5.2 POWER MANAGEMENT · capacity expansion and lowers the cost per watt of products. ... based on copper indium gallium di-selenide (CIGS ) have shown the highest laboratory electrical

High Altitude Airship: Detailed Project Report

RESTRICTED DOCUMENT 114

5.2 POWER MANAGEMENT:

The power required by the airship is given by the power needed to operate the onboard systems

and payload and the power needed to overcome the drag on the airship and maintain station

over a specified location.

Above the cloud layer there is an abundant amount of solar energy. The solar flux at the orbit,

say, of Venus is 2600 W/m2, which is much greater than the 1360 W/m2 available at Earth orbit.

This nearly 100% increase in solar flux can significantly increase the performance of solar

powered vehicles. Even within or below the cloud layer there may be sufficient solar energy to

operate an airship [52].

Solar arrays mounted on the airship envelope are required to generate electrical

power during daytime, enough to allow power management of both the propulsion system and

water electrolyzer. The solar cells provide electric power for the airship and mission operations

in daytime.

There are some technical challenges that need to be overcome before solar energy can provide

a practical power source for our application. The weight of the solar cells and the batteries

necessary to store and manage the collected power poses serious difficulties for airship

applications. The power required to propel an airship scales with the square of its length while

the payload available scales with length cubed, so the weight consideration can always be

accommodated if the airship is made big enough [53].

Solar cells work by absorbing light and converting it to electrical power, referred to as the

photovoltaic effect. The majority of commercial solar cells in use today are made of silicon

The performance of a solar cell is measured in terms of its efficiency in converting sunlight into

electricity. Typical commercial solar cells have an efficiency ranging between 6% and 18%,

meaning that for every 1,000 watts of sunlight striking a solar module, 60 to 180 watts of

electricity will be produced [54].

5.2.1 Different Systems that require power on the airship:

The applications and uses of HAAs are extensive and might include earth surveillance, such as

for scientific and weather monitoring. Commercial applications include monitoring and

controlling the ever-increasing complexities of aerial and maritime transportation and



telecommunication networks. Such a scenario of HAA applications requires a substantial

amount of power to operate surveillance systems, probes, sensors, telescopes, radar systems,

etc. A large amount of power for the envisioned roles of HAAs requires onboard power

generation or harvesting and storage systems. The propulsion and maneuverability of HAAs

also requires a significant amount of power. The weight growth of HAA is limited by the long-

term operation requirements and the limit on weight growth (possible lighter than air

requirement). Accordingly, it can offer no room for fuel-carrying power generators which limit the

operational time or otherwise increase the overall weight. The power technology for HAA

maneuverability and mission-oriented applications must come, at least in part, from its

surroundings, e.g. solar power [65].

Power for House-Keeping: The internal power requirement is determined from the power

consumption by the PMC station move over the guide rail, communication equipment, avionics,

and system monitoring devices.

Power for Radar/Guidance: The onboard radar operation is necessary for monitoring other

aircraft and air traffic control and management. The area of coverage provided by the 21 km

cruise altitude is in excess of 300 nmi thereby allowing one airship to control an extensive

amount of airspace

Storage system/battery:

The excessive energy harvested during the day time can be stored on onboard battery

system, used to produce hydrogen by the breakdown of water molecules collected from fuel cell

operation, or used for other utilities.

Most of the Lidar systems for monitoring atmospheric constituent gases or pollutants

require substantial power to operate. A Lidar system onboard the HAA can utilize the excessive

power for its environmental monitoring operation or the excessive power can be wirelessly

transmitted to a remote area, such as deserts or Antarctic bases, where electricity is not readily

available. Microwave rectenna array or laser can be used for wireless power transmission [66].

The power harvested by the inventive ATE (advanced thermo electric) generator can also be

utilized for propulsion for position correction and maneuver. The wind at an altitude of 21 km

(70,000 feet) or above is substantially lower than typical seasonal jet-streams that exist within

the northern hemisphere. Nevertheless, the large cross-section of the HAA is vulnerable to



drifting along with wind. Continuous positioning and maneuvering operation of the HAA against

the wind is necessary and crucial for the stationary operation and maximum solar exposure over

solar angle variation. Otherwise, the HAA will drift away to an undesirable location where the

use of onboard devices may be impossible. The propulsion for position correction and

maneuvering is also required during the night time [65].

5.2.2 Thin Film Photovoltaic:

Solar cells and modules made from certain thin film semiconductors have been shown to

be much less expensive to produce in larger volume and requiring much less raw material to

produce than silicon based PV cells.

Thin film photovoltaic products exhibit the following attributes:

• Scaleable, low cost manufacturing: Thin film solar cells and modules require a structural

"substrate" to support them, such as glass. Applying the films on low cost glass

substrates enables continuous and scaleable manufacturing. As much of the equipment

to process these substrates is used in other industries, the capital expenditure required

to establish large-volume thin film PV product manufacturing plants enables rapid

capacity expansion and lowers the cost per watt of products.

• Lower material cost: The substrate and raw materials used in thin film PV products are

less expensive than the cost of most semiconductor materials. With increasing thin film

manufacturing capacity and process yield improvements product costs are reduced.

