For the typical space mission there are four different power sources available for use: batteries, fuel cells, solar power, and Radio Isotopic generators. The figure below demonstrates which source would be appropriate for different lifetime and burst power requirements [2].
For the Google Lunar X-prize mission there are two distinct power supplies needed: for the rover, and for the moon base station. Although the table indicates that a combination of fuel cells with the primary battery may be enough to power the entire mission, a careful determination of power requirements must first be performed.
Some basic information about each type of power supply is needed before considering the power requirements of the mission.
Solar Cells: Solar cells are commonly used to power satellites and other remote applications. Developing technology is continuing to increase the efficiency while decreasing the cost of photovoltaic cells and thus making solar cells more and more appealing, however there are still some serious limitations. To begin with, the amount of power collected by a solar cell is proportional to the angle of incidence of the sunlight. This relationship is seen in the equation below.
Since solar cells can rotate to follow the sun, this problem is mostly overcome although that movement will require additional power thus limiting power supplied to the other systems. A more important drawback may be that the amount of power collected by a solar cell is directly proportional to its area leading large fragile and expensive arrays for high powered applications. Nonetheless, solar cells can often provide the best option to long term power requirements.
Fuel Cells:
The key to fuel cells is the electrochemical "cell" that combines fuel and and oxidizing agent which then converts the chemical energy directly into electrical power[3]. For many applications, a stack of these cells will be used in order to create higher power. Unfortunately, for short term applications at moderate power levels, the mass of the fuel cell becomes significant and prevents the current technology from competing with batteries.
Since 1965 with the launch of the Gemini spacecraft, NASA has been using Fuel Cells on a number of missions. Two types of fuel cells are used: Proton Exchange Membrane Fuel Cells (PEMFC) and Alkaline Fuel Cell (AFC) technology. Used strictly for manned missions where much higher power levels and longer discharge times have been required, they have seen little or no use in unmanned science missions. Although there is current research into the use of PEMFC cells for science missions, development of “power dense” and “energy dense” technology must be advanced before fuel cells would be a viable solution for such missions.
Radioisotope Thermoelectric Generator:
A radioisotope thermoelectric generator converts the heat released by radioactive decay into electric energy. There are many benefits to an RTG system with the most important being that they are lightweight, compact, and when compared against solar arrays and batteries RTGs produce significantly more power per mass. Since the technology has been proven safe, the major disadvantage becomes the cost of the RTG which is considerably more than that of other power supply systems.
RTGs have been typically used for extremely long-term missions, often when the solar panels are no longer feasible. Missions where RTGs have been used include the following Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini and New Horizons. In addition, the two Viking landers used RTGs as did the scientific experiments left on the moon by the Apollo missions.
Batteries:
On recent missions including the Mars Exploration Rover , Lithium Ion batteries have been the battery of choice. With no memory effect, high power density, and a slow loss of charge, and the ability to recharge this type of battery is ideal for space missions. The table below shows specifications for various batteries and reveals the significance increase in the energy density of lithium batteries.
For missions short to mid term missions requiring low to moderate amounts of power, batteries offer the cheapest solution.
