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Energy efficient heat exchangers for HVAC applications

Final Report Summary - ENE-HVAC (Energy efficient heat exchangers for HVAC applications)

Executive Summary:
The objective of the EnE-HVAC project has been to develop novel nanotechnological approaches to achieve a significant reduction of the energy requirements for HVAC (heating, ventilation and air-conditioning) systems. To achieve these savings, the EnE-HVAC project approaches all aspects of the HVAC system, developing solutions for improving heat transfer and transport throughout the whole system.

During the project, three technological approaches have been brought into play to enhance the overall energy efficiency of the complete HVAC systems.

Nanotechnological coatings limiting ice formation on HVAC systems
Frost formation on the surface of heat exchangers is a great challenge for the energy efficiency. Periodic defrosting by heating is required, but that consumes energy. A heat pump requires app. 13% of the total energy consumption of the heat pump for periodic defrosting at ambient temperatures below +7°C. Even if frost formation is not prevented completely, longer cycles between de-icing intervals would save energy significantly.
Through this project, super hydrophobic coating systems have been developed to slow the formation and spreading of ice on cooled surfaces. These systems have been developed through extensive laboratory development at Danish Technological Institute (DTI) and Tekniker IK 4, in close collaboration with Luve S.p.A. EXHAUSTO A/S and DVI A/S. Further testing of these surfaces in full-scale heat exchanger systems at EXHAUSTO and Luve have shown a significant delay of ice formation. At EXHAUSTO, the run time between de-icing intervals was increased from 5:45h to 11:30h, and ice formation on systems from Luve was decreased by 18%.

Nanostructured surfaces for increased heat transfer in refrigeration systems
When improving heat exchanger efficiencies of evaporators and condensers, it is important to look at how the boiling behaviour of these systems can be optimised in order to give a decreased energy consumption.
Through this project, nano- and microstructured surfaces as well as sol-gel based surface coatings have been developed to increase the boiling efficiency of refrigerant-based heat exchanger systems, and the overall performance and energy efficiency of these systems has been increased. Through laboratory development at DTI and Tekniker IK4, surfaces showing a significant improvement in boiling heat transfer for both CO2 and NH3 was developed. These systems were scaled for application on full-size heat exchangers at Vahterus Oy and an increased efficiency of 8% was shown for NH3 systems.

Development of nanofluids for increased efficiency of brine systems
The objective of this work was to develop nanofluids to improve the heat transfer across heat exchanging surfaces. Nanofluids are nanoscale colloidal suspensions containing condensed nanomaterial in a fluid. The potential of doping refrigerants with nanoparticles to increase the heat transfer from a heat exchanger surface to the refrigerant has been investigated. Development on nanodiamonds from Carbodeon Oy have been made to enable suspension of these in the refrigerants CO2 and NH3. For NH3 surface modifications were found to enable this suspension, but unfortunately no significant effects were observed for the boiling behaviour. In addition, nanoparticle doped brine systems were investigated but had no significant effect on the heat transfer.
To support the approaches above, ESI software Germany GmbH has developed simulation models for prediction of performance on heat exchanger systems with improved surfaces and/or refrigerants.

Project Context and Objectives:
The EnE-HVAC project will achieve significant energy savings in future Heating, Ventilation, and Air Conditioning (HVAC) systems via new and innovative technologies. These technologies include nanotechnological coatings and various types of surface treatment for improved heat transfer; new nano- and micro-materials for improved efficiency of the refrigerants, and improved efficiency and heat transfer capabilities of coolants via new nanotechnological additives.
These goals can be realised by tackling the efficiencies in all parts of the HVAC systems. The technologies used will address the heat exchanger 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. In order to obtain such large energy demands, heavy demands will be made on the refrigerants that are used; to ensure the largest possible environmental effects, there will be significant focus on the use of “green” refrigerants avoiding HFC and CFC gasses throughout the project.
To decrease the overall energy demand, it is vital to look for new and innovative technologies to increase the efficiency of currently applied state-of-the-art HVAC systems. These new technologies are:

• Nanostructured coatings including sol-gels and PVD coatings for increased heat transfer.

