Multi-WP – High-efficiency multivalent heat pump concepts for the thermal use of external air with geothermal storage

Optimization of multi-WP systems consisting of air-source heat pumps and geothermal storage with regard to increasing flexibility and efficiency from 30 kW for individual buildings as well as neighbourhood solutions and addressing aspects such as PV utilization optimization, mode of operation, utilization conflicts and noise pollution from air-source heat pumps. The project will establish the use of the heat source external air in combination with seasonal storage as a particularly efficient alternative for heating, cooling and hot water supply.

Short Description

Starting point / motivation

In order to achieve the heat transition, locally available, non-fossil heat sources are of great importance. External air is such a heat source, which is available everywhere and is already frequently used for heat pumps. Seasonal storage offers the possibility of making better use of heat sources that are available for a limited period of time.

The two technologies can benefit from each other in combination: Air-source heat pumps have experienced high market increases in recent years, but usually have lower efficiency ratios (annual performance factors) compared to heat pumps using other sources. Seasonal storages require heat sources for regeneration (reheating) to stabilize the temperature over the long term.

Contents and goals

The aim of this project is to significantly improve this situation by combining air-source heat pumps with geothermal probes as large-scale storage, both for seasonal and short-term load shifting, to decouple generation and consumption over time. The air-source heat pumps can be controlled to operate above a certain outdoor temperature or, in the case of PV use, also depending on the available solar yield.

The design of heat pumps and storage systems is highly important, as is the control logic, in order to be able to provide the required temperature levels and to maximize the annual performance factor of the system.

The technical feasibility as well as the ecological and economic viability depend on a number of factors such as system size, climate, mix of uses, the question of new construction or refurbishment, thermal density, etc., which are investigated in the project. Such systems are not yet established and questions of application areas as well as optimization have not yet been solved – this is where the project Multi-WP comes in.


In this project, multi-WP systems – air-source heat pumps with PV system and seasonal storage – are designed for different case studies (different energy consumption, mix of usage, etc.) and optimized within the scope of a variation calculation. Furthermore, a comparison is made with heat pump systems without seasonal storage (air mono/bivalent, groundwater and ground as sources) with regard to efficiency/work coefficient as well as economic and ecological aspects.

With seasonal storage systems, too high heat demand can lead to undercooling of the subsurface; a seasonally balanced heat balance is therefore essential. Aspects such as conflicts of use, lock-in effects, and noise pollution from air source heat pumps will be addressed within the case studies. The project results will be prepared for political decision makers, heat network operators and developers/builders and disseminated in various articles and events.

Expected results

The goal of this project is to improve the overall efficiency and cost-effectiveness of heat pump systems through load shifting. In addition, this will allow locally available heat sources to be used for heat generation relatively independently of location.

With the realization of this project another market segment for air source heat pumps opens up, moving away from a focus on the simplest and least expensive installation possible for renovation properties to a particularly efficient way to use environmental heat in new construction and redevelopment areas.

The efficient exploitation of the heat source air in combination with seasonal storage can significantly expand the possibilities of using locally available, non-fossil resources.

Project Partners

Project management

Austrian Energy Agency (AEA)

Project or cooperation partners

  • Ochsner Process Energy Systems GmbH (OPES)
  • Geological Survey of Austria (GBA)

Contact Address

Dr. Franz Zach
Mariahilfer Str. 136
A-1150 Wien
Tel.: +43(1) 5861524-106