All energy harvesting solutions include an energy harvesting source, an end device you want to power, a storage element (usually a battery) to store the harvested power, and a charge controller to manage the relationship between the panel and the battery.
Thermoelectric generators produce power when you have heat on one side and cooling on the other. The temperature differential between the two is what generates energy. A reasonable estimate is up to 0.6mW/cm2 per degree Kelvin.
With this device, you must ensure you don’t reach thermal saturation. You must continue to cool on the cool side and have heat on the heat side. This can become very difficult as the cooling side can eventually become as hot as the heat source, decreasing the power you can generate.
Piezoelectric is a vibration-type generator converting mechanical motion into magnetic energy that then generates usable power. Identifying a specific estimate of power generation is difficult. Still, as a general rule, the number will be significantly lower than comparably sized thermal electric, radio frequency, and photovoltaic energy harvesters.
Radio Frequency (RF)
Airwaves surround us, meaning RF energy, can be harvested using an antenna. This includes AM/FM radio, ZigBee, and Wi-Fi. All of these RF sources can be harvested to generate power, up to about 1.6mW/cm 2.
Photovoltaics take any light source, such as the sun, LED, or indirect sunlight, and produce electricity. This method produces about 3.2mW/cm2 per 100 Lux. For reference, grocery stores and retail lighting are often around 1000 Lux, making these locations ideal for indoor light energy harvesting.
The power (voltage × current) available to the load varies with changing light levels as illustrated in this family of curves (Figure 2).
Solar modules, for the most part, are very linear in their output, current tracks with light intensity. However, that changes when you drop below 10 percent of the full sunlight intensity of 1000W/m 2. Once it drops below that, the series resistance of the solar panel starts to take over and the open circuit voltage starts to drop. As can be seen, for lower light levels, the maximum available voltage at no load (Voc) is reduced. That reduces the amount of output power available.
The maximum power point, the maximum voltage x current product, on these curves is at the knee of the curve. The knee of the curve gets flatter in varying light conditions, from 200 to 1000 Lux. For the illustrated panel, it varies from 2.4V at 200 Lux up to 3V at 1000 Lux.
Why Charge Controllers?
The power available from the photovoltaic (PV) system is a function of both the output of the PV source and the charge state of the storage battery. The function of a charge controller is to maximize the power delivered to the load by adjusting the balance between these two.
Two common types of charge controllers are maximum power point tracking (MPPT) and pulse width modulation (PWM).
True MPPT does just that, it tracks the maximum power of the solar panel in real time. This technology holds the solar panel at its maximum power point and provides the maximum power from the panel in changing lighting conditions.
A PWM charger connects and disconnects the panel to and from the battery in short bursts and monitors the voltage of the battery to determine the state of charge. It will only work with solar panels that have a higher voltage than the battery they are charging. The greatest disadvantage of the PWM circuit is the loss of power between what’s available from the solar panel and what actually goes into the battery. On average, you get about 30-35 percent less power from a PWM circuit over an MPPT. Advantages include fewer parts and lower cost.
Comparison of MPPT and PWM Charge Controllers
For a 12-volt lead acid battery, the range of voltages from fully discharged to fully charged is generally 10.5V to 13.5V. The graphs in Figure 3 compare the IV curves for the two different types of charge controllers at constant light input for a discharged battery at 10.5 volts and a charged battery at 13.5 volts.
The orange section shows you a PWM charge controller power transfer. When a battery is fully discharged, the PWM pulls the solar panel down to the battery voltage. By doing so, you run on the flat part of the curve, the short circuit element of it. This will charge the battery at a slower rate than a maximum power point tracking (MPPT) charge controller will.
MPPT controllers harvest more energy and finish charging storage elements faster than PWM. MPPT controllers hold the panel at the maximum power point on the curve and convert all of that additional power (shown in the blue section) to current. On a fully discharged battery, you can generate about 55 percent more power with an MPPT charger vs. a PWM. Even when the battery is fully charged, you can harvest more power. The graph on the right shows the battery at 13.5V (the panel being used has a power point of 17V or 18V) in this case you can generate up to 20 percent more power vs. PWM.
Variations of Maximum Power Point Tracking
There are several MPPT methods, including fixed power point tracking (FPPT), ratio power point tracking (RPPT), and true maximum power point tracking (MPPT).
Fixed Power Point Tracking
Figure 4 shows each of these methods. The yellow line illustrates an FPPT charge controller. FPPTs require users to set/program the controller at a specific voltage. While they don’t track the solar panel, they still convert power that matches the battery voltage. They do this while holding the solar panel at the user-programmed specific voltage.
Ratio Power Point Tracking
With this method, you program the desired ratio depending on the fill factor of the solar module. If you look at the graph, the fill factor is the ratio of VPP × IPP ÷ V oc × Isc. Said another way, it’s the maximum power point divided by the endpoints. As the fill factor lowers, the panel’s maximum power point drops and the power point shifts down and to the left. You may be dead on with a ratio power point tracker with a particular cell at a certain time, but as the illumination of that cell changes based on different Lux conditions, the power point may drift off some, though the net result may only be a few microwatts.
Maximum Power Point Tracking
True MPPT does just that, it tracks the maximum power of the solar panel in real time. This technology holds the solar panel at its maximum power point and supplies the maximum power from the panel in changing lighting conditions.
There are three types of MPPT topologies, buck, boost, and buck/boost. Buck controllers take a higher-voltage solar panel and buck it down to your battery voltage. Buck controllers have traditionally been used with large solar arrays, which almost always run higher voltage than the battery they’re attached to. Boost topology takes a lower-voltage solar panel and boosts it up to your battery voltage. In energy harvesting scenarios, having a single-cell, two-cell, or three-cell device is much more desirable due to space constraints and the interconnection associated with multiple cells. A typical boost topology will take a 0.6-1.2V single cell and boost it up to a 3-5V battery.
Power Management Integrated Circuits (PMICs)
PMICs perform a number of useful functions for chargers. They harvest tiny bits of power from energy sources, all with ultra-low power consumption and over-voltage and under-voltage protection. This is critical for any lithium battery technology and is also valuable for other battery technologies. PMICs often have programming to utilize supercapacitors and provisions for adding a primary battery. Primary batteries are used as backup when the main system battery fails, or if not enough energy has been harvested, a primary battery can kick in and take over.
How to Determine the Best Charging Methods for Your Application
There is no one size fits all answer. If you need the least expensive option, direct charging works. If you can spend more, you can opt for a PWM circuit. If the price is secondary and you need the absolute best performance, a true MPPT may be the best solution for your application.