Energy harvesting transducers & ICs
The core component of a thermoelectric device is a thermocouple, which consists of an n-type and a p-type semiconductor connected by a metal plate. Electrical connection at the opposing ends of the p- and n-type material complete an electric circuit. Thermoelectric generation (TEG) occurs when the couple is subjected to a thermal gradient, in which case the device generates a voltage and causes current to flow, thereby converting heat into electrical power by what is known as the Seebeck effect. A thermoelectric module is then formed from arrays of these thermocouples connected in series. If heat is flowing between the top and bottom of the module, a voltage will be produced and an electric current will flow.
In the case of a typical airplane engine, its temperature can vary anywhere from a few 100ºC to 1,000 to 2,000ºC. Although most of this energy is lost in the form of mechanical energy (from combustion and thrust), a portion is dissipated purely as heat. Since the Seebeck effect is the underlying thermodynamic phenomenon that converts thermal heat to electric power, the main equation to take into consideration is:
P = ηQ
where P is electrical power, Q is heat and η is efficiency.
Larger TEGs that use more heat, Q, produce more power, P. Similarly, the use of twice as many power converters naturally produces twice the power, given that they can capture twice the heat. Larger TEGs are created by putting more P-N junctions in series; however, while this creates more millivolts per delta T (mV/dT), it also increases the series resistance of the TEG. This increased series resistance limits the power available to the load. Therefore, depending on the application requirements, it is sometimes better to use smaller TEGs in parallel rather than using a larger TEG. Regardless of which choice is used, TEGs are commercially available from a number of suppliers, including Tellurex Corp.
Piezoelectricity can be generated by applying stress to an element, which in turn creates an electric potential. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of an electric potential when stress is applied) also exhibit the reverse piezoelectric effect (the production of stress and/or strain when an electric field is applied).
In order to optimize a piezo transducer, one needs to characterize their source for vibration frequency and displacement. Once these levels have been determined, a piezo manufacturer can design a piezo that is mechanically tuned to the specific vibration frequency and size it to provide the necessary amount of power. The vibration in the Piezo material activates the Direct Piezo effect, which results in the accumulation of charge on the output capacitance of the device. This is usually pretty small so the AC open circuit voltage is high—on the order of 200 Volts in many cases. Since the amount of charge generated from each deflection is relatively small, it is necessary to full-wave rectify this AC signal and accumulate the cycle-by-cycle charge on an input capacitor. Once again, there are a number of piezoelectric transducers commercially available from a number of suppliers, including AmbioSystems, MIDE Technology Corp. and Advanced Cerametrics Inc.
However, what has been missing until now has been a highly integrated, high efficiency DC/DC converter solution that can both harvest and manage the energy from either a thermal or piezoelectric source. Linear Technology’s revolutionary LTC3108 and LTC3588-1 will greatly simplify the task of harvesting surplus energy from a variety of sources.
The recently introduced LTC3108 is an ultralow voltage step-up converter and power manager specifically designed to greatly simplify the task of harvesting and managing surplus energy from extremely low input voltage sources such as thermopiles, thermoelectric generators (TEGs) and even small solar panels. Its step-up topology operates from input voltages as low as 20mV. This is significant since it allows the LTC3108 to harvest energy from a TEG with as little as 1°C temperature differential – something a discrete implementation struggles to meet due to its high quiescent current.
The circuit shown in Figure 2
uses a small step-up transformer to boost the input voltage source to a LTC3108 which then provides a complete power management solution for wireless sensing and data acquisition. It can harvest small temperature differences and generate system power instead of using traditional battery power.
Figure 2: The LTC3108 used in a wireless remote sensor application powered from a TEG (Peltier cell)