CHALLENGE - Highly efficient use of hot gas and waste heat in air/water heat pumps for plus-energy buildings and quarters

CHALLENGE aims to further develop the system concept for air-to-water heat pumps in such a way that they can be used efficiently and without the above mentioned negative effects in densely built-up urban areas. In concrete terms, validated simulations and a functional model of the overall system on a laboratory scale are to be used to demonstrate that the concept can save 10% of electrical energy, reduce the noise of outdoor units in summer to a minimum and prevent the formation of local heat islands.

Short Description



Starting point / motivation

State-of-the-art air-to-water heat pumps systems are hardly ever used for heating, cooling and hot water in large-volume plus-energy buildings and quarters, as they are less efficient and noisier than brine-water heat pumps, and contribute to the formation of local heat islands in summer.

On the way to urban plus energy buildings/quarters, renewable energy sources are to be used in a resource-efficient and emission-free way on site and the building systems are to be designed as energy-efficient as possible. Studies have shown, that the combined systems of PV modules, heat pumps and various energy storage technologies currently used to cover space heating, cooling and domestic hot water requirements in large nZEB and plus energy buildings have to be highly efficient. Furthermore, the buildings have to meet highest energy standards to comply with the efficiency criteria for nZEB.

Large-volume buildings in densely populated urban quarters will become increasingly important in the "city of the future". However, they show characteristics that make it difficult or impossible to meet the efficiency criteria of Plus Energy buildings with state-of-the-art heating and cooling solutions. In addition to limited areas for the utilization of electrical energy from PV, only air-to-water heat pumps can be used in large numbers due to lack of suitable other heat sources. These are often less energy efficient, emit disturbing noise during operation and create local heat islands.

Contents and goals

CHALLENGE aims to further develop the concept of an air/water heat pump system developed by the consortium in the H2020 project HYBUILD. Various innovations shall make sure, that it can be used as heating, cooling and domestic hot water system in densely populated urban areas in large-volume plus-energy buildings or quarters without any negative effects in future.

In concrete terms, experimentally validated functional models are to be used to prove that the CHALLENGE concept saves 10% of electrical energy, prevents the formation of local heat islands, and reduces the noise of the outdoor device to a minimum or even completely avoid noise in summer operation.


To achieve this goal, a latent storage/heat exchanger, integrated into the refrigeration circuit, the refrigeration circuit, and the outdoor unit of the heat pump, will be completely redesigned. In addition, a robust and practical overall system control is being developed. It is adapted to the new cooling circuit in order to optimally integrate the individual apartments.

The unsolved research questions from HYBUILD and new research questions resulting from the CHALLENGE concept at low technology readiness level (TRL) will be answered at component, refrigeration circuit, overall system and building/quarter level. For this purpose, numerical simulations and experimental measurements will be carried out at the different levels.

Finally, the annual energy consumption and economic efficiency of the concept are calculated and evaluated under different boundary conditions based on annual simulations. The function of the concept will be verified with a functional model of the overall system on a laboratory scale.

Expected results

Successful implementation of the CHALLENGE concept will strengthen Austria's role as technological leader in innovative environmental technologies and will allow Austrian partners to build up competitive advantages. Under the premise of successful project development, it will also result in a holistically optimised technical solution for the provision of heating, cooling and domestic hot water for energy-plus buildings/quarters in densely populated urban areas in the medium term.

Project Partners

Project management

AIT Austrian Institute of Technology GmbH

Project or cooperation partners

  • AKG Verwaltungsgesellschaft mbH
  • Pink GmbH
  • Ochsner W√§rmepumpen GmbH

Contact Address

AIT Austrian Institute of Technology GmbH
DI Dr. Johann Emhofer
Giefinggasse 4
A-1210 Wien
Tel.: +43 50550 6061