Ambient light is a prime source of harvestable energy for low-power electronics devices. Most photovoltaic cells, however, are designed to work with direct sunlight and are ineffective indoors. Now, an organic light cell is available that can glean power from lighting as low as 20 Lux (lumens per square meter), opening the door for indoor energy harvesting in the IoT.
As the EDN article Energy harvesting expands IoT options points out, there are many applications for which even battery power is impractical. The cost and logistics of changing batteries can quickly become too burdensome for a widespread product installation. Outfitting a large apartment complex with traditional keyless electronic door locks, for instance, would require periodic replacement of hundreds of batteries, an unappealing prospect for building operators. Harvesting the energy of hallway lighting, however, could eliminate that requirement by continuously storing power against the lock’s occasional use.
The trouble with traditional photovoltaic cells in such an application is their poor response to low light levels. Such cells are designed to be effective in direct sunlight, hence the name “solar cell,” which has a light density of 30,000 to 100,000 Lux. Typical indoor home lighting, though, ranges from 50 to 150 Lux. At these levels traditional solar cells produce almost no usable power.
This inability of solar cells to be effective at low light levels has limited their utility for indoor energy harvesting applications. A new light cell from Swedish company Epishine promises to solve that problem. The company has created an organic photovoltaic cell optimized to work efficiently at illumination levels from 20 to 1000 Lux from artificial lighting such as LEDs and fluorescents.
The Epishine light cell is fabricated by printing the active elements onto plastic films that get pressed together. The result is a thin (0.2 mm), flexible product that produces about 18 µW/cm2 at an illumination of 500 Lux. The finished cell has a 10-mm minimum bend radius, so it can be curved (Figure 1) and fabricated to a custom shape to fit installation requirements.
Figure 1 This indoor light cell is printed on a plastic film, giving it a flexibility that opens installation options. Source: Epishine
In addition to its flexibility, the Epishine light cell has another handy attribute – it is essentially transparent to radio waves. The active organic electronic layer is only around 100-nm thick, so there is almost no RF absorption. This gives IoT designers the space-saving option of positioning their antenna behind the light cell rather than alongside.
Stock versions of the light cell are available in three sizes – 50-mm wide and 20-, 30-, and 50-mm in height. They come in two configuration options, six and eight series-connected cells. Each cell has an open-circuit voltage (VOC) a little over half a volt, so six cells in series yields VOC of 3.8V and eight cells yields 5.05V, making them compatible with common battery-pack voltages. As shown in Figure 2, a 50×50 mm, six-cell configuration would yield at least 40 μA at 3V in normal home lighting.
Figure 2 Power generation matching the needs of many small microcontrollers is available from normal room lighting with the Epishine light cell. Source: Epishine
While microamperes seem like a pretty meager energy supply, they can be more than enough to power today’s tiny, energy-efficient microcontrollers – at least while the lights are on. For applications needing to operate in darkness or to supply intermittent power at higher levels, the light cell can be paired with an energy storage device, such as a rechargeable battery. Properly sized to accommodate battery and light cell degradation over time, the energy harvesting power system can outlast the design’s installed operating life.
Rich Quinnell is a retired engineer and writer, and former Editor-in-Chief at EDN.