RV1 Literature OverviewV2

7/31/2019 RV1 Literature OverviewV2

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A Review of Energy Technologies for High altitude Ultra long EnduranceUAVs

By [email protected]

2011

Content:

1. Introduction2. Current UAVs market3. Common mission profile and requirements4. Pure Hydrogen systems

a. UAV Fuel Cellsb. Tanksc. Delivery systems and sub components

5. Pure Solar systems

a. Cell technologyb. Battery technologyc. Power management

6. Hybrid systems7. Waste energy8. Low power energy scavenging

a. Thermoelectricb. Piezoelectric

9. Summary

Introduction

Over the recent years the desire for unmanned aerial vehicles with increasingly longerflight endurance has gained importance, due to the obvious civil and militaryapplicability. Common civil mission profiles include hurricane tracking, continuousand comprehensive border control, telecommunication relay and weather observation.If an autonomous vehicle is able to fulfil the requirements for these missions then itmay have clear advantages in terms of cost, over present satellite systems.Long flight endurance demands an optimal vehicle on a multi disciplinary level, withfocus on parameters, such as aerodynamic efficiency, weight and the power density of

the energy sources. To maximize the viewable ground area, cruise/loiter at highaltitudes is required, which in turn complicates the lift generation due to the low airdensity. Apart from the difficulties associated with energy demand for long flightendurances, other problems such as large aeroelastic effects resulting from the lightstructures and usually long wing spans as well as wind gusts and their effects on thelow flight speed. This review however will only focus on emerging and existingtechnologies associated with the power generation on the vehicle.Common technologies include Photovoltaic electricity generation, a variety of fuelcells (Proton exchange membrane, Solid oxide fuel cell) and combustion turbines.Due to its high energy density, hydrogen is the favourable chemical energy carriercombined with fuel cells and turbines. Battery technology is the second most common

energy carrier, due to their recharging ability with solar arrays.


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Current UAV development

Global Observer (AeroVironment)

This UAV is designed for an endurance of up to 7 days at an altitude of around 20

km. It solely relies on liquid hydrogen as its energy carrier and uses a Protonexchange membrane (PEM) fuel cell to generate electric power for the engines. Aprototype has been developed and is now undergoing flight tests. Hydrogen poweredflights of 5 hrs. at an altitude of 2km have been performed as well as 7 day flightendurance ground testing of the equipment.

Orion (Aurora)

With originally a similar mission design requirement as the Global Observer, thisvehicle also relied on liquid hydrogen as the energy source, however the power isgenerated in a combustion engine and no fuel cell is used. The development has now

morphed into a medium altitude concept with an endurance of up to 120 hrs.

Phantom Eye (Boeing)

Boeings design will be able to lift a pay load of 190 kg and stay aloft for around 4days. A prototype demonstrator has been presented, featuring a single fuselage withconventional tail and wing configuration. It will run on hydrogen, with 2.3Lsupercharger engines from the Ford Company.

Zephyr (QinetiQ)

With a wingspan of just over 20 meters, this vehicle is relatively small. It fully relieson solar energy and batteries to sustain long endurance flight. A demonstratedendurance of 14 days has been achieved at an altitude of 23 km. Due to its small sizethe payload capacity is limited and just around 3 kg

Odysseus (Aurora)

This is a proposed UAV system with ultra long endurance with solar energy as itsmain energy source. The Design is supposed to fulfil the requirements of the DARPAVulture program. The concept consists of 3 individual wings, joined into a single Z

for maximum solar power harvesting.

SolarEagle (Boeing/QinetiQ)

Building on QinetiQs experience with zephyr a larger design, also contending in theVulture program, is to be developed with a wingspan of 120 m. Again solar energyand electric engines will be incorporated.

(..) Lockheed Martin

The third contender in DARPA project includes a flying wing design with multiple

rotating tail surfaces to capture solar energy efficiently. A number of pods aredistributed along the span to help load the wing evenly with payloads.


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Mission profile and requirements

DARPA Vulture program has set out the following requirements for the mission of aUAV with endurance in the year region:

'Pseudo-satellite' aircraft

454 kg of payload

5kw of payload Power

99% on station probability with reference to wind speeds

It is obvious that a design which meets these requirements will be reliable on externalenergy, since it is not feasible to carry all the energy on board for endurances beyond30 days [1]. Therefore all 3 contenders rely on photovoltaic energy to power thevehicle.

