Low Temperature District Heating System Research Articles

Possibility assessment of the low-temperature district heating systems implementation in Ukraine

These researches concern the application of renewable energy sources in district heating systems. In Ukraine, district heating systems cover approximately 50% of the demand for thermal energy in the residential and communal sector. District heating systems 2G are most often used, which are characterized by high temperatures of the heat coolant, the lack of accounting for heat energy consumption during transportation of the heat coolant, and the use of fossil fuels. In the countries of the European Union, the introduction of district heating systems is considered one of the key directions for the transition to a decarbonized, environmentally safe and efficient energy system. The development of district heating systems technologies makes it possible to lower the temperature of the heat coolant in heat networks and increase the use of renewable energy sources. Ukraine will eventually become a full-fledged member of the European Union, and this determines the need to find ways to bring Ukraine's heat supply systems to the 4G level, in particular to low-temperature district heating systems with the most efficient use of renewable energy sources and waste heat. This article examines climatic, physical-geographical and social features, regulatory, technical and financial-economic opportunities and barriers to the implementation of low-temperature district heating systems in Ukraine. As a result of analytical studies, it was established that there are prerequisites for the introduction of low-temperature heat supply systems in Ukraine, however, a number of technical, regulatory, social and financial and economic measures need to be implemented to bring district heating systems up to 4G indicators. These studies allow establishing measures that require further research for the possibility of introducing low-temperature district heating systems in particular and environmental safety of heat supply systems in general.

Impact of Flexibility Implementation on the Control of a Solar District Heating System

Renewable energy sources, distributed generation, multi-energy carriers, distributed storage, and low-temperature district heating systems, among others, are demanding a change in the way thermal networks are conceived, understood, and operated. Governments around the world are moving to increase the renewable share in energy distribution networks through legislation like the European Directive 2012/27 in Europe, and solar energy integration into district heating systems is arising as an interesting option to reduce operation costs and carbon footprint. This conveys an important investment that adds complexity to the management of thermal networks and often delays the return on investment due to the unpredictability of renewable energy sources, like solar radiation. To this end, this paper presents an optimisation methodology to aid in the operative control of an existing solar district heating system located in the northwest of France. The modelling of the system, which includes a large-scale solar field, a biomass boiler, a gas boiler, and thermal energy storage, was previously built in Dymola. The optimisation of this network was performed using MATLAB’s genetic algorithm (GA) and running the Dymola model as functional mock-up units, FMUs, using Simulink’s FMI Kit. The results show that the methodology presented here can reduce the current operation costs and improve the use of the daily storage of the DH system by a combination of mass flow control and the implementation of a flexibility function for the end-users. The cost-per-kWh was reduced by as much as 16% in a single day, and the share of heat supplied by the solar field on this day was increased by 5.22%.

Comparison of traditional district heating with low temperature district heating systems featuring organic Rankine cycle and heat pump

Paper presents a comparison between a traditional district heating system with conventional boilers as a source of heat and electricity purchases from power utilities and a low temperature DH system incorporating the CHP with organic Rankine Cycle providing electricity and local heat pump raising the temperature of the DH fluid to the required temperature in the dwelling, meaning that both heat and electricity are produced within the system. Additionally, such “island” system may feature or not the heat pump. Comparisons are made based on the results obtained using a developed simple analytical model enabling calculations of the efficiency of respective systems. The analyses have a general character and can be used in studies of complex networks however presented in the paper simple examples show merely the fundamental capabilities of the developed tool. The objective of the heat pump in the system is to increase temperature of hot water to the required level locally at the dwelling and provide the heating in such way, contrary to the traditional system, where water heating is considered by the use of electric heaters to increase parameters of water for central heating purposes and preparation of utility hot water. Presence of the heat pump in the system allows for significantly lower temperatures of distributed in the network heat carrier.

