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Power System

We are designated to design the power systems of Lunar Positioning System for astronauts on moon walks. We use solar panels to provide sufficient power for lander working in the daytime as well as charging the batteries for nighttime use. For astronauts, we equip the appropriate rechargeable batteries on them to support all their work when doing moon walks. These are the basic ideas about our power systems.

Power Source of Astronaut

There are various kinds of batteries on the market. Because we need a battery which is suitable for the lunar environment, we focus on finding the battery which has following properties: high power-to-weight ratio, rechargeable, large operating temperature range, and low cost.

Based on the properties above, we find that Lithium batteries are the best for us. Lithium batteries have several advantages such as higher energy density, higher open circuit voltage, low self-discharge rate, no memory effect, and so on. Though they are not so stable compare to traditional batteries, i.e. they might explode because of misuse of batteries, lots of protection circuits or other techniques have been proposed to prevent the occurrence of accidents. They are used widely around the world today. These are the reasons why we chose this kind of battery.

Among several types of Lithium based batteries, we chose to use Polymer Li-ion battery because of its higher energy density, popularity, and lower cost. Below are some specifications we care about the most. The detailed specification can be found in the following links.

Specifications:

Figure 1: The Battery

Power consumption Analysis

We divide the power consumption of astronaut into three parts, one is for the link from astronaut to lander, another is for the link between astronaut and RF tags, and the other is for astronaut’s equipments.

Our consideration about how much power we can use without harming human body is based on the criterion applied in cellular phone. They defined a measurement named “Specific absorption rate (SAR)”. It’s a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency (RF) electromagnetic field. It is defined as the power absorbed per mass of tissue and has units of watts per kilogram. Up to 2 W/Kg of tissue is the constraint for cellular phone. We’ll take this criterion into consideration when we calculating power related stuff.

Based on our calculation of communication links, the first part is around 0.25 W. The second part is around 10 W. And the third part is about 10W. So the peak power output we may use is 20.25W.

Minimum Maximum Estimated Lifetime

We know the specifications of our chosen battery and we also know the peak power usage of our systems, so we can calculate the minimum maximum estimated lifetime as follows. (Since we use the peak power to calculate this, so we only can get the smallest maximum estimated lifetime.)

The battery we've chosen can produce 15.725Wh per cell. We can use total 10 cells to support astronauts up to 7.8 hours. And it only costs 0.77Kg weight in the earth gravitational field.

References:

[1] http://www.powerstream.com/
[2] http://www.powerstream.com/LLL-low-temp.htm
[3] http://www.powerstream.com/LLL-polymer.htm
[4] http://www.gmbattery.com/English/index.htm
[5] http://www.gmbattery.com/English/LiPo_Battery.html
[6] http://en.wikipedia.org/wiki/Lithium-ion_battery

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