A separate study [2] defines a possible Hurricane Science Mission and aCommunications Relay Mission with the following requirements:

1. Communication RelayEndurance 14 days

Payload 136kg

Payload power 1Kw

Loiter Altitude 18km

2. Hurricane ScienceEndurance 30 days

Payload 200kg

Payload Power 1Kw

Loiter Altitude 18km

Each mission profile consists of the common five stages

(NASA/TM2009-215521)

The loiter time on the ground plays an important role for vehicles using liquidhydrogen, due to increased boil off rate at high temperature differences between tankand ambient.


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Hydrogen Powered Systems

Due to its high energy density (~ 120MJ/kg compared with JP-8 at 43 MJ/kg)hydrogen as a chemical energy source is attractive for long endurance missions whereenergy is carried on board the vehicle. Looking at hydrogen in terms of energy per

volume, a different picture emerges. Even at very high pressures its density is muchlower compared to JP-8. Using cryogenic liquid hydrogen is slightly denser but stillnot comparable to conventional fuels. Because of this fact, the storage and deliverybecomes a critical issue and significantly complicates the energy conversion system.

Common UAV Fuel cells

I. Proton exchange membrane fuel cell (PEMFC)

The PEM energy converter uses the hydrogen together with oxygen to directlygenerate an electrical current. This is achieved by the use of a polymerelectrolyte membrane, which only enables hydrogen ions (protons) to passthrough it forcing the electrons to flow through the load to the cathode. Herethe oxygen is fed into the cell and combines with the ions and electrons toproduce water. Splitting up the hydrogen is done efficiently via a platinum

anode; where as a suitable cathode to split up the oxygen has to beendiscovered yet. The advantages of this fuel cell include a low operationtemperature ( ~ 80 100 C) and an estimated efficiency of around 55%. Theydo however encounter problems with temperature, water and air management,as a constant temperature, the correct extraction and addition of water plus air,is crucial to prevent dry out and material failure. The Fuel cells efficiency alsodepends on the amount of current drawn by the load. Generally, for higherloads the efficiency lowers due to an increased voltage loss across the cell.


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II. Solid Oxide Fuel Cell (SOFC)

The process is similar to the PEM fuel cell, with the difference that here themembrane composed of a dense layer of ceramic, only allowed Oxygen ionsto pass through. An important difference to consider is the raised workingtemperature for the cell to function properly. Sophisticated cells operatebetween 1300 and 1700 C adding a substantial temperature management subsystem to overall energy conversion process. Operation efficiencies lie in the45% range.

Tanks

In order to be able to carry as much mass of hydrogen in a minimal storage container,the gas may be pressurized and cooled to cryogenic temperatures. The level ofpressurization is governed by the structural integrity of the tank and the pressurerequirements of the hydrogen delivery system. [3]identified 30k psia as an optimumfor a given mission and vehicle design with a tank temperature of 20.4K. Note that theboiling temperature is at 23K and boiling off always needs to be considered in a tankdesign, due to the external heating through the insulation. 3 Different insulation types,a vacuum jacketed tank with a multi layer insulation (MLI) in the vacuum gap, avacuum tank without MLI and a single walled tank with spray on insulation; wereanalysed for the 14 day endurance mission [4]. A boil off mass of liquid hydrogen up

to 244 kg for the total mission was calculated which would reduce the endurance to 9days if it was not compensated for. The application of active thermal controlequipment for zero boil off was investigated [3] which however results in largerincrease in mass when compared to the boil off mass for this mission duration.

Apart from storing liquid hydrogen, it may also be contained in a metal hybrid tank.Here the hydrogen is stored within a metal and requires a certain temperature torelease it. Therefore temperature management of the tank is crucial to achieve therequired inflow to the energy converter. The energy density of this storage approach islower but the system is simpler to implement and less risky.


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Liquid Hydrogen supply system

The general system only requires a low powered liquid hydrogen pump if a separatepressurization system (e.g. with the use of helium) is in place [3]. Fill and drain valvesas well as venting systems are needed for the tank itself and in the delivery process

1. Internal combustion system

The stored liquid hydrogen is required to be vaporized in a heat exchanger andis then fed to the combustion engine. A limited number of sub systems arerequired; however the heat exchanger requires a significant power input.

