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Renewable and Sustainable Reviews 44 (2015) 96–108

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Renewable and Reviews

journal homepage: www.elsevier.com/locate/rser

Reviews of methods to extract and store energy for solar-powered aircraft

Xian-Zhong Gao, Zhong-Xi Hou n,1, Zheng Guo, Xiao-Qian Chen

College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, Hunan, PR China article info abstract

Article history: is considered to be the most reliable source in the future, and applying solar energy for Received 15 June 2013 flight is one of the most promising utilizations of renewables. Since the invention of solar-powered Received in revised form aircraft in 1974, the methods to extract and store energy for it have become the important research 14 April 2014 points all around the world. Currently, the main methods to do so are photovoltaic cell, rechargeable Accepted 2 November 2014 battery, and component of maximum power point tracking (MPPT). These years, many institutions and scholars have dedicated great efforts to the relative research. Besides, there are also several other Keywords: methods in consideration, such as to extract energy from wind shear and store energy by gravitational Photovoltaic cell potential. However, it is still not a simple task for the designers of solar-powered aircraft to select a particular technique from a number of existing techniques, since each technique has certain advantages Maximum power point tracking and disadvantages with respect to different performance indices to fulfill the special requirements. Solar-powered aircraft Thus, the aim of this paper is to review in detail the working principle of different methods to extract and store energy, and to compare their performances on the basis of desirable features applied on solar- powered aircraft, and to provide some guidance principles for designers to select proper methods. & 2015 Published by Elsevier Ltd.

Contents

1. Introduction...... 97 2. The power components and system ...... 97 3. Photovoltaic cells ...... 97 3.1. The energy conversion efficiency...... 97 3.2. Classification and application of photovoltaic cells ...... 98 3.2.1. Silicon photovoltaic cell...... 98 3.2.2. Thin-film photovoltaic cell ...... 99 3.3. The trends of photovoltaic cell applied on solar-powered aircraft ...... 100 4. Rechargeable batteries...... 100 4.1. The energy density ...... 100 4.2. Classification and application of rechargeable battery...... 100 4.2.1. Li-ion and Li-ion polymer batteries ...... 100 4.2.2. Lithium–sulfur batteries ...... 101 4.2.3. Regenerative cell system...... 101 4.3. The trends of rechargeable battery applied on solar-powered aircraft ...... 102 5. Maximum power point tracking ...... 103 6. The other methods to extract and store energy...... 104 6.1. Energy extraction from wind shear ...... 104 6.2. Energy storage by gravitational potential ...... 105

n Corresponding author. Tel./fax: þ86 731 84573189. E-mail address: [email protected] (Z.-X. Hou). 1 Permanent address: No. 109, Deya Street, Kaifu District, Changsha 410073, Hunan, PR China. http://dx.doi.org/10.1016/j.rser.2014.11.025 1364-0321/& 2015 Published by Elsevier Ltd. X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108 97

7. Conclusions...... 106 References...... 106

1. Introduction cells and rechargeable batteries, as shown in Fig. 1. The former is covered on the wing and used to extract energy from the sun to Today, the world is facing many tough challenges, among them supply power to the propulsion system and the control electro- energy crisis and environmental pollution are at top of the list [1]. nics; the latter is installed on the fuselage or other place of aircraft, Keeping in view of this scenario, great efforts have been made by it is charged in daytime when the energy is surplus and discharged many countries towards renewable and sustainable energy sources. to supply the power required in nighttime. Solar energy is considered to be the most reliable source for the future, Due to unpredictable nature of solar energy and the operational fl and applying solar energy for ight is one of the most promising condition of solar-powered aircraft, it is necessary to design a utilizations of renewables [2]. Since the can be considered sophisticated energy and mission management to achieve a fi inexhaustible, it has the potential to rede ne the endurance capabil- continuous flight during more than one day [68]. The key ities of aircraft [3]. Additionally, by using an autopilot system to the component to do so in solar-powered aircraft is named as Energy aircraft,itispossibletoworkanywhereintheworld[4].Thus,the Management System (EMS) [69]. In order to get the highest solar-powered aircrafts can be employed as an ideal communication amount of energy from the photovoltaic cell, a component named platform and are especially suitable for the high-altitude, long- MPPT is required. The MPPT is basically a DC/DC converter with fi endurance (HALE) missions, such as border surveillance [5],forest re variable and adjustable gain. It contains electronics that will fi ghting, ground tracking [6], precision [7] and even the monitor the current and the of the photovoltaic cells and future exploration of Venus [8,9] and Mars [10,11]. rechargeable batteries [70]. By changing the gain, it enables the fi From the rst solar-powered aircraft Sunrise-I took off at Camp available energy extracted from the photovoltaic cells to be Irwin, on the 4th of Nov. 1974 [12] to now, the solar- maximum [71]. For the most of solar-powered aircraft, the MPPT powered aircrafts have been quite well developed and widely is integrated in the EMS, as shown in Fig. 2. applied in practice. However, there are still some techniques need further refinement and advanced research are required to achieve reliable, safe and low cost flights, such as structural material, 3. Photovoltaic cells aerodynamic efficiency, control system, and power supply system [13]. Among all of them, power supply system is especially 3.1. The energy conversion efficiency important and crucial, because it does affect solar-powered air- crafts’ working duration and mission completion [14]. The main The energy conversion efficiency ηPC of a photovoltaic cell is the methods for power supply system to extract and store energy are percentage of the solar energy to which the cell is exposed that is to employ the photovoltaic cells and rechargeable batteries. For converted into . This is one of the most important the reason that the photovoltaic cells have non-linear I–V char- parameters to evaluate the performance of photovoltaic cells. ηPC acteristics and output power depends on power load severely, the can be calculated by Eq. (1) on the standard test conditions (STC), maximum power point tracking (MPPT) technique is crucial for where Pm is the photovoltaic cell’s power output at its maximum photovoltaic cells to enhance overall energy conversion efficiency. These years, many institutions and scholars have dedicated great efforts to research the technologies about solar cells [15–31], recharge- able batteries [32–42] and MPPT techniques [2,43–58]. Besides, there are also several other methods in consideration, such as to extract energy from wind shear [59–63] and store energy by gravitational potential [64–67] for solar-powered aircrafts. Although lots of works have been done, it is still not a simple task for the designers of solar- powered aircraft to select a particular technique from a number of existing techniques, since each technique has certain advantages and disadvantages with respect to different performance indices to fulfill the special requirements. Thus, the main purpose of this paper is to make an attempt to review the working principle of different methods to extract and store energy, and to compare their performances on the basis of desirable features applied on solar-powered aircrafts. The paper is organized as Fig. 1. The power components of solar-powered aircraft. follows: The power components and energy management system (EMS) in solar-powered aircrafts are introduced in Section 2. The photovoltaic cells, rechargeable batteries, and MPPT in solar- powered aircrafts are discussed in Sections 3–5, respectively. The methods to extract energy from wind shear and to store energy by gravitational potential for solar-powered aircrafts are presented in Section 6.Finally,theconclusionsaregiveninSection 7.

