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Better batteries boost drone duration.

Until recently, electric drones have been either lightweight battery-powered aircraft with an endurance of one hour or less, or long-span solar-powered devices that (with energy storage for night-time operation) can stay above the weather for days. Leaps in battery technology are now making possible quiet, efficient, small to medium electric drones with an endurance of several hours.

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Although still inferior to reciprocating engines in power/weight ratio, electric power emits lower noise and thermal signatures and avoids the logistical problems of battlefield fuel availability.

Driven by commercial demands in the cell phone, computer, automotive and satellite business sectors, major advances in battery performance are expected to continue. Fuel cells will soon provide a further option for unmanned aircraft.

Small military drones are like hobbyists' aero models, but new demands call for stealth, sensor and weapons carriage, data transmission, increased radius and endurance, (ideally) day/night adverse weather operations and a greater degree of autonomy to facilitate use by unskilled operators and at longer distances.

For hobbyists, the typical electrically powered model aircraft now has a lithium ion polymer (LiPo) battery and a brushless motor turning a composite propeller. A wide variety of brushless motors are available off-the-shelf from manufacturers such as America's E-flite, the Czech Republic's AXI, China's Hyperion and Israel's Bental.

Experience in Afghanistan and Iraq has underlined the fact that armed forces need the capability to operate in high ambient temperature areas. The Nevada-based Lew Aerospace, testing its 9.1-kilo Inventus E reconnaissance drone, reports measuring airframe temperatures of over 70[degrees]C. Such conditions can easily cause electric motors to fail.

ThinGap's motors, in which the iron core is eliminated and the coil is wound on an aluminium structure, conducting heat directly to the mounting, have proved capable of reliable operation despite these high temperatures.

Batteries

A battery consists of two or more cells, which may be linked in series (to provide multiples of cell voltage) and/or in parallel (to provide more ampere hours at the resulting voltage). For example, a Flight-Power Evo-7400 LiPo battery with a '6s2p' configuration has two parallel sets of six cells in series. It weighs 1.117 kg, and provides a capacity of 7.40 Ah. Since a LiPo cell has a nominal voltage of 3.7 volts, that of this battery is 22.2 volts.

Nickel-metal hydride (NiMH) and nickel-cadmium (NiCd) cells have a nominal voltage of 1.2 volts, thus requiring three in series to give the voltage of one LiPo.

Most batteries used in advanced applications employ secondary (rechargeable) cells, although primary (disposable) cells offer some performance advantage. The AeroVironment Raven can achieve 60 to 90 min flights with rechargeable batteries, but 80 to 100 minutes with single-use units. The same company's Puma is reported to have an endurance of three hours with a rechargeable battery, but four hours with a disposable unit.

Extreme endurance basically depends on employing two (or more) batteries in parallel, and cells of high specific energy (Wh/kg). Longer flights are more easily achieved by larger drones, since a basic sensor load represents a lower percentage of take-off weight.

Endurance can be traded against payload. For example, the 19-kg version of the CyberBug being developed by the Florida-based Cyber Defense is designed to carry a 5.4-kg payload for up to two hours, but (if payload is exchanged for extra batteries) it is predicted to have a maximum endurance of around eight hours. Likewise, the 35-kg Elbit Skylark II has a maximum payload of 9.0 kg, which can be exchanged for extra batteries to give an endurance of up to six hours.

Specific Energy

Specific energy has recently increased dramatically, from the 60-Wh/kg of the NiCd battery to 80 for NiMH, 160 for Li-ion and 200 for today's LiPo.

The main emphasis for drone applications has been on reducing battery weight, thus making possible increases in payload and/or endurance. However, today's batteries are still not completely user-friendly.

Early NiCd batteries had to be fully discharged before recharging, otherwise they would not take a full charge. The NiMH batteries that followed can cause corrosion and may explode if abused.

Likewise, Li-ion batteries (with applications including the wing structure of the 210-gm AeroVironment Wasp) can be dangerous if mistreated. The nominal voltage for a Li-ion cell is 3.6 v, and (like the LiPo cell) it can be seriously damaged by discharging below 2.8 v. If the cathode should reach 100 deg C it will emit pure oxygen, producing a high risk of fire.

The LiPo battery appeared in the mid-1990s. The anode is carbon in which lithium ions are dissolved, while the cathode is lithium cobalt oxide or lithium manganese oxide. The electrolyte is a polymer film in which lithium is dissolved. The electrodes and electrolyte form three sheets, which are laminated together, thus avoiding the need for a metal casing.

The LiPo cell can be extremely thin, and is normally manufactured as a long sheet which is rolled or folded, then sealed in a soft plastic cover. If wired in parallel, these cells must be 'balanced' to ensure uniform voltages, so that none will drop below 2.8 v.

The solid polymer electrolyte of a LiPo is not flammable; hence such batteries are somewhat less hazardous than their Li-ion forbears. However, they need to be 'broken in' with a series of short discharges, and they need careful charging. If more than 4.235 v is applied, they can explode and catch fire, and the flames cannot be extinguished with water. Shorting the battery can also cause an explosion.

Over-discharging LiPo batteries, which would render them useless, is avoided by using a low-voltage cut-off (LVC), which prevents it falling below 3.0 volts.

