Mybe Your needs maximum voltage? The maximum voltage is at resonance frequency.  The resonance frequency of Receiving is very close to resonace frequency of Transmitting. Maximum power depends on load impendance. To calculate power Your needs model.

Here reference to Piezoelectric model:

Kubov V.I. Piezoelectric Echo System LTspice-model. Proceedings of the XXXI International Scientific Conference ELNANO 2011, p.117

http://elnano.kpi.ua/el_baza/cont_2011.pdf

I don't think that such a system is going to be very efficient. Maybe a fraction of 1%? And piezoelectric transducers are basically small capacitors in series with voltage sources so you will (a) need a device to convert hundreds or even thousands of volts at a microamp into useful electricity and (b) an alternating drive mechanism capable of driving the piezo crystal to just short of fracture. I wish you success of course but I don't think it's going to be easy!

 Thank you, it is good information.