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Energy Fundamentals 
Energy Conversion 
In many cases useful work is done when energy is converted from one form to another.  Any device that effects such a transformation called an energy convertera device that enables energy to be changed from one form to another, enabling work to be done in the process.a device that enables energy to be changed from one form to another, enabling work to be done in the process. or an energy conversion device (Figure 1).
	
	Figure 1 Energy Conversion
 
Figure 2 provides examples of a wide range of energy conversions.  Plants convert electromagnetic (light) energy into chemical potential energy (glucose) in the process of  photosynthesis. Heterotrophic organisms such as animals, fungi and most bacteria convert the chemical energy produced by plants into useful work (growth, locomotion) in the process of respiration.  In society, a  lamp converts electrical energy into electromagnetic (light) energy; a toaster converts that electricity into heat. Other important energy transformations involve multiple conversions.  In coal-fired power plant, a furnace converts the chemical potential energy into heat, a boiler converts that heat into steam, a turbine converts the steam into mechanical energy, and a ggenerator converts that mechanical energy into electricity.
	
	FIGURE 2 Energy Conversion Matrix
 
Efficiency of Energy Conversion
Every energy conversion can be characterized by its efficiency, which is the ratio of the useful energy or work output to the energy input:
 
 
Note that efficiency is a quantitatively dimensionless number; it is a number between 0 and 1, or between 0 and 100 percent. The larger the number, the greater the efficiency.  Efficiency cannot exceed 100 percent because such a device would violate the First Law of Thermodynamics.  
The key word in the definition of efficiency is useful;  if we did not specify the work output as useful then the efficiency of any device would be 100 percent, because the law of conservation of energy instructs us that energy cannot be created or destroyed.  The intended use of a device defines what is '"useful."  For example, the purpose of an incandescent bulb is to provide illumination; that is the useful work out.  The input of 100 J of electricity to an incandescent bulb produces about 5 J of light and 95 J of waste heat.  Thus, the efficiency of an incandescent bulb is 0.05 or 5 percent.  Figure 3 illustrates how efficiency is calculated for some common types of energy conversion.
	
	FIGURE 3.  Efficiency of Energy Conversion and Energy Transfer  The term "isentropic" means constant entropy. An isentropic process is an idealization of an actual process, and serves as a limiting case for an actual process.  Source: Adapted from Weston (1992). 
The efficiency of a machine that transmits mechanical power is measured by its mechanical efficiency, the fraction of the power supplied to the transmission device that is delivered to another machine attached to its output (Figure 3a). Thus a gearbox  for converting rotational motion from a power source to a device driven at another speed dissipates some mechanical energy by fluid and/or dry friction, with a consequent loss in power transmitted to the second machine (Radovic, 1997).  The efficiency of the gearbox is the ratio of its power output to the power input.  For example, wind turbines have a gear box that connects the rotor shaft to the electrical generator (Figures 4 and 5).  The gearbox increases the rotational speeds from about 15-20 rotations per minute (rpm), the turning speed of the blades, to about 1,000-1,800 rpm; this is the rotational speed required by most generators to produce electricity.  The best wind turbine gearboxes convert mechanical energy into mechanical energy about 98 percent efficiency, the remaining 2 percent being lost to friction.
	
	
	FIGURE 4.  Schematic of a Wind Turbine.  Source: NREL
	FIGURE 5.  Schematic of a Wind Turbine.   Gear box for a 1.5 MW wind turbine. Source: NKE
 