Smart heat tariffs in transition to free market

Innovative pricing mechanisms should motivate heat suppliers and consumers to move toward more sustainable energy systems and introduce low-temperature district heating systems and sector coupling in smart energy systems. Therefore, district heating regulation regimes should also be changed to stimulate transformations in the energy sector. The district heating tariffs depend on many factors, including fuel prices, operational parameters, taxes, investments, and other criteria. Therefore, an analysis of the DH tariffs has been implemented to find solutions to motivate DH enterprises towards energy efficiency and climate neutrality. The analysis results are based on the decision-making assessment approach by selecting various criteria and evaluating them from five significant aspects: engineering, environmental, climate, economic and socioeconomic. The central elements within the developed fuzzy cognitive mapping model are investment costs, heat production costs, and primary energy consumption. Considering the set boundary conditions, the most beneficial method for smart heat tariff definition could be heat tariff benchmarking with integrated energy efficiency standards for DH operators.

Multiobjective optimization and analysis of low-temperature district heating systems coupled with distributed heat pumps

Developing district heating can help decarbonizing energy systems. Modern concepts of district heating revolve around low operational temperature to reduce heat losses and enhance the efficiency of the supply plants with technologies such as cogeneration, condensing boilers or geothermal heat pumps. However, temperature reduction in district heating networks may be limited by technical requirements on the building side. Booster heat pumps on the end-users’ side can be a solution to bridge the network-building temperature gap. However, the current state-of-the-art on such systems does not provide ways for optimizing in real-time the operating temperature, which is needed to fully exploit its benefits, and proposed control strategies focus solely on reducing costs without considering environmental aspects. This article proposes a new multi-objective optimization framework to determine the real-time supply temperature for low-temperature district heating with booster heat pumps. The optimization approach uses rolling horizon and both operating costs and GHG emissions are minimized simultaneously. The model on which the optimization relies includes efficiency curves for equipment and water transport delays in the network. Many scenarios are simulated to understand how key factors affect the optimal supply temperature, such as the fuel type (i.e., wood chips or natural gas), price of electricity, emission factor of electricity, and relative weight of costs and emissions in the objective function. Results show that the proposed framework can indeed reduce costs and emissions, but the viability of the system depends strongly on the above-mentioned factors. In the case-study with typical electricity costs and emissions for Quebec (Canada), the average operating temperature could be reduced to 42 and 66 °C depending on the type of boilers in the system. While emissions were always smaller than with a conventional network, operating costs could be higher (with wood chip boiler) or lower (with natural gas boiler). This work can contribute to the deployment of efficient low temperature networks.

Energy, exergy analysis and optimization of insulation thickness on buildings in a low-temperature district heating system

In the study, energy and exergy analysis of the buildings on a campus in Turkey are conducted by using actual operating data and taking measurements in the district heating system as a case study. The energy and exergy demands, losses that stem from all buildings are calculated according to average daily outdoor temperature data. Due to the high heat losses in the buildings, determining the optimal insulation thickness for the exterior wall should be investigated. Therefore, optimal insulation thicknesses, energy savings, fuel consumptions and payback periods of the insulation material on the exterior wall of the building are examined by using Life Cycle Assessment and P1–P2 method for natural gas. Optimal insulation thicknesses are calculated for different insulation materials such as XPS, glass wool, rock wool and EPS for the climatic regions (HDD=800–4250°C days). According to average exergy losses from the building components per unit area, the average total exergy loss is calculated as 2.39×10-2 kW/m2.year and 1.42×10-3 kW/m2 (5.92%) of this loss stems from the exterior walls, 1.93×10-3 kW/m2 (8.07%) from the floors, 7.37×10-4 kW/m2 (3.08%) from the roofs, 1.58×10-2 kW/m2 (65.99%) from the windows and doors, 4.04×10-3 kW/m2 (16.92%) from the ventilation with infiltration. Energy requirement values of the building are found between 2.68–25.70 kWh/m3 towards from the warmest to the coldest climatic region for the uninsulated wall. In the un-insulated state, fuel consumption varies between 1.93-18.48 m3/m2 from the warmest to the coldest region. The optimal insulation thickness values of the building’s exterior wall are calculated as between 2.3–10.0 cm according to different climatic regions. In-state of exterior wall insulation of 3 cm, fuel consumption decreases by 46.63%–53.46% compared to different insulation materials and climatic regions compared to the un-insulated state.

