Overview
Cold district heating represents a technical variant of district heating networks designed to operate at significantly lower transmission temperatures than conventional systems. This approach enables the simultaneous provision of both space heating and cooling to connected consumers, allowing buildings to utilize thermal energy independently. The system functions as a bidirectional thermal network where heat can be exchanged between different users or stored in the ground, enhancing overall energy efficiency and flexibility in urban energy infrastructure.
Operating Temperatures and Thermal Dynamics
The defining characteristic of cold district heating is its low-temperature operation. Transmission temperatures typically range from approximately 10 to 25 °C (50–77 °F). These temperatures are well below those found in traditional district heating systems, which often operate at 60 °C or higher. The lower temperature range reduces thermal losses during transmission, particularly in insulated pipes, and allows for greater integration with renewable energy sources and waste heat recovery systems. The thermal energy balance in such systems can be conceptually represented by the equation Q = m × c × ΔT, where Q is the heat transfer rate, m is the mass flow rate of the fluid, c is the specific heat capacity, and ΔT is the temperature difference between supply and return lines.
Heating and Cooling Mechanisms
Heating is achieved through water heat pumps installed at the consumer level. These heat pumps extract thermal energy from the network's low-temperature water and upgrade it to the desired indoor temperature. For cooling, the system can provide direct cooling by circulating the relatively cold network water through building heat exchangers. Alternatively, if lower temperatures are required, chillers can be used to further reduce the temperature of the network water. This dual capability allows buildings to heat and cool simultaneously, depending on their specific thermal needs and the time of day.
Classification as 5th Generation District Heating and Cooling
In scientific terminology, cold district heating systems are collectively referred to as 5th generation district heating and cooling (5GDHC). This classification distinguishes them from previous generations that primarily focused on heating or used higher temperature ranges. The 5GDHC concept emphasizes the integration of renewable energies and the ability to balance fluctuating production from sources such as wind turbines and photovoltaic systems. By operating at low temperatures, these networks can effectively utilize low-grade waste heat and geothermal energy, contributing to a more sustainable and potentially greenhouse gas and emission-free heat supply. The system is sometimes also referred to as an anergy network, highlighting its ability to capture and utilize ambient thermal energy from the environment.
History of cold district heating
The development of cold district heating emerged from the need to optimize thermal networks for lower transmission temperatures. The concept traces its origins to the late 1970s, with the first networks established in Arzberg and Wulfen in 1979. These early implementations demonstrated the feasibility of operating district heating systems at temperatures significantly lower than conventional setups. The technology evolved to address the limitations of traditional district heating, which typically required higher temperatures to overcome thermal losses over distance.
By the 21st century, the concept gained traction in Europe, driven by the integration of renewable energy sources and the need for flexible heat and cooling supply. The term "5th generation district heating and cooling" (5GDHC) was adopted to describe these advanced systems, highlighting their ability to provide both heating and cooling simultaneously. This dual functionality is achieved through water heat pumps in buildings, which extract thermal energy from the network for heating or reject heat for cooling. The low temperature range of 10 to 25 °C allows for efficient operation and reduced thermal losses.
As of 2018, there were 40 schemes in operation across Europe, indicating a steady growth in the adoption of cold district heating. These systems are considered a promising option for sustainable heat supply, as they can be operated entirely by renewable energies. The technology contributes to balancing the fluctuating production of wind turbines and photovoltaic systems, enhancing the overall efficiency of the energy grid. The collective term "anergy network" is also used in scientific terminology to describe these systems, emphasizing the use of ambient thermal energy.
The historical progression of cold district heating reflects a shift towards more sustainable and flexible energy solutions. The initial networks in Arzberg and Wulfen laid the groundwork for future developments, which have since expanded to include a variety of applications. The integration of heat pumps and the ability to provide both heating and cooling have made these systems increasingly attractive for urban areas seeking to reduce greenhouse gas emissions. The growth in the number of operational schemes underscores the potential of cold district heating to play a significant role in the future of energy infrastructure.
How does cold district heating work?