2. The power components and energy management system

The concept of solar-powered aircraft is quite simple: An aircraft equips with power components which are photovoltaic Fig. 2. The structure of energy management system [67]. 98 X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108

Nomenclature mb the energy density of rechargeable battery MCFC molten carbonate fuel cell

ηPC efficiency of power conversion MPP maximum power point CdTe MPPT maximum power point tracking – CuInSe2 copper indium diselenide Ni Cd nickel cadmium E the stored energy in rechargeable battery Ni–MH nickel metal hydride EMS energy management strategy PAFC phosphoric acid fuel cell EVA ethylene-vinyl acetate PET polyethylene terephthalate G the input solar irradiation Pm the output power of photovoltaic cell at maximum GaAs power point HALE high-altitude long-endurance P–V power–voltage I–V ampere-voltage SC the surface area of photovoltaic cell Li-ion lithium-ion SOFC solid oxide fuel cell Li–S lithium sulfur TiO2 titanium dioxide

mb the mass of the rechargeable battery

power point, G is the input solar irradiation, and SC is the surface solar-powered aircraft due to the consideration of energy conver- area of photovoltaic cell. sion efficiency, weight of substrate, cost-effectiveness, environ- mental adaptability and reliability [2]. The silicon is the most η ¼ PM ð Þ pc 1 widely type of photovoltaic cell used in solar-powered aircraft; the G Sc type of thin-film has been proposed to applied on the The energy conversion efficiencies vary from 6% for amorphous aircraft by some scholars, but has not been demonstrated in flight silicon-based photovoltaic cells to 40.7% with multiple-junction test yet. research lab cell and 42.8% with multiple dyes assembled into a hybrid package. The efficiency of the 3rd generation photovoltaic technologies have achieved 44% [29]. Fig. 3 shows the evolution of 3.2.1. Silicon photovoltaic cell the best research-cell efficiencies since the middle of the 1970s. The silicon photovoltaic cell consists of mono-crystalline sili- con, multi- and . As indicated 3.2. Classification and application of photovoltaic cells in the blue line of Fig. 1, the improvements of silicon photovoltaic cell in efficiency have come in ‘steps’, and each of these steps Nowadays, the industry’s productions of photovoltaic cells can corresponds to a new design feature. Since the early 1980s, there be mainly cataloged into five classifications: silicon (consists of have been two major surges that have taken silicon cell efficiency crystalline silicon and amorphous silicon), organic and polymer close to 25% [16]. However, the efficiencies for commercially cell, thin-film solar cell such as cadmium telluride (CdTe), gallium available silicon photovoltaic cell are around 16–22%. Table 1 gives arsenide (GaAs), titanium dioxide (TiO2), and hybrid photovoltaic the typical solar-powered aircrafts powered by silicon photovoltaic cell [73]. However, there are only few of them can be applied on cell and their performances.

Fig. 3. The best-research cell efficiencies chart (Reprinted with permision from the National Laboratory) [72]. X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108 99

Table 1 The aircrafts powered by silicon photovoltaic and performances.

