Photovoltaic solar energy is the conversion of sunlight into electricity, while solar thermal energy can convert about 90% of the solar radiation into heating water and air. Almost all the energy that makes up life on Earth comes from the sun. Solar energy is created in the core of the sun when four hydrogen atoms fuse to form one helium atom by nuclear fusion. Gravity pulls the entire mass of the sun inward and creates such a strong pressure in the core that atoms fuse together. Every second, 700 million tons of hydrogen are converted into helium. This nuclear process creates massive heat that causes atoms to expel energy as photons. It takes about 100,000 years for a photon from the core to reach the radiant surface of the sun, where it emits around 63 MW/m2 of energy. Given this amount of energy, the amount of radiation that hits the Earth’s atmosphere is about 1,367 W/m2, and the sun provides 15,000 times more energy each year than everyone on the planet consumes. However, we can only collect limited amounts due to various factors. About fifty percent of the energy that hits the atmosphere reaches the Earth’s surface. The energy as gamma rays, X-rays, and UV rays is absorbed by the atmosphere and converted into heat.
There are two types of solar panels: solar photovoltaics and solar water collectors. The word “photovoltaic” is a combination of “photo,” which means light, and “voltaic,” which means electricity. They generate electricity for buildings and transportation. Solar thermal energy generates heat, not electricity, and consists of a collector with a series of pipes through which water circulates and is heated by the sun.
Solar panels consist of modules, and these modules consist of solar cells connected in series and in parallel. Whether the solar cells are connected in series or parallel depends on the desired current and voltage. Connecting the panels in series generates a greater voltage, while connecting them in parallel generates a greater current.
Solar cells are the main component of solar panels and consist almost entirely of silicon, one of the most accessible minerals found on earth. Silicon consists of 14 electrons arranged in three electron shells. The first shell consists of two electrons, the second shell of eight electrons, and the outer shell is half full with four electrons. This means a silicon atom always seeks to fill its outer shell with four other electrons, and is willing to share its electrons with another atom to do so. A silicon atom in the crystal lattice absorbs a photon of incident solar radiation, and when the energy of the photon is high enough, an electron is released from the outer shell of the atom.
The main problem with pure crystalline silicon is its poor conduction of electricity. This is due to the limited motion of the electrons. To help, silicon cells are doped at the level of parts per million of boron (3 valence electrons) and phosphorus (5 valence electrons) to produce P-type and N-type silicon, respectively. The P-type has a free opening and is positively charged, and the N-type has an extra electron and is negatively charged.
When valence electrons absorb heat or light in a semiconductor, they can jump from the valence band into the conduction band, where they can move freely. Each electron that jumps to the conduction band leaves behind a vacant position or hole in the valence band, a process called electron hole pair generation. The holes usually disappear spontaneously, since electrons tend to recombine with holes. However, the recombination process can be reduced by creating a potential layer between the N-type and the P-type layers. This barrier is known as the depletion zone, where a static charge is present and electricity is generated. The barrier inhibits the free migration of electrons, which leads to a buildup of electrons in the N-type silicon layer and a deficiency of electrons in the P-type silicon layer. N-type solar cells have the solar cell structure built with a silicon wafer doped with boron as the base. Compared to P-type, N-type solar cells are increasing market share, since they are more efficient and not affected by light-induced degradation (LID).4 Eventually, equilibrium is reached, as it gets harder for electrons in the N side to cross over to the holes in the P side, leading to an electric field that separates the two layers and creates the cell voltage. If an external circuit connects these layers, then current flows through that circuit from the N-type silicon layer to the P-type silicon layer.
As the electrons flow, they do the work that makes PV cells function. Electrical contacts are formed by metal bases on the bottom of the cell, and by metal grids or meshes on the top layer. These top layers must be largely uncovered to allow photons to pass through. Also, silicon has a high reflectivity, so it must be coated so it does not reflect most of the sun’s radiation. A glass cover is usually placed over the solar panel to protect it from the environment. Much care must be taken in choosing a glass cover, making sure it has a high transmissivity. If transmissivity is low, much of the sun’s radiation will be blocked by the glass.
Solar cells vary in cell material, design and manufacturing methods. Amorphous or polycrystalline silicon, cadmium sulfide, gallium arsenide, and other semiconductors are used for cells. There are several limiting factors for solar cells, such as the maximum wavelength at which photons can create an electron-hole pair. For silicon, the maximum wavelength is 1.15 nm. From this factor alone, the maximum theoretical efficiency of silicon cells is 23%. Solar cells are rapidly evolving, but more ways are being discovered to increase their efficiency. Cell modules can be purchased inexpensively on the market, which have an efficiency of 15% to 20% and a life span of 25 to 30 years. Experimental single crystalline silicon cells were produced with efficiencies of 25%, and cells with multiple junctions with efficiencies of more than 30% were constructed.
What do people living autonomously off the utility grid, who rely on solar power, do when the sun is not shining? The two main options are backup generators or batteries. A backup generator is not clean, but affordable, while storing energy in batteries is clean, but expensive. Batteries have only a 5 to 15-year lifespan considerably less than the 25 to 30-year lifespan of a PV solar system, and they can be dangerous due to the energy they store and the acidic electrolytes they contain. When using a battery, a deep cycle battery needs to be used. A deep cycle battery can discharge its stored energy longer, like at night or during extended power outages.
