Unlocking Energy Efficiency: Exploring the Possibility of Low-Temperature Operation at Existing District Heating Systems

Low-temperature district heating has emerged as a promising and cost-effective technological solution towards the achievement of renewable energy systems with minimal emissions. This innovative system acts as an essential link between low-energy buildings and low-grade (i.e., low-temperature) sources, facilitating efficient utilization of thermal energy for diverse applications. Moreover, this approach reduces the overall amount of wasted energy during both production and transportation stages by optimizing heat distribution in buildings through intelligent control mechanisms. Therefore, widespread implementation of such cutting-edge technologies can significantly enhance sustainability practices while simultaneously addressing challenges posed by climate change mitigation efforts.

Early generations of district heating systems (either 2G or 3G) are dependent on high-temperature operation since the existing buildings have poor energy efficiency, necessitating transitional measures to low-temperature district heating infrastructures in the future. Hence, this report seeks to cast doubt on whether low-temperature operation can be used in existing district heating systems without sacrificing thermal comfort at the end-user site. The first goal is to highlight the technical challenges associated with a low supply temperature operation at existing district heating systems, with special emphasis on building retrofitting measures, dimensional considerations of the indoor heating system at the end-user site, network concerns, and control strategies. On the basis of various reputable publications, approaches to implementing low-temperature operation at the existing systems are demonstrated, taking into account the space heating and the domestic hot water production units as well as control schemes and cutting-edge techniques. The last point before the closing thoughts highlights the steps already performed to reduce the system operation temperature scheme at the existing district heating systems through successful case studies.

Technical Challenges

It is important to have a complete understanding of the system boundaries, taking into account the design, capacity, and constraints, before implementing a low-temperature operational scheme at the existing district heating systems. Since the demand characteristics affect how a district heating system is designed and operated, consideration must be given to the operational temperature restrictions established by end users, with a focus on lowering supply temperatures for existing buildings.

Building Retrofitting: Considering the space heating systems, it is evident that the best way to lower system temperature is to take steps to minimize the heat demand through increases in building energy efficiency. Building standards have already been put in place in several countries to reduce the amount of heat that current buildings require. The more stringent regulations already in place have made it possible to lower design temperatures throughout time in order to meet building energy needs.

Temperature Scheme: When choosing the operational temperature strategy for a district heating system of a certain size, there is always some flexibility, which means that the same heat output can be maintained under diverse operational modes. For example, it might be viable to reduce the supply temperature at the expense of increasing mass flow rate in a network at a certain heat load. In light of this, consideration should be given to the overall system performance, with a focus on the enhanced cooling provided by the end-user substations (i.e., a decrease in the return temperature). It should be noted that return temperature is a system feature that varies depending on the design, operation, and control of a district heating system rather than a parameter that can be controlled directly. In addition, it should be emphasized that performance enhancements at end-user stations should be taken into account as a way to reduce the supply temperature as well as the return temperature, or both.

Thermal Length: It is evident that for a given rate of heat transfer, the supply temperature need decreases with the increasing total surface area of the heat emitters. Use of the current heat exchangers at lower supply temperatures and larger mass flows could result in decreased power delivered to the building, which would result in poor thermal comfort, as well as further issues with the supply of domestic hot water. Sometimes it may not be possible or practical to improve the performance of a building; in these cases, lengthening the thermal length may be the best option.

Over-Dimensioning: The degree of radiator oversizing, which is frequent in the existing building stock, is another element influencing the operational temperature level required in the existing district heating systems. Intentional or unintentional over-dimensioning may occur during the system's design phase when large safety margins are taken into account, or it may happen as a result of retrofitting procedures. Also, it's crucial to keep in mind that heat interface units are often designed to endure the worst-case scenario, which would be the coldest time of the heating season, without considering internal heat or solar gains. Considering the fact that the heat interface units are oversized at these times due to the relatively low heat demand, these periods can result in lower operational temperatures than in the design scenario.

Domestic Hot Water: The need for a high-temperature regime to prevent Legionella growth at the domestic hot water unit is one of the key obstacles to lowering the supply temperature in district heating systems. In actuality, a temperature of 45 °C might be thought of as adequate for domestic hot water use on a regular basis. Based on that, a supply temperature of 50 °C from the district heating network may be adequate to give the necessary hot water temperature via instantaneous heat exchanger units without risk of Legionella. If a storage unit-equipped substation design is desired, the storage unit may be configured to hold the network heat carrier medium rather than the final domestic hot water medium, therefore reducing the volume of hot water in the end-user circuit. If the total volume of residential hot water is less than 3 litres, there is no restriction regarding minimum temperature, according to German standard (DVGW 551).

