@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} } @article{IgiligiVielhauerEhrenwirthetal.2023, author = {Igiligi, Anthony and Vielhauer, Armin and Ehrenwirth, Mathias and Hurm, Christian and Summ, Thorsten and Trinkl, Christoph and Navarro Gevers, Daniel}, title = {Assessment of Conservation Voltage Reduction in Distribution Networks with Voltage Regulating Distribution Transformers}, volume = {16}, pages = {3155}, journal = {Energies}, number = {7}, publisher = {MDPI}, address = {Basel}, issn = {1996-1073}, doi = {https://doi.org/10.3390/en16073155}, year = {2023}, abstract = {The application of voltage reduction in medium and low voltage grids to reduce peak power demand or energy consumption has been implemented since the 1980s using several approaches. Conservation Voltage Reduction (CVR), as one such approach, uses a voltage control device to reduce or increase the voltage setpoint on a busbar, thereby reducing or increasing the amount of active and reactive power supply in the network. Voltage regulation for CVR is always implemented according to established network planning standards in each country. Research in this field has proven that a CVR factor (CVRf) of 0.7-1.5 for peak demand reduction can be achieved. This is an evaluation metric of CVR. The aim of this research is to determine and validate CVRf for peak demand reduction by comparing actual results obtained during regular tap changes with other randomly distributed periods outside tap change operations, using a set of measurement data. It is important to understand CVR deployment capability by evaluating CVR potentials from historical random tap operations before a robust network-wide deployment is introduced. This research provides such guidance. It also provides a novel approach to determining tap changes from voltage measurements using a time-based algorithm. A CVRf ranging from 0.95 to 1.61 was estimated using a measurement dataset from a test field. The result of the entire evaluation shows that the CVRf are smaller during peak PV production and greater during peak demand periods. Further evaluation using statistical hypotheses testing and a control chart was used to validate the evaluation.}, language = {en} } @article{SummEhrenwirthTrinkletal.2023, author = {Summ, Thorsten and Ehrenwirth, Mathias and Trinkl, Christoph and Z{\"o}rner, Wilfried and Pischow, Kaj and Greenough, Richard and Oyinlola, Muyiwa}, title = {Effect of argon concentration on thermal efficiency of gas-filled insulating glass flat-plate collectors}, volume = {2023}, pages = {120657}, journal = {Applied Thermal Engineering}, number = {230, Part A}, publisher = {Elsevier}, address = {Amsterdam}, issn = {1873-5606}, doi = {https://doi.org/10.1016/j.applthermaleng.2023.120657}, year = {2023}, abstract = {Insulating glass flat-plate collectors can save cost by being produced quickly and automatically in insulated glass production facilities, and they can be filled with argon to reduce heat loss. During its lifetime, the collector is likely to lose argon because of gradual material degradation of the sealing. However, information on the influence of the argon concentration on the collector efficiency is limited. Therefore, the objective of this research work was to analyse this effect. A theoretical material property calculation of argon-air mixtures was carried out to determine the convective losses with variable argon concentrations. Thermal collector performance was measured experimentally using an outdoor solar tracker test rig. The results strongly suggest, that the influence of argon concentration on both the convective losses and the thermal efficiency is non-linear. The measurements revealed that an argon concentration of 90 \% can increase average thermal performance by 6.7 ± 4.8 percentage points. An increase in argon concentration from 0 \% to 50 \% has almost twice the effect on average thermal efficiency as an increase from 50 \% to 90 \%. Concluding from these results, an argon leakage threshold of 2.5 percentage points per year is proposed to avoid disproportionate loss of efficiency over time.}, 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} } @article{BachseitzSheryarSchmittetal.2024, author = {Bachseitz, Michael and Sheryar, Muhammad and Schmitt, David and Summ, Thorsten and Trinkl, Christoph and Z{\"o}rner, Wilfried}, title = {PV-Optimized Heat Pump Control in Multi-Family Buildings Using a Reinforcement Learning Approach}, volume = {17}, pages = {1908}, journal = {Energies}, number = {8}, publisher = {MDPI}, address = {Basel}, issn = {1996-1073}, doi = {https://doi.org/10.3390/en17081908}, year = {2024}, abstract = {For the energy transition in the residential sector, heat pumps are a core technology for decarbonizing thermal energy production for space heating and domestic hot water. Electricity generation from on-site photovoltaic (PV) systems can also contribute to a carbon-neutral building stock. However, both will increase the stress on the electricity grid. This can be reduced by using appropriate control strategies to match electricity consumption and production. In recent years, artificial intelligence-based approaches such as reinforcement learning (RL) have become increasingly popular for energy-system management. However, the literature shows a lack of investigation of RL-based controllers for multi-family building energy systems, including an air source heat pump, thermal storage, and a PV system, although this is a common system configuration. Therefore, in this study, a model of such an energy system and RL-based controllers were developed and simulated with physical models and compared with conventional rule-based approaches. Four RL algorithms were investigated for two objectives, and finally, the soft actor-critic algorithm was selected for the annual simulations. The first objective, to maintain only the required temperatures in the thermal storage, could be achieved by the developed RL agent. However, the second objective, to additionally improve the PV self-consumption, was better achieved by the rule-based controller. Therefore, further research on the reward function, hyperparameters, and advanced methods, including long short-term memory layers, as well as a training for longer time periods than six days are suggested.