Topic: Energy Flow and Matter Cycles Lesson: Energy Systems, Net Energy and Energy Quality

Objectives of the lesson Calculate system efficiency State the concept of net energy Define energy quality Review questions on the topics covered in the 1 st quarter

Introduction - 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

Introduction -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, 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

Energy system analysis encourages us to look at each step of the process of energy transfer from source to end-use. In this examination we usually confine our bookkeeping to direct energy input and loss. For example in a coal electricity generation energy system we do not take into account the energy used to build the trucks and tractors, or to build the buildings, transmission lines, transformers, etc. In a nuclear plant energy system, it takes enormous amounts of energy to enrich the uranium in the fuel rods and build the thick stainless steel vessel that surrounds a nuclear reactor. 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

Energy quality High-quality energy is energy that can easily be used to move things or to generate electricity Mechanical and electrical energy is considered to be high in quality energy Organized energy is high-quality energy. “Organized” means that the molecules and atoms of the system under study are behaving in an orderly way Mechanical energy has the highest quality. When a car is in motion all the molecules and atoms are moving forward, at least in part. Electrical energy is also high-quality energy because the electrons in the wires move in a predictable (non-random) way.

Energy quality Energy quality is also high when energy is concentrated. Gasoline and sugar are ranked high on the quality scale; energy is concentrated in their highly structured chemical bonds. Focused sunlight is also high-quality energy. Ambient temperature heat (heat at a temperature near that of the surroundings) has the lowest quality. You can’t move things with it. You can’t cook with it. You can’t even heat with it. Cars and electric power plants are designed to release it into space.

Energy quality The quality of an energy source is a measure of its ability to be used to produce mechanical or electrical energy. It is difficult to rank all sources of energy on a quality scale. A crude ranking would look something like the information in Figure 3.44 below.

Energy quality People use energy to accomplish certain tasks. We want to move the car, watch TV, heat the house, and cook dinner. The task accomplished is called the end-use. It is useful to rank the tasks (or end-uses) we use energy for in terms of quality. It makes sense to match energy sources to end-use tasks. High quality sources should be saved for high-quality tasks. Using high-quality energy to do low-quality tasks is wasteful, both from an energy and an economic point of view. One can kill flies with a sledge hammer, but much effort has been wasted