Solar energy and Piezoelectricity
Objective
The objective of this article is to document the learning process involved in exploring with the materials which shows how to make use of a solar panel and the charge stored from it, featuring two circuit designs for two separate boards: Sparkfun’s Sunny Buddy MPPT Solar Charger and Energy Harvester Breakout.
Suggested Readings
Materials
1.5V 400mA 80x60mm Solar Cell
3.7V 1000mAh Polymer Lithium-Ion Battery
Sparkfun Sunny Buddy MPPT Solar Charger
LiPo Charger Basic Micro-USB
Energy Harvester Breakout
Miniature Solar Cells
Piezo-elements (or Piezo-discs)
Background Information
The solar panels acquired are 1.5V 400ma cells with the dimensions of 80×60mm. The maximum power point is found using the voltage at maximum power (VMP) and the current when the power output is at its peak (IMP). The equation to use is:
V=IR
Where V is the voltage, I is the current, and R is the resistance. The VMP is usually not the maximum voltage, which is 1.5V in this case, but less. But in an ideal case, the MPP, in Watts, will be:
P = IV
Realistically, the solar panels will provide currents less than indicated by the manufacturer due to the weather or the seasons.
These photodiodes act as mini solar cells that are sensitive to a wide range of light wavelengths (430-110nm). Open circuit voltage: 350 mV, short circuit current: 47μA.
Sunny Buddy MPPT Solar Charger
Maximum Power Point Tracking (MPPT) controllers make sure that the system is at its max power point. Solar panels cannot be connected directly to a battery and requires a solar charger. The sunny buddy board is intended to charge Lithium Polymer (Li-Po) batteries with solar panels. The default maximum charge current is 450mAh with a maximum recommended output of 20V, and a recommended minimum of 6V. The smallest battery should have a 450mAh rating as it is dangerous to charge a battery more than its capacity.
This rechargeable battery outputs 3.7V at 1Ah and could only be charged with special Li-Po chargers. The battery has built-in protection against over voltage, over current, and minimum voltage. For wearable projects, conductive threads may cause a short in the circuit which will create sparks and heat. (Be careful with the sewing!)
Lithium-Polymer charger basic with a Micro-USB dock
This docks enables charging devices plugged into the system with a Micro-USB dock (e.g. Android chargers) with a Lithium-Ion battery, which will be charged using solar energy and piezoelectric systems.
This board can be used for harvesting piezoelectric and solar energies. The board can also be used as a standalone nanopower buck regulator. Pre-configured for an output of 3.3V and an input protective shunt is set at 20V for greater storage capacity.
Piezo-elements (or Piezo-discs)
The piezo-elements generate electric charge in response to mechanical stress, in form of compression, which includes tapping, knocking, or squeezing motions on the surface of the discs. Stress is the force over the cross sectional area of an object, as shown below:
Parts
Sunny buddy board parts
Energy harvesting breakout parts
Circuits
Design 1: Energy harvesting breakout
This design requires capacitors, and a diode. In the illustration above, a Schottky diode is used and each of the 4 supercaps have a 2.5V rating, which totals to a 10V rating that matches the maximum voltage of the solar panel in this example. The energy harvesting breakout is allowed to store amounts of charge from both solar and piezoelectric systems.
The piezo-discs could be directly connected to a LED indicator in a circuit, like a battery connected to a load, to show when it creates voltage. Snapshots below of a piezo-disc connected to a LED as it is slapped shows that the LED indicates the generated voltage as it is under the force.
The charge from the piezo-discs could be stored in a capacitor or a battery, which would accumulate over time. Ultimately, the goal for the long-term project is to be able to make use of the charge acquired from both the solar and piezoelectric systems as part of a wearable device.
Design 2: Sunny buddy
This design with a Redboard shown above requires no load, yet no way to indicate whether the solar panel works or not except for the built-in indicators in a Redboard. However, with a Sunny buddy board, a lithium-ion battery could be directly plugged into the battery charge connection dock. In both cases, the solar panel is connected to the primary barrel jack footprint which requires a barrel plug with the diameter: 5.5mm outer / 2.1mm inner.
A photo below of the circuit using the sunny buddy features two 1.5V solar panels in a series, which totals 3V, equivalent to 2 AA batteries.
The LED indicator shows that the system is either charged by the Lithium-Ion battery or the solar panels, or both. To verify the system is charged by the solar panels, the panels were covered and uncovered from the light source repeatedly which dimmed the LED significantly.