• Nanotechnological coatings with anti-freezing properties to limit over-icing of heat exchangers.

• Nanofluids for the improvement of heat transport.

Ice formation on air fins from EXHAUSTO A/S heat exchanger.
The nanotechnological coatings will be applied on the air side of the air-air as well as the liquid-air heat exchangers. Sol-gel coatings that significantly will decrease ice formation and adhesion to heat exchanger fins are being developed at the two research institutes: Teknologisk Institut (Denmark) and IK4 Tekniker (Spain). These can be heat exchangers and heat pumps used for residential or commercial buildings, where ice formation can be a large problem. By avoiding ice formation or ice adhesion, de-icing cycles can be minimized or completely avoided thus giving rise to significant energy savings. To ensure the best performance and applications, Italian LuVe S.p.a Danish “Dansk Varmepumpe Industri” and EXHAUSTO A/S are included in the consortium to help develop and demonstrate the technologies.
On the refrigerant side of liquid-air and liquid-liquid heat exchangers, there are two approaches for improving heat transfer. In boiling heat transfer, micro- and nano-structured surfaces will be developed at Danish Technological Institute to achieve large increases in the boiling efficiencies of the refrigerants and in that way allow for a reduced energy usage. Sol-gels developed at IK4 Tekniker will also be applied on the liquid side of heat exchangers. By manipulating the polarity of these surfaces, the wetting capabilities and thus the heat transfer capabilities of both refrigerant and brine can be improved. Again, relevant heat exchanger manufactures and developers are an integrated part of the project consortium. Finnish Vahterus Oy will be producing and testing modified liquid-liquid heat exchangers, while Italian LuVe S.p.a and Danish “Dansk Varmepumpe Industri” focus on liquid-air heat exchangers for residential and commercial applications.

Dispersion of Nanodiamonds in liquid CO2.
A third approach to increasing the efficiencies of the heat transfer is the use of nanodiamonds from the Finnish company Carbodeon Oy. Nanodiamonds have shown promise for increasing heat transfer in heat exchanger applications. Using single digit nanodiamonds developed at Carbodeon we expect to achieve significant increases in the efficiency of the refrigerant with very small amounts of nanodiamonds. This increase has previously been demonstrated in other refrigerants, but this project has focus on natural refrigerants such as CO2 and NH3. The addition of nanodiamonds to these refrigerants can only be achieved through a close collaboration with a company such as Carbodeon. The company has considerable control with the functionalities of the nanodiamonds and can therefore modify the diamonds to achieve the best possible results.

To maximize the output of the project, ESI group, the German pioneer in digital simulation software for prototyping and manufacturing processes that takes the physics of materials into account, is included in the project. The capability of tailoring surfaces towards specific physical/chemical properties will be assessed using ESI’s Multiphysics suite of solvers ACE+. Coupling nano-phenomena with large-scale heat transfer models and fine-tuning the surface structures toward achieving the desired goals (anti-ice surfaces / improved condensation / improved evaporation / enhanced heat transfer) will enable predictive modeling of surface effectiveness.
Accurate simulations of heat transfer accounting for nano-scale phenomena with models describing complete heat exchangers or other HVAC components require the modeling of unsteady free surface flow driven by surface tension, turbulence, heat-transfer, buoyancy and phase-change. The computational simulation must be able to maintain stability, accuracy and low turnaround times.
The entire project was divided into different overlapping “phases”. The phases comprise:

• Lab-scale primarily focused on the development and test of the selected technologies at lab- scale.
• The small tests focused on scaling the technologies from laboratory samples to a scale where they can be applied on real heat exchangers.
• Full-scale is the phase where the technologies go from testing to real demonstration.