Name Year Type and size Energy conversion efficiency Output power (%) (W)

Solar Impulse HB-SIA 2009 Mono crystal silicon 10,748 on the wing, 880 on the horizontal stabilizer 18 6000 [74] Sunrise I [75] 1980 Mono crystal silicon – 400 Sunrise II [75] 1985 Mono crystal silicon 1120 (2 4 cm) 16.1 580 Gossamer Penguin [76] 1981 Mono crystal silicon 3920 (2240 (2 4 cm), 700 (2 6 cm), 980 13.2 540 (2.4 6.2 cm)) Xihe [77] 2009 – 16 – Pathfinder [78] 1997 – 14.5 8000 So long [79] 2005 Mono crystal silicon 76 – 225 Skysailor [70] 2004 Mono crystal silicon REW32 216 16–18 84 Sunriser [80] 2000 Mono crystal silicon 256 – 30 [81,82] 2001 SunPower Mono crystal silicon 62,000 16 – Zephyr7[83] 2010 Amorphous silicon 19 – Heliplat [13,84] Start from Mono crystal silicon 22 1500 2004

Fig. 5. Install photovoltaic cells on the Helios prototype [86].

Fig. 4. The method to install photovoltaic cells on the exact airfoil shape.

It can be seen from Table 1 that most of the solar-powered aircraft choose the mono crystal silicon to be the component for Fig. 6. The flat-paneled airfoil special designed for solar-powered aircraft [87]. energy extraction, and its energy conversion efficiency is about – 13 20%. Table 2 The problem for the application of silicon photovoltaic cell on The aircraft proposed to be powered by thin film. solar-powered aircraft is how to encapsulate solar-cell to be bent fi along the contour of airfoil, for the reason that the crystalline Name Year Type of thin- lm Energy conversion efficiency (%) materials are very brittle and cannot be easily shaped into a smooth aerodynamic contour which is crucial for solar-powered Proposed in Skysailor 2004 GaAs triple Junction 27–28 aircraft to retain high aerodynamic efficiency. [70] cell There are two methods to solve this problem. The first method Proposed in Mars 1990 Gallium arsenide 25 is to bend the solar array to fit the airfoil shape. Because the fragile aircraft [90] solar cells silicon photovoltaic cell can be bent in a small range, the cell can fit the airfoil very well by the adhesiveness of EVA (Ethylene-vinyl 3.2.2. Thin-film photovoltaic cell acetate) substratum and the stress of PET (Polyethylene terephtha- Thin-film photovoltaic cells are basically thin layers of semi- late) skin [85], as shown in Figs. 4 and 5. But, by this method, the conductor materials applied to a solid backing material. Gallium exact airfoil shape must be compromised due to limitations on the arsenide (GaAs) [88], cadmium telluride (CdTe) [19], copper

flexibility of the crystalline solar cells and manufacturing techni- indium diselenide (CuInSe2) [89] and titanium dioxide (TiO2) que, which leading to a loss in aerodynamic efficiency. [20] are materials that have been mostly used for thin film The other method is to use a flat-paneled airfoil, and the photovoltaic cells. Among all of thin-film solar cell, the attention photovoltaic cells can be installed on the wing without bending, of the solar-powered aircraft’s designers is mainly concentrated on as shown in Fig. 6. By this method, the loss of efficiency attributed the GaAs, because of its constantly increasing energy conversion to bending during encapsulation can be avoid, and the loss of efficiency. As indicated in the purple line of Fig. 1, for the single- efficiency due to irradiance differences over a curved solar array junction GaAs device, the best result to date is 29.1%, and for the can be also reduced [87]. Furthermore, if the bottom of airfoils multi-junction tandem GaAs-based photovoltaic cell, the best is flat, then this place can be used to mount additional solar result is up to 44%. cells to collect earth radiation for additional solar-energy The typical solar-powered aircrafts have been proposed to be harvesting. powered by GaAs-based photovoltaic cell are listed in Table 2. 100 X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108

