Topic: Energy Flow and Matter Cycles Lesson: Energy Systems

Objectives of the lesson Realize that efficiency of different energy convertors vary widely Explain how system efficiency and conversion of energy are related Calculate system efficiency

Energy and Efficiency The efficiency of a machine is the ratio of the desired output (work or energy) to the input. Efficiency (useful energy or work out/energy or work in) 100% The first law of thermodynamics (the conservation of energy) places a limit on how high efficiency can go. Since you can’t create energy, no efficiency can be greater than 100 percent. In most real world situations, efficiencies are much less than 100 percent. For example, in industry, much energy is lost as heat caused by friction. For that reason, industrial engineers try to reduce friction. Their efforts have included machining smooth surfaces, developing new lubricants, inventing better electrical conductors,and so on. Though the losses may be small, they are still there.

Energy and efficiency Any device that converts heat energy into mechanical energy is called a heat engine. Heat engines are important. The internal combustion engine in our cars, jet engines, and the coal- fired steam turbines at electric power plants are all examples. Unfortunately, heat engines are very inefficient.

Efficiencies of various energy converters. Devices that are inefficient are at the bottom of the illustration. Devices with high efficiency are at the top. Note that electric motors, generators and batteries are highly efficient. Heat engines are inefficient devices. Heat engines made of metal must be quickly cooled, which adds to their inefficiency. Devices that are used for lighting are also very inefficient.

Energy Systems and Strategies In modern societies, energy is usually not used in one or two simple steps For instance making a piece of toast requires many steps. Farmers must first plant, fertilize, and harvest grain. The grain must then be shipped, stored, ground, and the resulting flour hauled to a bakery. At the bakery, the flour is made into dough that is baked into a loaf, packaged, and then shipped to a supermarket. Consumers buy the loaves and take them home to make toast. The energy to power the farm equipment, trucks, and cars and the electricity to power the toaster are also produced in many step systems. A system, in this case, is a series of steps designed to produce a useful product or service.

System Efficiency Examine a system for lighting an incandescent light bulb using energy that originally comes from coal

System efficiency The coal that is mined contains a certain amount of energy, but energy was used to mine the coal. This energy must be counted as part of the input. After washing and sorting, the coal must be transported to the power plant. It takes energy to transport coal. This must be added to the input side of the record. Coal (1) is burned in the furnace (2) at the electric power plant. The heat produced is used to create steam. The steam (3) turns a turbine, which turns the generator (4). Each of these conversion steps has its own efficiency. In the end, only about one-third of the energy of the coal at the plant appears as electrical energy (6). The other two-thirds is wasted as heat. It goes up the stack (5) with the hot exhaust gases or into the cooling tower (7). Turbines won’t work unless the pressure is high where the steam goes in, and low where it leaves.

System efficiency The electrical energy must be transported over high-voltage transmission lines (6) and a lower-voltage distribution system It must flow through transformers to step the voltage up or down Energy is lost in transmission as it heats up the conductors and disappears into the surroundings. The remaining energy arrives at the lamp

System efficiency

System efficiency At the lamp, most of the energy is lost in the process of heating the lamp filament to give off light. Only 5 percent of the energy that enters the light bulb is actually converted to visible light. This means only 5 percent of the energy that goes into the bulb lights up the room. This light energy is finally absorbed by objects and converted into heat. The efficiency of the process of using energy to obtain light does not depend just on the efficiency of the final conversion. Instead, it depends on the efficiency of each step of the flow. The cumulative efficiency of the whole process is called the system efficiency.

System efficiency Many of our energy conversion processes require several steps. For example, to produce electricity, we usually mine, crush, and transport coal. Then we burn it to turn a turbine to turn a generator. Finally, the electricity must be brought to our homes by wires. It is then transformed to some end use, such as lighting a bulb. At each conversion of energy along some path, some energy is “lost” in the form of waste heat. Waste heat is discharged into the atmosphere or into our rivers and lakes by the electric utility plants

System efficiency The waste heat is also given off by the wires that carry electricity, lost through our poorly insulated houses and commercial buildings, lost by inefficient furnaces, and Lost to the atmosphere by the inefficient engines of our automobiles. In each conversion of energy in a multi-step process, a “heat tax” must be paid because some of the energy is “lost” as far as future use is concerned. As a result, the overall system efficiency is equal to the product of the efficiencies of the various steps in the process

Energy system efficiency of an automobile

Energy system efficiency of electric lighting

Questions 1 Calculate the system efficiency of operating an electric car from the original energy stored in coal. On a separate piece of paper, write the steps and their percentage of efficiency; then calculate the cumulative efficiency percentage.

Question 2 Calculate the system efficiency for space heating with electric heaters, fuel oil, and natural gas. From a total energy use perspective, which system uses energy most wisely?

Net Energy

Net Energy The net energy of an energy production system is the : Ratio of the total energy produced over the lifetime of the system to the total energy, direct and indirect, used to produce that energy