Onoriu Puscasu

Semi-flexible bimetal-based thermal energy harvesters

This paper introduces a new semi-flexible device able to turn thermal gradients into electricity by using a curved bimetal coupled to an electret-based converter. In fact, a two-step conversion is carried out: (i) a curved bimetal turns the thermal gradient into a mechanical oscillation that is then (ii) converted into electricity thanks to an electrostatic converter using electrets in Teflon?. The semi-flexible and low-cost design of these new energy converters pave the way to mass production over large areas of thermal energy harvesters. Raw output powers up to 13.46??W per device were reached on a hot source at 60??C?with forced convection. Then, a DC-to-DC flyback converter has been sized to turn the energy harvesters? raw output powers into a viable supply source for an electronic circuit (DC@3?V). At the end, 10??W of directly usable output power were reached with 3 devices, which is compatible with wireless sensor network powering applications.

Innovative thermal energy harvesting for zero power electronics

Thermal gradients, commonly present in our environment (fluid lines, warm fronts, electronics) are sources of energy rarely used today. This paper aims to present innovative approaches of thin and/or flexible thermal energy harvesters for smart and autonomous sensor network applications. The harvester system will be based on the collaborative work of interrelated energy nodes/units, which will be either piezo-thermofluidic converters (use of rapid thermal cycles of a working fluid) or piezo-thermomechanic converters (use of the mechanical energy developed by rapid snapping of micro-switches). The two kinds of energy nodes convert a heat flux into storable electrical energy through a piezoelectric transducer. Miniaturization of the energy nodes will lead to increased thermal transfer rates and consequently increased harvested power. To effectively use thermal energy sources in varying environments, the nodes will be adaptive versus different thermal gradients (in a predefined temperature range) and will possibly influence each other. The concept is unique in the sense that it is based on a matrix structure of micro or mini energy nodes which will work together in a collective approach to optimize the harvested energy, and which do not require the use of radiators as classical Seebeck approach, thanks to the controlled thermal resistance. This opens the door to new properties and features of the object, with better performances. It could therefore be declined on flexible substrates, allowing conformability around the sources of potential heat for low power applications.

Piezoelectric and electrostatic bimetal-based thermal energy harvesters

This paper reports on innovative thermal energy harvesters (TEH) turning heat fluxes into electricity in a two-step conversion, involving (i) a curved bimetallic strip converting thermal gradients into mechanical oscillations, which are then (ii) converted into electricity by a piezoelectric or an electret-based electrostatic transducer. This work mainly focuses on (i) the optimizations of the piezoelectric devices, (ii) a first demonstration of a Wireless Sensor Node powered by our electrostatic transducers, validating the viability of bimetal-based thermal energy harvesters, and (iii) the possibility of future scaled scavengers by a micrometric silicon approach to improve efficiencies and power densities.

An innovative heat harvesting technology (HEATec) for above-Seebeck performance

An innovative approach to heat energy harvesting (HEATec) is proposed in this paper. It consists of a two-step conversion of heat into electricity. The first step is a thermo-mechanical conversion by a bimetal and the second is an electromechanical conversion by a piezoelectric. The first developed prototypes show natural thermal resistance matching between their body and the interface with ambient air, and therefore do not need a heat sink in order to work. The available mechanical power (2.7 mW/cm2 measured in practice for a single bimetal, and extendable to theoretical 27 mW/cm2 for 100 bimetals occupying the same surface) that can be converted into electricity may lead to a superior performance compared to the best commercial Seebeck devices. Analytical scaling laws for our technology have been established and show power density gain equal to the scaling factor, making it LSI integration favorable.

Scale laws for enhanced power for MEMS based heat energy harvesting

An innovation approach to thermal energy harvesting is presented. It consists of a two step conversion of heat into electricity. The new technique can be used for powering ultra-low power electronics and autonomous systems. One of the keys to improve the generated power density is downscaling of individual devices. Laws modeling downscaling have been established in this paper and show that the miniaturization of the devices by a factor k increases the generated power density by the same factor, due to the increased speed of heat transfer. The scaling laws predict increasing power gain when miniaturizing the devices with use of e.g. VLSI technologies. This can help in providing a strong alternative to Seebeck devices.

A disruptive technology for thermal to electrical energy conversion

A disruptive approach to thermal energy harvesting is presented. The new technique can be used for powering ultra-low power electronics and autonomous systems. We propose a two step conversion of heat into electricity: thermal to mechanical and mechanical to electrical. Devices can work in a wide range of temperatures: from -40°C to 300°C, and the available mechanical power density is in the order of 1 mW/cm2. We evidenced that one of the keys to improve the generated power density is downscaling of individual devices. To demonstrate this point, laws modeling downscaling have been established and show that the miniaturization of the devices by a factor k increases the generated power density by the same factor, thanks to the higher speed of heat transfer.

Thermal energy harvesters with piezoelectric or electrostatic transducer

This paper describes the idea of the energy harvester which converts thermal gradient present in environment into electricity. Two kinds of such devices are proposed and their prototypes are shown and discussed. The main parts of harvesters are bimetallic spring, piezoelectric transducer or electrostatic transducer with electret. The applied piezomembrane was commercial available product but electrets was made by authors. In the paper a fabrication procedure of electrets formed by the corona discharge process is described. Devices were compared in terms of generated power, charging current, and the voltage across a storage capacitor.