3.3. The trends of photovoltaic cell applied on solar-powered aircraft day, it is necessary for many types of stand-alone photovoltaic systems to even out irregularities in the solar irradiation and Generally speaking, as demonstrated in Figs. 4–6, it can be seen concentrate the solar energy to higher power in certain circum- that selecting a kind of proper photovoltaic cell and covering it stance by rechargeable battery [92]. Do as one typical stand-alone firmly on the wings are significantly important for the designing of , undoubtedly, solar-powered aircraft also the solar-powered aircraft. They would also influence the total needs the rechargeable battery to store the surplus power in weight, price and the flight duration. Arraying the photovoltaic cell daytime and supply the power in nighttime. following the optimum tilt angle is crucial for solar-powered For the rechargeable battery, the energy density is the key aircraft to retain high aerodynamic efficiency. While, if a flat- figure to evaluate its performance. The energy density is defined as paneled airfoil was used in solar-powered aircraft, the loss of the amount of energy stored in a given system per unit mass, or efficiency attributed to bending during encapsulation can be avoid, region of space per unit volume. Generally, the energy density of a and the loss of efficiency due to irradiance differences over a rechargeable battery can be calculated by Eq. (2), where E is the curved solar array can be also reduced. stored energy and mb is the mass of the rechargeable battery. Comparing to Tables 1 and 2, it also can be found that: although E the energy conversion efficiency of GaAs photovoltaic cell is greater m ¼ ð2Þ b m than that of silicon photovoltaic cell, most of designers choose the b latter one rather than the former one to power the aircrafts. Because the energy density is so critical for most applications, The reason is rooted on two problems: the first one is the weight especially for solar-powered aircraft, considerable efforts world- of substrate; the second one is the cost-effectiveness. The substrate of wide have been made to improve the energy density of recharge- thin-film solar cell is three times heavier than that of crystalline able battery [93]. Fig. 7 compares the energy densities and power silicon [70], which is very disadvantageous for its application on density of some common energy storage technologies. solar-powered aircraft, especially for HALE aircraft. Because the more mass of HALE aircraft, the more power is required for them to keep 4.2. Classification and application of rechargeable battery cruise speed [14]. While the price of thin-film solar cell is about several times of crystalline silicon, so the thin-film solar cells are Rechargeable batteries come in many different shapes and mostly used in [91] rather than solar-powered aircraft. sizes, ranging from button cells to megawatt systems. The current energy storage technologies including: alkaline, lead-acid, nickel cadmium (Ni–Cd), nickel metal hydride (Ni–MH), lithium-ion 4. Rechargeable batteries (Li-ion), Lithium-ion polymer (Li-ion polymer), lithium sulfur (Li–S) and fuel cell [94]. For the solar-powered aircraft, the 4.1. The energy density selection of rechargeable battery is the most critical issue because it represents the most important part of the total weight [11]. Due to the solar power is not an idea energy source and the Thus, the relatively low energy density technologies such as photovoltaic cell can only generate power at certain times of the alkaline, lead-acid, NiCd, Ni–MH are always out of consideration. The Li-ion, Li-ion polymer, Li–S and fuel cell are the candidates of energy sources in a wide variety of solar-powered aircraft for their relatively high energy density, as shown in Fig. 7.

4.2.1. Li-ion and Li-ion polymer batteries Li-ion batteries are the members of a family of rechargeable battery types, in which lithium ions move from the negative electrode to the positive electrode during discharges, and back

Fig. 7. The practical energy and power density of current energy storage technol- ogies. The plot is based on data from Refs. [39,93,94]. Fig. 8. The configuration of .

Table 3 The aircrafts equipped with Li-ion or Li-ion polymer batteries.

Name Year Type of BATTERY Specific energy (W h/kg)

Solar Impulse HB-SIA [74] 2009 Lithium-ion polymer battery, weigh 400 kg, or more than 1/4 of the total mass of the plane 240 Sunrise II [75] 1985 Lithium-ion polymer battery 145 Xihe [77] 2009 Lithium-ion polymer battery 196 Green flight challenge: 2011 High energy density lithium-polymer cells, which consists of three groups of 88 series-connected cells. In practice, 180 Taurus G4 [102] total capacity exceeds 90 kW h. Total mass of battery is about 500 kg So long [79] 2005 120 Sanyo 18,650 lithium-ion polymer battery 5.6 kg, 1200 W h 214 Skysailor [70] 2004 Including a margin of 20%, the battery of aircraft composed of eight E-tec1200 cells in series, six in parallel, 172 which lead to a capacity of 207.36 W h with 7200 mA h at 28.8 V and a total weight of 1.2 kg X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108 101 when charging [95]. Lithium-ion polymer battery has the same permit it could recharge at temperatures as low as 60 1C. property of Li-ion battery but is lighter in weight and can be made In order to do so, an advanced electronic control system is used in any shape desired. This kind of batteries, in particular, have a to maintain battery temperature and condition during flight. wide range of applications [96]. Although the technology behind the Li-ion and Li-ion polymer battery has not yet fully reached maturity [97–99], comparing to 4.2.3. Regenerative fuel cell energy storage system other energy storage technologies available today, Li-ion and Fuel cell is an important enabling technology to provide a Li-ion polymer batteries have been always selected as the quite high electrical energy by reaction of fuel and oxygen without rechargeable battery for solar-powered aircraft, as listed in Table 3. combustion. The fuel cell has many advantages over other Table 3 shows the energy density of Li-ion and Li-ion polymer rechargeable batteries, such as higher efficiency, less pollution battery applied in solar-powered aircraft is about 140–240 W h/kg. and so on. Depending on type of the electrolytes, there are several Although the Li-ion and Li-ion polymer battery are two of the fuel cells, for example, phosphoric acid fuel cell (PAFC), molten best rechargeable batteries to have a higher energy density and a carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC) [110]. very slow loss of charge when not in use, their energy density is The use of hydrogen and oxygen gases is necessary to achieve a still not high enough. This has become the crucial constraint for reasonable power density in fuel-cell system [111]. the solar-powered aircraft to support a HALE flight. The designers Fuel cell technology also has the potential to achieve a real of Sky-sailor, Noth et al. [100,101] have concluded that the design technological breakthrough in the aircraft field [112]. Many of solar-powered aircrafts as a ‘hen and egg problem’ caused by designers have proposed or have equipped the fuel cell on solar- the weight of rechargeable battery: The required power during powered aircraft, as shown in Table 5. After the evaluation of night needs more battery, but at the same time, the additional current state-of-art rechargeable battery systems, and their poten- weight of battery will consume more energy. tial in achieving high specific energy density using advanced Thus, for the solar-powered aircraft equipped with Li-ion and material in the near future, it can be concluded that the regen- Li-ion polymer battery, it requires a drastic reduction of the rest erative fuel cell based energy storage system holds the promise part of plane, and to optimize the energy efficiency of whole of achieving the energy density in the order of 400 W h/kg system. The high aspect ratio wing with a low-speed profile is and above. also necessary to maximize the aerodynamic performance, as shown in Fig. 8.

