Electric thrusters in general are about 1.5 times as efficient as a good chemical propulsion system. The only problem is that the electric thruster can only produce a small amount of force, but it can run for long periods of time. A parameter called an impulse comes into play here. An impulse is a force multiplied by the time applied. A chemical engine produces large thrust over short time and electric engine produces small thrust over large amounts of time. Essentially the impulse created by the electric engine can equal or exceed that of a chemical engine. The electric engine can get you where you need to go, but it will take longer to get there.
So why use an electric thruster? If the mission is not time sensitive an electric propulsion system will use less fuel, and therefore cost less to launch into space. On average it currently costs about $10,000 for every kilogram launched into low Earth orbit. Therefore if time is not a crucial issue the delay can be worth the money saved at launch.
The idea of using electricity instead of combustion for a propulsion system is a relatively young field. There are three main types of electric spacecraft propulsion that have been tested: electrostatic, electromagnetic, and electrothermal.
The electrostatic category consists of engines such as the ion engine that flew aboard Deep Space 1 and the hollow cathode thruster used in this project. The propulsion system works by creating ions out of a stable gas such as helium, neon, xenon, etc. The heavier the gas the more thrust you can achieve. The most common way to generate the ions is through electron bombardment. The electron impacts with the neutral particle, which knocks off an electron from the neutral particle creating an ion and giving it a +1 charge. Once the gas is ionized it is attracted to the electric field of the engine. The accelerated ion then pass through a grid and expelled creating the thrust. The ions are moving much faster than particles resulting from combustion so you get more thrust per kilogram.
The electromagnetic category consists of engines such as the Hall effect thruster. This type of propulsion system is the same as an electrostatic thruster with the addition of a magnetic field. The magnetic field condenses and directs the plasma flow to give a larger amount of thrust.
The electrothermal propulsion systems are the most similar to a chemical propulsion system. The electrothermal engine relies on high temperatures, high pressures, and flow through a nozzle to create the thrust. The only difference between the chemical and the electrothermal systems is that in chemical propulsion the heat is created from combustion and in electrothermal propulsion the heat is created electrically. The electrothermal engine is capable of reaching higher temperatures than a chemical engine and therefore more thrust per kilogram of fuel. The gas is not ejected as fast in the electrothermal engine as it is in the electrostatic or electromagnetic engines so it has less thrust per kilogram of fuel than those two.
Find out about chemical spacecraft propulsion systems...