Introducing NORB's first amateur satellite!
Space is something that has always really fascinated me. There's so much out there that we yet just can't put our finger on, and so much to learn. After spending almost two years in the HAB world, gradually accumulating vast quantities of knowledge along the way, I'm now in a position where I believe I have the potential to design and build my very own amateur satellite!
Initial features of the satellite:
- SSDV and telemetry downlink using AX25 packet structure and 1200 baud AFSK
- CW Transponder
- JPEG Serial Camera
- Ability to stay charged for full lifetime in orbit from solar and possibly thermoelectric sources
- Possibility of an on board packet repeater to be used in the UKHASnet network.
Firstly, the power system:
One of the biggest challenges I will face along the journey of this project is the power system for the satellite. When deployed in LEO (low earth orbit), the satellite will have an orbital period of approximately 90 minutes, providing a duration of around 45 minutes in sunlight, and 45 minutes in darkness. The question is, how do you get an amateur satellite to sustain an acceptable lifetime in LEO using only the energy gained from sunlight? And the answer, if you want to do it properly, is not such an easy thing to find. Therefore, the first challege of the first challenge is to meet the following objective:
- Establish a PV cell configuration that maximizes the power generated to charge a specially-suited battery during the satellite's orbital lifetime.
In order to meet this objective, many things will have to be considered along the way:
- Basic operation of a PV cell
- The diode equation
- Effect of series and shunt resistance
- Open circuit voltage
- Short circuit current
- Solar spectrum (outer space)
- Effect of temperature
- AR coating and light trapping techniques
- Effect of solar cell thickness
- Formation of a superposed IV characteristic
- Fill factor
- Thin film, amorphous and polycrystalline materials
Basic operation of a PV cell
The role of a solar cell is of course to convert sunlight into electricity, and this is all possible due to the existence of semiconductor theory. Without getting too lost into this, the basic principle is that you have a valence band and a conduction band; the gap between them known simply as the "band gap." In order for an electron to be transmitted across the band gap, the electron must be provided with a certain energy from the photon. If the energy of an absorbed photon is greater than or equal to the band gap, and electron is transmitted into the conduction band. This type of setup is known as a "P-N junction."
The current through a diode can be expressed as a function of voltage through the diode law as:
- I = the current in the diode
- I0 = the dark saturation current
- V = the voltage across the diode
- q = the absolute value of electron charge
- k = Boltzmann's constant
- T = absolute temperature in Kelvin
- n = ideality factor.
It must be noted that when an electron or hole recombine within the cell, that E-H pair no longer contributes to the current generation and therefore the power. Thus, the more efficient PV cells have less recombination than other cells, and this recombination factor is linked directly with I0, the dark saturation current. In short, when looking for optimum performance of a solar system, cells with a lower dark saturation current have higher efficiency.
Effect of temperature
The saturation current is also very closely related to the device temperature. A device operating at a higher temperature will reduce the band gap of the junction. This effects the open circuit voltage more so than the short circuit current in the following way:
With increasing temperature, the short circuit current typically increases slightly, while the open circuit voltage decreases further.
More on this to come, but gosh is this a big topic!