Energy storage technologies and their combinational usage in micro/mini unmanned aerial vehicles: a review

2.1. Hybrıd electric propulsion systems

Electric drive systems have a quiet operation, low thermal signature, and low emissions. however, due to the low specific energy of the battery technology commonly used in existing systems, their usage times are short. Hybrid electric propulsion systems can combine the advantages of multiple systems by reducing fuel consumption, aircraft noise, and thermal signature [13-17].

More than one power source is used together in the hybrid electric drive system. The first source is an electric motor, while the other source is usually a conventional internal combustion engine (can also be a fuel cell, battery, or supercapacitor). In systems where electric motors and internal combustion engine are used together, the internal combustion engine is added to increase the range [16]. Aircraft powered by internal combustion engines can fly for long periods due to their high energy density. The range of a vehicle driven by a hybrid electric drive system is shorter than a vehicle driven solely by an internal combustion engine, but its efficiency is higher [15]. The combined thrust of the two power sources allows them to be designed in a smaller size [18].

Furthermore, the advantages of electric propulsion systems and conventional combustion engines can be combined in a single system. Hybrid electric propulsion systems are energy efficient and can be operated over long distances. An energy-efficient aircraft also saves costs and contributes to the conservation of resources. All these advantages also increase the system’s life [19-20].

2.1.1. Series hybrid electric propulsion system

In this architecture, the mechanical power produced by the internal combustion engine is converted into electrical energy with a generator. This power can drive the electric motor or be used to charge the battery. The internal combustion engine is not used directly to generate thrust, so it operates efficiently at its optimum operating point regardless of driving conditions [21]. It is the electric motor that provides the thrust. Fig. 1 shows the series hybrid electric propulsion structure. The dashed lines in the figure show the electrical connection paths.

Control of the system with serial architecture is simple and easy. This system is also compatible with multiple motor and fan structures. However, in serial architecture, a total of 3 drive components are needed together with the generator, therefore the system is heavy. Due to its weight, serial architecture is not preferred in aviation [13]. Also, since the electric motor provides all the thrust, it must be sized accordingly, creating aviation problems. There is a lot of energy loss in the serial architecture and additional components cause an increase in cost and mass. The serial hybrid electric drive architecture is suitable for short trips in aviation [23].

Fig. 1Series hybrid electric propulsion structure [22]

2.1.2. Parallel hybrid electric propulsion system

The electric motor and the combustion engine are connected to the same shaft in a parallel architecture. They can provide the thrust together or separately [18, 21]. In this architecture, two drive components are needed; the electric motor also acts as a generator [23]. Fig. 2 illustrates the parallel hybrid electric propulsion structure.

Fig. 2Parallel hybrid electric propulsion structure [22]

The control strategy of the parallel hybrid electric propulsion system is complex, so it is essential to ensure energy management. In addition, the system cannot operate silently [24]. In addition, the parallel hybrid system has a high specific power and specific energy. Although the specific power is increased in the series hybrid architecture, no systemic changes can be made as long as the battery power remains the same [25]. The parallel architecture consists of fewer drive components, which makes it lighter [26]. For this reason, the parallel structure in aircraft comes to the fore.

2.1.3. Series/parallel hybrid electric propulsion system

In this system, the combustion engine can provide the thrust directly. In addition, the battery or the generator driven by the combustion engine powers the electric motor, and this electric motor can also provide the thrust [22]. Fig. 3 shows the series/parallel hybrid electric propulsion system structure.

This architecture may have some advantages as well as disadvantages to serial and parallel architectures separately. There has been an increase in cost and mass due to the number of drive components. As the number of potential power flow paths increases, so does the power management complexity. The serial structure has the least number of potential power paths. The maximum potential power path is in a series/parallel structure. In other words, it can be sorted as Serial < Parallel < Serial/Parallel according to power management complexity. Serial/parallel structure is preferred in land vehicles because weight is not a problem, but weight is a significant disadvantage for aircraft [26].

Fig. 3Series/parallel hybrid electric propulsion structure [22]

2.2. All electric propulsion system

In hybrid architectures, batteries and gas turbine engines provide thrust at different times, and gas turbine engines are also used to charge the battery. In all-electric systems, the only source of propulsion in the aircraft is the batteries. Fig. 4 shows the construction of an all-electric propulsion system.

