Report from Selected Technical Sessions
Decarbonisation was the backdrop to the entire Vienna programme, but a distinct group of sessions addressed it directly: the transition to low-GWP working fluids, the displacement of fossil boilers in buildings and networks, and the increasingly quantitative assessment of what these changes actually deliver in emissions terms.
The refrigerant transition dominated. The direction of travel set by the Kigali Amendment and the European F-gas Regulation was treated throughout as settled, and the research question has moved to how systems are designed around the fluids that remain available. Work presented from Japan on the development of refrigeration and air-conditioning technologies for next-generation low-GWP refrigerants set out the practical difficulty plainly: many low-GWP candidates carry properties, from flammability to reduced volumetric capacity, that require the surrounding system to be reconsidered rather than simply refilled. Several papers examined heat transfer characteristics of low-GWP fluids in smooth horizontal tubes, including R454C, R455A and ternary blends, supplying the property data that heat exchanger design depends on.
Propane attracted particular attention. One contribution analysed releasable refrigerant charge in a residential R290 heat pump using one-dimensional dynamic distribution modelling, addressing the safety question that governs where a hydrocarbon unit may be installed, and doing so in the context of large multi-family buildings in European cities where gas boilers remain widespread. A companion study examined how far propane heat pumps can be made more compact through sensitivity analysis via dynamic modelling. Together these illustrate a pattern visible across the conference: charge reduction is no longer a secondary optimisation but a primary design constraint that shapes compactness, component selection and installation options.
Carbon dioxide and ammonia both featured as routes to larger-scale decarbonisation. A field-performance evaluation was presented of a 1.5 MW reversible R744 air-source heat pump installed at the University of British Columbia Okanagan for an ambient district energy system, described as the largest known CO2 air-source heat pump deployed in a district energy context. Separately, heat pumps using a solution cycle with ammonia and water were presented for high-temperature applications and district heating, with the argument that these natural refrigerants have no greenhouse effect and contain no PFAS, giving long-term regulatory certainty that synthetic alternatives cannot match. The PFAS question surfaced repeatedly in discussion, and it is increasingly shaping fluid selection independently of global warming potential.
Assessing what decarbonisation delivers
A notable feature of the Vienna papers was the growing rigour of the assessments themselves. Several contributions moved beyond coefficient of performance to combined energy, economic and environmental evaluation. One such study assessed low-GWP combined air-source heat pump systems for residential heating and domestic hot water, evaluating energy performance, economic feasibility and environmental impact together. This matters because the three criteria do not always point the same way, and the sessions made clear that the fluid or configuration with the lowest direct emissions is not automatically the one that will be installed.
Hybrid systems were treated with more nuance than in previous conferences. A simulation and experimental sensitivity study on the impact of electric heat pump size within a hybrid heat pump examined how the sizing of the electric unit relative to the gas condensing boiler determines actual emissions reduction. The framing is pragmatic rather than ideological: in building stock where full electrification is not immediately feasible, the sizing decision determines whether a hybrid installation is a genuine step towards decarbonisation or a way of postponing it. Similar reasoning appeared in a study on large-scale gas absorption heat pumps for distributed heating, presented as a preferable alternative to gas boilers in contexts where electric heat pumps face constraints.
Networks, storage and the system view
At network scale, the decarbonisation of district heating and cooling was addressed through the combination of seasonal thermal energy storage and large heat pumps. The argument advanced was that scalable decarbonisation of district heating requires integrating renewable heat while reducing reliance on peak boilers, and that seasonal storage coupled with large heat pumps is among the few combinations that can do both. This connects directly to the work reported in the thermal networks sessions and in the positive energy districts workshop, and it was one of the clearer points of convergence across the programme.
Flexibility appeared here as a decarbonisation instrument rather than purely an economic one. A fully reversible heat pump and organic Rankine cycle system using fixed-speed scroll machines was presented with start-up behaviour and first operational results, framed around the need for energy systems that can convert, store and dispatch across multiple vectors. The underlying claim, echoed in the opening keynotes, is that the emissions benefit of a heat pump depends on when it runs as much as on how efficiently it runs.
Materials, lifetime and the wider footprint
A smaller but growing strand looked past operational emissions. Work on heating and cooling terminal units designed to maximise recyclability of materials argued that as building standards converge on nearly zero and zero energy performance, the embodied and end-of-life impacts of the equipment become proportionally more significant. This connects to the circularity workshop held under HPT Project 65, where the conclusion was that the barriers to circular heat pumps are commercial and regulatory rather than technical. Related work on tribology, examining hybrid ceramic and surface-treated rolling bearings lubricated with oils heavily diluted by refrigerant, addresses a less visible aspect of the same problem: component lifetime under the operating conditions that low-GWP fluids and high-temperature duty impose.
Monitoring ran through the whole area as a precondition. Contributions on intermittent refrigerant mass flow rate sensing for performance monitoring made the point that the contribution of heat pumps to climate targets can only be claimed if real-world coefficient of performance and heat delivery are actually measured, rather than assumed from rating data. The same theme is taken up in the digitalisation and in the standards sections of this magazine.
Summary
Taken together, the decarbonisation sessions in Vienna showed a field that has largely finished arguing about direction and is now working through consequences. The refrigerant transition is under way and is reshaping system design rather than merely substituting fluids. The assessment methods are becoming more honest about cost and about lifecycle impact. And the emissions case for heat pumps is increasingly being made with measured data from operating installations rather than with modelled potential.
All conference papers are available from the searchable HPT TCP database: https://heatpumpingtechnologies.org/publications
