Solar Powered Airplane Literature Review

1 SOUTHERN ILLINOIS UNIVERSITY CARBONDALE

Solar-Powered Airplane Literature Review Senior Design Team 96 3/19/2009

Solar-powered Airplane

2

Abstract This literature review studies previous designs of solar-powered airplane. The paper covers important areas prior to the actual design of the model, such as: structural design, motor design, solar power and energy storage. Different designs are compared by size, weight, battery used, energy power, flight altitude and control system. In conclusion, senior design team 96 will determine the best options for the solar-powered airplane project based on the previous attempts. Nomenclature TRL

technology readiness level

RFCS

regenerative fuel cell system

SOFC

solid oxide fuel cell

LiPo

lithium –polymer

UAV

unmanned aerial vehicle

MPPT

maximum point power tracker

PV

photovoltaic

AR

aspect ratio of wing

b

wingspan

Cd, Cl

drag and lift coefficients

e

Oswald’s efficiency factor

g

gravity

η

efficiency

Ps

power of sunrays

Pse

power of sunrays converted to electricity

1. Introduction Today, the vast majority of transportation vehicles around the world are powered by fossil fuels. Moreover, this reality presents challenging problems to future generations. First, crude oil and natural gas reserves are running out while the world’s demand is exponentially increasing. Second, access to these resources will become more difficult to small companies and families as gas or energy prices rise. Lastly, the extraction and production from fossil fuels results in a huge environmental impact. Considering the three factors previously mentioned, it makes sense to find other clean and sustainable ways to power our society. The concept of creating an airplane that flies without using any fuel is not a new idea by any means. In the past three decades there have been many attempts to develop this type of plane. Throughout the last thirty years, a common constraint was that technology could not meet the requirements of the respective models. Nowadays, present advances in nanotechnology, composite materials and photovoltaic have significantly changed the scenario. Pathfinder, Helios, Solar impulse, Sky-sailor and Centurion, just to name a few, are great examples of solar-powered airplanes. These designs can be separated in two groups, manned or unmanned. A manned airplane needs to account for a larger number of factors such as extra weight, size, power and security. This literature research intends to study prior attempts in the incursion of the solarpowered airplanes in order to understand the basics of this challenging concept. The literature review describes subsystems such as solar panels, energy storage, type of motor and transmitters. Furthermore, after the conclusion of this report, and based on the information collected from the research, senior design team 96 will begin its own solar-powered airplane project.

2. History of Solar Powered Flight There have been several attempts in solar flight since the first ever solar powered model took flight on November 4, 1974. Solar Power involves collecting energy from the sun's rays and using it for power or storing it for later use. Lockheed contracted AstroFlight, a United States (US) company to research alternative fuel sources. Sunrise, as it was called, used just over 1000

solar cells yet only produced 450 watts of power. This would be enough to get its 26 lbs and 32 ft wingspan to 20, 000 ft, which was a world record until the same company produced the Sunrise II in 1975. Sunrise II used 4480 solar cells and delivered 600 watts of power. The succeeding model was superior because it weighed just less than 4 lbs and could climb to an altitude of 75,000 ft. The next attempt would not be for another 5 years, when in 1980 AeroVironment a US based company backed by the Dupont Corporation started to build the solar aircraft called The Gossamer Penguin, a fragile and not very airworthy aircraft model. The name would later be changed to "Solar Challenger". It had a wingspan of 47 ft and weighed 200 lbs. The frame of this aircraft was covered with 16,128 photovoltaic cells which produced 2,600 watts of power. Solar Challenger was able to fly 163 miles from Paris to Manston in the United Kingdom at 12,000 ft. This was a world record at the time for distance covered using only solar power. This achievement sparked an exodus into the world of solar power. In 1983 AeroVironment, the same company that built Solar Challenger, got to work on the High Altitude Solar Aircraft (HALSOL). Unlike previous attempts, this prototype lacked the airplane model. HALSOL was a simple flying wing with a wingspan of 98 ft (30 m). The main wing was made of carbon fiber Styrofoam and Kevlar which was covered by a thin Mylar plastic film. The overall result was a light and extraordinarily strong material. There were 9 test flights done with the HALSOL, but after further review it was determined that the photovoltaic technology had not advanced enough to continue. Ten years later NASA restarted its own HALSOL project, naming it Pathfinder. Pathfinder took 3 years to perfect but was able to capture the altitude record for a propeller driven aircraft at 80,201 ft. Pathfinder was followed by an improved version called Pathfinder-Plus that had solar cells made of silicon, which improved efficiency by 5%. This improvement was seen when power was boosted from 7,500 watts in Pathfinder to 12,500 watts in Pathfinder-Plus. The Centurion project, later renamed Helios, was larger than any previous attempt. It applied hybrid technology; solar energy using photovoltaic cells by day and fuel cells by night. “The Helios was designed to be the forerunner of high-altitude unmanned aerial vehicles that could fly ultra-long duration environmental science or telecommunications relay missions,

