Alternative Energy: Solar Power

Written by: Nicholas Connor

In spite of their evident disadvantages, fossil fuels have been the dominant energy source of the 20th century. Perhaps as Brian Cox has asserted in Chain Reaction, “we are addicted to petroleum”. Environmental consequences thereof, such as carbon dioxide emissions and recent industrial spills, implicate that the contiguous manufacture and consumption of these fuels are perilous for future generations. Although society celebrates consumer technology, Max Planck’s conjecture is pertinent: “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it”(“Science” 272). Scores of clever and innovative methods for the development of energy have transpired during these past several decades and thus warrant consideration.

Of several viable alternative energy sources to hydrocarbon-derived energy, solar power is the most promising.  In justifying this option, one must first understand some principles of heliophysics, the study of the Sun and its connection to the Earth. This intricate science concerning solar radiation is multifaceted, whereas the common observation thereof might allude to simplicity. Beckman and Duffie state that solar radiation originates in the extraneous temperatures of the interior where several hydrogen molecules are capable of bonding, or fusing, to form helium gas (2). Consulting a periodic table reveals that this helium gas is actually lighter than its constituents. In the 1912 Manuscript on the Special Theory of Relativity, Albert Einstein’s calculations implicate that the mass and energy of any substance are directly proportional (42), and thereby this fusion of hydrogen releases massive energy in the form of heat.

Sol Wieder explains that gravitational forces contain the gaseous matter in a dense sphere, allowing this fusion process to occur continuously, unlike that of a hydrogen bomb explosion. The heat created is subsequently dispersed throughout the sphere (2). These internal reactions function as a thermostat for the visible surface, which is accordingly named the photosphere (3). Generally, this region is the source of sunlight and thereby a primary concern of the solar applications to be explained. Young and Freedman offer a laymen understanding of the photosphere, stating that it is not unlike familiar models such as molten steel and light bulbs (1122). The thermal radiation or sunlight being emanated into space is resultant of the surface temperature discussed.

Ignoring quantum electrodynamics for the moment, one must take notice that light travels as a wave and exists in differing lengths (1122). That is of course why there are many colors. Remarkably, only a third of the sunlight reaching the Earth is visible and therefore the available radiation is three times that perceived (Wieder 15). For this reason the potential efficiency of solar power is deceptive. The quantity of solar radiation received here is contingent on the Earth’s position along its orbit. Many aspects regarding the orbit are familiar, especially those of night, day, and the seasonal solstices. Ironically, the terrestrial orbit is closest to the Sun during December. Winter is the coldest season because the axis of rotation is cockeyed at an angle of 113.5°, whereas during the summer its angle is 66.5° relative to the Sun (Wieder 22). These variations of orbit determine the hours of sunlight per day and the direction by which light reaches the atmosphere. Precise measurements of the Earth’s position are imperative for efficient solar applications.

Furthermore, other conditions in the atmosphere, such as clouds and dust, sometimes significantly reduce the amount of solar energy reaching the Earth’s surface. The intensity of solar energy on a surface oriented perpendicular to the Sun’s rays above the Earth’s atmosphere has been measured by NASA to be between 1,365 and 1,367 Watts/m2. This energy is transmitted through the atmosphere and reaches the Earth’s surface at a rate that varies over time at a particular location (Wieder 35). Weather patterns and other atmospheric conditions scatter these incoming rays. Clouds are the predominant atmospheric conditions that inhibit solar energy collection.

Thus, the annual average daily solar radiation in the United States is highest where the atmosphere is very dry. Facilities designed to produce solar power must be placed strategically because sunlight levels vary across the country. For example, in the western desert regions of Arizona, Nevada, and California, the annual average of daily solar radiation ranges from 6.5 to 8.0 kiloWatt*hr/m2 in some locations (Duffie 47-50). Most locations along the Pacific coastline likely have higher moisture levels because radiation levels drop to around 5.0 kiloWatt*hr/m2, and would be less suitable for a solar facility. Similarly, the direct solar radiation is lower along the Gulf of Mexico coastline in Texas and Louisiana than it is slightly inland at the same latitudes (Duffie 47-50). Accurate predictions of the climate and atmospheric conditions are important for competitive solar power applications (Martin 1).

Sunlight can be converted to solar power using two methods: active and passive. Solar thermal technology is considered passive because it converts solar energy directly into heat for immediate use. Also, the heat can be stored in a thermal medium, such as dry rocks, or converted to mechanical and electrical energy. Approaches in the latter category comprise what are commonly referred to as concentrating solar power technology, which utilizes mirrors to concentrate solar radiation and capture heat. That heat is then converted to electricity by conventional technology. There were advances in this method during the energy shortage of the 70’s. In the 1980’s, a commercial implementation of concentrating solar was achieved with the construction of nine parabolic trough plants in the California Mojave Desert, totaling 354 megawatts (Planning, Budget, & Analysis 187). Concentrating solar power technology encompasses three approaches: trough, tower, and dish. All three methods can produce electricity for use on the central utility grid. The modular character of the dish system makes it convenient for local applications too (PBA 208). For easy comparison with hydrocarbon applications, emphasis has been placed on viable solar systems that can deliver electricity to consumers over a central utility grid.

