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 (nearly zero-energy building) 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.

Therefore, CHALLENGE aimed to further develop the concept of an air/water heat pump system developed by the consortium in the H2020 project HYBUILD. Various innovations should 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.

To achieve this goal, a latent storage/heat exchanger integrated into the refrigeration circuit, the refrigeration circuit itself, and the heat pump's outdoor unit were completely redesigned. In addition, a robust and practical overall system control was developed that is tailored to the new refrigeration circuit and optimally integrates the individual apartments. Numerical simulations and experimental measurements were carried out at various levels. Based on annual simulations, the annual energy consumption and economic efficiency of the concept were finally calculated and evaluated. The functionality of the concept was demonstrated with a functional model of the entire system on a laboratory scale.

The most important finding from the extensive work carried out in the CHALLENGE project is the confirmation that both the RPW-HEX storage and the refrigeration circuit configuration developed on this basis work. The target of saving at least 10% of electrical energy over one year of operation was verified using simulation calculations; the savings in electrical energy compared to an identical heat pump are as high as 38%. The experience gained and unexpected problems encountered in the design and implementation of the functional models have significantly increased the know-how of all partners, giving the companies a clear knowledge advantage over their competitors.

The know-how regarding modeling/simulation (especially in the form of the models now available) and experiments with latent heat storage systems and their combination with compression heat pumps at the component (refrigeration circuit) and system level (in particular control concepts) has been greatly expanded and international visibility has been further increased. The knowledge gained will definitely be incorporated into further R&D projects, as will the expertise gained in optimizing refrigeration circuits, component selection, and control technology (in particular the interaction of a central heat pump with decentralized storage units used in apartments for heating, cooling, and hot water supply in the apartments). will be incorporated into the economic activities of the industrial partners.

Apart from the subsequent direct marketing of heat pumps and storage systems, primarily to end customers, the target groups for the exploitation of the know-how and scientific methods (1D and CFD modeling as well as laboratory experiments at component and system level) are industrial companies such as storage and heat pump manufacturers who can develop and optimize refrigeration circuit concepts and components for these.

Despite the extensive modeling work carried out in CHALLENGE, further research is needed in the field of latent heat storage, particularly with regard to increasing storage density and improving performance. In order to integrate the knowledge gained in CHALLENGE into the product portfolio of a storage manufacturer such as AKG, a possible follow-up project should include the design of further storage systems (in particular with different geometric structures) and validation through experiments.

In the future, a major focus should also be placed on adapting the concept to more future-proof refrigerants such as propane. Based on the knowledge already gained, a laboratory prototype should be followed by a demonstration in which the innovative overall control concept can also be applied to prevent summer heat islands and noise emissions through economical operation of the heat pump outdoor unit. A non-technical challenge here is certainly the necessary persuasion work required in cases of mixed ownership in plus-energy buildings and districts to invest jointly in this sustainable and innovative energy concept. This persuasion is all the more necessary given that the main criterion, the levelized costs for space heating/cooling and hot water do not yet fall below those of the reference systems, at least as long as CO₂ emission taxes are neglected – despite all the advantages that the CHALLENGE concept offers over the reference systems, in particular energy efficiency, minimal CO₂ emissions, moderate space requirements, minimal noise emissions, and the avoidance of heat islands — a holistically optimized, highly efficient technical solution for providing heating, cooling, and hot water in densely populated urban areas.

Project management

AIT Austrian Institute of Technology GmbH

Project or cooperation partners

• AKG Verwaltungsgesellschaft mbH
• Pink GmbH
• Ochsner Wärmepumpen GmbH

AIT Austrian Institute of Technology GmbH
DI Dr. Johann Emhofer
Giefinggasse 4
A-1210 Wien
Tel.: +43 50550 6061
E-mail: johann.emhofer@ait.ac.at
Web: www.ait.ac.at