Piezoelectric energy harvesting : enhancing power output by device optimisation and circuit techniques

Energy harvesting; that is, harvesting small amounts of energy from environmental sources such as solar, air flow or vibrations using small-scale (≈1cm) devices, offers the prospect of powering portable electronic devices such as GPS receivers and mobile phones, and sensing devices used in remote applications: wireless sensor nodes, without the use of batteries. Numerous studies have shown that power densities of energy harvesting devices can be hundreds of μW; however the literature also reveals that power requirements of many electronic devices are in the mW range. Therefore, a key challenge for the successful deployment of energy harvesting technology remains, in many cases, the provision of adequate power. This thesis aims to address this challenge by investigating two methods of enhancing the power output of a piezoelectric-based vibration energy harvesting device. The thesis starts with a survey of the power requirements of a range of electronic devices and systems. It then moves to a review of the various energy harvesting technologies available for harvesting from the different types of energy source, including: solar, air flow, thermal, pressure variations, radio frequency radiation and mechanical. For each energy source, the most commonly employed mechanism for energy conversion is detailed, relative advantages and disadvantages are outlined, and power output levels that have been achieved through experiment are reported. A comparison is then made between power levels harvested, and power requirements of electronic devices and systems. Following this, the piezoelectric concept is introduced, both in historical and technological contexts, and a review of the history and state-of-the-art in power enhancement of piezoelectric generators is given, which includes both power enhancement through advances in the design of the generator and power enhancement through use of the harvesting circuitry. The thesis then moves to studying two methods of enhancing the power output of a piezoelectric-based vibration energy harvesting device. The first method concerns the harvesting device itself. Through initial investigation, the power output of a prototype device, fabricated from a commonly available piezoelectric buzzer, was ascertained using a simple test setup built around the ‘sweep in’ function of a function generator; the result was 8. 18μW. The prototype device was built without any guidance on how the geometric dimensions of it might affect its power output. Therefore, to achieve a higher power output, it was considered that a modelling approach might be useful. However, it was considered that the model should not simply predict outputs of the device (e.g. power or voltage) when provided with dimensions and material characteristics, rather, the model should be used with an optimisation algorithm to provide the device design itself; i.e. the model should be designed for use with a computer-based optimisation algorithm, such that the output of the optimisation process is a list of values for the device dimensions that together constitute a design that has been optimised for maximum power output. To this aim, an analytical model has been developed. The model results in an expression for the power output of the device that, because it has within its arguments all of the dimensional parameters of the device, can be used with a computer-based optimisation algorithm to optimise the geometrical parameters of the device. An example of how the model can be used is given using the complex conjugate optimisation algorithm provided by Mathcad 2000 Professional software, and a design for a device optimised for maximum power output given a volume constraint of 1cm was obtained. The optimised device was then fabricated and tested using an improved test setup based on National Instruments hardware and software, and the maximum power achieved from the optimised harvesting device, under vibration acceleration amplitude of ±0.23g, was 370. 37μW, which occurred with a 325kΩ resistive load. A voltage amplitude of 15.52V was measured at this maximum power level, and the resonant frequency of the device with a 325kΩ load was 87Hz. The second method investigated concerns the harvesting circuitry. Initial investigations were performed using the prototype harvesting device and two bridge rectifier circuits: one using silicon rectifier diodes and the other using schottky-type diodes. It was found that although schottky-type diodes have a lower forward bias voltage, their higher reverse current meant that more energy was harvested using the silicon rectifier diodes (13.5μJ in comparison with 7.6μJ). Since the bridge rectifier is the circuit that is most usually used as an interface to energy harvesting devices, the operation of it was also analysed via circuit simulation, and two main mechanisms that inhibit power transfer from the harvesting device to load were observed: 1) power is lost due to the internal impedance of the generator, 2) during steady-state operation, the generator spends much of its time in the open-circuit condition. The latter mechanism occurs because under steady-state conditions the smoothing capacitor is already charged, and therefore the diodes don’t conduct until the voltage output of the harvesting device reaches the capacitor voltage plus the forward bias voltage of one of the diodes in the rectifier. Following this, a new harvesting circuit concept was proposed. The new concept comprises: a Synchronised Switch Harvesting on Inductor (SSHI) functional block, which boosts the output voltage of the piezoelectric generator; a ‘storage’ functional block, in which charge is collected per half-cycle of the piezoelectric generator output and then dumped into a large storage capacitor; and a ‘DC-DC converter’ block, which uses the reservoir of charge in the storage block to power a buck-boost DC-DC converter, which in turn can provide regulated DC power to an end application system. The new concept holds several advantages over the traditional bridge rectifier, which are: 1) the concept takes advantage of the increase in the voltage output of the harvesting device that results from using the SSHI technique; 2) the concept can harvest during the whole of each AC cycle, unlike the bridge rectifier circuit; 3) the concept inherently converts from AC to DC, by the accumulation of charge in two storage capacitors (one collecting positive charge and one collecting negative charge) which are then connected together in series to form an energy reservoir with a positive terminal and a negative terminal. This negates the need for four rectifier diodes and allows for the connection of a buck-boost DC-DC converter; and 4) through impedance matching to the piezoelectric generator, maximum power transfer can occur at all times. In addition, because the concept adopts the philosophy of storing charge over time such that an energy reservoir is built up (from which the application system can then be powered), higher power applications can be powered than would be possible by using a ‘continuous powered’ approach, such as the bridge rectifier. The concept also uses pre-existing maxima and minima detection circuitry (since it is already required for the SSHI technique) therefore a minimum of extra components needs to be added, and the concept is simple: if built into a full energy harvesting system, the controlling microprocessor only needs to fulfil a few functions and can operate with a very low clock rate, and therefore it can be very low power and a small size. A prototype circuit that can implement the concept was designed and built, using a mixture of discrete and integrated components, and the performance of the concept was assessed through two different approaches: (1) an experimental approach was adopted to compare it with a bridge rectifier circuit in terms of the amount of energy that could be harvested into storage capacitance (2) a simulation approach was adopted in order to compare it with another technique reported in the literature in terms of maximum average power output over time. Through the experimental approach, it was found that the new concept harvested 247% more energy into storage capacitance than the bridge rectifier circuit in a similar length of time and for the same input vibration conditions. Through the simulation approach, it was found that the maximum average power output of the piezoelectric harvesting device when the new concept was used was nearly eleven times more than when the technique reported in the literature was used. Both the analytical model and new harvesting circuit concept developed in this thesis provide methods by which the power output of a piezoelectric-based vibration harvester can be enhanced: by using the model, the power output of the optimised device was an improvement by a factor of 45 over the power output of the prototype device, and through use of the new circuit concept, 247% more energy can be harvested into storage capacitance than if the bridge rectifier circuit is used. The simulations also show that the new concept enhances the power output of the harvesting device, by as much as eleven times that achieved through the use of another technique given in the literature. An enhanced level of output power means that a greater number of potential applications can be realised, and this study goes some way towards closing the gap that exists between the power requirements of electronic devices and systems, and the amount of power that can be harvested by piezoelectric conversion of vibration energy. Page intentionally blank