How to collect heat and vibration energy for IoT devices

Alfred Piggott, founder and chief technology officer of Applied Thermoelectric Solutions, said: “Some examples of thermoelectric materials are bismuth telluride, lead telluride, cobalt triantimonide and silicon germanium, which can provide good performance. Using these materials, In an ideal application with a properly designed thermoelectric generator, an efficiency of up to 9-11% can be achieved. Which material is the best depends on many factors, but mainly depends on the application, budget and the design of the thermoelectric generator .”

Translated from ――EEtimes, Maurizio Di Paolo Emilio

Advanced power management is the key to maintaining the rapid development of digital technology. The use of energy harvesting solutions can become an important turning point for ultra-low power solutions for the Internet of Things.

The realization of so-called zero-power sensors requires energy from the environment. To narrow the range of available energy options, the next criterion will be the amount of available energy and required energy.

For example, the collection of solar and wind energy can provide a solid foundation for high-power solutions.

At the same time, heat energy is often easily obtained from by-products of engines, machinery, and other sources. Thermal gradient collection refers to the process of obtaining environmental heat and using it. In many ways, using energy and environmental phenomena, the use of piezoelectric devices to convert vibration into electrical energy seems to be an effective method. Depending on the size and construction density, it has the capacity to produce hundreds of microwatts (?W/cm2).

Thermal gradient

Collecting energy through temperature gradients is the use of thermoelectric solutions. The use of cogeneration is limited because it requires a variable temperature input, while others can provide hundreds of thousands of hours of uninterrupted operation, but the efficiency is very low. The thermoelectric solution is solved by the Peltier battery module.

Alfred Piggott, founder and chief technology officer of Applied Thermoelectric Solutions, said: “Some examples of thermoelectric materials are bismuth telluride, lead telluride, cobalt triantimonide and silicon germanium, which can provide good performance. Using these materials, In an ideal application with a properly designed thermoelectric generator, an efficiency of up to 9-11% can be achieved. Which material is the best depends on many factors, but mainly depends on the application, budget and the design of the thermoelectric generator .”

The ideal thermoelectric material should have lower thermal conductivity, higher electrical conductivity and higher Seebeck coefficient.

The thermoelectric effect based on this energy harvesting was proposed by the German physicist Thomas John Seebeck. In thermoelectric devices, voltage is generated when different temperatures are combined. Similarly, there will be a temperature difference when voltage is applied. The ability of a material or device to generate voltage per unit temperature is called the Seebeck effect.

The materials commonly used to create p and n regions (bismuth telluride, or Bi2Te3) allow to obtain an output voltage of 0.2mV/K per cell, and if the thermoelectric converter uses multiple double p and n (20mV, in ?T= 10K uses 10 cells), a higher value can be obtained. The equivalent model of the power supply is represented by the Thevenin generator with RT output resistor, and the maximum power that can be provided to the load is obtained by the resistance impedance adaptive Rload=RT.

The temperature difference between the two points causes heat energy to flow from the highest temperature point to the lowest temperature point. The heat flow will always exist until it reaches thermal equilibrium and can be used to collect reusable energy. The process of extracting energy from heat exchange is governed by the laws of thermodynamics.

Later, Jean Charles Athanase Peltier discovered that heating or cooling occurs by passing current through the intersection of two different conductors. The direction of the airflow determines the direction of the temperature change, up or down. The heat generated or absorbed is related to the current, and the proportional constant is called the Peltier coefficient.

Piezoelectric effect

Mechanical vibration is another way to provide enough energy for Electronic systems. The vibration of piezoelectric transducers has been widely used in energy harvesting applications in recent years through the use of special masses and special systems that allow movement.

Piezoelectric converters use the direct piezoelectric effect, which is the characteristic of certain crystals that produce a potential difference when subjected to mechanical strain. This effect occurs at the nanometer scale and is reversible. In recent years, polymer plastic-based piezoelectric materials (Pvdf) have been extensively developed, and new materials and more and more advanced manufacturing processes are constantly being sought.

The piezoelectric effect converts kinetic energy into electrical energy in the form of vibration or shock. Piezoelectric generators (energy harvesters) provide a reliable solution by converting vibration energy wasted in the environment into usable electrical energy. They are ideal for applications that need to charge batteries, supercapacitors, or directly for remote sensor systems (Figure 1).

