EXPECTS – Exploiting scale effects for high efficiency thermal systems (2013–2017)
At microscopic length scales the thermal properties of materials are strongly dependent on the size of the system and differ from those in the macroscopic limit. In this project we aim to exploit these scale effects for developing new high efficiency thermal solutions and materials.
The research project is divided into three main tasks (click for further details):
Each task covers one of the key components of a thermal energy storage system.
The expected results will have extensive repercussions and a wide impact on the energysystems in general. For instance, CHP power plants (Combined Heat and Power) would greatly benefit for the new heat transfer fluids, and with improved thermal insulation materials the heat losses of buildings could be substantially reduced. The research will be performed in collaboration with four schools at Aalto University: School of Science, School of Chemical Technology, School of Engineering, and School of Electrical Engineering. The multi-disciplinary combination of scientific excellence of the research groups here adds significant value to the proposal and facilitates scientific and technological break-throughs.
Traditional heat storages (water, standard phase change materials etc.) lose all their thermal energy when the storage is kept in a cold environment for a long time. Using supercooled materials, the latent heat (typically much greater than the sensible heat) is always preserved even if the storage is left uninsulated to a cold environment for years. This way huge storage volumes are not needed and storing heat into small distributed storage units without substantial energy losses is possible. Moreover, the storage potential (MJ/m3) of the supercooled liquids is much greater compared to traditional non-phase changing materials.
How do nanoemulsions work?
Emulsions are dispersions of one liquid phase in another immiscible liquid phase, in nanoemulsions the dispersed droplets are 1-100 nm in diameter. In our research idea, the storage unit is a nanoscale droplet and to achieve high storage potential, the concentration of the droplets needs to be high.
Based on our earlier findings, the following advantages of nano/microscale storage units are identified:
- Some promising materials (e.g. xylitol and similar compounds) crystallize very slowly, meaning that the heat release of these macroscale units is very inefficient. This could be solved by dividing the storage into numerous nano/microscale units.
- Most standard phase change materials (e.g. most salt hydrates) have sufficient speed of crystallization for efficient heat release. Unfortunately, they behave poorly when supercooled as macroscale units. It is well known that the stability of the supercooled state increases when the size of the particle decreases and when liquid has no contact to solid surfaces.
- One major problem of these materials is that they cannot keep a uniform composition during continual crystallization and melting cycles; the phases separate and their storage potential drops dramatically. We expect that very small units would minimize the phase separation effect.
Moreover, as the thermophysical properties depend on size of the system at nanoscale, we expect that we can adjust the storage potential and phase change temperature by developing suitable nanoemulsions and their mixtures.
The effective thermal conductivity (“lambda”) is the characteristic performance value of insulators, which takes into account all forms of heat transfer. For typical glass fiber insulation applied in buildings this value is about 35 mW/mK.
By exploiting the size effect, surprising options may become available. As one example, we have evaluated the performance of insulation material constructed from aluminum “foam” in vacuum. Our calculations show that thermal conductivity can be below 5 mW/mK, if the walls of the foam are of nanoscale thickness. Moreover, this value is reached at 800 °C; at room temperature the “lambda” values would be even smaller as the thermal radiation flux is substantially lower.
The size effect can be exploited by several ways:
- Periodic microscale structures can be applied for damping thermal radiation by destructive interference
- At microscale, the thermal conductivity of gas trapped inside the insulators starts to diminish. At nanoscale, the conductivity is only a small portion of the macroscale value.
- Thermal conductivity of the solid skeleton material of the insulator diminishes at nanoscale.
The target is to develop thermal insulation materials, whose effective thermal conductivity is below 5 mW/mK at 0 °C, and still possess sufficient strength and properties to transfer moisture. In addition, we aim to develop high temperature insulation materials for thermal storage and industrial processes to minimize thermal losses.
Nanofluids (fluids containing nanosized (1-100 nm) particles) are emerging class of heat transfer fluids due to their enhanced heat transfer performance as compared to base fluid. Typically studied nanofluids for these purposes contain solid nanoparticles, such as metals or metal oxides, dispersed in conventional base fluids.
Addition of nanoparticles into the base fluid will significantly increase the heat transfer characteristics of the fluid. The enhancement is much greater than standard mixture theories predict but the physical reasons behind the anomalous phenomenon are yet unsolved.
Dispersed nanoparticles do not significantly sediment under gravity, making nanocolloids structurally more stable than microcolloids. Based on our current success with nanofluids, a 3% increase in electricity production of CHP-plants (Combined Heat and Power) could already be obtained. Moreover, the improved fluid will promote the energy efficiency of many other kinds of heat transfer systems while minimizing technical changes in the system: With increased heat transfer of the working fluid, the maximum temperature level needed in the primary circuit can be lowered and it can still deliver the same amount of heat to the secondary circuit.
In CHP-plants this means a lower final temperature and pressure of condensing steam, and thus much better efficiency electricity production. If water is replaced with the targeted fluid as a working fluid of distributed heating systems, an 8% increase in electricity production of a CHP-plant can be obtained while keeping the production of heat unchanged.