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Surface heating under thermal insulation is being installed on an exterior wall for the first time as part of a renovation project, whereby the capillary tube mats (pictured) are applied to the concrete facade and then plastered over with adhesive mortar.
© IZES gGmbH
Innovative energy sources system

With 21 °C, the supply temperature is greater than the room temperature, so that an effective flow of heat is created through the existing wall into the room. Around 2 W/m2 of heat is fed into the room.
© IZES gGmbH

The ice storage tank consists of a ten-cubic-metre, water-filled concrete cistern that is sunk into the ground next to the building.
© IZES gGmbH

Heating buildings via their exterior walls

Surface heating systems facilitate the integration of renewable energy sources and reduce energy costs. However, since the retrofitting of underfloor heating is very expensive, thermally activated walls provide an option in existing buildings. For the first time scientists at Saarland University are testing the use of capillary tube mats in exterior wall heating.

Until now, capillary tube mats have been principally used in interior walls, ceilings and underfloor heating systems. What is new is their use in exterior walls. As part of a research project conducted at Saarland University, the mats are being applied to a 160-square-metre concrete facade. Once they are installed they will disappear under a layer of mortar with good thermal conductivity. This enables a homogeneous temperature distribution in the wall and is also required because a final layer of thermal insulation will then be applied on top. The capillary tube mats are made of six-millimetre-thick tubes. These contain a water-glycol mixture and lead to supply and return lines at the base of the facade.

Heating with low supply temperatures

The thermal activation of the 34-centimetre-thick concrete wall enables low supply temperatures. Since the transfer area is relatively large, the heat transfer medium does not have to be heated so much as with conventional heating systems. In addition, a large thermal mass is available. This therefore enables the heat generation and consumption to be better decoupled time-wise, which facilitates the integration of renewable energies into the system.

“The location of the radiant heating system between the existing wall and the new thermal insulation enables very low supply temperatures to be used that are less than 20 to 25 degrees Celsius. This is because supply temperatures that are only slightly above the idle temperature in the heating plane can change the heating flow through the existing wall. The idle temperature refers to the temperature in the heating plane in the idle state, in other words when the wall is not thermally activated,” explains Christoph Schmidt, Project Manager at the Institut für ZukunftsEnergieSysteme (IZES). Supply temperatures greater than the room temperature can compensate for transmission heat losses from the covered wall surfaces and, in addition, supply the room with heat to meet the remaining heat losses.

As with any heating system, there are also heat losses with wall heating: “In the buildings previously investigated with simulations, the ‘efficiency’ of the wall heating was always in a range between 80 and 90 percent. That means that 10 to 20 percent of the energy incorporated in the heating plane is lost to the outside through the thermal insulation,” says Schmidt. The efficiency of the external wall heating can be expressed as the ratio between the heat transmission resistance of the entire wall structure and the heat transmission coefficient of the newly applied thermal insulation.

Meeting the heating requirement with as little electricity as possible

12 PVT collectors with a total gross area of approximately 20 square metres help to provide the necessary energy. They provide both solar thermal heat and solar electricity. To achieve this, they are coupled to a brine-water heat pump that is partly electrically driven by the PV system; the rest of the electricity requirement is met by the grid. The heat pump produces heating or cooling energy as required. It draws energy from an ice storage tank that is sunk into the ground next to the building. The storage system regenerates itself partly from the soil, but mainly via the solar thermal system. Schmidt: “In terms of the energy efficiency, this combination is ultimately aimed at achieving the highest possible annual performance factors for the system. These depict the ratio between the useful energy for the building heating and the electricity requirement of the system. This therefore provides us with precise information about the direct thermal utilisation of the solar thermal energy and the efficiency of the heating provided by the heat pump system.”

A plant room controls and monitors the system on the basis of measurements, whereby it is continuously fed with data from all the important parameters. In addition to values from the temperature sensors in the outer wall, these also include values for the room temperature, humidity and occupancy. Since all the rooms have their own exterior wall heating circuit, they can be individually controlled and regulated. In addition, the entire hydraulic system and the electrical components are also metrologically recorded.

“The system offers a variety of modes and optimisation possibilities,” says Danny Jonas. He is a PhD student at the Institute of Industrial Automation and Energy Systems under Professor Georg Frey and is investigating the various operating strategies for the system. “We can choose both a cost-effective mode of operation as well as the most efficient or self-sufficient operating strategy,” he explains.

The construction work on the building is scheduled to be completed by the end of this year. The test phase with measurements will then begin. As part of the project, the scientists will record and evaluate two heating and cooling periods. The project will be completed in the middle of 2017.



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Universität des Saarlandes

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Fachverband der Stuckateure

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