Thermoelectric Generator Kits Allow Energy Harvesting Investigation
EverGen kits from Marlow offer a range of source-to-application, thermoelectric-based energy harvesting (EH) systems for evaluation and testing. Each EverGen kit integrates an electrically-matched, thermally-optimized, and custom thermoelectric generator with a heat sink and voltage step-up converter. This combination allows designers to provide power under a wide range of temperature conditions. Designers can customize these systems to meet size, form factor, or temperature constraints. Systems can also scale to multi-watt outputs.
The EverGen series offers existing energy solutions for sensor networks that currently rely on expensive wired systems or short-lived batteries. These thermal energy harvesters employ a thermoelectric generator (TEG) to power wireless sensors without a battery, reducing maintenance and replacement costs. For more information on thermoelectric generators, see the sidebar “What Is A Thermoelectric Generator?”
By converting small temperature differences into milliwatts of electrical power, these thermoelectric EH solutions can perpetually power wireless sensors for the lifetime of the application. By recycling wasted heat, they provide a reliable energy source for sensors, actuators, valve solenoids and other small devices. Characteristics of the EverGen series are listed in Table 1.
The three EverGen energy harvesting kits are designed to suit a wide range of environmental conditions, including convection and liquid processes. The kits are as follows:
- EverGen Liquid-to-Air: Harvests the thermal energy between a higher temperature fluid stream and ambient air via natural convection, for conversion to electrical power.
- EverGen Liquid-to-Liquid: Harvests the thermal energy between a higher temperature fluid stream and lower temperature fluid stream for conversion to electrical power.
- EverGen Solid-to-Air: Harvests the thermal energy between a higher temperature solid surface and ambient air via natural convection for conversion to electrical power.
Each kit integrates an electrically-matched, thermally-optimized, custom thermoelectric generator and heat sink with a Linear Technology LTC3108 voltage step-up converter (see sidebar, “DC-DC Converter Boosts Output Voltages”) to provide the tools and flexibility necessary to evaluate a wide range of test conditions. For example, the EHA-PA1AN1-R02-L1 is intended to harvest power from the temperature difference between a warm solid surface and the surrounding ambient air.
The EHA-PA1AN1-R02-L1 assembly should be mounted to a hot surface. A layer of graphite is adhered to the mounting surface on the EHA-PA1AN1-R02-L1 to ensure a good thermal contact between the mounting surface and the EHA assembly. Performance will vary with ambient conditions. For best results, mount assembly in ideal orientation as shown in Fig. 1. For maximum reliability, the recommended continuous operation is below 85°C. Fig. 2 shows the short circuit current and estimated power output for variations in temperature difference.
Output from the EHA-PA1AN1-R02-L1 is a function of heat source, ambient temperature, and electrical load downstream of the converter. Data in Fig. 2 is representative of typical system performance under steady-state flow and thermal conditions. The data should not be taken as comprehensive or representative of every possible operating condition.
Flexible configuration
Marlow can modify this EH system to maximize power output, or to meet size, form factor, or temperature constraints for any thermal energy harvesting application. In special cases the EH system can be customized to accommodate alternating temperature differences or a reverse temperature difference.
Fig. 3 shows the LTC3108 inputs and outputs for a TEG application. The LTC3108’s output is regulated to the voltage set by pins VS1 and VS2 shown in Table 2. A 220μF capacitor is connected to pin 1, and auxiliary capacitor can be paralleled with it. The EHA-PA1AN1-R02-L1’s interfacing pin connections match those of the LTC3108, as shown in Table 3.
The LTC3108 includes a low current output of its low dropout (LDO) regulator, designated VLDO. This output provides a regulated 2.2V output for powering low power processors and other low power ICs. VLDO output is currently limited to 3mA.
There is a Power Good (PGD) output that monitors VOUT of the LTC3108. This can drive a microprocessor or other chip I/O and is not intended to drive a higher current load, such as an LED. When VOUT is within 7.5% of its programmed value, PGD pulls up to VLDO through a 1MΩ resistor. If VOUT drops 9% below its programmed value, PGD goes low.
The LTC3108’s VSTORE output can be either a storage capacitor or battery. The storage element on VSTORE can be used to power the system in the event that the input source is lost or is unable to provide the current demanded by VOUT and LDO output. A large value capacitor may be connected from this pin to GND to power the system in the event the input voltage is lost. It will be charged up to 5.25V. Note that it may take a long time to charge a larger value capacitor, depending on the input energy available and the loading on VOUT and VLDO. The maximum current from VSTORE is limited to a few milliamps, so it can safely trickle-charge NiCd or NiMH rechargeable batteries for energy storage when the input voltage is lost. Note that the VSTORE capacitor cannot supply large pulse currents to VOUT. Any pulse load on VOUT must be handled by the VOUT capacitor.
