Wireless chargers are used to refill sensors' power supply in a wireless rechargeable sensor network (WRSN) so that the WRSN can operate sustainably. Since wireless chargers are costly, the problem about how to deploy as few as possible chargers to make a WRSN sustainable is important. This paper proposes a greedy algorithm, named adaptive pair based greedy cone selection (APB-GCS), to consider the Friis propagation model for solving the problem under the assumption that chargers are equipped with directional antennas and can be deployed on grid points at a fixed height and that the sensors are deployed on the floor or object surfaces. According to simulation results, the APB-GCS algorithm outperforms others in terms of the number of deployed chargers with moderate computation complexity.