Taking advantage of ambient power sources like heat, motion or light can be very useful when you’re looking for an alternative to non-renewable sources aka batteries. But is it enough to power your device? And, how would you calculate that?

We’re gonna give you a brief overview over how this investigation could look like. We will show you how to measure the energy consumption of the device, how to measure the harvested energy of your solar panel and eventually, how to calculated whether the harvested energy can cover the energy consumption of the device.

Step 1: Measure the energy consumption of the device

For the sake of this article, we have chosen to work with the Generic Node from The Things Network as our example device. Throughout this article we’re gonna our Otii Arc as well as the Otii Automation Toolbox to measure energy consumed and harvested.

You want to start by setting up your device, in this case connect the Generic Node to Otii Arc and measure its energy consumption. We did this both with transmission rates locked at Spreading Factor (SF) 9 and 12.

Fig 1: Locked at SF9 it resulted in 6.19uWh during the active period measured for 6.4s and 52.1nWh in the sleep period measured for 30s

Fig 2: Locked at SF12 it resulted in 35.4uWh during the active period measured for 7.4s and 52.1nWh in the sleep period measured for 30s

Step 2: Measure the harvested energy of the solar panel

There is a sheer endless variety of different energy harvesters on the market. For this article we have chosen two solar panels from SparkFun – the Energy Harvesting Modules .08mA@2.9V 200Lux Solar Module (COM-16356) and the Mini Solar Panel – 0.3 Watt, 2 Volt (ETFE) (PRT-18723).

Both of these solar panels are going to be tested in two different conditions. In one condition referred to as the “window condition” the respective solar panel will be placed close to the window at a December afternoon with clear sky, but no direct sunlight. In the second condition referred to as the “lab condition” the panels will be placed 1m away from the window and closer to the fluorescent lamp on the ceiling.

To display the harvested energy we will draw up a I-V curve to show the solar panel’s voltage when loaded with a discharge current and a power curve to show how much power the panel generates. The power curve will then also give us the maximum power.

For this you will need your Otii Arc and the Otii Automation Toolbox. Explaining scripts?

Here is the overview over the maximum power for both solar panels in both conditions:

Fig 11: Overview maximum power

Step 3: Calculate whether the harvested energy can cover the energy consumption of the device

So, now to the tricky part – the calculation. After measuring both the energy consumption of the Generic Node and the harvested energy of the two solar panels, you want to calculate whether the energy harvested is sufficient to power the device.

To do so, we’re going to assume that if the solar panel’s harvested energy is higher or equal to the energy needed for the device to function properly, then it is self-powered. And, to make the calculation even easier we’re going to make the following assumptions:

• the device is always in SF9/12
• 100% of the solar panel energy is usable
• no leakage in energy storage (battery/capacitor)
• same light condition all the time (i.e. no night, office light always on, etc.)

Calculation

Energy needed for the device = Energy consumed in active mode + Energy consumed in sleep mode

Sleep mode time = Cycle time – Time in active mode

Harvested energy from solar panel = Solar panel power * Cycle time

Results

The Mini Solar Panel (PRT-18723)

The Energy Harvesting Modules .08mA@2.9V 200Lux Solar Module (COM-16356)

What does that mean exactly now? Let’s look at the PRT-18723 in the “lab condition”: If the Generic Node is in SF9, it can perform a send/receive every 159s and still be self-powered. In SF12, the same number is 910s.

Conclusion

Now you have measured the energy consumption of your device, you have measured how much energy different solar panels can harvest in different conditions and you have calculated whether it is sufficient to power your device. So, you have everything you need to go ahead and pick the right model for your specific use case.

If you want to know more about Otii Arc, Otii Ace and the Otii Automation Toolbox make sure to book a demo with our tech expert!