Give away medical masks when you place an order. learn more
Battery size constraints in energy harvesting systems necessitate creativity and innovation in order to maximize efficiency of the power source. This article will address several existing novel techniques and applications that will assist designer choices in implementing the best power solution for use in a particular energy harvesting system.
Rather than a limit on total energy, harvesting transducers impose a limit on the instantaneous power available. Since sensor networks have a physically embedded nature, they are especially well suited to exploit ambient energy sources available in certain applications.
Numerous harvesting techniques (solar, vibrational, biochemical, and motion-based) have been tried and tested. One of the challenges of harvesting energy from ambient is to convert, transfer, and store the usable power effectively. In this context, there is a need to understand and design efficient energy harvesting power management circuitry.
The typical architecture for energy harvesting power (Figure 1) consists of a power processing interface (voltage rectifier or doubler in the case of an AC signal), electrical energy storage mechanism and voltage regulation (if needed, probably using a switching regulator for efficiency).
Figure 1: Power management energy harvesting circuit in blue. (Courtesy Engineering and Physical Sciences Research Council)
In this case, we are examining a variable frequency and amplitude source of energy, and for this vibration example in particular, we look at the piezoelectric transducer.
The power processing interface—AC to DC voltage rectifier
Piezoelectric sensors respond to vibration with an AC output signal (If the energy harvesting sensor has a DC output, then no rectifier will be needed and this block is simplified). Designers will most likely want to power some sort of semiconductor device which usually will require a DC voltage. Then, our first step is to rectify the AC sensor output in the piezoelectric case.
One efficient way to construct the rectifier architecture is the AC-DC doubler. Compared to a full-wave rectifier with only one large capacitor, the two half-wave rectifiers in cascade need two large capacitors, but the output DC voltage is doubled. For low vibration levels, the doubler output VS can be high enough to be used, in many cases, directly by the load without using an additional DC to DC converter. The full-wave voltage doubler is composed of a pair of series-stacked half-wave rectifiers (Fig. 2). The bottom rectifier charges C1 on the negative half cycle of input. The top rectifier charges C2 on the positive half-cycle. Each capacitor takes on a charge of 5 V (4.3 V considering diode drop). The output at node 5 is the series total of C1 + C2 or 10 V (8.6 V with diode drops).
Figure 2: Full-wave voltage doubler consisting of two half-wave rectifiers operating on alternating polarities. (Courtesy of Tony R. Kuphaldt, author, “All about circuits”)
Note that the output v(5), in Figure 3 reaches full value within one cycle of the input v(2) excursion.
Figure 3: Full-wave voltage doubler: v(2) input, v(3)voltage at mid-point, v(5) voltage at output. (Courtesy of Tony R. Kuphaldt, “All about circuits”)
Electrical energy storage
Once the harvested sensor signal is rectified to DC, we will want to store it somehow. Replacing batteries is usually the goal, so typically a super capacitor is used for this storage role. One advantage of the super capacitor is that it has more charge and discharge cycles than a battery can supply. Other advantages of the super capacitor are high power density, high cycle efficiency, longer lifetime, and lower toxicity of material used as compared to batteries. An issue with super capacitors in energy harvesting systems is that of “cold booting.” This is a condition that occurs when the system starts running from zero stored energy. If the system starts booting up as soon as it has harvested enough energy, it is likely to drain the energy shortly after booting, forcing the system to reset and repeat the cycle of futile attempts to boot up. The better solution is to hold off booting until sufficient energy has been harvested, although being too conservative translates into increased latency. Typically comparators have been used for this function.
Cymbet has some energy harvesting evaluation boards that use their thin film EnerChip energy storage devices.
Voltage regulation—DC to DC step up or down regulator
Our next step is to stabilize the voltage with a regulator and convert it to the needed voltage to power our circuit. A regulator will also lower the ripple seen at its input. This regulator circuit must have low quiescent current and we must maximize its efficiency with the device chosen. Light load modes or discontinuous switching mode is preferable for optimum efficiency management at light loads. When light loads are sensed, the regulator enters into a discontinuous current mode (DCM), Pulse-skipping begins and quiescent current typically drops to 10s of uA. Texas Instruments’ TPS62120 is such a device for energy harvesting with integrated MOSFET’s. If a switching regulator is selected without internal MOSFET’s, then TI has the very efficient NEXFET Series, we show as an example an N-channel part CSD16406Q3 in this link.