lasting weeks or months without consumable fuels or emitting airborne pollutants" [1]. The wingspan of Helios was 247 ft and it used 62, 120 bi-facial solar cells to power it. The first manned aerial vehicle powered by solar energy was developed by Solar Flight. Eric Raymond, founder of the company, flew Sunseeker I for the first time in 1986. The concept was further developed and Sunseeker II replaced the previous model in 2002. Currently, Sunseeker II is the only flying manned aerial vehicle in the world. Other recent projects in Europe are Zephyr, Sky-Sailor and Solar Impulse. Zephyr was designed by QinetiQ as a high altitude platform for satellite communications. Sky-Sailor is a first prototype model in a project aiming to develop a solar-powered aircraft able to conduct missions in Mars. Finally, Solar Impulse is an attempt to complete the first solar-powered flight around the world without any stops. This incursion was initiated by Bertrand Piccard, commander of the first non-stop round the world balloon flight. Piccard and Andre Borschberg, CEO and co-founder of the project, will pilot the airplane in 6 hours intervals. The first flight of Solar Impulse is scheduled in 2009. The history in the solar powered airplane incursion is summarized by Table 2.1, which compares the past attempts previously mentioned. Table 2.1: Characteristics of various solar-powered airplanes Model

Year

Solar Cells

Energy (W)

Weight (lbs)

Wings pan (ft)

Altitud e (ft)

Batte ry N/A

Sunrise I

1974

1000

450

26

32

20,000

Sunrise II Solar Challenger

1975

4480 16,12 8

600

4

32

75,000

2600

200

47

12,000

HALSOL *

1983

N/A

N/A

98

Pathfinder PathfinderPlus

1993

N/A 1980

N/A N/A

Centurion

1998

Helios Sunseeker I Sunseeker II

1999 1986 2002

N/A Silico n Silico n Silico n 62,12 0 N/A N/A

SoLong

2005

76

1998

N/A N/A

7,500

560

98

50,500

12,500

700

121

N/A 12,500

1,900

206

12,500 N/A N/A

2,048 N/A N/A

247 N/A N/A

80,201 100,00 0 100,00 0 N/A N/A

225

25

15.6

26,250

N/A LiPo NiCd LiPo LiPo

Control Remot e Remot e Remot e Remot e Remot e Remot e Remot e Remot e Pilot Pilot Remot e

LiPo Sky-Sailor

2004

216

90

2.5

3.2

6,500

Zephyr

2007

1,500

66

59

60,000

Solar Impulse

2007

N/A 12,00 0

6,000

3307

200

27,887

LiPo

Remot e Remot e

LiPo Pilot

3. Important Factors to Consider 3.1

Weight of the aircraft One of the main challenges to overcome during the design of this project is to

achieve the perfect balance between total weight of the aircraft and solar power generation. This relationship between the mass of the elements and the energy they can store or transform is known as power density. In order to obtain enough power density to fly the airplane during day and night, the most recent and efficient technologies must be utilized. Production of light weight parts is possible thanks to the new advances in composite materials and nanotechnology. Important breakthroughs in the latter, have positively affected another determinant factor, energy efficiency. Out of all the parts of the airplane, the battery is the biggest weight contributor. Taking Sky-sailor as an example, the LiPo battery accounted for 44% of the total 2.595 kg of weight [23]. In the SoLong model, the battery weight was 4.35 kg out of a total mass of 10.8 kg [22]. Eric Raymond [28], a pioneer in this matter, stated that light weight batteries are the single most important improvement that is needed. Currently, lithium batteries are the best option in the market. Considering all previous aircraft projects, it is important to choose the lightest materials available, while simultaneously meeting the model requirements at the same time. The most problematic weight restraint is the battery of the plane.

