Solar Power For Consumer IoT

Posted on 06/22/2018 at 12:00 by Samuel Jones


Solar Power For Consumer IoT Title Graphic

Previously in this 3 part IoT series, we looked at how using solar energy harvesting to power automation systems can reduce costs and increase efficiency in commercial applications.


In the commercial world, it's all about money and the bottom line.


In the consumer world, convenience is king.


Consumers want devices and systems that are easy to use and dependable. The best consumer IoT solutions are those that people can set up and forget about. Batteries are easy to use and will work, but eventually, they will stop working. Then what?


When batteries die, the consumer has two options; replace batteries or toss the device. Both options are a headache. “Where’s my screwdriver?” “What type of batteries do I need?” “Now I need to run to the store.” “Do they still sell my device?” “Will a different device work with my system?” Finally, the dreaded, “I think I need to call customer support.”  


Many times the consumer opts to toss the device and replace it, or they forget about the device entirely. The National Fire Protection Association reports that 20% of US homes have smoke alarms that are not operational due to dead batteries. Even something as serious as a safety hazard isn’t enough to overcome the inertia of inconvenience.


This concept holds true for all sectors of the consumer IoT space.


The smart homes of the near future could easily have over 100 nodes. Even if battery life is five years, that averages out to changing two dead batteries a month. It wouldn’t take many months for the consumer to start asking themselves if these systems are worth the hassle.


Every time a consumer takes off a wearable to charge its batteries, it’s an opportunity for them to stop using it. Half of all consumers who buy wearable devices stop using them within a few months, and charging is a significant reason for that.


For smartphones, convenience has been the driver of many of the advanced features that make our lives easier. Even the simple inconvenience of plugging something in is currently driving the evolution of wireless charging tech.   


What's the solution? Remove the hassle.


Ambient energy is readily available in more places than you might expect and can be extracted and utilized through energy harvesting. Advances in microelectronics technology have enabled ultra-low-power devices capable of running on these ambient energy sources.


One of the simplest and most accessible forms of energy to utilize is light. Whether you are in your home, driving to work, or walking down the street, light is present for a majority of the waking day.


So how much power is available, and what kind of devices can be run in these places?


The power available depends on how intense or bright the light is. Since our eyes are so good at adjusting to bright and dim light, you might be surprised that outdoor light can be over 1000 times more intense than indoor light. Unfortunately, that means 1000 times less power available in dimly lit indoor environments.


The good news is that PowerFilm’s Indoor Light Series solar panels have been optimized to efficiently extract energy in these dim, indoor environments.


A standard unit of measurement for light intensity is lux. Put simply, lux is how many light rays are passing through an area at a given time. Brighter light equals more light rays and a higher lux measurement.


An interior room with no windows and standard lighting will generally measure between 200-400 lux. At this level, wireless sensors, beacons, and other low measurement rate (>10 seconds) devices can be powered with a reasonably sized indoor solar panel (1-10 square inches). How often these rooms are used and how long the lights are on in these rooms will significantly affect available power.


Interior rooms with windows will have more power available and can measure between 500-1000 lux. At this level, indoor solar can power the devices stated before but also can support higher data rates (1 second or less). These higher data rates are essential for lighting, security, and other control systems where a short delay between detection and action is desired.


Power available will jump up in outdoor environments, measuring around 10,000 lux in shaded areas and 100,000 lux in direct sunlight. Outdoor solar can support higher power consumption devices such as cameras, actuators, lights, electric locks, and more.


So you have devices or products that you want to power with solar, what’s next?


As an example, let’s go through the steps to define a solar solution for the Visonic MCT320 Wireless Contact Sensor, used to detect when windows and doors open/close.  


First, determine the average daily power consumption of the device. The easiest way to do this is by looking at how long the current batteries last. Manufacturers usually list battery information in the manual or technical specifications. Visonic states that a 850mAh 3V CR-2 Lithium battery can last up to 5 years. Always convert units to power (Watts) to avoid voltage mismatch issues.


850mAh x 3V = 2550mWh power consumption over 5 years.


To get daily power consumption, divide by battery lifetime, and correct for battery self-discharge.


A primary lithium battery will self-discharge 10% of its capacity over five years.


2550mWh x 0.9 / (365 x 5) = 1.26mWh per day


Next, characterize the environment where the device is going to be installed and how much power is available. Assuming the device is placed on a window, 1000 lux for 5 hours per day is a modest estimate. At this level, PowerFilm indoor solar can generate 0.032mW/cm^2. Now we can determine how much solar is needed to power the device.


1.26mWh / 5h / (0.032 mW/cm^2) x 1.2 = 9.45cm^2 indoor solar panel


A 20% margin was added to account for reliability and losses during charging. A custom 3.6V PowerFilm Low Light solar panel of this size could be mounted directly onto the device’s enclosure.


Additionally, a charge controller and storage element need to be chosen. The BQ25570 energy harvesting IC offers a diversity of features that will fit this application well. It can be configured to efficiently harvest energy from the solar panel, store it in a 40mAh lithium battery, and provide a constant 3V output. This exact configuration is implemented in the Solar Development Kit (DEV-BASIC), which could be used as a direct plug and play solution.  


In an indoor environment, this system will be subject to minimal degradation and will provide maintenance and hassle-free operation nearly indefinitely.


Solar harvesting opens up opportunities in the consumer IoT space that would otherwise be held back by the inconveniences of batteries.


As you develop your next line of smart products or tinker with wireless sensors and controls, PowerFilm encourages you to keep in mind how much consumers hate batteries and what you can provide as a better solution.


PowerFilm is a leader in custom solar solutions, both outdoors and indoors, and can work with you to find the best solutions for your needs.


Contact us, and let’s get started on your solution today.


Categories: Internet of Things