Electrical energy does not exist naturally in any convenient form and it must be converted from some other energy form when needed. Chemical energy is the most practical source and is generally used in one of two ways. Fuel can be burnt in a heat engine, such as a petrol or diesel engine, or a gas turbine, which then drives an electrical generator. This process is inherently inefficient. Alternatively, the fuel may be consumed in an electrolytic reaction in a battery or fuel cell.
For low power (less than 50W) and short duration missions, batteries are the logical choice. Engine generators and fuel cells are the preferred choices for applications greater than 500W, as illustrated here. The following sections highlight the salient points of each technology.
Diesel (or gasoline) generators produce electricity in a multi-step process. The energy in the fuel is first converted into rotary mechanical energy in the engine. Finally, the rotary mechanical energy is converted to DC or AC electrical energy by a generator.
As this is a multi-step energy conversion process, involving a heat engine with moving parts and, thus, frictional losses, it is an intrinsically inefficient method for producing electricity. Diesel generators typically have efficiencies in the range of 20% to 30%. Gasoline generators usually exhibit efficiencies in the 10% to 15% range.
Diesel and gasoline generators are also characterized by a high operating temperature, noise, air pollution, and their requirement for regular maintenance. On the other hand, diesel and gasoline generators are inexpensive and readily available. They are also easily, even continuously, refuellable. The energy available from the generator is limited only by its supply of fuel.
A battery is an electrochemical energy conversion device. It converts the energy in the fuel (active material of the electrodes) directly into DC electricity. There is no combustion, no multi-step energy conversion, and no frictional loss. The battery is intrinsically efficient, silent and non-polluting. In most embodiments, the battery is also a low temperature device that produces power immediately upon demand.
However, batteries suffer from the limitation that all the available fuel is contained within the battery case. When a non-rechargeable (primary) battery has consumed its fuel, it is discarded and, with it, the energy conversion device and the "fuel tank". In a rechargeable (secondary) battery, one can reuse the energy conversion device and the "fuel tank", but one must wait for several hours for the recharging process to be completed. One cannot operate a battery continuously, as one can operate the diesel generator.
The fuel cell is a device that converts a fuel and an oxidant directly into electricity by an electrochemical process. The fuel is not burned to produce heat and the efficiency of energy conversion is not limited by the Carnot Cycle limits placed upon heat engines. The fuel cell, like a battery, has no moving parts in the energy conversion device, and thus suffers no frictional losses.
Realistic energy efficiencies in excess of 50% can be achieved with fuel cell systems in generator set applications. Fuel cells combine the advantages of both diesel generators and batteries, while eliminating the major drawbacks of both. A fuel cell is essentially the electrochemical energy conversion device for the battery, engineered in such a way that it is continuously refuelable, like the diesel generator.
A fuel cell is intrinsically energy efficient, non-polluting, silent, and reliable. In some embodiments, it is a low temperature device that provides power instantly upon demand, and exhibits a long operating life with minimal maintenance.
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