Fly Ash VIPs

Vacuum insulation panels (VIPs) offer high-performance, but are expensive and have a fairly high environmental impact. This article outlines recent research into VIPs that use fly ash as their core material, offering the potential for greater sustainability and for lower cost VIPs to be made.

Vacuum insulation panels (VIPs) are among the highest-performing insulation materials available. They combat radiation, convection and conduction to provide excellent thermal insulation at low thicknesses. At their heart is the core. This comprises a light-weight powder (usually pyrogenic silica), fibres to give mechanical strength, and opacifiers to reduce heat transfer via radiation. The core is mixed, pressed, surrounded by a high-barrier foil, and the air evacuated to give a typical internal pressure of ≈ 1mbar, before the envelope is sealed.

This approach leads to typical λ ≈ 7mW/(mK), but at a high cost. This is due to both the cost of the materials used and the production process itself. Up to half of the cost is from the use of pyrogenic silica. which is very fuel intensive. The high cost of VIPs has limited them to a relatively limited range of high-end applications, including cold-chain logistics, industrial appliances and cooling devices. In their current form, VIPs are uneconomic for the vast majority of construction applications. They are also hampered in building applications due to the fact that they cannot be customised at the job-site, although they do offer good insulation at low thickness, leading to a higher useable area.

Towards improved VIPs

In order to reduce the cost of VIPs, while also improving their environmental credentials, research was conducted research into VIPs that replace some or all of the pyrogenic silica with fly ash. This produces FA-VIPs. The project was carried out within the INNODAEMM network alongside the Institute für Strukturleichtbau und Energieeffizienz (ISE), HSI Turbinenstahlbau and Vaku-Isotherm, a VIP producer. The aims were to: 1. Develop a range of FA-VIPs; 2. Develop these for building applications; 3. Use the FA-VIP in the construction of cold-chain containers.

Literature and FA-VIP project aims

Other researchers have previously used ashes in VIPs, with varying results. Awuye et al (2017) used 45 - 70wt% fly ash in VIPs that were evacuated to a pressure of 10mbar, achieving λ = 7-19mW/(mK).¹ Li et al (2016) used rice husk ash at 6 - 46 wt% in VIPS evacuated to 1mbar, reaching λ = 4.8 - 9.6mW/(mK).² The inclusion of ashes generally increases λ, but this effect can be accepted due to the lower cost and amount of energy consumed in production.

However, these results in the literature are difficult to compare and not comprehensive. The FA-VIP project thus used fly ash from hard coal, fly ash from brown coal and rice husk ash (Figure 1) in the same study. Each was used in its unprocessed form, as well as a processed form (via sieving or grinding), to give a total of six different ash samples. Their particle size distributions are shown in Figure 2.

The aim was then to identify the parameters needed to generate core materials that were both processable - mostly defined by the bending strength - and that also exhibited sufficiently low λ values. Different portions of the processed and unprocessed ashes, pyrogenic silica and supporting fibres were combined, as well as a sample that contained no ash. This showed that adding ashes gave higher densities, as one would expect. The porosity of each mixture was also assessed using its bulk density.

A range of tests showed that the bending strength - and hence processability - did not increase linearly with increased %wt of fly ash, indicating an optimum point at around 40 - 60%wt ash. Varying the pressure applied during core production also indicated a step-change increase in bending strength above 0.7N/mm², after which there was no additional benefit to raising the pressure. The bending strength was found to increase almost linearly with the proportion of fibres included in the blend. Most notably, the bending strength was highest with unprocessed rice husk ash (Figure 3). This is thought to be due to its elongated shape, which allows it to interlock with other particles.

Heat transfer coefficient

Table 1: Heat transfer coefficients of FA-VIPs made during the study.

Table 1 compares the heat transfer performance at the centre of panel (without ageing and edge effects) of a conventional VIP containing pyrogenic silica alongside those made with processed ashes. A conventional VIP has a λ ≈ 20mW/(mK) at atmospheric pressure, which reduces to λ ≈ 4mW/(mK) when evacuated. With all of the ashes, the conductivity rises, but there are some important differences.

Firstly, the second and third lines of Table 1 show that the reduction in core compression pressure leads to lower heat transfer. This was a more significant effect than when we compare the second and fourth lines, which differ in the proportion of reinforcing fibres. The lowest thermal conductivities were achieved by processed fly ash from brown coal. This was due to this ash having the smallest particles.

Cost comparison

Table 2: Cost comparison between standard VIP and one that contains 60%wt fly ash. Thickness = 20mm. Area = 1m².

Pyrogenic silica currently costs around Euro8/kg, while fly ash costs just Euro0.07 - 0.30/kg, depending on the type and whether it is processed or unprocessed. Table 2 shows that, to fill the core of a standard 20mm thick VIP of 1m², it would cost Euro23.50. The overall VIP cost would be around Euro55.50. By substituting with 60% fly ash, we can cut the cost for core material down to Euro7.50, giving a total VIP cost of Euro39. The researchers expect the lifespan of such a panel to be broadly the same as a conventional VIP.

This reduction greatly increases the number of applications in which VIPs, specifically FA-VIPs are commercially viable, including in some construction applications. As part of the FA-VIP project, ISE has investigated the use of FA-VIPs in floor insulation.

Conclusion

This project has shown that it is possible to use various fly ashes in the production of VIPs. The optimal ratio is 40 - 60%wt of ash, although there are multi-dimensional trade-offs between density, processability and thermal conductivity. Most notably, there is an increase in thermal conductivity, although this is balanced by lower cost and lower use of energy in the production of panels. Possible continuation of the project includes research into the commercialisation of FA-VIPs.

References

1. Awuye, D.; Chen, Z.; Li, B.; & Xuejia, W.: ‘Alternative Hybrid Core Material For Vacuum Insulation Panels (Silica Fly Ash Glass Fiber),’ International Journal of Scientific & Technology Research, pp. 75 80, Vol. 6, 2017.

2. Li, C. D.; Saeed, M. U.; Pan, N.; Chen, Z. F.; & Xu, T. Z.: ‘Fabrication and characterization of low cost and green vacuum insulation panels with fumed silica rice husk ash hybrid core material,’ Materials and Design, pp. 440 449, Vol. 107, 2016.