As an example, consider an energy harvesting-based industrial monitoring system, such as a pipeline in the remote wilderness that needs to constantly monitor the flow rate, temperature and pressure of a pipeline for every 50-meter section of pipe. Each node has temperature, pressure and flow sensors built into the wall of the pipeline. Measurements must be taken and reported every five seconds. As the pipeline is hundreds of miles long, running power and information lines would be very expensive and subject to constant maintenance, potentially requiring expensive repairs. Replacing batteries periodically would also be very expensive due to their vast number and the ruggedness of the remote terrain. What is needed is a power source that can continually generate sufficient power, which is readily available and sustainable. One of the most popular and readily available energy sources would be a small solar cell combined with a storage device such as a battery or supercap to deliver continual power thru night time and poor weather conditions.
With the introduction of very low power sensors and microcontrollers with integrated wireless transceivers, average power requirements have been dramatically reduced. This makes them ideal for energy harvesting powered applications. Their power requirements range from 10uW in sleep mode, to about 50mw to 75mW during processing and transmission (in 1ms to 2ms bursts). The microcontroller requires a consistent source of power typically at 2.2V whereas the wireless transceiver generally uses 3.3V. Although a single photovoltaic cell only 1cm² in size can easily provide the required power, its output voltage ranges from 0.25V to only 0.6V, which is too low to power the rest of the system. This is where an energy harvesting IC comes into play. It must boost the very low voltage source to a level capable of charging a single cell Li-Ion battery, generally around 4.1V. Additionally, it must not pull too much current from the solar cell as it will collapse its internal voltage. As allowable current draw varies with illumination, the harvester IC must continually monitor the solar cell’s voltage and limit current accordingly. Finally, the harvesting IC must be as efficient as possible over a very wide range of charging currents and require the minimum level of quiescent current while the charging circuit is asleep in order to minimize the size of the battery.
The energy harvesting IC
Linear Technology recently introduced the LTC3105 - an ultralow voltage step-up converter specifically designed to dramatically simplify the task of harvesting and managing energy from low voltage, high impedance alternative power sources such as photovoltaic cells, TEGs (thermoelectric generators) and fuel cells. Its synchronous step-up design starts up from input voltages as low as 250mV, making it ideal for harvesting energy from even the smallest photovoltaic cells in less than ideal lighting conditions. Its wide input voltage range of 0.2V to 5V makes it well suited for a wide array of applications. An integrated maximum power point controller (MPPC) enables operation directly from high impedance sources, like photovoltaic cells, preventing the input power source voltage from collapsing below the user-programmable MPPC. Peak current limits are automatically adjusted to maximize power extraction from the source, while Burst Mode® operation reduces quiescent current to only 18uA, optimizing converter efficiency.
The circuit shown in Figure 2
uses the LTC3105 to charge a single-cell Li-Ion battery from a single photovoltaic cell. This circuit enables the battery to continually charge when the solar source is available, and in turn, the battery can power an application such as a wireless sensor node from the stored energy when the solar power is no longer available.
Figure 2: Single photovoltaic cell Li-Ion trickle charger