We study lifetime of linear and two-dimensional Wireless Sensor Networks (WSNs) for the case where wireless sensors are not able to change their transmission power. In this case, unlike majority of existing works, power control cannot be used to increase network's lifetime. Instead, other approaches such as adding extra nodes and careful node placement and routing can be employed to improve network's lifetime. For linear WSNs, we consider two scenarios. In the first scenario, every node is required to periodically send packets to the base station while, in the second scenario, only a subset of nodes (e.g., a subset that covers the whole network) are required to do so. For the first scenario, we derive bounds on the maximum lifetime achievable. For the second scenario, we establish a lower bound on the minimum number of nodes required to achieve any specific lifetime, as in this scenario, lifetime can be arbitrarily increased by adding enough number of nodes in the network. Next, we extend our work to two-dimensional WSNs. As before, we do not use any power control mechanism. Similar to the first application in linear networks, we assume that all nodes send reports to the base station, periodically. However, rather than node placement, we focus on routing. Also, we assume that each node can aggregate all the received packets into a single packet. Our primary objective here is to improve network's lifetime. As a secondary objective, we aim at reducing network's delay while preserving network's lifetime.