Data-driven assessment for the supervision of District Heating Networks

There is an ongoing trend towards temperature reduction in District Heating Networks, allowing for the reduction of distribution heat loss and enabling the integration of low exergy heat production systems. There is a clear scientific consensus on the improved sustainability of such systems. However, there is not sufficient knowledge on how to deliver a successful transition to a low temperature District Heating system, while ensuring the operational levels of the existing system. This paper presents the experience on the progressive temperature reduction of a district heating subnetwork over the 2018–2021 period in Tartu, Estonia. Data from heat meters is extensively used to assess the capacity of substations and network branches to deliver the required heat and quality levels. Faulty substations are identified for targeted assessment and improvement works. Several substations have been identified as missing some of the performance criteria. This has led to further analysis, closer supervision and interventions in the operational conditions of the network. This is an ongoing process, expected to remain in the established procedures of the DH network operator. At the end of the process, a temperature reduction of 7 ºC has shown an improvement of 4.8% in network heat loss.

Feasibilities of utilizing thermal inertia of district heating networks to improve system flexibility

The use of district heating network inertia (DHNI) has been regarded as an efficient and cost-saving method to improve the flexibility of the energy systems. However, the acclaimed benefits in most studies are found in specific cases with traditional middle-temperature systems. The cases where DHNI is not feasible are unclear and the applicability of DHNI in future low-temperature district heating (LTDH) systems requires further investigations. Therefore, this study applied a top-down methodology where the practical storage potentials of DH networks are evaluated based on field investigations of 134 Swedish DH networks and 25 Chinese DH networks with various sizes and demand densities. Empirical relationships between the heat density and storage potentials are established and analyzed. Then, bottom-level analysis from technical and economical aspects are conducted on a variety of application scenarios for DHNI, including different temperature levels, heat sources, control strategies and renewable energy profiles. It is found that in LTDH system, by raising the network temperatures to actively use the DHNI, the heat source efficiency is reduced regardless the size and density of the network and, thereby, making the DHNI infeasible. This implies that the DHNI is only applicable in middle-temperature systems with combined heat and power plant (CHP) as a heat source in the extraction mode. Furthermore, the back-pressure mode is not economically attractive. In summary, the results from a multi-scenario analysis identified limited benefits of the DHNI, implying a proper consideration of its roles in future works.

Variable heat pricing to steer the flexibility of heat demand response in district heating systems

Flexibility from low-temperature district heating (LTDH) systems favors the integration of renewable energy in power systems. Flexibility from LTDH systems may be revealed and steered through dynamic pricing for heat. For the purpose of optimal heat pricing, the heat market with variable heat pricing is modeled as a Stackelberg game (i.e., with a bi-level structure). The upper-level problem of the heat provider is to maximize net profit by issuing hourly heat prices day-ahead, while the lower-level problem of heat consumers is to minimize costs by adjusting heat demand based on the dynamic price. The proposed model is tested on a groundwater-based LTDH system with one provider and three consumers. It is evaluated based on the operational cost of the heat provider and the average heat unit price for the consumers, compared to flat heat pricing.

Network configurations for implemented low-temperature district heating

This paper presents the findings and conclusions from an inventory of network configurations implemented in several early projects concerning low-temperature district heating systems implemented in both existing and new networks. The main findings are presented for each configuration group, including configuration layouts, typical temperature levels and several implemented installation examples, together with the advantages and disadvantages of each network configuration. In the assessment, a classification system comprising six different groups of typical network configurations was identified for low-temperature heat distribution. Together with eight variants within three of these six groups, fourteen possible network configurations were identified for low-temperature district heating. The main feature became the choice between a cold or warm network for the heat distribution, while the suitability of each network configuration depends on the temperatures of the available heat sources.

