Environmental and Design Considerations
Over the past three decades, solar cell optimization has greatly increased the efficiency and reduced the price of photovoltaic arrays for use here on earth. Through the use of clever material processing techniques and multijunction construction, manufacturers have been able to increase the raw efficiency of a solar cell to over 43%. Figure 1 shows how research has increased the performance of many solar cell technologies.
Figure 1. Solar Cell Efficiencies
However, certain technologies are not well suited to the environment and challenges of space. Some of the significant considerations for operating a solar cell in space are:
- Insolation is ~1400W/m2 (AM0) in space, whereas insolation is ~1000W/m2 (AM1.5) on earth.
- Spectral distributions of the incident solar energy are ‘blue shifted’ in space.
- There is no wind, rain, salt, hail, trees, birds, gravity, etc. in space.
- There are charged particles and other forms of radiation in space.
- Price of the panel is less important for space cells than for earth cells.
- Mass and volume are severely limited during launch to space.
The two most valuable metrics for determining a solar cell technology for use in space are the specific power (amount of power which can be produced per unit mass), and the volume of the stored array during launch. Based on these requirements, it is clear that crystalline forms of photovoltaics, although cheap, common, and well-understood, are a poor choice for use in space. These types of cells have traded efficiency for mass, which gives only incremental gains in the specific power and increases the volume needed during launch.
Thin Film Solar Cells
The deployed PV array for the SSP satellite should be large, thin areas filled with cells of low cost, good stability, and high specific power. While the ~20% cell efficiency of thin film technology lags the crystalline and multijunction efficiencies, the thin film materials are approximately 100 times thinner than their crystalline counterparts.
Two approaches to thin film arrays are most prevalent in today’s industry: amorphous silicon (a-Si:H) and polycrystalline Cu(Ga,In)Se2 (CIGS). Figure 2 shows a layered view of a CIGS thin film solar cell. These materials are deposited on substrates and processed into solar cells, yielding ~5µm thick structures which have excellent radiation tolerance with minimal or no protective coverings. With proper materials use and cell optimization, laboratories have built thin film arrays that can offer impressive specific powers of 16.8 kW/kg.
Figure 2. Cigs Cell, Layered View.
These thin film arrays can be stowed in a folded or rolled configuration during launch (Figure 3A,B), and then deployed in space using extensions of methods already proven for smaller satellites. This feature vastly reduces launch costs for delivering the solar array into space.
Figure 3a. Folded Panel Deployment
Figure 3b. Rolled Panel Deployment
Array Sizing and Orientation
The array sizing must account for the desired amount of power to be transmitted to earth, practically achievable physical dimensions, manufacturing capabilities, and possibility of future performance gains. The solar arrays for the SSP satellites will have an operational unit size of 5GW, and each unit will require 9km2 of collection area in space. As solar technologies continue to advance, more efficient or lighter designs will be installed on later satellites which will create significant launch cost savings.
In contrast to terrestrial solar power, arrays in space receive approximately constant solar irradiation. In order to take advantage of this condition, the SSP solar array must be oriented such that the incident sunlight is normal to the cells at all times. The spacecraft will be built such that the solar panels can be independently pivoted, allowing the panels to always face the sun while the microwave beam can be directed at a rectenna on earth. The movement of the panels will be guided by sun sensors and actuated by small thrusters to achieve optimal orientation. Additionally, satellite operators could purposefully orient the panels out of direct alignment with the sun if necessary for load balancing on earth.
Resources
http://www.nrel.gov/pv/thinfilm.html
http://www2.jpl.nasa.gov/basics/bsf11-3.php
http://www.orbital-power.com/home/thin-film-solar-cells/
http://www.propagation.gatech.edu/ECE6390/project/Fall2011/SatCom_Project_2011.pdf
Shimazaki, K.; Kobayashi, Y.; Takahashi, M.; Imaizumi, M.; Takamoto, T.; Ito, T.; Nozaki, Y.; , “Progress in development of ultra-lightweight solar panel using space solar sheet,” Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE , vol., no., pp.000725-000730, 20-25 June 2010
