In a wireless rechargeable sensor network (WRSN), sensor nodes can harvest energy from wireless chargers to refill their power supplies so that the WRSN can operate sustainably. This paper considers wireless chargers equipped with 3D-beamforming directional antennas, and assumes they can be deployed on grid points at a fixed height to propose two greedy algorithms solving the following critical problem: how to deploy as few as possible chargers to make the WRSN sustainable. The first algorithm is the node based greedy cone selecting (NB-GCS) algorithm trying to optimize the number of chargers based on node positions. The second algorithm is the pair based greedy cone selecting (PB-GCS) algorithm trying to optimize the number of chargers based on node pairs. We conduct simulation and analyze the time complexity of the NB-GCS and PB-GCS algorithms. As will be shown, the latter is better in terms of the number of chargers, while the former has lower time complexity.