Control Strategy: It is crucial to evaluate whether the control and operation pose an obstacle to fully utilizing the existing heating systems. Despite the ease with which a constant flow approach may maintain the hydraulic balance in the distribution network, a variable flow strategy is preferable since it is more adaptable and responsive to demand changes. Due to the potential of obtaining the lowest possible supply temperature in line with the load state, the risk of overconsumption is decreased, and the thermal comfort is regulated. The techniques used to control the supply temperature in the current district heating systems must also be taken into consideration. One common contemporary practice in conventional systems is to keep the supply temperature constant for the duration of the heating season. During the prolonged off-peak times, the supply temperature must be decreased, which is possible with variable supply temperature regulation using weather compensation curves.

Equipment Faults: Although it can be argued that existing infrastructure is suitable for low-temperature operation without the need for retrofitting during off-peak periods, system faults and malfunctions are one of the biggest obstacles to the usage of low temperature heating. The need for greater supply temperature levels than necessary is a result of the current state of the district heating systems, which are hampered by faulty indoor heating systems and substations. The reason for this is because when consumers complain about a lack of thermal comfort, the usual response by the operator is to either increase the supply temperature or the pump pressure head, or both. Thus, in order to prevent a restriction for the supply temperature reduction in the existing district heating systems, fault detection and diagnosis are very important.

Effective Solutions

Some authoritative publications, mostly PhD theses, have already covered the adaptation of low-temperature functioning at the existing district heating systems, which can be viewed as solutions to the technical issues previously highlighted.

Hakan ?brahim Tol | District Heating in Areas with Low Energy Houses – Detailed Analysis of District Heating Systems based on Low Temperature Operation and Use of Renewable Energy | PhD Thesis

Download: https://orbit.dtu.dk/en/publications/district-heating-in-areas-with-low-energy-houses-detailed-analysi

The second case study in this PhD thesis states that it will take into account the regional energy planning scheme with low-temperature operation in already-existing district heating systems that base currently on high-temperature operation.

Because the peak winter periods are shorter than the rest of the heating season, the idea for using a low-temperature district heating system here entailed using an operational control technique that bases on raising the supply temperature during those months (keeping low-temperature operation during the rest periods). The need for high flow rates can be eliminated while still achieving thermal comfort by this control strategy, allowing for smaller pipe diameters in the event that switching from a natural gas grid is explored (there is a need for a new district heating system design).

When contemplating the transition from high-temperature scheme to low-temperature operation in an existing district heating system, it is crucial to account for a transmission period in an established settlement. This entails considering existing buildings that are equipped with over-dimensioned radiator units until such time as their head demand can be reduced to levels compatible with lower energy consumption. In order to allow the current network to be adequate until the old buildings are refurbished, the control strategy in this case plays a crucial role in decreasing the mass flow requirements on the network during this transition period. After the transition period, low-temperature operation can be imagined providing low-energy buildings, with the control approach in question serving as a lifesaver by lowering the mass flow need resulting from the low-temperature operation.

In light of this, a demand-responsive control strategy may be employed, altering the set point of the supply temperature in accordance with the cooling capabilities of the end-user substations (with either direct or indirect hydraulic configuration). As a result, this novel control method raises the supply temperature when poor performance is detected at the end-user site (a critical node in the network), while it lowers it when enough cooling is detected. You may access the details regarding this control strategy in this patent file - Link.

Dorte Skaarup ?stergaard | Heating of Existing Buildings by Low-Temperature District Heating | PhD Thesis

Download: https://orbit.dtu.dk/en/publications/heating-of-existing-buildings-by-low-temperature-district-heating

This thesis explores the potential of utilizing low-temperature district heating to provide space heating for existing buildings. The study includes three main parts: investigating current heating elements, evaluating control and operation barriers in case study buildings, and exploring tools for implementing low-temperature district heating. The study suggests that lower temperatures can be used for space heating as most of the current heating systems are over-dimensioned, with potential reductions possible throughout much of the year. The design of these systems caters to rare outdoor temperatures, and low-temperature district heating has successfully heated certain single-family homes. However, inadequate control and system design caused high return temperatures in some cases due to hydraulic short-circuits or faulty thermostats while occupants misused thermostats or had small radiators. Also, focus is placed on identifying critical radiator units in existing structures, which act as an obstacle to realizing the full potential of low-temperature operation. Additionally, it is claimed that improperly sized radiator units and malfunctioning thermostatic radiator valves are to blame for the unreasonably high set points needed for the supply temperature in some buildings.?

