# Thermal-Mechanical Behaviour of Road-Embedded Wireless Charging Pads for EVs
## 10 Discussion
### Sources of Deviation between the Experiment and Simulations  
Despite the generally good agreement between the experimental and numerical results, minor deviations were observed. The average percentage difference between the experimental and simulated strains is presented in Table 2. As expected, there is a clear increase in deviation with increasing system complexity. Differences between the simulation and experimental results increased between the partial and complete pads. The increase in the RMSE at 13 \({}^{\circ}\)C ambient temperature was the most significant because of the offset of experimental strains on Ferrite 4 of the complete pad at 13 \({}^{\circ}\)C ambient temperature. Differences between the experimental and simulated strains increased further with the addition of the embedment material. The RMSE values at 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C ambient temperatures were lower than that of the unembedded pad; however, the embedded pad was the only one where the general trend between strain and ambient temperature did not match in the experiment and simulation. A notable uncertainty in the FE model was the coefficient of thermal expansion (CTE) of the coil. As discussed in Section 3.4, the coil was assumed to have the isotropic CTE of copper (\(1.67\times 10^{-5}/^{\circ}\)C). The sensitivity of ferrite strains to this parameter was investigated by simulating the pad using the following transverse and axial CTE: \(1.67\times 10^{-6}/^{\circ}\)C, \(1.67\times 10^{-5}/^{\circ}\)C (original) and \(5.01\times 10^{-5}/^{\circ}\)C. Only one CTE (axial or transverse) was changed at a time. It is clear from the results, Figure 22, that the numerically obtained strains in the ferrite are not sensitive to the transverse CTE of the coil; however, they are sensitive to axial CTE. This was because of the significant length of the wire: \(2.8\) m. A CTE of \(1.67\times 10^{-6}/^{\circ}\)C results in a much closer match to the experimental results. In the case of an un-infiltrated litz wire, the thermal expansion along its length would be non-linear and thus difficult to predict, especially because of the twisting of the individual wire strands. It is possible that the CTE of certain areas is even lower than that of copper, due to the localized buckling of the individual strands of wire. From Sections 9.2 and 9.3, it can be seen that the temperature underprediction in the ferrite was around 6 \({}^{\circ}\)C for the suspended pad and 7 \({}^{\circ}\)C to 10 \({}^{\circ}\)C for the embedded pad. The reduced accuracy of temperature and strain prediction after the pad embedment points to variations in the properties of the simulant pavement material. The larger underprediction in ferrite temperature was likely due to voids in the aggregate-epoxy composite, which reduced the composite's thermal conductivity. Samples used for conductivity measurements did not have visible voids, because they were cast into a small mould which was easily shaken and vibrated by hand to release trapped air. Because the modulus of the pavement material was extremely sensitive to temperature between 20 \({}^{\circ}\)C and 60 \({}^{\circ}\)C, a large change in the stiffness of the pavement can result from a relatively small temperature error. Inaccuracies in convection boundary conditions are likely to have contributed to some error in the temperature prediction of the embedded pad. The convection coefficients applied to the model were calculated using horizontal and vertical flat plate correlations, which assume that the plate is at a uniform temperature. This was roughly true for the