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Light contains energy. When light hits a conductor (or semiconductor) some of the energy is translated into moving electrons, creating current. We can harness the current using solar cells (aka photovoltaic cells). When the sun shines on a solar cell, the current output is mostly constant, which is known as direct current, DC. DC is easy to use and great for charging batteries and powering microcontrollers, but using DC from solar cells can be tricky.
Solar Cell Huge - 5.2W 180x220mm (7.09x8.66")
Solar panels come in many varieties. When shopping for solar panels for your system, there are a few specifications you need to follow. The specs below will refer to the Solar Cell Huge, shown above.
Output voltage range: The panel above will output a voltage between 0V and 10V depending on the intensity of light and what you have attached to the solar panel (i.e. the load). You will need to make sure whatever you attach to the solar panel will be able to handle at most, 10V. This is called the open circuit voltage.
Power: 5.2 Watts is the total power this solar panel can deliver at its maximum efficiency. The solar panel is most efficient at a point called the maximum power point, which is surprisingly not the maximum voltage, 10V, but a little less, 8V. When the panel provides its peak power, the current draw is around 650mA. This is called the short circuit current. Multiply 8V by 0.650A and you get 5.2W.
Maximum power point: With no load (nothing connected to the solar cell), in full CO sunlight, the solar panel will output around 10V (Voc). When you start to draw more current (increasing load), the voltage starts to decrease, and the efficiency of the solar panel hits a sweet spot, then tapers off.
Thanks to pveducation.org for the graph.
The sweet spot for operation is called the maximum power point (top of blue curve) and is around 8V @ 650mA for this specific panel. Sophisticated solar power systems use MPPT controllers to monitor and balance the system to maintain the max power point. To check if your system is at this point, simply measure the voltage on the solar cell with your load attached to see if you are close to 8V. It isn’t too big of a deal if you are not exactly at the maximum power point, the panel will still work, but you will be losing efficiency. Notice, in our testing, we saw 550mA short circuit (Isc), which is outside of the maximum power point.
If you wanted to use this solar panel to power your RedBoard, make sure the voltage regulator can handle at most 10V on the input (which it does). It is almost this simple. Realistically, the solar panel will not be in direct sunlight most of the time and the panel could provide as little as 100mA or less on shady days. The RedBoard running the blink sketch (blinking the on board LED) draws a peak current of 60mA. This setup will not allow much room for additional hardware, due to the inconsistent and limited power draw from the panel.
Another design choice for solar power is to use the Energy Harverster board.
The Energy Harvester board is simply an efficient voltage regulator with an input window that allows charge to accumulate on a battery or supercap, up to a defined voltage, before turning on and powering your system.
In this setup, supercaps are attached to the input to give more control over when and how much power is delivered to the output at 3.3V. Supercaps act in similar ways to batteries, however they don’t hold charge as well, but they do last through millions of charge and recharge cycles. The above design uses 4, 2.5V supercaps in series to equal a 10V rating. The Schottky diode is in there to prevent the caps from sending current back to the panel once charged.
As sun shines on the panel, the supercaps begin to charge. If you measure the voltage on the caps, you should see the voltage slowly increase from 0V as more sunlight hits the panel. When the supercap is charged over a certain voltage (between 4V and 5V, defined by the rising UVLO threshold, see page 3 in datasheet), the board will turn on, the caps will discharge through the board and output a regulated 3.3V to power your system. This is nice, because it allows a defined amount of charge to accumulate on the input caps and when the voltage hits a certain level, they are discharged through the board to power your system. The more capacitance you have on the input the longer the board will run, but the caps will take longer to charge. You can think of this system as supplying bursts of power, when it becomes available.
An simple application of this setup is a simple solar power robot using the Ardubot (no microcontroller required).
When the input capacitors charge up, the motors turn and propel the beast forward, until the caps deplete and everything is turned off. Once the caps charge again, the motors turn on and the cycle repeats. FOREVER. Well, maybe not forever, but much longer than the life of any battery! Whisker switches can be attached to each motor control to steer.
Here are some things to watch out for when designing for low power:
Everything draws power: wires (use thick ones), any resistor, capacitors also have an ESR, which contributes to power loss, LEDs, regulators, etc. All of these items will draw power from your system. Deciding what can stay and what can go is a first step to minimize excess power consumption. Check out the tutorial on How to Power a Project for more information.
What are you trying to do? For example, running motors takes much more current (10-100 times as much) than running an I2C sensor. If you are drawing large amounts of current, your storage device (battery or capacitor) will need a large capacity. The Energy Harvest board will also allow for supercaps to be attached to the output to support larger loads.
Aerogel supercapacitors are a great choice for power storage if you are really needing to save power. They hold charge almost as well as a battery! Regular supercaps are also okay if you need a lot of storage (more capacitance = more storage), but will drain quickly.
The uses for solar power are many and diverse. Let us know how you used solar in your project!