Energy Harvesting

[From the last episode: We saw how wireless powerTechnology that allows a battery to be charged from a small or large distance, over the air, without having to connect a charger. may make it easier to charge IoTThe Internet of Things. A broad term covering many different applications where "things"  are interconnected through the internet. device batteries.]

The number and kind of opportunities for IoT devices is incredibly diverse. As we’ve seen, some get their powerThe rate of energy consumption. For electricity, it’s measured in watts (W). from the wall, but many rely on batteries for their power. And there’s one thing about batteries: they drain.

Yes, some of them are rechargeable. But that means you need a way to charge them, which typically means plugging into wall power somewhere along the way. And recharging circuits add to the cost of a device.

So what happens if you’re deploying, say, sensorsA device that can measure something about its environment. Examples are movement, light, color, moisture, pressure, and many more. on a bridge so that you can keep track of the structural health of the bridge? This is an important growing area, but clearly you’re not going to be plugging it in on the bridge. It’s going to need a battery.

For simplicity’s sake, most folks just put – say – a coin cell in there that should last 5 years. I mean, who the heck knows what’s going to be happening in 5 years? That’s an eternity in the electronics world.

But here’s the thing: apparently, batteries aren’t lasting as long as expected. That could be because of variability in how much energy they can store, or perhaps the circuit folks underestimated how much energy they’d use. It does vary from sensor to sensor, so the battery may at least be part of it. And if the battery may drain in 2 years instead of 5, you kind of have to plan on that.

And when that time comes, someone needs to go out onto the bridge and replace the batteries. That’s expensive.

Better than Batteries?

There is an alternative – one that’s gotten going by fits and starts. That alternative is to use what’s called energy harvestingTechnology that can grab energy from the environment for local use with low-power devices.. The idea is to pull energy from some aspect of the surroundings. A small solar cell might work for outside stuff. Or you can take advantage of temperature differences: if there’s a hot side and a cold side, you can use that to generate some energy. Or maybe vibration could be used – there may be vibration on the bridge as traffic crosses.

There are many different options, and they all have their strengths and weaknesses. One thing is for certain, however: none of them generates gobs of energy. This is for super-low-power sensors and other circuits.

The original vision is to have these sensors be completely self-powered using harvesting. That has proven hard to do, however. So what might be more common now is that the harvesting supplements the battery. You have a management circuit that controls the energy flow, and it uses the harvested energy where it can. If that’s not enough, then it pulls from the battery.

In this case, you still need the battery, but because it’s got some help in the harvesting department, it can last a lot longer.

There is one other challenge when doing this, and anyone that has followed the renewable-energy field will be familiar with this: The energy isn’t always generated exactly when you need it. If you use solar, for example, you still need the sensor to work at night.

A Place to Store the Energy

So you do need some kind of energy storage so that you can stow away all the energy you can while it’s available, and then you can pull from the storage when you need to for the circuits. The question is, what kind of storage should you use?

There are, broadly speaking, two different choices. One is obvious: a rechargeable battery. Here you would need to add that charging circuitry to the systemThis is a very generic term for any collection of components that, all together, can do something. Systems can be built from subsystems. Examples are your cell phone; your computer; the radio in your car; anything that seems like a "whole.". The other may be more unexpected: it’s a thing called a supercapacitorA type of capacitor that can be used for storing energy..

We’ve seen that capacitorsCapacitance is the ability of an electron or hole to “feel” other electrons or holes on the other side of a small gap. The gap prevents actual current from flowing, but, if small enough, electrons and holes can pile up on either side of the gap. Specific devices that make use of this are called capacitors. store charge, typically for a short while, and we would typically use them to smooth out varying voltagesVoltage is what gets electrons to flow. It's analogous to water pressure, which gets water to flow. Voltage is measured in units of "volts." or filter out noise. But those capacitors are relatively leaky: when you put energy in them and don’t use it, it leaks out.

Supercapacitors are like regular capacitors on steroids. And more recent technology has made them leak way less than in the past. So they can hold energy for long enough to be useful in energy harvesting.

So which do you use? Battery or supercap? Here’s the thing: batteries can store more energy in a given space. But they’re slower to charge or give the energy back out. Supercaps are super fast – you can charge and discharge them very quickly. But they can’t hold as much energy as a battery. We express this by saying that batteries have greater energy density, while supercaps have greater power density. And that, along with the circuits and application needs, will determine which gets used.

Ooo, That’s Gonna Cost Ya…

Of course, then there’s the other bit of reality: cost. Coin-cell batteries are super cheap. Energy harvesting, once you add things in, can cost five times – or even more – than that coin cell. So anything that can work with a coin cell and has to be cost-competitive will go with the coin cell. No harvesting. It would be nice not to have to change the battery in the smoke detector, but folks aren’t going to want to pay extra for that.

Where energy harvesting makes the most sense is where it’s expensive to replace a battery. Like the bridge example, where trucks must roll. That’s where energy harvesting pays for itself.

There is one other consumer area where it might work: wearables. Clothes with built-in sensors and such that you don’t want to have to charge. There’s lots of body motion that can be harvested, so that may be something we see in the future.