“Costs, manufacturing processes and markets have constrained the development of this technology, and it has been more than a decade since energy harvesting systems were touted as the best approach for consumer electronics and devices. Solar, wind and hydroelectric technologies have proven successful in powering large factories and even cities, but very little in powering small devices.
This article is compiled from: Semiengineering
After several years of stagnation, energy harvesting technology is seeing a new trend in certain markets.
Costs, manufacturing processes and markets have constrained the development of this technology, and it has been more than a decade since energy harvesting systems were touted as the best approach for consumer electronics and devices. Solar, wind and hydroelectric technologies have proven successful in powering large factories and even cities, but very little in powering small devices.
“I always thought it looked promising, but it never really caught the eye,” said Joe Ward, senior director of North American sales and business development at e-peas. “But I think it’s becoming more and more important now. To really implement energy harvesting technology, you have to realize the pain of battery systems. Customers need to have real trouble with life, replacement, disposal.”
Although not universally adopted, energy harvesting is being performed today in some specific applications. Consumer applications may be limited to wearables, but certain industrial markets will rapidly drive forward as long as it pays off.
Arm Distinguished Engineer James Myers said: “The prospect of energy harvesting microcontrollers that can run forever without maintenance remains exciting. But many challenges remain and adoption has slowed.”
“Sensor networks that do not require wiring and do not require battery replacement are the most promising systems,” said Christian Bretthauer, Principal Engineer at Infineon’s PSS RF division.
At least that’s the promise. “Apart from the usual small-area solar/photovoltaic power generation, some thermoelectric generators and large electromagnetic vibration harvesting devices, there are currently no energy harvesting systems that can power the industrial IoT and wireless sensors in trains.” Inviza CEO Robert Andosca Say. “Nobody has deployed MEMS-based technology at scale.”
How to define energy harvesting
Energy harvesting refers to the ability to harvest operational energy from some aspect of the operating environment. It can come from light, temperature differences, vibrations, RF waves or many other physical phenomena. Usually, only low power circuits have enough energy.
Figure 1: Energy harvesting is suitable for low-power circuits, ideally without battery replacement, but a more realistic effect is to extend battery life.Source: Renesas
There are qualitative differences in the way energy is generated. One of them involves real harvesting of free environmental energy. Solar energy is a good example. Otherwise, the captured energy will be wasted.
A slight variation on this might be thought of as energy “stealing,” ie, drawing energy from other parts of the system. For example, one might use tire deformation to power a tire pressure sensor. Here, the rotation of the tire comes from the electric motor which has the main fuel or energy source, so this consumes a very small part of the energy. Theoretically, this could increase tire friction and increase the load on the engine, but such an effect would not be noticeable. Energy isn’t strictly “free,” but it’s close enough to be considered free.
A common form of energy harvesting is large-scale photovoltaic (PV) technology. This would harvest energy from the sun, but it’s a very different market compared to other energy harvesting worlds, which tend to consume only a small amount of energy to run small sensors and other circuits. Therefore, for practical purposes, PV for the grid is generally considered to be separate from energy harvesting.
Energy harvesting system resurrected
Energy harvesting is like MEMS and sensors were a decade ago. This is partly driven by the idea of using MEMS technology for energy harvesting. However, energy harvesting can involve many technologies besides MEMS. They all fuel excitement and technological thinking, and spawn companies that never really get the spotlight.
The promise of energy harvesting was (and still is) the ability to equip sensors and other devices that will be installed in remote locations with the ability to harvest electricity from the surrounding environment. The real benefit is that there is no need to replace the battery on a regular basis. These devices will be self-powered, greatly reducing their management costs.
However, one of the main barriers compared to batteries is the cost of energy harvesting devices. “For consumer electronics, they’re talking about sub-dollar or sub-dollar products,” said Ken Imai, senior manager of the product marketing, IoT and infrastructure business unit at Renesas Electronics. “Energy harvesting systems cost three to five times more.”
For consumer applications, device makers simply cannot demonstrate whether price-sensitive devices can afford the increased cost. Yes, for smoke alarms, don’t loudly change new batteries at 2am. But if energy harvesting is used, consumers may pay for it. “I can do this with a 50-cent battery, or I can save $3 to replace the battery,” Ward said. “In the end, cost still wins.”
So the real cost to the customer made the technology never work out, so most things were put on hold.
This is especially true of MEMS-based energy harvesting, which is typically harvested on silicon wafers using circuit-building techniques. As with chips, the number of finished dies per wafer drives the economy. The typical solution to circuit problems is to continue shrinking the chip size to smaller sizes. But the physics of energy harvesting requires a certain size to achieve good power density.
