Enhancing District Heating System Efficiency: A Review of Return Temperature Reduction Strategies
Abstract
:Featured Application
Abstract
1. Introduction
1.1. Background
1.2. Problem Statement
1.3. Significance
1.4. Literature Review
1.5. Aim, Objectives, and Scope
1.6. Review Approach
2. Thematic Review and Analysis
2.1. The Distribution Network
2.1.1. Hydraulic Balancing
2.1.2. Bypass Applications
- ‘Thermostatic bypasses’, which redirect supply water to the return line during low-heat demand to prevent excessive cooling.
- ‘Minimum flow bypasses’, which ensure a minimum flow rate when there is no demand.
- ‘Flow control bypasses’, which are equipped at constant-speed pump outlets to return excess flow to the return line and adjust the DH flow rate.
- ‘Admixing bypasses’, which are installed at building substations to lower the supply temperature and prevent scalding.
- Thermostatic Bypasses
- Hydraulic Bypasses
- Admixing Bypasses
2.1.3. Cascading Applications
- Designing home radiators without over-dimensioning necessitates a peak plant with a larger capacity to raise the supply temperature from 60 °C to 90 °C.
- DH return temperatures exceeded 60 °C during cold spells below −7.5 °C, preventing effective supply from the geothermal well.
- Radiators would need to be oversized by 160% to maintain a 60/30 °C operation scheme without a peak plant.
- Economic optimization is achieved by using radiant panels alongside peak plant temperature boosting.
- A peak plant boosting the DH supply temperature to 75 °C.
- Radiator-equipped homes, oversized by 46%, cooling to a return temperature of 50 °C.
- Radiant panel-equipped homes, receiving a return temperature of approximately 35 °C from the return pipeline, which is then redirected.
- The first option utilizes a heat exchanger with inlets from both the DH supply and return pipelines to boost temperature when return temperatures are low, maintaining the required supply temperature for the low-temperature sub-network.
- The second option involves a two-stage setup with series-connected heat exchangers. The lower heat exchanger pre-heats the return water from the low-temperature sub-network, while the upper heat exchanger further raises the supply temperature to the desired level using an additional inlet from the DH supply pipeline. Mixing occurs at the DH side when the upper heat exchanger is operational.
2.2. Heat Storage Systems
- For an 80/60 °C temperature scheme, a peak flow rate of 2.4 L/s necessitates a storage volume of 2880 liters for a 20 min discharge duration.
- For an 80/40 °C temperature scheme, a peak flow rate of 1.2 L/s requires 1440 liters of storage capacity for the same 20 min discharge duration.
2.3. Control Strategies
2.3.1. Flow Control
- The Constant-Flow Strategy
- The Variable-Flow Strategy
2.3.2. Supply Temperature Control
2.3.3. Low-Flow Operation
2.4. Heat Source
2.4.1. Exhaust-Gas Condensation
2.4.2. Combined Heat and Power
- Back-Pressure
- Extraction-Condensing
- Extraction and Back-Pressure
- Fuel-Cell micro-Cogeneration
2.4.3. Heat-Only Boilers
2.4.4. Heat Pumps
2.4.5. Solar Collector
2.4.6. Industrial Excess Heat
2.5. System-Level Optimization
2.5.1. Operation Temperature
2.5.2. Optimization Dilemmas
2.6. Implementation Strategies
2.6.1. Tariff Structures
- Motivation Tariff
- Determination of Reference Levels: The reference return temperature level was initially set by measuring the average return temperature of 28.5 °C during the 2001/2002 period.
- Threshold Establishment: Based on these measurements, thresholds were established with a natural band of 5 °C. A bonus was awarded if the supply–return temperature difference exceeded 35 °C, while a penalty was imposed if it fell below 30 °C.
- Consumer Engagement: Efforts were made to promote consumer awareness of this new tariff and to educate local heating and plumbing engineers about return temperature reduction techniques.
- Initial Results: In the first year of implementation (2004), 1000 consumers received bonuses for meeting the desired return temperature criteria, while 4500 others received penalties. This incentive structure effectively motivated changes in consumer behavior.
