Picture of satellite with PV array

Light Energy
As we all know, the sun in our solar system is a star. Scientists have learned that stars in other solar systems release energy in different forms. Many stars release large amounts of x-ray energy or radio signals. Our star, the sun, releases 95% of its energy in the form of visible light.

graphic of sun and plant

The light energy from sun is responsible for practically all forms of energy here on earth. Photosynthesis is the process of storing energy through the chemical combination of Carbon Dioxide and water. This process is essential for plant life and can only happen in the presence of sunlight.

The remains of the plant life from prehistoric times is what we now call “coal”. Coal, a combustible mineral, can trace its ancestry back to the time of the dinosaurs. Along with petroleum and natural gas, it is a “fossil fuel”. The term “fossil fuel” means that is it energy that can trace its beginnings back to once-living organic materials. Coal began as trees, ferns and other plants that existed and died in tropical forests from 1 to 400 million years ago.

Over large periods of time, many layers of plants were buried under prehistoric forests and seas. Geological processes involving pressure and temperature compressed and altered the plant remains, increasing the amount of carbon present. Millions of years later, the material that once had been living plants dependent on light energy was transformed into what we now know as coal.

pictures of coal mining, oil wells and wind turbines

The sun also creates wind power that turns windmills and wind turbines. The uneven warming of large air masses causes rising and circulating air currents.

Hydropower harnesses the power of falling water. These power plants are usually near rivers. The water in a river is continuously replenished through the cycle of water evaporation and condensation from cooler air as it rises. The sun also controls this process.

Infrared rays, visible light rays, and ultra-violet rays are three significant parts of the solar energy spectrum. The infrared and ultraviolet rays are invisible forms of solar energy. The solar energy in the infrared region is the sun’s energy we feel as heat. The amount solar energy in the infrared region contains less energy than the visible light region of solar radiation. The solar energy in the ultraviolet region, that can cause our skin to tan, contains much more energy than the visible light region.

picture of three regions of solar energy
(infrared, visible light, UV)

X-rays and Ultraviolet rays contains large amounts of solar energy that can be harmful to humans. Our atmosphere absorbs and filters out most of these dangerous forms of solar energy. Scientists feel that we have damaged the outer atmosphere with gasses like “freon”, that have escaped into the atmosphere. Freon is a man made gas found in air conditioners and refrigerator compressors. It has damaged the “ozone” layer of our atmosphere, the layer that filters out the UV rays of the sun. Have you noticed that sun tan lotions have ratings that indicate how much protection they offer? These products are designed to filter out the high levels of solar energy. If we do not use these products we can get a very bad sun-burn. A sun-burn is actually our skin cells being permanently damaged or killed by solar energy. Skin cancer can also result from extreme exposures to damaging x-Ray and UV rays from the sun.

The sun releases enormous amounts of energy into our solar system. Only a small fraction of this energy reaches the earth’s atmosphere. In one minute the amount of solar energy that reaches the earth’s surface is greater than all the energy consumed by the world’s population in a full year. The developers of PV cells realize that photovoltaics’ work much more efficiently in space outside the earth’s atmosphere. An average of 1367 Watts of solar energy strikes every square meter of the earth’s outer atmosphere each second. Scientists are predicting that someday much of the earth’s electric energy will be generated in space using large PV arrays. Solar panels have already become very common in remote areas where electricity cannot be supplied by transmission wires to operate devices or to recharge batteries.

How PV Cells Work
The physical process that a PV cell changes light energy into electrical energy is called the Photovoltaic Effect. The word “photovoltaic” is a compound word containing the root words “Photo”, meaning light and the word “voltaic”, meaning electricity. The light rays from the sun are made up of packets of solar energy called “photons”. The photons contain different amounts of electrical energy. When a PV cell is exposed to photon energy the energy can either be reflected by the material or absorbed by the material. The amount of energy absorbed depends on the intensity and orientation of the light energy source with the PV arrays. PV cells operate at their highest efficiency when the light rays are directly hitting the solar cells’ perpendicular with the surface of the panels. If the position of the solar panel changes causing the angle of the light rays to be less than 90 degrees, the output voltage of the PV cells will be reduced.

Illustration showing light rays at 90 degrees and less than 90 degrees.

This is sort of like the automobile crash tests we have seen on TV. If the car hits a wall directly all of the energy is transferred into the wall. If a car hits a wall at an angle only a portion of the energy is transferred into the wall.

The material in the PV cell is a combination of man-made molecular compounds (semi-conductors) that easily exchange electrons from atom to atom, much like copper wire and other conductors. It is the absorbed photons that create electric energy. The photons attach themselves to the electrons of the atoms in the PV cell and are then passed on to other atoms that are depleted of their electrons creating a current flow. This semi-conductor current flow is very similar to basic electric current flow from a battery through a circuit.

a graphic of light energy, PV cell in an electric circuit

The ISS Solar Arrays
Each of the individual components of the ISS (Unity, Zarya, Destiny, Zvezda) are equipped with clusters of solar panels called solar arrays.

A single PV cell will typically produce 1 to 2 watts of electrical energy. This alone is far from the amounts of energy required to operate the International Space Station (ISS). PV modules are built by connecting 20 – 40 single cells together. PV arrays are simply formed by connecting together multiple modules. A PV array can be built with as many modules as you would want. The larger the array, the more electrical energy you can generate.

The PV array’s that are used to provide power to the ISS are very large. At completion there will be eight solar arrays on the ISS. Each of the arrays will measure 108 feet in length. They will be connected to a center truss 310 feet long. Each array will consist of many solar modules that are attached to each other so they open and close like an accordion. When they are fully extended or open each array will cover an area of 27,000 square feet. That is almost the size of a football field. They are the largest deployable structures ever to be sent into space.

Picture of ISS solar arrays

The PV arrays will generate high levels (160 volts) of electrical energy necessary to operate the ISS. However, before the electrical energy can be used it must first be converted to a level that the on board equipment requires (124 volts). The energy levels are reduced or “stepped down” by transformers. This is similar to the way electric power is stepped down by transformers on power poles before it is brought into our homes for use in electric equipment.

Graphic of power poles, transformer and wires to home and electrical appliances

The energy must also be stored in a battery for use when the ISS is in the earth’s shadow and the PV cells are incapable of generating power. The PV arrays are connected to banks of batteries. The battery banks store enough power to provide a constant 78 kW of electric power during the eclipse portion of the orbit.

In order for the PV cells to operate at their highest efficiency they must always be pointed directly at the sun. When the satellite is moving this can become a very complicated task. Computers continually control motors or “gimbals” that adjust the position and angles of the solar arrays with the sun. If these arrays are not positioned correctly, the power generated can be reduced by as much as 50%.