Energy Revolution

The Physics and the Promise of Efficient Technology

Mara Prentiss

• Publisher: Belknap Press
• Year: 2015
• Print ISBN:
  9780674725027
• eBook ISBN:
  9780674744974
• Topic: Physical Sciences
• Subtopic: Public Policy

Energy can be neither created nor destroyed—but it can be wasted. The United States wastes two-thirds of its energy, including 80 percent of the energy used in transportation. So the nation has a tremendous opportunity to develop a sensible energy policy based on benefits and costs. But to do that we need facts—not hyperbole, not wishful thinking. Mara Prentiss presents and interprets political and technical information from government reports and press releases, as well as fundamental scientific laws, to advance a bold claim: wind and solar power could generate 100 percent of the United States' average total energy demand for the foreseeable future, even without waste reduction.

To meet the actual rather than the average demand, significant technological and political hurdles must be overcome. Still, a U.S. energy economy based entirely on wind, solar, hydroelectricity, and biofuels is within reach. The transition to renewables will benefit from new technologies that decrease energy consumption without lifestyle sacrifices, including energy optimization from interconnected smart devices and waste reduction from use of LED lights, regenerative brakes, and electric cars. Many countries cannot obtain sufficient renewable energy within their borders, Prentiss notes, but U.S. conversion to a 100 percent renewable energy economy would, by itself, significantly reduce the global impact of fossil fuel consumption.

Enhanced by full-color visualizations of key concepts and data, Energy Revolution answers one of the century's most crucial questions: How can we get smarter about producing and distributing, using and conserving, energy?

III

ENERGY LINKS

7

DISTRIBUTING ELECTRICITY

To paraphrase a mid-twentieth-century detective novelist, our romantic lives and our electrical delivery systems are similar: if they are working well we ignore them, but if they are not working well we can think of little else. In industrialized countries, electricity is distributed to almost all residences and businesses using wires. This conveniently delivered energy allows us to heat, cool, and light our houses. It also runs our appliances and powers our electronic devices. Many people hope that in the future it will provide the energy that will run our cars. Yet, for all of the importance of electrical power, we rarely pay much attention to it.

As discussed previously, commercial electrical power is provided by a current of electrons flowing along wires. In SI units, that flow of electrical current is measured in amperes. The electrical power consumed by a device is given by the product of the current, I, and V, the voltage decrease resulting from flowing through the device. In equation form this becomes

Power = I × V.

The voltage difference corresponds to the difference in electrical energy between the two sides of the device. Similarly, if g is the gravitational constant, then g× h corresponds to the gravitational potential energy difference per unit mass between two positions with a height difference of h.

It turns out that “voltage” has no absolute meaning. The only thing that is meaningful is the difference between two voltages. A similar issue arises in connection with gravity. We can call sea level height zero, in which case Denver has a height +1 mile, or we can call Denver height zero in which case sea level has a height of −1 mile. In both cases, Denver is one mile higher than sea level. If one is trying to lift a weight from sea level to Denver, it does not matter if one describes the process as moving from −1 mile to zero or from zero to +1 mile because the same amount of work is required. A similar situation occurs for voltage. People often speak of 1 volt or 5 volts or 100 volts, but those numbers are meaningful only if they are compared with something. Just as most people define sea level as zero height, in which case the Dead Sea and Death Valley are at negative heights, most people define “zero voltage” as the electrical potential energy of the earth. Sometimes people who are careful actually refer to this as “earth ground.” In every house, there is a connection to a pipe (usually a copper water pipe) that is in electrical contact with the earth. This represents the “earth ground” value for that house. The voltage delivered to a house is described as the potential difference between the wire from the power company and earth ground. Since all of the houses are connected to the same earth, this earth ground value is universal. Of course, the power company is also attached to earth ground, so most discussion of electric power simply refers to a voltage, even though what is really meant is the voltage with respect to earth ground.

For most electricity delivery systems in the world, the voltage delivered can be expressed as

Voltage Out = Vo sin (2π t/Period + ϕ),

where the voltage out is the potential difference between earth ground and the wire. In the United States, the period is one-sixtieth of a second, so the system completes sixty cycles in one second. In Europe, the period is one-fiftieth of a second, so the system completes fifty cycles in a second. Such systems are called alternating current or AC systems: the direction of the current alternates with time because the voltage oscillates between positive values, which pull electrons from the ground toward the power plant, and negative values, which push electrons away from the power plant toward the ground.

INSIDE THE HOUSE

In order to use delivered electricity, people plug devices into wall plugs. Figure 7.1 is a picture of a typical grounded U.S. wall plug. Each outlet has three holes: two long vertical holes and one hole that looks like a D tipped on its side. The D is the earth ground. The little slot is the “hot,” which means that its potential difference with respect to the ground is given by the equation above. The longer slot is called neutral. The size difference between the neutral and “hot” slots makes it impossible to connect an electrical device backward with the hot connected to the neutral. This is a safety feature to avoid electrical accidents.

When something is plugged into a wall socket, the electrons flow from the “hot” and move through the electrical device. The electrons that enter a device from the “hot” wire leave the electrical device through the neutral wire.