3.2

Size and Shape The purpose and functionality of the airplane will significantly change the size of

the model. Many factors will determine the appropriate size of the aircraft. For example,

if the plane is designed to be controlled by a pilot or fly for long periods of time (day and night), a large wing surface area is required for additional solar panels in order to account for the extra weight or power needed, respectively. Solar Impulse is a good example for a large size airplane. The aircraft, with a wingspan of 61 m [29], is designed to be controlled by a pilot and flight continuously around the world. On the other hand, unmanned airplanes designed for short flights do not need much power, therefore needing less wing surface area for solar panels. As a result, the aircraft model becomes smaller and lighter. Sky-sailor, Zephyr and SoLong are great examples for small airplane models with a wingspan of 3.2 [27], 12 [1] and 4.75 m [22], respectively. On another note, the size of the airplane seems to determine the shape of the model. More specifically, the shape of larger size aircrafts is similar to an actual airplane. In contrast, the shape of smaller size aircrafts is similar to gliders, taking advantage of the aerodynamics and light weight model in order to use less power. However, projects developed by NASA do not follow this trend. Pathfinder-plus, Helios and Centurion are long flying wings with multiple motors designed for long flights and high altitudes.

3.3

Wing design A solar powered aircraft need wings that maximize both surface area and lift.

Designs that have shown promise for solar flight include the flying wing, blended wing, and conventional wing. Conventional wings are what are found on most aircraft in the sky. They provide good stability since most of the weight falls along the planes center of gravity. Although they are the most common design there is still room for improvement, as the wings do not generate enough lift to carry very heavy loads in relation to total airplane weight. An improvement of this design first came with the flying wing. The flying wing is defined as no visible fuselage or cargo area, only a large wing which houses the payload. It is theoretically the most fuel efficient aircraft in terms of aerodynamics and structural weight due to the entire craft acting as an airfoil and generating lift. This is its main advantage over other types of aircraft. However, this comes at a price because what is gained in improved lift is lost in the ability to control the aircraft as the giant wing with

no rear tailfins has proven hard to stabilize in the air. This has caused the emergence of hybrid wings such as the B-2 bomber as well as a modified design of the blended wing [12]. The blended wing is an adaptation of the flying wing, but it has separate, visible fuselage and cargo area which are blended into the rest of the large wing. Some advantages of this design are increased fuel economy, as the whole aircraft is generating lift and not only the wings. Another important advantage is improved payload capability due to the extra lift that is generated. The blended wing craft also has a heavier structural weight making it a much more solid, rigid aircraft. One of the downfalls is that the cargo area is more spread out so it is harder to maintain the majority of the weight along the planes center of gravity. The blended wing is better suited for slower speed aircrafts since the large front surface area of the giant wing and body create a much larger amount of drag at high speeds than conventional aircraft [13]. Drag, the only force other than gravity opposing the airborne plane, is created by the aircraft physically moving through the air. In order to maintain level flight aircraft must maintain a balance between the lift and the drag. They are both determined by the same equation, the only difference is the Coefficient of drag is used instead of Coefficient of lift. The coefficient of drag can be approximated by the equation [14]

(3.1) This is then used to determine the power needed by the airplane to maintain level flight using the following equation [14]

(3.2) Where g is gravity, m is mass of entire plane, and b is wingspan.

4. Overall Design Considering the design and construction of a solar-powered airplane, there is a number of parts common to all past projects. These elements are: solar panels, battery, motor and a propeller. The necessity of solar panels is evident, due to the purpose of the project. Solar panels lay on the top surface of the wings in order to avoid sun blockage from other elements. The number of cells will vary depending on the size of the airplane and the power is needed to maintain flight. Today, solar panels made out of Silica are widely used because have a better efficiency. Recent solarpowered airplanes projects use maximum point power trackers (MPPT) to increase power efficiency from the solar panels. The addition of MPPT’s is certainly beneficial considering their light weight and positive effects on the overall efficiency of the solar cells. The power generated by the solar cells is storage in batteries. As previously mentioned, batteries are the heaviest part of the plane. Today, LiPo batteries have the best power to weight ratio, thus most of the previous projects have selected them. Then the power flows from the batteries to the motor. In some cases, a DC/AC converter is used before the motor. However, since the brushless DC motor is the most popular choice, a converter is rarely necessary. Finally, the motor moves the propeller to begin flight. Regarding structure materials, carbon fiber is the number one choice due to its light weight and strength. Kevlar is widely used for more strength and resistance too. Depending on the purpose and function of the plane, other parts are added to the model. Some examples are cameras, radios, altimeters, GPS, sensor, etc. Figure 4.1 and 4.2 show the electric system diagram for the Solar Impulse and Sky-Sailor, respectively. A complete illustration of the Sky-Sailor model, with its parts and location, is given in Figure 4.3.

