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Methods of reducing the district heating return temperature from the local substations
Xiaochen Yang, PostdocDepartment of Civil Engineering, DTUContact: [email protected]
Sensible regulation and smart control
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Background
• 1. Why LTDH?
• 2. Why low return temperature?
• 3. Difficulties− Thermal comfort− Dynamic heating load− Deviation between the design and
practical situation
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Reasons & substation layout
• Typical substation layout− HEX for DHW preparation
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Reasons & substation layout
− Storage tank for DHW preparation
1. Less peak load ???2. Set point T of the tank3. Circulation is
necessary for comfort requirements
1. Problematic tank charging program
2. Large circulation heat loss
3. High DH return temperature
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Proposed solution
• Piecewise large charging flow
• Small constant charging flow
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Determination of the minimum charging flow
• Maintain required DHW temperature during any draw-off period
• Provide sufficient heat including the DHW consumption and heat loss
• Return temperature as low as possible
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Demonstration building and dynamic modeling
• Typical multi-storey building in Copenhagen, 15 flats, 750 L tank, dailyheat supply 142kWh, 50% for heat loss
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Demonstration building and dynamic modeling
Dynamic results:1. All temperatures2. Heat flowrates in any
heat transfer process
3. System hydraulic regulation
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Model resultsPiecewise charging
Constant charging flow
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Model results
• Reduce the heat loss 50%-> 30%
Necessary charging flowrate Critical temperatures
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Results comparison
Measurements Simulated cases
Reference case
50% heat loss 30% heat loss
PeakFlowrate [kg/s]
0.25 0.25 0.04 0.025
Avg. Return T[°C]
57.6 55.9 38.0 33.1
Average cooling[°C]
10.6 14.1 32 36.9
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Further improvement
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Conclusion
• The DH return temperature from the conventional storage tank system can be reduced with better regulated charging flow
• Reduce the system heat loss can help to reduce the average DH return temperature
• Pracital DHW load profile is of great importance for the control
• Theoretically, the DH return temperature can be further cooled down by installing a circulation heat pump
• Future work:− Investigate the system performance with heat pump− Compare possible systems with different layouts
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Xiaochen [email protected] of Civil EngineeringTechnical University of Denmark