}, language = {en} } @article{SummOyinlolaKhattaketal.2024, author = {Summ, Thorsten and Oyinlola, Muyiwa and Khattak, Sanober and Ehrenwirth, Mathias and Trinkl, Christoph and Z{\"o}rner, Wilfried}, title = {Towards maximum cost-effectiveness: multi-objective design optimisation of insulating glass flat-plate collectors}, volume = {2025}, pages = {124594}, journal = {Applied Thermal Engineering}, number = {258}, publisher = {Elsevier}, address = {Amsterdam}, issn = {1359-4311}, doi = {https://doi.org/10.1016/j.applthermaleng.2024.124594}, year = {2024}, abstract = {A significant challenge in the advancement of solar thermal heating systems lies in the unexplored techno-economic potential of insulating glass flat-plate collectors. These collectors are constructed in accordance with the specifications of standard insulating glass units and have emerged over the past decade as a promising design concept for enhancing the cost-effectiveness of solar thermal systems. However, substantial findings regarding the techno-economic viability of their production are still pending. The aim of this paper is to optimise insulating glass collector designs for solar district heating applications by identifying key design parameters that maximise cost-effectiveness. This study employed a five-stage methodology. It included thermo-hydraulic collector modelling using MATLAB/Simscape and the CARNOT Toolbox. The model was validated against experimental performance tests. A Latin hypercube computational design with 250,000 samples was set up to train supervised machine learning metamodels and perform a multi-objective optimisation using an elitist genetic algorithm. The study identified the argon concentration, collector length, and width as critical parameters influencing efficiency. Larger, thinner collectors demonstrated superior performance due to reduced convective losses and increased aperture-to-surface ratios. The optimisation revealed that the insulating glass collectors could achieve a 7.7 percentage point increase in efficiency, a 19.4 \% reduction in material cost, and a 14.5 \% decrease in weight compared to market-available flat-plate collectors. However, the direct economic comparison was not considered strong in evidence due to a lack of economic data from technology providers. The most cost-effective designs featured an argon concentration of 99 \%, sealing thickness of 31.2 mm, and a glazing thickness of 4.1 mm, and 4.5 mm, while collector length and width varied more significantly. The research findings indicate the techno-economic potential of insulating glass collectors, demonstrating their ability to outperform conventional flat-plate collectors in terms of cost-effectiveness and efficiency. Future studies should focus on producing and testing larger modules and incorporating production costs to fully realise their potential for solar district heating applications. This study provides valuable guidelines for IGU designers and producers aiming to develop cost-effective and efficient solar thermal collectors for district heating systems.}, language = {en} } @article{SummOyinlolaKhattaketal.2023, author = {Summ, Thorsten and Oyinlola, Muyiwa and Khattak, Sanober and Trinkl, Christoph and Z{\"o}rner, Wilfried}, title = {Statistical analysis of solar thermal collectors in the Solar Keymark Database}, volume = {2024}, pages = {103581}, journal = {Sustainable Energy Technologies and Assessments}, number = {61}, publisher = {Elsevier}, address = {Amsterdam}, issn = {2213-1396}, doi = {https://doi.org/10.1016/j.seta.2023.103581}, year = {2023}, abstract = {Experimental, analytical, or numerical investigations are ordinarily conducted to reveal optimisation potential for solar thermal collectors. At the same time, the 'Solar Keymark Database' contains more than 2,000 test reports from certified laboratories featuring properties including thermal efficiency, dimensions, or optical properties. This dataset offers untapped potential for statistical analyses as an alternative optimisation approach. Hence, this paper aims to provide a) the first statistical findings of solar thermal collector properties listed in the Solar Keymark Database and b) insights into statistical relations between these properties. The key correlations observed from the analysis of flat-plate collectors were between efficiency and both gross height (R = 0.30) and gross area (R = 0.27). We concluded that preferable collector designs may be featured with larger area to height ratios. The analyses of evacuated tube collectors revealed a strong correlation between efficiency and transversal incidence angle modifier (R = -0.65) as a result from different tube spacing. It was noticeable that the quasi-dynamic test method reported significantly higher efficiencies (7.14 percentage points) for evacuated tube collectors, which should be carefully considered for future test procedures. Overall, the statistical analysis was in accordance with conventional bottom-up analyses and revealed insightful dependencies for the present collector data.}, 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{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{EmbergerNairLichteneggeretal.2025, author = {Emberger, Lukas and Nair, Gireesh and Lichtenegger, Klaus and Moser, Maria and Summ, Thorsten and Natiesta, Thomas and Fan, Jianhua and Wang, Pengcheng and Unterberger, Viktor and Ohnewein, Philip}, title = {Open data in the solar thermal community: Status, barriers, and opportunities}, volume = {2025}, pages = {100114}, journal = {Solar Energy Advances}, number = {5}, publisher = {Elsevier}, address = {Amsterdam}, issn = {2667-1131}, doi = {https://doi.org/10.1016/j.seja.2025.100114}, year = {2025}, abstract = {Open data in the renewable energy sector could contribute to a greener future and reduce carbon emissions. This study investigates existing open datasets in the solar thermal domain, challenges in publishing and using open data and its potential impact. The results support the common belief that open data is useful, offering considerable benefits for data re-users and the solar thermal community. While substantial open data in the solar thermal domain already exists, some datasets like plant statistics and cost data are hard to utilize due to licensing, accessibility, and quality issues. The study shows that data owners benefit less from publishing data compared to data re-users and service providers. While data sharing seems appealing to data owners, several barriers discourage data sharing. Suggestions to promote the use and publications of open data in solar thermal domain are discussed.}, language = {en} }