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

Innovative, dynamic control concepts had been 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 had been evaluated and possible business models had been developed.

Short Description


completed (January 2023)

Starting point, contents and results

The use of the building mass and in particular the floors or suspended ceilings as thermal energy storage is becoming increasingly important, especially driven by the trend towards decentralized heat supply systems based on solar energy (solar thermal, photovoltaic or wind power), which supplies its energy independently of demand and must therefore be stored in some form. In addition, significantly more volatile electricity prices are entering the market, which also increasingly increases the need for energy storage. While any kind of extra installed storage (water storage, battery) sometimes causes considerable costs and space requirements, the building itself is available for free, so to speak, and "only" needs to be activated.

The problem essentially lies in the issue of living comfort (too high/too low temperatures due to overheating/undercooling) and potentially increased heat losses from the building. The solution lies in storing the right amount of energy at the right time, in the right place, with the right heat amount, while taking advantage of the dampening effect of the building's storage masses. At the same time, the heat pump heating system must be as efficient as possible, i.e. the heat must be generated at the lowest possible temperatures. This applies not only to heating, but even more so to hot water production as buildings consume less and less heating.

The aim of the project is the development and testing of innovative but simple control concepts in combination with outdoor air heat pumps, which lead to the best possible efficient heat storage of electricity (PV self-consumption or grid surplus electricity) in the building mass of residential buildings (single-family/multi-family buildings) with the best possible but also variable comfort parameters through (individual) room control in combination with increases or decreases of room temperatures or control of the heating circuits (flow temperature or mass flow).

As an essential part of the work in the "Energieschwamm" project, detailed simulation models for the heat pump with desuperheater and internal switching for simultaneous cooling and heating, the buildings as single- or multi-family building with room-wise zoning, and corresponding hydraulic systems for single- or multi-family building were created in the TRNSYS simulation environment for detailed simulation studies with a wide range of variation options.

Thus, in the end, more than 300 simulation variants were created and evaluated for the single-family house and the multi-family house, respectively, from which an excerpt is presented to illustrate the key result in this report.

Laboratory measurements of special operating situations were also carried out with the heat pump in order to be able to model and parameterize the simulation models realistically. These experiences were supplemented by internal data of the monitoring systems of the test buildings integrated in the heat pump and building control systems, which particularly concerns the dynamic behavior during changes of operating situations in real operation.

Practical tests in occupied test buildings could be used to compare the effects of the control concepts with the simulation results or to identify operational practice-related problems and integrate them into the concept development. Findings on user acceptance and system understanding of all parties involved (occupants, operators, installers, system suppliers) from the practical tests in the real buildings are also important basic information for future implementation potentials or strategies.

Basically, it should be noted: The findings from this project are based on a very large number of assumptions for specific boundary conditions and therefore cannot be generalized to quantitatively produce the same results for other cases. However, the results can show in which direction it can go and which orders of magnitude are possible.

For a PV-air heat pump system in a single-family house designed as a low-energy house, the grid electricity consumption can be halved using a standard buffer storage and a building mass activation via underfloor heating compared to a PV-air heat pump system without an overheating concept. Different boundary conditions and parameters were investigated, leading to the result that already the standard screed thickness (0.08 m) of an underfloor heating system with 3 degrees increased space heating flow temperature in case of PV overheating leads to an additional operating cost saving of about 30% compared to a standard control concept, i.e. without extra costs for the thermal battery "building mass".

PV self-consumption for heat pump operation alone can be quadrupled. If the household electricity consumption for the whole system is included, the PV self-consumption can still be doubled. Although the target room air temperature is increased from 21°C to 24°C as a default during PV overheating, the final resulting room air temperature as an average value only increases by about 0.3 deg during the winter period. The daytime peak temperatures hardly increase thanks to the delay caused by the thermal mass, but the period from late afternoon to (after) midnight remains about 0.5 deg to a maximum of 1 deg higher after a sunny day.

According to the simulation results, the increased heat losses of the building with PV overheating concept, with 6 to 12% higher electricity consumption, are significantly lower than those that electrical storage devices such as batteries or even large-scale pumped storage power plants have as electrical losses.

The effort to implement such a PV overheating concept is very small (once the control system has implemented the few basic functionalities), since ultimately only the control system has to be parameterized accordingly. The most important influencing parameter is the psyche of the user himself, who must have the "courage" to accept "unusual" parameter settings such as a setpoint room temperature for the PV overheating mode that is 3 degrees higher than the usual comfort temperature. The risk is at best a few days "too warm" and a correction or even the "reconstruction" of the heating to a standard system is limited to changing a few parameters in the control.

Important for the near future would be the dissemination of this concept especially in the training and education sector as well as the implementation and scientific monitoring and evaluation of good demonstration objects over several years. The potential should actually be very large with the urgent general social need for thermal refurbishment of residential buildings, the switch from fossil-fueled heat generators to heat pumps powered by renewable electricity, and the installation of PV systems.

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