.

Fig. 24: Innovative concepts for room cooling create a pleasant atmosphere – especially in workshops and other workplaces.
© Lichtblau Architekten

Fig. 26: Storage concepts for active integration into heating and cooling systems.
© H. Mehling

Fig. 27: Schematic methods for active systems with air as the heat-transfer fluid.
© ZAE Bayern

Fig. 28: Schematic principle of an active PCM cooling ceiling.
© ZAE Bayern
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Concepts for building services technology

Storage systems for heating and cooling of rooms with various heat-transfer fluids are very energy-efficient and have already been developed as products in certain cases. Large additional heat storage capacities can be achieved using PCM slurries as fluid storage media that can be pumped.

The heat and cold storage devices that are integrated into innovative building solutions are based mainly on three different concepts. Figure 26 shows the most familiar system on the left, where the storage material is contained in a storage tank and the heat-transfer fluid (HTF) flows through channels into a heat exchanger. With the second concept, the PCM ismacro-encapsulated in PCM modules that are located in the storage container, and the heattransfer fluid flows around the capsules. In the case of the third concept, the PCM is a component of the heat-transfer fluid and increases its capacity to store heat. It can thus be pumped to any given location in the system in order to release or absorb heat directly. The heat-transfer fluid and the PCM form a pumpable storage medium together, also known as a “PCM slurry”.

Air, water or other fluids can be used as the heat-transfer fluid in the first two concepts, whereas the third concept is only suitable for fluids.

Systems with heat transfer to the air

Storage in thermo-active building systems (TABS) only uses free convection in the air and radiation exchange as the mechanisms of heat transfer to the rooms. Their performance is thus limited. This is particularly true in connection with PCMs as heat storage devices, as the temperature difference between the storage material and the room air is only a few degrees Celsius here. One solution is the use of forced convection, meaning that the air is actively blown across the surface of the storage material. For simplicity, this approach is used in the same way for both charging and discharging. Possible installation forms include a ceiling construction, integration into the floor, or a separate unit. As air is used as the heat-transfer medium, the provision of cooling using cool night-time air is the aim in the case of most systems. These systems are also called “free cooling” systems as the cooling is freely available. They are very energy-efficient, as no energy is consumed in the provision of cooling. However, the channels should be designed in such a way that cleaning of the air path is possible.

  • Systems for room cooling using a ceiling construction

…are currently being tried out in pilot systems. PCM bags filled with salt hydrates are already being used in passive cooling ceiling systems. These are easy to install. However, the cooling capacity is limited because of the low thermal conductivity of the surface boards facing into the room – e.g. gypsum boards. Active rear ventilation improves heat transport and allows for higher cooling capacities during the day along with planned night-time regeneration of the system using cool outside air. The temperature profile calculated for an office room with a rear-ventilated ceiling construction with a PCM shows that the peak temperatures can be reduced by around 2 K using the PCM. This type of “rear-ventilated cooling ceiling with PCM” is being investigated in the “PCM demo” project. Another example here is the CoolDeck developed by the Swedish company Climator. It features specially designed air flow paths where the ceiling of the room forms the upper limit of the air duct. This means that the PCM itself and the ceiling are used for storage. The system has been installed in the offices of Stevenage Borough Council in England as part of a demonstration project. The maximum indoor air temperature was reduced by 3-4 K in summer. As cooling is provided by the night-time air alone, the only energy consumption was that of the ventilator. This results in a coefficient of performance (COP) in the range of 10 to 20, according to Climator. The system has already been installed in a number of buildings.

  • Systems for room cooling using a floor construction

...have only been tested on a laboratory scale so far. This type of system, with granulate PCM integrated into a double floor, has been tested at the University of Hokkaido in Japan. To cool the room, air is extracted from the room through a ventilation opening, cooled when it flows through the granulate PCM, and then fed back into the room through openings in the floor covering. A chiller is also connected to the air circuit by means of heat exchanger in order to provide a backup to remove stored heat during the night. The commercial implementation of this concept is planned.

  • Systems for room cooling using a separate unit

...are already available as products on the market. Imtech has developed just such a system (as part of the German Federal Ministry of Economics and Technology’s “Low-Ex” research focus). Figure 30 shows the structure schematically. A PCM graphite composite material that combines a high storage capacity with high performance at low temperature differences is used as a storage medium. It stores around 30 Wh/kg (108 kJ/kg) in the temperature range of 18 °C to 22 °C. The storage material is integrated into the device as a stack of storage plates around which the air has to flow. Each device contains about 35 kg of the storage material – this corresponds to a storage capacity of around 1 kWh. Here too, the night-time air is used as a cold source. Depending on the setting of the outside air flap, the indoor air in the room can then be cooled using the stored cooling capacity. In ventilation mode, it is also possible to draw in fresh air from the outside before it is fed into the room.

  • Systems for room heating

...are used in solar-air equipment, for example. These have the advantage that they can combine ventilation and heating in one system. Research has been in progress on suitable latent heat storage devices for a number of years now, and they have already been tested in pilot systems. One example is the storage unit developed by Grammer in cooperation with ZAE Bayern as part of the “Innovative PCM technology” project: During charging, the temperature peaks in the air caused by the solar collector are smoothed and, during discharging, the air temperature is increased by 5–8 K over a period of a number of hours. This storage unit was in operation from February 2003 to December 2007, and no detectable changes in its thermal behaviour were recorded over the entire operating period relative to the unit’s performance when it was new.

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