Efficiently rectify 18-20V to 12V AC-DC for wireless charging
Make sure the diodes are good for whatever frequency is being run.
You're using a linear regulator; the most efficient such a regulator can ever by is \$V_{in}/V_{out}\$.  A switching regulator will be much better.
You can probably get an incremental gain by using a voltage doubler into a switcher, because you'd only have one diode drop in each cycle on the input side -- but then you'd need a switcher that can work from a higher voltage.  You'd need to do some up-front engineering to verify what's best.
A synchronous rectifier feeding a choke input filter may work better -- but the size and complexity may be prohibitive.