@inproceedings{SchmittReumSummetal.2024, author = {Schmitt, David and Reum, Tobias and Summ, Thorsten and Trinkl, Christoph and Schrag, Tobias}, title = {Investigation of the Flexibility Potential by Decoupling Building Mass and Room Temperature}, booktitle = {International Sustainable Energy Conference - Proceedings}, editor = {Fink, Christian and Brunner, Christoph}, publisher = {TIB Open Publishing}, address = {Hannover}, issn = {2976-2030}, doi = {https://doi.org/10.52825/isec.v1i.1165}, year = {2024}, abstract = {Using the thermal building mass as a thermal storage received increasing attention in research during recent years. Due to the large mass of concrete, it offers a large storage capacity and thus a high potential for flexibility. However, passive heat losses during cool down of a thermally activated building influence room temperature and thus limit its flexibility potential. In this contribution a multi-layer activation concept was investigated which thermally decouples the building mass and room air. The study aims to analyse the cool down of a thermally activated building in terms of different charging parameters while considering other heat sources such as solar gains. A dynamic building simulation of a demonstration building was set-up and compared to simulative studies from literature to proof the validity of its dynamic behaviour. In the simulation model the room temperature could be kept above 19 °C between 100 - 190 h. However, when charging the building structure quickly, room temperatures above 24 °C are reached easily. Considering other heat sources such as solar gains, advanced control algorithms are required for efficient operation of the heating system.}, language = {en} } @article{ReumSchmittSummetal.2024, author = {Reum, Tobias and Schmitt, David and Summ, Thorsten and Trinkl, Christoph and Ochs, Fabian and Schrag, Tobias}, title = {Experimental analysis of parallel operation of two heat sources in a dual-source heat pump incorporating two compressors}, volume = {2025}, journal = {International Journal of Refrigeration}, number = {171}, publisher = {Elsevier}, address = {Amsterdam}, issn = {1879-2081}, doi = {https://doi.org/10.1016/j.ijrefrig.2024.12.013}, pages = {124 -- 138}, year = {2024}, abstract = {Dual-source heat pumps have the potential to improve heating efficiency and / or the space required for heat sources. Parallel operation of two heat sources using two compressors can increase these benefits but poses technological challenges - specifically in the increased refrigerant cycle interconnection complexity, refrigerant and compressor oil management. These benefits and challenges have yet to be thoroughly investigated which is the primary objective of this research paper. An experimental analysis was conducted on a dual-source heat pump that utilized both an air heat source and a ground heat source. It included both individual and parallel operation modes of the heat sources. A prototype with a novel two-compressor interconnection was designed, manufactured, and tested in laboratory conditions. Analysis of the energetic benefit included efficiency evaluation at different ambient conditions and the corresponding loads on the ground source heat exchanger. Energetically, the single source operation showed comparable results in the coefficient of performance to conventional heat pumps despite the increased interconnection complexity. Parallel operation could efficiently cover heating loads at reduced load on the individual heat sources (at -7 °C, a reduction to 41 \% on the ground-source and 58 \% on the air-source heat exchanger). Technological challenges were highlighted in refrigerant and compressor oil shifts. While a control-based solution for the refrigerant shift was demonstrated, the oil shift requires further analysis of oil conditions in failing compressors.