• Performance attributes: In addition to cost per watt advantages, thin film photovoltaic

technologies exhibit performance advantages in generating energy in low light level and

increased temperature environments. This positions them particularly well for

applications in regions with less direct sunlight, such as in Northern Europe.

5.2.3 CIGS:

Of the three thin film technologies currently being commercialized, solar cells and modules

based on copper indium gallium di-selenide (CIGS) have shown the highest laboratory electrical

conversion efficiency, can be produced on low cost glass substrates, hold promise for low cost

manufacturing, and are highly scalable to high volume manufacturing [54].



5.2.4 Regenerative fuel cells:

The current world trend is tending to avoid the use of batteries in emergency back-up power

systems and use a more environmentally friendly and reliable technology in the form of a fuel

cell system. A fuel cell is a device that converts the chemical energy of a fuel (hydrogen, natural

gas, methanol, gasoline, etc.) and an oxidant (air or oxygen) into electricity. In principle, a fuel

cell operates like a battery. Unlike a battery however, a fuel cell does not run down or require

recharging. It will produce electricity and heat as long as fuel and an oxidizer are supplied.

A fuel cell is a device that converts the chemical energy of a fuel (hydrogen, natural gas,

methanol, gasoline, etc.) and an oxidant (air or oxygen) into electricity. In principle, a fuel cell

operates like a battery. Unlike a battery however, a fuel cell does not run down or require

recharging. It will produce electricity and heat as long as fuel and an oxidizer are supplied [56]

Fuel cells are classified by their electrolyte material. At the present time, there are

several types of fuel cells that are being developed for applications as small as a cellular phone

(0.5 Watts) to as large as a small power plant for an industrial facility or a small town (10

Megawatts).

• Alkaline Fuel Cell (AFC)

• Phosphoric Acid Fuel Cell (PAFC)

• Molten Carbonate Fuel Cell (MCFC)

• Solid Oxide Fuel Cell (SOFC)

• Proton Exchange Membrane Fuel Cell (PEMFC)

• Direct Methanol Fuel Cell (DMFC)

• Zinc-Air Fuel Cell( ZAFC)

The Regenerative Fuel Cell (RFC) is a system that can operate in a closed loop and could serve

as the basis of a hydrogen economy operating on renewable energy. Fuel cells generating

electricity, heat, and water from hydrogen and oxygen would be used throughout the economy,

powering factories, vehicles, and houses. The hydrogen would be generated from the

electrolysis of water, splitting it into its constituent components of hydrogen and oxygen, using



renewable energy sources such as wind, solar, or geothermal. Such a system would not require

any specific type of fuel cell, but would need an infrastructure to deliver hydrogen to the many

fuel cells in use. The goal is to incorporate both photovoltaic solar cells and a regenerative fuel

cell on board of an aircraft/airship. The solar cells will power the aircraft during the day and

generate a supply of hydrogen that would be stored for use by the fuel cell overnight. Such a

system would then be capable of flights lasting many days [57].

Fig 5.7: Illustration showing the working of RFC

Benefits:

The RFC can provide significant benefits including:

• Seasonal energy storage: near-zero self discharge

• High levels of storage at a reasonable cost

• No practical limitations on the Depth of Discharge



• Easy and accurate state of charge measurement

• Ability to store energy from multiple energy sources

• Virtually unlimited cycle life [58]

Fig. 5.8: A typical Layout of a Regenerative Fuel Cell System [68]

5.2.5 The Unitized Regenerative Fuel cell (URFC):

Running on hydrogen fuel and oxygen from the air, a 50-kilowatt fuel cell can power a light

weight car without creating any undesirable tailpipe emissions. If the fuel cell is designed to

operate also in reverse as an electrolyzer, then electricity can be used to convert the water back

into hydrogen and oxygen. This dual function system is known as a reversible or unitized

regenerative fuel cell (URFC). The functions of both the electrolyzer and the fuel cell are carried

out by a single stack that can operate alternately in each mode. Lighter than a separate

electrolyzer and generator, a URFC is an excellent energy source in situations where weight is

a concern [59].



Fig 5.9: Unitized Regenerative Fuel cell [59]

Also, in applications where the unit operates all the time in either electrolyzer or fuel cell mode

(i.e., no idle periods) the URFC may be a better choice since it would not require auxiliary

heating, while the separate components would require auxiliary heating during periods when

they are not in operation[60].



Table 5.3: Specific Energies for URFC and Rechargeable Batteries

The regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest

energy density - about 450 watt-hours per kilogram - ten times that of lead acid batteries and

more than twice that forecast for any chemical batteries. Today, fuel cells are being used for

Space Shuttle on-board power, power plants, and a variety of experimental vehicles.

For space-borne applications, as well as for HALE aircraft, lightweight energy storage would

best be achieved by coupling unitized regenerative fuel cells (URFCs) with lightweight pressure

vessels that are integrated into the vehicle’s structure. Hydrogen and oxygen that are made by

the solar-powered electrolysis of water can be used for energy storage, or as propellants for a

high specific impulse, high thrust-to-weight ratio rocket propulsion system. The nontoxic nature

of water, and the ability to convert it into rocket propellants on the fly, may provide the strongest

incentives to develop space-borne applications, which also pose the toughest development

challenges [61].

