Introduction
In the previous chapter, the BLDC motor has been selected to be the propulsive motor of the EUAV. The basic requirement for unmanned aircraft is a portable power source of electrical energy, which is transformed to mechanical energy in the electric motor for UAV propulsion.
Therefore various DC power supply sources are discussed in this chapter, highlighting their advantages and disadvantages, in order to select the proper DC power source to be considered as the EUAV supply system.
UAVs’ power supply sources must possess the following properties: – High power and energy densities – Fast dynamic response. – High efficiency. Defining the following terms to be used throughout the thesis; specific power density is the maximum available power that a supply can deliver per unit weight (W/kg) and per unit volume (W/L), respectively, while specific energy density is defined as the source’s energy storage capacity per unit weight (Wh/kg) and per unit volume (Wh/L), respectively [24]. The DC power supply sources to be discussed are: batteries, solar energy, super-capacitors and fuel cells. 3.2 Batteries It is a constructed device from one or additional electrochemical cells with exterior connections to supply power to electrical devices [25]. In more details, a battery has a positive terminal (anode) and a negative terminal (-ve) terminal (cathode). The -ve terminal is the source of electrons that will flow and transfer energy to an external device while connected to an external circuit. When that connection happens, electrolytes are capable of move as ions within, permitting the chemical reactions to be fulfilled at the separate terminals and so supply energy to the external electric circuit. What allows current to flow out of the battery to achieve work is the movement of those ions within the battery [26]. Primary batteries (single-use or disposable) are used one time and discarded; the electrode materials are irrevocably changed during discharge. Secondary batteries (rechargeable) can be charged and discharged several times; the genuine composition of the electrodes may be recovered by reverse current. . Lead-acid and lithium-ion battery are examples of a secondary battery are used in UAVs and in portable electronics, respectively. 3.2.1 Battery Selection The main types of rechargeable batteries used or being considered for electric and hybrid vehicle applications are: 1) Lead-acid. 2) Nickel-Cadmium (NiCd). 3) Nickel-metal-hydride (NiMH). 4) Sodium Sulfur (NaS). 5) Lithium-ion (Li-Ion). 3.2.1.1 Lead-Acid Battery The oldest type of rechargeable battery is the lead-acid battery, founded by Gaston Plant” in 1859. The weight of the Lead-acid battery is a little concern, is more economical for large power application. Lead acid batteries have many advantages such as; high reliability, high rates of discharge capability, low need for maintenance, low cost and low level of self-discharge. However, they have some drawbacks for example; they cannot be stored in a discharged condition, their electrolyte and lead content are environmentally unfriendly. Also, they have low energy density, poor cold temperature performance, in addition to their short calendar and limited full discharge cycles; are among the obstacles to their use in EUAVs.
3.2.1.2
Nickel-Cadmium Battery This type of batteries has the advantages of superior low-temperature performance, flat discharge voltage, long life, excellent reliability, and low maintenance requirements. But their biggest drawbacks are the high cost and the toxicity contained in cadmium. 3.2.1.3 Nickel-Metal Hydride Battery This type of batteries has approximately two times the energy content of lead-acid batteries of the same weight [26]. It also has a good average specific power and is also environmentally friendly.
About their disadvantages; they have relatively high cost and low cell efficiency. Electrode chare efficiency is highly affected by temperature, so there is a rapid drop in electrode charge efficiency at temperatures over 40”C. Also storage at high temperatures results in limited discharge current, degradation, limited service