Satellites, space and solar panels
In this blog we will be asking the following questions: Solar in space. What panels are used? How are they used? How are they tested and what are some future applications for PV as a world power source.
Why is solar technology used in space?
- Solar technology can be a political football on the ground
- In space, it encounters little opposition
- The power alternatives in space are politically difficult
The options are radioisotope power systems, or RPS.
RPS converts heat generated by the natural decay of the radioactive isotope plutonium-238 into electricity.
Why is solar technology used in space?
Also Governments are not keen on radioactive elements falling to earth!
What kind of solar cells are used in space?
There are two types of solar cells used: silicon cells and multi-junction cells made of gallium arsenide and similar materials.
The International Space station uses:
- Silicon cells covered by thin glass to avoid degradation from radiation
- This makes up the 16 arrays up there
- They are the largest representation of solar in space, occupying enough area to cover most of an American football field
Gallium arsenide, the other solar
- Multi-junction cells made of gallium arsenide and similar materials resist degradation better than silicon
- They are the most efficient cells currently made
- Energy conversion efficiencies up to 34%
- “Junction” refers to the number of light-absorbing layers in the cell
- Common in space today are three-junction cells, but four and six are on the way
Why do solar cells degrade in space?
There are 4 sources of radiations: the Earth’s radiation belts (also called Van Allen belts), galactic cosmic rays (GCR), solar wind and solar flares
The Van Allen belts and the solar wind contain mostly protons and electrons, while GCR are in majority very high energy protons, alpha particles and heavier ions
Solar panels will experience efficiency degradation over time as a result of these types of radiation, but the degradation rate will depend strongly on the solar cell technology and on the location of the spacecraft.
Solar power satellite
To start with, the space system itself can be divided into two major parts. The first one is the satellites, these are usually earth-orbiting objects. Due to which the satellite receives nearly constant solar irradiation. The main purpose of the satellite is to give a bird’s eye view on the earth’s large portion for the collection of data from various land and ocean areas. besides, the other purpose is to transfer data for communication links. Additionally, to achieve the maximum working efficiency, most of the satellites rotate in the Low Earth Orbit (LEO) [180-2000km above the earth’s surface]. The second major part is the spacecraft, these are usually used for the deep space probe missions and interplanetary explorations. But for now, let’s just concentrate on the earth orbiting solar satellites.
For more such amazing content, do follow our LinkedIn page.
1] Solar array location:
a) Body mounted arrays: These arrays are directly mounted on the body of the satellite. This type of mounting is mostly used for small sized satellite such as CubeSat, SmallSat etc. due to space constraints. Also, the solar panels can be efficiently packaged along with the payload of the system.
b) Deployed arrays: This type of mounting is mostly used for large sized satellite with high power requirement. In this type the solar cells are mounted on wings which are deployed once the spacecraft reaches to its defined mission orbit. The arrays should be flexible in nature as the wings must also be stowed during launch and this stowed volume influences the size and design of the wings. On the other hand, for low altitude orbits, the atmospheric drag can be a major factor and this increases as the area of the wing increase. also, large wings will increase the overall mission cost and launch cost. But the solar satellites require large solar array wings to generate a high amount of power, hence to overcome this problem the solar satellites mostly use Geosynchronous orbit (GEO) [36000 km above the earth surface] as there is no atmospheric drag is present.
2] Space solar array construction:
a) Cover glass: Solar cells are fitted with cover glass to reduce the optical reflection and the damage done by radiation in space and to improve the surface thermal emittance. Two materials are almost exclusively used in current technology: Corning 7940 fused silica, and Corning 0211 micro sheet glass. The fused silica is more resistant to darkening by ultra-violet and electron radiation; this may be a significant factor in regions where thick cover glass are required. Micro sheet, on the other hand, is less costly, and small. Adequate for use in regions where a relatively thin shield can provide insulation. The electrical efficiency of solar cells is enhanced by a thermal design of the array which provides the coolest possible operating temperature for the cells. Materials and combinations of materials are used which minimize the ratio of solar absorptivity to blackbody emissivity. Hence, both front and rear surfaces of the array should be covered with cover glass for temperature optimization.
b) Adhesive: Adhesives are used to connect the solar cell to the substrate, and the glass cover to the surface. The types of adhesives that are usually used are silicones and epoxies. The adhesives are applied uniformly in thicknesses ranging from 0.025 to 0.050 mm. The adhesives based on epoxy are often adjusted by adding a plasticizer to provide stability at low temperature rates. On the other hand, silicones are stable over a wide range of temperatures.
c) Honeycomb: Honeycomb panels are advanced sandwich elements that consist of low-module lightweight cellular (honeycomb) core sandwiched between high-module, high-resistance face sheets. The assembly maximizes the ratio of rigidity to weight and the ratio of bending strength to weight, resulting in a panel structure that is especially effective in carrying both in and out of plane loads.
