The amount of energy available from the Sun is almost unimaginable. One hour of sunlight falling on the earth has enough energy to power the entire earth for a year. If engineers can capture 1 percent of this energy, the energy to power the earth for a year could be captured in 100 hours. Assuming 8 hours per day collection time, it would take 12.5 days of captured solar energy to power the earth. Even if only one-tenth of this amount can be captured, it would only take 125 days of captured solar energy to power the earth. Solar energy also drives the winds and ocean currents. A month’s worth of energy captured from wind could power the earth. There is also the geothermal energy of the earth itself. As fossil fuels get more expensive and harder to find, alternative fuels will become more and more affordable. Remember that coal replaced wood as society’s fuel as the industrial revolution was beginning partially because wood became scarce (Gore 2009, 52–57).
Electricity can be generated from sunlight using mirrors to concentrate solar energy to heat a fluid, which then creates steam to turn a turbine to produce electricity. Electricity can also be generated directly from photovoltaic panels.
Concentrated solar thermal energy plants are large utility owned installations. There are three varieties. One has a large array of two axis tracking mirrors that reflect solar energy onto a receiver mounted on a tower (Figure 32.1). The high temperature fluid is then used to create steam to drive electric generators (Gore 2009, 64). The Ivanpah Valley, California solar tower power plant has 173,500 computer controlled mirrors focusing solar energy on a receiver at the top of a 46 story tower. Water is heated to 1,000 degrees Fahrenheit, creating steam to generate electricity. Enough electricity will be generated to power 140,000 homes (Bernstein 2014). Another method of concentrating solar energy is an array of linear parabolic reflectors that focus the Sun’s energy on a pipe filled with a specially designed fluid. This heat is then used to generate steam in a heat exchanger. The steam is used to generate electricity. The parabolic trough can be a smooth parabolic mirror or it can be made from many flat mirrors arranged in a parabolic shape. The third method involves a parabolic dish mirror that tracks the sun focusing solar heat on a receiver attached to the mirror dish. The heat generated is used by a Stirling Engine to generate electricity (Gore 2009, 64).
FIGURE 32.1 A solar tower power plant in Daggett, California.
Source: Wikimedia Commons, Attribution-Share Alike 3.0, Photo by Kjkolb 2003.
FIGURE 32.2 Photovoltaic panels produce electricity directly from the Sun by exciting electrons in the photovoltaic material.
Traditional photovoltaic panels are made from silicon (Figure 32.2). A thin top layer has a small amount of phosphorous in it, which supplies extra electrons. Silicon has four valance electrons. Phosphorous has five valance electrons. A thicker lower layer has a little boron in the silicon. Boron has three valance electrons. Thus the upper level has a slight negative charge and the lower level has a slight positive charge. Because of the slight deficit of electrons in the lower level, the electrons have more room to move around and have slightly more energy than the electrons in the upper level. Where the two layers join together there is a resistance to the passing of electrons. Sunlight comes through the thin upper layer and excites the electrons in the lower layer enough so they can jump across the boundary between the two layers. This creates an excess charge in the upper layer, which is collected and sent through circuits to provide electricity to a use (Strong and Scheller 1993, 15–18). Photovoltaic panels produce direct current. Most household and com mercial appliances and the utility grid use alternating current; thus, a solid state controller is necessary to produce alternating current.
Photovoltaic panels lend themselves to distributed use. They can be mounted in small, medium, or large arrays almost anywhere (Gore 2009, 68). When a utility grid is available the most straightforward method of storing excess electricity production during the day is to send the extra electricity back into the electric grid. If the utility grid is not available, then batteries are necessary to store electricity generated during the day for use at night or during cloudy weather. Batteries are sized by determining the amp hours they need to deliver. Amp hours are calculated by dividing the watt hours of energy required by the building per day by the voltage of the system and battery bank. Often this is 24 volts. The amp hours per day are multiplied by the number of days of storage required and by a 1.2 discharge factor since batteries can only discharge 80 percent of their total charge (Grondzik et al. 2010, 1342).
Another form of photovoltaic panel is thin film amorphous silicon. These panels are less expensive and less efficient. Traditional silicon cell panels are from 10 to 15 percent efficient depending on their temperature. Amorphous silicon panels are from 5 to 7 percent efficient also depending on temperature. The efficiency is highest when the panels are cool, 50 to 60 degrees Fahrenheit, and least efficient when they are in the range of 110 to 120 degrees Fahrenheit (Humm and Toggweiler 1993, 99). Thus, it is important to mount photovoltaic panels so that they can ventilate away heat. It is also important to mount photovoltaic panels so that they are not shadowed at all during most of the day. Even a small amount of shadowing drastically reduces output.
Photovoltaic panels of either type come with a voltage and a current rating. Connecting the panels in series adds individual panels’ voltages together so that it increases the voltage of the array of panels. Connecting the panels in parallel adds the individual panel’s current together so that it increases the current output of the array of panels. Most photovoltaic arrays are connected in a combination of parallel and series connections to provide the voltage and current required (Figure 32.3).
FIGURE 32.3 Nellis Air Force Base 15 megawatt photovoltaic power plant.
Source: US Air Force photo, public domain.
The National Renewable Energy Laboratory (NREL) provides a website (www.nrel.gov.rredc/pvwatts