Attività

An electric generator is a device that converts mechanical energy obtained from an external source into electrical energy as the output.

It is important to understand that a generator does not actually ‘create’ electrical energy. Instead, it uses the mechanical energy supplied to it to force the movement of electric charges present in the wire of its windings through an external electric circuit. This flow of electric charges constitutes the output electric current supplied by the voltage dip generator. This mechanism can be understood by considering the generator to be analogous to a water pump, which causes the flow of water but does not actually ‘create’ the water flowing through it.

The modern-day generator works on the principle of electromagnetic induction discovered by Michael Faraday in 1831-32. Faraday discovered that the above flow of electric charges could be induced by moving an electrical conductor, such as a wire that contains electric charges, in a magnetic field. This movement creates a voltage difference between the two ends of the wire or electrical conductor, which in turn causes the electric charges to flow, thus generating electric current.

Extensive efforts have been made to harvest energy from water in the form of raindrops1,2,3,4,5,6, river and ocean waves7,8, tides9 and others10,11,12,13,14,15,16,17. However, achieving a high density of electrical power generation is challenging. Traditional hydraulic power generation mainly uses electromagnetic generators that are heavy, bulky, and become inefficient with low water supply. An alternative, the water-droplet/solid-based triboelectric nanogenerator, has so far generated peak power densities of less than one watt per square metre, owing to the limitations imposed by interfacial effects—as seen in characterizations of the charge generation and transfer that occur at solid–liquid1,2,3,4 or liquid–liquid5,18 interfaces. Here we develop a device to harvest energy from impinging water droplets by using an architecture that comprises a polytetrafluoroethylene film on an indium tin oxide substrate plus an aluminium electrode. We show that spreading of an impinged water droplet on the device bridges the originally disconnected components into a closed-loop electrical system, transforming the conventional interfacial effect into a bulk effect, and so enhancing the instantaneous power density by several orders of magnitude over equivalent devices that are limited by interfacial effects.
In this paper, we will present a comprehensive state of the art of TEGs. This paper differs from other reviewing papers (Siddique et al., 2017, Patil et al., 2018) in presenting the different types (planar, vertical and mixed) and technologies (silicon, ceramics, and polymers) of TEGs. We will also investigate the latest thermoelectric materials and keys for generating high-efficient power factor with the different TE materials arrangement (conventional, segmented and cascaded). Furthermore, we will present the use of TEGs in high and low-power applications (medical, wearable, IoT, WSN, industrial electronics, automobiles and aerospace applications).

There are three design approaches of TEGs which differs according to the thermocouples’ arrangement on the substrate regarding the heat flow direction (Glatz et al., 2009), which are: (i) Lateral heat flow, lateral TCs arrangement; (ii) Vertical heat flow, vertical TCs arrangement; and (iii) Vertical heat flow, lateral TCs arrangement.

The first TEG design uses a lateral TCs arrangement to convert a lateral heat flow, -Q. In this design, called also planar TEG, thermocouples are printed, patterned or deposited on the substrate surface (Fig. 2a). The main advantage of this approach lies in its ability to manipulate the thickness and the length of each thermocouple arm combined to its suitability with thin film deposition, which allows creating thinner and longer thermocouples compared to other types (Glatz et al., 2009, Kao et al., 2010, Qing et al., 2018). Besides, this arrangement increases the thermal resistance of the thermoelements compared to other TEGs designs because of using lengthy TCs arms which leads to a temperature gradient increasing along these latter, and eventually an output voltage rising.

The second TEG design, i.e. vertical TEG, is made of TCs arranged vertically between the heat source and the heat sink (Fig. 2b) (Aravind et al., 2018). Thus, the heat is flowing vertically along the thermoelement arms and the substrates. This arrangement is similar to the Peltier-based module for refrigeration. This kind of TEGs provides high integration density, and is the most commercialized because of its simplicity, high TCs integration, and high output voltage (Leonov, 2013).

The last TEG design, referred to as mixed, is made by TCs mounted laterally on the substrate, while the heat flows vertically (Fig. 2c) (Sawires et al., 2018, Yan et al., 2019, Huu et al., 2018). The vertical heat transfer was instigated through the integration of micro-cavities into the substrate, located under the thermocouple arms (Ziouche et al., 2017). This technique could be achieved in silicon when using CMOS standard technology, or by a lift-off process in polyimide/polymer-based flexible foil. This latter consists of creating a wavy form in the substrate containing the patterned thermocouples (Hasebe et al., 2004).