Factors affecting space solar array
1] Temperature: The temperature of any entire spacecraft is regulated entirely by radiation, since there is no convection or conduction medium. Optimizing radiation from the array is necessary to keep the PV array cool. Avoid absorption of excessive energy, often using “tuned” cover glasses on the cells to resist the IR and UV irradiation the cells are unable to use. Maximize heat flow away from the array using surfaces with high emissivity to radiate heat to deep space. There will be significant temperature differences between full sunlight at the front of the array and darkness at the rear. In sunlight, operating temperatures typically vary between 30 and 90C in the eclipse, the temperature of the array may range from.10C for a LEO-mounted body panel to.140C (worst case for a deep eclipse of the deployed GEO array).
2] Van Allen radiation belt:
In this the protons, electrons, and heavier ions “caught up” by the magnetic field of Earth in “belts” named for the discoverer “Van Allen”. Electron energies range from a few keV to around 7 MeV. While the protons and ions energies range from a few keV to around 500 MeV. A small number of particles have higher energies than these values but from a spacecraft design point of view, they are not important. The spatial distribution varies according to several orders of magnitude with altitude and orbit inclination. Besides, the South Atlantic anomaly – the 11C disparity between the magnetic and spatial poles and the offset of the Earth’s spatial and geomagnetic centres creates a magnetic disturbance over the South Atlantic and charged particles are trapped at lower altitude here (about 400 km). Therefore, Van Allen belts are a problem for any orbits because the spacecraft must travel through them.
3] Atomic oxygen: At altitudes of 200-700 km, atomic oxygen is the major constituent of the atmosphere (109 atoms cm-3 at Shuttle orbital heights of 250 km, 107 atoms cm-3 at International Space Station orbit of 500 km). Atomic oxygen is very reactive and erodes material, especially exposed silver on interconnects. It can lead to array failures and is a factor in the determination of mission lifetime. The array should have all vulnerable surfaces protected (e.g. with the combination of cover glass and adhesive). Also, the erosion of surfaces can lead to thermal, electrical and optical changes, so altering the operating environment of the solar array.
4] Micrometeoroids and debris: Micrometeoroids are the remnants of comet orbits and exposure is increased whenever the Earth’s orbit intersects with that of a comet. The orbital debris includes spent rocket stages and fragments of rockets and spacecraft. There are at least 19,000 known man-made objects of at least 10 cm in size in LEO and GEO and probably hundreds of thousands of smaller items. They cause damage from collision and can increase other operational problems (e.g. atomic oxygen interactions, array charging). Also, impacts can change the thermal, electrical or optical properties of the array. the arrays are designed with redundant contacts to allow for damage to parts of cells without taking out the whole string. Besides, the risk of damage is increased for long mission duration, increased spacecraft size and certain altitudes and orbits. The large solar wings are particularly vulnerable.
5] Magnetic moment control:
The movement of the electrical currents in the array in the presence of a magnetic field leads to torque on the array. To eliminate this phenomenon, the array should be designed for magnetic field cancellation by using a mirror configuration for the strings.
Advantages and Disadvantages
1] Advantages:
- The full solar irradiation would be available would be always which results in about 5 times the solar energy as compared to the best terrestrial sites on Earth.
- The antenna could be directed at any Rectenna located on the Earth.
- The zero gravity and high vacuum condition in space would allow much lighter, low maintenance structures and collectors.
- No fuel required.
2] Disadvantages:
- The entire structure is Humongous.
- High cost and requires a huge amount of time for construction.
- The risk involved with malfunction.
- High power microwave source and high gain antenna can be used to deliver a burst of energy to a target and thus can be used as a weapon.
Why is solar power needed on satellites?
Spacecraft and satellites in space need a tremendous amount of energy to be operational. Before solar was a viable solution for providing this power, batteries were used. The only problem is that batteries have a set capacity, and without any means to recharge these batteries, they become useless when they run out of energy.
Solar panels paired with batteries are a much better option because they provide a constant stream of renewable energy. Right now, solar is used to provide electricity to the computer systems and other systems that are used to monitor and control various parts of the spacecraft.
The ultimate goal, however, is to use solar energy to propel spacecraft and minimize or completely remove the need for other sources of fuel. This would have serious implications for space travel in a very positive way.
What solar technology do spacecraft use?
There are two types of solar cells that are common in spacecraft:
- Silicon cells covered in thin glass, and
- Multi-junction cells made up of gallium arsenide and other similar materials.
The silicon cells that are covered with glass are pretty similar to conventional solar panels, but they are further improved to handle radiation and extreme temperatures. This type of panel can be found on the International Space Station, which currently holds the majority of solar panels found in space.
The solar cells that are made up of gallium arsenide are much more efficient, and as a result, are sometimes a better option when physical space is a concern. These panels can reach up to around 34% efficiency vs. the 15-20% that most commercial solar panels can reach.
High-efficiency gallium arsenide panels of the Dawn satellite
Satellites in space are also equipped with solar panels that can follow the direction of the sun to maximize their absorption of sunlight. Sun rays in space are even more abundant than on Earth, due to the absence of an atmosphere. About 55-60% of solar energy gets either reflected or absorbed on its way to Earth’s surface through clouds, gases, and dust.
