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Abstract

This report studies the fundamental principles of solar electricity, economics, effects on the environment, issues, and future research. Solar PV (photovoltaics) converts sunlight directly into electricity, whereas, solar thermal can convert ~90% of solar radiation into heat for hot water and heating (air) applications.

Introduction

Almost all the energy that makes up life on Earth originates from the sun. Solar energy is created in the core of the sun when four hydrogen atoms fuse into a helium atom via nuclear fusion. At the core, gravity pulls all the sun’s mass inward, creating such intense pressure as to force the fusion of atoms. Every second, 700M tons of hydrogen is converted to helium. This nuclear process also creates massive heat that causes atoms to expel energy in the form of photons. It takes about 100,000 years for a photon to travel from the core to the radiative surface of the sun. The sun emits approximately 63 MW/m2 of energy. Given this amount of energy, the amount of radiation received by the earth’s atmosphere is around 1,367 W/m2 and every year the sun delivers 15,000 times more energy than is consumed by every human on the planet. However, we can collect only limited amounts due to various factors. Only about fifty percent of the energy that is received in the atmosphere passes through to the earth’s surface. The energy in the form of gamma rays, x-rays, and ultraviolet radiation is absorbed by the atmosphere and turned into heat energy.

Solar Panels

There are two types of solar panel collectors. There are solar PV panels and solar water panels. The word photovoltaic is a combination of “photo” meaning light, and “voltaic” meaning electricity. Solar PV panels are being used in all types of applications, from the residential market with powering homes to the transportation sector with automobiles and airplanes. Solar water panels consist of a series of pipes in the collector which are heated by the sun. This hot water is then used either for heating purposes or domestic hot water.

Solar Cells

Solar electric systems are made up of modules, and these modules are made of solar cells that are connected in series and parallel. Whether the solar cells are connected in series or parallel depends on the current and voltage desired. Connecting the panels in series produces a greater voltage, whereas connecting them in parallel produces a greater current.

Solar cells are the main component of solar panels and are made of almost entirely silicon. Silicon is one of the most easily accessible minerals found on Earth making the manufacturing of cells relatively easy. Silicon consists of 14 electrons arranged within three shells. The first shell consists of two electrons and the second shell eight electrons, making both shells full. The outer shell, filled when it has eight electrons, is only half full having four electrons. This means that a silicon atom will always look to fill its outer shell with four other electrons, and it is willing to share its electrons with another atom to do so. This is a very important quality when it comes to PV cells. An atom of silicon in the crystal lattice absorbs a photon of the incident solar radiation, and if the energy of the photon is high enough, an electron from the outer shell of the atom is freed.

The main problem with pure crystalline silicon is attributed to its very poor conduction of electricity. This is due to its electrons’ limited movement. To help with this, the silicon cells are usually doped with exceedingly small amounts (of the order of one-part in 106) of boron (3 valence e’s) and phosphorus (5 valence e’s), to create P-type and N-type silicon, respectively. P-type (hence positively charged) has a free opening and N-type has an extra electron.

The freeing of electrons results in the formation of hole-electron pairs. The holes normally disappear spontaneously as electrons tend to recombine with holes. However, the recombination process can be reduced by building between the N-type and P-type layers a potential barrier known as the depletion zone where a static charge exists, and the generation of electric current occurs. The barrier inhibits the free migration of electrons, leading 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 are 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, thus leading to an electric field that separates the two layers and creates the cell voltage. If these layers are connected by an external circuit 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 made 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. Another thing to consider is silicon has an extremely high reflectivity, therefore 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 that it has a high transmissivity. If transmissivity is low, much of the sun’s radiation will be blocked out by the glass and not make it to the cell.

There are many other variations on solar cells, with differences in cell material, design, and methods of manufacture. Amorphous or polycrystalline silicon, cadmium sulfide, gallium arsenide, and other semiconductors are used for cells. There are several limiting factors for solar cells; one of these is 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, however, and more ways are being discovered to increase their efficiency. Cell modules can be purchased inexpensively on the market that is 15% to 20% efficient and has design lifetimes of 25 to 30 years. Experimental single crystalline silicon cells have been produced with efficiencies of 25% and cells with multiple junctions have been constructed that have efficiencies of more than 30%.

Alternative Power

What do locations that are off the utility grid, living autonomously, and rely on solar energy for most of their power do when the sun is not shining? The two main options are backup generators or batteries. Having a backup generator is not clean but is affordable whereas storing energy in batteries is clean but is expensive. Batteries have only a 5 to 15-year lifespan that is considerably less than the 25 to 30-year lifespan of a PV solar system and they can be dangerous because of 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 over a longer time such as at night or during lengthy 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 the local power companies. This is not the case in all locations; however, as some power companies do not buy back energy. Special safety considerations must be made when connecting to the utility grid. Previously there was a hazardous situation called islanding, in which solar panels can provide power to lines that linemen think are downed due to power outages or shut down for repairs. This could result in electrocution leading to serious injuries and even death, however, now the solar panel systems automatically shutdowns if the grid is down with a new feature called module-level rapid shutdown that was 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.

Environmental Impact

Solar energy constitutes one of the bases of clean alternate power solutions and perhaps could represent a viable solution to some energy problems in the world. The more we improve the economics and efficiency the greater 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 possibilities of an environmental accident almost to zero. Sunlight will not run out, but fossil fuels will. Additionally, they need to be mined or drilled which causes an extremely negative environmental impact even in safe conditions. It is worth noting, the fewer the resources used to obtain silicon the lesser the environmental impact. The environmental impact stems from the manufacturing, transportation, and construction of solar systems, and not during energy use, ranking it one of the cleanest forms of energy on Earth. Critical against global warming, solar energy is more environmentally friendly, abundant, available, and flexible than any other energy source.

Economics

When deciding on whether it is worth it economically, we should not solely focus on its capital costs and compare it to fuel when it is such a different form of energy. For example, it is vastly different compared to coal in terms of maximum availability and efficiency. Another expense of coal use is the long supply chains 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. It will require smart individuals to rethink the supply and distribution network since the construction and operation of the solar cell distribution grids 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.

Future

Solar cell technology is evolving rapidly, with new, more efficient cells being developed and the costs of manufacturing coming down. The various ways in which solar cells are being used is growing. One of the new ways solar energy is being used 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.

Another form of solar cells that are being implemented, and have a promising future, is flexible solar cells. These cells are being placed on clothing and accessories and can be used to directly charge personal appliances like cell phones, iPods, and PDAs. Another form of these solar cells is roof shingles. The complex installation of solar panels can be discouraging to 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 those of 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 with 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.

Another developing technology is the batteries that store the extra energy produced by the solar panels.  Adding batteries to solar systems can be a very expensive step. To combat these 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 that are 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 near 58%, the Spectrolab scientists believe that, by using more than three junction cells, concentrator cells could achieve efficiencies of 45% to 50%. 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.

References

  1. Duffie, John A. & William A. Beckman. “Solar Engineering of Thermal Processes”. John Wiley & Sons, Inc., 2006.
  2. Toothman, Jessika & Aldous, Scott. “How Solar Cells Work”. http://science.howstuffworks.com/environmental/energy/solar-cell3.htm.
  3. Gilpin, Lindsey. “8 crazy new solar research breakthroughs”. Innovation. http://www.techrepublic.com/article/8-crazy-new-solar-research-breakthroughs.
  4. Pickerel, Kelly. “The difference between n-type and p-type solar cells”. Solar Power World. 2018. https://www.solarpowerworldonline.com/2018/07/the-difference-between-n-type-and-p-type-solar-cells.

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