4.2.2. Lithium–sulfur batteries Just because the relative low energy density of rechargeable battery puts great constraint to the design of solar-powered aircraft, the demand for advanced rechargeable batteries with high energy density increase drastically. Under this circumstance, Li–S battery has been developed by the company of Sion Power since 1994 [103,104]. Based on the lithium/elemental sulfur redox couple, Li–S battery has a theoretical specific capacity of 1600 mA h/g, and a theoretical specific energy of 2600 W h/kg assuming the complete reaction of lithium with sulfur to Li2S Fig. 9. The lithium–sulfur battery used on Zephyr [103]. [105]. This is the highest theoretical energy densities of any rechargeable battery chemistry. After a great deal of research [38,40–42,106,107], many scholars have pointed out that the Li–S battery is a very promising technology for high energy applica- tions and may succeed Li-ion cells because of its higher energy density and reduced cost from the use of sulfur. This technology is especially suitable for the application in solar- powered aircraft where the weight is a critical factor. The Zephyr 7whichhasflew over 336 h in Yuma, Arizona on July 2010, reaching an altitude of 21.6 km, is the first and the solo one equipped with Li–S battery in open literatures [108],asshowninTable 4. As shown in Fig. 9, the Li–S batteries used on Zephyr 7 are laminar. They are installed as a thin-film sandwich within the wing, as shown in Fig. 10. Because the operational altitude of Zephyr 7 is about 17–21 km, it is important to keep the Li–S battery in the suitable temperature and condition that would Fig. 10. The place to install the lithium–sulfur batteries in Zephyr.

Table 4 The aircraft equipped with Li–S battery.

Name Year Type of battery Specific energy (W h/kg)

Zephyr7 solar powered high-altitude 2010 The lithium–sulfur battery pack was designed and assembled by SION Power and consisted of 500–600 Achievable long endurance UAS [83,104] 576 cells built into a battery configuration of 12 cells in series and 48 in parallel. The battery pack [109] 350 demonstrated was carefully engineered to minimize total pack weight. In addition to providing flight power, the battery pack supplied power to a special internal pack heating system to maintain the batteries at 0 1C throughout the cold night 102 X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108

Table 5 The solar-powered aircraft equipped with rechargeable fuel cell.

Name Year Type of battery Specific energy (W h/kg)

Proposed in Mars 1990 The regenerative fuel cell, used for energy storage, used gaseous hydrogen and oxygen as the reactants 440 aircraft [90] Heliplat [13,84] 2004 The fuel cells act as energy storage. It consists of the fuel cell array, the hydrolyser as well as hydrogen, oxygen and water 550 tanks. The gases are stored at high pressure 120 bar and used during the night to feed the fuel cells which supply the motor; the water feeds the electrolyser to finish the cycle Helios [81,82] 2003 The energy storage system uses a PEM fuel cell and electrolyzer stacks, weight gas and water storage tanks and a 500 specially designed autonomous monitoring and control system

Fig. 11. The schematic diagram of hydrogen–oxygen fuel cell [113].

The Helios is the first successful flight-test solar-powered aircraft equipped with fuel cell. The tank of Helios carries pressur- ized hydrogen and oxygen, which are combined to produce electric power, heat, and water, as shown in Fig. 11. As long as these gases are supplied, the unit continues to produce power. Not only are these systems attractive from an environmental stand- point, but since they have very few moving parts, these systems have the potential of very high reliability [113]. For Helios, if the required power were supplied by the ordinary rechargeable batteries, such as Li-ion or Li-ion polymer, it would be too heavy to meet either altitude or endurance goals. So the fuel cell is crucial for flight-test of Helios. However, not as the Li-ion battery or laminar Li–S battery, the weight of fuel cell cannot be laterally distributed along the wing of Fig. 12. The fuel cell tank of Helios [115]. aircraft. As shown in Fig. 12, the Helios requires the heavy fuel cell pod located at the centerline of the aircraft and 2 high-pressure hydrogen tanks located at the center of each wing tip panel. The 4.3. The trends of rechargeable battery applied on solar-powered structural flexibility and the large masses associated with the fuel aircraft cell system cause the 3-point mass effects and introduce sub- stantial complexity into the aircraft’s flight dynamics, which By the discussion in Section 4.2, it can be concluded that: contribute to causing the persistent high dihedral and resulting The most widely used rechargeable batteries in solar-powered in the aircraft fell into the ocean at last [114]. aircraft are still the Li-ion and Li-ion polymer batteries, but their X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108 103

