(Illustration Source: Linear Technology)
To describe the vast number of stars in the observable universe, during his television series Cosmos the late astrophysicist Carl Sagan emphasized that there were “billions and billions,” placing exaggerated embellishment on both“ b’s”.
Were he alive today Dr. Sagan could easily apply his catch phrase to describe the number of connected devices in the “Internet of Things” (IoT).The total installed base of connected “things” on the IoT is forecast by market research firm IC Insights to be 13.2 billion units worldwide this year. The firm further predicts new connections will increase 40% in 2015 with 574 million new Internet connections expected to be attached to embedded systems, sensors, instruments, vehicles, controllers, cameras, wearable electronics, and other objects.
Consider, now, that most of those billions of connected things are wireless, sensor-based terminals collecting data, that many of them are quite small and that they may well be relatively inaccessible. So how do you power these stand-alone IoT nodes and do so at a low cost? Batteries? Nope. Struggling to fit a battery into a tiny package in an inaccessible space is not an effective solution. What’s more the cost of maintaining, replacing and discarding billions of batteries would be astronomical (not to mention the enormity of the human labor issue).
Recognizing that providing the power needed to keep all these sensors functioning for their expected lifespan was a linchpin that could potentially short-circuit the IoT, engineers have been looking to scavenge energy from the IoT node’s environment. The umbrella term for this group of scavenger technologies is called Energy Harvesting. Energy harvesting techniques use power generating elements to convert light (solar), heat (thermoelectric), vibration (piezoelectric), or RF energy (such as that emitted from cellphone towers) into electricity in a stable manner and without a great deal of loss. Energy harvesting permits the design of systems able to operate for years on these ambient power sources, eliminating the battery-change problem.
This is not to say that there is no use for a battery in these systems. Once the ambient energy is gathered it must be then stored to provide the required current at a time when: 1) it is needed (IoT nodes have low duty cycles, that is, a sensor taking periodic air temperature samples might only be active a few milliseconds per hour, and can be in sleep mode the rest of the time) or 2) when the source of energy is not available (the sun’s rays, for example, are not present at night). Conventional rechargeable coin cell batteries can be used in this manner and so can thin film rechargeable solid-state batteries as well as supercapacitors.
High-efficiency energy harvesting (EH) designs convert these relatively low levels of energy into an amount that can provide enough power for an IoT node. The illustration below shows the major components of a wireless sensor system including the EH transducer, energy processor (including power conversion and storage), sensor, microcontroller and the wireless radio.
Major elements of a wireless IoT node (Source: Cymbet)
Let’s take a closer look at the four main categories of ambient power: light, temperature differential, vibration and RF.
The table below shows the amount of harvested power available from these energy sources with solar (ambient outdoor light) and vibration (such as from industrial machinery) offering a considerable power advantage over the others; keep in mind, however, that not all applications will require that much energy.
Energy-harvesting sources.and harvested power. (Source: Texas Instruments)
Let the sun shine
One of the most well-established energy harvesting technologies is solar energy, which has been used for many years to power small devices such as wristwatches and more recently to provide back-up emergency power for cellphones. Solar power collects sunlight and converts it into electricity, but light-to-electricity conversion efficiency is not very high; typical solar panels are rated at 15% or 20% efficiency and that’s under optimum conditions; solar panels can endure rain, cloudy skies (reducing output to about 20 to 30% of the current generated in bright sunshine) time of year differences (solar intensity is reduced in winter), changes in the number of hours of available daylight and other factors that negatively impact output.
Semiconductor companies have developed controllers to optimize energy harvesting from solar panels. These controllers utilize Maximum Power Point Tracking (MPPT). This technique optimizes the match between the solar array (PV panels), and the battery bank or utility grid. The function of MPPT has been described as analogous to the transmission in a car. When the transmission is in the wrong gear, the wheels do not receive the maximum possible power. An MPPT algorithm monitors the input current and voltage and controls the duty cycle to maintain the MPP set point needed to maximize energy output from the photovoltaic module. Essentially, a controller IC looks at the output of the panels, compares it to the battery voltage then determines what the best power is that the panel can put out to get the maximum Amps into the battery. Most MPPT's are around 93-97% efficient.
Thermal energy can be tapped for the IoT by taking advantage of available transducers and converter ICs. Using energy extracted from thermal gradients, low-power circuits can operate for years without the need for battery replacement. Two scientific “effects” are in play here and need a bit of explanation. The “Peltier effect” is usually used to create a temperature difference by applying a voltage between two electrodes connected to a sample of semiconductor material, usually to provide solid-state cooling. But the reverse process can also be applied-- using a temperature difference to generate a current. The “Seebeck effect” is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors is converted into an electrical potential, or voltage.
For example, an energy-autonomous sensor for measuring airflow temperature might use copper to conduct warm air inside the tube where air is flowing, while a heat sink on the outside conducts the cooler ambient air, creating a difference in temperature between the sides of a Peltier element, and generating power for the sensor.
Right now waste heat is being converted to renewable energy from factory equipment such as pumps and motors for Industrial Internet of Things (IIoT) applications. But soon, usable waste heat could be captured from the human body to power some of the wearable health and medical implant sensors being developed (for typical indoor air temperatures, a harvester attached to a person’s skin will be able to use a ∆T of up to 10°C.)
Shake it up baby
Kinetic energy can be converted into electrical energy by means of the piezoelectric effect; mechanically deforming a piezo crystal (or Micro Electro Mechanical System piezo-MEMS) with tension or pressure to generate electrical charges that can be measured as voltage on the electrodes of the piezo element. Piezoelectric vibration energy harvesters convert this mechanical vibrational energy into alternating electrical energy (AC). This AC is then electronically converted to DC, which can be used to drive wireless IoT applications or recharge a battery. Piezoelectric energy transducers deliver maximum energy when operating at the resonant frequency of the vibrational source, and when operating into a load designed to match the piezoelectric output impedance. The key for harvesting the vibrational energy with any piezoelectric material is to fully understand the vibration environment, and the best way to do this is to measure the vibration using an accelerometer.
RF for the taking
Because of the widespread growth of wireless communications and a concomitant increase in the number of radio transmitters, especially for mobile base stations and handsets, there is a lot of essentially “free” RF energy around. Along with ubiquitous Wi-Fi sources, engineers can find RF energy sources ranging from short-range wireless technologies such as Bluetooth and ZigBee, to long-range cellular services. For RF energy harvesting, a receiving coil serves as the energy source, generating a voltage in response to electromagnetic coupling with the RF transmitter. The challenge in RF energy harvesting lies in maximizing the output from the transducer at a given ambient energy level. Efficient RF energy-harvesting design has become simpler thanks to available components and ICs from a variety of manufacturers.
RF energy harvesting systems can deliver renewable energy by converting radio waves to DC power for IoT applications. (Source: Powercast)
In summary, harvesting ambient energy requires only a few basic components, including an energy source transducer--delivering energy that may come at random times, and in random, usually very small amounts--a rectifier, an energy storage device, and an output regulator. We’ve looked at the four major energy harvesting sources: light, heat, vibration and RF. To successfully design an energy harvesting system for a wireless IoT node engineers have to consider the source, type of transducer available for that renewable energy source, where the node is located and the conditions under which it will operate, the required power levels of the node (including, usually, an MCU and radio) as well as the estimated system efficiency and some knowledge of the power-management electronics available for that type of energy harvesting system.