The second law of Thermodynamics states that energy will naturally flow from more organized states to lower level states that are more distributed and disorganized. Traditionally, the energy loss was something that was just accepted, but scientists have increasingly been paying attention to this ‘waste’ energy and have developed ways to harness and recover energy that would otherwise naturally dissipate.
The term energy harvesting has been applied to the principle of converting otherwise spare light, heat, vibration, and radio frequency energy into electricity. While energy harvesting is limited to small amounts of energy, advances in recovery methods and low power electronics have let us cross a threshold and enter a time when practical and useful applications exist for recovered energy.
As with many technological advances, the ability to “do” must be matched with a compelling “why” before a technology achieves wide acceptance and use. In the case of energy harvesting, the “why” is already well established. The first and most obvious reason is that harvested energy is essentially free once equipment installation is complete. However, this is often not the most compelling reason to use harvested energy to drive operation of devices. More practical reasons include avoiding the labor costs of installing power lines to equipment installations. This often requires use of certified or licensed installers, and in dense or widely distributed installations, installation costs for power runs can add up rapidly. Even devices that are traditionally battery operated benefit from energy harvesting; while the cost of batteries is relatively low, the cost of manual labor to travel to each installation to monitor and replace batteries can be significant. Hazardous areas complicate this scenario even more, as equipment may need to be turned off or de-energized before maintenance can be performed. Energy harvesting can eliminate the need for batteries or significantly increase their lifespan, thereby allowing the additional costs of energy harvesting features to be recovered rapidly.
Several types of energy harvesting exist. The most established and widely understood is solar energy, which has been used in different forms for centuries. The relatively recent conversion of solar energy to electricity has been used mostly in medium and large scale installations, and typical medium sized systems can provide houses and buildings with a supplemental power source that is frequently tied to both battery packs for power loss protection as well as the AC mains to ‘sell back’ excess energy to the local electric utility. Small scale examples such as solar powered calculators have been around for decades due to their excessively low power needs, but few other devices operated on low enough power to make solar practical for small devices. Further materials and efficiency advances have resulted in the ability to create useful small-scale systems that can provide energy to other small low-power devices. Today, small-scale solar is used for a wide range of devices including watches, wireless sensors, RFID tagging, security lighting, battery chargers, and more. Several factors influence the efficiency of solar energy harvesting, and different materials have been developed to balance size, cost, and efficiency for different applications. Available spectrum and light intensity are generally the most important factors that influence efficiency. Indoors, the artificial light spectrum is typically limited to the lower end of the light spectrum, and power output from typical indoor lighting can be 100-500 times lower than that of full sunlight.
Piezoelectric materials offer another fairly efficient method for energy recovery. These materials become electrically polarized when submitted to mechanical stress or strain. Because they are typically made with ceramic materials, they are very durable and have high heat resistance, and this makes them good choices for industrial applications and other harsh environments. Two primary types exist: resonance and off-resonance. Resonance materials are often used to recapture vibrations, and ensuring a good match between the device and the vibration frequencies is crucial to efficient energy capture. Resonant materials are increasingly used for structural sensing in bridges and buildings. These sensors gather energy from vibrational stresses and can provide power to small transmitter devices under severe stresses. Off-resonant piezoelectric devices generate a charge when they are compressed, and they are usually intended for single-event activation. Gas grill starter elements and some prototype energy harvesting sidewalks are better-known examples of off-resonant piezoelectric devices in action, and newer novel examples include self-powering light switches that offer radio control of nearby lighting when energized by switch activation.
Thermoelectric materials generate an electrical charge when placed across a temperature gradient, due to their characteristic low thermal conductivity and high electrical conductivity. When a significant enough gradient exists, the charge can be harvested by high-efficiency electronics. Traditional thermoelectric materials used rare elements and were thus very costly. However, new nano-scale manufacturing techniques have allowed physical structures in common semiconductors to replace the exotic materials. This lowered the costs of manufacturing considerably, and allowed new architectures which are ushering in new possibilities for thermoelectric applications. Despite the advances in materials, large temperature gradients and efficient heat transfer on the hot and cold sides of the thermoelectric device are still required for the most efficient operation. Designing effective thermoelectric systems thus requires a comprehensive approach that identifies the maximum thermal gradient in a system and provides efficient heat transfer on the hot and cold sides.
Radio Frequency (RF) energy harvesting is the other predominant area that researchers are focusing on. Due to the nature of today’s connected world, RF sources are everywhere. This yields an environment that is awash in spare RF energy, and scientists have been focused for some time on methods to convert some of this spare RF energy back into electricity. Today’s solutions are limited to systems that employ dedicated directional or beam-forming antennas and tuned receiving antennas to achieve a high enough energy density to make harvesting possible. In the future, however, the combination of broadband radio services that are evolving to get more use from the available spectrum and the continued expansion of RF services into new frequencies will result in a more energy-dense environment where broader-spectrum RF energy harvesting can become feasible.