}, language = {en} } @inproceedings{RammEhrenwirthSchrag2019, author = {Ramm, Tobias and Ehrenwirth, Mathias and Schrag, Tobias}, title = {Modelling of the central heating station within a district heating system with variable temperatures}, booktitle = {Proceedings of the 13th International Modelica Conference}, editor = {Haumer, Anton}, publisher = {Modelica Association and Link{\"o}ping University Electronic Press}, address = {Link{\"o}ping}, isbn = {978-91-7685-122-7}, issn = {1650-3686}, doi = {http://dx.doi.org/10.3384/ecp19157567}, pages = {567 -- 576}, year = {2019}, language = {en} } @inproceedings{SchragEhrenwirthRammetal.2020, author = {Schrag, Tobias and Ehrenwirth, Mathias and Ramm, Tobias and Vannahme, Anna and Trinkl, Christoph}, title = {Solar energy use in district heating networks}, booktitle = {ICREEC 2019: Proceedings of the 1st International Conference on Renewable Energy and Energy Conversion}, editor = {Belasri, Ahmed and Beldjilali, Sid Ahmed}, publisher = {Springer}, address = {Singapur}, isbn = {978-981-15-5443-8}, issn = {2352-2534}, doi = {https://doi.org/10.1007/978-981-15-5444-5_1}, pages = {3 -- 10}, year = {2020}, language = {en} } @inproceedings{VannahmeRammEhrenwirthetal.2019, author = {Vannahme, Anna and Ramm, Tobias and Ehrenwirth, Mathias and Schrag, Tobias}, title = {Comparison of a usual heat-transfer-station with a hydraulic modified version under the aspect of exergy saving}, booktitle = {Proceedings of the 13th International Modelica Conference}, editor = {Haumer, Anton}, publisher = {Link{\"o}ping University Electronic Press}, address = {Link{\"o}ping}, isbn = {978-91-7685-122-7}, issn = {1650-3686}, doi = {http://dx.doi.org/10.3384/ecp19157647}, pages = {647 -- 654}, year = {2019}, language = {en} } @inproceedings{RammHammelKlaerneretal.2017, author = {Ramm, Tobias and Hammel, Carolina and Kl{\"a}rner, Michael and Kruck, Alfons and Schrag, Tobias}, title = {Energy storage and integrated energy approach for district heating systems}, volume = {2017}, booktitle = {Energy Procedia}, number = {135}, publisher = {Elsevier}, address = {Amsterdam}, issn = {1876-6102}, doi = {https://doi.org/10.1016/j.egypro.2017.09.515}, pages = {391 -- 397}, year = {2017}, abstract = {This paper deals with the potential of using the thermal storage capacities within local district heating systems to balance the low and medium voltage grids by the use of heat pumps (HP) and combined heat and power (CHP) plants. The state-of-the-ar of district heating systems as well as the coupling of the heat and power grid is discussed. The research work of the projec NATAR (local heating grids with lowered temperature as provider of balancing power) and the local heating grid of Dollnstein which serves as an use case, are further described. A first analysis of measured data of the district heating system indicates an optimisation potential regarding to the control strategy of the combined heat and power plant as well as the usage of the available thermal storage capacities.}, language = {en} } @inproceedings{SchmittReumSummetal.2024, author = {Schmitt, David and Reum, Tobias and Summ, Thorsten and Trinkl, Christoph and Schrag, Tobias}, title = {Potential of a Long-Term Predictive Controller for Managing Energy Storages to Reduce the Electric Grid-Load of an Air-Source Heat Pump}, booktitle = {Proceedings of the International Renewable Energy Storage and Systems Conference (IRES 2023)}, editor = {Droege, Peter and Quint, Lelia}, publisher = {Atlantis Press}, address = {Dordrecht}, isbn = {978-94-6463-455-6}, doi = {https://doi.org/10.2991/978-94-6463-455-6_21}, pages = {205 -- 211}, year = {2024}, abstract = {Electrifying the domestic heating sector is mandatory to decarbonize the buildings sector but leads to additional peak loads in the public grid. Air-source heat pumps the mostly used technology in this regard even though, they suffer from lower efficiency during times of high heat demands. The usage of energy storages and predictive controllers enables operation shifting and hence offers potential to decrease the peak loads in the electric grid. This potential can be increased by using a longer prediction horizon as extended operation shifting can be performed. In this contribution, a methodology to derive the theoretical potential on the grid peak load reduction of such a long-term predictive controller is introduced. The electric load profile of an air-source heat pump in a typical German single-family home was generated based on different weather data scenarios for the city of Ingolstadt. Based on the profile the dependency between peak load reduction and required storage capacity was identified. In the case study a reduction of the peak load from 21 - 31\% for an unrenovated building was achieved using a 500 L buffer storage. Further investigation needs to be done to derive a more realistic potential for renovated or new buildings.