The solar panels found in many satellites in space also include a folding structure that allows the panels to expand while the spacecraft is in orbit. This format is also used in the International Space Station.
Lastly, the solar panels in space do not need to convert DC electricity into AC. On Earth, your electricity all of your electronics run on AC power. This is why it is necessary to have a solar inverter to convert the base DC electricity from your panels into AC. AC power is also useful for transmitting electricity over long distances.
Because the electricity that a satellite in space or other spacecraft does not need to travel these distances, it can stay in the DC format. This also helps reduce the amount of hardware needed for these systems.
Space Solar Tech is Built Durable and Efficient
Overall, there are many similarities between space-based solar panels and conventional solar panels. They both include cells that are made of conductive material (usually silicon) and are fit into arrays. The biggest difference has to do with the overall quality and durability of the modules.
In space, there is extreme heat, cold, and radiation. This is accounted for in space-based solar panels and naturally influences the state of the hardware. Also, NASA is constantly experimenting with different semiconductor materials for producing better solar cells for space. Gallium arsenide is one example of this, and there should be many new innovations on the way!
Propellants Tanks
Northrop Grumman is the world’s leading producer of titanium propellant tanks used in government, scientific and commercial satellites, launch vehicles, and space explorers, with more than 600 designs qualified and more than 6,400 tanks delivered. The tanks have been a part of nearly every large U.S. launch vehicle and geosynchronous earth-orbit satellite, with 100 percent reliability.
Northrop Grumman propellant tanks have landed on Mars, Venus and the Moon, and have visited every planet in the solar system. They have been an integral part of nearly every major space-exploration vehicle, including Cassini, Mariner, Pioneer, and Voyager. Launch vehicles with Northrop Grumman tanks have included Delta III and Delta IV, the space shuttle and the entire Atlas family of vehicles.
Northrop Grumman’s capabilities include design, manufacture, and testing of propellant and pressurant tanks such as pressure management (PMD) tanks, diaphragm tanks, pressurant tanks and motor cases.
Electrostatic Clean Solar Array Systems
Northrop Grumman has provided electrostatic and magnetically clean solar array panels for several spacecraft including the Themis constellation and C/NOFS. These unique spacecraft components use a fully conductive surface to provide less than 1V of surface charging potential and less than 0.1mA-m2 to prevent interference with scientific instruments.
Stiff and stable deployment of large sensors is best accomplished with a purpose-built folding truss, and Northrop Grumman is a world leader in producing such systems.
For example, we were selected by Ball Aerospace to design and build the Reflector Deployment Assembly for the Global Precipitation Measurement Microwave Imager. The RDA is a folding, statically determinate hexapod truss used to support a 1.2-meter-diameter reflector dish.
Our RDA structure exceeded the stringent requirements for placement repeatability (less than 0.001 inches, 3-sigma, more than 10 deployments) and thermal stability (less than 0.01 inches and 18 arcsec each axis over each orbit). Deployment is powered and controlled by a robust spring/damper drive unit that, once deployed, does not apply loading to the structure to ensure the final reflector position is driven entirely by the precision of the folding struts.
Northrop Grumman also produced the Extendible Support Structure for the RADARSAT-2 mission for MDA and the Canadian Space Agency. The structure is currently in service in a wide variety of roles, including sea-ice identification and ship routing, iceberg detection, agricultural crop monitoring, marine surveillance for ship and pollution detection, terrestrial defense surveillance and target identification, and surface mapping.
Performance Features
- Linear, preloaded structures
- Sub-millimeter deployment precision and stability
- Active or passive fully controlled deployments
- Predictable, deterministic structures
- Passive compensation of satellite or bus distortion
Application Benefits
- Ideal for 1 m2 to 100 m2 structures
- Motor- or spring-driven deployment
- On-orbit retraction capability
- Performance validated at subscale level
Thermal Technologies
Northrop Grumman is the world’s leader in two-phase thermal management of spacecraft and has more than 30 years of experience designing and manufacturing space-qualified products for various government, military and commercial customers. By combining engineering expertise in thermal and structural design with hardware fabrication, we provide leading-edge thermal-management solutions not only to the aerospace community but also to customers for terrestrial cooling communication electronics.
The company works closely with engineering experts, customer staff and vendors to provide a systems-oriented, multidisciplinary approach to successfully meet customers’ needs, whether those needs are for design-to-performance or build-to-print programs.
Capabilities include design, manufacture, testing and integration of complete two-phase thermal-control systems such as heat pipes (-260 C to 175 C); heat pipe radiators and structural equipment panels; two-phase-loop heat pipes (LHPs); deployable radiator assemblies; phase-change materials (PCM); multi-layer insulation (MLI) blankets; thermal preparation; cryogenic cooling applications; and composite structures.
Facilities and resources include 105,000 square feet of engineering and manufacturing space featuring flight-certified integrated assembly areas, processing ovens, thermal vacuum chambers, large environmental chambers, paint booths and full machining and inspection capabilities.