Energy-Sponge: The Building as an Energy-Sponge - Electricity In - Heat Out

Innovative, dynamic control concepts shall be developed which enable (air) heat pumps in combination with PV- or renewable grid electricity to use the building mass of a multi-familiy house as heat storage. User acceptance shall be evaluated and possible business models shall be developed.

Short Description




Starting point / motivation

The use of the building mass, especially the floors and suspended ceilings, as thermal energy storage is increasing in importance, driven by the trend to decentralized heating systems on the basis of solar energy (solar thermal, photovoltaic, wind energy), which delivers energy independently of the demand and therefore requires some form of storage.

In addition, the market shows significantly more volatile electricity prices, which also increases the demand for energy storage. While any type of additionally installed storage (water storage, battery) partly causes considerable costs and space requirements, the building mass is available free of charge, and has "only" to be activated.

Contents and goals

Simulation studies that have already been carried out as well as realized buildings show a very large storage potential by activating the building mass. The storage of electrical energy in the form of heat can be done most efficiently via a heat pump, since the heat to be stored can be increased by a factor of 3 to 4 compared to the input electrical energy by means of environmental heat.

The problem is to store the right amount of energy with the right power at the right time and at the right place, considering the question of residential comfort (too high / low temperatures due to overheating / undercooling) and the potentially increased heat loss of the building.

At the same time, the heat pump heating system must be as efficient as possible, so that the heat is generated at as low temperatures as possible. This is true not only for the heating demand but with buildings requiring less and less heating especially for domestic hot water preparation.


The goal of the project is the development and demonstration of innovative, dynamic control concepts in combination with (ambient air) heat pumps, which enable the best possible heat storage of electricity (PV self-consumption or excess electricity from the grid) in the building mass of multi-family buildings with the best possible but also variable comfort parameters by means of individual room control with overheating/undercooling of room temperatures.

Furthermore, the domestic hot water system will also be designed to be significantly more efficient at lower temperatures thanks to innovative (hygiene) measures, and to have a significantly increased storage potential for PV or excess electricity from the grid.

The domestic hot water preparation in summer by combining excess electricity and space cooling by means of a reversible heat pump is intended to further increase the overall efficiency as well as the decentralized electricity storage potential. But also the decisive factors and motivators for a good user acceptance are to be determined as a basis for the development of potentially successful business models.

Expected results

As a result, at least approximately confirmed control concepts through practice tests are expected, which lead to a significant electricity storage potential in the form of heat storage through building mass activation with a high overall system efficiency. Insights into user acceptance from practice tests in real buildings will provide an important basis for the implementation and final development of business models.

Project Partners

Project management

University of Innsbruck, Department of Structural Engineering and Material Sciences, Unit for Energy Efficient Building (UIBK)

Project or cooperation partners

  • Graz University of Technology, Institute of Thermal Engineering (IWT)
  • iDM Energiesysteme GmbH (IDM)
  • Pink GmbH Energie- und Speichertechnik (Pink)
  • Graz Energy Agency (GEA)

Contact Address

Dipl.Ing. Alexander Thür, PhD
Technikerstrasse 13
A-6020 Innsbruck
Tel.: +43 (512) 507 - 63653