Design challenges when using energy harvesting – Virtual Roundtable (part 1 of 2)

The first part of EEWorld’s two-part “virtual roundtable” discussion on energy harvesting considers technical challenges and design tradeoffs for engineers using energy harvesting power sources. Joining us for this virtual roundtable are: Dr. André Mansano (AM), CTO at Nowi; Graeme Clark (GC), Marketing Manager, Energy Harvesting Business Unit, at Renesas Electronics, Ken Imai (KI) Sr. Manager, Product Marketing, IoT and Infrastructure Business Unit, also with Renesas Electronics; Eric Biel (EB), Principal Engineer at Powercast; and Alessandro Nicosia (AN), Product Marketing Engineer for Energy Harvesters with STMicroelectronics.

JS: What is the biggest challenge engineers face when first using energy harvesting power sources?

GC & KI: The biggest challenge for many engineers is the need to consider exactly how their application needs to work in a low energy environment, what functions are really required and when they are required, to minimize energy use and to maximize system reliability.

AN: The energy harvesting power sources market is very heterogeneous as any harvesting source has its own characteristics in terms of the input-output electrical curve like shape, working point at maximum power, MPP (Maximum Power Point) ratio to maximum Vout/Iout, minimum generated power, sensitivity, conversion efficiency, etc… All those factors will influence the designer to make one choice or another when selecting the most appropriate source. Once done, the biggest challenge of an engineer would be using the best technology and implement the most optimized harvester to maximize the energy extraction by approaching the MPP as close as possible, along with keeping the power losses as low as possible. In such a scenario, the power source technology is fundamental because from it depends on the minimum and maximum power that can be harvested, the capability of switching on the given application, and the algorithm that must be designed to implement the MPPT (Maximum Power Point Tracking), that is the maximum power transfer between input and output. Typically, this is achieved by using the Impedance Matching Principle, that ensures the full transfer of the harvested power to the load and the maximum extraction of energy from the source.

AM: The biggest challenge is the application environment. Harvesters are very sensitive to their environments, and it is not always possible to have stable conditions that are very different from one application to another. Moreover, even for the same application, the ambient conditions can vary a lot in a short period of time or from one position to another. Making estimations based on average conditions can work for some applications. However, it is not always the case. Therefore, it is very challenging to make accurate estimations for selecting the suitable harvester.

EB: Let me preface my answers by explaining that Powercast, established in 2003, specializes in harvesting radio frequency (RF) energy as the power source to create long-range over-the-air wireless power solutions.  These RF-based wireless power technologies work in the far-field (up to 80 feet) to provide power-over-distance, eliminate or reduce the need for batteries, and power or charge devices without wires and connectors.  We were founded with the vision of enabling untethered devices powered over the air in all kinds of environments, including industrial, commercial, consumer, and public health.

There are two main hurdles we seek to overcome with our customers when implementing a wireless power solution: availability of an adequate RF transmitter power source and the receiver antenna design.

First, if there is not enough RF power available in a certain environment, then harvesting is simply not a good option. If the power isn’t there, then the device can’t harvest it – it’s that simple. We almost always recommend that a Powercast transmitter or another reliable source of RF power (such as an RFID reader) be installed within range of the device to ensure that it will always be receiving enough power to charge or operate.

Secondly, the RF power receiving antenna configuration is crucial. Without a high-performing receive antenna, where it is tuned and oriented correctly in the end device, available RF power can go un-harvested. Factors to consider when designing an energy-harvesting antenna that will deliver excellent results include the enclosure material, circuitry design, PCB dielectric material and thickness, the presence of nearby metal as well as the RF harvesting frequency itself.

JS: What is the most misunderstood aspect of energy harvesting? How does that translate into specific design challenges?

STMicroelectronics SPV1050 is an ultra-low-power energy harvester with an integrated battery charger that helps designers achieve high efficiency and small form factor.

AN: Usually, most people believe that energy harvesting must fully replace the mains in terms of continuity and power capability. Indeed, given its intrinsic prerogatives, the real goal of an energy harvesting source is as an add-on of the main power source to get the fastest battery charge in order to prolong the system’s autonomy as much as possible. In other words, it has to be considered an alternative power outlet but only when available, and, with limited power capability. Taking the above considerations into account, we can then say that from a certain standpoint, the design challenges derive from the assumption that the design engineer must find out how to grab any uW achievable from the source. This means timely checks of the changed source’s working conditions, tuning the output stage impedance to track the new maximum power point, minimizing the losses when the algorithm is not active, and allowing the application start-up with the worst energy-wise working conditions within the expected time frame and the rules dictated by the system design.

AM: The most misunderstood aspect is the importance of using power management ICs combined with the harvester to provide the best performance. One of the main blocks of energy harvesting solution is the DC-DC converter, which plays an important role in adjusting the voltage level provided by the harvester. Without a DC-DC converter, it is difficult to guarantee a suitable voltage level for the rest of the circuit. There are several types of DC-DC converters varying between boost, buck, and buck-boost, which can be combined with different types of harvesters.

