Wireless power transfer systems with multiple transmitters promise advantages of higher transfer efficiencies and
focusing effects over single-transmitter systems. From the standard formulation, straightforward maximization of the power
transfer efficiency is not trivial. By reformulating the problem,
a convex optimization problem emerges, which can be solved
efficiently. Further, using Lagrangian duality theory, analytical
results are found for the achievable maximum power transfer
efficiency and all parameters involved. With these closed-form
results, planar and coaxial wireless power transfer setups are
investigated.