Energy in form of light, heat and movement is abundant in our environment and the last two can even be produced by our own body. However, ambient sources are typically of transient nature, meaning that the energy level is changing in magnitude over time. We experience different light intensities when changing locations, due to weather conditions and during day and night. Some days we move constantly, others we spend almost entirely stationary.
Our environmental temperature is also fluctuating over the course of a day. The changes in energy availability are also reflected in the power output of energy harvesters. A photovoltaic cell will harvest a great deal of power in full sunlight but will fail completely during the night. A movement harvester is great for sports applications but will perform poorly on a bedridden patient. Using body heat is a comparably constant energy source as the body is regulating its temperature as long as we are alive. Regardless, the actual energy output depends to a lot on external parameters such as the ambient temperature, wind conditions and activity of the wearer. As a result of the transient power profiles, almost all energy harvesting systems include an energy buffer that levels out the fluctuations and provides a constant power to the connected load.
The idea of hybrid harvesting systems is to combine different energy harvesters into one device to compensate for energy fluctuations. Because different harvester types can have different characteristics and require distinct hardware and electronics architecture, the choice needs to be carefully evaluated in respect to the final application. Thermoelectric generators for example deliver direct current at comparably low voltages. Hence, they don’t pair well triboelectric movement harvesters that typically develop short spikes of alternating current at high voltages. A more suitable combination is photovoltaic with thermoelectric harvesters. Both share similar electric characteristics (resistance, voltage, current, etc.) and can therefore use identical electronics for power management.
In good light conditions, the photovoltaic harvester can produce significant power levels from a small area. In a dark surrounding, the lower efficiency but constantly available thermoelectric body heat harvester takes over. In this fashion, constant energy production is possible in all ambient conditions. At the same time the impact on device complexity and architecture is limited.
Hybrid energy harvesting systems allow to tap into different energy sources in parallel in order to compensate for power fluctuations. For wearable applications, the combination of thermoelectrics and photovoltaics is an interesting option as both harvesters complement each other in power generation while sharing identical electronic infrastructure.