Table 6 5. Maximum power point tracking Comparison of different MPPT techniques [52]. The green are suitable for solar- powered aircraft. Conceptually, MPPT is a simple problem: There exists only one – MPPT Technique Complexity Tracking Energy Transient Efficient point, called maximum power point (MPP) on the P V curve of accuracy tracking tracking for partial photovoltaic cell. The function of MPPT is to match the power of factor speed shading load to this point, and to lock the operating point at MPP to extract maximum power from the solar-cell array. However, even in VMPPT S Low Low Slow No normal condition, photovoltaic array has non-linear I–V character- CMPPT S Low Low Slow No Look up table S Low Low Slow No istics and its output power varies with and Curve fitting S Low Low Slow No ambient temperature. For aircraft, the operational condition of P&O S Medium Good Medium No photovoltaic varies drastically with the changes of altitude and fi Modi ed P&O C High Very Fast Yes attitude, and the partial shading conditions also will be encoun- good – – INC M High Good Fast Yes tered, which make the I V and P V characteristic more complex. Modified INC C Very Very Fast Yes Thus, tracking the correct maximum power point can sometimes high good be a challenging task [55], especially for solar-powered aircraft. INR C High Good Fast No Since the MPPT plays a significant role in enhancing overall RCC C High Very Fast Yes power conversion efficient, many algorithms for MPPT have been good Power feedback S High Good Medium No reported and each with its own features. Here, we do not intend to BST C High Good Medium No discuss the working principle of each MPPT algorithm. Our aim is Slide mode control M Medium Very Fast Yes to provide some guidance principles for the designers to select a good proper MPPT according to the characteristics of solar-powered Temperature M High Low Medium No Gradient descent M High Good Medium No aircraft. Numerical based C High Good Medium No Many designers have realized that the energy is the crucial Intelligent control C Very Very Fast Yes constraint for the success flights of solar-powered aircrafts based: FLC high good [10,14,114], so, the tracking accuracy and energy tracking factor Intelligent control C Very Good Fast Yes are important for the MPPT applied in solar-powered aircraft. based: ANN high Intelligent control C Very Good Fast Yes For example, the Zephyr had been equipped with an advanced based: PSO high power management system to ensure the photovoltaic cells Load parameters C High Low Slow No operate at or near their peak power point whether the cells are based working in propulsion or energy storage mode [108]. β -Method C High Good Fast Yes fl Three point MPPT C High Good Fast No During the ight of aircraft, the operational condition of Parasitic C Very Good Medium Yes photovoltaic varies drastically with the changes of altitude and capacitance high attitude, and the partial shading conditions also will be encoun- One cycle control M High Good Fast No tered. So the high transient tracking speed and the ability to deal Variable inductor M Medium Good Medium No with partial shading are also required. Current sweep C Medium Low Medium No Array C Low Low Slow Yes In Ref. [52], Bhatnagar et al. have compared against each MPPT reconfiguration algorithms in terms of some critical parameters, such as the Linearization S Very Good Fast No number of variables used, complexity, accuracy, speed, hardware based high implementation, cost, tracking efficiency and so on. According on State space based C High Good Medium Yes PV output S High Good Medium Yes above discussion and based on the application requirements of senseless solar-powered aircraft, the MPPT algorithms in green part of Biological swarm C Very Good Fast Yes Table 6 are suitable for the designers to select. chasing high After designing and constructing the algorithms of MPPT in the System oscillation C High Good Medium No hardware, it is usually embedded in a DC/DC converter which control DC-link capacitor C High Very Fast Yes inserted between the photovoltaic module and the load, battery or droop good other power components, as shown in Fig. 13. Fig. 14 illustrates a completed hardware of MPPT installed in the fuselage of a solar- powered aircraft [2]. energy density is not high enough and becomes the crucial constraint to support a HALE flight. The fuel cells hold the promise of achieving the highest energy density about 450–550 W h/kg, as shown in Table 5. Although it has the potential to achieve a breakthrough in the aircraft field in theory, its concentrated mass will introduce a substantial complexity into the aircraft’s flight dynamics. So, in current technological level, it is not appropriate to apply the fuel cell on the solar-powered aircraft. By the comparison of all the types of rechargeable battery which have been installed on solar-powered aircraft, we recom- mend that the most promising rechargeable battery in solar- powered aircraft is Li–S battery. Since it not only has a relatively higher energy density, but also can be installed as a thin-film sandwich within the wing and make the weight of rechargeable battery laterally distribute along the wing. Fig. 13. Function of MPPT controller in solar-powered aircraft [116]. 104 X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108

Fig. 14. MPPT installed in the fuselage of solar-powered aircraft [2].