A trough system consists of a large field of single-axis concentrators, or parabolic mirrors, that are arranged in parallel rows and focus sunlight onto focal line receivers. Their design allows them to track the Sun from East to West in order to keep radiation focused on the linear receiver (PBA 187). Fluids are pumped through the linear receiver and absorb heat. Next, they are circulated through a series of heat exchangers to generate pressurized steam. Conventional steam turbines are used to produce electricity. The remaining steam is recycled in a condenser and a cooling tower removes the excess heat so the fluids can be transferred back to the receivers (PBA 188). One square meter of current solar troughs can produce over 300 megaJoules of energy per year.

In tower systems, fields of large two-axis mirrors, or heliostats, direct sunlight onto a tall central receiver (PBA 169). The solar heat is absorbed by a working fluid, such as molten salt or water, which generates steam and powers a turbine. In molten salt systems, the solute is pumped from a cold tank and cycled through the receiver, heated, and then deposited into a hot tank where it will be ready for use. The hot tank is used to supply a steam turbine that can supply power to the grid (PBA 169). Effective storage designs can run a turbine for thirteen hours without additional input (PBA 169). This allows for electricity production during periods of high demand and low sunlight. The first of such tower systems, Solar One, was built in southern California and operated during the mid-1980s. Standing 91 meters tall, it used a water system and 1,818 heliostats to produce around ten megaWatts of energy (PBA 171). Solar One was later converted into Solar Two with the addition of heliostats and replacement of water for a “molten nitrate salt system” (PBA 179).  Another system was recently built in Spain, the Solar Tres, which has a similar design but even greater power capacity. Recent developments in manufacturing and material science have allowed tower systems to become economically competitive.

The third concentrating solar power technological approach, the dish system, uses a mirrored concentrator, which is around five times larger than a satellite TV dish, to focus sunlight onto a thermal receiver and heat engine. Dish assemblies should track the sun across the sky to ensure maximum energy generation. The receivers are integrated into a high-efficiency combustion engine. The engine has thin tubes containing hydrogen or helium gas that run along the outside of the engine’s four piston cylinders and open into the cylinders. As concentrated sunlight falls on the receiver, it heats the gases in the tubes to high temperatures, causing the hot gases to expand inside the cylinders. The expanding gas drives the pistons. The pistons turn a crankshaft, which drives an electric generator. The receiver, engine, and generator comprise a single, integrated assembly mounted at the focus of the mirrored dish. This type of solar technology has been in use for over two centuries (PBA 214). Dishes can be grouped in clusters of any magnitude. Dish technology typically achieves higher power efficiencies than its thermal counterparts.

These systems in concern seem quite complex in their entirety, but certainly one might imagine without effort this concept: the same radiant body, which nourishes the biological hierarchy, could also sustain man’s plethora of electronics and machinery. Opponents of solar energy claim that its facilities would require too much land, but Williams articulates that a technology with 10% efficiency on a mere 1.5% of U.S. land area could have supported the U.S. energy demand in the 70’s (1). Martin Lamonica of CNET has reported a Silicon Valley start-up that reached 43.5% efficiency in a solar technology, photovoltaic cells. Photovoltaics are the active power producers. The solar radiation in the 48 States has an overall average rate of about 1,800 kiloWatt*hr/m2 per year (Williams 3). Offshore panels are viable for solar power production too. Manufacturing techniques and material science breakthroughs are making solar technology cheaper and more effective every day. The nation should follow President Barack Obama’s instruction from his 2011 State of the Union Speech:

In America, innovation doesn’t just change our lives. It’s how we make a living.  [. . . .] This is our generation’s Sputnik moment. Two years ago, I said that we needed to reach a level of research and development we haven’t seen since the height of the Space Race, […] especially [in] clean energy technology – an investment that will strengthen our security, protect our planet, and create countless new jobs for our people. [….] With more research and incentives, we can break our dependence on oil [….] So instead of subsidizing yesterday’s energy, let’s invest in tomorrow’s.

 

Works Cited

Beckman, William, and; John A. Duffie. Solar Engineering of Thermal Processes. New York: John Wiley & Sons, 1980. Print.

Einstein, Albert. Einstein’s 1912 Manuscript on the Special Theory of Relativity.

Trans. Anne Beck. New York: George Braziller, 2003.

Lamonica, Martin. “Solar Junction Claims Cell Efficiency Record.” CNET News. 14 April 2011. Web. 16 April 2011.

Martin, Luis, et al. Prediction of Global Solar Irradiance Based on Time Series Analysis: Application to Solar Thermal Power Plants Energy Production Planning. 31 July 2010. PDF file.

NASA Science. “The Inconstant Sun.” Science News. RSS, 17 Jan. 2003. Web.

15 Apr. 2011.

Obama, Barack. “State of the Union.” United States. Washington D.C. 25 Jan. 2011. Address.

U.S. Department of Energy: Planning, Budget, & Analysis. Renewable Energy Technology Characterizations. 10 Aug. 2008. PDF file.

Wieder, Sol. An Introduction to Solar Energy for Scientists and Engineers. United States: John Wiley & Sons, 1982. Print.

Williams, Richard. Solar Energy Technology and Applications. Michigan: Ann Arbor Science, 1975. Print.

Young, Hugh D., Roger A. Freedman, and A. Lewis Ford. Sears and Zemansky’s University Physics. United States: Pearson Addison-Wesley, 2007. Print.