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Airship design method based on available solar energy in low stratosphere

Dumas A., Anzillotti S., Trancossi MUniversit di Modena e Reggio Emilia

Copyright 2009 SAE International

ABSTRACT

The actual applicative research concerning airships andtheir use as HAP (High Altitude Platforms fortelecommunications and military use) define the

possibility of new applications of these systems, alsoconcerning energetic high altitude production. This paperpresents the energetic balance of a photovoltaic platformwith capability of static hovering at high altitude realizedby electric powered propellers. This is the first steptrough the design of the P.S.I.C.H.E. (PhotovoltaicStratospheric lsle for Conversion into Hydrogen asEnergy vector) airship concept: a stratospheric airshipwhich can serve a platform for hydrogen and oxygenproduction.This paper analyses the energetic design process for aHigh Altitude Platform based on photovoltaic energycapture, but the process could be generalized in order to

be applied to any airship project. It is considered anairship shapes equipped with large PV array that coversenergy request during the day. Surplus photovoltaicpower supplies electrolysis equipments producinghydrogen and oxygen from water, brought up by anauxiliary airship. Hydrogen and oxygen are liquefied andstored in gas cylinders, in order to satisfy supplementarypower requirements by a fuel cell system during night-time insufficient solar irradiance. The StandardAtmosphere Model is used to evaluate PV performanceat various operative altitudes.A propulsion system with electric motors guaranteesairship manoeuvrability and hovering. Energy balance ofPV-hydrogen energy supply system has been analyzedfor three airship shapes with equal volume with concernof overabundant hydrogen and oxygen production.

Hydrogen annual production for PV square meter hasbeen evaluated in relation to ground production at thesame latitude.

INTRODUCTION

A new interest of airships has begun about 15 years agoand it is witnessed by futuristic and fascinating projectsNowadays it seems that some projects are going tobecome reality, despite the economic crisis. Specializedaerospace magazines presented the project JHL40teamed by Boeing with the Canadian company SkyHookIt is a heavy-lift aircraft that combines helicopter rotorsystems with a neutrally buoyant airship. It is consideredthe first heavy transport hybrid which is going to beoperative.Some high altitude platforms are currently in pre-designphase for civil purposes, essentially telecommunications

and digital broadcasting, and military systems such as aidefence and surveillance.Due to their heavy operative tasks both transport airshipsand high altitude platforms need large energy supply fovery long enduring mission. A concrete possibility isconstituted by photovoltaic energy production associatedto a fuel cell system for hydrogen and oxygen. It is aclosed mass loop cycle based on renewable solaenergy, except a fractional dissipation of water in thehydrolytic process, which produces hydrogen and oxygendissociation from water by electrolysis, coupled with afuel cell system for night-time energy supply fed by O2and H2 tanks.This paper investigates hydrogen and oxygen productionby stratospheric lighter-than-air platform with large hulphotovoltaic surface, at operative altitudes above 12 km

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which is defined in order to avoid dangerousmeteorological phenomena like summer cumulonimbusand clouds shadowing in order to maximize solar energyintercepted in a year.

SHAPE DESIGN

Lighter-Than-Air history presents a large variety ofspecialized configurations for different uses. The mostcommon shape is the conventional streamlined designs(ellipsoidal), but many projects use unconventional andinnovative shapes [3] even specialized for special tasks.It is possible to observe different shapes: spherical,lenticular, toroidal, deltoid, flat-body, multi balloon, etc.,and to analyse the characteristics of every kind of shape:- spherical shape:

- provides the optimum lift efficiency- it is not so efficient in aerodynamics;

- ellipsoidal (conventional) shapeapproximates a bodyof revolution about a longitudinal axis creating astreamlined body:- has a good aerodynamic performance;- presents a low efficiency in terms of lift and a

high sensitivity to crosswinds and it couldpresent problems at low speeds and duringmooring.

- lenticular shape or flying saucer is an envelopegeometry generated by the revolution of a flat shapeabout a vertical axis:- the aerodynamics of this shape has a reduced

affection by horizontal wind direction andpresents a large upper surface for PV array;

- it could present some problems of stability in thecase of not perfectly horizontal winds.

