Solar Cells

Abstract

Various types of solar cells were studied and their feasibility is evaluated. Solar cell parameters and connections were studied.

Description

The various types of solar cells available are

 Monocrystalline Silicon
 Polycrystalline Silicon
 Thin Film
 Amorphous Silicon
 GaAs based

The parameters to be considered during the selection of solar cells are:

 Efficiency (Most important)
 Temperature coefficient
 Power Density
 Radiation resistance
 Weight
 Cost


Silicon type solar cells are inexpensive but are relatively inefficient compared to GaAs based Photovoltaic cells. Also, they have a relatively high-temperature coefficient and low resistance to radiation. As space would mean exposure to heavy radiation and extreme temperatures, Silicon-based PV cells are ruled out. The only feasible type of solar cells that can be used in space are GaAs based solar cells.
In space, high efficiency is the main requirement. So Triple-junction GaAs solar cells are used to capture a wide range of light spectrum. Commercially available triple junction GaAs cells have efficiency up to 24 to 28%. GaAs cells are also said to be space qualified.
Now that the cells are selected, the question is about how to connect them. Connecting all cells in series would give a high enough voltage but it suffers from “Weakest link”. If one cell goes down, the entire chain goes down. Connecting all cells in parallel would not have the weakest link problem but it suffers from low voltage. So something in middle seems to be optimum. Connecting a few cells in series and connecting a number of these series chains in parallel is feasible.
During the design of the power system, we need to find out how much power the solar cells can generate.
Taking a simple example of a CubeSat in low earth orbit, the CubeSat is illuminated on either zero, one, two or three sides simultaneously.

Survey - Pico and Nano-Satellites

About three-quarters of all pico-and nano satellites are equipped with solar cells. From 1997 this is even 85%, thus there are still some pico-and nanosatellites which run on batteries as electrical power supply only. Gallium Arsenide(GaAs)solar cells are used the most since they provide a very high conversion efficiency up to 30% and are widely available. Silicon solar cells are also used even in recent pico-and nanosatellites. Although they have lower efficiencies, the cost of such cells is very

low compared to the GaAs cells. About 16% of the satellites have deployable solar panels, all others which do have solar panel shave them body mounted. In this case, the size of the structure is limiting the area of the solar array significantly.

The average power available ranges from ten milliWatts to seven Watts. For 43 pico- and nanosatellites, the available bus power could be divided by the total mass of the satellite. What can be noticed is that the average specific power increases when the mass of the satellite becomes lower. This can be logically explained by the fact that the mass of the satellite is related to the volume (third power), while the effective area of body mounted solar cells is related to the area of the sides of the satellite (second power). One outlier can be noticed, which is StenSat. This is one of the smallest satellites but is very flat with a relatively large solar cell area combined with sun pointing.

The conversion method of raw available power from the solar cells to power on the spacecraft bus is Direct Energy Transfer (DET) or Peak Power Tracking (PPT) for most pico- and nanosatellites. The DET method takes the power at a predetermined voltage point on the current-voltage (IV) characteristic of the solar cells and shunts excessive power. This is a very simple and reliable method, but because the IV-curve shifts with temperature and degradation of the solar cells, the point should always be taken with a margin from the maximum power point. The PPT method just follows the IV-curve from the open-circuit voltage with DC-DC converters but can lead to problems if there is a too large instantaneous current surge. The maximum power point tracking (MPPT) is the most elegant method since it will retrieve the maximum power from the solar cells. Excessive power can be easily measured and either shunted or used advantageously. Due to the increased complexity of the MPPT method, only 7% of the pico- and nanosatellites are using this.

The satellites with non-rechargeable batteries used Mercury batteries in the early pico- and nano satellites and Lithium batteries for pico- and nanosatellites in the last decade. Most satellites with solar cells have rechargeable batteries of Lithium-ion or Lithium-polymer type, although some use Nickel–Cadmium or Lithium– Chloride batteries. Delfi-C³ is the only nanosatellite known which does not have any battery at all.