From above discussion, it can be seen that designing an efficient and lightweight MPPT device to make voltage and current for a maximum power is an important way of reaching to a high endurance flight [2].

6. The other methods to extract and store energy

Currently, the photovoltaic cell, rechargeable battery, and MPPT are the main research points about the methods to extract and store energy for solar-powered aircraft all around the world. However, the major handicap associated with the development of solar-powered aircraft is still the limited on-board energy capacity [60,117–119]. Therefore, it is necessary to improve the battery technologies as well as finding the other methods to extract and store energy for solar-powered aircraft. Among the many methods, the methods to extract energy from wind shear and store energy by gravitational potential are considered to be the most promising way to enhance the HALE ability of solar-powered aircraft. Fig. 15. The average strength and direction of wind in Changsha (28.21N, 112.61E).

6.1. Energy extraction from wind shear

The idea of energy extraction from wind shear comes from the observations of albatrosses who can fly long distances even around the world almost without flapping their wings [120]. Rayleigh [121] is known as the first one to propose that the birds can extract energy in a horizontal but non-uniform wind field in the year of 1883, and named this phenomenon as dynamic soaring. After his pioneering works, many researchers [61,63,118–120,122] have studied this phenomenon, and try to design an aircraft to extract energy from wind shear. The idea of dynamic soaring now have been widely accepted by scholars in aviation, especially for aircrafts which may be con- trolled automatically to extract energy from wind shear to greatly extend flight duration and distance [123]. Since wind shear is Fig. 16. The dynamic soaring trajectory of solar-powered aircraft for loitering [124]. persistently distributed in the upper atmosphere from altitude of 10 to 20 km [124], as shown in Fig. 15, incorporating with the rapid In engineering, Langelaan and Roy considered that the perception- technologies development of solar-powered aircraft, dynamic sensing and persistence-staying are the crucial technical challenges soaring may be considered as an alternate energy source for for UAVs to fly like birds [123]. Similar to their standpoints, here, we aircraft both in day and night. argue that there are at least four major issues which need further A typical flight trajectory of dynamic soaring is shown in Fig. 16, study to pave the way to long-endurance flight by the way of energy by repeating this maneuver, the aircraft can continue flying almost extraction from wind shear, and we believe the clues to solve these indefinitely without having to put in much effort, in effect, problems can be found from research of the birds in nature. it is extracting energy from the wind shear. Incorporating with First, the energy gain mechanisms. There are many theories to dynamic soaring, it can be expect that the long-endurance interpret how the albatrosses to extract energy from wind field, performance of solar-powered aircraft will be greatly enhanced the typical theories are dynamic soaring proposed by Raleigh [121] since the power requirements can be reduced without adding and sweeping flight proposed by Wilson [125]. The former additional weight on the aircraft. believes that the birds make use of the gradient in wind velocity X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108 105 near the surface of the sea to extract energy. The later considers power requirement and the trajectory planning. Shiau and Ma [77] that the birds make incidental use of updrafts to keep flight. realize this problem and jointly optimize the design-parameters Recently, the study of Richardson, P. L. has shown that, under and cruise speed during level flight to design a solar-powered typical wind and wave condition, albatrosses draw 80% to 90% of aircraft. For the case they studied, the optimization reduces the their total flight energy from dynamic soaring, and the other mass of the aircraft and increases the cruise speed simultaneously. comes from the so called sweep flight [120]. Sachs and Bonadonna Klesh and Kabamba [133,134] also prove that level flight trajectory give a more intuitive interpretation about energy gain mechan- planning for solar-powered aircraft has a great impact on their isms in dynamic soaring by GPS tracking of albatrosses [126]. . Spangelo and Gilbert [6] formulate a new These studies are very important for people to understand the optimization problem about flight trajectories for solar-powered flight of birds, while for the designers of UAVs, they are more eager aircraft with specified closed ground tracks. By their study, they to know what kinds of configuration are more suitable to extract find that the average input power to battery with variable speed energy from wind and which parameters in UAVs are tightly and altitude is greater than that with constant speed and altitude. coupled with the energy extraction efficiency. Other relative works such as optimal level turn can be seen in [3]. Second, wind field estimation. The detailed patterns of alba- All these works prove that the power trajectory planning is crucial trosses’ foraging behavior on a small to medium scale have been for solar-powered aircraft to have a better performance, but they studied by ornithologists for many years [127], and they found don’t pay attention on the problem of how to operate solar- that the small-scale flight paths show typical zigzag patterns, powered aircraft without solar irradiation which is very important which are tightly coupled with respect to wind conditions. The fact for aircraft to achieve the aim of long-endurance. that albatrosses can adjust their searching behavior according to The most common method in engineering application to deal wind indicates they have the ability to sense the changes of wind with this problem is to let the solar-powered aircraft carry in environment very well. However, how albatrosses do this is still rechargeable battery [135]. While under current technological a puzzle, and how to do this for an small UAV with standard level, the weight of the rechargeable batteries needs to occupy