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To improve energy efficiency and give more access to renewable energy sources, low-temperature district heating (LTDH) is a promising concept to be realized in the future. However, concern about Legionella proliferation restricts applying low-temperature district heating in conventional systems with domestic hot water (DHW) circulation. In this study, a system with decentralized substations was analysed as a solution to this problem. Furthermore, a modification for the decentralized substation system were proposed in order to reduce the average return temperature. Models of conventional system with medium-temperature district heating, decentralized substation system with LTDH, and innovative decentralized substation system with LTDH were built based on the information of a case building. The annual distribution heat loss and the operating costs of the three scenarios were calculated and compared. From the results, realizing LTDH by the decentralized substation unit, 30% of the annual distribution heat loss inside the building can be saved compared to a conventional system with medium-temperature district heating. Replacing the bypass pipe with an in-line supply pipe and a heat pump, the innovative decentralized substation system can reduce distribution heat loss by 39% compared to the conventional system and by 12% compared to the normal decentralized substation system with bypass. [ABSTRACT FROM AUTHOR]

District heating in Denmark is going through the transition from 3rd generation (80/40 °C) to 4th generation (50–55 °C/25 °C) systems in preparation for district heating based completely on renewable fuels by 2035. However, concern about Legionella growth and reduced comfort with low-temperature domestic hot water supply may be discouraging the implementation of low-temperature district heating. Aimed at providing possible solutions, this study modelled various proposals for district heating systems with supply temperatures of 65 °C, 50 °C and 35 °C and for two different building topologies. Evaluation models were built to investigate the energy, economy and exergy performances of the proposed domestic hot water systems in various configurations. The configurations of the devised domestic hot water substations were optimised to fit well with both low and ultra-low-temperature district heating and to reduce the return temperature to district heating. The benefits of lower return temperatures were also analysed compared with the current district heating situation. The evaluation results show that the decentralized substation system with instantaneous heat exchanger unit performed better under the 65 °C and 50 °C district heating scenarios, while the individual micro tank solution consumed less energy and cost less in the 35 °C district heating scenario. [ABSTRACT FROM AUTHOR]

This study investigated the performances of five different substation configurations in single-family houses supplied with ULTDH (ultra-low-temperature district heating). The temperature at the heat plant is 46 °C and around 40 °C at the substations. To avoid the proliferation of Legionella in the DHW (domestic hot water) and assure the comfortable temperature, all substations were installed with supplementary heating devices. Detailed measurements were taken in the substations, including the electricity demand of the supplementary heating devices. To compare the energy and economic performance of the substations, separate models were built based on standard assumptions. The relative heat and electricity delivered for preparing DHW were calculated. The results showed that substations with storage tanks and heat pumps have high relative electricity demand, which leads to higher integrated costs considering both heat and electricity for DHW preparation. The substations with in-line electric heaters have low relative electricity usage because very little heat is lost due to the instantaneous DHW preparation. Accordingly, the substations with in-line electric heaters would have the lowest energy cost for DHW preparation. To achieve optimal design and operation for the ULTDH substation, the electricity peak loads of the in-line electric heaters were analysed according to different DHW-heating strategies. [ABSTRACT FROM AUTHOR]

• Detailed measurements and Modelica models are performed to evaluate the building heat flexibility. • Integrated heat flexibility of the low-energy building was analyzed numerically. • Heating control was optimized considering the energy and economy objectives. • The load shifting performance and the heat rebound effect were analyzed. • Actual occupancy and dynamic heat price were applied to analyze the flexibility. Along with the increasing application of the renewable energy in the energy systems, the future energy system will be more stochastic and fluctuated. Therefore, it is of great importance to effectively coordinate the energy supply and demand side considering their different dynamic characteristics. This study analyzes the impact of the integrated energy flexibility from the building mass and the storage tank on the heating system operation for a low-energy building connected to district heating (DH). According to the actual room occupancy and dynamic heat price, the strategy to unlock the energy flexibility is developed and optimized. The detailed thermal performance of the system is simulated by Modelica models. Compared to the reference case, the integrated energy flexibility is helpful to make better use of the low-price heat. Moreover, a variety of quantification indices were developed to evaluate the flexibility and rebound effect. The results show that the fully charged thermal mass of the low-energy building can help maintain the comfort indoor temperature for up to 43 h. Combined with the storage tank, the heat flexibility potential of the case building reaches 78 kWh and saves 1.1 DKK of the daily heat cost with the optimized heating strategy compared to the reference scenario. Moreover, the flexibility factor is improved by shifting the peak heat load to the low-price period. [ABSTRACT FROM AUTHOR]

• The optimal radiant floor configuration is investigated with different DH scenarios. • The control strategy was developed to optimized the comfort, energy and economy. • The heat flexibility potential of the radiant floor was analyzed numerically. • All cases reach flexibility factor of 1 for NZEB with acceptable thermal comfort. The radiant heating is cost effective way of space heating while providing favourable thermal comfort. Due to the large thermal flexibility, it is possible to utilize the pipe embedded radiant heating for low-temperature heating and demand management, thereby giving more access to the low-grade heat sources such as the renewable energy or the industrial waste heat. In this study, we investigate the optimal configuration of the radiant floor heating system considering different district heating (DH) scenarios. We propose the scheduled control strategy considering the comfort requirements, the occupancy and the dynamic energy price. Parametric simulations regarding the pipe spacing, the thickness of the concrete core, and the DH supply temperatures are performed by Modelica models. The critical levels of the radiant floor configurations of applying medium-temperature DH (MTDH), low-temperature DH (LTDH) and ultra-low temperature DH (ULTDH) are investigated. The indoor comfort, the thermal performances, the economy performances and the energy flexibility as the crucial factors of different cases are analysed and compared. The results show that the pipe spacing of 300 mm achieves better comfort and shorter charging time for all DH scenarios of the case nearly zero energy building (NZEB). The larger thickness of the concrete core can stabilize the indoor temperature variation and buffer the impact from the environmental disturbance more effectively. [ABSTRACT FROM AUTHOR]