Table I Configurations of the tested shapes.

Configuration (A) presents a nearly hemispherical shapewith the best surface-volume ratio while configuration (B)

has been constructed starting from a segment of sphereand has the largest upper surface. Configuration (C)represents an intermediate surface between (A) and (B).

SHAPE DESIGN PRELIMINAR ANALYSIS

The history of recent large blimp projects has beenstarted by aerodynamic performances and lift capacityonly. Most projects have got in trouble by the energeticproblem. This kind of problems has become moresevere approaching photovoltaic powered systems.

This paper evaluates the energetic definition of HAP foenergetic production and TLC, but results could beapplied in any case. In order to solve this kind oproblems it has been decided to use an energetic designapproach.The first objective limit is connected to the volume of thesystem. This is a completely defined parameter bystructural weight and service ceiling. In a first pre-designphase we have defined three different layouts, byanalysing the main weight of structure, photovoltaic and

energy conversion equipments, electrical engines andrelated accessories, etc.By this component analysis and considering a serviceceiling of 20 km, we have determined a total volume ofabout 2.2 10

6m

3.

Figure 1 Shape configuration (A)

Figure 2 Shape configuration (B)

Figure 3 Shape - configuration (C)

Considering these prescriptions it has been evaluated anoverall system mass about 2*10

5kg. By this value, it is

possible to evaluate the influence of the shape on energysystem performance and payload capacity in the case ofdifferent shapes with the same volume (table I). Theshapes represented in the figures above (Figure 1, 2, 3)have been analysed.

AERODYNAMIC CONSIDERATIONS

To realize our research we have obtained and usedmeteorological and atmospheric data at the location ofmeteorological station of San Pietro Capofiume, locatedlatitude 44.65, longitude 11.63 and elevation 6 meters.It has been calculated mean velocity, density, andtemperature profiles in the atmosphere up to 25 kmbased on disposable measures of the last 15 years. Aldata have been presented in [1]. Here it is shown, fobrevity, for the only month of January the velocity profileits standard deviation and the maximum ( fig. 4), and in

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figure 5 is shown the directions of the wind and the itsstandard deviation.

0 10 20 30 40 50 60 70 80 900

5

10

15

20

25

[m/s]

Quote

[Km]

Wind Speed (Jan)

Mean+

std

-std

max

Figure 4 Monthly average velocity, SD andmaximum profiles.

50 100 150 200 250 300 3500

5

10

15

20

25

[deg]

Quote[Km]

Wind Direction (Jan)

Mean

+std-

std

Figure 5 Monthly average wind directions and SD.

In Fig. 6 Reynolds number profiles has been calculatedbasing on V

1/3as reference length (Tab. 1).

The platform needs energy and power for hovering,manoeuvring, and supplying board systems andequipments. Mean monthly wind speed is the basis forcalculation of total necessary energy on a year, indicatedas mean power. The maximum value and the SD of thewind have to be utilized for sizing the peak powerrequired to counteract the action of wind. Anotherparameter, here not considered is the maximum shift

allowed to the platform.The drag force FD and the lift component force FL havebeen calculated for the three considered shapes by CFDsimulation realized by the Fluent code, utilizing themonthly data of the locations of San Pietro Capofiume.The determination of the overall Drag force FD and theoverall Lift component force FL for each shape consentsto evaluate the Lift and Drag coefficients by theirdefinitions, respectively given by

22 L

L

FC

v A(1)

and

22 D

D

FC

v A(2)

Profiles of the annual energy requirements show (Figure9). a maximum in around 1012 km, due to jet streamsThe characteristic peak correspondent to jet streamaltitude is common to all the quantities related with windspeed and at higher altitude it reduces considerably. It isnoteworthy that, at every altitude, (B) and (C) shapes

present a less need for energy due to aerodynamicprofile.

Figure 6 Reynolds number profiles at different altitudes

It has been possible obtain the profiles of the twoparameters respect the Reynolds number and thedifferent shapes, as shown in figures 7 and 8

Figure 7 Drag coefficient CD vs. Re.