autopilot sensor suite is a complex problem for the scientists and around 30% of the total mass of solar-powered aircrafts to achieve engineers in aviation. Langelaan et al. [128] and Lawrance et al. the aim of long endurance over days [11,84]. Although more [129] have done some pioneering works for this problem, but the rechargeable battery can provide more energy during night, the estimation accuracy and robustness need to be further improved. weight of additional batteries needs more energy to sustain Third, path planning in varied wind conditions. The study of continuous flight at the same time, as shown in Ref. [14]. Thus, Weimerskirch et al. has shown that the foraging range, travel and how many rechargeable batteries should be taken for long- flight speeds, and even the mass of albatrosses are changed with endurance purpose is a crucial problem needed to be solved for wind pattern [130]. Their observations show that the increased the designers of solar-powered aircraft. Prof. Sachs G. formulates intensity and poleward westerly winds resulted in a foraging range the trajectory planning problem of solar-powered aircraft from a shift towards the pole as well as an increase in flight speeds. This new viewpoint: The surplus energy of solar powered during day implies that, given different wind conditions, the birds will take can be stored by gravitational potential [65]. He has demonstrated different foraging movements to reduce flights cost. To study the in the paper that with an appropriate trajectory control, it is internal relationships between the wind condition and foraging possible for the solar-powered aircraft to stay aloft during night movement of albatrosses has great significance for the path with minimum or even no solar energy to be stored in recharge- planning of UAVs. The aims of this study are to answer two able battery. If the solar-powered aircraft can work in this way, questions raised by the designers of UAVs: (1) what kinds of flight then the mass penalty of rechargeable battery can be removed. path is the most efficient in energy cost, and (2) what wind Undoubtedly, it will bring great change in the conception of conditions are required for a UAV to maintain perpetual flight. designation and development of solar-powered aircraft. Motivated Finally, the aerodynamical characteristics of slight morphing of by his ideas, the properties of the maximum endurance path of the wing. Although the albatrosses do not flap their wings over gravitational gliding have been studied in Ref. [66], and a new long distances [120], they need a slight morphing in wing to energy management strategy has been proposed in Ref. [67], other change their aerodynamic characteristics and control their flight relative works about the energy stored by gravitational potential path. Lentink et al. show that the morphing or the sweep of the for bird and UAV can be seen in [136,137]. wing can halve sink speed or triple turning rate for swifts [131] The solar-powered aircraft-Solar impulse has successfully used and SmartBird demonstrated that the aerodynamic efficiency of a the method to store energy by gravitational potential. During the wing can be improved from 30% with passive torsion to 80% with day, the pilot ascends the aircraft to a higher altitude (about active torsion [132]. Similar studies with albatrosses in dynamic 8000 m) in thinner atmosphere to convert the energy from soaring have not yet been taken. The observation about the slight photovoltaic to gravitational potential in altitude. As the sun change of albatrosses’ wing in dynamic soaring with some degree begins to set on the horizon, the pilot reduces the motors and of accuracy and particularity is necessary. This study is propitious initiates a gentle descent (about 0.4 m/s) to a low night loitering to reveal how albatrosses control their aerodynamic characteristics altitude of 1000–1500 m meters, as shown in Fig. 17. The aircraft in dynamic soaring and sweep flight. It can also give some can glide for 4–5 h consuming almost no electric energy. When the inspiration for scholars in aviation to design the wing and control lowest altitude is reached, usually long after sunset, the motors law for UAVs. will be powered by the batteries and used to maintain a level flight We are confident to believe that incorporating with the rapid until the morning. As the breathtaking tones of the sun on the development of modern sensors and small on-board computers horizon start filling the sky with warmth, the aircraft can once with modern control techniques, the flight feat of birds may again begin its ascent [138]. become a new way to explore the energy from nature and may The method to store energy in gravitational potential is not as enlighten people on the road leading to long-endurance flight. the method to store energy by rechargeable battery, it can store the energy without any weight penalty, and thus it is a very 6.2. Energy storage by gravitational potential promising technological route to achieve the HALE flight for solar- powered aircraft. The power requirement of aircraft is decided by the flight Fig. 18 gives an intuitionistic description about the influence of trajectory, thus it is worth to study the correlation between the the duration of solar irradiation and initial altitude on the 106 X.-Z. Gao et al. / Renewable and Sustainable Energy Reviews 44 (2015) 96–108

To select a proper MPPT algorithm for solar-powered aircraft, the performances of tracking accuracy, energy tracking factor, transient tracking speed, and the ability to deal with partial shading need to be considered. Since the energy is crucial during the flight of aircraft and the operational condition of photovoltaic varies drastically with the changes of altitude and attitude, and the partial shading conditions also will be encountered. The methods to extract energy from wind shear and store energy by gravitational potential are the most promising ways to enhance the high-altitude, long-endurance ability of solar- powered aircraft, since dynamic soaring can be considered as an Fig. 17. The flight cycle of solar impulse [138]. alternate energy source for aircraft to solar both in day and night, and the gravitational potential can store the energy without any weight penalty.

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