Have you ever gone on a vacation and felt like you needed another one immediately after? I sure do, my last beach holiday was completely ruined by constant thunderstorms. Unpredictable weather always is always a dilemma when I’m planning ahead for my next vacation, especially if it involves outdoor activities.
I take the same cautious approach when I’m designing solar powered embedded systems intended for outdoor applications. It is a totally different beast than embedded systems that run off of a regulated power supply. As usual, I learned to be cautious the hard way, since my first solar powered prototype didn’t even last a day in the rain.
There are many aspects to consider and plan to ensure that your solar powered embedded system continues to work for days without sunlight.
Variables to consider when designing solar power embedded systems
1. Solar Panels
It goes without saying that the solar panel is the most critical part of a solar powered system. Monocrystalline is the preferred choice of solar panels since it is more efficient than polycrystalline or thin-film, and it performs well in hot weather. There are panels that can convert up to 22% of sunlight to electricity. That being said, the efficiency of monocrystalline and polycrystalline may differ depending on their supplier, so it is good to confirm these details in advance.
2. Battery Capacity
When it comes to a solar powered embedded system, an important parameter is the duration of the system when the solar panel is reduced to 0% efficiency. Environmental factors can result in your solar panel not receiving any sunlight for days or weeks. You’ll need a battery that as an adequate capacity, and you’ll also need to ensure that the solar panel’s charging rate is higher than the usage rate of the battery. It’s not very efficient if it takes 5 hours to charge the battery and only 2 hours for it to be drained by the system.
3. Sunlight Exposure
In a way, solar technology is pretty straightforward. Without sunlight, no electricity is generated. However, having 8 hours of daylight does not necessary means that your solar panel is producing electricity efficiently for 8 hours. There is another term called “peak sun hours” where the sun is at its highest in the sky and when your solar panel is at its most efficient. It is good to be aware of this variable and to calculate what your peak sun hours are.
4. Blockage and Dust
There was one time when one of our solar powered open space parking machines kept running out of power. After hours of checking out every single piece of hardware, we realized that the machine was installed under a tree and the shadows overlapped a part of the solar panel. The efficiency of a solar panel can be drastically reduced if a small part of it is blocked by dust, shadows or a fallen leaf. That is why it is good to plan your design specifically for the location it will be used.
A single leaf can reduce a solar panel’s efficiency close to zero percent.
5. Power Intensive Module
Power intensive modules will put a faster drain on your battery. Yet certain applications require power intensive modules like thermal printers, WiFi or a GSM module. If this is the case, it is necessary to understand and predict the power usage of the module so that you can budget the capacity of the solar panel and battery. For example, a system may only need to activate the GSM module twice a day to transmit the information to the data center. Proper calculation of the estimated size of data and the speed of transmission will provide a figure of how much power will be consumed during the transmission.
6. Firmware Architecture
While firmware programmers have the luxury of pushing microcontrollers to the limit in non-solar powered applications, solar power makes this a more delicate process. Take the time to get the right firmware structure. It can result in your solar powered embedded system lasting for weeks instead of days in cloudy weather. The best approach in developing firmware for a solar powered system is to put the microcontroller in deep sleep mode whenever it is not in use. The microcontroller will only wake up from its deep sleep mode by selected interrupts or scheduled timers.
7. Power Efficient Hardware Design
It is essential to minimize idle current when you’re designing hardware for a solar powered embedded system. Saving 1mA might be insignificant in a non-solar powered system, but in a solar powered system, it can extend the operational time on a cloudy day. A good strategy is to provide a separate power channel to logics and peripheral ICs that are controlled by the microcontroller. This eliminates unnecessary power consumption when the system is not in use, regardless of the microcontrollers operational mode.
The best solar powered embedded systems consume minimal power when they’re idle.
Why you’ll need to Analyze your power delivery network
When you’re trapped in the desert, you’ll realize how precious water is, especially when you’re almost at your last drop. The same principle applies to power efficiency in solar powered embedded systems. A power delivery network (PDN) analysis allows you to evaluate if the copper traces on the PCB are sufficient for delivering power efficiently to the load. You’ll want to avoid narrow spaces of copper or vias that are too small between copper planes. This will result in resistive losses and generates unnecessary heat. You can prevent this potential waste of power in the design phase, so it’s worth taking advantage of this feature if your software provides it.
If you’re designing a solar powered embedded system, built in tools like Altium’s PDN Analyzer will help ensure that your design does not exceed its power budget before it is manufactured.
Need more help designing a solar powered embedded system? Contact an expert at Altium.
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