Marek Brand | Heating and Domestic Hot Water Systems in Buildings Supplied by Low-Temperature District Heating | PhD Thesis

Download: https://orbit.dtu.dk/en/publications/heating-and-domestic-hot-water-systems-in-buildings-supplied-by-l

The second section of this PhD thesis examines whether it is feasible to employ low-temperature district heating to provide space heating systems in both new and existing buildings while the first section focuses on the performance issues in relation to the domestic hot water production without risk of Legionella. The objective of this study is to ascertain the lowest supply temperature that can be maintained while still maintaining thermal comfort and the ideal operation of the district heating system. When space heating is taken into consideration, even without renovations being made to the building envelope, the supply temperature set point can be decreased during the long off-peak periods and raised during the peak hours. The maximum DH supply temperature can be lowered further if a minor renovation like changing the windows is taken into consideration. Extensive building envelope renovations and/or the installation of low-temperature radiators in place of the current radiators allow for still further set point reductions as needed for supply temperature. According to this analysis, replacing the existing substation heat exchangers, so they can operate at low temperatures, is necessary if the supply temperature is lowered below 60 °C.

Xiaochen YANG | Supply of Domestic Hot Water at Comfortable Temperatures by Low-Temperature District Heating Without Risk of Legionella | PhD Thesis

Download: https://orbit.dtu.dk/en/publications/supply-of-domestic-hot-water-at-comfortable-temperatures-by-low-t

A barrier to achieving low-temperature operation is the comfort and hygiene standards as requested for the domestic hot water units. This PhD thesis plays a significant role in focusing on sterilization procedures and emphasizing design approaches for domestic hot water units; alternate strategies instead of adopting a high-temperature regime to reduce the danger of Legionella. Together with the substation heat exchanger unit, local heating devices like electric heating and a micro heat pump are also given attention since they enable district heating systems to run continuously at low temperatures.

Per-Olof Johansson | Buildings and District Heating – Contributions to Development and Assessments of Efficient Technology | PhD Thesis

Download: https://lup.lub.lu.se/record/1962823

The use of add-on fan blowers mounted on radiators, optimized control of space heating systems, and use of alternative connection principles in district heating substations are three techniques to improve the temperature performance of the buildings that are highlighted in the first section of this PhD thesis. Existing buildings can be converted to operate at low temperatures without requiring significant alterations to the end-user indoor heating system.

Add-on fan blowers can help increase the heat output from the radiator units, allowing simultaneous consideration of lowering both the supply and return temperature degrees, according to field research and simulations.

Given the emphasis on optimal control, it is possible to modify the flow and supply temperature set points to enhance the performance of the end-user substation, permitting low return temperatures during any arbitrary operation.

Success Stories

Brescia, Italy: With an annual supply of 1 TWh and coverage of 70% of the district's total heat demand, the district heating system in Brescia is one of the biggest in Italy. The annual heat losses are quite considerable because of the high operational temperature levels, which can reach 130 °C in the winter. A demo site for the study looking at temperature reductions is situated on a peripheral branch in the southern portion of the network and provides energy to 35 end users with a total contract capacity of about 700 kW.

Viborg, Denmark: In order to utilize large-scale heat pump units for the heat supply, the temperatures of the district heating network in Viborg, Denmark, have been gradually decreased (down to operational temperature scheme of 68/40 °C). In order to lower district heating temperatures, care was taken to lower the supply and return temperatures in the buildings. Examination of the apartment building revealed that the radiator system lacked the necessary tools for hydronic balance, and that actions by the consumers had a detrimental effect on the temperatures in the heating system. A saltwater electrolysis-based disinfection system was installed in the building to lessen the risk of Legionella contamination.

