What is Thermoelectric Energy Harvesting?

The world around us is in flux - the last two decades have seen technological progress steam to a breakneck pace. Just look around yourself at your room and think about what it looked like in 2010. It’s truly astonishing to see how central a position technology has taken in our lives. This is no coincidence and by no means is it an aberration - human progress has always been driven by faster and more efficient means of data processing. 

A few hundred years ago, it was the invention of the printing press by German inventor Johannes Gutenberg, that catalysed the spread of enlightenment ideas in Europe. It allowed information to be circulated faster and to more people than ever before. We all know what that enabled in terms of elevating standards of living and slashing poverty rates. 

Today, we stand at the precipice of yet another revolution, possibly just as monumental in it’s promise - The internet of things. IoT is possibly a term you have encountered quite a lot in recent times and for good reason -  it is starting to feature more and more in our daily lives and this trend is only set to continue. This 21st-century trend is no different, in essence, to the printing press and why it became such a catalyst for change - Both these trends are about data transfer. They’re about increasing the reach and penetration of information. 

IoT technology allows us to get things done better, faster and more efficiently. It does this by allowing us to harness the power of massive data collection and fast processing and transfer. Today IoT technology powers literally hundreds or even thousands of everyday applications around us - one barely resembling the other - But, all these disparate applications are based on the simple precept of more efficient data processing = better results.

Energy harvesting technologies broke into the scene a few years ago as the next logical step in this chain of reasoning. If IoT technology was all the rage because of how it allowed us to vastly expand the scope of our substantial computing/data crunching prowess, energy harvesting truly poured fuel onto an already raging fire. Energy harvesting technology was a crucial innovation that allowed IoT to truly spread its wings and break previously insurmountable barriers. 

Energy harvesting comes of age

Little did we know during the heady early days of IoT that we’re going to run into a major bottleneck in the form of batteries. Batteries were just not cutting it in terms of being a viable power solution for IoT devices. They came with serious limitations - even the best of them only had a limited lifespan which meant that deploying sensors in remote areas would prove to be an expensive hassle. Moreover, batteries limited the scope of development when it came to designing newer, sleeker and more cutting-edge edge devices. Moreover, batteries came with one truly horrendous environmental cost, which was just impossible to ignore. 

But, innovation and ingenuity have always served us well in overcoming roadblocks and making sure we push civilization forward. The IoT battery bottleneck was no exception - energy harvesting emerged as the go-to alternative power solution for IoT edge devices, a few years back. Since then, energy harvesting has come a very long way from being the alternative power source of choice to being the default power solution for all manner of IoT setups. 

To be fair, this is not hard to understand - energy harvesting simply makes more sense in every possible way. Self-powered nodes that use energy harvesting technology come with a whole host of advantages that battery-based solutions simply can’t hold a candle to - They are significantly more economical to run over the course of a project’s lifetime and do not require much in the way of maintenance and upkeep, i.e they are essentially maintenance-free. Moreover, and this possibly the most critical benefit that they bring to the table, they are super environmentally friendly. They don’t use rare minerals, toxic chemicals or slave labour. Add to that the fact that they don’t contribute to our growing e-waste problem, and you shouldn’t have any trouble seeing why they’re the gold standard when it comes to power solutions for IoT projects. 

Energy harvesting comes in many flavours - some of them like Triboelectric energy harvesting, piezoelectric energy harvesting and RF energy harvesting are very popular and well known. But in addition to these mainstays, all sorts of weird and wacky energy harvesting modalities have cropped up in recent years.

Thermoelectric energy harvesting is one such promising energy harvesting technology that has emerged out of the woodwork. In the ensuing sections, we’ll take a look at the basic principles involved in thermoelectric energy harvesting, some of the popular uses of thermoelectric energy harvesting and examine why it’s so promising. 

The basics of thermoelectric energy harvesting

The fundamental principle that underlies any type of energy harvesting technology is that at any given point of time, there is an enormous amount of untapped energy around us. Energy harvesting taps into this ubiquitous store of energy and converts it into usable electrical energy that can power nano-devices. This seemingly simple idea forms the basis of the massive IoT power revolution that we are seeing today. This process, also known as energy scavenging, can theoretically be extrapolated to fit any form of ambient energy, be it heat, vibration or electromagnetic energy.

So, this ambient energy sounds all swell and everything but, it sounds a bit abstract right? How does it work in real life? If you’re reading this, it is probably safe to assume that you’ve got a wi-fi router around you somewhere. That router is emitting packets of electromagnetic waves that are now all around you. Your computer uses this field to bring you your favourite cat videos and allows you to use this energy to write diss-posts on reddit. What’s more is that wi-fi is just one of the many kinds of electromagnetic waves that are constantly around us and furthermore, electromagnetic waves are just one of the several forms of energy that are present in abundant quantities around us. 