Energy and cost savings with continuous low temperature heating versus intermittent heating of an office building with district heating

Future low-temperature district heating systems (LTDH) require supply and return temperatures as low as 55 °C and 25 °C, respectively. In this direction, the use of night setbacks in heating of office buildings is a problem. This article investigated in a typical office building with large distribution system by tests and simulations the energy and cost savings by changing the control of the heating system from a continuous high-temperature operation either to a high-temperature intermittent heating or to a continuous low-temperature operation. Both strategies secured thermal comfort and resulted in similar energy savings of approximately 11% in the specific building. The reduction of the return temperature was higher under continuous low-temperature operation, resulting in additional cost savings due to a motivation tariff for low-temperature operation used in Denmark. As a result continuous low-temperature operation can achieve the highest total cost reduction for heating of 23.1%. Even if the results refer to the specific building, continuous low-temperature heating may result in significant energy savings in other buildings with large distribution systems. Moreover, implementing motivation tariffs towards LTDH may provide additional economic incentives. Therefore, it is interesting to investigate continuous low-temperature heating in each building and quantify the cost and energy savings towards LTDH.

A multi-vector community energy system integrating a heating network, electricity grid and PV production to manage an electrified community

Electrification in energy supply–demand plays a critical role in domestic heating and road transport, delivering an electrified community to reduce carbon emissions. This solution, however, places a significant power demand increase on the distribution networks. To ensure the security of electricity supply, an efficient energy system and energy demand reduction are essential. In this paper, a multi-vector community energy system, applying an electrified heating network, electric vehicle smart charging, community-scale peak shaving and photovoltaic (PV) generation, is demonstrated in three models to manage an electrified community. Firstly, a heating network model, comprising a central ground source heat pump, low-temperature district heating system, electric heaters and thermal storage, is established to measure the optimum distribution temperature. Next, an electrified community model illustrates hourly electricity demands and performances of a community energy system, which is then used to identify the required degree of housing thermal efficiency improvement (i.e., heating demand reduction). The third model evaluates decentralised PV/storage units to maintain the power demand below a targeted power. Modelling results show that the demand ratio of domestic hot water to space heating determines the distribution temperature, which indicates the temperature is increasing with growing housing thermal efficiency. Moreover, the electrification of a community could increase the peak power demand on the highest demand day by over five times, converting heating demands into electricity directly. This significant peak demand can be possibly reduced to only a 33% increase by employing a community energy system. The model of PV/storage units is validated through a 12-week assessment. Ultimately, a modelling tool is developed by assembling the mentioned models, providing four pathways to attain electrification. Users can adjust specific parameters and database to align with the local conditions. The results indicate the requirements of building a community energy system and electricity demands in the highest consumption period.

Comparison of traditional with low temperature district heating systems based on organic Rankine cycle

This paper compares a traditional district heating (DH) system with a low temperature DH system based on a combined heat and power (CHP) system using Organic Rankine Cycle (ORC). Both systems supply the same amount of heat, but it is shown that the system based on CHP using ORC is more effective and uses less fuel. This in turn results in less carbon dioxide emissions. The most advantageous configuration improving the real efficiency of the ultra-low temperature heating system seems to be a system combining a central heat pump and a station based on modern ORC technology which also provides an independent (off grid) electricity supply.

