The energy conversion chain

Every time we use energy, whether it's to heat our home, or fuel our car, we are converting one form of energy into another form, or into useful work. In the case of home heating, we are taking the chemical energy available in natural gas, or fuel oil, and converting that into thermal energy, or "heat," by burning it in a furnace. Or, when we drive our car, we are using the engine to convert the chemical energy in the gasoline into mechanical work to power the wheels. These are just two examples of the "Energy Conversion Chain" which is always at work when we use energy in our homes, offices, and factories, or on the road. In each case we can visualize the complete energy conversion chain which tracks a source of "primary energy" and its conversion into the final end-use form, such as space heating or mechanical work. Whenever we use energy we should be aware of the fact that there is a complete conversion chain at work, and not just focus on the final end-use. Unfortunately, many proposals to change the ways in which we supply and use energy take only a partial view of the energy conversion chain, and do not consider the effects, or the costs, that the proposed changes would have on the complete energy supply system. In this chapter we will discuss the energy conversion process in more detail, and show that some proposed "new sources" of energy are not sources at all, and that all energy must come from only a very few "primary" sources of energy.

A schematic of the global "energy conversion chain" is shown in Figure 2.1. Taking a big-picture view, this chain starts with just three "primary" energy sources, and ends with only a few end-use applications such as commercial and residential building heating, transportation, and industrial processes. Taking this view, our need for energy, which can always be placed broadly into one of the four end-use sectors shown on the far right in Figure 2.1, anchors the "downstream" end of

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Energy Conversion Chain
Figure 2.1 The energy conversion chain.

the conversion chain. This energy need is always supplied, ultimately, from one of the primary sources of energy listed on the far left-hand side of the diagram. In between the primary source and the ultimate end-use are a number of steps in which the primary source is converted into other forms of energy, or is stored for use at a later time. To take a familiar example, in order to drive our car, we make use of a fossil fuel, crude oil, as the primary energy source. Before this source provides the motive power we need, however, the crude oil is first "processed" by being converted into gasoline in an oil refinery, shown in the second step in Figure 2.1. The result of this processing step is the production of a secondary form of energy, or what is usually called an energy "carrier." Also, in this step there is usually some loss of energy availability in the processing step, as indicated by the branched arrow joining the processing block to the energy carrier block. There are, again, relatively few energy carriers, as shown in the third step of the diagram. Broadly speaking, these are refined petroleum products (gasoline in our car example), electricity, natural gas, and potentially, hydrogen. Once the primary source has been converted into the carrier of choice, it is usually stored, ready for later use in the final energy conversion step. In our automobile case, the gasoline is stored in the fuel tank of the vehicle, ready for use by the engine. When we start the engine, and drive away, the final step in the energy conversion chain is undertaken. This is the final end-use conversion step in which the chemical energy stored in the gasoline is converted into mechanical work by the engine to drive the wheels. In this step there are usually large losses of energy availability, due to the inherent inefficiencies of the end-use conversion step, and this is again indicated by the branched arrow in this step. If this step is representative of an automobile engine, for example, these energy losses may be on the order of two-thirds of the energy in the gasoline. This is, of course, just one example, but any energy-use scenario can always be followed through the complete energy conversion chain illustrated in Figure 2.1. In some cases, not all steps in the chain are required, but energy end-use can always be traced back to a primary energy source. For example, in most cases when electricity is the energy carrier it is used immediately upon production, due at least in part to the difficulty of storing electricity.

One striking lesson to be learned from Figure 2.1 is that there are only three primary sources of energy: fossil fuels, nuclear energy, and renewable energy. This means that every time we make use of an energy-consuming device, whether it is a motor vehicle, a home furnace, or a cell-phone charger, the energy conversion chain can be traced all the way back to one (or more) of these three main sources of primary energy. Also, in today's world there is currently very little use made of renewable energy (with the notable exception of hydroelectric power) as a primary energy source, so realistically we can almost always trace our energy use back to either fossil energy or nuclear power. And, finally, since nuclear power provides only a small fraction of the total electrical energy being produced today, fossil fuels are by far the most important source of primary energy. Fossil fuels can be broken down into three main sub-categories: coal, petroleum (or crude oil), and natural gas. Today, coal is a significant primary source of energy for electrical power generation, as is natural gas, while petroleum provides the bulk of the primary energy used to power our transportation systems. It can also be seen from Figure 2.1 that there are only three energy carriers that are of significance today; refined petroleum products, natural gas, and electricity. Hydrogen, often billed erroneously as an energy source of the future, is in fact an energy carrier, and not a primary source of energy. We shall discuss this issue in more detail in a subsequent chapter, but for the moment we simply show it as a possible energy carrier, as it is not presently used in this way to any significant degree.