Figure 4.1: Electric system of Solar Impulse [9]

Figure 4.2: Electric and control system of the Sky-Sailor [23]

Figure 4.3: Parts and location of components of the Sky-Sailor [23]

5. Solar Energy Solar energy is typically harnessed by two methods: passive and active. A passive solar energy uses the sunrays directly to heat liquid or gas, whereas an active system converts the sun’s energy into electrical energy by using a photovoltaic semiconductor called a solar cell. The passive solar system demands enough solar power density to make it viable and it is most effective during the daytime. The solar power density needed is calculated by

(5.1) Where t is the hour of the day using the 24 hour clock, pmax is the maximum solar power density of the day at t0 (noontime in the equator), σ is the standard deviation of the normal distribution function. The highest efficiency solar cells were able to convert 40% of the available energy into usable electrical power until recently when Boeing created a solar cell able to attain 40.7% efficiency. This cell was developed using a unique structure called a “multi-junction” solar cell,

which achieves a higher efficiency by capturing more of the solar spectrum. A multi junction solar cell is designed in layers such that every layer can obtain some part of the sunlight when it passes through. These layers combined allow the cell to capture more of the solar spectrum and convert it into electricity.

Figure 5.1: Photovoltaic cell

The total efficiency of the solar cell is calculated by

(5.2) Ps

power of sunrays reaching the solar cell

Pout

output electricity power consumed by the load

V

voltage

I

current

ρ

solar power density at the PV surface

A

area of the PV cell facing the sun

Table 5.1: Solar Panel Specifications [3]

Kyocera Model

Watts

Amps

Volts

Weight (lbs.)

Item

Price

KC 130GT

130

7.39

17.6

26.8

1121300

not available

KC 130TM

130

7.39

17.6

26.8

1101300

$555

KC 175GT

175

7.42

23.4

35.3

-

not available

KC 200GT

200

7.61

26.3

40.7

1102001

not available

KD135GX-LP

135

7.63

17.7

28.7

1101351

$570

KD180GX-LP

180

7.63

23.6

36.4

1101801

$756

KD205GX-LP

205

7.71

26.6

40.8

1102051

$799

KD210GX-LP

210

7.90

26.6

40.8

1102101

$874

KC 40T

40

2.24

17.9

10

1100401

$265

KC 50T

50

3

16.7

10

1100501

$280

KC 65T

65

3.75

17.4

13.2

1100651

$350

KC 85T

85

4.75

17.4

18.3

1100850

$410

6. Maximum Point Power Tracking (MPPT)

A Maximum Power Point Tracking (MPPT) system allows a PV array to produce the maximum power they are capable of. It does this by electronically varying the electrical operating point [4]. The optimal MPPT technique varies depending on application and ease of implementation. In conventional controllers, a battery is charged by the controller at the batteries voltage. The MPPT, the maximum power is calculated by the controller and the batter is charged at this voltage level.

Figure 6.1: Typical 75W PV Module at Standard Test Conditions [4]

Ease of implementation is greatly dependent of the individual’s familiarity with certain areas, especially analog or digital circuitry. Other factors are the number of sensors required for each application and the cost. For example, in a project with a very high cost the MPPT will be chosen on its speed and accuracy, while for other applications these factors may not be as important as the cost. The advantages and disadvantages of each technique is summarized in Table 6.1 to aid in the decision making process. Figure

5.2 shows the three MPPT’s used in the Sky-Saylor project with a total weight of 23.3 grams [23].

Figure 5.2: Three MPPT’s from Sky-seeker [23]

Table 5.1: Characteristics of MPP techniques [5]

7. Energy Storage

An integral part of the design of a solar powered airplane is energy storage. It is critical that the plane maintains flight at times when the solar energy available is not sufficient. The two most viable options for our application is battery or a fuel cell. The main advantage for batteries is their efficiency and technology readiness level (TRL). They have an efficiency level of 95% and a very long lifespan in terms of charge/discharge cycles. Batteries are also what is presently used to store energy in the application of satellites, due to the advanced TRL. The biggest disadvantage for batteries is the energy density. The newest rechargeable lithium battery technology is still only able to achieve 200 W*H/kg [6]. A quickly emerging option for energy storage is the regenerative fuel cell system (RFCS). The efficiency of the RFCS is not nearly as high as that of batteries, but the energy density is much greater making it an attractive option.

Figure 7.1: Schematic of SOFC [7]

Technology Round Trip Efficiency Battery 95% SOFC