Fig. 4All-electric propulsion structure [22]

3.1. Energy storage with flywheel

Flywheels are mechanical batteries. Flywheels have advantages such as long life, high efficiency, high power density, not being affected by temperature, and zero emissions. On the other hand, flywheels are mechanical systems and can break or explode. This means they carry risks. Besides the risk of explosion, flywheels need frequent maintenance. Additionally, due to mechanical movement, friction losses occur in the energy of the flywheels. Flywheels were used to drive the hybrid electric bus Gyrobus in Sweden in the 1950s. Thanks to the flywheel, some of the energy the bus lost during braking was recovered. However, in this bus, the volume occupied by the flywheel was very large, which narrowed the available space. From this perspective, it can be said that the energy that needs to be stored for flywheels should be proportional to the size of the flywheel. Large size also means large weight. Although it is possible to use a flywheel instead of a battery or with a battery in aviation, it is not a preferred system for small UAVs. The flywheel was also used in the Formula 1 championship in the 2009 season. The system in the Formula 1 vehicle is called KERS (Kinetic Energy Recovery System) and it is known that the rotor weight of the flywheel in this vehicle is 5 kilograms. Since the flywheel is a rotating mass, it needs to be balanced and balance errors cause vibration. To prevent this, additional weights are placed on the flywheel. This again shows that flywheels are not a good option for UAVs [31-32].

3.2. Energy storage with SMES (superconductor magnetic energy storage) system

In this system, there is a coil wrapped with superconducting wires, and electrical energy is stored in the magnetic field in this coil. The system efficiency is high, and the response time is short (in the order of milliseconds). The stored energy can be given back to the system at any time [30]. The cooling system used for SMES can be quite complex and costly [33]. In addition, SMES are more expensive than other storage technologies and require a larger working volume [31]. They are not preferred for small-sized UAVs.

There are different energy storage technologies used or in use in aviation. In this study, energy storage technologies used in small-sized UAVs are examined in detail. These technologies are batteries, fuel cells, and supercapacitors. Batteries and fuel cells store energy chemically, while supercapacitors store it physically [34].

3.3. Energy storage with batteries

Batteries are the most widely used energy storage technology in electric aircraft. Batteries are systems that store chemical energy and convert it into electrical energy when necessary [35]. A typical battery; consists of two electrodes, an anode and cathode, an electrolyte, and a separator between them [36]. Fig. 5 shows the battery structure. While the anode is corroded during electrolysis, an electric current occurs at the cathode due to ionic exchange. The electrical energy formed from this reaction is used in various tools [37].

The electrical model of batteries can be expressed in several different circuits. The difference between them consists in emphasizing the importance of each circuit element. The simplest of these models is the internal resistance model. The Fig. 6 shows the internal resistance model of the battery. The internal resistance model describes the relationship between voltage drop and operating current.

Fig. 5Chargeable cell/battery diagram [28]

Fig. 6Battery electrical model [38]

In this model, the required power is calculated by multiplying the terminal voltage ($V_t$) and the battery current ($I$). Power equation:

Terminal voltage ($V_t$) is expressed by equation. Terminal voltage equation:

where $V_{oc}$ is the voltage value calculated based on the SOC value of the battery. $I$ stands for battery current and $R_e$ stands for internal resistance [38]. These are the basic elements of the battery. The electrical circuit can be revised according to the feature. desired to be emphasized.

Batteries are divided into three classes according to the usage situation. The first of these are the batteries, which are called primary batteries and cannot be recharged once used. On the other hand, secondary batteries are rechargeable batteries that can be used repeatedly. Secondary batteries are also called accumulators [37]. The third class consists of special batteries designed to fulfill a specific purpose [35]. Many types of batteries vary according to the material used in the battery. The choice of material depends on the expected performance and cost of the battery. In addition to the electrode material, batteries are also available according to liquid (aqueous and organic) and solid (polymer and ceramic) electrolyte types [39]. The types of secondary batteries used in UAVs are explained below.