}, language = {en} } @inproceedings{ReumNataleSchmittetal.2025, author = {Reum, Tobias and Natale, Christian and Schmitt, David and Dongellini, Matteo and Naldi, Claudia and Summ, Thorsten and Morini, Gian Luca and Schrag, Tobias}, title = {Control Strategies for Active Regeneration in a Dual-Source Heat Pump}, booktitle = {Proceedings of the 15th REHVA HVAC World Congress - CLIMA 2025: Volume 1}, editor = {Zilio, Claudio and Busato, Filippo and Mazzarella, Livio and Noro, Marco}, publisher = {Springer}, address = {Cham}, isbn = {978-3-032-06806-4}, doi = {https://doi.org/10.1007/978-3-032-06806-4_55}, pages = {564 -- 574}, year = {2025}, language = {en} } @inproceedings{SchmittReumSummetal.2025, author = {Schmitt, David and Reum, Tobias and Summ, Thorsten and Schrag, Tobias}, title = {Comparison of Heat Pump Modelling Approaches for Predictive Controllers to Optimize the Efficiency of Modulating Air-Source Heat Pumps: A Case Study from South Germany}, booktitle = {Proceedings of the 15th REHVA HVAC World Congress - CLIMA 2025: Volume 1}, editor = {Zilio, Claudio and Busato, Filippo and Mazzarella, Livio and Noro, Marco}, publisher = {Springer}, address = {Cham}, isbn = {978-3-032-06806-4}, doi = {https://doi.org/10.1007/978-3-032-06806-4_59}, pages = {611 -- 620}, year = {2025}, language = {en} } @inproceedings{ReumSchmittSummetal.2024, author = {Reum, Tobias and Schmitt, David and Summ, Thorsten and Schrag, Tobias}, title = {Energetic Potential of Parallel Operation of Two Heat Sources in a Dual-Source Heat Pump}, booktitle = {International Sustainable Energy Conference - Proceedings}, editor = {Fink, Christian and Brunner, Christoph}, publisher = {TIB Open Publishing}, address = {Hannover}, issn = {2976-2030}, doi = {https://doi.org/10.52825/isec.v1i.1158}, year = {2024}, abstract = {Dual-source heat pumps can mitigate disadvantages of single source heat pumps: They have fewer geological requirements compared to ground-source heat pumps while having higher efficiencies compared to air-source heat pumps. Parallel operation of two heat sources can also make electric heaters for peak loads obsolete, leading to economic benefits in the operational costs. Parallel operation has not been analysed thoroughly at different evaporation temperature gradients. To address this gap in research, four possible interconnections of two heat sources were analysed using a refrigerant cycle simulation, two with similar and two with separate evaporation pressures. The energetic potential of each interconnection is evaluated and compared to single source operation with an air-source and a ground-source heat pump. The results showed that only the interconnections with separate evaporation pressure allowed significant reduction in evaporation power from the ground source. As expected, the efficiency - compared to single air-source operation - increased for all parallel interconnections but decreased compared to ground-source operation. Efficient peak load coverage with small ground-source collectors therefore requires a more complex interconnection of completely split evaporator branches at different evaporation pressures. While the efficiency and heating power compared to single ground-source operation decreased slightly (by 4\% and 6\%, respectively), the power load on the GSHX and ASHX reduced to about 54\% and 66\% compared to the corresponding single-source operation, respectively. This allows high efficiency at reduced GSHX size and ASHX noise emission. Additionally, this interconnection also allows increased flexibility for improved heat source management.}, language = {en} } @article{VannahmeEhrenwirthSchrag2021, author = {Vannahme, Anna and Ehrenwirth, Mathias and Schrag, Tobias}, title = {Enhancement of a district heating substation as part of a low-investment optimization strategy for district heating systems}, volume = {10}, pages = {53}, journal = {Resources}, number = {5}, publisher = {MDPI}, address = {Basel}, issn = {2079-9276}, doi = {https://doi.org/10.3390/resources10050053}, year = {2021}, abstract = {In an ongoing project, low-investment measures for the optimization of district heating systems are analyzed. The optimization strategies are collected in a catalog, which is the core of a guideline. The application of this guideline is demonstrated using two concrete district heating networks as examples. In this study, the improvement of an analog controlled district heating substation by an electronic controller is investigated. High supply temperatures and heat losses are often a challenge in district heating networks. The district heating substations have a major influence on the network return temperatures. The comparison