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Supply and return temperatures for the district heating network for water and the paraffin-water dispersion.
© RWTH Aachen. E.ON Energieforschungszentrum

Heat losses from the district heating network for water and the paraffin-water dispersion
© RWTH Aachen. E.ON Energieforschungszentrum

Test rig for testing the dispersions with different heating technology components
© RWTH Aachen. E.ON Energieforschungszentrum

Comparison of the boiler output for the heating network for water and the paraffin-water dispersion
© RWTH Aachen. E.ON Energieforschungszentrum

Heating losses and efficiencies with a direct heating connection and a paraffin-water dispersion
© RWTH Aachen. E.ON Energieforschungszentrum
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New libraries for Modelica

To simulate the different energy systems, the scientists are using the Modelica language. The programming environment is freely available and allows the simulation of complex physical systems, including the transport of heat in hydraulic systems. However, paraffin-water dispersions have special characteristics that were not previously recognised in the program libraries. The fluids belong to the non-Newtonian fluids, whose flow behaviour depends on the acting pressure. The simulations were also previously unable to accurately describe the phase change of the paraffin. The dispersions tend to supercool, in other words they behave differently during heating and cooling.

With a specially constructed test facility on a laboratory scale, the researchers identified the properties of the dispersions. With their data, they expanded the library to include functions such as pressure loss, heat transfer, enthalpy, density and entropy for non-Newtonian fluids.

Hardly any efficiency gains with transfer stations

The simulations calculate a lower supply temperature and a higher return temperature for the paraffin-water dispersion relative to water . In the balance, the heat losses in the network are reduced by 5%.

However, the energy efficiency of the entire system increases by only 1.5%, since the viscous dispersion requires higher pump outputs. For this reason and owing to the higher temperatures in the heating network, the exergy efficiency of the system is less than that for the water-based system. The exergy efficiency is mainly determined by the large temperature difference between the combustion temperature in the boiler and the required supply temperature. Therefore, the exergy efficiency of the paraffin-water dispersion could be greatly improved if renewable energy were to be used for the heat production.

An interesting effect is shown by the load distribution: not only does the entire energy generation shift but, in particular, the output peaks are also reduced. The researchers see an opportunity here to smooth loads with ambitious schemes such as the “Smart Cities” project.

Greater efficiency with direct connection

The energy efficiency improves when the supply and return temperatures are lowered, as this reduces the heat losses. However, this is limited by the necessary temperature difference in the heat exchanger.

In a further simulation, the scientists therefore investigated the possibility of feeding heat from the heating network directly into the underfloor heating without using heat exchangers. After adapting the system, they reduced the supply temperature and the melting point of the paraffin-water dispersion to 40 °C. With this configuration, the efficiency increased by up to 25%. However, the results cannot be transferred directly into practice because the district heating network is operated at a different pressure level than the heating systems in the buildings. This would mean that a separate pumping system would have to be installed at each load, which would initially throttle the pressure from the district heating network and then increase it later in the return system.

The additional costs would be offset by eliminating the need for heat exchangers. The large main pumps for maintaining the pressure could also be eliminated. The effects of the different pump and pressure systems can only be conclusively clarified, however, through further investigations.

A home is simulated

A fictional house in the Aachen climatic region was used to demonstrate the effects of paraffin dispersions in underfloor heating, as brine in a heat pump system or as solar fluid in a solar thermal system. The house is insulated according to the German Energy Savings Ordinance from 2009 and, with an area of 132 m2, has a standard heating load of 5 kW. 

Characteristic periods from different seasons were simulated: 10 days in February with low solar irradiance, 10 days in April with high solar irradiance and average outside temperatures, and one week in July with high temperatures and high solar irradiance. When comparing the dispersions as an alternative to water, the scientists investigated both the energy and exergy efficiency. They assessed, for example, the electricity consumption of the circulating pumps for the different viscous media.

Underfloor heating

The researchers investigated the underfloor heating system with supply temperatures of 46 and 36 °C, and with different control strategies. The use of a paraffin-water dispersion only provided energy efficiency benefits with the low supply temperature and with low mass flows. In exergy terms, the advantages relative to water are almost completely negligible. As a positive side effect, the higher storage capacity of the slurry reduced the on/off cycling of the heating system.

Heat pumps do not benefit

In the Aachen model example, the efficiency of a monovalent heat pump changed only slightly when the brine for the borehole heat exchanger was replaced with the paraffin-water dispersion. The flow through the borehole heat exchanger was able to be reduced by two thirds owing to the greater heat capacity of the fluid. However, even under optimised conditions with adjusted mass flows, the performance factor only improved by less than 1%.

Solar thermal system increases coverage in winter

The reference house was then equipped with flat-plate collectors in combination with a thermal energy storage system. If the heating power is insufficient, an electric water heater can be switched on to meet demand. In theory, the higher thermal capacity of the fluid should increase the collector efficiency significantly, since the temperature in the collector decreases. The solar fraction does in fact increase in winter by up to 19%. In the transitional period, however, it does not improve, since the phase change in the working fluid is not fully completed. This is also confirmed by experimental investigations. The researchers conclude that the dispersions can only be used effectively if the collector temperatures do not vary considerably.

Cooling ceilings become more efficient

The researchers modelled the use of dispersions in cooling systems using the example of a 10 m² cooling ceiling. At night, this disperses the absorbed heat via a solar collector surface. The simulations demonstrate the considerable degree to which the efficiency of the slurries depends on an optimal system design: when the melting range of the paraffin-water dispersion is around 22 °C, the cooling capacity of the system increases by 20%. At lower melting points, however, the energy efficiency reduces until it is less than that of water, since the dispersion cannot completely solidify at night.

Projektinfo 18/2015:
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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
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