“For the most part, any energy harvesting is area-dependent,” Andosca said. “You can only shrink them so far. Then, you reach the physical barrier of the power density limit.”
This limits how small the energy harvesting device can be. With expensive silicon processing technology, it’s hard to keep those numbers down. In one example, the cost of a cantilever used to collect vibrations is at least $10 when forward predicted cost reductions.
Using other materials such as glass as a substrate may help, large format processing is another option. “The flat panel Display industry uses the same precise technology that the semiconductor industry or the MEMS industry uses to make sensors,” Andosca said. “They just use different sized substrates and some different types of substrate materials.”
Eliminate or extend battery
Interest in energy harvesting appears to be quietly reviving these days, especially in smart cities and industrial markets. One reason is that batteries are draining faster than expected, and replacing them is getting more expensive. This brings the break-even point closer to using energy harvesting.
Ward of e-peas explained: “People are realizing that the battery life is not going to be longer than stated. For some reason, they just can’t reach the end goal of five-year battery life. Replacement costs are going up because you need to go in Truck chassis or sending someone to replace the battery. If you’re going to use thousands of these sensors, it’s a nightmare. Then they have to dispose of the dead battery.”
One of the challenges is that batteries fail at different times. “Some of these batteries may last four years, but customers have to start planning for the first failure very early, which cuts the expected product life in half (or more),” he added.
Energy Harvesting System Composition
There are two key parts to an energy harvesting solution. Transducers are true generators and come in many different types. Some of them, such as thermoelectric generators (TEGs) and PV cells, generate DC voltages. Other devices, such as vibration collectors, will generate alternating current that requires rectification for use.
“PV remains the easiest, cheapest and most reliable option for outdoor applications,” Bretthauer said. “Vibration collectors can be interesting on all mobiles. Thermoelectric generators are also on machines that generate a lot of heat. Might be a good option.”
The second part of the solution is the power management chip or PMIC. e-peas calls this an Environmental Energy Manager (AEM). While it is possible to rectify and transfer voltage to the transducer, this will manage the power produced by the transducer, which requires a clean power supply at its input. There can be different versions for different transducers, as the physics of the transducers determine the voltage they produce.
Figure 2: Energy management helps use harvested energy to power circuits and direct excess energy to storage. Source: e-peas
Even though energy harvesting is mainly due to its battery-free nature, building an energy-harvesting circuit that is better than break-even has been a challenge. These circuits themselves require power, and if they use all the power being produced, they won’t work. A circuit can only function properly if it can handle power management and the remaining power is sufficient to handle the operating load.
This can be difficult, depending on the type of load being driven. In many cases, energy harvesting does not completely remove battery power, but rather replenishes it. It extends the life of the battery. The job of the PMIC, then, is to use the harvested energy first and deliver it to the battery if necessary.
In such an arrangement, there will typically be a second form of energy storage. Energy harvesting systems will need to store the energy they produce to decouple energy production from consumption. This may require supercapacitors or rechargeable batteries (compared to “primary” batteries, also known as “secondary batteries”).
Where power requirements are higher, supercapacitors are better at capturing and releasing stored energy faster. Batteries have higher energy density, but not as much power density. The quality of supercapacitors has been significantly improved, reducing leakage to levels well below that of standard capacitors.
While some energy harvesting devices are implemented as separate units, they can also be combined with other common circuits. Renesas Microcontrollers (MCUs) with energy harvesting management functions that can power the MCU as well as other circuits. The VDD pin can usually be used as a power supply for the input or as an output. The company is creating an ecosystem for a variety of different transducer types.
Figure 3: Energy harvesting MCU.Source: Renesas
One of the most responsible operations from an energy standpoint is radio communications. But Renesas says it can power many low-power wide-area (LPWA) protocols. Specifically, the company says that customers have actually installed LoRaWAN and LTE Cat M1 radios.
This is unlikely to work for high-power protocols, which also don’t make sense for ultra-low-power applications. “You really don’t need to send and receive a lot of data,” said Ashraf Takla, Mixel’s president and CEO. “Usually, you’re communicating slowly changing control signals, possibly between many different sensors and collection devices.”
Dynamic tuning and data buffering may also be required if the power supply is inconsistent. Imai added: “If we occasionally limit the energy input to the harvesting system, we can reduce the energy release by limiting the amount of radio communications and keeping the data in on-chip memory. When we have enough energy input, we can transmit the storage The data.”
application is important
Due to cost challenges, energy harvesting has focused more on applications where batteries are expensive. Wearables are a consumer application area of interest to consumers. The Casio G Shock watch is one example of using energy harvesting, and clothing would be another.