2. Solid Oxide Fuel Cell System

Due to its higher operating temperature substantial extra equipment is requiredto increase the temperature of the hydrogen and the oxygen supply. Bycombining the hot exhaust gases from the cells anode and cathode, theincoming oxygen and hydrogen can be preheated efficiently without therequirement for continuous extra power in heat exchangers. However, duringthe start up phase this extra power will be required.

3. Proton exchange Fuel Cell

Since a PEM cell requires humidification of both the hydrogen and air theseextra systems need to be included. The exhaust gas of the cell may beliquefied and recovered to form a closed loop water system. Both thevaporizer for the liquid hydrogen and the air from the intake require and heatinput to reach the required operating temperatures. These are lower comparedto the SOFC preventing the reuse of hot exhaust gases.

The 3 different systems described were thermodynamically analysed and the PEMsystem resulted as the best performer with an achievable endurance of 16 days for thedefined reference mission [3]. This was due to the low fuel consumption of the celland the lower power required for the conditioning of the hydrogen and the air.


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PEM Fuel system - NASA/TM2009-215521

Pure Photovoltaic Systems

As shown by the 3 contenders for the Vulture project, when it comes to flightendurances longer than 1 month solar energy is the obvious choice for energyconversion. A number of important factors play a role when using solar energy. In theshort term, one needs to consider the large surface area required to expose the cells,the optimal angle at which light rays hit the cells, the operating temperatures of thecells; however for longer endurances the aircraft position on the globe becomes ofimportance due to variable energy densities with time and position.The biggest problem is however the night flight. Since no more solar energy can beconverted some form of storage device is required which functions as the energysource to sustain the cruise/loiter altitude. This storage may be undertaken in terms ofmechanical energy (in altitude and flight speed) and released by gliding to loweraltitudes in the night [5]. However common missions require the UAV to cruise at aconstant altitude continuously.

So the combined efficiency of the solar cells and storage medium has to be high for asuccesses full design, as well as the energy per mass of the storage medium.


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Cell technology

The PV cell is a semiconductor device which, when struck by photons dislodgeselectrons which move through the cell, creating and filling holes. This movement of

holes and electrons generates the current across the load. The efficiency of thisprocess depends on the PV material and the band gap of the semiconductor. Recentdevelopment includes the use of multiple layers of different materials to capture morephotons at different energy levels.

Battery Technology

Power management

Since PV cells are not an ideal power source it is of crucial importance to optimize theuse of the power when it is available. The internal resistance of a cell depends on boththe current drawn and the light intensity, making it important to adjust appropriately

to continuously draw the maximum power at variable operating conditions. Thisprocess is referred to as maximum power point tracking (MPPT). Additionally the


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system will have to manage the charge and discharge of the batteries according torequirements and power availability.

Since a UAV may require sudden peak power due to control manoeuvres, suddenvoltage and light intensity changes at the load and the PV cell need to be handled.

Hybrid Systems

A number of hybrid systems have been proposed [1; 3; 6] offering combinations ofsolar and hydrogen technology applied to long endurance UAVs.

One interesting proposal was the combination of a solid Oxide fuel cell and a gasturbine [1]. This proposed energy conversion system can be very thermodynamicallyefficient due to the reuse of hot exhaust gases as well as burning unused hydrogen.

(Nasa/TM)-2006-214328

By using waste air from the cathode and splitting it to provide the optimal mixture tothe burner remaining chemical energy can be converted to heat. These hot gases arethen used to: heat and vaporize the hydrogen, and drive a turbine to generateelectricity. The whole system is enclosed and insulated to minimize heat waste to thesurroundings. A study was then carried out to minimize the systems mass for a givenflight endurance. Even though the systems efficiency is high, its power density is lowdue to the increase in components. This does however not limit its applicability to theproposed mission endurances of 20 days.


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Further hybrid systems were analysed in a concept study [2], highlighting thedifficulties with solar powered regenerative fuel cells. In theory the process allows acyclic use of water by using solar powered electrolysis during the day and electricitygenerating fuel cells at night.

The concept was a flying wing with a span of 100m, wing area of 600 m2 and PV cellefficiency of 20%. If such a system is balanced over a day night cycle, then the vehiclecan remain airborne over a long time. The study showed that the baseline assumedtechnology (study from 2007) did not enable to possibility for such a mission with afeasibility of just 29-30% (where 100 = feasible). Possible technology advances andmission requirement changes were investigated showing possible feasible concepts

(AIAA 2007-4936)

It can be noted that a substantial improvement in solar cell efficiency / Energy storagesystem (ESS) specific energy/ or ESS round trip efficiency is required to make thecurrent mission feasible.