There are thermistor leads for hot side and cold side TEG temperatures. The temperature readings from the on-board thermistors monitor the hot side and cold side of the embedded TEG. In application, these temperatures will be different than the source and sink temperatures used to define the system temperature difference included in the performance plot. Besides monitoring the on-board thermistors, it is recommended that the designer place and monitor external thermocouples or thermistors/RTD’s on both the heat source and heat sink (or ambient conditions).
For applications involving bursts of pulses, size the VOUT capacitor to provide the necessary current when the load is pulsed on. The required capacitor value will be dictated by the load current, the duration of the load pulse, and the amount of voltage drop the circuit can tolerate. The capacitor must be rated for the voltage selected for VOUT by VS1 and VS2.
COUT = Output capacitor in µF
ILOAD = Load current in mA
tPULSE = Pulse on time in ms
DVOUT = Maximum allowable drop in VOUT
Note that there must be enough energy available from the input voltage source for VOUT to recharge the capacitor during the interval between bursts of load pulses. Reducing the duty cycle of the load pulse will allow operation with less input energy.
Sizing the storage capacitor
The VSTORE capacitor may be a very large value (thousands of microfarads or even Farads), to provide holdup at times when the input power may be lost. This capacitor must charge to 5.25V (regardless of the settings for VOUT), so ensure that the holdup capacitor has a working voltage rating of at least 5.5V at the temperature for which it will be used. The VSTORE capacitor can be sized according to:
Where:
IQ = 6µA quiescent current of LTC3108
IVL = Load current on VOUT in between bursts
ILDO = Load on the LDO between bursts in mA
IBURST = Total load current during the burst in mA
t = Duration of the burst in ms
f = Frequency of the burst in kHz
TSTORE = Storage time required in seconds
VOUT = Output voltage required
For a 5V programmed output voltage the VSTORE capacitor cannot provide any beneficial storage time. Storage capacitors requiring voltage balancing are not recommended, due to the current drawn by the balancing resistor.
In many pulsed load applications, the duration, magnitude and frequency of the load current bursts are known and fixed. In these cases, the charge current required from the LTC3108 to support average load is:
Where:
ISLEEP = Sleep current on VOUT required by the external load between bursts in µA
IBURST = Total load current during the burst
t = Burst time duration
T = Transmit burst rate period
The liquid-to-liquid and liquid-to-air versions of the EverGen thermelectric generator kits operate in a manner similar to that of the solid-to-air unit.
What is a Thermoelectric Generator?
A thermoelectric generator (TEG) is a semiconductor-based electronic component that functions as a small heat pump. By applying low voltage DC source to a TEG, heat flows via the semiconductor elements from one face to the other. The electric current cools one face and simultaneously heats the opposite face. Consequently, a given face of the device can be used for heating or cooling by reversing the polarity of the applied current.
A typical single stage cooler consists of two ceramic plates with “elements” of p-type and n-type semiconductor materials (bismuth telluride alloys) between the plates, which yields a single stage cooler. The elements of semiconductor materials are connected electrically in series and thermally in parallel. When a positive DC voltage is applied, electrons pass from the p-type to the n-type element, and the cold-side temperature decreases as the electron current absorbs heat, until equilibrium is reached. The heat absorption (cooling) is proportional to the current and the number of thermoelectric couples. This heat is transferred to the hot side of the cooler, where it is dissipated into the heat sink and surrounding environment.
Shown in Fig. 4 is Marlow Industries recently introduced Triton ICE thermoelectric module series that can chill electronics as much as 2°C (4.24°F) below current offerings. These coolers can dramatically improve customer electronic systems in thermal performance, cost, noise, weight, size or efficiency. Triton ICE modules exceed the industry standard in cooling capacity, rate and efficiency while using the same input power.
History
Initial discovery of this effect occurred in the early 1800s when Jean Peltier found there was a heating/cooling effect when passing electric current through the junction of two conductors. Alessandro Volta and Thomas Seebeck found that holding the junctions of two dissimilar conductors at different temperatures creates a voltage. William Thomson (Lord Kelvin) showed that over a temperature gradient, a single conductor with current flowing in it has reversible heating and cooling. With these principles developed and the introduction of semiconductor materials in the late 1950s, thermoelectric cooling became viable.
DC-DC Converter Boosts Output Voltages
The LTC®3108 is a highly integrated DC-DC converter ideal for harvesting and managing surplus energy from extremely low input voltage sources such as TEGs (thermoelectric generators), thermopiles and small solar cells. The step-up topology operates from input voltages as low as 20mV. The LTC3108 is functionally equivalent to the LTC3108-1 except for its unique fixed VOUT options.
Using a small step-up transformer (Fig. 2), the LTC3108 provides a complete power management solution for wireless sensing and data acquisition. The 2.2V LDO powers an external microprocessor, while the main output is programmed to one of four fixed voltages to power a wireless transmitter or sensors. The power good indicator signals that the main output voltage is within regulation. A second output can be enabled by the host. A storage capacitor provides power when the input voltage source is unavailable. Extremely low quiescent current and high efficiency design ensure the fastest possible charge times of the output reservoir capacitor.