A common theme for all energy harvesting technologies is that efficiency depends on a good pairing between the installation environment and the technology used. For this reason, designs must carefully consider the end use before picking an energy harvesting strategy to employ. Applications on trains, for example, will have consistent, repeatable vibrations that make them a good match to piezoelectric applications. Engines and heat-intensive industrial applications provide sources of large temperature differentials, and this makes them an ideal location for thermoelectrics. Outdoor applications are ideal for solar, and examples abound of remote devices that use solar to keep batteries charged and provide 24/7/365 operation. Finally, indoor installations can leverage solar cells tuned for artificial light.
No matter the source of the energy, TI is leading the way with ultra-low power energy harvesting technologies to convert this energy into a form that can be used in end devices. Several devices in TI’s BQ line of battery management products are tailored to nano-scale energy applications, and TI has published application examples that highlight the capabilities of these products and allow investigations into new and novel applications. The TI BQ25505 is an ultra-low power step-up converter with dual-battery management for energy harvesting applications. It can be started with a Vin as low as 330mV and continue operating down to Vin=100mV, and features a programmable maximum power point tracking (MPPT) sampling network that optimizes the transfer of power into the device. The MPPT function allows the BQ25505 to periodically check the input and adjust the load on the source to ensure the power source is operating in its peak output range for its present state. This allows the overall energy harvesting system to operate at its highest possible level of efficiency for the present environmental conditions. The BQ25505 also features battery monitoring and multiplexing, and contains programmable under-voltage and over-voltage levels for a primary rechargeable battery. When the battery passes into its low charge state, the BQ25505 can switch operation to a secondary non-rechargeable battery to ensure the primary battery is not damaged. The BQ25505EVM evaluation module enables all of the features of the BQ25505 and provides test points and jumpers to enable easy circuit monitoring and component changes during design development.
TI has also provided several reference designs for an energy-harvesting system with integrated MCU and sub-1GHz radio. The TIDA-00488 design highlights the ability of wireless sensors to control HVAC, lighting, and other systems in response to the current temperature, humidity, and natural light environment. Applications of this design include energy saving systems that adjust interior illumination or HVAC based on the amount of natural light and solar gain that is occurring. The system leverages energy harvesting using TI’s BQ25505. The BQ25505 is paired with a TI SimpleLink CC1310 ultra-low power wireless MCU and several TI sensors. It is powered by an IXYS IXOLAR high efficiency SolarBit cell (Part KXOB22012X1L), and the overall design is efficient enough to power itself in most lighted situations.
A second reference design outlines a maintenance-free Bluetooth Low Energy (BLE) beacon subsystem based on an indoor light energy harvesting design. The TIDA-00100 design pairs TI’s BQ25505 and CC2541 to provide BLE beaconing for building automation, smart retail, smart signage, and proximity marketing. The design is efficient enough to operate off of ambient indoor lighting as low as 250 LUX and features an 8mF Super Capacitor to provide an energy reservoir capable of providing the supplemental power needed during beacon transmission.
The BQ25504 and BQ25570 are other popular energy harvesting boost charger ICs from TI. The BQ25504 is similar to the BQ25505, but exchanges the secondary battery interface for a programmable Battery-Good output flag that can be used to enable or disable system loads or place attached microcontrollers into a low power state. The BQ25570 also adds a secondary buck converter to the functionality of the BQ25504. This buck converter can be used to power a secondary MCU host controller, sensor, or radio communication module. The TIDA-00242 energy harvester reference design leverages the BQ27707 and a solar panel to acquire and manage the microwatts to milliwatts of power needed for ultra-low power applications. The storage method is a 47nF Super Capacitor that is charged and maintained by 4 series low power solar elements using MPPT tracking.
Another recent reference design with bq25570 is an energy harvester booster pack reference design TIDA-00588. This design can harvest energy from a wide variety of current sources or from the onboard solar cells to power any low power TI LaunchPad. There are three storage methods on this board, including a 47mF super capacitor, a LIR2032 coin cell, and an external battery connector.
Aside from solar harvesters, TI also has a reference design for thermoelectric generators. This TIDA-00246 is a fully programmable state-machine which runs at 60nA and can enable and disable key functions as they are needed by the system to optimize power consumption.
TI’s nano-power energy harvesting technologies allow the most efficient low power designs today. With several devices that feature low quiescent currents and >90% conversion efficiency, TI enables self-powered and battery-based devices across many different applications using solar, thermal-electric, piezoelectric and other energy sources.
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