Exergetic optimisation of the district heating supply in Ulm
Initial situation in Ulm
The heating market in the town of Ulm, which has approximately 120,000 inhabitants, is unusual: district heating currently meets about 50% of the entire heating requirement. The supply area extends to the inner and western part of the town, Böfingen/Eichberg, the University District on the Eselsberg hill, the Kuhberg area as well as to the Donautal industrial area and the Wiblingen residential district in the southern parts of the town, which are still independent of the rest of the grid. Lowering the temperature and a holistic approach to converting the steam network provide the bases for the next stage in achieving exergy optimised district heating. In order to further pursue this process for increasing the exergetic efficiency, the Fernwärme Ulm GmbH utility company (FUG) is systemically searching for further possibilities to lower the current supply and return temperatures without compromising the supply quality to customers. Lowering the return temperature is generally seen as a proven tool in order to be able to also lower the network supply temperature in the long term. However, this places technical limitations on the return feed connection to heating customers. For example, at a certain limit to the return feed output, extensive areas need to be adapted to the return temperature since further individual connections no longer make sense.
Conversion of the steam network as a basis for exergy optimised district heating
With the development of a detailed concept for converting the inner-city steam network to hot water operation, a cornerstone has been laid for the structural redesign of the district heating: by considerably reducing the supply temperatures (steam network: 180 °C, future hot water network 120 °C, variable) and the associated increase in the electrical generating efficiency, the energy generated in the central CHP plant in Magirusstrasse will be used with a greater exergetic efficiency. As part of the detailed investigation into converting the steam network, the district heating system is being examined holistically. FUG is also systematically searching for other possibilities to lower the current supply and return temperatures without compromising the supply quality to customers.
Goals, Work approaches
In Ulm, two approaches have been examined:
- Reduction of the supply temperature for the university connection from a constant 180 °C at the moment to a maximum of 120 °C with variable operation.
- Systematic reduction of the network temperatures, in particular the return feed temperatures in the existing district heating network for supplying new buildings and refurbished old buildings.
Both approaches require close collaboration with the most important customers and ultimately lead to a better use of the heat provided by the generation plants. The improved use of the heat goes hand in hand with exergetic optimisation, since both cases lead to reduced return temperatures. This in term enables the electrical generating efficiency to be increased in the CHP plants.
Sub-project 1: Reduction in the temperature of the university connection
FUG has been operating the university connection since 1995 as a hot water supply connection from the CHP plant on Magirusstrasse to the university campus on Eselsberg hill. Two additional gas-fired heating boilers are also available there for covering peak loads. The university and its downstream customers have demanded a supply temperature of 180 °C throughout the year. In meeting this requirement with the supply feed, FUG also supplies several customers along the connection pipeline from the return feed, which is generally at a temperature of at least 90 °C. In practice it can be observed that the return temperature during low load periods in summer or in the transitional periods is considerably higher with up to 120 °C, since the work and capacity provided are not required. The high supply temperature of 180 °C is used on the scientific campus by a two-stage absorption chiller that operates a district cooling network. However, neither FUG nor the responsible university department have any firm knowledge at the moment as to which other processes currently require these high supply temperatures and which intentions in terms of temperature requirements are currently being pursued by individual high-temperature heating customers in the “Science City”. It can merely be determined that measures will be carried out during the next few years, such as in the surgery faculty, and that the early presentation of a new temperature and supply concept can prevent a manifestation of the current temperature requirements for the next 10 to 15 years.
As part of an exergetic optimisation and harmonisation of the network parameters, it needs to be asked whether these high supply temperatures, which furthermore are kept constantly high throughout the year, are actually required. By lowering and, if required, varying the supply temperatures, two results can be expected that improve the overall efficiency of the supply: a reduction in the transport losses and an increase in the electrical generating efficiency in the Magirusstrasse CHP plant through using a more favourable decoupling level (e.g. with lower pressures).
Sub-project 2: Reducing the return temperatures in the existing network
The return temperature in Ulm’s inner-city district heating system is currently about 62 °C at the Magirusstrasse CHP plant. It can generally be assumed that the older customers have a return temperature of around 65 °C whereas newer customer connections can maintain 50 °C as a return temperature. In order to further develop the exergetic quality of the district heating, one possible solution would be to supply customers from the return feed. This would then achieve the aim of having lower return temperatures, which could lead to a changed price structure that is coupled to the return temperature. This in turn would promote the implementation of LowEx building services technology, which would also lead to lower return temperatures. The solution has not only a model character in Ulm: these days district heating is predominantly found in denser urban districts that often require large-scale refurbishment. For example, it is often residential blocks belonging to housing associations that, as is the case in Ulm, are supplied with district heating: housing associations generally refurbish such residential blocks, which often have considerably more than 100 residential units, on a comprehensive basis. Consequently, such residential blocks cannot be considered as appropriate for district heating in the conventional sense because the thermal densities are too low. A typical limit is a thermal density of about 30 MW/km2. A similar situation currently applies in Ulm for new development areas which, in contrast to areas of old buildings undergoing refurbishment, are rarely found in the inner city but also have an extensive character. In addition to the connected loads, the temperature requirements also drop. This means that a provision is also feasible with lower supply temperatures than is currently usual in district heating. However, the hygienisation takes on a particularly important role in domestic hot water heating in order to prevent Legionella bacteria.
