The proliferation of ultralow power wireless sensor nodes for measurement and control, combined with new energy harvesting technology, has made it possible to produce completely autonomous systems that are powered by local ambient energy instead of batteries.
This is a clear benefit when battery replacement or servicing is inconvenient, costly or dangerous. Sensor nodes powered by harvested energy can be deployed in such diverse areas as building automation, wireless/automated metering, predictive maintenance and many other industrial, military, automotive and consumer applications. The benefits of energy harvesting are clear, but an effective energy harvesting system requires a clever power-management scheme to convert the miniscule levels of free energy into a form usable by the wireless sensor system.
It’s all about the duty cycle
Many wireless sensor systems consume very low average power, making them prime candidates to be powered by harvested energy. Because sensor nodes are often used to monitor physical quantities that change slowly, measurements can be taken and transmitted infrequently, resulting in a very low duty cycle of operation and a correspondingly low average power requirement.
For example, if a sensor system requires 3.3V at 30mA (100mW) while awake, but is only active for 10ms every 10 seconds, then the average power required is only 0.1mW, assuming the sensor system current is reduced to microamps during the inactive time between transmit bursts.
Microprocessors and analog sensors that consume only microwatts of power and small, low cost, low power RF transceivers are widely available. The missing link in enabling practical energy harvesting has been a power converter/power management block that can operate from one or more of the common sources of free energy. With its ability to start up at input voltages as low as 20mV, the LTC3108 provides the missing link for thermal energy harvesting, by providing a compact, simple, highly integrated power management solution for powering wireless sensors from a thermoelectric generator (TEG) with temperature differentials (?T) as small as 1°C.
Referring to Figure 1, the LTC3108 uses a small step-up transformer and an internal MOSFET to form a resonant oscillator. With a transformer ratio of 1:100, the converter can start up with inputs as low as 20mV. The transformer secondary winding feeds a charge pump and rectifier circuit, which powers the IC and charges the output capacitors. A 2.2V LDO output is designed to be in regulation first, to power a microprocessor as soon as possible. The main output capacitor is then charged to the voltage programmed by the VS1 and VS2 pins (2.35V, 3.3V, 4.1V or 5.0V) for powering sensors, analog circuitry, or RF transceivers.
Figure 1: LTC3108 block diagram.
(Click on image to enlarge)
The VOUT reservoir capacitor supplies the burst energy required during the low duty cycle load pulse when the wireless sensor is active and transmitting. A switched output (VOUT2) is also provided for powering circuits that do not have a shutdown or sleep mode. A power-good output alerts the host that the main output voltage is near its regulated value.
Once VOUT is in regulation, harvested current is diverted to the VSTORE pin for charging an optional storage capacitor or rechargeable battery. This storage element can be used to power the system in the event that the energy harvesting source is intermittent. (There is also an LTC3108-1 which is identical to the LTC3108 except that it provides a different set of selectable output voltages: 2.5V, 3.0V, 3.7V or 4.5V.)
The basics of thermoelectric generators
Thermoelectric generators (TEGs) are simply TECs (thermoelectric coolers) operating in reverse. TEGs convert a temperature differential across the device (resulting heat flow through it), into a voltage via the Seebeck effect. The magnitude and polarity of the output voltage is dependent on the magnitude and polarity of the temperature differential across the TEG. If the hot and cold sides of the TEG are swapped, the output voltage changes polarity. A TEG can be modeled as a temperature-dependent voltage source with a series resistance (specified as an AC resistance).
TEGs are available in a wide variety of sizes and electrical specifications. Most modules are square, ranging in size from 10mm to 50mm per side, with a typical thickness of 2mm to 5mm. Their open-circuit output voltage is in the range of 10mV/K to 50mV/K, depending on size. Generally speaking, larger modules provide a larger Vout for a given ?T, but have a higher AC resistance and a lower thermal resistance. The size of the TEG required for a given application depends on the ?T available, the maximum average power required by the load, and the thermal resistance of the heat sink used to cool one side of the TEG.
To extract the maximum amount of power available from the TEG, the converter input impedance must provide a reasonable load match to the TEG AC resistance. The LTC3108 converter presents an input impedance of about 2.5Ω, which falls in the middle of the range for most TEG AC resistances (0.5Ω to 7.5Ω).
When placing a TEG on a warm surface for purposes of energy harvesting, a heat sink must be added to the cool side of the TEG to allow heat transfer to the ambient air. The ?T seen across the TEG will be less than the difference between the warm surface and the ambient, due to the thermal resistance of the heat sink, because the TEG has a relatively low thermal resistance (typically in the range of 1°C/W to 20°C/W).
Referring to the simple thermal model in Figure 2, consider an example where a large piece of machinery is operating with a surface temperature of 35°C in a surrounding ambient temperature of 25°C. A TEG is attached to the machinery, with a heat sink on the cool (ambient) side of the TEG.
Figure 2: Simple thermal model of the TEG and heat sink.
(Click on image to enlarge)
The thermal resistance of the heat sink and the TEG dictate what portion of the total 10o
C ?T exists across the TEG. Assuming the thermal resistance of the heat source (RS) is negligible, if the thermal resistance of the TEG (RTEG) is 4°C/W, and the thermal resistance of the heat sink (RHS) is 4°C/W, the resulting ?T across the TEG is only 5°C.
Large TEGs have lower thermal resistance than smaller ones due to their increased surface area, and therefore require a larger heat sink to be of benefit. In applications where a relatively small heat sink must be used due to size or cost constraints, a smaller TEG may provide more output power than a large one. A heat sink with a thermal resistance equal to or less than that of the TEG will maximize the electrical output by maximizing the temperature drop across the TEG.
(Part 2 will look at a pulsed-load application design example, and Thermal-harvesting applications requiring autopolarity.)
About the author
David Salerno is a Design Section Leader with Linear Technology Corporation.