The purpose of this paper is to introduce a transmission power control scheme based on the remaining energy level and the energyharvesting status of individual sensor nodes to extend the overall lifetime of wireless sensor networks (WSNs) and balance the energy usage. Ambient energy harvesting has been introduced as a promising technique to solve the energy constraint problem of WSNs. However, considering the tiny equipment and the inherent low and unbalanced harvesting capability due to

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... mental issues, there is still a long distance from perfectly solving the problem. In this paper, a wind and solar power joint-harvested WSN system has been demonstrated, which uses ultracapacitor as energy storage. By analyzing the power recharging, leakage, and energy consumption rate, a novel energy-level-based transmission power control scheme (EL-TPC) is produced. In EL-TPC scheme, the transmission power is classified into various levels according to the remaining energy level. By adapting the nodes' operation pattern, hierarchical network architecture can be formed, which prioritizes the use of high energy level, fast charging nodes to save the energy of uncharged nodes. The simulation and demonstration results show that EL-TPC scheme can significantly balance the energy consumption and extend the entire network lifetime. 2 International Journal of Distributed Sensor Networks energy can be converted to electrical energy and used directly or stored in the means of energy storages, for example, batteries and ultracapacitor. However, using rechargeable sensor nodes also suffers from many problems, such as the technical challenges in tiny ambient energy collection device production and implementation. The extremely low recharging speed due to typical ultra-low-power ambient energy and the unbalanced recharging speed also generate problems. The lifetime of energy storage component is also a design challenge under the situation of frequently recharging and consuming. The purpose of this paper is to solve the problem of the unbalanced energy consumption and harvesting speed in ambient powered WSNs, which applies a transmission power control scheme to enhance or reduce the communication distance of sensor nodes based on the energy level situation. By applying such a scheme, the sensor nodes with higher remaining energy resource or higher energy harvesting capability will take more responses to the network data packet delivery, while the other nodes will be in an idle state for longer time. Our previous work has been presented in [6]; a building surface was mounted, wind power was collected, and wireless sensor network system has been demonstrated, which aims to monitors the usage pattern of air conditioners (ACs), the outdoor temperature. The idea of energy-level-based transmission power control scheme (EL-TPC) was firstly introduced in [6] as well. It achieves energy saving and balancing by modifying the transmission power level on different nodes according to the remaining energy level, power recharging, and leakage speed of the nodes. This paper is extended from [6]; more clear descriptions and more details of EL-TPC will be provided. Theoretical analyses are made, and the corresponding network simulations are also extended to highlight the outstanding performance of EL-TPC. Solar energy-harvesting sensor nodes are also introduced in the demonstration system. The remaining of this paper is organized as follows. In Section 2, the related works of energy harvesting and the corresponding management schemes are introduced and compared. Our previous work of EL-TPC is also briefly introduced. The demonstration WSNs architecture and hardware component design are described in Section 3 with the measured experiment results of the device parameters. The transmission power control scheme EL-TPC is introduced in Section 4 in details. In the following Section 5, the network simulation and real BSMSN system demonstration are set up and carried out; the performances are evaluated. Finally, the conclusion is given in Section 6.