How to Colonize an Asteroid

Basic Power Systems

Simple PV system block diagram. The combination of a power source, storage batteries, distribution wires, and controls together make up a power system. One of the more simple examples of a power system would be the radio subsystem in the shed, and the next step up the ladder would be the shed's primary power system.

Let's take a system similar to the shed's radio subsystem and look at all of the individual parts. This hypothetical system is based on solar photovoltaic cells, which convert the light from the sun into electricity (electrons flowing in the wires).

The power goes into a charge controller which regulates the flow of power to the batteries so that they do not get overcharged, and controls power distribution to the rest of the system. Another function of the charge regulator is to block the batteries from discharging through the PV panels when it gets dark.

The batteries store the electricity until it is needed for use, and supply power to the rest of the system for use at night when the sun is not shining.

The charge regulator then regulates the power coming from the batteries and sends it to the distribution system to ensure an even flow of power. The distribution system consists of wires, light sockets, and connector panels which help to distribute the power to the different places where it is needed.

Each of the individual subsystems mentioned above will also generate waste heat. On earth this is not too much of a problem as we have air in the atmosphere to transport the heat away, but in space we need to add thermal radiators to prevent things from getting too hot.

When we design a power system such as this we need to start at the opposite end - what do we need the power for and how much do we need to supply with our system?

As an example, let's say that we want to operate 2 dome lights from a car (10 watts each) and a radio which draws .2 amps when operating, and we want 3 hours of light per day, and 5 hours of radio listening per day.

The first step is to calculate the daily total power draw in amp hours. The radio is simple - .2 amps times 5 hours equals 1 aH (amp-hours) per day. For the lights, we first need to use the power formula to convert watts to amps. If the primary supply voltage is 12 volts dc, then one light would draw 10 watts divided by 12 volts equals .83 amps.. We then multiply this by the 3 hours per day we want them to be on and we come up with about 2.5 aH per light per day. Multiply this by 2 lights, and we get 5 aH for lights and 1 aH for the radio, for a total draw of 6 amp-hours per day. Add about 30 percent to this to make up for thermal loss and other inefficiencies and we find that we need to generate at least 7.8 aH per day.

In addition to the actual current draw required by our lights and radio, we should have an excess capacity equal to a 24 hour period without power in the event that it gets dark or we need to make repairs to the solar cells, so the capacity of our battery should be at least 15.6 amp hours.

The additional battery capacity also means that the solar cells need to generate more power in a day than we will consume to keep the battery charged with a reserve capacity. Let's go with a conservative 10% extra power generation. This means that the minimum generation for our solar panels should be 7.8 aH plus 10% (.78aH) for a total generating need of about 8.6 aH per day. In space we have sunlight 24 hours a day, so the panels need to produce 8.6 aH divided by 24 hours equals .36 amps continuous, or 360 milliamps, at 12 volts - this is no biggie - you can purchase an off the shelf solar cell with this capacity for about $50.00 (US - in fall 2001). If we want to use the same system here on earth, we have to take into account the fact that the sun does not shine 24 hours a day. In my area, the yearly average is only 2.3 hours of full sun per day. To calculate the size of the solar panels needed we have to divide the 8.6 aH by 2.3 hours (instead of the 24 hours per day in space) for a total of about 3.74 amps continuous. We can also purchase off the shelf components for this system, but we are looking at around $460.00 just for the solar cells. Granted, neither of the cells specified here is qualified for use in space, but we can see that the spaced based system can be much smaller that that which we would need here on earth.