Emulsions for supply systems: New fluids as heat transfer mediums in air conditioning and refrigeration technology
Reducing peak loads with PCS
The new materials offer several advantages: they help save energy when cooling and air conditioning buildings and also help to improve the comfort for users. These phase change slurries (PCS) facilitate better use of environmental heat sinks and can shift the cooling to night-time. The night-time cooling storage (peak shifting) enables them to cap peak outputs during the day.
This means that the chiller can be made smaller and it also works more efficiently with the lower ambient temperatures during the night. Because heat can be stored at a temperature very close to the target temperature of the building being cooled, the chiller does not have to cool down so much and cooling losses are minimised in the pipes and storage system.
In contrast to passive latent heat storage systems in which the night air generally provides the only regeneration possibility for recooling, active flow-through systems with pumpable PCS enable considerably improved heat transfer as well as the use of additional cooling sources such as ground water, refrigeration units or heat pumps for the recooling.
In addition, the fluids can increase the capacity of the existing cooling networks. Compared with plants and buffer storage systems based on water, a smaller storage volume is sufficient. With CHP units and district heating networks, the storage system allows the heat generation and consumption to be decoupled and the peak outputs and demand fluctuations to be balanced.
By using phase change fluids, designers and operators can increase the capacity of existing air conditioning systems and design new systems with more compact dimensions.
Storing surplus energy with systems with high benefit costs but low operational costs such as absorption chillers and heat pumps is particularly worthwhile: the smaller the system can be made by using a buffer, the greater the savings effect.
The use of PCS is particularly interesting for applications that only permit a small temperature difference such as building cooling and air conditioning. Conventional air conditioning systems with water as the heat transfer medium require high volume flows and large storage volumes, since there is normally only a very small temperature difference between the supply and return. Using slurries as the coolant and storage medium increases the energy density. They are generally suitable for all kinds of heat transfer and for storing heat in temperature ranges in which the phase change takes place. A good heat transfer in the storage medium is very important for economic operation.
In the plant, the latent heat volumes absorbed or released during the phase change supplement the sensible heat volumes and thus increase the thermal capacity of the storage system. By choosing an appropriate type of paraffin, a temperature range that is suitable for the system can be set for the phase change.
Composition and function of PCS
Phase change slurries (PCS) consist of a carrier fluid in which the phase change material (PCM) is either micro-encapsulated as a 35-40% suspension or is dispersed as tiny droplets in a 50% emulsion. With micro-encapsulation, tiny paraffin droplets are encapsulated in ultra-thin shells made of Plexiglas, melamine resin, polyurethane, rubber or gelatine. With the emulsions, a predefined mixture of surfactants is used to finely distribute the particles in a hydrophilic carrier fluid (generally water). Emulsions are more mechanically durable whereas micro-encapsulated PCMs are more stable thermally and in storage, but have a somewhat lower enthalpy as a result of the encapsulation.
The capsules should have a wall thickness that is as thin as possible in order to increase the active volume. The shells must be mechanically stable. With PCM emulsions, the stability depends on the PCM, emulsifier and nucleating agent used as well as on the particle size. Small particles and a narrow particle size distribution provide a more stable emulsion/suspension. Their better surface-to-volume ratio enables quicker charging and discharging but also increases overcooling (hysteresis) during solidification, which can be reduced, however, by means of additives.
In order that the plant functions properly, it is important that the slurry does not settle, remains stable for decades and is still pumpable after the phase change of the storage material. Because the slurries have a greater viscosity than water, more energy is required for the pumping. The viscosity increases with the concentration of the phase change material and when it solidifies. One of the focus points of the researchers at Fraunhofer UMSICHT is to ensure that the slurries can flow and prevent deposits forming in the pipe systems.
All in all, the ideal PCS should be cost-effective and meet a diverse range of technological requirements:
- High energy storage density (including sensible heat, if possible over 200 MJ/m³)
- High thermal and mechanical stability
- Low viscosity
- Particles should have a high surface-to-volume ratio to ensure rapid charging and discharging
- Phase change temperature should be freely definable
- Minimum overcooling (hysteresis)
- Narrow temperature window for the phase change (less than 8 K)
- Non-toxic and chemically inert
Researchers are optimising the fluid properties
With micro-capsule PCS, the researchers at the Fraunhofer Institute for Solar Energy Systems ISE have achieved a stability of more than 20,000 cycles. Based on accelerated ageing tests to establish the long-term stability of the suspensions, the researchers believe that a service life of at least six years can be achieved. They are working on producing PCM fluids that offer considerably greater thermal storage capacities in the melting range than water and which can also be used just as easily in conventional plants.
For micro-encapsulated paraffin, the researchers have established that the increase in capacity per volume unit is two and a half times that of water. Together with various research partners, Imtech is working on developing even more efficient materials: a fourfold energy density appears achievable. Newly developed PCS have been tested in pilot plants, whereby the aim is to achieve a long-term stability of up to 10,000 cycles. Following the tests in laboratories and pilot plants, the suspensions and emulsions with phase change materials will now be used in demonstration plants. Researchers and developers are working on cost-effective PCS for use in practice that achieve a service life of 20 years in the building sector and 30 years in the energy sector.
Researchers from Fraunhofer UMSICHT and RWTH Aachen have jointly developed simulation models that map complex hydraulic networks with phase change materials. Based on these results, they have produced a PCS whose properties provide the best possible compromise between the energy density and viscosity. In addition, Fraunhofer UMSICHT is also developing emulsions for a wide range of applications, such as for example for controlling the temperature of batteries in the electromobility sector or as a refrigeration medium for solar cooling. The team at RWTH Aachen constructed a building with cooling ceilings and PCS for the Solar Decathlon 2012.