The next stage of development is represented by solid-state lithium batteries, giving 300 to 400 Wh/kg. In late 2006, Qinetiq flew its Zephyr Hale (high altitude, long endurance) drone with a rechargeable Sion Power lithium-sulphur battery using cells rated at 350 Wh/kg. Whereas in earlier flights using LiPo cells the Zephyr had an endurance of 70 minutes, the new cells lasted more than two hours. When used in conjunction with solar arrays it is anticipated that in time lithium-sulphur batteries will sustain such aircraft for the nighttime portion of multi-day missions.

Despite these advances it is noteworthy that the specific energy of gasoline (20 kWh/kg) is around 50 times that of the best electric battery foreseen in the near-term.

Fuel Cells

The principle of the fuel cell was discovered by a German scientist (Christian Schonbein) in 1838, and the first such unit was produced by a Welsh scientist (Sir William Grove) in 1843. Interest then appears to have lapsed until 1959, when a 5-kW fuel cell was produced by British engineer Francis Thomas Bacon.

Although a single fuel cell typically produces a voltage of only 0.86 v, the concept has more potential than conventional batteries, and is particularly applicable to spacecraft equipped with liquid oxygen and hydrogen.

In a hydrogen/air fuel cell the metallic anode and cathode are normally separated by a proton exchange membrane (Pem). The anode is coated in a catalyst such as platinum, and fed with hydrogen. This breaks down into positively charged ions (protons), which pass through the Pem to the cathode, and negatively charged electrons, which can produce a current through an electrical load en route externally to the cathode. The cathode is supplied with oxygen (in the form of air), which combines with the hydrogen protons to produce hot water or steam.

The fuel cell is thus a very clean form of power. In the case of a fuel cell using a hydrocarbon fuel (such as diesel or methanol), the waste products are carbon dioxide and water.

To return to the history of fuel cells, in the 1960s Pratt & Whitney bought a licence to employ Bacon's patents in the US space programme. This led to UTC Power supplying fuel cells for the Apollo missions and the shuttle programme, and developing large (200 kW) ground units.

Ground warfare now involves many devices (such as radios, laser rangers, night vision goggles and hand-held computers) that require mobile electrical power, and this has led to army requirements for power sources that are lighter than conventional batteries.

Germany's SFC Smart Fuel Cell claims to be the market leader in such applications, with its own fuel cartridge supply infrastructure. It has already delivered its 'Jenny' direct methanol fuel cell (DMFC) units to eight defence forces. DuPont Fuel Cells aims to be the leading supplier of Pem membranes and components.

DuPont and SFC are collaborating on the US Army's M-25 Land Warrior Soldier Power Generation System (LW-SPGS), which will produce weight savings approaching 80%, relative to conventional power sources.

For drone applications, a fuel cell can take oxygen from the air. The problem is to find the best form or source of hydrogen. One approach is the DMFC with methanol in plastic cartridges, as proposed by Toshiba (and others) for cell phones and hand-held audio devices. It is anticipated that such products will be widely marketed during 2007.

Alternatively, metal hydrides such as sodium borohydride or lithium aluminium hydride in the form of powder or pellets will react with water to produce hydrogen.

The Massachusetts-based Protonex Technology, developing a hydride-based soldier power system fuel cell for US ground forces, indicates that 200 gm of sodium borohydride requires 860 gm of water. Interestingly, pure water is not essential, and--if necessary--urine can be substituted.

The first fuel cell-powered micro-drone was the 170-gm AeroVironment Hornet, which carried hydrogen pellets and water, and first flew in March 2003. It had fuel cells on top of the wing, taking oxygen from the ambient airflow.

In late 2005 the US Navy Research Laboratory flew the 2.54-kg Spider-Lion drone, fitted with a 95-Watt fuel cell jointly developed with Protonex Technology. This consumed only 15 gm of compressed hydrogen gas in a flight lasting 200 minutes.

Protonex was later awarded a contract by the Air Force Research Laboratory (AFRL) to develop fuel cell power systems using chemical hydrides for long-endurance micro drones.

The Protonex ProCore UAV system is a 700 gm fuel cell fed by a 1300 gm sodium borohydride cartridge that provides 770 Wh of energy. It can generate 50 to 200 watts and is expected to achieve a drone endurance of six to twelve hours.

However, Boeing Integrated Defense Systems has reportedly concluded that fuel cell propulsion is not suitable for large drones, and this view appears to be supported by Lynntech, another leader in fuel cell development. An AFRL spokesman has stated that the best near-term option for micro air vehicles is the combination of an advanced battery and a fuel cell.

Solar Power

The feasibility of solar-powered high-altitude drones has been demonstrated successfully, notably by the AeroVironment Pathfinder/Helios series, which aimed to develop a regenerative power system combining solar and fuel cells for day/night operation. The gaseous hydrogen fuel cells of Helios had a specific energy of 500 Wh/kg. In daytime excess solar energy powered a Pem electrolyser, splitting water into hydrogen and oxygen, to be stored in high-pressure tanks. At night these gases were fed to a Pem fuel cell, providing electrical power.

In May 2005 AeroVironment flew a sub-scale Global Observer, the first drone to have a fuel cell using liquid hydrogen. The company is now seeking funds to build the full-scale product, the 1800-kg GO-1 and 4100-kg GO-2, offering endurances of seven or eight days. The Global Observer is being proposed in competition with the General Atomics MQ-9 Predator-B and Northrop Grumman RQ-4 Global Hawk in the context of high-altitude, long endurance surveillance missions.
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Title Annotation:Drones: propulsion
Author:Braybrook, Roy
Publication:Armada International
Date:Apr 1, 2007
Words:1945
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