3.4. Energy storage with super capacitors

They are also known by names such as supercapacitors, electrochemical capacitors, ultracapacitors, and electric double-layer capacitors. These names have been given by different manufacturers of supercapacitors [49]. Supercapacitors are structurally similar to batteries. But unlike batteries, supercapacitors have a separator membrane that separates the two electrodes. In addition, while both electrodes are usually made of the same material in batteries, the electrodes in supercapacitors can be made of the same or different materials [50].

A simple capacitor is formed by placing an insulating material between two metal electrodes. Fig. 7 shows the capacitor structure. On the other hand, Supercapacitors have an electrolyte between two electrodes with a raised surface. Figure 8 shows the supercapacitor structure. The electrodes are connected to each other by a power source and current flows from one electrode to the other. Charge separation naturally occurs at each electrode-electrolyte interface, and energy is stored in this way. The voltage continues even if the power is disconnected. Equal but opposite charges attractions in the electrolyte in solid electrolytes. This is where the term ‘double layer’ comes from [51].

Fig. 7The capacitor structure [52]

Fig. 8A supercapacitor with a surface-enhanced electrolyte [52]

The storage capacity of conventional capacitors is in the micro and mini farad range. In capacitors, energy charges accumulate on the plates, while in supercapacitors, charges accumulate at the electrode-electrolyte interface. The most important factor determining a capacitor’s capacitance is the width of the surface area to which the electrons are attracted. Supercapacitors have a very large surface area, and the capacity of a supercapacitor can be defined in kilofarads, unlike other capacitors [53-54].

Due to its porous structure, the carbon material is very suitable for being an electrode material [54]. In addition, carbon nanotubes, which are allotropes of the element carbon, have high conductivity and good electrochemical stability, but carbon nanotubes exhibit a low capacitance [55]. Activated carbon is a form of graphite with a porous structure. Activating carbon increases the surface area and pore volume [56-57]. Supercapacitors can store more charge than others because activated carbon is used as the electrode material, and activated carbon has a spongy structure. Thus, the maximum area that the loads can hold is provided [52]. Carbon can be activated in different ways. There are many studies on obtaining activated carbon from biomass wastes. Activated carbon could be obtained from various biomass wastes such as sugar cane pulp, corn cob, coconut shell, rice husk [58]. It is easy and relatively inexpensive to obtain in this way [59].

Supercapacitors are devices with high specific power, and electrical energy loading into the device occurs rather quickly compared to the battery [60]. No chemical reaction takes place inside supercapacitors and energy is physically stored. Therefore, the charge-discharge cycle is almost unlimited. In addition, they do not need service during their lifetime [61]. Since they do not contain dangerous and toxic substances, their production and disposal do not have a negative impact on the environment. They can also operate smoothly in a wide temperature range (–40 °C to +85 °C) [30, 62-64].

Their energy densities are relatively low [64-65]. However, this also makes them less hot and they can operate in a wide temperature range. They perform much better than batteries at very low temperatures.

Super capacitors are an emerging, new and promising technology. Using them together with the battery in UAVs improves the battery and increases the system performance.

3.5. Energy storage with fuel cells

Fuel cells are systems that generate electrical energy from chemical energy and use pure hydrogen or oxygen in the air as fuel [66]. As in batteries, energy conversion in fuel cells occurs through redox reactions that take place at the anode and cathode electrodes.

Fuel cells have a higher energy density compared to batteries. With this feature, it provides a longer flight and a longer range. However, the chemical reaction in it causes long response times, as in batteries. This means they cannot rise to an advanced level in terms of power density. Fuel cells can operate in different temperature ranges depending on their type. The operating temperature range for PEM (Proton-exchange membrane fuel cell) fuel cells used in UAVs varies between 80 °C and 1000 °C [67]. It is more advantageous in weight than batteries. A fuel cell is much lighter than a lithium-ion battery with the same energy capacity. In addition, it is suitable for use in covert operations thanks to its lower thermal trace and low noise characteristics [68-69]. Although its use on UAVs is becoming widespread, it has certain limitations.