Cold district heating operates as a technical variant of district heating networks, functioning at significantly lower transmission temperatures than conventional systems. These networks typically maintain water temperatures in the range of approx. 10 to 25 °C (50–77 °F). This low-temperature operation enables a unique capability: different consumers can simultaneously heat and cool their buildings independently of one another. This system is scientifically classified as 5th generation district heating and cooling (5GDHC) and is sometimes referred to as an anergy network.
Thermal Exchange Mechanisms
The system provides both space heating and cooling through distinct mechanisms. For heating, buildings utilize water heat pumps. These heat pumps extract thermal energy from the low-temperature heating network to produce hot water for the building. The efficiency of this process relies on the temperature lift required to raise the network's baseline temperature to the building's demand level. Conversely, cooling is provided either directly by utilizing the cold heat network or indirectly via chillers when necessary. This dual capability allows the network to serve as a thermal reservoir, absorbing excess heat during cooling periods and releasing it during heating periods.
Prosumer Dynamics and Grid Integration
A defining feature of cold district heating is the shift from passive consumers to active prosumers. Because the network operates at low temperatures, buildings can both draw from and feed into the thermal grid. This dynamic allows for better balancing of fluctuating renewable energy production, particularly from wind turbines and photovoltaic systems. By integrating entirely with renewable energies, these networks contribute to a sustainable, potentially greenhouse gas and emission-free heat supply. The ability to operate at low temperatures reduces thermal losses and enhances the overall efficiency of the district energy system, making it a promising option for modern urban energy infrastructure.
What are the main heat sources for cold networks?
Cold district heating networks rely on low-grade thermal energy sources that align with their operational temperature range of approx. 10 to 25 °C. Because the system uses water heat pumps to upgrade this energy for space heating, the primary requirement is a stable, accessible source of ambient or waste heat rather than high-temperature combustion. The flexibility of the 5th generation district heating and cooling (5GDHC) model allows for the integration of diverse energy inputs, supporting the system’s role in balancing fluctuating renewable production.
Groundwater and Geothermal Sources
Groundwater and shallow geothermal reservoirs are common heat sources for these networks. These sources provide a relatively constant temperature year-round, which is ideal for the efficient operation of water heat pumps that extract thermal energy from the network. The stability of groundwater temperatures helps maintain the low transmission temperatures characteristic of cold local heating systems.
Industrial Waste Heat and Solar Thermal
Industrial processes often release low-grade waste heat that would otherwise be lost to the environment. Capturing this heat contributes to the emission-free heat supply potential of the network. Additionally, solar thermal energy can be integrated, further enhancing the share of renewable energies in the system. These sources support the network’s ability to operate sustainably and reduce greenhouse gas emissions.
| Heat Source Type | Characteristics in 5GDHC |
|---|---|
| Groundwater | Stable low-grade temperature; suitable for heat pump extraction |
| Geothermal | Shallow reservoirs providing consistent thermal energy |
| Industrial Waste Heat | Low-grade heat recovery from industrial processes |
| Solar Thermal | Renewable input supporting emission-free operation |
Network configurations and components
Cold district heating networks utilize specific pipe configurations to manage simultaneous heating and cooling demands. The most common configuration is the single-pipe system, where a single network supplies heat to buildings equipped with water-to-water heat pumps. In this setup, the network fluid temperature typically ranges from 10 to 25 °C. Some systems employ two-pipe or three-pipe arrangements to optimize hydraulic balance and thermal efficiency. Four-pipe systems are also used in certain applications to provide independent heating and cooling circuits. These configurations allow different consumers to heat and cool simultaneously and independently of each other.
Pipeline materials and insulation
Polyethylene is a common material for cold district heating pipelines due to its thermal and hydraulic properties. The low operating temperatures reduce heat loss compared to conventional systems. Proper insulation is essential to maintain the temperature range of 10 to 25 °C. The choice of material affects the system's longevity and maintenance requirements. Polyethylene pipes are often selected for their flexibility and resistance to corrosion.
Seasonal heat storage
Seasonal heat storage is a key component of cold district heating systems. It allows for the balancing of fluctuating production from renewable energy sources such as wind turbines and photovoltaic systems. Thermal energy can be stored in underground aquifers, borehole fields, or large tanks. This storage capability contributes to the system's ability to operate entirely on renewable energies. It also helps in managing the thermal load over different seasons.