So, think. What other form of energy is around us all the time - yeah that’s right, heat!

Thermoelectric energy harvesting essentially refers to the process of converting heat energy into electrical energy. There are two important terms you are likely to encounter when it comes to thermoelectric energy harvesting - the Seebeck effect and TEGs.

Without further ado, let’s see what these terms mean. 

First off, what’s the Seebeck effect? The Seebeck effect is a physical phenomenon that was first explained by a scientist named, well you guessed it, Seebeck. When there is a temperature gradient between two different kinds of electrical conductors or semiconductors, there is a voltage gradient that develops between them as well. This is the Seebeck effect in a nutshell. To put it even more simply, a temperature gradient between two conductors also results in the formation of a voltage gradient. Thermoelectric energy harvesting exploits this phenomenon to convert heat energy into electrical energy. 

What is a TEG then?

TEGs or thermoelectric generators are solid state devices that use the Seebeck effect to generate electricity using heat flux. In essence, they do exactly what heat engines do but are lighter and contain fewer moving parts, both of which are desirable attributes in the context of remote sensing and IoT applications in far-flung, remote areas. 

TEGs tend to be more expensive than heat engines due to the higher costs involved in producing them. There is a limited number of materials that can be used to manufacture TEGs. These materials are called thermoelectric materials. Each of these materials is ranked on a so-called thermoelectric list in order of their thermal conductivity. 

Alloys of bismuth, tellurium and selenium are some common examples of materials that are used to make TEGs. TEGs are used in a wide range of applications - usually, they are used in places where more efficient but heavier engines like Stirling engines aren’t feasible. Among other things, you’d find TEGs in space probes, automotive generators, solar cells and off-grid power generators. 

Energy harvesting using TEGs

In TEG systems, the optimisation of the design plays a huge role in the efficiency and functionality of the final energy harvesting system. Efficiency is the name of the game when it comes to any energy harvesting setup - how good is the system at converting input into output. The less energy a system dissipates and squanders back into the atmosphere, the better it is considered to be. WIth TEG’s however, efficiency is not the main selling point. 

Thermoelectric generators are particularly desirable from an energy harvesting standpoint because of how quietly they operate.They’re also quite universal in their functionality because they use heat as their source energy. It doesn’t take a heart surgeon to know that there’s more areas with an abundance of heat energy than say RF waves. Yet another advantage is that they are ideally suited to be deployed in high-temperature environments, where other types of energy harvesters may not be quite as right a fit. Usually, thermoelectric can handle temperatures of up to 250°C. But here’s the cool part - they can also be used in colder climes. After all, it’s not like they necessarily need heat to generate electricity so much as a temperature gradient. So, given the right conditions, they can also be deployed on cold surfaces. 

And of course like all other energy harvesters, they are a green energy source, being completely eco-friendly. A TEG device, much like any other energy harvesting device, is a step in the right direction towards a fossil-fuel independent future. 

Real world applications of TEGs

We’ve seen how TEG’s have a number of favourable properties that make them a particularly compelling option for a wide variety of IoT applications such as military, aviation, space technology, medical implants, biotechnology and consumer wearables. 
The only challenge that a lot of manufacturers face with TEGs is their low efficiency. But, many are finding ways to circumvent this problem by employing waste heat recovery.

In this section, let’s examine some of the major use-cases for TEGs.

1. Military

Military systems are very reliant on energy availability. This energy is mostly in the form of fossil fuel, whether we like it or not. Alternative energy sources are still not nearly viable when it comes to heavy-duty use cases that the armed forces need. 

Military avionics, infrared detectors, thermal viewing devices, cooling systems and missile testing are just a few examples of military applications where thermoelectric energy harvesting is understood to have vast potential and huge promise. 

The American military, for example, has been on the lookout for a company that could help it build a brand new missile guidance system tester. They identified a promising thermoelectric firm that could collaborate with them on building a test-system that could produce 500 Watts. 

2. Space vehicles

Thermoelectric energy harvesting is used in space vehicles where relying on batteries to power devices is not very practical. 

You might find yourself wondering though, “hey wait a minute, it’s also not practical to go about setting up temperature gradients in outer space right?”

