Report from Selected Technical Sessions
The existing building stock remains the hardest problem in the field, and the urban sessions in Vienna reflected that. The papers divided between thermal networks at city scale, ground-source systems and their design methods (in the IGSHPA track), and the awkward particulars of retrofitting heat pumps into buildings that were never designed for them.
Each section is based on a report contributed by the organiser or session chair and has been edited by the HPT Magazine editor for length and consistency.
Heat pumps in thermal networks
Report contributed by Carsten Wemhöner, OST – Eastern Switzerland University of Applied Sciences
Session THN1 covered heat pump integration in district heating across a wide capacity range, from small machines to the multi-megawatt units recently introduced to the market. Peikani assessed natural refrigerants in heat pumps for micro-district heating with dual-source configurations, finding that natural refrigerants can outperform common HFCs and that a dual-source configuration can increase performance by up to 30 percent, particularly at lower outdoor temperatures, which supports their use despite the additional complexity. Meier examined covering peak loads in district heating networks using ground-source heat pumps, proposing that ground-coupled heat pumps and chillers installed locally at larger clients can provide free cooling and ground regeneration in summer while contributing to peak load coverage in winter, displacing fossil peak plant. The arrangement is potentially a mutual benefit: the network carries more base load, and the decentralised peak capacity can be used to connect new customers.
Wittstadt reported measurement analyses from the research and development commissioning phase of multi-megawatt heat pumps at five German locations, monitored since 2021. The paper summarised the challenges of integrating both the thermal and the electrical side and presented performance data reflecting the particularities of each site. The conclusion, that the installations show a steep learning curve and confirm the potential of large heat pumps for district heating, is one of the more valuable findings in the conference precisely because it is based on commissioning experience rather than design studies. Related work was presented under the German living lab on large heat pumps in district heating networks.
Session THN4 turned to local networks and source configurations. The keynote by Gholamrezaie investigated the impact of local heat pump characteristics on the performance of a CO2-based thermal network combining centralised and decentralised zone-wise machines, a configuration resembling water-loop heat pumps within a building. The advantages identified were reduced grid losses and heat recovery between zones with simultaneous heating and cooling demand. Passarelli evaluated seasonal performance of large air-source heat pumps with multiple evaporators affected by frosting and defrosting, simulating configurations and time-based strategies under Harbin conditions; the difference between best and worst strategy reached 20 percent, with fin pitch the most influential geometric parameter, though the optimal approach combined field configuration and control rather than either alone.
Further contributions addressed the selection of natural refrigerants for district heating heat pumps, the identification of zeotropic mixtures of natural refrigerants for geothermal district heating, and an exergetic and energetic comparison of reverse Rankine, Brayton and Stirling cycles for district heating applications, reflecting the diversity of network temperature regimes. Adsorption cooling with biochar thermochemical storage was presented as an integration strategy for district heating, decoupling heat generation from cooling demand.
Ground-source systems and design methods (IGSHPA track)
Report contributed by Signhild Gehlin, The Swedish Geoenergy Center
The IGSHPA research track opened with a keynote from Spitler on designing the next generation of geothermal heat pump systems. He pointed to growing interest in larger systems serving large buildings and networks of buildings, where land area becomes a design constraint and the available area may be irregularly shaped and scattered between buildings, with varying demand profiles and multiple sources adding complexity. He introduced preliminary work on an open-source design tool that automatically places and sizes an optimal number of boreholes within a land area to meet given heating and cooling loads.
Bernier presented the methodology for evaluating uncertainty in the design length calculation of vertical ground heat exchangers that went on to receive the Best Scientific Paper Award, comparing calculation methods across five test cases and finding local and global uncertainty of the order of plus or minus 11 to 14 percent. Angelotti addressed groundwater flow in borehole heat exchanger design, modifying the widely used ASHRAE sizing method to include medium and long-term advection effects, and concluding that above a Darcy velocity of 10-7 m/s these effects cannot be neglected, with the benefit most pronounced for shorter borehole spacing and unbalanced loads. Liu introduced GeoWISE, an online tool for preliminary design and techno-economic analysis now extended from individual buildings to district geothermal heating and cooling systems, drawing on a database of more than 125 million existing US buildings and using AutoBEM and EnergyPlus to predict thermal loads. He also presented a sensitivity analysis of design parameters in the RowWise borehole layout algorithm, which generates and optimises configurations within irregular polygonal land boundaries.
Supporting work included a high-order finite line source solution for simulating straight boreholes, a theoretical model for the thermal response of slender boreholes under short-term transient conditions, analysis of variable borehole thermal resistance and heat pump behaviour on sizing and energy consumption in a Swedish residential context, and the modelling of subsurface temperature fields around borehole heat exchangers from monitoring data. Mine water geothermal energy was presented both as a heat source and as seasonal storage, with attention to the conditions under which it becomes economic.
Retrofit, and the buildings that resist it
The retrofit papers were candid about difficulty. One contribution examined optimal retrofit levels for heat pump installation in residential buildings, noting that while heat pumps in new build are becoming routine, electrifying the existing stock remains a significant challenge because low-temperature heat distribution is often not feasible in existing dwellings without substantial intervention. Another presented low-temperature heating through façade-integrated thermal activation as a minimally invasive retrofit route, addressing the disruption that conventional emitter replacement entails. Work on air-to-air heat pumps in existing buildings examined the influence of building thermal mass on consumption, and a comparative analysis of heat pump applications in multi-family buildings set theoretical concepts against real-world installations, complementing the HPT Project 62 workshop.
Vienna itself provided a case study, with a paper on the flexible deployment of heat pumps for urban decarbonisation in the city, presenting a portfolio of heat pump based solutions complementing the rapidly expanding district heating system in pursuit of climate neutrality by 2040. Mine water heat storage was presented as a decarbonisation solution for urban areas, and a study of performance degradation of an air-source heat pump in a dormer room enclosure offered experimental evidence on a space-saving roof-integrated approach for dense urban settings.
Acoustics, a recurring obstacle to urban deployment, was addressed through work under HPT Project 63 on the psychoacoustics of air-source heat pumps, studying human response to the cumulative impact of noise. Together with the space, ownership and cost constraints identified in the HPT Proejct 62workshop on heat pumps in multifamily buildings, it underlines the central message of these sessions: in the existing urban stock, the binding constraints are frequently not thermodynamic.
All conference papers are available from the searchable HPT TCP database: https://heatpumpingtechnologies.org/publications