For the United States, the voltage difference between the hot and ground in a wall plug is described by the following equation:

Voltage = 170 sin (2π 60t + ϕ),

which is indicated by the red line in Figure 7.1. For convenience in electrical calculations, the voltages supplied by the power company are usually described in terms of their root-mean-square value, which is their peak value divided by the square root of 2. For the United States, this value is 120 volts, which is why most people think of the United States as a 120-volt system. In contrast, Europe uses a 240-volt system, which means a peak-to-peak excursion of more than 600 volts. This high European voltage used to pose serious problems for Americans traveling in Europe because electrically powered equipment designed for one place would often not work correctly in the other. Worse, the systems could sometimes fail catastrophically, destroying the device and potentially injuring the user. Now most electronic devices use digital circuitry to manage input voltages, so computer power supplies and cell phone chargers can be plugged in either in Europe or in the United States without difficulty. All that is required is a different plug; however, it is useful to know that when plugging a U.S. device into a European circuit the U.S. plug should be inserted into the converter before the converter is inserted in the European plug. The reverse order frequently results in rather unsettling spark generation.

Figure 7.1 Illustration of electric power delivery to U.S. consumers. The upper image shows a standard U.S. grounded plug, highlighting the connections for the ground, neutral, and hot wires. The lower image shows voltage differences as a function of time. The red line shows the difference between the hot and the neutral, and the black line shows the difference between ground and neutral. The horizontal gray line shows the output of a typical solar cell, which does not change with time. The approximately 17-volt constant solar panel output appears as a straight line just above zero volts. Clearly, converting solar panel outputs into wall plug outputs requires additional processing, which is part of the cost of a solar panel system.

In contrast with the time-varying voltages characteristic of the generating systems that exploit Faraday’s law, the solar panel output varies with the amount of sunshine hitting the panel, over the less than one-second time scale shown in Figure 7.1. The output of a solar panel is effectively constant over minutes, though it changes slowly over a day as the sun rises and sets. Such constant current sources are called direct current, or DC sources. A typical voltage from one individual cell is approximately 0.5 volts; however, in commercial solar panels individual cells are combined to provide somewhat higher voltages. Outputs of 12 to 24 volts are not uncommon. The gray blue line in Figure 7.1 shows a 17-volt constant output versus time for a typical solar panel. Obviously, this output voltage is not the same as that provided by wall plugs. Thus, if one wants to replace the power provided by U.S. power plants with the power generated from a solar panel on a building’s roof, additional electronics will be required to convert around 12 volts of fairly time-independent voltage into a time-varying voltage that reaches 170 volts sixty times a second.

Wind sources use rotating shafts and Faraday’s law to create voltage that varies in time as the shaft rotates; therefore, like fossil-fuel-burning electrical power stations and hydroelectric power plants, wind turbines naturally produce a time-varying voltage. In wind-driven systems, the faster the wind is blowing, the faster the generator shaft rotates, increasing the number of voltage cycles per second. Of course, one can use gear boxes and other techniques to reduce the variation with wind speed, but if wind power is to be supplied to the U.S. power grid and delivered to customers over the same wires as power generated by fossil fuel–burning plants, then the voltage generated by the wind turbines has to have the same time dependence as the voltage from the fossil fuel plants. In practice, it is often easier to convert the wind-generated voltage from a time-varying or alternating current (AC) signal to a constant-voltage or direct current (DC) signal. The power can then be shipped as DC and converted back to AC to match the fossil fuel plants that contribute power to customers. A later section of this chapter will consider different long-distance electricity-delivery strategies, but the next section will consider the last interface between the power company and an individual consumer.

JUST BEFORE THE HOUSE

In the United States, power is transported to customers at much higher voltages than the 120 volts that are characteristic of wall plugs inside houses. As will be discussed in the next section, high voltage transport is used because the losses in the wires decrease as the voltage increases. At present, in the United States, the delivery voltages are usually 7,200 or 14,400 volts, but there is a push to increase those voltages in order to increase energy efficiency. In order to be useful to consumers, the high voltage that passes through the wires from the power plant must be converted into a 120-volt source for U.S. wall plugs. The devices that make this voltage change are called transformers.

Transformers use Faraday’s law, which says that a time-changing magnetic field flux through the area enclosed by a wire loop generates an electrical current that will flow around the wire loop in which the magnetic field flux is changing.¹ The time-changing magnetic flux in the secondary loop causes current to flow in the secondary loop, as illustrated below:

Time-changing current from electrical company

→ time-changing magnetic field in the space occupied by the secondary loop

→ time-changing magnetic field flux through secondary loop

→ current flows in the secondary loop in accordance with Faraday’s law.

The two wire loops are not electrically connected. Faraday’s law allows power to be transmitted between the coils without the coils touching. Since Faraday’s law depends on a time-changing field, such a power transfer system works only in AC systems. This is the wonder of AC power systems. It is possible to take electrical power from a wire without ever making electrical contact with the wire. Shorting out the secondary loop does not short out the primary loop. This is extremely important. It means that it is easy to connect and disconnect people from the power grid since the wires that are connected to the power plant are never touched. DC systems must use a different strategy.

One can control the ratio of the voltage in to the voltage out by controlling the number of loops in each wire. The voltage out is equal to the voltage in times the ratio of the number of loops in the secondary loop divided by the number of loops in the primary. If the secondary has fewer loops, the output voltage is smaller. If it has more loops, the voltage is higher.

If the device were perfectly efficient, the power in would be equal to the power out. The electrical pow...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Contents
• Introduction: U.S. Energy Use—Past, Present, and Future
• I. Foundations of a Renewable Future
• II. Renewables Are Enough
• III. Energy Links
• Conclusion: A Renewable Future
• Appendix A: Carnot Efficiency
• Appendix B: Electricity from Heat
• Appendix C: Recommended Steps toward a Renewable Future
• Acknowledgments
• Index