Another important feature illustrated in Figure 2.1 is the release of emissions, both in the initial processing step and in the final end-use conversion step. Again using the automobile example, these are primarily in the form of carbon dioxide (CO2), carbon monoxide (CO), unburned hydrocarbon gases (HCs), and nitrogen oxides (mainly NO and NO2, but usually just described as NOx). Some of these are released in the refining process, but most of them are released during the final conversion from chemical energy to useful work in the vehicle engine. This emission of pollutants from both the primary energy processing step, and the end-use step, provides an extremely important link between energy use and the environment. The reaction of unburned hydrocarbons and NOx, in the presence of sunlight, for example, is responsible for smog formation, which has become a major problem in urban centers. This has been alleviated somewhat in the developed world by the introduction of stringent regulations to limit emissions from vehicles and power stations, but will continue to be a very serious problem with the growth in vehicle ownership, particularly in large developing economies.

The emission of CO2, on the other hand, results in a quite different environmental problem; global warming brought about by the "greenhouse effect.'' We will discuss this effect in more detail in the next chapter, but will simply note here that the CO2 molecules (and other greenhouse gases, such as methane) act like a selective screen, or "blanket," which allows short-wavelength radiation from the sun to pass through to warm the earth, but trap the longer wavelength energy which is normally re-radiated back out into space by the earth. This provides a net gain of energy by the earth's atmosphere, so that over time the global temperature increases. Although this has been somewhat controversial in the past, most scientists and observers now agree that global temperatures have increased by approximately 0.75 °C over the past 200 years, primarily due to anthropogenic, or man-made, increases in CO2 concentration in the atmosphere. This concentration is some 370 parts per million (ppm) today, and has risen from a long-term average of 280 ppm before the industrial revolution of the eighteenth century. The Intergovernmental Panel on Climate Change (IPCC) has suggested that by the end of the twenty-first century the global concentration of CO2 will be somewhere between 550 and 900 ppm, resulting in an increase in the average global temperature of between 1.4 and 5.8 °C. The consequences of such a large increase in average global temperature are somewhat uncertain, but it is quite likely that it would result in a shrinkage of the polar ice caps and a spread of severe drought conditions in some areas of the world. The IPCC has also suggested that the global mean sea-level could increase by between 0.1 m and 0.9 m by the end of the century, which, at least at the high end of the estimate, could have very serious consequences for coastal communities. Of course global warming could also mean an extension of the growing season in some parts of the world, so there may even be some positive benefits. The consensus appears to be, however, that any significant global warming would result in serious environmental degradation in many vulnerable parts of the world.

The energy storage block depicted in Figure 2.1 is not an energy conversion process, but it is a critical part of many energy systems. In many cases it is necessary to store the energy in its intermediate form as an energy "carrier" before the final end-use step. In such cases it is simply not practical to use the energy directly as it is produced in the initial conversion from primary energy to energy carrier. This is the case for the automobile, of course, as it would be completely impractical to feed a continuous supply of gasoline from the refinery to the vehicle's engine. The intermediate energy carrier is therefore stored after manufacture, often in several different stages, before ending up in the automobile's fuel tank. For example, gasoline is usually first stored in large tanks at the refinery, then transferred by delivery tanker trucks for secondary storage at filling stations, and finally pumped into the vehicle fuel tank when required. In fact, one of the major benefits of gasoline (or any liquid hydrocarbon fuel) is that it is easily stored, and has a very high "energy density," as we shall see later. Electricity, however, is quite difficult to store in large quantities, and it normally moves directly as an energy carrier to the final end-use conversion step. In this case the final end-use conversion is usually done by an electric motor, or a resistor-type heating element, and these are directly connected, through the electricity distribution system, to a generator at a power station. Because electricity can be moved through wiring efficiently over long distances, storage is not a requirement for fixed applications in our homes, offices, and factories. For transportation applications, however, other than for electric trains, or trolley buses, the storage of electricity is a major challenge. Batteries are very effective for small-scale application of electricity to devices such as laptop computers and other portable electronic devices, but do not yet have sufficient energy storage density for widespread application to electric cars, for example. We will examine this challenge in more detail in a subsequent chapter.

