Distribution of District Heating: 2Nd Generation

Article Technical paper

Title TitleDistribution of district heating: 2nd Generation

Jan Eric Thorsen, Oddgeir Gudmundsson, Marek Brand, Anders Dyrelund Author(s), Danfoss Published at ...

districtenergy. danfoss.com www.danfoss.com Jan Eric Thorsen, Oddgeir Gudmundsson, Marek Brand, Danfoss District Heating Application Centre, Denmark Anders Dyrelund, Senior Market Manager Energy, Romboll, Denmark

Distribution of district heating: 2nd Generation

District heating (DH) is here to stay. Looking back on the history of DH it goes quite some years back. During the years it has developed to fulfill the demands as they came up, typically driven by the demand for reduced investment and heat costs, but also lower equipment space demands, concerns of energy efficiency and lower fire risks. The development has been categorized in 4 generations which each indicate major changes in the technology. Currently most DH schemes being operated are categorized as being at the stage of 3rd generation DH technology starting its transition to the 4th generation to address the challenge of the future non-fossil and renewable based energy system. To meet the demands of the future energy system, existing DH schemes will develop into the next generation, the 4th and the most advanced generation. The intention of this series of articles is to describe the typical principles of each generation and discuss the motivation behind each generation and the main development drivers.

The main characteristic of the second The 2nd generation DH was dominating generation. The 2nd generation also generation DH system is that the heat in DH systems from the 1930s to paved the way for fuel savings by is transported by pressurized super- the 1980s. The distribution network applying CHP in larger scale than heated water at temperatures above typically consisted of two steel pipes, before, driving down the cost of 100°C. This is a significant difference a flow pipe and a return pipe. To electricity generation and increasing compared to the 1st generation DH some extent also open systems were both the living comfort and the systems, where steam is used to applied mainly in former Soviet Union air quality level for the population transport the heat. By moving from countries, in which consumers tapped in urban areas. The super-heated steam a number of benefits were domestic hot water directly from the hot water district heating allowed achieved, for example: district heating pipes. A special case is using the surplus heat from large • Higher efficiency of condensing heat seen in Iceland where only the supply power plants much more efficient, plants. pipe was installed due to abundance of where some of them could even • Return water was easily collected free geothermal energy and sufficient be modified to CHP extraction in a and the low grade energy in the water quality. After extracting the heat simple way. Although the decreased return water was further used. from the supply, the water is flushed to electricity generation could be • Better energy quality and energy the sewers. Another difference to the as high as 0,2 MW per megawatt st usage match. 1 generation steam based system was heat extracted, it reduced the fuel • Simple to build and operate the the centrally placed circulation pumps consumption for heating by 50% system, even under varying load. providing sufficient head to drive compared to the 1st generation DH. • Simple way of metering heat the water through the distribution Moreover it opened for use of surplus consumption. network and allow heat extraction heat from waste incineration plants. • Thermal storage in larger scale at the consumer site. Large pumps Altogether, including peak load boilers nd became possible, decoupling were available for the 2 generation and heat losses, the consumption demand and supply. systems, leading to the situation that of fossil fuels could be reduced • Moving from steam to pressurized steam pressure based water circulation by 50-70% for heating compared water allowed significantly larger was not a precondition for DH systems with the 1st generation DH. Finally distribution networks, which anymore. the heat storages tanks at the CHP reduced requirements to plant The main driver for the development plants allowed to optimize the heat location. of the 2nd generation DH system was production and extract the heat in • Reduced risk and consequences in increased operational safety, increased off-peak periods without reducing the case of leakages efficiency of both the heat distribution power capacity in power peak hours. • More reliable and secure supply, and operation of heat plants and The flow and return steel pipes were e.g. in case of flooding due to heavy the possibility to use a wider variety placed underground into concrete rainfall of heat sources compared to the 1st ducts, similarly as for the 1st generation