- Alternative Pricing Models
- Tariff in Network Cascading
2.6.2. Ownership Border
2.7. Novel Concepts
2.7.1. Pressure-Independent Thermostatic Radiator Valve
2.7.2. Decentralized Pumps
2.7.3. Advanced Thermo-Hydraulic Fluids
2.7.4. Heat Pump Sourced by Return Medium
2.7.5. Monitoring and Control
3. Discussion
4. Conclusions
5. Future Directions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
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Cases | Substation Type | Control Type | Control | ||
---|---|---|---|---|---|
Control at Indoor Heating System | Control at Substation | Radiator Inlet Temperature | |||
Case A | Direct | Variable Flow | Thermostatic Radiator Valve | - | Same as the DH network (85 °C) |
Case B | Direct | Constant Flow | - | Supply temperature adjustment by admixing bypass and 3-way valve | Set to weather compensation |
Case C | Indirect | Variable Flow | Thermostatic Radiator Valve | - | Fixed at 80 °C |
Case D | Indirect | Constant Flow | - | Supply temperature adjustment by flow control at DH side of heat exchanger (no flow adjustment at the indoor heating system) | Set to weather compensation |
Category | Consideration | Description |
---|---|---|
Technical | Hydraulic Balancing | Ensures even heat distribution and prevents return temperature spikes. |
Cascading Applications | Utilizes residual heat effectively before rejection, reducing return temperature. | |
Supply Temperature Control | Adjusting supply temperature dynamically for optimal return temperature control. | |
Smart Control Systems | Advanced real-time control mechanisms optimize system performance and reduce inefficiencies. | |
Heat Storage Systems | Thermal storage solutions to stabilize temperature variations and improve efficiency. | |
Operational | Bypass Regulation | Minimizing unnecessary bypass flows that elevate return temperatures and impact efficiency. |
Adaptive Flow Control | Flow adjustments based on dynamic heat demand variations for better temperature regulation. | |
Demand-Driven Pumping | Decentralized pumping solutions that enhance energy efficiency and return temperature reduction. | |
Consumer Behavior and Awareness | Encouraging behavioral changes, maintenance practices, and awareness campaigns for efficient energy use. | |
System Integration Challenges | Addressing system-wide integration of new technologies while maintaining efficiency objectives. | |
Economic | Investment Feasibility | Evaluating cost–benefit trade-offs of return temperature reduction measures for economic viability. |
Tariff Structures and Incentive Mechanisms | Implementing pricing strategies that incentivize efficient heat utilization and penalize inefficiencies. | |
Cost Savings from Efficiency Measures | Reducing heat loss and operational costs through better system design and optimization strategies. | |
Economic Impact of Lower Return Temperatures | Lower return temperatures improve fuel efficiency and enhance economic performance of DH systems. |
Methods | Benefits | Limitations |
---|---|---|
Hydraulic Balancing | Improves overall network stability, reduces temperature imbalances. | May require retrofitting in existing networks with older pipe infrastructure. |
Bypass Minimization and Smart Flow Control | Directly reduces unnecessary heat losses and avoids return temperature spikes. | Requires advanced monitoring and control, potential operational complexity. |
Cascading Applications | Maximizes residual heat utilization, enhances efficiency. | Requires well-structured temperature zoning, may not be easily implemented in legacy networks. |
Supply Temperature Optimization | Reduces heat losses and improves efficiency at the source. | In older networks, reduced supply temperature may result in inadequate heating performance. |
Heat Storage Integration | Stabilizes network fluctuations, enhances flexibility. | High upfront investment, space requirements. |
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Tol, H.İ.; Madessa, H.B. Enhancing District Heating System Efficiency: A Review of Return Temperature Reduction Strategies. Appl. Sci. 2025, 15, 2982. https://doi.org/10.3390/app15062982
Tol Hİ, Madessa HB. Enhancing District Heating System Efficiency: A Review of Return Temperature Reduction Strategies. Applied Sciences. 2025; 15(6):2982. https://doi.org/10.3390/app15062982
Chicago/Turabian StyleTol, Hakan İbrahim, and Habtamu Bayera Madessa. 2025. "Enhancing District Heating System Efficiency: A Review of Return Temperature Reduction Strategies" Applied Sciences 15, no. 6: 2982. https://doi.org/10.3390/app15062982
APA StyleTol, H. İ., & Madessa, H. B. (2025). Enhancing District Heating System Efficiency: A Review of Return Temperature Reduction Strategies. Applied Sciences, 15(6), 2982. https://doi.org/10.3390/app15062982