A further option includes the modification of the mission requirements making theconcept study feasible at the current technology level.


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Waste Energy

The previously mentioned systems all produce waste heat in some form or another,which can be classified as waste energy since it is not utilized for the system itself. Areduction in this waste energy, by either using part of it as an input to another system

or by optimizing the main system itself, is very desirable for high overall efficiencies.Traditionally an optimization on the main system level is the main focus, however asshown by some proposed hybrid systems, improvement can also be achieved on alower level.

All systems described so far include some form of waste energy, which may possiblybe harvested to produce low to medium useful power outputs. This may relieve someof the traditional equipment required to power smaller subsystems, such as payloads,avionics, pumps and valves.

Low power energy harvesting

Possible low power conversion systems could include thermoelectric generators(TEGs), producing electricity from a temperature difference, piezoelectric materialsreacting to vibration movements.

Thermoelectric Generators

Based on the Seebeck effect, an electrical potential can be generated within isolatedconducting materials exposed to a temperature gradient. The power output from sucha device is directly proportional to the heat flux across its thickness making hightemperature differences desirable. Positive features include, direct conversion toelectricity without moving parts, long life, high reliability and easily withstandingvibrations and high accelerations.

(AIAA 2003-6092)

On the other hand, 2 major problems exist with the devices. TEGs tend to have highoutput resistances, especially when a number of modules are connected in series,


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making the power transfer only efficient for high resistance loads. Furthermore due tothe material properties the p and n type semiconductors tend to have poor thermalconductivity. This makes them bad heat sinks, and not applicable to locations wherethis property is required.Their efficiency has been continuously improved due to advancing materials,

however wide commercial application has not taken place, because of their cost andsize. A major hurdle to commercialisation is the size - efficiency relationship,preventing the miniaturization of the generators. However work is under way [7] totackle this problem.Even at the current technological level TEGs may be applied in a long enduranceUAV context and provide low power outputs. A detailed system model, includingTEGs is required to confirm this.

TEG Performance

In a 2003 study [8] thermoelectric modules were incorporated in a small combustion

engine driving a propeller, to produce electricity from the temperature differencebetween the hot engine and the cold propeller air downwash. A test flight of a smallmodel UAV was conducted highlighting good operational capabilities of the TEGsin a turbulent flight environment.

Product Manu. DesignhotTemp

C

DesigncoldTemp

C

Maxeffic.

ModulePowerdensity

W/kg

ModuleWeight

g

DesignPower

W

Cost

HZ-14 Hi-Z tech. 230(300)

30 4.5% 158 82 15 140$

HZ-20 Hi-Z tech. 230(300)

30 4.5% 165 115 21 200$

HZ-2 Hi-Z tech. 230(300)

30 4.5% 185 13.5 3 69$

10CX1 Custom.Thermoelectric.

300 30 245 60 14.7 108$

05CQ Custom.Thermoelectric.

300 30 286 25 7.15 52$

04CQ Custom.Thermoelectric.

300 30 204 25 5.1 52$

TEG2-40-40-4.7/100

EURECAMesstechnikGmbH

120 180 26 4.7 18

TEG2-40-40-10/100

EURECA 120 416 24 10 97

TEG2-40-40-19/200

EURECA 200 500 19 36,50

TEG2-50-50-40/200

EURECA 200 1052 38 40 67,30

This table includes some commercially available TEGs with their given

characteristics. Single module output power ranges between 2 and 40 watts, whichmay be extended with multiple modules, if the hot surface area is available.


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Piezoelectric

A further possible approach to harvesting low power from ambient or waste sources

may be through structural vibrations via the piezoelectric effect. Hereby a material isemployed which has an appropriate coupling between its mechanical and electricalstate, i.e. a compression or extension of the material will result in a voltage field andthe other way around. Materials with these properties are usually of crystalline orceramic nature. Design of the crystals behavior to bending of the material is importantto optimize their performance at certain excitement frequencies.