of the two substation setups with analog and electronic controllers is carried out by laboratory measurement. It can be shown that the return temperatures can be reduced by an average of 20 K in winter and transition, as well as 16 K in summer. The district heating network losses are calculated for one of both specific district heating networks. They are calculated from the ratio of network losses to generated energy. The generated energy is the sum of network losses and consumer demand. The thermal losses of the network can be reduced by 3\%. The volume flow in the heating network can be reduced to a quarter. Therefore, the pumping energy requirement drops sharply since these changes cubically affect the volume flow}, language = {en} } @article{WernerEhrenwirthMuschiketal.2021, author = {Werner, Melanie and Ehrenwirth, Mathias and Muschik, Sebastian and Trinkl, Christoph and Schrag, Tobias}, title = {Operational experiences with a temperature-variable district heating network for a rural community}, volume = {46}, journal = {Chemical Engineering \& Technology}, number = {3}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0930-7516}, doi = {https://doi.org/10.1002/ceat.202100114}, pages = {482 -- 490}, year = {2021}, abstract = {The potential of utilizing temperature-variable district heating systems in rural areas was investigated. A local district heating system in Germany and the related project NATAR are briefly described; measurement data is evaluated. Operational experiences with a seasonal temperature reduction and consequent advantages for different heat generation systems and heat storages are discussed. The analysis of measurement data reveals a high optimization potential for solar thermal systems in case of an integration both into a low- and a high-temperature storage. Furthermore, a combined heat-and-power plant in combination with a CO2 heat pump promises sector-coupling potential and high electricity self-consumption.}, language = {en} } @article{SlonskiSchrag2019, author = {Slonski, Matthias and Schrag, Tobias}, title = {Linear optimisation of a settlement towards the energy-plus house standard}, volume = {12}, pages = {210}, journal = {Energies}, number = {2}, publisher = {MDPI}, address = {Basel}, issn = {1996-1073}, doi = {https://doi.org/10.3390/en12020210}, year = {2019}, abstract = {Future buildings will use technologies that are either well-known, innovative, or a combination thereof in order to be environmentally friendly and feasible at the same time. To evaluate and compare such systems through simulation, adaptive tools need to be available. This paper describes a conceived method for planning quarters and settlements. The novelty of this work emerges from the combination of a building simulation with a linear economic optimisation of the energy system, to achieve the energy-plus house standard for a settlement. Furthermore, the tools applied are adaptive or open source. In this article, a hypothetical basic example is given for a predefined idealised settlement, which consists of 132 single-family houses of one building type. The hourly demand for electrical energy and heat is established for three energy-efficiency classes for the building type with a dynamic simulation in MATLAB/SIMULINK using the CARNOT toolbox. This toolbox is also used to calculate the specific electrical energy production by photovoltaics. The components for the energy system of the settlement are implemented in the open source linear optimisation tool URBS. An economic optimum for the energy system of the settlement is found for each of the energy efficiency classes for an accumulated energy demand of the buildings. In this way, a lossless energy hub between the buildings is assumed. The results of the conducted simulations indicate that the optimal ratio of air/water to ground/water heat pumps shifts towards air/water heat pumps with more energy efficient houses. This is due to the lower specific investment costs, which outweigh the operational costs when less energy is required. The lowest costs for the entire energy system are for the one with the most energy efficient settlement. This is the case, as the costs for the higher energy standard of the buildings are not considered in the calculations. The behaviour of the optimisation is tested and discussed through a sensitivity analysis for one efficiency class. By presenting this simple, comprehensible example, an impression of the possible applications for this methodology is conveyed.