Bladeless rotational piezoelectric energy harvester for hydroelectric applications of ultra-low and wide-range flow rates

Highlights

• A bladeless rotational piezoelectric hydroelectric energy harvester is proposed.

• The proposed structure harvests energy from liquid with ultra-low energy density.

• A magnetic wheel fixed to the tipping bucket is introduced.

• The proposed structure works at wide-range flow rates.

• A 940 μJ of electrical energy was stored in the 470 μF capacitor.

Abstract

In this paper, a bladeless rotational piezoelectric energy harvester using a tipping bucket is proposed to overcome the energy collection problem from liquid with ultra-low flow and ultra-low head. It is typically difficult to collect ultra-low flow liquid energy using the blade rotation of hydro turbines. To overcome this problem, a magnetic wheel fixed to the tipping bucket is introduced in the proposed structure, which is intermittently turned multiple times through the shaft under the continuous excitation of ultra-low flow. Consequently, the mechanical energy of the ultra-low flow can be intermittently converted into rotational energy through passively adjusting the center of gravity of the tipping bucket. Then, the rotational energy is converted to electric energy through the non-contact nonlinear magnetic interaction between the magnetic wheel and a piezoelectric cantilever beam with a small magnetic mass at the free end. Based on the theoretical model of the proposed structure, the various parameters affecting the energy harvesting performance are discussed in detail. Experiments were also conducted to validate the energy harvesting performance of the proposed structure. The results show that 940 μJ of electrical energy can be stored in the capacitor for powering small electronic devices when the proposed structure is used to charge the 470 μF capacitor using the rectifier circuit at 1000 mL/min.

Introduction

Hydropower technologies are widely used for large-scale electrical power generation systems where hydropower turbines drive generators to produce electricity. For large-scale hydropower generation, there are two main types of hydraulic turbines [1] classified according to different water head and flow rate conditions that are used: impulse turbine and reaction turbine. The impulse turbine is a horizontal or vertical wheel driven by powerful jets of water generated by a high head of water striking the buckets or blades on the wheel, thus causing rotation. Presently, the main type of impulse turbine employed is the Pelton turbine for high heads and low flow rates. In contrast, the reaction turbine is activated by water whose pressure changes as it flows through the turbine and releases energy. A frequently used type of reaction turbine is the propeller turbine for low heads and high flow rates.

However, these hydro turbines may not be feasible for small-scale low hydropower generation (i.e., generating power from microwatts and milliwatts to watts), especially for low heads and/or low flow rates. The small-scale low hydropower generation can provide energy to supply remote sensors in natural environments, such as forests where solar or wind energy sources are not suitable or available. For areas in the wild where water resources are limited, small hydroelectric structures may provide sufficient power to supply sensors over long periods. Based on the principles of fluid mechanics, computational fluid dynamics modelling [2], [3], [4], [5], [6], [7] can use numerical methods and algorithms to solve fluid energy harvesting problems that involve fluid flows. To convert water flow energy into vibration energy under different water flow environments [8], a number of research works related to low hydropower generation techniques, such as vortex shedding [9], [10], [11] and flapping motions [12], [13], have been conducted.

For small-scale energy harvesting methods [14], [15], [16], [17], the use of piezoelectric materials [18], [19], [20], [21], [22], [23] afford many advantages, such as high power density and relatively high voltage. Song et al. [24] proposed an upright vortex-induced piezoelectric energy harvester inspired by the vortex-induced vibration of a cylinder in flowing fluid. The results showed that the maximum output power is 84.49 μW with an energy density 60.35 mW/m² at a velocity of 0.35 m/s. Wang et al. [25] proposed a flapping airflow energy harvester based on the oscillations of a horizontal cantilever beam facing the airflow direction. The results indicated that power can be enhanced by up to 30% when the external magnetic excitation is properly integrated. Tang et al. [26] proposed a new energy-harvesting device called the flutter-mill, which utilises the self-induced vibrations of a thin flexible cantilevered plate in axial flow. They demonstrated that the flutter-mill can be designed with a compact size and achieve high performance.

In addition, Lee et al. [27] proposed a piezoelectric flow energy-harvesting structure working in an internal flow environment. Their study results showed that the harvester can achieve 20 mW at a flow rate of 20 L/min. Morais et al. [28] utilised a commercial hydro-generator as an energy-harvesting device for small stationary data acquisition platforms placed in irrigation pipes. A small quantity of water was derived from the pipes to drive the turbine coupled with a direct current (DC) generator. The results showed that the hydro-generator produced 80 mW at a load and flow rate of 100 Ω and 30 000 L/min, respectively. Wang et al. [29] developed a piezoelectric film energy harvester to generate power from pressurised water flow. They found that the maximum generated voltage and instantaneous power were 72 mV_pp and 0.45 nW, respectively, when the excitation pressure oscillates with an amplitude of 20.8 kPa and a frequency of approximately 45 Hz. Bao et al. [30] proposed a piezoelectric hydro-energy harvester using a special cylindrical container for low head and low flow rate applications. The container is fixed on the free end of a piezoelectric cantilever beam. The self-turning function of the container enables the beam to have a self-excited function for hydro energy harvesting. They also proposed two rain energy harvester structures [31], [32] based on the self-turn function of the special container that can be used together with the traditional raindrop energy collection structures, thus enhancing the energy harvesting performance under light rain conditions.

However, in the previous studies on hydro energy harvesters using a self-flip container, only the gravitational potential energy of the water in the container was collected in various forms; the rotational kinetic energy of such a container cannot be harvested. In addition, the previous studies may not be compatible with medium and high flow rates. In other words, at medium and high flow rates, previous structures cannot capture more kinetic energy of water flow which will be converted into the rotational kinetic energy of the bucket. In this research, to overcome this limitation, a rotational piezoelectric energy harvester using a tipping bucket is proposed for ultra-low flow and ultra-low head applications. At the same time, it can also capture the kinetic energy of medium and high flow. Thus, the proposed structure is suitable for energy harvesting from fluids with wide water flow rate. The proposed structure consists of a magnetic wheel, tipping bucket, and piezoelectric cantilever beam with a small magnetic mass at its free end. In several studies, a rotating structure was employed for rotational piezoelectric energy harvesting [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. Different from previous research, the magnetic wheel in the proposed structure is fixed to the tipping bucket with a self-flip function. The magnetic wheel fixed to the tipping bucket interacts with the cantilever beam in a non-contact manner, which does not waste energy and can also improve the service life of the energy harvesting structure. Owing to the change in the centre of gravity of the tipping bucket under continuous ultra-low flow excitation, when the bucket is full, it can self-flip to release its water content over a brief period and then reset itself. As the bucket rotates, the magnetic wheel intermittently turns multiple times. This intermittently converts the mechanical energy of ultra-low into the rotational energy of the magnetic wheel. Through the non-contact nonlinear magnetic interaction between the magnetic wheel and beam with the small magnetic mass at the free end, the rotational energy is converted into the elastic potential energy of the beam. Then, the elastic potential energy is converted to electrical energy through the piezoelectric effects. The paper includes six sections. After a brief introduction on the area and significance of the work, the proposed energy harvester is briefly described in Section 2. In Section 3, the theoretical modelling of the proposed energy harvester is presented. Section 4 discusses the theoretical results, and Section 5 explains the experimental validation of the proposed energy harvester. Finally, the conclusions are summarised in Section 6.