Figure 8 Lift coefficient CL vs. Re.

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Figure 9 Required annual energy for hovering

Using the meteorological data of San Pietro Campofiume(Bologna, Italy) the energetic consumption for hoveringof a platform fixed at the altitude of 15 km is comparedwith that one of platform that changes its altitude of

hovering, between 15 km and 17 km, following lessvelocity of the wind. In all cases, A, B and C.It is possible to find a little but not significant reduction inconsumption (figure 10).

Figure 10 Energy required for hovering: fixedaltitude of 15 km and variable altitude 15-17 km

SOLAR IRRADIANCE EVALUATION

Three components of solar radiation, direct, diffuse andreflected, are taken into account. Also for theseparameters the envelope has been reduced to smallplanar surfaces. The calculation, by quarter of hoursteps, has involved direct and reflected irradiance on theentire shape.

Direct solar radiation

Reduction of atmospheric filtration is computed by usualoptic techniques using the following traditional calculationschema (Fig. 11), which is used also for ground based

installations. Direct solar radiation increases with altitudebecause of the solar radiation has a shorter way in theatmosphere. It reaches a value which is similar to theextra-atmospheric one about an altitude of 30 kmDifferent models has been evaluated and compared.

Figure 11 Geometric model for calculations

DRUMMOND AND LAUE MODEL - For calculationpurposes one of the most significant equations is theequation of solar radiation modified by Drummond, Laueand others [4, 5, 6,]. This equation expresses solar

irradiation as a function of altitude and could beexpressed as:

1/sin, 1

Sc A

nI A h SF a h e a h SF (3

with a=0.14.

OKI AND SHIINA MODEL A more recent study by MOki and H. Shiina [7] based on experimental analysisbased on different localities all over the world. They haveobtained the following equation which is function of Aand h, expressed in meters:

54650sin 30 325 8 10 1180 3

AI h

(4)Both the presented models are studied in order torepresent the irradiation conditions in high altitudelocalities and loose significance over 6 km altitude. Acomparison of the considered equation results has beenplotted (Fig.12).

Figure 12 Solar irradiance vs. altitudes(Drummond-Laue Model and Oki Shiina model)

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ASHRAE CALCULATON METHOD - An alternativeformulation is that one adopted by ASHRAE [10]. Directnormal solar radiation assumes the following expression:

0sin0, /

B p

A pd nI A p p C SF e (5)

Cn is dimensionless, and it represent the relationshipbetween normal radiation calculated using the averagevalue of water vapour in a sunny day and the samequantity calculated by the tabulated value of the

ASHRAE Handbook [10]. At considered altitudes(820km), the atmospheric physical properties areconforming to the Standard Atmosphere model. By thishypothesis it is possible to assume Cn=1. The relationp/p0 is the correction as function of the altitude. The Bcoefficient is the dimensionless coefficient of Threlked eJordan, reported in ASHRAE calculation method.

Table II Coefficient B, ASHRAE model

Figure 13 - Solar irradiance vs. the altitude (ASHRAEModel)

Diffuse solar radiation

W. Knaupp and E. Mundschau [8, 9] have analyzed thesolar irradiance using the MODTRAN code, with aconstant zenith angle. The code uses data oftemperature, pressure and atmosphere composition ofUS 1976 Standard Atmosphere Model. They calculatephenomena of solar radiation absorption and dispersionof solar spectrum at various level of atmosphere by wave

length and produce spectral and global values at variousaltitudes. They assume that diffuse irradiation over 7.5km results lower than 40 W/m

2and in this work is

prudently, considered negligible.

