Heating and cooling for buildings and industry accounts for 50% of the European Union’s annual energy consumption. More specifically, heating and cooling accounts for 13% of oil consumption and 59% of total EU gas consumption (direct use only) – which equates to 68% of all gas imports1. Although most of the energy currently consumed for these purposes is accounted for by heating, demand for cooling solutions is on the increase.
The calculated total EU space heating load is 2823 TWh, of which 1702 TWh (60.3%) is in the residential sector, 677 TWh (24%) is in the tertiary sector and 443 TWh (15.7%) in the industrial sector2. The space heating load in the EU is expected to remain more or less stable, as growth in demand is offset by improved insulation, optimised ventilation (with heat recovery), increased urbanisation (heat islands) and global warming. However, space cooling demand is expected to continue to rise. According to Ecodesign Impact Accounting3 projections, space cooling demand is expected to rise to 305 TWh (+38%) in 2020 and 379 TWh in 2030, with residential air conditioners expected to account for most of this growth. To reduce overall energy consumption in the heating and cooling sector, the European Commission has identified a compelling need to seek new and innovative technologies and to fund projects developing efficient heating and cooling systems.
One such project is EnE-HVAC (Energy efficient heat exchangers for HVAC applications). This project, funded under the European Commission’s Seventh Framework Programme (FP7), aims to achieve significant energy savings in future HVAC systems by using new and innovative technologies that achieve a far better heat transfer efficiency in heat exchangers, with energy savings of up to 50% on the total energy consumption in an HVAC system compared to current conventional commercially-available systems.
In a comment to SETIS, EnE-HVAC Project Coordinator Dr Jacob Ask Hansen stressed that, to minimise energy consumption, it is vital to secure new and innovative technological solutions that significantly improve the energy efficiency of current state-of-the-art HVAC systems. He said that new technologies would be brought into play in order to achieve a far more efficient heat transfer in heat exchangers. This will significantly reduce the energy consumed in modern heat exchangers for cooling and ventilation.
The EnE-HVAC project brings three novel technological approaches into play to improve heat transfer and transport throughout a HVAC system, thereby enhancing its overall energy efficiency. Nano-technological coatings limit the formation of ice on HVAC systems, nano-structured surfaces increase the heat transfer in refrigeration systems and nano-fluids increase the efficiency of brine systems. These three technological approaches contribute to the overall efficiency of the system, bringing it closer to its goal of a 50% energy saving.
"This very ambitious goal can be realised only by tackling the efficiencies in all parts of the HVAC systems. The technologies used will address efficiency on both the air- and liquid side of heat exchangers such as condensers/evaporators, and on heat recovery systems. Furthermore, this project will address the heat transport system to ensure high efficiency throughout the HVAC system. To achieve these large energy savings, it does put up large requirements on the refrigerants used; to ensure the largest possible environmental effects, throughout the project there will be a significant focus on the use of “green” refrigerants avoiding HFC and CFC gasses,” Hansen said.
Nano-structured coatings and surface treatments, such as Sol-gels and PVD4 coatings improve heat transfer in HVAC systems. Sol-gel functional coating technology can be used to make thin and transparent coatings with tailor-made functionalities. Sol-gel coatings are applied by standard methods such as spray-coating and are widely used due to their excellent adhesion to metals and their high chemical and wear resistance. Typical film thicknesses reach from a few nm to a few μm. This low thickness makes them extremely interesting for enhancing heat exchangers where good heat transfer is required in combination with other properties such as low adhesion of ice, improved bubble formation, antifouling, etc.
The project’s work with nano-fluids is aimed at developing fluids that improve transfer across heat exchanging surfaces. Nano-fluids are nanoscale colloidal suspensions containing condensed nanomaterial in a fluid. The project has investigated the potential of doping refrigerants with nanoparticles to increase the heat transfer from a heat exchanger surface to the refrigerant. Modifications to the particle surface were found to enable their suspension in the refrigerant but no significant effects were observed for the boiling behaviour. Likewise, nanoparticle doped brine systems were investigated but were found to have no significant effect on the heat transfer.
Frost formation on the surface of heat exchangers is a great challenge for energy efficiency. Periodic defrosting by heating is required, which consumes energy. Approximately 13% of a heat pump’s total energy consumption is spent on periodic defrosting at ambient temperatures below +7°C. To counteract this, EnE-HVAC has developed super hydrophobic coating systems to slow the formation of ice on cooled surfaces. Even if frost formation is not prevented completely, longer cycles between de-icing result in significant energy savings. Testing of these surfaces in full-scale heat exchanger systems has shown a significant delay of ice formation, with an associated reduction in energy consumption for defrosting.
In order to test the heat transfer improvement of the structured surfaces, a test setup simulating pool boiling has been developed. Furthermore, a large scale heat transfer setup simulating flow boiling is being built and will be used to test the most promising structured surfaces. A well-orchestrated synergy between simulation and experimental tests helps reduce the number of physical experiments required by the project.
By reducing the energy consumption of HVAC systems, nano-technology will help to ensure that future systems are more efficient, thereby reducing the overall heating and cooling load in Europe and enabling the EU to meet the anticipated increase in demand for cooling without an associated increase in energy consumption.
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