Project Results:
Please see attached pdf file

Potential Impact:
Initially, several pathways for achieving energy savings in HVAC applications where envisaged:
• Nanostructured coatings including sol-gels and PVD coatings for increased heat transfer
• Nanotechnological coatings with anti-freezing properties to limit ice formation on heat exchanger surfaces
• Nanofluids for the improvement of heat transport

Two of these strategies have shown very promising results, namely the nanostructured surfaces for increased heat transfer and the nanotechnological coatings for anti-freezing properties; these are addressed separately below:

Nanostructured coatings for increased heat transfer
Development of nanostructured surfaces for increased heat transfer in heat exchangers using the fluid phase change refrigerants CO2 and NH3 have been demonstrated in lab-scale, however, it has only been possible to verify these effects in large scale for 500nm nanostructured TiO2 surfaces using NH3 as refrigerant. The tests run at Vahterus with low LMTD values indicate improvements of 15% in the evaporation heat transfer coefficient in the whole LMTD range. The durability of these surfaces have been further demonstrated in long-term tests at DTI.

For Vahterus, these improvements are very interesting, but the 15% improvements are not enough to implement a change of production, as a 15% increase can be achieved by scaling the size of the heat exchanger (number of heat exchanger plates), without increasing the size of the total heat exchanger assembly too much. These limited improvements compared to results obtained from laboratory experiments can be attributed to a lack of complete understanding of the flow and boiling regimes within Vahterus’ heat exchangers. The use of nanostructured surfaces is expected to have an effect on the boiling heat transfer, but it is not completely known to what degree boiling heat transfer is dominating in a flow-system like the one used at Vahterus. These systems are expected to be a mixture between liquid film evaporation, boiling heat transfer and convection regimes, where the nanostructures will increase efficiency in the boiling heat transfer regime only.

However, development of nucleation boiling models at ESI can have a very large potential impact on the future design of heat exchangers from Vahterus, as these models can help improve the overall geometry of the heat exchanger plates in order to achieve significantly increased efficiencies.

However, the use of nanostructured surfaces for fluid phase-change heat exchangers does still have a large potential for specialized applications. Laboratory investigations have shown massively increased heat transfer efficiencies in systems dominated by boiling heat transfer. These effects can be implemented in non-flow systems, such as thermosiphons (heat pipes) used for cooling of, e.g. power electronics. In these applications, the predominant heat transfer will be through pool boiling, and size will be a very important factor, thus making these systems very relevant.

From this project, a very important secondary result with potential impacts has been the development of a very cost-effective nano-micro structuring technique that is scalable and enables the structuring of large industrial-scale systems.

Nanotechnological coatings for anti-freezing properties
Through this project, it has been demonstrated that developed sol-gel coatings can significantly increase the time it takes for ice to build up on an air heat exchanger. From the demonstration run at Exhausto on their complete heat exchanger set-up, the time before defrosting is necessary was increased from 5h 45’ to 11h 30’. That increase is gained without changing the temperature efficiency of the coated heat exchangers. For further optimization a change of the de-icing flow should be considered, so the coated exchanger can become 100% de-iced before the unit returns to normal operation. Similarly, demonstrations carried out at LuVe S.p.A have revealed a reduction in the build-up of ice of 18% on their complete cooler systems using air-liquid cooling. In both demonstration cases, the frost formation on the heat exchangers was significantly different from non-coated heat exchangers, and it is evident that the frost spreading does not follow the normal patterns, and frozen droplets are observed instead of the more homogeneous frost layer that normally is observed.

The prolonged time before defrosting is necessary will result in a significant decrease in the energy used for de-icing, as, e.g. in a climate like the Danish, the number of periods with frost conditions that last longer than 10-12 hours (a night) is very limited compared to periods of 5-6 hours with frost conditions.

Already now, it is being discussed how and when this solution can be implemented in large scale, but there are still some obstacles that have to be overcome. For instance, the cost of producing the sol-gel coating, and the cost of the formulation of the sol-gel solutions for use in large-scale production. If this technology should be applied on a large scale, formulations based on other less toxic solvents will be preferred.

Nanofluids for improved heat transfer
Unfortunately, this approach did not yield the expected results. There have not been any significant positive effects of the use of nanodiamond-doped refrigerants, nor of the use of PCM materials or nanoparticles in brine systems.

However, a large knowledge base has been established by Tekniker and Carbodeon regarding the modification and use of nanoparticle systems. For Carbodeon, this means that they now have the tools for tailoring their nanodiamond systems for other applications. Thta will be used especially for enhanced polymer materials, where tailoring nanodiamond systems will significantly aid the integration of these into different polymer systems.