Novel Solar-Driven Low Temperature District Heating and Cooling System Based on Distributed Half-Effect Absorption Heat Pumps with Lithium Bromide

Novel Solar-Driven Low Temperature District Heating and Cooling System Based on Distributed Half-Effect Absorption Heat Pumps with Lithium Bromide

Individual Heat Substations Integrated with Heat Pumps for District Heating Systems in Ukraine

Energy systems around the world require a sustainable way for energy supply that does not add carbon to the atmosphere and thus does not enlarge the greenhouse effect, which is seen nowadays as the main cause of climate change. Urban housing consumes the most energy in European countries accounting for up to 43 % of final energy, while 65 % of it is spent on house heating and hot tap water supply. The reduction of carbon dioxide emissions and increasing of energy efficiency of power plants in Ukrainian cities, where district heating systems are centralised, and heat carriers are supplied from central combined heat and power stations based on fossil fuels, can be performed by modifying the existing district heating systems. One of the promising ways is the transition to low-temperature heat carriers from combined heat and power stations, which suppose the use of renewable energy sources, which can be integrated with individual heat substations of houses, and should satisfy the needs of residents in heating and hot water, with the possibility of air conditioning. In the present work, the possible implementation of individual heat substations integrated with heat pumps for low-temperature district heating systems is discussed. The analysis of the possibilities to increase the energy efficiency of the dormitory building with old construction is provided. The method for the design of individual heat substations for heating and hot water supply systems is discussed, including the recommendations for the building renovation.

Successful implementation of low temperature district heating case studies

Low temperature district heating is recognized as a key technology for the (cost-) efficient integration of renewable energy and waste heat sources in our energy systems. Several studies indicate that a deployment of local district heating schemes is a key measure for reaching the politically set climate goals. Further, implementation of low temperature district heating systems are recommended for taking maximum advantages of synergies with other sectors for decarbonization of the heating sector. Within the IEA DHC Annex TS2 project already realized low temperature community energy system concepts as well as planned or designed systems are identified and visualized. Furthermore, projects showing an innovative heat use or operation of buildings are also included in the analyses. The different projects are assessed and compared. The presentation of the demonstrators is set up in such a way that knowledge is generated about the indoor heating system, the district heating system and of the competitiveness of low temperature district heating systems, giving the evidence that these systems are feasible, efficient and reliable under various boundary conditions. The demonstrators further indicate that there are existing challenges, where further research on innovative district heating concepts for integrating decentral feed-in of renewable energy is required. The demonstrators included in the IEA DHC Annex TS2 are analysed in regard to which elements of new knowledge they can generate. For each demonstrator there are specific innovations in focus. Furthermore, the case studies show that the related business models for the utilities change when system temperatures are lowered. The paper presents and discusses the results from current research work within of the IEA DHC Annex TS2 on Implementation of Low Temperature District Heating Systems (Averfalk et al., 2021; Annex TS2, 2021 [1]).

Analysis of heat losses during heat supply of an urban-type settlement using a heat pump

Centralized heat supply systems include a network of pipelines connecting buildings and structures in such a way that entire districts and quarters are supplied with a heat carrier from a single boiler plant. District heating is a well-studied part of the energy system. Despite this, low temperature district heating is still underdeveloped, although it is one of the key technologies for energy efficient urban heating. Low-temperature heat supply systems are the basis of 4th generation district heating systems, since they can significantly reduce heat losses during transportation of the heat carrier to the heat energy consumer and allow the use of heat from renewable sources, for example, heat pumps. The study focuses on analyzing the heat loss of a district heating system by comparing simulation results at a standard flow temperature and at a temperature typical of the 4th generation heat supply schedule. On the basis of the created model, the calculation of heat and hydraulic losses of the low-temperature district heating system was carried out, as well as a comparative analysis in comparison with the standard heat supply system.

