CECS Spotlight: Solar energy for soldier mobility

With frontline soldiers becoming ever more reliant on electronic aids, the issue of battery power is as crucial as firepower for mission effectiveness, and technology to capture the sun's energy will soon provide vital support.

Currently, soldiers are dependant on battery power provided by a wide range of conventional battery types. Each has different endurance and reliability levels, and each rechargeable type requires its own kit, compounding the bulk and weight needing to be carried. According to recent studies done on batter usage, 20 kg of spent lithium batteries are typically discarded by a single soldier during a five-week deployment, and up to 88 AA-cell primary batteries may be consumed on a five-day mission.

While battery technology research has delivered considerable improvements on this situation, the goal of a small, lightweight power storage system, capable of sustaining all electronic equipment for as along as the soldier can stay in the field, is seen to be still a long way off.

This being so, attention has turned towards ways of harvesting ambient energy using devices such as vibration transducers, thermoelectric converters, radio frequency collectors and photovoltaic cells. Of these, the solar option is currently seen to be the most viable and the most likely to provide a field power generation capability soonest.

Investigating the solar option

Work on developing person-portable solar power generation technology for use by the ADF is being undertaken by DSTO through a Capability and Technology Demonstrator (CTD) Program project with the Australian National University (ANU).

"A photovoltaic cell developed by ANU, referred to as monocrystalline elongate cells, is seen to hold the most promise for creating a system that meets ADF requirements," explains DSTO researcher Dr Vinod Puri.

Traditionally, silicon cells are made from slices cut from silicon ingots, forming a rigid brittle wafer about 150 mm in diameter and 0.2 mm thick. Under sunlight, each cell producing 0.5 volts, with output current proportional to cell area.

Elongate cells are thin silicon strips, typically 50 to 100 mm long, 2 to 3 mm wide, and only about 50 microns (thousands of a mm) thick - similar to the thickness of a sheet of paper or a human hair.

One form of elongate cell, called 'planks' is made using chemical etching and laser cutting techniques to divide a wafer of 10 to 20 microns thickness into hundreds of 2 mm-wide strips. Being fabricated at present in small numbers in ANU's laboratory, they are seen to perform well in terms of energy conversion efficiency as well as flexibility.

A similar type of elongate cell, called 'sliver cells', is formed by finely slicing through wafers of 2 mm thickness with a laser to produce thousands of strips just tens of microns thin, using the cut edge as the solar collection surface. This form of cell was also developed at the ANU, and is currently being commercialised by Australian company, Origin Energy.

These modes production ensure that much greater use is made of a given volume of silicon - an expensive material due to the extremely high purity levels required - thus reducing the cost of producing solar panels, the original goal of ANU's work.

Outshining its photovoltaic rivals

A further key attribute of ANU's approach is the use of monocrystalline silicon, considered the 'gold standard' material of choice for photovoltaic systems. This form of silicon offers high and reliable performance, with proven efficiency in converting sunlight into electricity of greater than 20% over the course of 20 to 30 year panel lifetimes.

It also delivers better performance than 'thin-film' cells of non-silicon material, which are currently finding use in flexible panels.

Made by depositing a layer of photovoltaic material onto a substrate surface of polymer or metal, think-film cells perform less well at converting sunlight into electrical energy, and are less durable. For some applications, however, these downsides are offset by lower costs of production and the flexible lightweight nature of panels that can be made this way.

For frontline military use where minimising the weight and bulk of gear carried is the overriding consideration, monocrystalline silicon elongate cell technology stands out as a preferred option because of its capability to generate the most amount of power per kilogram of system weight.

A system good for bright sun and shadowy places

The ANU CTD is investigating the possibility of using efficient elongate silicon solar cells to obtain power-to-weight ratios of greater than 150 watts per kilogram (W/kg) - a performance level more than five times that of currently available systems, and about three times that of state-of-the-art thin-film systems.

The reason such a high target has been set is that, unlike domestic and industrial uses of the technology where panels can be fixed in place to maximise sunlight capture, soldiers on a mission may not be able to avoid shading due to the need for concealment, and sunny days might not occur during the mission anyway.

Hence, a frontline military-use system must be capable of generating power under full light as well as in the low-light conditions of clouded skies and shade.

Also, unlike domestic and industrial uses, a frontline military-use system must comprise a much smaller area for portability purposes, meaning that high conversion efficiency is of great importance.

The ANU CTD is developing modules capable of generating battery charging voltage levels from a very small unit area - about one square centimetre. The overall system is made from many unit areas wired in parallel. This setup enables the system to continue supplying charge to the batteries under conditions of partial shade, unlike conventional panels whose outputs drop to nearly zero when just one or two cells fall into shade.

A roll-up mobile power source

Being thin, elongate cells are not only very lightweight but also very flexible, to the degree that hey can literally be wrapped around a finger. Initial work by ANU has shown high tolerance of these cells to flexing, unaffected by flexing to a curvature radius of just two to three centimetres even after 100,000 cycles.

The university is developing a manufacturing process for fabricating micro modules of elongate cells using flexible transparent protective packaging. These modules are expected to have a bending radius of five centimetres, possibly less.

Because most of the weight of an elongate micro module is its transparent protective packaging, and the cells are highly efficient, very high power-to-weight ratios of hundreds of watts per kilogram are possible.

Even higher power-to-weight ratios are potentially achievable for short-term applications - above one kilowatt per kilogram - through the use of thinner albeit less robust, packaging. This option is made possible by the fact that silicon is a durable material, and can survive outdoors for long periods with minimal protective packaging.

"A kilowatt-rated module for short-term use could feasibly be packed within the volume of a wine cast," says Puri.

The researchers anticipate that this form of technology could be available to the ADF within two to five years time.

This article is reproduced with kind permission of DSTO. It first appeared in 'Defence Science Australia' Volume 1, Issue 1, Mar 2010. Photo: Elizabeth Thomsen, Research Fellow, Centre for Sustainable Energy Systems, ANU: Courtesy DSTO.