Mar 25 2011
There is a recent growing interest in the replacement of batteries by fuel cells for portable consumer electronics, such as cellular phones, laptop computers, digital cameras and so on. This is perhaps partly due to the success of lithium-based batteries in powering laptop computers, cellular phones and the like. The requirement for higher energy density or longer operational time between recharges was generally well served by the Li-ion battery and high-energy density nickel-based batteries, especially those based on metal hydrides. Safety and environmental factors were key considerations in addition to the high energy density of these batteries.
There is now growing pressure on battery manufacturers to further increase energy density for the next generation of portable electronic equipment, which will require much higher energy densities, if the equipment is to be conveniently portable. This is not just because of marketing and product differentiation – it is a technological requirement for high-bandwidth and advanced microprocessing applications, which demand much more power. The situation becomes critical as cellphones and laptop computers merge to provide users with broadband wireless and multifunctional portable computing capability. Unfortunately, battery technology is unlikely to keep pace with these growing power demands and laptop equipment manufacturers are already being faced with introducing various power down options and low-power components to save battery energy. In the future, this may severely limit the practical capability of the planned broadband computing devices.
Projections for the size and growth rate of these new product markets are impressive are beginning to inspire researchers and developers of fuel cell to address the issues of scale-down of their technology, which has generally and for some time now been targeting automotive and distributed power applications.
Because we are approaching the energy density limits of rechargeable commercial battery technology, small fuel cells are being considered as possible solutions to the problems of providing portable power, at least at a theoretical level. Although the primary interest is the advantageous energy content of electrochemical fuels, such as hydrogen or methanol, in terms of weight and volume, there are many factors that will influence and possibly hinder the changeover to a radically different powering technology. Customer considerations – especially cost, safety and convenience – will inevitably become the primary commercial factors in the success or failure of any new power source. These considerations will be especially critical for the successful introduction of fuel cells as replacements for batteries.
From the point of view of cost, fuel cells for the new applications look very promising. Their competition – batteries – becomes relatively expensive as the required energy storage rises for the new broadband computing applications. For example, the cost of adding 100 Wh of energy storage using methanol can be less than 1% of the cost of adding this energy in the form of battery materials. In addition, the carrying weight and volume of the stored energy is advantageously reduced by a factor of ten (in theory at least). The practical problem, of course, is the energy converter for the energy-dense fuel, i.e. the fuel cell itself. Scale-down of what is after all an energy-converting system consisting of a fuel cell plus a fuel supply, rather than a “simple” energy storing electrode couple, presents a technological challenge.
Unlike the relatively simple battery, subsystems for fuel cells are involved in the supply of both fuel and air and the removal of product water, and carbon dioxide in the case of hydrocarbon fuels. Miniature electromechanical delivery and control systems will add complexity which may affect the reliability and operational lifetime of the entire fuel cell system. These concerns will be in addition to the usual list of electrochemical and materials degradation factors which reduce the longevity of batteries, but which are less troublesome in fuel cells. Fortunately, since the main competition is a battery power source, with comparatively low energy content, the commercially acceptable or “allowable” cost of a small fuel cell may exceed US$ 5 per watt, even in mass consumer applications. This may allow for the introduction of more sophisticated and reliable miniaturized engineering solutions.
Safety concerns impeded the early introduction of the now successful Li-ion battery and must again be a primary consideration in the introduction of fuel cells. The safety consequences of the requirement for pure hydrogen in hydrogen-based fuel cells (e.g. PEMFC) must be addressed at the most fundamental design level for consumer applications. For instance, limiting the amount of free hydrogen in any part of such a fuel cell system would be important in satisfying safety regulations.
Another class of ambient temperature and pressure fuel cell being considered for portable equipment, is the liquid-fed direct methanol fuel cell (DMFC). From the safety point of view, this system suffers from the perception of toxicity of the methanol, which will have to be addressed.
