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Schedule for converting the steam system in Munich from 2003
© SWM Services GmbH

Network conversion in practice: Installation procedure during a conversion week
© SWM Services GmbH

Use of a mobile heating container
© SWM Services GmbH
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Converting Munich’s steam network

Even in 2000, the gradual growth in the network structures over many years meant that a large part of the Munich district heating system still had high flow temperatures throughout the year. Two thirds of the connection value was attained in sub-networks with flow temperatures greater than 100°C and peak temperatures well in excess of 130°C. Therefore it was not possible to use cheap pipe-laying technology with plastic jacket pipes.

High operating and maintenance costs for the capped ducts and numerous shafts meant that Munich’s district heating was not economic when taken as a whole. This particularly applies to Munich’s largest high-temperature network, the steam network, which has a total pipeline length of around 250 km, 4,400 customers and a connection value of 1,200 MW. In fact, most of these customers do not even require high temperatures: around 95% of Munich’s district heating customers require thermal heat at a temperature level less than 100°C. Only 5% use the high temperatures provided by district heating for generating steam for canteens, air conditioning, low-pressure steam heating and in absorption cooling systems.

For this reason, lowering the flow temperatures when converting the steam network into a hot water network is not just sensible but accords with market demands. The focus of the steam network conversion is on upgrading networks and constructing new customer stations. It is necessary to upgrade the piping, since the future hot water network will be operated with higher pressures. Although the existing pipelines are generally adequate for the higher pressures, network installations such as fittings and expansion joints must be modified. Because glass fibre-reinforced plastic pipes are used, the maximum return pressure is limited to 10 bars, which therefore limits the maximum flow pressure.

The customer stations are converted by installing new compact stations, which are much smaller than steam customer stations. This means that the customers have much more space available in their basements. The conversion only takes place during the summer months when there is no heating, each time within the period of one week, so that ‘normal’ customers are without hot water supplies for 5 working days. For customers where this could be critical, such as hospitals and hotels, hot water is made available during the conversion period. This is provided using mobile hot water containers in different sizes, which are supplied to specific areas or customers. If required, mobile heating generators can also be hired.

Since 2003, around 400 customers on average have been converted per year and 23 km of pipes have been upgraded. If required, the Munich municipal utility company, Stadtwerke München (SWM ), will coordinate the conversion process, procure new compact stations and commission heating installation companies. Thanks to an excellent public relations campaign and little damage, the public response has been overwhelmingly  positive.

Benefit from greater efficiency

With the conversion of the steam network, the Munich municipal utility company has also improved the efficiency of its cogeneration plants. This is because the new method for distributing heat with hot water enables the turbines in the CHP plants to produce steam more efficiently than before. Here the steam is extracted and, by means of heat condensers, used for generating hot water with low pressures. With the same amount of heat production and almost the same fuel consumption, more electricity is generated and the production costs are considerably reduced. This advantage can be directly used at the new gas and steam turbine power plant in southern Munich (with an electrical output of up to 450 MW). The heat exchanger stations for converting steam to hot water will be installed in all generating plants that do not require upgrading in the short and medium term. At the same time there is a reduction in both heat losses and emissions in the network: the reduced energy consumption saves on electricity imports from Germany’s grid, and there are potential CO2 savings of more than 100,000 t/a. In addition, there are practical advantages for district heating customers. For example, the costs for new connections are considerably reduced, and, because hot water systems are very inert, customers will not immediately suffer a cut in supplies should a generating plant fail.

Costs, economic viability

Internal studies by the Stadtwerke München (SWM) have shown that in relation to the pipeline length, the specific operating and maintenance costs for the steam network are on average twice as much as for hot water networks. The ensuing savings in converting to hot water amount to several million euros a year, although of course during the conversion years the savings will only be commensurate with the progress made with the conversion. The conversion of the steam network will be profitable in the long term, since the additional costs resulting from constructing a new hot water transport pipe (southern link), the network upgrading, subsidies for new customer stations and modifications to the generator plants, etc., will be more than offset by the savings made as a result of additional earnings from the increased electricity generation, the lower operating and network expansion costs, and the reduced heating losses. The main impact will only be felt, however, during refurbishment, when existing pipes in capped ducts can be replaced with cheaper plastic jacket pipe technology.

Summary

The most important finding  is that it is possible to convert the Munich steam network during the scheduled yearly phases and with the envisaged technical concepts. The upgrading of the network has functioned. Thanks to the preceding pressure tests made in the condensate system, less damage occurred after the conversion than had been expected. The customer conversion was also carried out as planned, and the difficult logistics in terms of coordinating the network upgrading with the customer conversion within a week for each conversion area was also solved successfully. Last but not least, the project also sets an example for local climate protection, since the successful conversion means that not only has it been possible to prevent the replacement of Germany’s largest steam network with a gas supply, and thus prevent a negative message being sent out to other steam network operators, but it has also been possible to achieve maximum energy savings and emission reductions in Munich. A welcome side effect is the fact that the use of cheap plastic jacket pipe technology creates new economic potential for utilizing district heating, which was previously not considered possible with the elaborate capped duct construction method.

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facilities-based carrier
Stadtwerke München GmbH