(a) unchanged state / (b) Compressed material / (c) stretched material/

A possible application in the UAV context has been investigated by [ref], byincluding layers of PZT materials in a representative wing spar with the combinationof thin film batteries for electricity storage. This is a very interesting concept as thePZT material is not an additional load to an existing structure anymore, but rather aload carrying part of the whole structure. The smart incorporation of the PZT layerswith a given maximum power output at minimum mass addition is an optimizationproblem. The electric power generation depends on both the thickness of the layers aswell as the excitation received, which in turn depends on the damping of the structure.Since the new spar will have variable damping at different layer arrangements, thedependent relation has to be optimized to a given criteria.

(AIAA 2009-2139)

Using PZT materials in such a way requires extensive material testing to verify theload bearing capabilities at different charging and temperature conditions [9].


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A further important issue to consider is the fact that PZT based harvesters employedin a vibrating cantilever type structure, will exhibit maximum power generation at asingle vibration frequency. This is not very effective as ambient excitation vibrationswill be broadband in the low frequency region. By making use of a bistablecomposite plate with mounted piezoelectric patches, nonlinearity is induced in the

harvesting process, improving the frequency range which generates low power [10].

Summary

Current long endurance UAVs and their propulsion and systems technologies havebeen reviewed. For an endurance of up to a month consumable energy sources maybe employed, where as longer flight endurances require external energy harvestingthrough for instance solar energy. Therefore, all 3 contender concepts for the ultralong endurance mission set out by DARPA are solar powered.

Low power scavenging methods through thermoelectric and piezoelectrictechnologies have been reviewed. TEGs seem a promising technology, if the highrequired temperature gradient is available in an existing vehicle, which is more likelyto be the case in hydrogen powered systems as solar powered ones.

The incorporation of PZT materials into the primary structure may provide additionalpower, however it comes with a number of challenges due to the lower structuralperformance of the material. Weight addition, tensile strength and dampingconsiderations have to be taken into account.


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References

[1] Himansu, A., Freeh, J. E., Steffen Jr, C. J., Tornabene, R. T. and Wang, X. J.(2006), "Hybrid solid oxide fuel cell/gas turbine system design for high altitudelong endurance aerospace missions", vol. 44, no. 15.

[2] Nickol, C. L., Guynn, M. D., Kohout, L. L. and Ozoroski, T. A. (2007), "HighAltitude Long Endurance Air Vehicle Analysis of Alternatives and TechnologyRequirements Development", vol. AIAA Paper 2007-1050.

[3] Marc, G. M., Robert, T. T., John, M. J., Mark, D. G., Thomas, M. T. andThomas, J. V. O. (2009), Hydrogen Fuel System Design Trades for High-Altitude Long-Endurance Remotely-Operated Aircraft, 215521, Nasa, Glenn

Research Center.

[4] Wang, X., Harpster, G. and Hunter, J. (2007), "Thermal analysis on cryogenicliquid hydrogen tank on an unmanned aerial vehicle system", vol. 45, no. 6.

[5] Sachs, G., Lenz, J. and Holzapfel, F. (2009), Unlimited Endurance Performanceof Solar UAVs with Minimal or Zero Electric Energy Storage, American Instituteof Aeronautics and Astronautics, 1801 Alexander Bell Dr., Suite 500 Reston VA20191-4344 USA, [URL:http://www.aiaa.org].

[6] Chen, H. and Khaligh, A. (2010), "Hybrid energy storage system for unmannedaerial vehicle (UAV)", IECON Proceedings (Industrial Electronics Conference),pp. 2851.

[7] Span, G., Wagner, M., Grasser, T. and Holmgren, L. (2007), "Miniturized TEGwith thermal generation of free carriers", Rapid Research Letters, , no. 6, pp. 241.

[8] Fleming, J., Ng, W. and Ghamaty, S. "Thermoelectric Power Generation forUAV Applications", 1st International Energy Conversion EngineeringConference; Portsmouth, VA; Aug. 17-21, 2003, Vol. AIAA Paper 2003-6092,American Institute of Aeronautics and Astronautics, Inc, Reston, VA, 20191-

4344, United States, .

[9] Anton, S., Erturk, A. and Inman, D. (2010), "Strength analysis of piezoceramicmaterials for structural considerations in energy harvesting for UAVs", vol. 7643.

[10] Arrieta, A. F., Hagedorn, P., Erturk, A. and Inman, D. J. (2010), "Apiezoelectric bistable plate for nonlinear broadband energy harvesting", AppliedPhysics Letters, vol. 97, no. 10.

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