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Paraffin-water dispersions can transport heat or cooling energy with a high energy density. Researchers are investigating their possible uses in buildings and district heating systems.
© RWTH Aachen: E.ON Energieforschungszentrum
Phase change materials
Projektinfo 18/2015

Without adaptations to the heating network, phase change slurries (PCSs) do not improve the efficiency.
© RWTH Aachen: E.ON Energieforschungszentrum
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Designing heat distribution with slurries

In heating and cooling systems, water is mostly used for transporting heat. For special applications, dispersions of paraffin and water could carry out this task more efficiently. These so-called slurries achieve high heat capacities within a narrow temperature range. Scientists at RWTH Aachen are simulating the complex behaviour of the fluids in hydraulic networks. With new simulation models they are investigating possible uses – ranging from underfloor heating in family homes to district heating networks.

When paraffin is heated above its melting point, it absorbs heat for the liquefaction and releases it again when it solidifies. This latent heat storage is used by dispersions consisting of finely dispersed paraffin in water. In a narrow temperature range around the melting point, these slurries exceed the heat capacity of water by more than double. Depending on their chemical composition, paraffins are theoretically suitable for freezer temperatures ranging from minus 20 to around 110 °C. However, until now the fluids have hardly been used for transporting heat. This means that only a few long-term stable products have been available until now and the experience and calculation methods for their practical use have been largely lacking. In experimental and theoretical research work, researchers at RWTH Aachen and the Fraunhofer Institute for Environmental, Safety and Energy Technology (UMSICHT) have significantly improved the bases for their practical use. New experimental equipment and methods now enable the dispersions to be better characterised. The scientists have also made progress in the production of temperature-stable fluids. The simulations presented here from the Aachen-based researchers reveal which specific energy systems would make an application worthwhile. Cooling applications and, in particular, district heating systems proved to be promising candidates.

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Heating networks

The use of paraffin-water dispersions in district heating networks proved to be promising. This was exemplified by the district heating network belonging to the Melaten campus, which the researchers modelled in simulations.

The campus at RWTH Aachen University has about 193,000 m2 of building area. A heating plant in the north of the campus supplies the buildings with heat. It also supplies other loads such as the laboratory areas and the University Hospital laundry. Depending on the load, several gas boilers generate up to 90 MW.

The six-kilometre-long district heating network connects the loads via two different hydraulic circuits with the CHP plant. Pumps generate a maximum flow rate of 1,000 m3/h of water.

The simulations capture only the sub-network that supplies the space heating and the laboratories. The temperatures in this circuit range between 90 and 130 °C. The second, not simulated circuit provides heat at a constant supply temperature to Aachen University Hospital.

In the simulation, the heat consumption of each building is recorded in tabular form in accordance with the ambient temperature, building type and building size. The scientists modelled the hydraulic circuit for the district heating network with transfer stations so that the heat and pressure losses can be calculated in the network. The complete system is controlled by a pressure difference at the ends of the network. The absolute pressure loss in the system is maintained at a constant 2 bar using pumps.

With these specifications, the researchers simulated the network both for a paraffin-water dispersion and for water. They reduced the temperature range relative to the actual district heating network, as otherwise the maximum phase change temperature of the paraffin would be exceeded. They chose a temperature of 70 °C as the melting point of the paraffin-water dispersion.

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Addresses

Experimental testing and simulations
RWTH Aachen, E.ON ERC, GRK

Development of paraffin-water dispersions
Fraunhofer UMSICHT

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BINE-Projektinfo 18/2015
(PDF, 4 pages, 654 kB)

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