For wearables to be successful, the technology must be invisible, including generating electricity. “I hate charging things, and I don’t charge my pants,” Andosca said.
The idea is to get energy from the flexing of the belt, the pressure on the foot, or any other movement you perform. Thin packages may help piezoelectric transducers, forming so-called benders.
Agriculture brings other opportunities. One example is the use of motion sensors on cow ears, which can help alert ranchers when a cow is in heat. This is a clear example of a sensor that offers a lot of utility but has serious limitations when it comes to changing batteries. Similarly, stationary sensors for soil conditions will benefit as they will be placed in remote areas where maintenance is expensive.
There are also some industrial equipment that can benefit from harvesting. While most heavy industries have huge wall-mounted power availability, being able to install sensors or devices in hard-to-reach locations without the need for a nearby plug provides convenience and flexibility. Imai mentions an Electronic faucet that, for example, can use a turbine in a water pipe or the temperature difference between hot and cold water to generate electricity.
Healthcare offers more opportunities for energy harvesting. “Everyone wants to seal the device so that no wires or batteries are exposed,” Ward said.
Other outdoor applications include environmental sensors such as weather stations and structural health sensors. The latter can be used to help maintain critical infrastructure such as oil and gas pipelines or bridges. Sensors can be installed in many hard-to-reach places, making battery replacement impractical.
Necessary battery life also varies by application. “Certain agricultural products require six months of continuous monitoring, whereas applications in smart buildings or smart cities take several years,” Ward said.
Automotive is another area where energy harvesting systems can help drive vehicle electrification. “When you compare a consumer electronics product or a building to an energy harvesting system in a car, it’s completely different,” said Puneet Sinha, head of Siemens EDA’s Mechanical Analysis Division.
Kinetic energy recovery systems represent the most sophisticated approach in today’s hybrid vehicles. “Since 2009, Formula 1 has been using a kinetic energy recovery system (KERS or ERS-K) to convert kinetic energy into electricity using an electric motor/generator set,” said Robert Schweiger, director of automotive solutions at Cadence.
He added: “Energy harvesting and recovery can also be done by converting the kinetic energy released by the shock absorber. And there have been attempts to use the deformation of the tire to power the pressure sensor.”
But further refinement may be required before deployment. “These applications (except regenerative braking) are still in the research phase,” Sinha said. “They need to be efficient, but more importantly, they need to be business-wise about how much extra cost has to be added to the system and the benefits of that.”
Although electric vehicles generate much less heat than internal combustion engines (ICs), another possibility is to exploit temperature differences. “Batteries are much more efficient than internal combustion,” Sinha said. “In an ICE, the maximum efficiency is about 35 percent, and the battery is almost 90 percent efficient. You’re not taking that much heat, so you have to reduce the heat dissipation, as well as reduce the heat consumption.”
In another example, a battery that doesn’t work well at low temperatures can be heated to improve performance. “Some companies want to put a converter in the battery pack to take the heat and use it for battery heating,” Sinha said.
While these are helpful attempts, they won’t be a breakthrough in driving range. “They can make a difference of a few percent, but they don’t travel more than 250 to 500 miles,” warns Sinha.
In the automotive world, energy management algorithms are perhaps more important than any other application. “Think about the Tesla Model S and the Audi e-tron,” Sinha said. “They’re the same size and class, and the battery power is the same in both. However, one car has 30 to 50 percent more range than the other.”
He continued: “Going forward, more and more companies are investing in updating software to improve the efficiency of extracting energy from batteries.”
Looking to the future
There is little progress in these technologies, and in fact it is still in the early stages. As product launches and market development take place over the next year or two, we may see energy harvesting finally reach levels of commercial viability that were not available in the past.
That said, the energy density of batteries has tripled over the past decade while costs have continued to decline. This makes the economics of energy harvesting a moving target, one that gets harder and harder over time.
In summary, Arm describes three key challenges faced when studying energy harvesting. “The first challenge is about application specificity,” Myers said. “You can’t use a solar cell in a dark cellar or in a thermal system that doesn’t have a temperature gradient. Second, there is variability in power. With a small 1 cm2 cell, You might harvest milliwatts outdoors, but only microwatts in office lighting. Third, low power density: The power output of an energy harvesting system is significantly lower than a similarly sized battery.”
But for applications that can meet these three challenges simultaneously, energy harvesting appears promising.