}, language = {en} } @article{HaslerHuberLeindeckeretal.2013, author = {Hasler, Ewald and Huber, Christian and Leindecker, Herbert C. and Schrag, Tobias and Stocker, Emanuel and Wartha, Christian}, title = {Bewertung unterschiedlicher Fassadenkonstruktionen unter Lebenszyklusaspekten mit einer Fallstudie B{\"u}rogeb{\"a}ude}, volume = {35}, journal = {Bauphysik}, number = {4}, publisher = {Ernst \& Sohn}, address = {Berlin}, issn = {1437-0980}, doi = {https://doi.org/10.1002/bapi.201310072}, pages = {250 -- 256}, year = {2013}, language = {de} } @article{WernerMuschikEhrenwirthetal.2022, author = {Werner, Melanie and Muschik, Sebastian and Ehrenwirth, Mathias and Trinkl, Christoph and Schrag, Tobias}, title = {Sector Coupling Potential of a District Heating Network by Consideration of Residual Load and CO2 Emissions}, volume = {15}, pages = {6281}, journal = {Energies}, number = {17}, publisher = {MDPI}, address = {Basel}, issn = {1996-1073}, doi = {https://doi.org/10.3390/en15176281}, year = {2022}, abstract = {The growing share of fluctuating renewable electricity production within the German energy system causes the increasing necessity for flexible consumers, producers, and storage technologies to balance supply and demand. District heating networks with combined heat and power units, Power-to-Heat applications, and thermal energy storage capacities can serve as one of these flexible options. In this context, a simulation model of the district heating network of the rural community Dollnstein, Germany, was built. With the residual load of different regional areas (Germany, Bavaria, Eichst{\"a}tt, Dollnstein) it is investigated, how the heat generators can operate in an electricity market beneficial way. Two different control algorithms were evaluated: Due to a correlation between the residual loads and the CO2 emissions of the electricity mix, the CO2 savings achieved by this control algorithm are determined. Another way to operate electricity market beneficial is to consider the current CO2 emissions of each region. The main outcomes of this paper are, that there is a high potential for sector coupling by shifting the operation times of a CHP and a heat pump according to the residual load. The electricity demand of the heat pump can be met in terms of low CO2 emissions of the electricity mix, while the CHP can replace electricity with high CO2 emissions. These results can be improved, by considering not the residual load but the current CO2 emissions in the control algorithm.}, language = {en} } @article{VannahmeBuschEhrenwirthetal.2023, author = {Vannahme, Anna and Busch, Jonas and Ehrenwirth, Mathias and Schrag, Tobias}, title = {Experimental Study of District Heating Substations in a Hardware-in-the-Loop Test Rig}, volume = {12}, pages = {43}, journal = {Resources}, number = {4}, publisher = {MDPI}, address = {Basel}, issn = {2079-9276}, doi = {https://doi.org/10.3390/resources12040043}, year = {2023}, abstract = {This study compares two district heating substation systems for implementation in rural district heating networks with non-retrofitted single- and two-family houses. The aim is to determine which system has the potential to provide lower return temperatures and/or lower power peak demand. A hardware-in-the-loop-test rig was utilized to measure the two district heating substations under real operation conditions. This experimental study demonstrates that load balancing of the district heating network is attainable with the district heating substation with storage. This is especially advantageous when there is a high demand for domestic hot water. Overall, both systems yield comparable return temperatures.}, language = {en} } @article{VannahmeEhrenwirthSchrag2024, author = {Vannahme, Anna and Ehrenwirth, Mathias and Schrag, Tobias}, title = {Development and application of a guideline for assessing optimization potentials for district heating systems}, volume = {2024}, pages = {131226}, journal = {Energy}, number = {297}, publisher = {Elsevier}, address = {Amsterdam}, issn = {1873-6785}, doi = {https://doi.org/10.1016/j.energy.2024.131226}, year = {2024}, abstract = {Integrating renewable energies into district heating systems has a large potential to reduce CO2-emissions in the heating sector. As district heating systems offer the possibility of incorporating renewable energies into the heat supply, new systems have to be built and the existing networks must be maintained. This