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Low temperature district heating (DH) system gives easier access to the renewable energy as heat sources and improves the heat distribution efficiency. From the exergy point of view, low DH supply temperature also better matches the exergy demand of space heating and domestic hot water. It is more beneficial to operate district heating system under lower temperature level for the heat-sparse area where the distribution losses accounts for a large proportion in the total heat supply. In this study, the actual performance of a case ultra-low temperature district heating (ULTDH) system in Denmark was investigated based on long-term measurements. The system combines the central heat pump and local boosters, while the impact of such configuration on the overall system performance was analysed. The energy, exergy and economy performances of the case system were compared to medium temperature district heating system (MTDH) and low-temperature district heating system (LTDH). The results show that the LTDH system without supplementary heating has the highest energy and exergy efficiency. While the ULTDH system has better performance compared to the MTDH system in energy, exergy and economy due to substantial savings from the distribution heat loss. [ABSTRACT FROM AUTHOR]

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]

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]

The hot water tank is widely used for domestic hot water (DHW) preparation. When supplied by district heating (DH), the operation of DHW tanks directly influences the DH return temperature, thereby affecting the DH system efficiency, supply capacity and the realization of the low temperature district heating. However, the conventional charging method often results in high DH return temperature. This study develops a new charging method aiming at reducing the DH return temperature without violating the comfort or hygiene requirements. The concept uses the multi-mode charging method considering the periodical characteristics of the load pattern. Multi scenarios are simulated by dynamic models using the practical DHW load profiles from a case study. Moreover, the impacts of the tank configuration, the location of the temperature sensor, and the distribution heat loss are investigated. The results show that the new control method can reduce the primary return temperature by 5–8 °C compared to the conventional control method. The distribution heat loss imposes great impact on the DH return temperature. In addition, the tank with the external heat exchanger performs better than the tank with the internal heating coil if the circulation heat loss is less than 50% of the DHW demand. • An innovative charging method for the DHW tanks with DH supply is proposed. • Different DHW systems are simulated by Modelica models dynamically. • The role of the circulation heat loss on DH return temperature is investigated. • The new charging method improves the system performance effectively. [ABSTRACT FROM AUTHOR]

Hydro-fracture geometry prediction is of great practical importance for optimizing construction parameters and evaluating stimulation effects. Existing physical simulation methods are computationally intensive. Deep learning-based methods offer fast model inference, yet typically require a large amount of field data for accurate model training and lack model interpretability in explaining the complex physical processes. This work presents a physics-informed surrogate modeling method for hydro-fracture geometry prediction. The proposed method encodes the hydro-fracture physical laws, in the form of partial differential equations, as a loss term to govern the training process of the surrogate model, aiming to alleviate the data requirement for model training. Experimental studies demonstrate that the proposed modeling method effectively reduces the training data requirement and improves model accuracy and interpretability. Hydraulic fracturing is a tremendous task, also is indispensable for oil/gas development. We propose a physics-informed surrogate model to predict HFG, i.e. width, length, and height of hydro-fracture during hydraulic fracturing. The proposed model incorporates the physical laws and demonstrates high data efficiency, model accuracy, and model interpretability. This work can help infield experts free from the high-cost numerical simulation and the lack of expensive infield data. [Display omitted] • Physics-informed surrogate modeling for hydraulic-fracture geometry prediction. • Leverage the physical laws in the form of PDEs to drive the model training. • Model with high data efficiency, prediction accuracy, and interpretability. • Timely and accurately support onsite optimization of hydraulic fracturing. [ABSTRACT FROM AUTHOR]

This paper presents a high-temperature cascade heat pump system with a high compression ratio for recovering the waste heat of the chemical plant and replacing the steam. To analyze the economic and the thermodynamic performance of the high-temperature cascade heat pump, a mathematical model, considering energy, exergy, economy and environment, has been developed under different heat source inlet temperature. The proposed system here indicated good stability and feasibility under various operating conditions according to the simulation results. However, as the heating capacity and the annual net profit were found to be conflicting with each other, a multi-objective optimization method was developed to solve this problem. In addition, the technique for Order of Preference by Similarity to Ideal Solution method was selected as the decision-making method to search for the final optimal result of the system. The optimization results showed that if the optimized heating capacity project was selected, the average heating capacity and the average annual net profit were 2.8% and 6.7% lower than their maximum possible values. At the optimal heat source inlet temperature of 71 °C and condensing temperature of 145 °C, the heating capacity of the system was 498.9 kW and the payback period was 3.9 year. [Display omitted] • A HTCHP system for recovering waste heat and replacing steam was studied. • Mathematical model was developed to analyze thermodynamic and economic performance. • Multi-objective optimization was used to solve the contradiction. • TOPSIS method was applied to selected the final optimal results. [ABSTRACT FROM AUTHOR]