Thermal Energy Harvesting: Converting Heat into Power

Energy harvesting refers to the ability to collect energy from the environment, or the system itself, to power electronics. Specifically, thermal energy harvesting converts thermal energy collected from a heat source into electricity.

Advantages of thermal energy harvesting

This technology offers several significant advantages:

Battery Elimination: By eliminating the need for batteries, thermal energy harvesting is particularly beneficial for portable devices and low-power applications.
Self-Sufficient IoT Devices: It enables the creation of self-sufficient IoT devices, crucial for standalone and mobile applications that require continuous operation without the need for battery recharging. This is particularly valuable for remote or difficult-to-reach locations within metropolitan infrastructure, significantly reducing maintenance and battery replacement requirements.
Wearable Solutions: Thermal energy harvesting facilitates the development of innovative wearable solutions for medical and consumer applications.
Green Energy Technology: By harnessing ambient heat sources, this technology contributes to the development of green energy solutions, reducing reliance on fossil fuels and minimizing greenhouse gas emissions.

Applications of thermal energy harvesting

Thermal energy harvesting can provide an autonomous and renewable energy source for a wide range of sensors and electronic devices. These devices can leverage temperature differences in their environment to generate power. The introduction of increasingly efficient devices will pave the way for new applications that fully capitalize on the potential of thermal energy harvesting.

One intriguing application is the use of thermal energy harvesting in wearable systems. By harnessing the temperature difference between the human body and the surrounding environment, small electric currents can be generated. Temperature disparities exist everywhere, both in natural and artificial settings. Thermoelectric energy can be produced by capitalizing on these temperature gradients.

Thermal energy

According to the laws of physics, the total energy of a system is conserved, although it can transform from one form to another. Various environmental energy sources can be harnessed for energy generation. The environment is replete with variations in temperature and heat movement.

Typical examples include:

Waste Heat from Engines: Heat generated during engine operation.
Geothermal Heat: Heat extracted from the Earth’s subsurface.
Industrial Waste Heat: Heat generated during industrial processes, such as cooling water in steelworks.

By utilizing a thermoelectric generator (TEG) and associated electronics, thermal energy can be converted into electrical energy and stored in a storage device. TEGs operate on the fundamental principle that heat flux, or the temperature difference, can be directly converted into electrical energy. They are ideally suited for low-power embedded devices due to their typically small size and lack of moving parts (solid-state).

Seebeck effect

The Seebeck effect is the phenomenon where an electrical voltage is generated when a temperature gradient exists across a material. The basic building block of a TEG is the PN junction, comprising a single structure of thermoelectric materials P and N, electrically connected in series and doped with impurities such as boron (P) and phosphorus (N).

Figure 1: A TEG is essentially a Peltier cell with a hot and a cold surface (Source: ResearchGate). — Figure 1: A TEG is essentially a Peltier cell with a hot and a cold surface (Source: ResearchGate)

Multiple PN pairs connected in series form the fundamental units of a TEG module. In this configuration, the PN pairs are arranged in parallel to produce a voltage proportional to the temperature gradient. For effective operation, the device’s hot (Th) and cold (Tc) sides must be maintained at different temperatures.

The performance of the thermoelectric material is quantified by the thermoelectric figure of merit, ZT, defined as:

ZT = (S² * σ * T) / κ

Where:

S is the Seebeck coefficient
σ is the electrical conductivity
κ is the thermal conductivity
T is the temperature at which the thermoelectric properties are measured

ZT measures the amount of electrical energy that can be generated at a given temperature gradient. Higher ZT values indicate better thermoelectric performance.

Improving the thermoelectric performance of a material can be achieved by:

Increasing the power factor (PF = S² * σ)
Reducing the thermal conductivity (κ = κe + κph), where κe and κph represent the electronic and phononic contributions to thermal conductivity, respectively.

The Seebeck coefficient, electrical resistivity, and thermal conductivity are the critical factors that determine the efficiency of this thermal process. These three distinct physical properties, collectively constituting the figure of merit, are interdependent. Therefore, improving one characteristic often adversely affects another.

Reducing the phononic contribution to thermal conductivity (κph) is the most promising strategy to enhance overall efficiency. This can be achieved by minimizing the size of the material.

Novel materials

While battery-based solutions continue to improve in terms of efficiency and size, they may not be sufficient for certain low-power applications, such as IoT sensors. In such cases, energy harvesting technologies offer significant advantages. The growing interest in energy harvesting has spurred the development of complementary technologies, including ultra-low power (picowatt) microelectronics and supercapacitors.

An ideal thermoelectric material should exhibit a strong Seebeck effect, high electrical conductivity, and low thermal conductivity. However, finding a material that satisfies all these requirements simultaneously is challenging, as electrical conductivity and thermal conductivity are typically interrelated.

Recent research breakthroughs have led to the development of novel materials with ZT values ranging between 5 and 6. These materials, composed of a thin layer of iron, vanadium, tungsten, and aluminum applied to a silicon crystal, have the potential to revolutionize the sensor power supply industry, enabling sensors to generate their own power from environmental sources.

Depending on the available temperature gradients, TEGs can generate power output ranging from 20 µW to 10 mW per cm².

Thermal energy harvesting solutions

The AEM20940 from e-peas is a thermal energy harvesting IC that functions as a fully integrated energy management subsystem. This device extracts DC power from a thermoelectric generator (TEG), enabling it to charge a rechargeable storage element while supplying two independent, regulated voltages to the system.

This innovative design allows engineers and product developers to extend battery life and eliminate the need for disposable batteries in a wide range of wireless applications, including industrial monitoring, home automation, wearables, and smart agriculture.

The AEM20940 (Figure 2) is capable of harvesting input currents of up to 110 mA. It includes an ultra-low-power boost converter, which charges storage devices such as Li-ion batteries, thin-film batteries, supercapacitors, or standard capacitors. The boost converter operates efficiently across a wide input voltage range from 50 mV to 5 V.

Figure 2: AEM20940 block diagram. — Figure 2: AEM20940 block diagram (Source: e-peas)

With its unique cold-start capability, the subsystem can begin operating even with empty storage elements at input voltages as low as 380 mV and input power as low as 100 µW. An optional external module can further reduce the cold-start voltage to 60 mV with an input power of just 150 µW.

The system provides two regulated voltage outputs: a low-voltage supply, typically used to power microcontrollers at 1.2 V or 1.8 V, and a high-voltage supply, suitable for radio transceivers at 1.8 V, 2.5 V, or 3.3 V. Both outputs are stabilized with highly efficient low-dropout (LDO) regulators, ensuring minimal noise and high stability.

The device’s configuration pins allow users to adjust operating modes by setting parameters such as overcharge and over-discharge voltages for the energy storage element and by selecting the output voltage levels for both high and low supplies. The AEM20940 integrates all active components required to power typical wireless sensors, needing only seven external components: five capacitors and two inductors, which are available in compact 0402 and 0603 sizes.

This high level of integration reduces the footprint, bill of materials (BOM), and overall design complexity, helping accelerate time-to-market and lower costs for wireless sensor network (WSN) designs. By leveraging the AEM20940, designers can achieve efficient and compact thermal energy harvesting solutions, paving the way for more sustainable and energy-autonomous wireless systems.

Conclusion

Thermal energy harvesting offers a sustainable and reliable power source for a diverse range of applications. Continued research and development efforts focused on improving material efficiency and device design will further expand the potential of this promising technology.