DH) systems. The district heating concept in general holds the key to address the fluctuating nature of renewables. To allowmaximal utilization, traditional DH temperature levels need to be reduced, which conveniently also solves the challenge of reduced energy demand in buildings. This development path is characterized as the 4th generation DH system. But how does this development path look and what can be learned from the transitions between older DH generations in order to manage the transition toward the 4th generation more efficiently? This article addresses some of the main learnings from the past transformation processes in order to advice on strategies towards the 4th generation. The generations of DH DH goes quite some years back. During the years, it has developed to fulfill the demands as they came up, e.g. the demand for reduced investment and heat costs, lower equipment space demands, and concerns of energy efficiency. Four generations are lined out, each indicating major changes in the technology. The main characteristic of the first generation DH systemwas the heat carrier of steam. Steamwas the obvious choice since electric motors were not yet available for more long-distance hot water distribution. The consumer groups were small urban industries using steam in their processes and large heat consumers such as hospitals and big residential complexes. Additionally, steamwas considered as a good heat carrier due to its high heat content. Furthermore, it was available from e.g. boilers or steam power plants, which did not run in a condensing mode and hence had high temperature steam available. This was when the concept of CHP was brought into operation. The first steam-based systems where built in the USA in the 1880s and became the normal DH design until 1930. Soon, the disadvantages of steam-based systems, including the high investment and operation costs, requirements for complex condensate system, and high heat losses due to high operating temperatures, became apparent.
The main characteristic of the 2nd generation DH system is heat transported by pressurized super-heated water at temperatures above 100°C. This is a significant difference compared to the 1st generation DH systems. By moving from steam, a number of benefits were achieved, for example: • Higher electrical efficiency of condensing heat plants. • Return water was easily returned and the low-grade energy in the return water was further used. • Better energy quality and energy usage match. • Simple to build and operate the system, even under varying load. • Simple way of metering heat consumption. • Large-scale thermal storage became possible, decoupling demand and supply. • Reduced risk and consequences in case of leakages. The 2nd generation DH was dominating in DH systems until the 1980s. The distribution network typically consisted of two steel pipes, a flow pipe and a return pipe. Another difference to the 1st generation steam based system was the centrally placed circulation pumps providing sufficient head to drive the water through the distribution network and allow heat extraction at the consumer site. Large pumps were available for the 2nd generation systems, leading to the situation that steam pressure based water circulation was not a precondition for DH systems anymore. The main benefit of the 2nd generation DH system compared to the 1st generation DH system is the increased energy efficiency due to lower operating temperatures and reduced risk and consequences in case of leakages. Furthermore, the challenges related to the corrosive condensate experienced in the 1st generation were eliminated.
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