Substation components
Building substations in cold district heating networks include water heat pumps for heating and chillers for cooling. The water heat pumps obtain their thermal energy from the heating network to produce hot water for space heating. Cooling can be provided directly via the cold heat network or indirectly via chillers if necessary. These components enable the system to provide both space heating and cooling. The efficiency of the substation components is critical for the overall performance of the network.
Worked examples: European pilot plants
Several pilot projects in Europe have demonstrated the technical and operational viability of 5th generation district heating and cooling (5GDHC) networks. These installations validate the concept of using low-temperature water loops, typically between 10 and 25 °C, to serve multiple consumers simultaneously for both heating and cooling.
Case Study: Arzberg, Germany
The Arzberg project serves as a prominent example of a 5GDHC system integrated with renewable energy sources. In this configuration, the network utilizes a low-temperature water loop that acts as a thermal reservoir. Consumers are equipped with water-to-water heat pumps to extract thermal energy for space heating. During the summer months, the same network can provide cooling by rejecting heat from buildings into the loop, allowing for simultaneous heating and cooling operations among different subscribers. This setup reduces the need for large-scale thermal storage at the source, leveraging the thermal mass of the network and connected buildings.
Case Study: Wulfen, Germany
In Wulfen, the 5GDHC network is coupled with a geothermal source. The system extracts low-grade heat from the ground, maintaining the network temperature within the optimal 10–25 °C range. This integration allows for efficient operation of heat pumps at the consumer end, maximizing the coefficient of performance (COP). The project highlights the potential for balancing fluctuating renewable energy production, such as wind and solar PV, by adjusting the thermal load on the network.
| Location | Key Feature | Status |
|---|---|---|
| Arzberg | Renewable integration, simultaneous heating/cooling | Operational |
| Wulfen | Geothermal coupling | Operational |
| Oberwald | Low-temperature network | Operational |
| Wüstenrot | 5GDHC implementation | Operational |
| Aurich | Renewable energy balancing | Operational |
| Herford | Urban heat network | Operational |
| Fischerbach | Thermal storage integration | Operational |
Other notable projects include Oberwald, Wüstenrot, Aurich, Herford, and Fischerbach, each contributing data on the efficiency and flexibility of 5GDHC systems. These examples collectively demonstrate that cold district heating networks can operate entirely on renewable energies, offering a sustainable and potentially emission-free heat supply solution. The ability to balance fluctuating renewable production further enhances their role in the broader energy transition.
Role in future energy systems
Cold district heating networks represent a critical infrastructure component for the decarbonization of urban energy systems. As a technical variant of district heating operating at low transmission temperatures, this system is classified as 5th generation district heating and cooling (5GDHC). Its operational design allows it to be powered entirely by renewable energies, positioning it as a promising option for achieving a sustainable, potentially greenhouse gas and emission-free heat supply. This capability is central to the transition away from fossil fuel dependence in the thermal sector.
Sector Coupling and Power-to-Heat
The integration of cold district heating into future energy systems relies heavily on sector coupling, specifically through power-to-heat mechanisms. The system utilizes water heat pumps to produce hot water for building heating. These heat pumps obtain their thermal energy directly from the heating network, which operates at transmission temperatures in the range of approx. 10 to 25 °C. This low-temperature operation is essential for the efficiency of the heat pumps and facilitates the simultaneous and independent provision of both space heating and cooling to different consumers. Cooling can be provided either directly via the cold heat network or indirectly via chillers, depending on the specific demand.
Integration with Renewable Energy Sources
A key advantage of 5GDHC systems is their ability to contribute to balancing the fluctuating production of wind turbines and photovoltaic systems. The flexibility of the network allows it to absorb excess electricity generated by variable renewable sources, thereby stabilizing the grid. This integration supports the broader energy transition by linking the electricity and thermal sectors, enhancing the overall efficiency and reliability of renewable energy utilization. The system's capacity to operate with mixed primary fuel sources further enhances its adaptability to local renewable energy availability.
See also
- Carbon credits: Mechanisms, markets and quality standards
- Coal-ash management by U.S. electric utilities: Overview and recent developments
- Lng import terminal
- Biogas production by anaerobic digestion of coffee husks and cattle manure
- Flywheel frequency regulation