4.1. Battery-super capacitor

In this combination, the battery provides long range and durability with its high specific energy, while the supercapacitor meets the sudden power need with its high specific power. Batteries and fuel cells cannot meet the power needs of the system in some situations, such as sudden maneuvers. The power used when the vehicle accelerates or slows down shortens the life of the battery. During high power demands, the battery voltage may drop below the minimum limit. Battery-supercapacitor hybridization can be used to protect system voltage from sudden drops. Supercapacitors and batteries can be used together to extend battery life and improve performance in electric vehicles. Since battery-supercapacitor hybridization increases efficiency, it also reduces costs in high-cycle applications. It has also been found that adding a supercapacitor in parallel with the battery increases the energy capacity by 20 % compared to discharging the battery alone [71].

In order to increase the power density in batteries, it is necessary to increase the size of the battery, but this causes additional cost increases and increases the weight. Weight gain is a disadvantage for aircraft. It has also been observed that the battery-super capacitor hybridization has higher specific power than the battery-powered system alone [72-73]. For this reason, it is seen as a rational solution to add an additional super capacitor to the battery instead of increasing the battery size.

4.2. Battery-Fuel Cell

This was one of the first hybrid electric systems proposed. The battery provides high power pulses and offers the required power density in this structure. The fuel cell converts the fuel into electrical energy at a constant rate to provide a high energy density for long-term missions, thus providing high efficiency [74].

Fuel cells are efficient and also have low emissions but require expensive filtration to prevent pollution. It is also heavy. In battery-fuel cell hybridization, the fact that both technologies are separately heavy causes the system to be overloaded, and this creates a disadvantage in aviation applications. Weight is a big problem, especially in UAVs. Additionally, the hydrogen used in fuel cells can only be stored by liquefying it. For this purpose, hydrogen is stored in high-pressure tubes and there is a possibility that these tubes may explode. In addition to the hydrogen storage problem, its availability also poses a disadvantage [4]. Although this hybridization is a system used in UAVs, its disadvantages cannot be ignored.

4.3. Super capacitor-fuel cell

The supercapacitor has high specific power while the fuel cell has high energy density. In this combination, the fuel cell meets long-term power requirements, while the supercapacitor serves high pulse power requirements at short intervals (energy control). However, due to the fuel cell’s undesirable features, such as the risk of explosion, high volume, and most importantly, the need for an additional source for the starting energy, batteries are preferred in addition to the supercapacitor instead of this structure.

In hybrid electric system configurations, the aim is to use one of the technologies as a power provider and one as an energy provider. The task undertaken by each technology in these configurations is briefly summarized in Table 2.

4.4. Hybrid electric drive system examples and combination technologies

The hybrid use of energy storage devices in electric vehicle technology, especially supercapacitors, is a topic that has come to the fore recently. the fact that it does not have a long historical background also limits the knowledge of the literature on this subject. in particular, most studies on aircraft are limited to theoretical knowledge. However, some experimental studies were also found. Examples of theoretical and experimental studies are as follows; in an article published in Bulgaria in 2010, it aimed to improve the flight time by connecting a supercapacitor in addition to the battery in the propulsion system of the UAV, and the system was designed. in the article, more attention was paid to the design of the energy management system of this structure. No experimental study has been conducted [75]. Xiong et al published a review article on battery-supercapacitor hybridization in 2018. This article examines energy management systems and hybrid propulsion system architectures for hybrid propulsion. as a result of the study, it was concluded that connecting an additional supercapacitor to the battery extends the battery life and reduces the overall cost [76]. in the study conducted by Gong and Verstraete in 2018, fuel cell, battery and supercapacitor were used together in the UAV propulsion system, and flight tests evaluated the performance of the UAV. This study aims to reduce the fuel cell’s load and thus increase its life by using the supercapacitor and the relatively high specific powers of the battery in situations that require sudden maneuvers such as landing and take-off. This system has been tested on a fixed-wing UAV with components connected in parallel. It was concluded that the triple hybrid propulsion system reduces the load of the fuel cell and increases its life [77]. the use of fuel cells in the study increased the weight of the propulsion system, and although this extra weight is not a problem for a fixed-wing UAV, it is undesirable for a micro/mini-class UAV. as a result, although using three technologies together creates a system with very good features by combining the advantages of all three, it is useless for micro/mini UAVs.