Design of a Solar Powered Battery-free Wireless Sensor Node

Baldev Grewal, Roman Nizamuddin, Ravneet Simak
supervised by Dr. Xavier Fernando

The design project is based on battery less solar-harvesting circuit that is tailored to the needs of low-power applications such as wireless sensor networks. The harvester circuit performs maximum power point tracking of solar energy under non-stationary light conditions, with high efficiency and low energy cost exploiting PV modules. The limiting factor in conventional sensor nodes is the battery, which prevents maintenance-free operation for long periods of time. An emerging energy storage device called the super- capacitor holds the solution to the low power and maintenance-free operation requirements of sensor nodes. By replacing the battery with a super-capacitor and a novel charging scheme, the possibility exists for an operational life of up to twenty years for a sensor node. The expected advantages of the super-capacitor, battery swap are significantly longer lifetime and decreased maintenance costs.

The amount of energy that can be harvested depends on various factors such as voltage level of storage device and the incident light intensity. Since, the output characteristics of a PV array vary non-linearly when temperature or irradiance conditions change. Therefore, maximum-power-point tracking (MPPT) techniques are exploited for adjusting the operating point of the solar panel in order to obtain the maximum output power from the PV module. The adoption of a simple MPPT circuit allows us to shrink the size of PV modules and to reduce the capacity of the energy reservoir. The model consists of three units: the MPP regulator, the MPP tracker, and the MPP power supply. The MPP regulator that is usually exploited in MPPT techniques is a buck power converter. It operates in continuous mode as the maximum capacitor voltage is lower than the nominal operating voltage of the solar cell. Since the input power varies continuously with the atmospheric conditions, pulse width-modulated (PWM) signal tracks the maximum operating point supported by a small capacitor. The core of the harvesting circuit is the MPP tracker that attempts to obtain the maximum achievable power from the solar cell. The ultralow-power comparator generates the PWM control signal for the buck converter. The MPP power-supply unit, with two low-threshold diodes, leads two possible power supplies to the harvester comparator, and only one source can be employed at a given time. The output from the super-capacitor is regulated using another buck converter to provide operating voltage to micro-controller/transmitter unit which is used to sense data (temperature, pressure, etc.) and wirelessly transmits it to another station.

Wireless sensor networks (WSNs) monitor environmental and physical conditions, and cooperatively communicate acquired data through the network to a central location. WSNs are gaining popularity in many markets such as, medical, industrial, environmental, and residential and any such application could be achieved using this design solution.

Project targeted applications: Wireless sensor applications. For example temperature, pressure, smoke, etc.