Bench Talk

(Source: Alberto Masnovo - stock.adobe.com)

Energy harvesting uses naturally occurring energy to extend the available power beyond the limits of what finite energy sources, such as batteries, can provide. This approach improves the overall efficiency of the energy source, lengthening the time between charges and improving device performance from unreliable renewable energy sources. The movement toward more sustainable solutions motivates design engineers to move beyond these finite energy sources toward a fully renewable solution: solar cells. Also called photovoltaic (PV) cells, these devices convert the sun’s radiative energy into usable electrical energy in one step through the photovoltaic effect. This effect contacts positively and negatively charged silicon to create an electric field.

These independent microgenerators power local loads exclusively to reduce demands on the grid system while also cutting owners’ utility bills. Applications of solar cells include “off-grid” homes and smaller loads such as roadside electronic signage and IoT equipment—including sensors and actuators—that install remotely and connect wirelessly. Because the energy source of solar cells is free, the ideal state of this type of energy is to “install and forget” it, using the sun as an infinite energy source reservoir. However, engineers must overcome several challenges to realize this approach in the IoT.

The most direct challenge is whether the energy balances demand at various use conditions. Short- and long-range IoT sensors could require between 7 and 16.4uA of energy on average. Design engineers optimize PV cells for a given energy source and add capacitors to reduce dependence on auxiliary power or complete bridge dark periods. The capacitors, in turn, are optimized to minimize leakage current. And designs also ask for a sensor interface for sampling data, storing, and transmitting information.

Fortunately, an innovative product solution and a fundamental understanding of the details of the challenges can solve these problems.

The DC-DC converter receives power from the photovoltaic array and produces electricity available to power devices in a small solar micro-generator. Two of the critical performance metrics are high energy efficiency and power quality. These metrics are essential because the user expects a small device that delivers consistent performance. However, the variability of solar energy supplied to the PV cell presents three principal challenges for self-sustaining photovoltaic IoT sensors:

• Insufficient input energy through PV radiation
• Limited storage and drive capability through the capacitor
• Excessive peak current occurring too frequently through sensor sampling, data storage, or transmission device

Other challenges are temperature limits, voltages that do not fit, conversion loss, energy discharge at the wrong times, and aging/degradation of the sensor. With these inefficiencies and losses, the power supply is unstable and may contain sag or ripple.

In addition, solar energy is only supplied part of the day and enters the photovoltaic cell at varying intensities. These inconsistencies further reduce the IoT device’s performance quality. Challenges with low or consistent input energy coupled with limited storage and excessive peak current create the need for a backup solution to level the energy load.

A PV energy harvester can provide this critical backup/augmented power using an advanced supercapacitor like the Hybrid Storage 196 HVC ENYCAP^™ Capacitor. ENYCAP is a hybrid storage device, meaning it stores both electrostatic and electrochemical energy that can supply backup power. With robust voltage flexibility from 1.4V for single cells up to 8.4V for multiple, the ENYCAP is available in radial (stacked through-hole), surface mount flat, or lay flat orientations.

It is polarized with high capacitance and offers a high 13Ws/g energy density to minimize package space. The capacitor is also durable up to 1000 hours of run time at 85°C without maintenance or service. In addition, the 196 HVC ENYCAP does not require cell balancing, which is a time-saving advantage over traditional supercapacitors, and it contains a non-hazardous electrolyte. It also exhibits a lower self-discharge when compared with existing market supercapacitors.

The 196 HVC ENYCAP is more than a traditional capacitor suitable for energy harvesting. It is a hybrid energy storage capacitor, a backup system for applications such as miniaturized systems, memory controllers, SRAM/DRAM, cache protection, industrial PC/controls, and emergency lights and micro UPS power sources.

The ENYCAP can provide up to 2mA of harvest power with ultra-low leakage in the IoT sensor application described above. This performance level minimizes reverse current to improve performance and efficiency.

In light of the growing market share of renewable solar power across consumer and industrial electronics, device manufacturers must ensure that their products consistently deliver high performance. Moreover, designers need to consider renewable energy's variability as well as its primary benefit of free, near-infinite supply. Incorporating an energy harvesting enabler like the Vishay 196 HVC ENYCAP hybrid storage capacitor provides a high-performing hybrid storage solution and solar cell backup.

Adam Kimmel has nearly 20 years as a practicing engineer, R&D manager, and engineering content writer. He creates white papers, website copy, case studies, and blog posts in vertical markets including automotive, industrial/manufacturing, technology, and electronics. Adam has degrees in chemical and mechanical engineering and is the founder and principal at ASK Consulting Solutions, LLC, an engineering and technology content writing firm.