The process of obtaining the necessary energy to power electronic devices from a source other than the system itself, such as the surrounding environment, is known as energy harvesting. Thermal energy harvesting, in more detail, is the process of converting thermal energy collected from a heat source into electrical energy.
The following are some of the advantages of harvesting thermal energy:
- It is possible that the need for a battery will be eliminated. This advantage is particularly significant for low-power applications and portable devices.
- The ability of Internet of Things devices to generate sufficient power on their own. This component is required for the manufacture of standalone and mobile devices that can operate without interruption and do not require the battery to be recharged on a regular basis.
- Thermal energy harvesting enables smart sensing to be applied to more difficult-to-reach or remote locations within a metropolitan infrastructure by reducing the need for maintenance and battery replacement.
- The ability to create innovative wearable solutions for a wide range of applications, including those in the medical and consumer sectors.
- The development of methods for producing sustainable energy. This will reduce the consumption of fossil fuels as well as greenhouse gas emissions.
This technology has the potential to provide a self-sustaining and renewable energy source for a wide range of sensors and electronic devices, allowing them to generate energy from temperature differences. If certain conditions are met, the development of increasingly efficient devices may pave the way for innovative solutions that make optimal use of thermal energy harvesting.
Creating small electric currents that harvest thermal energy as a difference in two temperatures—the body’s and the surrounding environment—is an intriguing method for using thermal energy harvesting technologies in wearable systems. This method is one of several intriguing applications of these technologies. Temperatures, whether natural or artificial, can vary greatly from one location to another. Using these temperature differences, also known as gradients, can result in the generation of thermoelectric energy.
The sum of a system’s energy forms is always preserved by physics laws, even if those forms can be transformed into one another. Energy can be obtained from a wide range of natural environmental sources.
The air we breathe contains a wide range of temperatures, and heat circulates constantly all around us. Heat generated by engine waste, heat derived from geothermal activity in the soil, heat generated by cooling water in steelworks, and various other types of industrial operations are common examples.
A thermoelectric generator (TEG) and certain electronic components can be used to convert thermal energy into electrical energy. This electrical energy can then be stored in a battery. The TEG is built on the fundamental idea that heat flux, also known as temperature difference, can be converted into electrical energy. It is an excellent choice for low-power embedded devices due to its typically very small size and lack of any moving parts (solid-state).
When two sides of a material have different temperatures, an electrical voltage can be generated as a result of a phenomenon known as the Seebeck effect. A thermoelectric generator’s basic component is the p-n junction (TEG). This junction is made up of a single structure of thermoelectric materials P and N, which are electrically connected in series and doped with impurities such as boron (P) and phosphorus (P) (N).
The fundamental elements of a TEG module are multiple p-n pairs that are connected in series. The p-n pair junctions in this configuration are connected in parallel to generate a voltage proportional to the temperature difference. The hot (Th) and cold (Tc) sides of the device must be at different temperatures for the process to be successful. The thermoelectric material’s performance, as measured by the thermoelectric figure of merit ZT, is given by the following equation:
- Where S denotes the Seebeck coefficient, R denotes electrical resistivity, T denotes thermal conductivity, and T denotes the temperature at which thermoelectric properties are measured. A material’s ZT value indicates how well its thermoelectric properties work; in general, a higher ZT value indicates better thermoelectric properties. The amount of electrical energy that can be produced at a given temperature gradient is measured by ZT. It is possible to improve a given material’s thermoelectric performance by either increasing its power factor, denoted by the equation PF = S2, or decreasing its thermal conductivity, denoted by the equation = e + ph, where e and ph represent the electronic and phononic contributions, respectively.
Three factors influence the efficiency of this thermal process: the Seebeck coefficient, electrical resistivity, and thermal conductivity. These three distinct physical characteristics are interdependent on one another and, when combined, form the figure of excellence. As a result, improving one without negatively impacting the other is difficult, if not impossible. ph is the only variable whose value can be changed arbitrarily without affecting the other variables (T). As a result, shrinking the operation is the most promising strategy for increasing overall efficiency.
