Fig. 30: Schematic representation of the active PCM cooling module from Imtech.
© Imtech

Fig. 31: PCM heating storage device.
© Alfred Schneider GmbH

Fig. 32: Arrangement for a cooling system consisting of a latent heat storage device (1), cooling ceiling with capillary tube mat (2), and facade heat exchanger (3).
© TU Berlin, Hermann-Rietschel-Institut

Fig. 33a: Phase change fluids (slurries) consist of a carrier fluid and a PCM suspended or emulsified in this fluid. The particle sizes present result in a white liquid.

Fig. 33b: Phase change fluids (slurries) consist of a carrier fluid and a PCM suspended or emulsified in this fluid. The particle sizes present result in a white liquid.
© Fraunhofer ISE

Fig. 33c: Phase change fluids (slurries) consist of a carrier fluid and a PCM suspended or emulsified in this fluid. The particle sizes present result in a white liquid.
© Fraunhofer ISE
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Concepts for building services technology II

Systems with heat transfer to water

There are numerous examples of systems that use water or other fluids as a heat-transfer fluid: Storage tanks with heat exchangers or with macro-encapsulated PCM modules are familiar. These storage devices are generally used as ice storage tanks in combination with chillers to provide cooling for buildings. The most important energy benefit is the increase in the coefficient of performance due to the use of night-time cooling. Another factor is the optimal operation of the chiller. The use of a smaller chiller, which is sized for an intermediate load, leads to reduced investment costs. The consumption costs can also be reduced thanks to the increased coefficient of performance, and the amount of electricity used at peak tariffs can also be lowered. In the area of building heating, research on latent heat storage devices initially concentrated on use in solar heating systems which aim to increase the solar energy share. The goal right from the beginning was to store heat for a number of days with the same volume or a smaller volume. The first products have been commercially available for a number of years now. A storage tank from Alfred Schneider GmbH uses salt hydrate as its storage medium. It has already been installed in dozens of systems. Solar collectors, CHP plants and wood-burning systems can all be used as heat sources here (Fig. 31).

A storage system integrated into the facade that works with water as its heat-transport medium has been developed by TROX GmbH as part of a further research project. The latent heat storage device’s particularly compact design makes it possible to install the system for each room in the facade. This means that the system is also suitable for refurbishment projects. Figure 32 shows the system investigated, which provides room cooling and uses the ambient air as a heat sink: The system consists of the latent heat storage unit (1) with paraffin as the storage material, a cooling ceiling with capillary tube mats (2), and a facade heat exchanger (3). The excess heat present during the day is extracted from the room using the cooling ceiling. The principle here is that the water inside the capillary tube mats is heated up.

This heated water in the cooling circuit is then pumped to the latent heat storage unit. Here in the latent heat storage unit, the water is cooled and the PCM undergoes a phase change. The storage unit is regenerated once the latentmaterial is fullymelted or else when there is no longer a cooling requirement for the room. This process occurs during the night, making use of the low outdoor temperature. During this operating phase, the water circulates between the latent heat storage device and the facade heat exchanger. Inside the storage device, heat is transferred from the PCM to the water and then released by the facade heat exchanger to the surroundings by means of convection and radiation heat transfer. The latent material returns to its solid state and can then be used again for room cooling.

In addition, the system allows for direct overnight cooling for all surfaces that enclose the room without having to open windows, which might be undesirable from a security viewpoint. Experimental and numerical investigations at the Technical University of Berlin showed a temperature reduction of up to 4 K for office rooms under typical load conditions. To achieve this significant temperature reduction, 2 kg of paraffin per square metre of room area was required. In addition, overnight cooling using water was employed for the surfaces that enclose the room.

Phase change fluids

Water is the most commonly used heat storage material that is fluid or can be pumped. In many cases, mixtures of water and glycol are also used. At high temperatures, oils are often used too. These liquid heat storage materials all store heat in the form of sensible heat.

If large heat storage capacities are to be achieved, the user can work with large volumes or else employ a large temperature increase or reduction. If a system is able to work with large temperature differences between the temperature actually required and the temperature available from the storage device, high heat storage capacities can be achieved using sensible heat-transfer fluids. If, on the other hand, only small temperature differences are possible, the amount of heat that can be stored by sensible heat-transfer media drops very considerably. For example, if the storage temperature may only be 10 K above or below the application temperature, then only 42 kJ/kg of heat can be stored using pure water. A PCM-containing liquid would be a major advantage for this type of application. When the melting temperature is suitable, the heat capacity can be increased in precisely the desired temperature range.

Water/ice mixtures, which can still be pumped up to a certain level of ice crystals, are already common on the market. However, material constraints mean that they can only be used under 0 °C. Above 0 °C, two different technologies are mainly used nowadays for adding paraffins to water: They are either micro-encapsulated and then suspended in water, or else an emulsion of paraffin in water can be created using appropriate additives. Both processes are intended to prevent the paraffin coagulating to form larger drops and separating out from the water once the paraffin has melted. At the same time, dispersion ensures that the paraffin can be pumped in both liquid and solid form.

Cooling systems are particularly well suited for the use of phase change fluids (PCM slurries), as they fulfil the requirement for low temperature differences in the system. In addition, storage of cold is advisable here in order to achieve more favourable operating conditions for the chillers used and to ease the load on the public electricity grid during the day. If, for example, a building is to be cooled to 20 °C in this time, it is possible to charge the cold storage device with a temperature as low as 0 °C during the night, but this would lead to lower coefficients of performance from the chiller used and also to higher storage losses. When using a phase change fluid with a melting range of between 10 °C and, for example, 20 °C and with double the storage density of water, the same storage density could be already be achieved by cooling the storage device to 10 °C. Another potential advantage of slurries is the relative ease with which they can be used as heat-transfer fluids in existing cold storage devices in order to increase their storage capacity.


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