Model predictive control for a heat booster substation in ultra low temperature district heating systems

District heating systems may support an increased penetration of stochastic renewable energy technologies and a reduction in centralized combined heat and power plants to reduce carbon dioxide emissions. Ultra low temperature district heating minimizes transport heat losses while enabling the utilization of low-grade surplus heat. Local heat booster substations can heat water to useable temperatures using a heat pump and a hot water tank for storage and flexible operation. This paper proposes a hybrid model predictive control strategy in which an existing heat booster substation is modelled and its charging schedule optimized in real-time over a 24-h forecasted prediction horizon. This enables load shifting whereby scheduling of the heat pump minimizes operation costs. The realisation of energy flexibility can support greater utilization of renewable energy sources and surplus heat in energy supply systems to reduce primary energy consumption. The linear hybrid model predictive controller was successfully implemented in a real 22-flat multifamily building in Copenhagen to verify the control strategy. A comparison of the proposed model predictive control scheduling to the standard rule-based control showed average savings of 23 % on the electricity costs.

Applicability of thermal energy storage in future low-temperature district heating systems – Case study using multi-scenario analysis

With the flexibilities added from thermal energy storage (TES) technologies, low temperature district heating (LTDH) system can coordinate the heat and electricity sectors in a cost-effective manner. Such combinations have therefore become an important step to achieve a 100% renewable energy system. Despite the importance of TES has been demonstrated in previous studies, giving drastic changes compared to the current systems, the practical applicability of TES in the LTDH systems remains unknown. Furthermore, the proposed benefits of TES might deviate from the expectations considering the development of future characteristics, such as the low temperature levels and small space-heating demand. This study investigates the performances and benefits of four typical short-term TES technologies, including the use of central water tank (CWT), district heating network inertia, domestic hot water tank (DHWT), and building thermal mass, based on a case LTDH system in Roskilde, Denmark. Techno-economic analysis is conducted on a variety of scenarios, based on future changes in operation of the heat sources to the end-users. An integrated model is also developed to simulate the operation dynamics of the district heating system with regards to optimizing the use of the TES units. This study provides a performance map of the TES technologies in accordance with the transitions from current to future LTDH systems, indicating the relationships between the system characteristics and optimal TES applications. The CWT is found to be most preferable for integrating the variable renewable energy due to its ability to store heat for long periods. In the end-use side, with the improved building performances and reduced space heating demand in the future, there is less potential for the use of building inertia. In contrary, the benefit of the DHWT, which mainly comes from the reduction of bypass loss during the non-space-heating period, is increased in the future. Furthermore, raising the network temperatures for active storage is found to be infeasible under all future LTDH scenarios because this measure significantly influences the heat source efficiency.

An exergy-based minimum carbon footprint model for optimum equipment oversizing and temperature peaking in low-temperature district heating systems

Total decarbonization strategies are facing technical and environmental challenges according to the 2nd Law of Thermodynamics, such as equipment oversizing versus temperature peaking by heat pumps to accommodate low-temperature renewable and waste energy sources. As a method of this paper, the Rational Exergy Management Model (REMM) derived fourteen metrics, aiming to minimize the CO2 emissions responsibility. Two case studies are presented. One of them is a fifth-generation district energy system concept with a 250 MW design heating load of 20,000 residence-equivalent apartments at a supply temperature of 35 °C. Results showed that two heat pumps in a cascade achieved a 23% higher exergy utilization rate with an optimum temperature peaking to 45 °C and 25% radiator oversizing. The second case study is concerned with the strategy of the Chinese government to substitute the domestic use of coal and wood with local wind turbines for electric heating to combat global warming. Five alternatives were considered; 1-Wind electricity to resistance heating, 2-Wind electricity to heat pumps, 3-Wind electricity to mini hydrogen fuel cells, 4-Wind electricity to hydrogen and micro-cogeneration, and 5-Hydrogen district systems with biogas, geothermal, and solar energy. Results showed that Case 1 has the maximum carbon footprint, whereas a custom-designed hydrogen house has the least footprint leading to about 90% CO2 emissions responsibility emanating from exergy destructions. The paper concludes that low-temperature heating either in district energy systems, in private buildings, or prosumers is both environmentally, economically, and technically feasible.