Reflected radiation

More significant is the contribution of the reflectedcomponent, in particular that one due to the presence ofclouds under the station. In these conditions the albedo

coefficient assumes is nearly the maximum (0.9)Assuming the hypothesis of an isotropic radiation underthe station, it is possible to obtain

,

1 cos

2r t hI I (6)

Total radiation on an inclined surface, neglecting thediffusive component, is given by

cost d r n r I I I I I (7)The total radiant energy intercepted by the surface canbe calculate using the following expression

2

1

s

s

i

month t

n i

Q I d (8)

where s is given by

arccos tan tans (9)

It has been calculated the solar irradiance intercepted byplanar surfaces at different azimuth and inclinationpositioned from 10 to 25 km, by quarter of hour steps.The numerical integration of equation (8) gives theaverage monthly values of intercepted energy by asurface. The sum of those values is the total annuacaptured energy

12

1

i

monthyear

iQ Q

(10)Intercepted energy depends by the angle of tilt and theazimuth of the plane. In figure 14 are represented theresults for planar surfaces at constant azimuth. Figure15 presents the results obtained by planar surfacestracking the sun, characterized by constant differencebetween azimuth of the plane and that of the sun.Data presented in figure 14 and 15 can be used toevaluate the energetic production realized by the surfaceof the envelope, reduced in small planar surfaces. Inother papers [1, 2] the comparison among the shapes atconstant azimuth has been evaluated. Here it has been

presented the behaviour of the A, B and C configurationsunder the hypothesis of a rotating shape such as a solarfollower which guarantee that every flat surface has aconstant azimuth from the sun. Under these conditions ihas been verified that energy collection by PV panels issymmetric. The symmetry plane is a vertical planepassing through sun and the centre of the platform itselfThe two symmetric half-shapes defined by this planeintercept the same solar flux. This result shows that it ispossible to have a correct and efficient electricconfiguration, which guarantees a good behaviour of thesystem.

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Looking for a criterion of optimization on the PV surfaceamount which has to be installed, it has beeninvestigated the behaviour of two quantities, nominallyPV surface and the mean value of the solar radiationintercepted yearly by the planar surfaces of the envelope,against the minimum value prescribed of solar irradiance(cut-off value). The results are presented respectively infigure 16 and 17.It can be easy noticed that the same energeticproduction permit an important reduction of PV installed

surface in the tracking configuration if compared withfixed configuration.None of two quantities presents a particular behaviouragainst the cut-off value. In order to evaluate the validityof the system two different values of the cut-off may beconsidered. They correspond to the solar radiationintercepted by two different ground based configurations:

- PV fixed plan on a horizontal surface- PV fixed plan on the best tilt.

The choice between the two configurations dependsstrongly by the cost of PV surface and of the land areaoccupied.

Figure 14 Annual global irradiance on fixed flatsurfaces at constant azimuth.

Figure 15 Annual global irradiance on flat surfacesat constant azimuth from the sun

In this paper we have chosen the value of cut offcorresponding to best fix configuration intercepting thesun. (5400 MJ/m2). By this assumption it is possible tocalculate envelope surface that guarantees betteirradiance value (Figure 18) and, consequently, to cut thePV panels with less energetic production. (figure 19)

Figure 16 PV surface vs. value Irradiance cut-off

Figure 17 Mean yearly irradiance vs. value ofirradiance with cut-off

PV SYSTEM

PV energy production was carried out undeconsideration of operative conditions at high altitude. Inparticular PV efficiency is affected by high altitudeatmosphere conditions such as low temperature andwind speed. Equilibrium temperature of PV cells hasbeen calculated by the thermal balance equationsuggested by Colozza [11], involving radiant, convectiveand conductive heat transfer:

( ) s cond conv rad el cell f T (10)The photovoltaic efficiency has been computed fromequilibrium temperature and solar irradiation [12]

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Photovoltaic power has been computed all year long bysteps of 15 minutes. Energetic production over a yearhas been computed by numerical integration of thesevalues.

Figure 18 Yearly irradiance distribution configu-ration (B) tracking at an altitude of 15 km [MJ/m2]

Figure 19 Yearly irradiance distribution with cut-offvalue of 5400 MJ m

2- 15 km, configuration (B)

[MJ/m2]

Figure 20 PV power production and meannecessary propulsive power, 21 June, configuration(B) - [MW]

Figure 21 PV power and mean necessary power21 December configuration (B) - [MW]

Figure 22 Daily maximum PV power productionduring the year

Overabundant summer energy supplied by PV array(Figure 19) allows effective hydrogen and oxygenproduction by electrolysis but also during winter solstice(Figure 20) daily energy production is greater thanenergy requirements for hovering.The maximum daily power delivered by PV system isvalued for the three configurations (Figure 22)Configuration (B) reaches top production (almost 10MW) on summer solstice.