Demands placed on housing associations
A particular feature of the Ulm research project is that the customer side is involved in the form of Ulm’s two largest housing associations: ulmer heimstätte (uh) and Ulmer Wohnungs- und Siedlungs-Gesellschaft (UWS). As representatives of the housing industry, uh and UWS have stipulated several requirements for the heating systems: the solutions should be “caretaker-appropriate”, cope with high strains caused by a frequent change of tenants and have maintenance requirements that are as low as possible. At the same time, the solutions must provide maximum supply security. The housing associations’ aforementioned stipulations therefore preclude the use of surface heating on walls and ceilings since these are not compatible with the housing associations’ requirements regarding practicability. Problematic are the inherent restrictions caused by the high fluctuation of tenants and part of the tenant clientele. This means that conventional radiators and underfloor heating can be used as surface heating systems.
Demands placed on the domestic hot water heating
The domestic hot water heating with storage systems result in higher return temperatures that are greater than 60 °C for large parts of the year. This is due to the requirements for preventing Legionella bacteria in accordance with the Drinking Water Directive. As a technical solution, a connection shall be ensured in Ulm that provides the necessary temperatures with network return temperatures below 65 °C and domestic hot water operation but nevertheless enables maximum water volumes to be deployed from the primary return feed for space heating.
Effect of lowered return temperatures on generating and distributing heat
In terms of the heat distribution, the return temperature influences the efficiency of the district heating system in two ways: through reducing the heat losses and through reducing the pump power requirement. Reducing the return feed temperature reduces the heat lost during the heat distribution in direct proportion, since this reduces the temperature difference between the medium and the surroundings that causes this: the higher the heat transfer coefficient of the transmission pipe, the greater the savings in terms of the heat losses. The annual mean temperature differences in operating the supply and return feeds means that approximately 30 to 35% of the overall heat losses are caused in the return feed. Achieving a realistically possible reduction of the return feed temperature from 65 to 50 °C reduces the heat losses in the return feed by about 27%. However, since the return feed heat losses only make up about one third of the overall heat losses, the total heat losses only reduce by about 9%.
The effects of changing the return temperature on the power plant process were modelled using simulation calculations based on the two typical CHP generation configurations: non-condensing (back-pressure) CHP and extraction-condensing CHP. The result for power plant type 1: The decreasing return temperature improves the cooling of the steam in the heat condenser and the enthalpy of the condensate lowers. For a heat output of 100 MW, for example, the additional electricity generated for each K that the return temperature is lowered amounts to approximately 0.08 MW/K. With extraction-condensing CHP, there is essentially a shift from condensation electricity to cogeneration electricity. This is why the overall electricity generation changes slightly less than with the non-condensing (back-pressure) CHP.
Example project: Apartment building in Sedanstrasse
The Ulm Wohnungs- und Siedlungsgesellschaft (UWS) housing association is currently constructing an apartment building in Sedanstrasse with 70 residential units for which a connection value of 160 kW is being provided. Hot water is being supplied by a continuous flow freshwater system. A so-called triple connection is being realised for the new-build scheme. Once the district heating house station has been installed, a supporting measurement programme will monitor the trial operation of the house station and the first operational year. This measurement phase will be extended should there be a need for further optimisation measures.
Exergetic assessment of the temperature drop
For the new-build apartment building in Sedanstrasse, the exergetic efficiency of various supply solutions was compared:
- District heating supply from the supply feed with an annual mean temperature of 90 °C, district heating supply from the supply feed with an annual mean temperature of 70 °C
- District heating supply from the return feed with a return temperature of 65 °C for domestic hot water heating and 55 °C for the space heating
- Gas-fired condensing boiler
- Heat pump
Two balance circuits were examined for the assessment. The first balance circuit covers the provision of heat to the building as far as the transfer of the usable heat to the space and domestic water heating. The heat losses and the pump power requirements are integrated in the district heating balance circuit. The second balance circuit is broader in scope and also takes into account the building’s electricity requirement and the electricity provision. The district heating balance circuit includes the coupled generation of electricity and heat. The calculations have made it possible to assess the energetic efficiency of the heat and electricity provision for different heat supply systems for the new-build project.