2 DH systems. In the early days the ducts for the 2nd generation systems were built on site sized large enough Compesator for persons to walk along the pipes, later when more experience with the concept had been acquired ducts DH pipe became narrower. To speed up the installation time and to reduce the costs, pre-fabricated duct elements were invented. However chambers for U-turn e.g. valves, expansion joints, anchor Support elements and draining equipment were still built on site, which was both Guide & support time consuming and expensive. In the 2nd generation the ducts typically have a longitudinal slope, leading the intruding water into the duct towards Fixation a drain located at the lower locations of the duct system. The longitudinal slope of the pipe additionally gave the opportunity to vent the pipe at FIGURE 2: Principal pipe support principle, including supports, guides, U-turns, compensators and the higher locations and to drain the fixations. pipes in the lover locations, e.g. in connection with service works. The pipes were insulated on site using quality was kept high, the systems special attention to the expansion, typical mineral wool and wrapped have proven to last for many decades. e.g. with expansion units and bends with a protective layer of fabric. As Even today more than 40 year old underground. long as the insulation was dry and pipes are in very good shape. In general three operating principles undamaged the thermal resistance Due to the high heat losses associated were applied in the 2nd generation was high, but when the mineral wool with the main pipes some countries networks: became wet, e.g. when the ducts utilized separate and smaller • Constant flow from one plant to the were flooded, the thermal resistance distribution pipes for DHW purposes dropped significantly. Although the whole network. and hence minimizing the heat loss design heat loss for the 2nd generation • Fixed flow ratio to consumers. during the non-heating period. Figure DH pipes was quite low it typically • Variable flow, supply according to 1 shows four pipelines in a local 4-pipe became high over time due to damage consumer’s demands. system supplied from a group sub- of the insulation and loss of insulation In the cases where the variable flow station. The domestic hot water supply properties. principle was applied the heat could and circulation are wrapped in the In the case the steel pipes, have be supplied from several base load same insulation on top. The two lower and peak load plants, within the same been protected against corrosion pipelines are for DH supply and return. from outside (concrete ducts for network, in an optimized way. The To maintain proper pipe position in underground pipes or protection of tariff for the heat in the variable flow above ground pipes) and the water the ducts, or proper pipe position systems was based on both a fixed above ground, a number of guide capacity part and a variable energy elements were installed along the part. pipe. These include anchor elements, The heat exchangers applied were simple support elements and guide typically of the tube and shell type, elements. Besides this, compensators see Figure 3. They were used at the or U-turns were installed to absorb utility, in the DH network to separate the large thermal elongations due to primary and secondary networks and large temperature variations between in the buildings as well. Domestic hot highest winter operation temperature water was prepared by shell and tube and no operation during summer (e.g. heat exchangers for a larger group of up to 170°C supply), limiting the stress consumers. In case of supplying on in the pipe material. building level, the hot water storage Figure 2 shows a principal example of tank principle was applied. the pipe and the above mentioned FIGURE 1: 2nd generation DH pipes, U-turn The radiator systems in the buildings with supply and return pipe placed elements. were typically designed for supply in the lower pipelines, and a The first pre-insulated pipes were temperature of 90°C, either via direct domestic hot water and domestic developed in this period to be used or indirect connection with the DH hot water circulation wrapped together in same insulation, placed in networks with temperatures network. Due to the high supply in upper pipeline. typically below 130°C and often with temperatures the surface areas could

3 1970s resulted in the development of the 3rd generation of DH systems. The main technical improvements were related to: Reducing the operation costs by: • Reducing the temperature levels, leading to savings in distribution losses. • Lower temperature opens the FIGURE 3: Shell and tube heat exchangers, FIGURE 4: One pipe radiator system, with no opportunity for other low cost and/ controlled by manual valves. control devices. or renewable sources. • Increasing the thermal resistance of the insulation, leading to savings in be limited, leading to material and Benefits distribution losses. cost savings. But the drawback was The main benefit of the 2nd generation • Pre-insulated pipes, leading to long a higher thermal distribution loss in DH system compared to the 1st lifetime of the pipes. the network. The buildings heating generation DH system is the increased • Better efficiency of the CHP plant installations typically had one pipe energy efficiency due to lower due to lower temperatures. radiator system (with or without a by- operating temperatures and reduced • Components and systems of higher pass of the radiators) to save material risk and consequences in case of quality and efficiency. and labor costs, see Figure 4. leakages. Further the challenges Reducing the investment costs by: Reduction of supply temperature on related to the corrosive condensate • Pre-insulated pipes buried direct into st the building level was realized either experienced in the 1 generation were the ground without expansion joints. by traditional mixing loops with pumps eliminated. • Pre-fabricated material lean or by a jet pump concept. The building substations. installations for the 2nd generation DH Reasons for the 2nd generation to • Improved DH pipe construction systems were typically made on site develop into the 3rd generation methods. based on heavy and material intensive The increased focus on energy • More cost effective and larger components. efficiency and energy costs in the unpressurized heat storage tanks.

4 4 References • Lund, H. et. al. (2014) 4th Generation District Heating (4GDH). Integrating Smart Thermal Grids into Future Sustainable Energy Systems. Energy 68(2014) 1-11. • Frederiksen, S. Werner, S. District Heating and Cooling , Studentlitteratur, Lund 2013. • Dawid O. Meeker Jr. et al., District Heating and Cooling in the United States, National Academy Press, Washington DC. 1985. International Standard book number 0-309-03537-6. • http://www.res-h-policy.eu/downloads/Swedish_district_heating_case- study_(D5)_final.pdf • High Temperature Water Heating: Its Theory and Practice for District and Space Heating Applications, Dec 1963 by Paul L. Geiringer.

More information Find more information on Danfoss Heating products and applications on our homepage: www.heating.danfoss.com

Danfoss A/S Heating Segment • heating.danfoss.com • +45 7488 2222 • E-Mail: [email protected]

Danfoss can accept no responsibility for possible errors in catalogues , brochures and other printed material. Danfoss reserves the right to alter its products without notice. This also applies to All trademar ks in this material are proper ty of the respective companies . Danfoss and all Danfoss logotypes are trademar ks of Danfoss A/S. All rights reserved.