The near flat-top surface of the airship offers a wide area to accommodate a new energy

harvesting device from sun light, such as solar cells or advanced thermoelectric (ATE)

generators. The power harvested by the ATE generator can be utilized for power transmission

to microwave-powered aerial vehicles (MPAVs), for recharging on-board energy storage



systems (regenerative fuel cells), for night operation of the propulsion system, and for the

internal power requirements such as propulsion and control. If more power is required, it will

need to build a larger HAA [66].

Fig 5.10: Configuration of ATE device for solar energy conversion [66]

5.2.6 Reliability and life expectancy of Regenerative fuel cells:

Typical expected battery life is estimated to be between 10 and 20 years depending on the

battery selected. These life predictions only hold true when the ambient temperature is

maintained around 77o F. In harsh environment it is common to replace battery every 18

months.

The PEM regenerative fuel cell system has design life of 20 years, with proper maintenance of

the stacks. The electrolysis stack has expected life of 40000 hours of operation. With an

estimated backup usage of less than 500 hours per year, this equates to 80 years of service.

The fuel cell stack has an estimated life of between 5000 and 10000 hours. With the same back

up hours per year, that equates to between 10 and 20 years of service. Both the fuel cell and

electrolysis cell stacks normally operate at temperature between 120 and 180oF.



Batteries RFC’s

Life cost 200kwh

system

$120,000 $20,000

Incremental

additional

storage cost

$150 – 300/Kwh $30/Kwh

Life 5-8 years Systems:20 Yrs

with maintenance

Maintenance

required

Complete battery

replacement after

calendar life

reached

Cell stack only

refurbished after

60,000 hours

Environmental

operating hazard

Batteries need

indoor storage,

Acid present

H2 stored outside

system can be

either indoor or

outdoor

Disposal hazard Lead, Acid issues. Discharged

material has no

harmful materials

Table 5.4: Comparison between Batteries and RFC’s

5.2.7 Energy Storage Technology:

Energy storage devices such as batteries are necessary to power the payloads and

telemetry systems. The solar panels provide varying power but the onboard non-propulsive

systems require a steady and dependable power source. Batteries can act as a power filter

when charged by the solar panels. Numerous options for high capacity, or high specific energy,

non-regenerative batteries and fuel cells exist



Table 5.5: Specific energy and power of different batteries

5.2.8 Developer/manufacturer:

Uni-solar: The press release from Unisolar is given below to indicate on their engagement with

Lockheed Martin to develop solar technologies for HAA.

March 17, 2004,

Lockheed Martin gave a contract to develop and deliver solar cells on polymer substrates.

These solar cells will be used by Lockheed Martin in Phase 2 of the High Altitude Airship (HAA)

program, awarded by the Missile Defense Agency in September 2003.

Phase 2 includes developing an airship that can sustain operations for one month at 65,000 feet

while providing 10 kilowatts of power to a 4,000-pound payload [63]

May 15, 2007,

Uni-Solar announced that Air Force Research Laboratory (AFRL) in Kirtland AFB, New Mexico

exercised an 18-month contractual option for $9.1 million to develop new solar cell technology

to be used in space and airship vehicles addressing defense and homeland security

applications.

Uni-solar triple-junction modules, originally developed for terrestrial applications, are made of

amorphous silicon-based thin-film alloys, which are deposited on a 5-mil flexible stainless steel

substrate. By utilizing a polymeric substrate, space cells have already been developed that have

a specific power greater than 1000 Watts per kilogram (W/kg), which is significantly higher than

what is currently available. A high specific power is required for airship application. The radiation



hardness and superior high-temperature performance of amorphous silicon make it an attractive

material for space application [64]

5.2.9 Power estimations from other agencies:

DARPA:

Current battery technology (specific energy density of 0.5 kW-hr/kg)

Seeking energy storage technologies with specific energy density at least 2.0 kW-hr/kg.

A power draw of 100 kW may be considered typical.

Achieved:

Areal density- 90.6 g/m2

Fiber strength-to-weight ≥ 1000 KN·m/kg 1274 kN-m/kg

Specific energy density 779 W-hr/kg

Table 5.6: HALE-D performance parameters and characters.

5.2.10 Global status:

a. Japan (JAXA):

Technical review that was held during 2001 set some goals for development of mono crystal Si-

Solar cells with specific mass 3 g/W (including heat insulator) with 13% efficiency or more and

RFC specific energy density of 450 W.h/kg.

In feasibility study work, they discovered that an SPF airship of 245m long would require total

propulsion power of 181 kW for the design speed of 30 m/s at 20 km.

The manufacturing-test model has a sandwiched structure 1,035mm wide and 650mm high,

composed of a thermoplastic protection film, 256 cells of 60µm thick, uses silicone adhesive,

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5.2 POWER MANAGEMENT · capacity expansion and lowers the cost per watt of products. ... based on copper indium gallium di-selenide (CIGS ) have shown the highest laboratory electrical