study investigates ways to optimize existing district heating systems in order to ensure economic sustainability in the long-term. Previous case studies have elaborated on a variety of optimization measures. However, to date, these measures have neither been collected nor consistently assessed for a wider application range. Therefore, in the study presented here a system for assessing the ecological and economic benefits of optimization measures was developed and applied. The assessment method utilized showed that optimization of district heating consumer substations and adding of a central buffer storage tank has a high optimization potential in comparison to intermittent operation strategy, which has a significantly lower optimization potential. From this information and the transferability data, a district heating operator can determine which optimization measure should be prioritized, which is shown at the end of the paper on an example case.}, language = {en} } @article{VannahmePatelSchmittetal.2024, author = {Vannahme, Anna and Patel, Dharmik and Schmitt, David and Summ, Thorsten and Trinkl, Christoph and Schrag, Tobias}, title = {Technical and economic analysis of digitally controlled substations in local district heating networks}, volume = {2024}, pages = {133585}, journal = {Energy}, number = {312}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0360-5442}, doi = {https://doi.org/10.1016/j.energy.2024.133585}, year = {2024}, abstract = {Since the 1990s, there has been a noticeable increase in the establishment of local district heating networks in Germany, coinciding with the use of thermal energy from biogas and biomass facilities. Historically, the prioritization of economically efficient heat utilization was subdued due to favorable electricity feed-in tariffs. However, with the expiration of Renewable Energy Sources Act subsidies, operators shifted focus to the operational costs of district heating networks. This study aims to examine the effects of optimizing controllers and valves at district heating substations in single-family homes, utilizing two local district heating networks. The emphasis is on evaluating impacts on operating costs and determining whether the economic and energetic benefits justify implementation. Simulation studies in MATLAB/Simulink Simscape depict both consumers and district heating networks individually, without aggregation. In the most favorable scenario, investing in enhancing substation controllers in single-family homes is projected to yield returns after 13 years. Comprehensive optimization of all substation controls in single-family homes can potentially result in up to 9\% savings in thermal energy demand due to reduced heat losses and a corresponding 9\% reduction in electrical power consumption for the main pump.}, language = {en} } @inproceedings{MuellerStrobelFaigletal.2024, author = {M{\"u}ller, Simon Nikolaus and Strobel, Rainer and Faigl, Markus and Schrag, Tobias}, title = {Analysis of Industrial 5GDHC System in Ingolstadt}, booktitle = {International Sustainable Energy Conference - Proceedings}, subtitle = {A Step Towards CO2-Neutral Industry}, editor = {Fink, Christian and Brunner, Christoph}, publisher = {TIB Open Publishing}, address = {Hannover}, issn = {2976-2030}, doi = {https://doi.org/10.52825/isec.v1i.1152}, year = {2024}, abstract = {This study comprehensively examines the technical manifestation and planning process for a shared thermal energy network at a 75-hectare industrial area in Ingolstadt. Unlike traditional systems, this (5GDHC) network showcases a smart integration of energy flows across 70 buildings with a yearly heating demand of around 17 GWh and a yearly cooling demand of around 35 GWh. The network spans over 9,100 meters of piping with diameters up to 800 mm. With thermal power from various sources including 1.8 MW waste heat from a data centre and potential for 10 MW from the Danube River, the system epitomizes a dynamic balance of heating and cooling demands. The paper itself examines the planning process of the system and encompasses topics such as defining energy requirements and load profiles, assessing potential energy sources and sinks for enhanced system efficiency, and analysing pipe and network design. Through this investigation, the study provides valuable insights towards a methodology to facilitate the successful implementation of future industrial 5GDHC systems, furthering the cause of sustainable energy.}, language = {en} }