What is Energy Harvesting?
“Energy Harvesting.” This buzz word sounds straightforward, but what does it actually mean? Where does this energy come from, and how do you “harvest” it? Is it useful for real-life solutions? How can I use it? These are all great questions, so let’s dig in.
Energy harvesting has a bad reputation for being overly complicated, but it can be incredibly simple. Arduino revolutionized hobby electronics by simplifying its components down to the point that anyone could do it.
Energy harvesting has the same potential.
Energy harvesting consists of two key components -- an energy source and something to convert that energy into useful power, i.e., electricity.
Energy harvesting can take the form of vibrations from a car engine, body heat, or radio waves from a nearby cell tower. Piezoelectric modules, thermoelectric modules, and antennas can convert these energy sources into electricity.
Light energy, from the sun or artificial sources, is one of the most abundant energy sources in the environment. A long history of photovoltaic innovations has made solar energy harvesting particularly useful.
What can I Power?
A growing interest in energy harvesting has been fueled by the trend of electronics becoming lower and lower power. Many of the sensors and wireless devices of the booming IoT industry could be fully supplemented with solar energy harvesting, removing the need for batteries or wired power.
So, what type of sensors and wireless devices can be powered with solar energy harvesting?
The answer to the question will depend on three main factors: the light conditions of the environment, the device itself, and how the device is operated.
For example, if we have a Bluetooth sensor, we could update more or less often based on the lighting conditions of the environment.
In dim indoor environments, we may only have enough energy to update every couple of minutes. However, in brighter settings, we could update multiple times per second. In an outdoor environment, a Bluetooth sensor could update continuously.
As you can see, the way a device can operate changes drastically based on the light conditions of the environment.
In general, passive sensors, like temperature and humidity, draw very little power and can be configured to operate in any environment. Devices that require sampling, such as inertial, force and acoustic sensors, draw higher amounts of energy and may need a brighter environment to operate. Cameras, backlit displays, and lights are examples of higher power consumption devices that are more compatible with an outdoor environment.
The power consumption of a wireless device depends on what protocol it is running. The great news is that most common protocols such as Bluetooth, LoRa, RFID, and cellular variants, can be very low power. Similar to sensors, power consumption is mostly affected by how a protocol is configured and operated.
Often the most crucial parameter is data rate. How often does your device collect and transmit data? In many cases, this is directly proportional to the average power consumption. This means that increasing your data rate from 1 second to 2 seconds could potentially halve your overall power consumption.
Equally important is the device’s sleep current. Between transmissions, many protocols allow the device to go into a near shutdown state, drawing only micro-amps of current or less.
Finally, the power and time it takes to complete transmission will vary for each protocol. Bluetooth devices need less than 5ms and 30mW to transmit a data packet. Whereas, a cellular device could take several seconds and need more than 1W of power to complete a transmission.
The bottom line is that most protocols can be configured to operate in any environment.
How can I Start Prototyping?
Now it's time to get down to the nuts and bolts. Solar harvesting sounds useful and widely applicable but there's one big question.
Where do I Start?
If at all possible, you should start with a development kit. Having the right tools is vitally important during the prototyping process. Manufacturers of both energy harvesting modules and energy harvesting circuitry have done everything possible to make integrating their technology into solutions simple.
The prototyping process can be broken down into three simple steps.
1) Identify a device, concept, or idea.
2) Find a compatible development kit.
3) Prove your concept.
The first step is somewhat self-explanatory. After reading about what energy harvesting is and what it can power, you most likely already have several ideas floating around your head. Let’s move on to step 2.
Find a Compatible Development Kit
There are many off-the-shelf development kits available, which do the heavy lifting and connect all the components for you. They have the circuitry and brains to extract energy from the environment, store it in a battery or storage element, and provide power to the device.
The right kit is going to depend on the power supply requirements of your device and your stage of product development.
If you already have a device or finished product, looking at its power source can provide insight on which kit you need to meet its power supply requirements (voltage and current). Find your kit with an output that matches the voltage of the battery or input.
For example, many kits provide a 3V or adjustable power output, which can directly replace 3V lithium button cells. Likewise, many kits have built-in li-ion battery chargers, which can be connected directly across devices that are already using them.
If you are starting from scratch or only have a concept, development kits are available that combine the energy harvesting and wireless sensing components, such as PowerFilm’s BLE indoor solar development kits.
Prove Your Concept
The last step is to prove your concept. Put everything together and deploy it in the environment. Theoretical calculations can provide a rough estimate, but the most accurate way to see if your system will work is to test it in the field.
For an energy-harvesting application, the fundamental concept you are trying to prove is that your system can produce enough energy to sustain itself or extend operating lifetime.
Tracking battery level over time is one way to determine the power consumption of your device. Once you have collected field data, you can begin to make informed adjustments to your design.
It’s best to look at the device first and the solar panel second because it is often much easier to reduce the power consumption of your device than it is to collect more energy.
To decrease power consumption, try reducing your data rate and disabling unused features. On a recent project, a device was measuring higher than expected sleep current. After combing through software, a line of code was identified, which enabled the log function. After changing this single line of code, the devices sleep current dropped from 200uA to 2uA, and overall power consumption dropped by nearly a factor of 100.
Without modifying the device first, an entire table of solar would be necessary, now a solar panel the size of a credit card was ideal.
Once you are confident that you have optimized your device, then you can look at the solar solution.
Altering size, lighting conditions, mounting angle, or panel orientation can increase the performance of an indoor solar panel. Then iterate as needed. Make adjustments, redeploy, and collect more data until you are happy with the performance you are seeing.
Once you have a proof of concept, the next logical step is to begin to refine the design and make it look more like a final product. You can move forward with the next stages of product development and ultimately, a product launch.
Solar energy harvesting doesn’t have to be complicated. As you’ve seen, it can be quite straightforward.
Are you currently designing a product that could benefit from solar energy harvesting or have any questions? Please reach out and contact us. We would be glad to work with you to develop your next generation of self-powered electronics.
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