Power Generation
Contents
Solar Power
Most common source of power in satellites is solar. Solar panels use sunlight to generate electricity required to power the satellite. Photovoltaic modules use light energy (photons) from the Sun to generate electricity through the photovoltaic effect. The majority of modules use wafer-based crystalline silicon cells or thin-film cells. Recent advancements in solar cell technology make it possible to harness Sun’s energy with considerable efficiency (around 25%). Newer cells based on Gallium Arsenide (GaAs) have higher efficiencies as compared to Silicon (Si) cells and also degrade slower in the harsh space environment.
Selection of Solar Cells
- Solar panels need to have a sufficient surface area that can be pointed towards the Sun as the spacecraft moves to get enough power.
- Also the efficiency of panels need to be considered i.e. how much power is generated per square unit of area when it is sunlit
- The materials used to make the solar panels should be such that it can sustain in space where temperature and other conditions can change rapidly.
- We need also to determine the arrangement of PV modules (how many in series and in parallel). This is determined by voltage and current requirements of components.
Testing
Efficiency of power generated by solar panels is dependent on temperature, intensity and angle at which rays strikes the panels. [1] It is useful to know about these dependencies. Sometimes these can be obtained from datasheets of product. But even in that case testing can be done to verify if the given relations are accurate or not. These tests can be done using a light source such as halogen lamp with the optical filters to increase color temperature close to the value of sunlight above the atmosphere. Light source should have continuous spectrum without strong parasite peaks and also it should not be expensive. The light from halogen lamp is projected on solar arrays in desired condition (temperature of surroundings, intensity of light coming from lamp, angle halogen lamp makes with plane of solar panel) and current and voltage across are measured and power generated is calculated.
Safety Measurements for Protection of Solar Arrays
If a voltage higher than the open circuit voltage is imposed on the cell’s terminal, a negative current will appear. In this case, the cell becomes a consumer. This phenomenon can be avoided using a diode. This diode is placed on each parallel branch of the solar network. The voltage drop across the diode must be as low as possible.[2]
Simulations
It is necessary to find out how much power is being generated in an orbit to know whether that power be enough to sustain the satellite. This can be done by doing simulations[3]. The satellite receives light from three sources, direct solar radiation, sunlight reflected from Earth and Earth's thermal radiation. Direct solar radiation is the major component of these. At each point of orbit the intensity of solar light falling on panels is obtained separately for each one of the panels and so the power generated at each moment from each panel is calculated. A graph can be plotted and how much power is generated on an average by a panel can be calculated.
Fuel Cells
They are usually used where environmental conditions don’t allow solar panels to be used (either not enough sunlight or temperature beyond the range of operating temperature of solar panels).
A fuel cell combines a fuel (hydrogen or hydrogen source) with an oxidizer (oxygen or air) to produce electrical power. Like a battery, a fuel cell has two electrodes (a cathode and an anode) that are separated by an electrolyte. However, unlike batteries, the electrode is not consumed in a fuel cell, and the cell can produce electricity as long as more fuel and oxidizer are pumped through it.
At present fuel cells can not be used on cubesats but research is going on this topic.
Nuclear Power
Though solar power is mostly used for powering the satellite, but there are many satellites that use nuclear power for their operation. [4] It is not a new concept but dates back to 1961. They are usually used for unmaintained situations that need a few hundred watts or less of power for durations too long for fuel cells, batteries, or generators to provide economically, and in places where solar cells are not practical. [5]
Heat is obtained from the decay of a radioactive particles. Small portion of heat energy is converted to electricity. Rest heat is rejected through radiators. The wastage of heat energy can be reduced by using a portion of rejected heat for thermal control of spacecraft subsystems.
At present nuclear power generation can not be used for cubesats.
Comparison of the Power Sources
| Solar Power | Fuel Cells | Nuclear Power | |
|---|---|---|---|
| Power Generated | Lowest | Higest | Medium |
| Complexity | Simplest | Medium | High |
| Cost | Lowest | Medium | Highest |
| Use on Cubesat | Possible | Not Possible (as of 2017 but research is going on for the same) | Not Possible |
| Life after not getting Sunlight [6] | Least | Medium | Highest (upto 10.75 years) |
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References
- ↑ https://www.hindawi.com/journals/ijp/2014/537645/#sec4
- ↑ http://www.goldstem.org/Swiss%20Cube/03%20-%20Power%20systems/S3-B-EPS-1-1-Electrical_Power_System.pdf
- ↑ Power CDR of Pratham
- ↑ https://www.lpi.usra.edu/opag/schmidtstaif06.pdf
- ↑ https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator
- ↑ https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-851-satellite-engineering-fall-2003/lecture-notes/l3_scpowersys_dm_done2.pdf