ENERGETIC BALANCE

The energetic balance for hovering has been analyzed avarious operative altitudes.An annual energetic balance has been presented in Fig23. This balance produces different series of results forthe three tested designs.The x axis zero value represents the equilibrium pointThe gray area on the left of the zero vertical linerepresents the unacceptable condition in which theenergy supply system is not able to meet energyrequirements for hovering. Different energeticsustainability has been reached, varying from 11 km forlenticular configuration (B) to 13 km for a spheroid shape

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(A). The configuration (C) is intermediate. All the testeddesigns present a maximum value of annual energyproduction near the altitude of 18 km and it presentnearly insignificant variation over 16 km, which appearsin the evaluated atmospheric conditions the optimalworking altitude.

Figure 23 Net energy balance over a year [MJ]

The energy exceeding the immediate power absorptionof the propulsive system can supply the electrolysissystem obtaining hydrogen and oxygen from water. Apart of the produced gasses can be stored in gaseousform to satisfy the power required in order to guaranteestatic hovering [1, 2, 14, 15] during the night. The mainpart of hydrogen and oxygen can be liquefied and storedin cylinders tanks waiting to be transported to the ground.

Figure 24 Hydrogen annual net production for squaremeter normalized that one at ground level

It has been verified that lenticular configuration (B)presents the best performance in terms of net hydrogenproduction. It can realize a production, mean for squaremeter of photovoltaic array in high altitude, more thandouble, than the production which could be realized atground level by PV fixed surface at best tilt andorientation (figure 24). In particular, for the configuration(B) and the sizing used the simulation gives an H2 output

near 350.000 kg/year, corresponding to 4106

m3

of gasat 101325 Pa and 0C (Table III)

Three items have to been considered regarding theenergetic needs for the liquefaction:- any system producing hydrogen had to pay an

energetic cost for liquefaction or compression.- at high altitude the energetic cost of liquefaction, due

to lower temperature, is about sixty percent of thesame process at the ground level [16].

-a part of the liquefaction work can be partiallyrecovered during the necessary re-gasificationprocess.

Table III Annual surplus electrical energy or gassesproduction (B-shape)

CONCLUSIONS

This paper presents a case of general energetic designfor airships. The presented method has been developedin order to design a High Altitude Platform, but it could beeasily applied to any photovoltaic powered airship. In theconsidered case it possible to have an annual energeticproduction as calculated in Table III. In particular it hasbeen verified that the considered shape (B), whichpresents the best energetic production, could be good fora large surplus energy production.By this considerations take origin the project of a high

altitude platform for energetic production, formerlynamed PSICHE (Photovoltaic Stratospheric Isle foConversion of Hydrogen as Energy vector) [1,2,13,1415], a large high altitude platform designed in order tocarry out simultaneously more than one task: hydrogenand oxygen production, telecommunications and digitabroadcasting, air surveillance and control groundmonitoring, and low temperature - low pressure hightechnology laboratory.

REFERENCES

1. Anzillotti, S., Una piattaforma stratosferica per la

produzione di idrogeno mediante energiafotovoltaica. PhD Degree Thesys, Rel. Dumas A.

Universit di Modena e Reggio Emilia, XVIII ciclo

2006.

2. Dumas A., and Anzillotti S.: PSICHE: A

Stratospheric Platform Producing Hydrogen and

Oxygen 5th International Conference on

Sustainable Energy Technologies. Vicenza, Italy

2006

3. Khoury, G.A., and Gillet, J. D. Airship technology

Cambridge University Press, 1999

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4. Drummond, A. J., Proc. IES/ASTM Int. Symp. Solar

Radiation Simulation, Los Angeles, Inst.