Another feature of the energy conversion chain is the loss of some "usable" energy during every processing step. Although the laws of thermodynamics tell us that energy is always conserved, and is neither created nor destroyed, some of it becomes unavailable to us at each step in the conversion chain. This "unavailable" (or "lost")

energy usually ends up as low-temperature "waste-heat," and although this is still a form of energy, it is not technically or economically feasible to use it. If we again look at the case of the automobile, for example, usable energy is lost during the processing of crude oil in the refinery to produce gasoline, and again in the conversion of the chemical energy in the gasoline into useful mechanical work by the engine. This loss of usable energy, a consequence of the laws of thermodynamics, is usually quantified by an "efficiency" value, which is the ratio of usable energy produced, or work done, in an energy conversion process to the total energy available at the beginning of the process. In the case of the automobile the efficiency of conversion of crude oil into gasoline at the refinery is approximately 85%, while for conversion of the chemical energy in the fuel into mechanical work by the engine and drivetrain it is only about 20%. In other words, starting with 100 units of primary energy (usually measured in kilojoules, kJ) in the form of crude oil, we end up with 85 kJ of energy in the gasoline. When the gasoline is burned in the engine to produce mechanical power (the rate of doing work, measured in kW), this 85 kJ produces only 17 kJ (20% of 85 kJ) of useful work at the wheels. The overall energy efficiency of this process, from primary source to end-use, is therefore only 17%. The end result is that when we drive a typical car, some 83% of the primary energy ends up as "unavailable" energy, mostly in the form of low-temperature heat being rejected from the car radiator and exhaust gases, and from the refining process at the oil refinery.

This overall efficiency that we have just described, starting with the energy available at the primary source, and ending with the useful energy that we need to propel our car, or heat our homes and factories, is sometimes called the "well-to-wheels" efficiency, with obvious reference to the motor vehicle example we have just discussed. When comparing the performance of different approaches to meeting a particular end-use, whether it is an automobile, or a coal-fired powerplant, it is this "well-to-wheels" efficiency that is the best measure of the overall energy system performance. This efficiency describes the overall performance of the complete energy conversion chain, starting from the primary energy source and ending with the end-use application. A graphical illustration of this approach, using an "energy flow diagram," is sometimes very helpful, particularly for analyzing complex systems with multiple energy inputs and multiple end-uses. An example of such a diagram for the very simple case of the automobile that we have just discussed, is shown in Figure 2.2. The energy flow diagram, or Sankey diagram as it is often called, was first used by the

Crde Oil Oil Gasoline 100 k Refinery 85 k

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Waste Heat 15

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Tidal Power Simple Sankey Diagrams

Figure 2.2 Simple energy flow "Sankey" diagram for an automobile.

nineteenth century Irish engineer, M. H. P. R. Sankey, to provide a quick visual representation of the magnitude of energy flows in the energy conversion chain. The two energy conversion steps for the case of an automobile using crude oil as a primary energy source are shown as boxes for the oil refinery, which converts crude oil into the gasoline energy carrier, and the engine which converts the chemical energy in the gasoline into mechanical work to drive the wheels. The width of the boxes or arrows representing energy flows are often drawn so that they are proportional to the fraction of total energy flowing in that direction.

A quick inspection of the diagram shows that for every 100 kJ of energy in crude oil that is used the refining process results in 85 kJ of available energy in the form of gasoline, and from this amount of energy the engine produces 17 kJ of useful work to drive the vehicle. The unavailable energy resulting from both these energy conversion steps is shown as "waste heat'' in both cases. In the automobile, most of this waste heat is rejected to the ambient air from the hot exhaust gases and from the engine cooling water by the radiator. We can see, using this diagram as an example, that every time we use energy, our "end-use" is just one part of an extensive "energy conversion chain'' leading back to one of only three primary energy sources. In order to understand the complete effects of our energy end-use on the environment, and on the long-term sustainability of the planet, we need to always consider the complete energy conversion chain. It is not good enough to simply analyze the "link" in the chain closest to our end-use if we are to fully understand the consequences of our energy choices. In subsequent chapters we shall begin to lay the groundwork to enable us to conduct a full "energy conversion chain analysis.'' We will also see the benefit of quickly being able to visualize energy flows using Sankey diagrams such as that shown in Figure 2.2 when we examine the complex flows from primary sources to end-uses for a complete energy economy. The Sankey diagram provides a very useful "snapshot" of the energy conversion chain, and clearly shows where energy is being lost, or converted into unavailable energy. Similar diagrams can be constructed to account for the total flow of energy, from primary sources to end-uses, for complete economies, or even for the total global energy consumption. These are particularly useful in showing the degree to which primary energy becomes "unavailable," or is lost in the form of waste heat. We shall discuss this more general form of the energy flow diagram in Chapter 10, when we look at global energy balances in more detail.

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  • Sampsa Sillanp
    What is the energy conversion chain in a car?
    4 years ago

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