How to collect heat and vibration energy for IoT devices

S234-H5FR-1803XB piezoelectric crystal converts vibration into electrical energy

The overall performance of the system depends on many factors, such as the input vibration, the geometry and material of the sensor, the quality that causes the vibration, and the Electronic interface. Therefore, even in the early design stage, a fast and reliable quantitative estimation of the behavior of the transducer and circuit junction is very much needed to optimize the entire system.

The analysis of the piezoelectric effect can be represented by the circuit shown in the figure below.

How to collect heat and vibration energy for IoT devices

The inductance LM is the equivalent inertial mass; the capacitance CM is the elasticity of the transducer; the resistance RM is the mechanical loss. The mechanical part is the force generated by the generator FIN, the opposite feedback force generator α-VP, and the CP (inverse piezoelectric effect) developed on the output device controlled by the voltage. At the same time, the mechanical speed? Produces the direct influence of the current? Supply of two capacitive outputs (piezoelectric) and other possible electrical loads connected to the sensor. Therefore, model identification involves the following six independent parameters: LM, CM, RM, CP, α and β. α and β are systems related to thermal coefficients.

Power Management IC (PMIC)

The temperature difference can be used to generate electricity, thereby using excess heat that would otherwise be lost. Waste heat from solar and geothermal systems can be collected. The discharge stream of general household appliances can be used.

Suppose we use battery-powered wireless IoT devices that operate in an environment with thermal gradients generated by the human body, oven, and motors. Without energy harvesting, the batteries of these devices would need to be replaced because they release energy, which incurs operating costs. According to the available temperature gradient, the thermoelectric generator can generate 20?w?10 mw/square centimeter.

Thermoelectric generators and piezoelectric sensors combined with appropriate PMICs will charge batteries in IoT applications.

In order to design a better thermoelectric energy harvesting system, there are many characteristics that need to be considered. Including electrical and thermal requirements, the use of suitable thermoelectric materials, specific applications should consider durability goals, product sales prices and engineering budgets.

Vibration is a ubiquitous source of energy. Every car on the road will vibrate in the cockpit on the asphalt road. If we consider the length of the highway and the large number of flowing cars, the idea of ​​obtaining energy from vibration seems very attractive.

Maxim’s MAX17710 is a complete system for low-power, high-efficiency energy harvesting, charging and protection. It can manage energy harvesting devices with output power from 1FW to 100mW. It is the industry’s first IC integrated environmental energy harvesting power management function, charging And protection of micro-energy batteries and (MECS), a form of solid-state batteries. When operating at ultra-low current levels, the MAX17710 accepts energy from poor management and the output ranges from 100mW to a level of 1μW to harvest energy from various sources. Examples include light (captured by photovoltaic cells), vibration (captured by piezoelectric elements), heat captured by thermoelectric generators), and radio frequency (eg, near field communication (NFC)).

How to collect heat and vibration energy for IoT devices

Another PMIC is AEM30940, which is an integrated energy management subsystem that extracts DC energy from thermal generators, piezoelectric generators, micro-turbine generators, or high-frequency RF input, while storing the energy in rechargeable components , And provide two independent regulating voltages for the system. It integrates an ultra-low-power boost converter to charge storage elements, such as lithium-ion batteries, thin-film batteries, or super or traditional capacitors. It can start to operate with empty storage element input voltage to 380 mv and input power only 3μw.

The LTC3588-1 integrated circuit provides a complete energy storage solution that is optimized for high-impedance generators such as piezoelectric transducers. It is characterized by a low-loss full-wave rectifier and a high-efficiency synchronous buck converter, which can transfer energy from the input storage device to the output regulated voltage, and can provide loads up to 100mA. It can be packaged in a 3mm×3mm DFN or 10-conductor MSE.

How to collect heat and vibration energy for IoT devices

In order to effectively design a fully autonomous wireless sensor system, you need low-power microcontrollers and sensors that consume the least amount of power in a low-energy environment. The power solution for such a system may include storing available mechanical, thermal, or electromagnetic energy in the local environment of the sensor itself.

Supercapacitors are the technical prerequisite for effective use of energy. They are very large-capacity capacitors, and have the functions of electrolytic capacitors and rechargeable batteries, but the energy stored per unit volume or mass is 10 to 100 times that of electrolytic capacitors, and can be faster than ordinary rechargeable batteries. Accumulate charge at a higher rate, and go through more charge and discharge cycles unscathed than rechargeable batteries.

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