The third option would be to connect to the local utility grid. This way, when the solar panels are not meeting the energy demand, the local utility grid would supply the electricity needed and send a bill. In some cases, the solar cells provide more than enough energy (net-positive) and that energy can be sold back to local power companies. However, this is not the case in all locations, as some power companies do not buy back energy. Special safety considerations must be made when connecting to the utility grid. There used to be a hazardous situation called islanding, in which solar panels can provide power to lines that linemen expect to be downed due to power outages or shut down for repairs. This could lead to electrocution, with serious injuries and even death. However, now a solar panel system automatically shuts down if the grid is down, with a new feature called module-level rapid shutdown added to the National Electrical Code (NEC) in 2017. Starting Jan. 1, 2019, in certain state jurisdictions, all conductors within an array’s 1-ft boundary must be reduced to 80 V or less within 30 seconds of rapid shutdown initiation.
Solar energy is one of the bases of clean alternate power solutions, and is a viable solution to some energy problems in the world. The more we improve economics and efficiency, the more it will be adopted and replace fossil fuels. Solar does not produce carbon and other greenhouse gases during its operation. There is no need to burn oil, and no toxic wastes are produced compared with nuclear power, reducing the possibility of an environmental accident almost to zero. Sunlight, unlike fossil fuels, has essentially an unlimited life. Fossil fuels need to be mined or drilled, which causes an extremely negative environmental impact even in the best conditions. The environmental impact stems from the manufacturing, transportation, and construction of solar systems, not during energy use, ranking it one of the cleanest forms of energy. Critical against global warming, solar energy is more environmentally friendly, abundant, available, and flexible than any other energy source.
Solar power is vastly different from fossil fuel power plants in terms of maximum availability and efficiency. Coal, crude oil / petroleum (becomes petrol after distillation and refining), and natural gas all have an extensive supply chain with many processing steps. What has held solar energy back is the idea that it should be used to replace elements of the established fossil fuel structure. Smart individuals will have to rethink the supply and distribution network, since the construction and operation of the solar cell distribution grid accounts for half the cost. The advantage of solar electric systems over conventional fuel combustion is they convert incident sunlight, in a one-step process, to direct current in the cell. Burning coal requires complex and expensive process steps for sending the electricity to the grid.
Solar cell technology is rapidly evolving, with new, more efficient cells being developed and manufacturing costs decreasing. The ways solar cells are being used are growing. One way is through space-based P-type solar panels. California is forging ahead with a plan to beam power from space-based solar panels as early as 2016. In a deal approved by state regulators, Solarenwill would provide 1700 GW-hrs of power per year for 15 years to Pacific Gas and Electric (PG&E), using satellites to convert the sun’s rays into radio-frequency beams that can be transmitted to receiving stations on Earth. The biggest hurdle in this new technology is the high cost of launching the components into geosynchronous orbit, which starts at about $20,000/lb. Other entrepreneurs are pursuing similar systems, including a group of Japanese companies that last summer announced a $21B plan to test hardware in space and sell power by 2030.
Flexible solar cells have a promising future and are being implemented more. These cells are placed on clothing and accessories, and can be used to directly charge personal appliances. Another form of these solar cells is roof shingles. The complex installation of solar panels can be overwhelming for many homeowners. To open the market of solar panels on homes to more people, Dow Chemical created thin-film PV solar panels that are the size and shape of ordinary asphalt shingles. The installation of these shingles is much simpler than conventional solar panels. They can be nailed into the roof, just like asphalt shingles. These solar shingles are also easier to install on homes compared to traditional solar panels, because they do not use elaborate racking systems that penetrate a roof. Sales of the Powerhouse Solar Shingles are expected to begin late this year.
Batteries that store the extra energy produced by the solar panels are generally cost-prohibitive. To combat weak batteries, Donald Sadoway, a professor of materials chemistry at Massachusetts Institute of Technology, has designed an all-liquid metal battery that could allow solar power schemes to flourish. The battery works on the same principle as any other battery, but by using liquid metal for electrodes and molten salt as an electrolyte, it can absorb electrical currents 10 times higher than present-day high-end batteries. The Department of Energy’s idea factory, the Advanced Research Projects Agency-Energy (ARPA-E), is putting $6.9 million behind this project, released in late 2009, making it one of the biggest first-round funding awards for the agency.
Breakthroughs are being made in the design of solar cells. Scientists from Spectrolab, Inc., a subsidiary of Boeing, are working on multi-junction cells that can achieve an efficiency of over 40%. With theoretical values of 58%, Spectrolab scientists believe concentrator cells could achieve efficiencies of 45% to 50% by using more than three junction cells. These multi-junction cells divide the broad solar spectrum into three subcell band gaps that each capture a different wavelength of light, enabling each subcell to convert the light into electricity more efficiently.
- Duffie, John A. & William A. Beckman. “Solar Engineering of Thermal Processes”. John Wiley & Sons, Inc., 2006.
- Toothman, Jessika & Aldous, Scott. “How Solar Cells Work”
- Gilpin, Lindsey. “8 crazy new solar research breakthroughs”. Innovation
- Pickerel, Kelly. “The difference between n-type and p-type solar cells”. Solar Power World. 2018