Environmental Sciences, Mt. Prospect, Ill., 1965.

5. Drummond A. J. , Hickey J. R. , Scholes, W. J. , and

Laue E. G., The Eppley-JPL Solar constant

Measurement program, SOLAR ENERGY," VOL. 12,

pp. 217-232, 1968.

6. Drummond, A. J. , Hickey, J. R. , Scholes, W. J. ,

and Laue, E. G., New Value for the Solar Constant

of Radiation, Nature 218, 259 - 261 (20 April 1968);doi:10.1038/218259b0.

7. Oki, M. and Shiina H. , Preliminary study for an

estimation method for annual solar irradiance at

various geographical altitudes, Eighth International

IBPSA Conference Eindhoven, Netherlands, August

11-14, 2003

8. Knaupp, W., and Mundschau, E. - Solar electric

energy supply at high altitude. - Aerospace Science

and Tech. 8, 245254, 20049. Knaupp, W., and Mundschau, E., Photovoltaic-

hydrogen energy system for stratospheric platforms.- 3

rdWorld Conf. on PV Energy Conv. 2004

10. 2005 ASHRAE Handbook : Fundamentals : Inch-Pound Edition, USA 2005

11. Colozza, A. Initial Feasibility Assessment of a High

Altitude Long Endurance Airship. NASA/CR 2003-

212724, 2003.

12. Arvesen J. C., Griffin, R. N. Jr., and Pearson, B. D.

Jr., "Determination of Extraterrestrial Solar Spectral

Irradiance from a Research Aircraft," Appl. Opt. 8,

2215-2232, 1969

13. Dumas A., Anzillotti S. and Trancossi M., Available

Solar Energy in Low Stratosphere, II Congresso

Nazionale Associazione Italiana Gestione

dellEnergia, Pisa, 2008

14. Dumas A., Anzillotti S., Zumbo F. and Trancossi M.

Photovoltaic Stratospheric Isle for Conversion in

Hydrogen as Energy vector: Energetic and Economic

feasibility analysis, II Congresso Nazionale

Associazione Italiana Gestione dellEnergia, 2008

15. Dumas A., Anzillotti S., and Trancossi M.,

Photovoltaic Space Isle for Conversion in Hydrogen

as Energy Vector: the Concept of a Stratospheric

Airship for Energy Production, Telecommunications

and Territorial Surveillance, SAE 2008 Wichita Air

Congress and Exhibition, oral presentation, Wichita

(KS), 2008

16. Amoresano A.,Langella G.,Novello C. Analisitermodinamica ed energetica di un impianto

criogenico per idrogeno liquido alimentato a celle

fotovoltaiche. 53 Congresso ATI, 15-18 settembre

1998 Firenze, 1998 ISBN: 8886281293, ISBN

13: 9788886281294.

CONTACTS

Antonio Dumas: antonio.dumas@unimore.it;Stefano Anzillotti: stefano.anzillotti@unimore.itMichele Trancossi: michele.trancossi@unimore.it

DEFINITIONS, ACRONYMS, ABBREVIATIONS

DefinitionsA solar altitude( angular coordinate) ;h altitude of the site (m);I radiative flux (W / m

2);

It,h total radiation on the horizontal plain (W/m2);

i1 first day of the month;i2 last day of the month;Nm

3Cubic meter of gas at 0C and 1,01325 bar

p pressureP Power (W)

Qmonth Intercepted solar radiation in one month (J/m

2

);Qyear Intercepted solar radiation in one year (J/m2);

Greek symbols Thermal Flux [W/m

2]

albedo coefficient

s hour angle (coordinate angular) azimuth; (coordinate angular) declination ; (coordinate angular) trasmittance-assorbance PV panel coefficient

Pedici0 at sea level

cell referred to PV panelcond referred to thermal conductionconv referred to thermal convectiond directel referred to electric currentn normal directionr reflectedrad referred to thermal radiations solar

AcronymsHAP High Altitude Platform;PSICHE Photovoltaic Space Isle for Conversion in

Hydrogen as Energy Vector

PV photovoltaic;TLC Telecommunications