Maurizio Di Paolo Emilio is the author of Microelectronic Circuit Design for Energy Harvesting Systems, a book covering the design of microelectronic circuits for energy harvesting, broadband energy conversion, and new methods and technologies for energy conversion. Di Paolo Emilio recently talked with Electronic Design about energy-harvesting technology, its advantages, and its challenges, as well as current trends and future predictions for this space.

MG: How important is the role of energy-harvesting in IoT?

MDE: In recent years, much attention has been placed on the Internet of Things (IoT), or a whole range of commercial and industrial devices (IIoT) interconnected by wireless (and wired) protocol. Analysts estimate an increasing number of IoT devices—about 2 billion over the next five years. The advent of these devices poses a serious powering problem: the batteries that have to be purchased, maintained, and disposed of.

The energy harvesting technique is a simple solution to recharge low-power devices easily and economically, at the same time, by using clean energy. A fundamental requirement for IoT is the power management: Mobile devices obviously require batteries, but the possibility to replace them completely or restrict the replacement/recharge is a considerable factor of importance, given the advent of further devices connected in the near future.

Energy harvesting technologies can help. They use electric power generation elements such as solar cells, RF sensors, and piezoelectric and thermoelectric elements for converting light and vibration. Furthermore, when using energy harvesting, there is a point to be considered—a balance between generation and energy consumption. This is because the device does not work if power generation is less than the required power.

Maurizio Di Paolo EmilioMG: What’s the advantage of microcontrollers in energy-harvesting systems?

MDE: Integrated power management circuits designed for energy harvesting, as well as low-power MCUs, will help the growth of the Internet of things. Energy harvesting is a valid (but not a simple or easy) option for the IoT recharge design.

Although the generation of characteristics of the energy production elements is improving from year to year, and microelectronics continues the step of an ultra-low power management, it is difficult to continuously provide enough power for a device on an ongoing basis, and the need of a collection technique in a first phase could help.

The progress in microcontrollers with low consumption decidedly opens the target for products and applications in which a collection of light, kinetic energy, or heat can be used to power an intelligent product without an external power supply or battery replacement. The microcontrollers have a fundamental role in all electronic devices. This improved fuel consumption and power management methods are a design factor to allow easy adoption of the energy harvesting.

MG: What’s your take on powering microsystems? Why are they important? What are the most common wrong assumptions made by energy-harvesting designers?

MDE: The power conditioning circuits play an essential role in an energy harvesting system through various parameters (such as the input impedance), at the same time carrying out processing functions (such as power control and filtering). Advanced techniques actively influence the behavior of harvesting devices (such as piezo pre-biasing). The power limit of a system to be used was considerably reduced with conditioning circuits that operate at lower levels of power, by reducing the losses to increase the maximum efficiency of the harvesting system.

Power supplies are often intermittent and the excitation parameters may change over time. The purpose of the conditioning circuit is to avoid an oversized design, with a storage system for providing a correspondence between the temporal profiles of the power demand from the load source.

The challenge is always to optimize the energy and the associated conditioning circuits to cope with a system where the correspondence of the power profiles and operation dynamics hours is in some way optimized.

MG: What are the major limitations of energy-harvesting systems nowadays?

MDE: One of the many challenges for the designers is to assess the energy factor, estimating the energy requirement and designing its power configuration. The fundamental requirement of IoT is power management. In many application scenarios of low-power devices, it is difficult to obtain a continuous active mode of energy, resulting not only in electronics but also the environmental situations (low light). Then a method of accumulation, even within the tiny size—smaller than the general cases—is still necessary to ensure the proper functioning of a device. The recent trend is to the replace the rechargeable batteries with super capacitors characterized by charge-discharge "cycles unlimited" (> 100,000).

Di Paolo Emilio book coverMG: What are some of the energy-harvesting applications available in the market?

MDE: Energy harvesting makes use of ambient energy to power small electronic devices such as wireless sensors, microcontrollers, and displays. Typical examples of these environmental sources are sunlight and any artificial source such as vibration or heat from engines or the human body. The energy transducers such as solar cells, thermogenerators, and piezoelectrics convert this energy into electrical energy. A first field of application is the automation with self-powered switches. Further applications are the monitoring systems for large industrial plants or structural monitoring of huge buildings. Another promising market is the consumer area with purses and clothing, which show the energy transducers integrated in the form of solar cells or TEG or RF transmitters to recharge consumer products such as mobile phones or audio players.

The application of energy harvesting techniques in the railway sector is a very promising field. They conducted studies and experiments on the potential of energy recovery devices, with the aim to provide information on the electric power actually generated by the use of harvester placed on board railway wagons.

One of the emerging market segments covered under the IoT is the wearable electronics category. Regardless of the application, most of these devices require a battery as the main power source. The rapidly expanding of wearable devices is set to revolutionize the technology and current processes in all areas by creating new market opportunities and new business models.

MG: What are the most promising energy-harvesting technologies? How about RF wireless power technology?

Solar radiation has the advantage to be a green energy without the production of polluting waste. The limits that may be encountered are discontinuities caused by the alternation of day and night and weather conditions. The other is the low intensity, which implies the need to have large areas of energy storage.

Many quantities of energy harvesting devices are currently sold in the building automation industry and in the consumer market. In the consumer market solar cells represent the main choice, but more solutions in terms of vibration and RF are entering the market. The development of IoT promises to be exciting and have a great economic impact due to the development of semiconductor devices, and to the advancement of wireless technology, by allowing devices that are smaller and more efficient at the same time.

The RF solutions represent a harvesting technique to permanently remove the batteries from smartphones and ensure the auto-recharge by RF sources themselves. The wireless charging technology has demonstrated perfectly how it is possible, but this time you have to consider the possibility of gaining energy from the environment. With the advent of IoT, we are bombarded by many secondary RF sources, and then we will have more chances to get enough electrical current from the surrounding environment. The power supply is definitely the future of energy harvesting!