Also, unlike conventional PMICs for constant power sources, most energy harvesting PMICs have an algorithm to detect and track the maximum power delivered by the harvester in an uncontrolled environment. This feature is known as maximum power point tracking or MPPT, and it significantly increases the total efficiency of the energy harvesting solution despite the quick changes in the application environment.

EB: We think one of the biggest challenges is getting people to understand the capabilities and limitations of RF energy harvesting as compared to traditional powering/recharging of devices. People are used to using and charging their devices in a certain way – running the battery to close to zero and then either replacing the battery or manually plugging it in to charge. With RF energy harvesting, devices can continuously receive low amounts of recharging energy, constantly “topping-off” the batteries to give back the energy that the device has used. In some applications, it’s even possible to completely eliminate the need to EVER manually recharge a device’s battery. This constant but low recharging energy type of operation is much different than what most people are used to, and getting people to understand it can be challenging.

Most designs require some level of customization to achieve good RF energy harvesting performance, and designers need to pay close attention to everything, including the antenna design, circuit or layout, and even the device enclosure. In energy-harvesting designs, typical circuit design decisions (such as board thickness, dielectric material, trace width/spacing, etc.) become much more important to the overall harvesting performance when compared to non-energy-harvesting designs. These things can prove challenging and sometimes offer a slight barrier to entry for traditional designers.

GC & KI: Often engineers think they can just replace the battery with an energy harvesting power source and the system will work in the same manner, in most applications the reality is a little different as power is often unreliable and we have to manage the application on the limited energy available. This means we must carefully balance the real requirements of the application with the energy available, storing the energy we generate from the harvesting source until needed, and we often need the ability to respond to changing energy levels in the environment.

JS: How would you describe the tradeoffs between energy harvesting and small-format primary batteries?

GC & KI: Small format batteries may often be cheaper and can result in smaller form factors, and the use of these is well understood. However, many of the materials used in such batteries can cause issues with shipment and safety. The use of energy harvesting power sources can remove or at least minimize these concerns. Energy harvesting power sources can result in a significantly longer product life and reduce or remove the need for human intervention, no need for battery recharging or replacement.

AM: Primary batteries are available in different energy capacities, shapes as well as being commercially available at very low prices, which make them widely used for many applications. However, replacing primary batteries can be a major issue for some applications where the labor cost required can be higher than the cost of the batteries. Moreover, the energy required to manufacture batteries is much higher than the energy stored in batteries as a final product, which also adds more challenges in terms of sustainability and the need to recycle these products.

On the other hand, energy harvesting is relatively higher in terms of cost because of the need to have a harvester and a PMIC besides the storage unit. However, if we take into consideration the added value to the application (for ex: avoid replacing batteries), then the total cost of energy harvesting can be comparable to the conventional solutions based on primary batteries, especially in large volume production applications.

AN: Energy harvesting and primary batteries constitute two opposite trends: the second of which was the only one until about 20 years ago or so. The problems related to the waste of batteries and its impact on the environment imposed a different direction driven by the need to minimize the heavy and dangerous metals ending up in landfills. At the same time, system power needs have dramatically increased, leading to the necessity of changing the primary battery many times. The described trend is common in both industrial battery-powered applications as well as the typical consumer ones. As a direct consequence, this would mean much higher maintenance service costs, much more frequent shut-down periods, loss of production and/or of service provided to the user. As typical examples, look at IoT sensor nodes used for industrial monitoring, or asset tracking nodes based on GPS and RF transmission. Also, in consumer applications, we are seeing many more power-hungry systems, enriched time by time by new functions and a more complex firmware with higher computation power and a faster clock for peripherals management. For all the above reasons, we assist the current trend to a strong and fast ramp-up of energy harvester devices and see secondary batteries replacing the primary ones more and more.

EB: Small-format primary batteries typically require the user to either replace them and/or take the device out of service after the battery has died. Instead, energy harvesting can constantly recharge a system’s battery, thus eliminating the device’s downtime and removing additional user action from the equation.

The friction factor is the availability of RF in the environment – if an RF transmitter isn’t within range of the device, then limited or no harvesting will take place. If no harvesting is taking place, then the addition of a wireless power solution simply won’t be useful to the end user. The biggest tradeoff lies between known technology vs. new technology. The user must learn how to adequately install/use a device that has been outfitted with a power harvesting solution, and understand that it doesn’t function the way a typical device with a primary battery does.  Once that hurdle is overcome, then energy harvesting devices will be able to overtake or help supplement devices with small-format primary batteries in most cases.

JS: Thank you to our panelists for their insights into technical challenges and design tradeoffs for engineers using energy harvesting power sources! You might also be interested in reading, Energy Harvesting Applications Considerations – Virtual Roundtable (part 2 of 2).