Although all aspects of this project have not resulted in solutions that will be implemented in heat exchanger products for the partners within the consortium, a large amount of valuable knowledge has been gained by all partners. Knowledge that will be used for generating new or improved products by the respective partners and possibly opening new business areas:

• For Teknologisk Institut and Tekniker IK4, knowledge gained in micro and nanostructuring of large complex surfaces is foreseen to be developed further and exploited within cooling applications and also transferred into other possible applications. The use of these surfaces have opened the possibility for highly efficient compact heat exchangers. For Vahterus Oy, this is a technology of great interest, but unfortunately the cost-to-performance of these technologies is still not at the desired level.
• The development of new sol-gel based coatings at Teknologisk Institut and Tekniker IK4 has proven very successful, and this technology is expected to be brought to the market in co-operation with LuVe SpA, Exhausto A/S and DVI within a relatively short timeframe.
• Although the development of nanofluids for enhanced heat transfer was not successful within this project, the knowledge developed at Carbodeon Oy, Teknologisk Institut and Tekniker IK4, regarding synthesis and modification of nanodiamonds and encapsulated phase-changing materials for integration into cooling media, has matured the technologies significantly at the individual partners. That has given new possibilities for the use of these materials in other applications.
• For ESI GmbH and ESI Software Germany GmbH, new computer models incorporating nano and micro structure with boiling phenomena were developed. These models show great promise for use in prediction of boiling effects. Also, the tight collaboration between ESI and the involved heat exchanger producers have resulted in improved knowledge within these business areas for ESI, thus enhancing their competitive advantage. On the other hand, the involvement from the heat exchanger producers have opened up for a new understanding of their products, which enables them to improve their efficiency further.

Exploitation possibilities for the individual project results will depend on different factors, such as the degree of maturity of the specific result, the market situation of the sector where it can be introduced, the financial readiness of the partners trying to exploit the result etc.

Main dissemination activities
During the project period, the project partners have actively disseminated the project results through participation in seminars and conferences as well as publishing in trade magazines and for the scientific community. A dedicated work package was set up to manage the project dissemination activities.

To promote the project start-up and progress, a web-site was set up. It was set up with a public part for external dissemination and an internal part for internal information and file sharing.
Furthermore, active promotion of the project and project results was actively disseminated during the project period through press releases and company websites.

As part of the dissemination of new innovative technological findings, the project partners have participated in international fairs and conferences. In the start-up phase of the project, we identified a list of conferences and fairs where it would be highly relevant to participate with dissemination purposes.

Through this project, new technologies have been developed, and to ensure the dissemination of this knowledge to both the scientific and technical community as well as to end-users, the publication of these results has taken place in publications aimed at both the industrial community and the scientific community.

Involvement with other EU initiatives
During the project period, the EnE-HVAC project has been involved in the nano-EeB cluster - later under the AMANAC CSA. AMANAC-CSA is a long-lasting collaboration and coordination platform aiming to maximize the impact of the participating Advanced Materials and Nanotechnology projects towards the European industry and society.
In this cluster, EnE-HVAC was initially partnered with other projects within the HVAC thematic area (EeB.NMP.2012-4 Nanotechnology based approaches to increase the performance of HVAC systems), namely nanoHVAC, nanoCOOL and EnE-HVAC. The main focus at the beginning of this cluster was to find possible synergies between the participating projects, and to increase the potential impact of the individual projects. The project activities changed and therefore we choose to join the insulation thematic area, as it has a lot in common with the EnE-HVAC project. For this cluster, the main focus has been on sharing non-confidential knowledge between the participating projects and building databases on the data for use within the projects, but also for future projects within relevant areas. Apart from knowledge sharing, the projects within the insulation-HVAC thematic area have also supported the cross-dissemination of awareness of the different projects; therefore, initiatives such as links between project websites have been established on the relevant sites.

Secondly, the EnE-HVAC project was invited to participate in the “Engineering and Upscaling Cluster” with a start-up workshop in Brussels in February 2015. The focus on this cluster is how to overcome the barriers and obstacles for engineering and upscaling with regard to ensuring impact of the results produced through EU funded projects.

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