Yes you’d be right. That’s why Radioisotope thermoelectric Generators (RTGs) are used in outer space. An RTG or RITEG as it’s otherwise called, is a rather complex battery that uses a set of thermocouples to harness the heat energy that is released by the decay of an appropriate radioactive material, and convert this energy into electricity. RTGs are a robust and highly dependable source of electricity for space applications because they can function in vacuum and are able to handle high vibrations. They are famously used by NASA in expeditions which last several months or years - These are situations where sunlight can’t be relied upon and RTGs are considered to be the gold standard.

RTGs are usually made to be able to sustain high temperatures (upto 1000°C). Silicon germanium, Lead tin telluride, and tellurides of antimony, germanium and silver (TAGS) are semiconductor materials of choice for RTGs. 

Much like regular TEGs, RTGs achieve this using the Seebeck effect. 
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Thermoelectric energy harvesting is also employed in health monitoring systems (HMS) of aircrafts. One of the main building blocks of an HMS is a self-powered wireless sensor node. It’s not hard to see why energy harvesting sensors are preferred for this purpose. 

3. Consumer wearables 

TEG devices particularly shine in low-power applications. Moreover, being a semiconductor/IoT company, this is also the application of TEG that excites us the most. In recent years, many have tried their hand, with quite some success, at using TEGs to power wearable microelectronics.

In the past few years, wearables have exploded in a big way. Smartwatches, fitness trackers etc have become an essential part of people’s everyday lives. They are no longer considered to be the frippery gimmicks that were seen to be even just a few years ago. 

Why TEGs are a particularly compelling power solution for IoT wearables is that they can harvest body heat and use it to power the device. Imagine a wristwatch that literally runs on the heat emitted by your wrist. Moreover, the ultra-low power requirements of these kinds of devices play in the favour of TEGs and their biggest limitation i.e low energy efficiency. TEGs make use of the temperature difference between the wearer’s wrist and the surrounding atmosphere in order to generate a voltage, which in most cases, is more than enough to run the device. 

The same can be said about bio-medical implants, which are designed to be used for several years at a time, and that too inside the user’s body. It is highly impractical for these systems to use batteries. Heat is a constant source of energy that is available within the human body and with just a little bit of ingenuity, TEGs can be used very successfully in bio-implants.

4. Industrial

Factories and manufacturing plants are possibly some of the best candidates for TEGs. As such, industrial plants come under a lot of scrutiny for being massive contributors to greenhouse gas emissions. 

TEGs could be perfect in industrial setups for this very reason. Thermoelectric harvesters would be able to recover waste heat and convert it to electricity, which is a resource industrial plants are always happy to have more of. Plants have all manner of small electronic devices that could be powered by these TEGs, which would also add up to significant savings in the long run. Most of these microelectronics run on batteries today - using TEGs would significantly cut down on the maintenance costs associated with replacing batteries. 

5. Automotive industry

Thermoelectric energy harvesting has definitely captured the fancy of the auto industry. Again, the reason is fairly obvious - motorcycles and cars dissipate a lot of energy in the form of heat. The automotive industry is sure to pause and take a long hard look at any proposition that offers them a chance to harness even a small fraction of this tremendous amount of energy, that is so far going entirely untapped.

Even accounting for the low efficiency of TEGs, the industry stands to gain a lot. Modern automobiles have a lot of functionality that depends on batteries. Thermoelectric energy harvesting offers a brilliant way to kill two birds with one stone - making use of the tremendous amount of wasted heat, for which the industry faces serious scrutiny and also, potentially, breaking free of the battery, which would make a lot of economic sense. 

A TEG placed in the exhaust system of an automobile should theoretically be able to harvest enough energy to at least play an augmenting role to the battery.

6. Residential

Thermoelectric generators could be a great fit in smart home setups. Within a standard residence, there are enough temperature gradients at play - Whether it’s the kitchen which is hotter than the air outside or the vice versa for the air conditioned bedroom. 

There is a lot of scope of thermoelectric generators to be used in residences to power all manner of smart home devices, like smart light switches, smart fridges, smart bulbs etc. 

TEG systems could be mounted on domestic boilers, water heaters, stoves, or even solar energy systems.

Mounting environmental concerns

The discerning reader would have noticed a recurring theme across several sections of this post-climate change and impending ecological crisis. It is not news to anybody by now that worries about climate change are at an all-time high. 

More people than ever before are lending their voices to the cause of ecological awareness and a more sensible and equitable approach to human civilisation. Energy harvesting and TEGs are extremely relevant in this context because they offer us a glimpse into a future where we aren’t so dependant on fossil fuels to run our grand industrial enterprise. 

Energy harvesting may not be the big answer to all our problems today but it sure is opening up a hell of a lot of opportunities that just weren’t there even a few years ago. This article is meant to bring TEGs into public awareness and highlight the sheer scope of potential applications that they can enable.