Scott McGarry

The Potential for Harvesting Energy from the Movement of Trees

Over the last decade, wireless devices have decreased in size and power requirements. These devices generally use batteries as a power source but can employ additional means of power, such as solar, thermal or wind energy. However, sensor networks are often deployed in conditions of minimal lighting and thermal gradient such as densely wooded environments, where even normal wind energy harvesting is limited. In these cases a possible source of energy is from the motion of the trees themselves. We investigated the amount of energy and power available from the motion of a tree in a sheltered position, during Beaufort 4 winds. We measured the work performed by the tree to lift a mass, we measured horizontal acceleration of free movement, and we determined the angular deflection of the movement of the tree trunk, to determine the energy and power available to various types of harvesting devices. We found that the amount of power available from the tree, as demonstrated by lifting a mass, compares favourably with the power required to run a wireless sensor node.

Development and Successful Application of a Tree Movement Energy Harvesting Device, to Power a Wireless Sensor Node

Wireless sensor networks are becoming increasingly more common as a means to sense, measure, record and transmit data for scientific and engineering evaluation, remotely and autonomously. Usually, remotely located sensor nodes are powered by batteries which are recharged by solar or wind energy harvesters. Sometimes nodes are located in areas where these forms of energy harvesting are not possible due to local conditions, such as under the canopy of a forest. This article outlines the design and testing of a device capable of harvesting energy from tree movement, and shows the device powering a wireless sensor node continuously. The device uses the force and displacement of the movement of a tree trunk (of a 6 m tall tree) to drive an electromagnetic generator that recharges a nickel metal hydride battery. The battery stores the energy from which a ∼0.5 mW wireless sensor node is powered continuously. This demonstrated method of energy harvesting may allow the placement and powering of nodes in locations previously not possible.

A practical application of using tree movement to power a wireless sensor node

A novel energy harvester based around capturing the motion of trees has been built and tested. The device consists of an electromagnetic generator located close to ground level, attached via an inelastic cord to a point on the trunk of a 5-6 meter tall eucalypt tree. The device uses the movement of the tree to drive the generator in one direction, rotationally, and a mass to keep the cord taught when the tree returns to its resting position. The electrical output is sent to electrical circuitry that rectifies, stores and switches the electrical power to supply a wireless sensor node. The initial configuration stored energy in a super-capacitor, the voltage of which indicates storage charge level. Once there was sufficient power to operate the sensor node it transmits local information such as temperature, and energy state, in terms of capacitor voltage, to a base node located approximately 80m away. Results show that there is sufficient energy in this method to power a wireless sensor node continuously in wind as low as 3-4m/s. In order to allow continuous operation in lower wind speeds a number of alterations have been investigated. These are reported here and include: operation with a secondary battery in place of the storage capacitor, increasing the electrical storage capacity and varying the connection point on the tree and the electronic duty cycle.

Evaluation of flexible transducers for motion energy harvesting

Personal electronic devices such as mobile/cell phones, radios and wireless sensors traditionally depend on energy storage technologies, such as batteries, for operation. By harvesting energy from the local environment, these devices can achieve greater run-times without the need for battery recharging or replacement. Harvesting energy could also achieve a reduction in the weight and volume of the personal devices - as batteries often make up more than half the weight/volume of these devices. Motion energy harvesting is one such approach where energy from mechanical motion can be converted into electrical energy. This can be achieved through the use of flexible piezoelectric transducer materials such as polyvinylidene fluoride (PVDF). A problem with these transducer materials it that their behaviour is non-linear due to operating and environmental conditions. Hence, for this reason researchers have found it has been difficult to measure the harvesting performance i.e. mechanical-to-electrical conversion efficiency. At CSIRO we are currently evaluating the performance of flexible transducers for use as motion energy harvesters. Preliminary results suggest an overall energy harvesting conversion efficiency of 0.65% for a flexible transducer material.

Other Thermomechanical Heat Engines

In Chap. 2, the discussion centered on traditional heat engine cycles that were developed at large to very large scale for industrial power generation. This chapter discusses smaller scale methods of converting a thermal difference into mechanical energy that are applicable at a micro-electro mechanical systems (MEMS) scale. The methods examined include thermomagnetic engines, shape memory alloy (SMA) engines, and hydride heat engines.

Artificial muscles on heat

Many devices and processes produce low grade waste heat. Some of these include combustion engines, electrical circuits, biological processes and industrial processes. To harvest this heat energy thermoelectric devices, using the Seebeck effect, are commonly used. However, these devices have limitations in efficiency, and usable voltage. This paper investigates the viability of a Stirling engine coupled to an artificial muscle energy harvester to efficiently convert heat energy into electrical energy. The results present the testing of the prototype generator which produced 200 μW when operating at 75°C. Pathways for improved performance are discussed which include optimising the electronic control of the artificial muscle, adjusting the mechanical properties of the artificial muscle to work optimally with the remainder of the system, good sealing, and tuning the resonance of the displacer to minimise the power required to drive it.

An Introduction to Waste Heat Capture and MEMS

Waste heat is all around us. Every energetic process, regardless of its initial form (kinetic, chemical, or electrical), eventually ends as heat, which eventually degrades to ambient temperature. In many situations, the heat could be captured and converted to electrical energy.

Established Thermomechanical Heat Engine Cycles

This chapter describes and discusses the four most common external combustion thermodynamic cycles: Stirling, Brayton, Ericsson, and Rankine. Internal combustion thermodynamic cycles, such as Otto, Diesel, and rocket, will not be considered, because it is impractical to use them for waste heat capture.

Thermal to Electrical Energy Converters

The previous chapters discussed the generation of mechanical motion from thermal energy and the subsequent conversion of this to electrical energy. The additional step from thermal to electrical energy can introduce further losses, reducing overall efficiency. In this chapter, the conversion of thermal energy directly to electrical energy is discussed.

Mechanical to Electrical Conversion

As described in Chap. 3 many methods of harvesting thermal energy convert heat energy into mechanical energy; often, this is vibration. While mechanical energy may be of use in some systems, an additional stage of converting energy from mechanics or kinetics to electrical energy is generally required. In this chapter, devices that convert mechanical energy to electrical energy will be referred to as transducers.