Albertslund, Denmark: At first, a 110 °C supply temperature was used to operate the district heating network. The temperature has decreased over time, and the system now functions at a flow temperature of about 90 °C. A new low-temperature fourth-generation district heating system, where the supply temperature is dropped from 90 to 55 °C, is being developed in order to have the heat and power supply be CO2-neutral by 2020. The Albertslund Municipality is launching a refurbishment program for existing structures in order to prepare them for low-temperature operation. In the apartments, new low temperature instantaneous water heaters have been installed, providing domestic hot water at a temperature of 45 °C.

S?nderby, Denmark: The purpose of the full-scale demonstration project in S?nderby, Denmark, was to determine if it would be feasible and advantageous to supply low-temperature DH to existing building areas. The demonstrated area consists of 75 single-family homes, with supply temperatures in the current district heating system averaging about 80 °C. With the appropriate network upgrading, it is possible to supply low-temperature to this area because the buildings were equipped to be heated via floor heating and the domestic hot water storage tanks were replaced with heat exchangers. Twin pipes that further reduce network heat loss are being installed in place of old, inefficient pipes as part of the project. The main source of water for the low-temperature area is return water from the main district heating system, which operates at a medium temperature of 48°C, while during colder months, the supply temperature is raised to 55°C. The yearly network heat loss has decreased from 41% to 13-14%; the large reduction in heat loss is mostly attributable to lower network temperature, improved pipe insulation, and smaller pipes due to the use of high network pressure.

Concluding Remarks

Low-temperature operation at existing district heating systems is a promising solution for improving their efficiency, reducing energy consumption and environmental impacts, and can be achieved through the implementation of effective control strategies and building retrofits.

It is possible to think about the future of existing structures with performance enhancements, moving from poor energy performance to low-energy class structures. Due to the performance enhancements to be made at the building stock, adaptation of low-temperature operation shall therefore be taken into consideration for the district heating infrastructure currently in place.

Improvements in performance at the end-user heating circuit are crucial because they enable a reduction in the supply temperature. The importance of lowering the return temperature as well as worries about reducing the supply temperature level must be considered in order to achieve optimal performance at the system level.

Notwithstanding the advantages of low-temperature operation, the heating system need not always be run at low temperatures during the whole heating season. The key to achieving reductions at the operational settings of the system is to raise the supply temperature only during peak periods.?

References

1. Tol, H?. District Heating in Areas with Low Energy Houses – Detailed Analysis of District Heating Systems based on Low Temperature Operation and Use of Renewable Energy. PhD Thesis. 2015. Technical University of Denmark.

2. ?stergaard, DS. Heating of Existing Buildings by Low-Temperature District Heating. PhD Thesis. 2018. Technical University of Denmark.

3. Brand, M. Heating and Domestic Hot Water Systems in Buildings Supplied by Low-Temperature District Heating. PhD Thesis. 2014. Technical University of Denmark.

4. Yang, X. Supply of Domestic Hot Water at Comfortable Temperatures by Low-Temperature District Heating Without Risk of Legionella. District Heating. PhD Thesis. 2016. Technical University of Denmark.

5. Johansson, PO. Buildings and District Heating – Contributions to Development and Assessments of Efficient Technology. PhD Thesis. 2011. Lund University.

6.?R?m?, M; Sipil?, K. Transition to Low Temperature Distribution in Existing Systems. 2017. Energy Procedia.

7. Averfalk, H et al. Transformation Roadmap from High to Low Temperature District Heating Systems. 2017. Annex XI Final Report. International Energy Agency.

8. Benakopoulos, T. Strategy for low-temperature operation of radiator systems using data from existing digital heat cost allocators. 2021. Energy.

9. Guelpa E et al. Reduction of supply temperature in existing district heating: A review of strategies and implementations. 2023. Energy.

10. Reguis, A et al. Challenges for the Transition to Low-Temperature Heat in the UK: A Review. 2021. Energies.

11. TEMPO. Brescia: Existing High Temperature District Heating Network. 2022. https://www.tempo-dhc.eu/brescia/ (accessed 03/04/2023).

12. EuroHeat & Power. Temperature reductions in existing apartment building in Viborg, Denmark. https://www.euroheat.org/resource/temperature-reductions-in-existing-apartment-building-in-viborg-denmark.html (accessed 03/04/2023).

13. COWI. Municipality in transition to low temperature district heating – From 2nd to 4th generation district heating in an existing supply area. https://stateofgreen.com/en/news/municipality-in-transition-to-low-temperature-district-heating/ (accessed 03/04/2023).