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How Does Thermal Energy Harvesting Work?

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Emily Newton's picture
Editor-In-Chief Revolutionized Magazine

Emily Newton is the Editor-in-Chief at Revolutionized Magazine. She enjoys writing articles in the energy industry as well as other industrial sectors.

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  • Oct 6, 2022
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Most sustainable energy initiatives focus on finding cleaner methods to generate power. That’s an important goal, but making current systems less wasteful can also help improve sustainability and reduce costs. Thermal energy harvesting has emerged as a promising potential solution in this area.

Energy harvesting refers to collecting waste power to introduce it back into the system. Thermal energy harvesting, as the name suggests, is a subcategory of this field that focuses on reclaiming power in the form of heat waste. Here’s a closer look at how it works and what it could do.

Types of Thermal Energy Harvesting

Widespread elimination of carbon emissions is practically impossible, especially in the timeframe the world needs. Thermal energy harvesting aims to reduce emissions from less renewable sources in the meantime, helping achieve helpful improvements sooner. 

Heat is an ideal waste form to target in energy harvesting, as virtually all electric systems emit it. Thermal harvesting capitalizes on this waste in one of two ways:

  • Thermoelectric energy harvesting
  • Pyroelectric energy harvesting

Thermoelectric Harvesting

The most common way to turn heat into electricity is through thermoelectric harvesting. This centers on a phenomenon known as the Seebeck effect, where a temperature difference between two thermoelectric components produces an electrical voltage.

When one material or side of a circuit is colder than the other, electrons will flow from the hot area to the cool one. Thermoelectric generators (TEGs) use a semiconductor to provide an easy path for these electrons to pass through. Going through the semiconductor creates an electric current with no moving parts.

In practice, TEGs attach to a heat source like an engine or battery to use waste heat to kickstart this process. There are also a few ways to maximize the energy the Seebeck effect creates. Shrinking the semiconductor’s grain size can produce three times the electricity as larger, natural grains by providing an easier path for high-energy electrons.

Pyroelectric Harvesting

Thermoelectric harvesting may be the most widespread thermal energy harvesting method, but it’s not the only one. Pyroelectric harvesting is similar in that it capitalizes on temperature gradients, but it approaches it differently. It’s also worth noting that pyroelectricity can create alternating current (AC), whereas thermoelectricity creates direct current (DC).

Thermoelectrics use a spatial difference between hot and cold materials to generate power, while pyroelectrics use a time difference. The vibration of their atoms produces an electrical charge as pyroelectric materials’ temperature fluctuates. These spurts of energy last just a moment but are strong enough to power some applications.

Infrared motion sensors are one of the most recognizable examples of pyroelectrics in action. However, while these devices use the concept to generate enough electricity to trigger a sensor, many researchers propose others can use it to turn waste heat into power.

Design and Implementation Considerations

Thermal energy harvesting requires specific design and usage considerations to work properly. That starts with the materials they use.

TEGs must use materials with high electrical but low heat conductivity. Bismuth, telluride and silicon germanium are among the most common, but novel materials could help these devices work on larger scales. Recent research has found that purified tin selenide can be the most effective thermoelectric system to date under the right circumstances.

Pyroelectric energy harvesting requires different materials. The best option depends on the specific application at hand. Generally, items that can store more energy within an electric field are better for smaller areas, while large applications work better with low energy storage.

It’s also important to consider a harvesting system’s end use. Pyroelectric harvesting may be better for devices that perform specific functions according to specific triggers, while TEGs are ideal for more versatile, general-purpose items.

Potential Applications of Thermal Energy Harvesting

Roughly 65% of energy from major sources goes to waste, so reclaiming excess heat as electricity could improve many processes. One of the most enticing in today’s world is the Internet of Things (IoT).

There are more than 16 billion active IoT connections in the world today, and these devices are only becoming more common. While each gadget doesn’t consume much power, this massive overall scale represents a considerable amount of electrical demand. Thermal energy harvesting could help reduce this impact.

IoT devices typically don’t require much power, so even today’s imperfect energy harvesting systems could be effective in these applications. A battery or wired connection could provide the primary source while a TEG collects its waste heat to offer extra energy, extending battery life or reducing hardwired power withdrawals.

Cars could use TEGs or similar devices to power their electrical components from their engines’ waste heat. Wearable devices like smartwatches could go a step further and use thermal energy from the human body. As technology advances, these systems could eventually provide grid-scale energy harvesting, gathering the heat from power plants to reduce energy waste.

Remaining Challenges

Thermal energy harvesting holds substantial promise, but it still has some shortcomings. Most notably, thermoelectric and pyroelectric systems only generate a small amount of power. 

A large TEG can produce hundreds of watts with high enough temperatures, but most applications, especially those harvesting waste heat, don’t involve that much heat. Household electronics may only produce enough waste heat to generate a few milliwatts. That may be enough to power some processes, but it may not have a substantial impact overall.

More efficient electronics need less energy, but they also won’t produce as much waste heat. This trend could make it difficult to design thermal harvesting applications that provide significant waste reduction.

The materials required to create these devices also pose a challenge. Recent research has found some promising new candidates, but many of these materials are difficult or expensive to obtain or work with. That may limit the accessibility of thermal harvesting systems.

Thermal Energy Harvesting Has Significant Potential

Despite some lingering obstacles, thermal energy harvesting can make some energy systems more sustainable. These technologies will become increasingly common across various use cases as advances continue.

Thermal energy harvesting is also just one type of waste power reclamation. More of these systems are approaching the issue from different sides, and they could reduce waste and energy-related emissions at scale before long.

Discussions
Luiz Horta Nogueira's picture
Luiz Horta Nogueira on Oct 7, 2022

Thanks for this interesting post. I would add that, although a lot of energy is wasted as heat to the environment, most of that is at low temperature, near to ambient. In this situation, low thermal efficiency is an inviolable consequence of the Second Law of Thermodynamics, as all of you know. For instance, for a heat stream at 100 C, considering the surroundings at 25 C, a Carnot perfect reversible cycle would have 16% thermal efficiency, and actual cycles would have less than 10%. Anyway, maybe in proper conditions of heat rejection, well designed Organic Rankine Cycle can perform satisfactory.  

Emily Newton's picture
Thank Emily for the Post!
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