Investigation of wireless power transfer in through-wall applications

In this work, we proposed a through-wall wireless power transfer system and investigated effects of various wall materials. The power transfer system was based on inductive coupling of metal coils at 1.3-MHz resonance. Softwood lumber, concrete brick and drywall with insulation filling were tested at two different thicknesses. Two sets of coils, each set consisting of two coils with identical dimensions, having radii of 5 and 15 cm were utilized. Each experiment was conducted with sequential tuning in receiver circuit, operating frequency and in transmitter circuit to reach maximum output power or maximum power transfer efficiency. The output power and transfer efficiency as well as their changes were obtained before and after tuning for different media and thicknesses. It is concluded that the power attenuation with spacing distance dominates the output power and transfer efficiency, while tuning could counteract the parasitic effects in the material and recover the power lost in deviation from resonance. The power attenuation with distance requires design considerations in coil dimensions. With larger coils, more power can be collected through thicker walls and the system tolerates more variation in wall thickness. Tests on randomly chosen walls in our laboratory building were conducted to validate the system performance.