Every day, both the efficiency and the size of battery-powered solutions improve. There is no way to improve the battery life of certain low-power applications, such as sensors used in the internet of things. As a result, technology that collects energy from the environment will be extremely beneficial to those devices. Because of the growing interest in energy harvesting, several complementary technologies, such as ultra-low-power (picowatt) microelectronics and super condensators, have been developed.
To be a good thermoelectric material, a substance must have a strong Seebeck effect, conduct electricity as well as possible, and transfer heat as inefficiently as possible. Finding a material that meets all of these requirements can be difficult because electrical conductivity and thermal conductivity frequently go hand in hand.
Recently, scientists were successful in developing a novel material with a ZT value ranging between 5 and 6. This new material, which is made up of a thin layer of iron, vanadium, tungsten, and aluminum applied to a silicon crystal, has the potential to revolutionize the sensor power supply industry by allowing sensors to generate power from natural sources. A thin layer of iron, vanadium, and aluminum makes up the material.
TEGs can generate anywhere from 20 microwatts to 10 megawatts of power per square centimeter, but this is entirely dependent on the temperature gradients present.
There are several integrated circuits (ICs) on the market that are suitable for the thermal energy harvesting process. The BQ25570 from Texas Instruments can extract microwatts to milliwatts from TEGs, as can the AEM10941 from e-peas and various ICs from Analog Devices Inc. and Renesas Electronics.
The BQ25570 includes a power management system that prevents the battery from overcharging or exploding while simultaneously increasing voltages via double circuits. Rechargeable Li-ion batteries, conventional capacitors, supercapacitors, and thin-film batteries can all be used to store the harvested energy.
Supercapacitors are a technological requirement before energy harvesting can be used effectively in any setting. They are capacitors with extremely high capacities and the functional characteristics of both electrolytic capacitors and rechargeable batteries. To put it another way, they have the best of both worlds.
They can, however, store 10 to 100 times more energy per unit of volume or mass than an electrolytic capacitor, accumulate electric charges at a much faster rate than rechargeable batteries, and withstand significantly more charge-discharge cycles than rechargeable batteries.
The process can begin when there is a temperature difference between the TEG plates that is large enough to produce a voltage on the TEG terminals. The BQ25570, which contains both a boost charger and a nanopower buck converter, extracts the power, which can range in size from microwatts to milliwatts depending on the temperature differential. The output voltage is then increased to 3.3 V, and the integrated boost converter maintains a 93% efficiency throughout the process.
There are two methods for storing incoming power in energy harvesting: You can keep that charge going by using capacitors or a battery. When deciding whether to use a conventional capacitor or a supercapacitor, the designer can use the following guidelines to help them decide:
- Select a capacitator with an ESR of less than 200 m.
- The leakage current must be less than 1 microampere at 1.2 volts.
- Large capacitors, despite charging slowly, can store significant amounts of current. Small capacitors, on the other hand, charge very quickly, increasing the amount of time required for startup.
- The following formula can be used to calculate the capacitor value, but it will differ depending on the application:
- C is equal to 15 times VOUT multiplied by IOUT multiplied by TON.
- where VOUT is the energy-harvesting sensor’s output voltage, IOUT is the average output current from the energy-harvesting sensor, and TON is the time it takes for the IC to turn on.
If the sensor fails to provide enough power, the storage capacitors will keep the system running for a predetermined amount of time.
Furthermore, the power conditioning of a thermoelectric energy harvester is extremely important. A generator’s output voltage is low due to its low voltage, even when it is operating at its maximum power capacity.
When the energy harvester is used to recharge an existing battery, the power-conditioning circuit ensures that the battery is not overcharged. Similarly, power conditioning is used to stabilize the output voltage whenever the temperature changes.
Conditioning circuits, which are responsible for a wide range of functions such as input impedance, power control, and filtering, play an important role in an energy-harvesting system. Among the most important components are the power-conditioning circuit, the microcontroller, the storage device (supercapacitor), and the transducer. A thermal, photovoltaic, or vibrational source can be used as a transducer.