The load circuit—computational load
Understanding the load characteristics and need for energy is critical to the eventual optimal efficiency architecture we may choose. Computational load circuits are responsible for calculating the required parameters (e.g. damping, frequency, or operating point) and sending these demand signals to the energy harvester and the power processing interface. The computational load also does the work of the wireless sensor, containing the wireless transceiver, sensors, and application software if used.
Maximizing the energy conversion efficiency with energy adaptive maximum power point tracking (EA-MPPT)
For piezoelectric materials at a given vibration status (magnitude and frequency), there exists an optimum output voltage at which maximum output power can be extracted.¹ An ultra-low-power maximum power point tracking (MPPT) scheme in a time-varying environment can be considered that tracks and holds the maximum power point periodically using a very small duty cycle, and consumes only a fraction of the power required by prior MPPT schemes. All functional blocks are activated or shut down together.
Let’s examine the typical characteristics of a solar panel (Figure 1) to explain MPPT. Isc is a short-circuit current that flows through the panel when the panel is short circuited. It is the maximum current that can be obtained from the panel.
Voc is the open-circuit voltage at the terminals of the panel. Vmp and Imp are the voltage and current values at which maximum power can be obtained from the panel. As the sunlight reduces the maximum current (Isc) which can be obtained, the maximum current from the panel also reduces.
Figure 4: I-V characteristics of solar panel. (Courtesy of Texas Instruments)
Now let’s look at variation of I-V characteristics with sunlight (Figure 5). The blue curve connects the points of the maximum power at different values of insolation (a measure of solar radiation energy received on a given surface area in a given time).
The purpose of the MPPT circuit is to maintain the operating point of the panel at the maximum power point in different sunlight conditions. As seen from Figure 5, the voltage at which maximum power is transferred does not change very much with sunlight. Using this method is the most efficient method to connect the sensor to a battery when the sensor has a DC output.
MPPT in a piezoelectric device is similarly modeled. All power, current, voltage, capacitor, and switch designations are detailed in Figure 6 for a battery-less power management unit (PMU) that manages harvested low level vibration energy from a piezoelectric device for a wireless sensor node example.¹
Figure 6: Piezoelectric system operational diagram. (Courtesy of Integrated Power Electronics Laboratory, The Hong Kong University of Science and Technology)
A piezoelectric film is modeled by a sinusoidal current source I(t) = Ipsin(2πft) in parallel with the internal capacitance Cp. The amplitude Ip depends upon the vibration magnitude and f is the vibration frequency. If one uses transistors as active diodes, the forward drop Vdp is close to zero, and the average rectifier output current Iinavg and the average harvested power Pinavg are:
Where VS is the output voltage of the rectifier, maximum power is obtained when VS is:
From Equation 2 and 3 the maximum power is:
To obtain the voltage VS(opt), we use the MPPT method of Equation 2, which disconnects the load by opening SIN for a very short time and senses the open-circuit peak voltage of the piezoelectric source for control:
It is approximately equal to the optimum voltage VS(opt) when the voltage drop Vdp is small.
The power management unit (PMU) should activate different blocks as the input power level changes. For example, at very low input power, the whole system may be duty-cycled; at medium input power the critical blocks work continuously while the rest are duty-cycled; and at high input power, all blocks are activated. To achieve energy-adaptive MPPT control, information on the absolute or relative amount of available power is needed. The problem is how to obtain and utilize this information without using power-demanding computational methods such as quantizing voltages and/or currents.
Now, here comes the energy-adaptive MPPT technique that will greatly improve the power efficiency. Since the value of VS(opt) is not adequate by itself to tell whether the available power increases or decreases, if we want to adaptively operate the individual part in different modes, a method to detect the absolute or relative amount of available power is needed.
If we use a simple “load perturbation” technique¹ and watch for level crossing events, we will be able to obtain the information of the available power level relative to the load power level.
When using a microcontroller, Energy Micro has a well-suited solution for Energy Harvesting. The 32-bit EFM32 MCU is especially applicable for use in low-power and energy sensitive applications. It is a solution for applications that need to operate for as long as possible (even decades) without external power or operator intervention. Battery replacement is often not possible for reasons of access and cost.
In summary the microcontroller is characterized by very low active power consumption, reduced processing time, very fast wake-up time and ultra-low standby current. To achieve such characteristics, EFM32 has broken with microcontroller design convention in some very distinct ways. These characteristics should be in any microcontroller considered for energy harvesting: