.

Fig. 16: PCM ceiling cooling panels in an open-plan office.
© Julia Schmidt/Deutscher Drucker

Fig. 17: Schematic representation of active PCM systems for cooling.
© ZAE Bayern

Fig. 19: Cooling ceiling system with a PCM (Ilkatherm).
© Sven Meyer

Fig. 20: PCM screed underfloor heating.
© Maxit Deutschland
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Active heat management

The heat storage effect can be used in a controlled manner as regards time and intensity by employing active, water-carrying PCM systems. Surface building components can be used to decouple demand for cooling and cooling supply in terms of time. PCM cooling ceilings have already been implemented in various demonstration projects.

Passive cooling concepts – in combination with PCMs in particular – are subject to two main restrictions that could limit their applicability. Firstly, heat transfer between the walls and the air limits the amount of heat that can be absorbed and, more importantly, released again over a 24-hour cycle. Doubling the thickness of the plaster layer does not necessarily lead to double the actual heat storage capacity available. Secondly, the only source of coolness available is the night-time air. Particularly on hot summer nights, this can mean that heat cannot be released from latent heat storage, with the result that the system will not work properly the following day. However, the stored heat can be removed efficiently and reliably using cooling water circuits. These systems can be integrated into the walls or ceiling, or can also be installed as suspended ceiling elements. They can also be integrated into walls or floors for heating purposes.

Innovative surface cooling and heating systems

As part of the “PCM active” project, the Fraunhofer ISE, together with project partners, has investigated active flow-through surface cooling systems combined with PCM building materials. The main goal of this work was the development of a water-carrying cooling ceiling, based on available PCM building materials. The PCM in the cooling ceiling means that a large fraction of the heat that would have to be actively removed in conventional systems can be stored passively in an intermediate manner. It is only necessary to actively remove the remaining excess. Outside of the melting range, the ability of a thinlayer cooling ceiling to react quickly remains intact, however. Another advantage of PCMs in cooling ceilings is that cumulative cooling capacity is available. Conventional cooling equipment must be designed in such a way that it can deal with peak loads. However, the ability of PCMs to store cold makes it possible to use smaller cooling systems. On top of this, additional cold sources can be used that only contribute low cooling capacities – these could include environmental heat sinks, such as borehole heat exchangers.

Cooling ceilings can be operated in a demand-oriented manner so that they can absorb cold at times when it is favourable from an energy or economic point of view. One of the central issues addressed by the “PCM active” project was the determination of the optimal melting range for PCMs. This melting range should be at the upper end of the human comfort range for passive applications. It should be selected in such a way that the ceiling can be operated in a highly energy-efficientmanner in the case of active systems. So far, various series of tests and simulation studies have shown that a melting range of around 19 °C to 22 °C is ideal for cooling ceilings. This makes it possible to release heat at night with relatively high supply temperatures in the cooling circuit. Environmental heat sinks can achieve the same temperatures. It also allows for cooling ceilings to be operated at maximum surface temperatures of around 23 °C. Measurements of the cooling performance confirm, as expected, that there are no significant differences as compared to conventional plastered cooling ceilings with capillary tube mats.

The second main issue addressed was how to regulate a PCM ceiling with the aim of achieving energy-efficient ceiling control while at the same time adhering to comfort criteria. The operating periods of the ceilings are to be minimised, and the volumetric flow rates and cooling water temperatures, which can vary depending on the heat sink used, should also be taken into account. Operational investigations are currently being conducted on various test cooling ceilings. In addition, cooling ceilings are also being used in real buildings – for example, in five offices with a total ceiling area of 100 m² at the Fraunhofer ISE.

The first active cooling system to be distributed commercially is the Ilkatherm® cooling ceiling from ILKAZELL. It is based on PCM plasterboard, which is stuck onto the room-facing side of a polyurethane sandwich composite. Capillary tube mats are fitted between the smartboard and the rear insulation for activation purposes. The system has already been used in combination with borehole heat exchangers as heat sinks in a demonstration building, namely the Engelhardt & Bauer printing house in Karlsruhe. The cooling ceiling has a modular structure, and can be used as a full-surface suspended ceiling or as an individually rear-ventilated ceiling element.

A screed underfloor heating system was developed in cooperation with Maxit Deutschland to provide surface heating. Micronal® from BASF was the PCM used. The thermal benefit of using additional PCM is minimal, however, because of the already very high storage capacity of the screed system. A further advantage is that the layer thickness for the underfloor heating can be reduced by around 25% as compared to conventional screed underfloor heating.

Testing under conditions close to those in real applications is also necessary alongside product tests in laboratory test rooms. Firstly, manufacturers require concrete data on how efficiently their products actually are under real conditions; secondly, along with technical data, users also like to be able to visit show buildings that demonstrate how PCMs can be integrated into a lightweight construction architecturally and in terms of building services technology.

After the successful development of components and materials, the current task is to increase the acceptance of PCMs in building applications among planners and users, and, among architects in particular, to create an awareness of PCMs as an energy-saving alternative or supplement to active air-conditioning and/or heating technology. For this reason, research is currently under way on “Practically oriented testing of the performance of building components with PCMs in demonstration buildings” (German Federal Ministry of Economics and Technology’s “PCM demo” project).

The “Water-carrying cooling ceilingswith PCMs” subproject involves an investigation of a combination of macroencapsulated PCMs and a water-carrying cooling ceiling. Suspended water-carrying cooling ceilings can achieve high cooling capacities (max. 100 W/m²) with low response times; however, this often means that they require high peak loads from cooling supply. The integration of PCMs means that a purely passive basic cooling capacity of around 40 W/m² can be provided during the day when cooling load peaks occur. At night, the PCM can then be regenerated using cold water. In this way, load peaks can be avoided during the day and the cooling load can be distributed more evenly. There are particular advantages if shallow geothermal energy (borehole heat exchangers) is used as the cold source, as the borehole heat exchangers have to be designed for peak loads. If the PCM system is combined with conventional technology (partial use of PCM modules), the advantages of short “response times” are preserved, and only peak loads in excess of the basic loading have to be dealt with. Up-to-date progress reports on the investigations will be presented to the scientific community from autumn 2009 onwards.

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