The third customer consideration -convenience – is a demonstrable advantage of using fuel cells, in that recharging is eliminated, saving time and additional carrying weight for the consumer.
Reasonable comparisons between batteries and fuel cells can be made if, and only if, the total energy content, including converter, fuel and fuel tank, of the energy conversion device is compared with an energy storage device, such as a battery pack, have an equal amount of stored energy.
In other words, the total energy produced by each system must be measured under the identical load conditions (i.e. power profile) to obtain an accurate comparison between the two.
Fueled systems, which are in various stage of development, can be used to replace batteries or supplement them as part of a hybrid system.
Batteries have shown a continuous, steady increase in energy density for the last 50 years, from about 30 to 350 Wh/kg. The possibilities for further improvement are good and need to be pursued aggressively, but the pace of improvements is likely to continue to be slow compared with other areas of technology development. The most attractive candidates that have been demonstrated at high readiness levels are PEM/H2 systems. With the advent of small, lightweight stacks (e.g. Protonex), the performance could improve even further. A PEM/H2 system can include a Li-Ion battery, which could be used for brief peak loads. This system has the highest specific energy for long duration missions (24 hours or more) when one considers the total energy delivered. If hydrogen is unaccettable, DMFC could be developed further and could outperform primary or rechargeable batteries. There are opportunities for significant mass reduction in both of these systems, which will make these comparisons even better.
Energy conversion systems based on fuel cells become more attractive at the 72-hr mission length, where there is a potential for reducing the total mass by a factor of 4 or 5. They are all significantly better than rechargeable lithium ion and, with development, will be much better than primary Li/MnO2.
It should be noted that these systems are more complex than batteries alone and require that attention be paid to such things as start-up, fuel and oxidant control, water management, shutdown and storage below freezing.
Let’s give an example, comparing the two above mentioned energy storage systems for an application in which a constant power of 300W is required for a 200-hr mission (e.g. portable electro-medical devices, remote telecommunications displaced in off-grid locations, electronics-based equipment of soldiers, nautical setup and so on).
Using a Li-Ion 18650 rechargeable battery system, many pieces of battery would be required, with a total cost and a total weight of 55200 € and 332 kg, respectively. Power density would be of 180 Wh/kg and unit price of about 0.92 €/Wh.
Instead, if we use a PEMFC-based system with pre-packaged hydrogen (e.g. GENPORT300 Hybrid Fuel Cell with GENFUEL cartridges), the overall cost would be of only 7240 € (with a consumption of 150 “solid hydrogen” cartridges) and the total weight of around 124 kg. Therefore, the power density of such energy conversion system would be of more than 500 Wh/kg, with a unit cost of 0.12 €/Wh.
In essence, energy conversion systems based on fuel cells are somewhat better than batteries for long missions, but are more complex to operate. However, mission length is easily extended by fuel addition and the longer the mission, the more competitive energy conversion systems based on fuel cells become (see the under graphs). An added benefit is the ability to continue a mission (provided enough fuel is left) even if resupply is not forthcoming in 24 hours.
Fuel cell systems are currently one fourth as massive as rechargeable batteries for longer missions (≥ 72 hours). From a logistic perspective, pre-packaged fuels can be treated as battery packs as long as appropriate safety and handling procedures are developed. Such pre-packaged fuels (for instance in the form of appropriate cartridges) will enhance the attractiveness of fueled energy conversion alternatives. For the 72-hr missions, energy conversion systems are an attractive alternative to batteries and could offer fivefold to tenfold mass reductions. Finally, fueled systems become more attractive as power demand increase.
The various challenges in developing small portable fuel cells – including their cost, safety and technological feasibility – are being carefully examined as a solution to the growing portable power needs of the mobile workforce, and the major industries that support them.
An Ion Lithium Battery Pack includes Cells and a Battery Management System (BMS)
Genfuel reacts with water and generates pure Hydrogen.
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