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This paper investigates a class of mobile wireless sensor networks that are not connected most of the times. The characteristics of these networks is inherited from both delay tolerate networks (DTN) and wireless sensor networks. First, delay-tolerant wireless sensor networks (DTWSN) are introduced. Then, three main problems in the design space of these networks are discussed: Routing, data gathering, and neighbor discovery. An approach is proposed for deployment of DTWSNs based on the traditional opportunistic broadcast in delay tolerant networks with on-off periods. The delay and the throughput of the routing scheme were investigated in the DTN literature. However, the energy consumption was not studied thoroughly, which is focused here. Neighbor discovery in a sparse network could be a major source of energy consumption. Therefore, energy per contact measure is evaluated analytically based on the distribution of physical link duration. The results for 2D constant velocity model and random waypoint model are reported and the average PLD is suggested as an appropriate choice of beacon interval.