Designing of efficient energy harvesting systems for autonomous WSNs using a tier model

Energy harvesting systems (EHSs) are the key to perpetual operation of electronic devices in application areas with bad infrastructure or mobility. Wireless sensor networks (WSNs) are often used in such areas. Normal WSN nodes are powered by batteries. Therefore, the lifetime is limited and the batteries have to be replaced manually after a certain period of time. This problem can be solved by EHSs. They exploit energy sources of the environment and store the harvested energy in energy buffers. The EHS supplies the electronic device and ensures a continuous operation. WSNs can benefit from these developments, because the lifetime can be enhanced dramatically. However, the EHS have to be adapted to the requirements of the application area and of the supplied device. This enhances the overall efficiency of EHS. To be able to do that, the fundamental mode of operation of an EHS has to be well-understood. We introduce a novel tier model for EHSs. It structures the EHS into tiers with special functions. This enhances the design process of an EHS, because tiers can be adapted to each other and the overall efficiency of can be increased. The tier model is applied to RiverMote, a WSN node for in-river water level monitoring. Each node is supplied by solar cells and the energy is stored in double layer capacitors (DLCs). The hardware of the EHS of RiverMote is divided into the tiers of the model. These tiers are adapted to each other carefully. Although no maximum power point tracker has been implemented, it has been shown that the available power of the solar cell is greater than 80 % of the maximum power point if the energy level of the DLCs is between 42 % and 100 %. This result was only possible by a careful design and an adaption of the tiers.