	
	FIGURE 6.  Kaplan Turbine.  Source: Army Corp of Engineers
Another type of efficiency measures the internal losses of power in turbomachinery, machines that transfer energy between a rotor and a fluid, including both turbines and compressors (Figure 3b).  A turbine transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid.  Note that "fluid" in this case refers to the flow of gases and liquids.  Here the efficiency compares the output with a theoretical ideal in a ratio resulting efficiency ranges from 0 to 1 as a measure of how closely the process approaches an ideal or "perfect" process, i.e., one in which no energy is lost to friction or dissipative effects.  A turbine with an efficiency of 0.6, will, for example, deliver 60 percent of the power of a perfect turbine operating under the same conditions. Water turbines are used in hydroelectric plants to convert power in flowing water into the mechanical energy of a rotating shaft, which in turn drives a generator.  The Kaplan turbine shown in Figure 6, a common turbine used in hydroelectric plants, typically operates at about 90 percent efficiency.
The efficiency of a heat engine is determined by the amount of heat energy required to produced a given amount of work output (Figure 3c). Some of the heat energy input is exhausted as "waste heat." Heat engines extract energy from a hot source (e.g. hot combustion products in a car engine).  The engine does work on its surroundings and waste heat is rejected to a cool reservoir (such as the outside air). The efficiency of a heat engine is the ratio of the work done to the heat extracted from the hot source.  Well-known examples of the heat engine include  the steam engine, the diesel engine, and the gasoline engine in an automobile.  Heat engines such as automobile engines operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle.  The actual efficiency of a heat engine can be compared to its Carnot efficiency, which can be thought of as the most efficient heat engine cycle allowed by physical laws.
Efficiency of Energy Transfer
Refrigerators, air conditioners, and heat pumps are known as heat transfer devices becasue they just transfer the same form of energy (heat) from one place to another. We call them "heat movers" because they are heat engines that "pump" heat from colder to hotter bodies.  This always requires work because it opposes the spontaneous tendency for heat to flow from a warmer to colder body.  In effect, these devices pump heat "uphill" from the lower temperature body to a higher temperature body. 
	
	VIDEO:  How a Refrigeration Cycle Works
The efficiency of these heat movers is called the coefficient of performance (COP) (Figure 3d).  It is defined in the same way as the efficiency of an energy conversion device, namely the ratio of the useful energy output to the energy input (Figure 3d).  But there is one important difference between energy conversion devices and energy transfer devices. In a conversion device, only a portion of the energy input is obtained as useful energy output, and the efficiency is necessarily a number between zero and one. In a transfer device, the useful energy output is the quantity of heat extracted from the lower temperature body, and this is not a portion of the energy input. In fact, the useful energy output can exceed the energy input, and this is why heat pumps can be extremely attractive for space heating purposes. So the COP (sometimes also called “energy efficiency ratio”) can be a number larger than one, but this does not violate the First Law of Thermodynamics.  Obviously, the larger the COP, the more efficient the heat mover will be.
Examples of Energy Conversion Efficiencies
Table 1 lists the average efficiency for a range of energy conversion process in nature and in society.  The laws of thermodynamics explain some of the ordering in the table.  The
"easiest" conversions are those that are in the direction of increasing entropy, and in particular those that produce heat (thermal energy) (Radovic, 1997).   Vigorously rub your hands together and you demonstrate the relative ease of converting mechanical energy into heat. For this reason the electric drier and the electric heater are very efficient.  Thermal electric power plants are relatively inefficient because they require a sequence of energy conversions that "waste" much of the energy in the fuel to the environment as heat.  This important energy conversion process is discussed below in greater detail.
 
Heat Engines and System Efficiency
The Industrial Revolution was driven by a heat engine—the steam engine—and it continues to play an important role in society.  Most heat engines are powered by fossil fuels such as oil, gas, or coal, but to do so they must overcome a fundamental constraint:  the energy in fossil fuels is (chemical) potential energy, so it must be converted into other forms in order to use it.  The potential energy energy is converted into heat in the process known as combustion, which in turn is used in a heat engine to obtain work.  There are two steps in this overall process (Figure 7).  In the first, heat is produced.  Then in the second, the heat is converted to useful work.  System efficiency is the term that describes the overall efficiency of these sequential energy conversions.
	
	FIGURE 7.  Energy Conversion in a Heat Engine.  Source: Adapted from Radovic (1997)
The efficiency of a system is equal to the product of the efficiencies of the individual devices (sub-systems).  if the efficiency of the conversion of chemical energy to thermal energy is 0.7 and the efficiency of the conversion of thermal energy to mechanical energy is 0.5, then the system efficiency of converting chemical energy to mechanical energy is (0.7) x (0.5) = 0.32, or 32 percent.  Note that the overall efficiency is always lower than any of the efficiencies of the individual components.
System efficiency is illustrated by the energy conversions in thermal electric power plant, a power plant in which the prime mover is steam-driven (Figure 8). Almost all coal, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power plants, are thermal stations.
	
	FIGURE 8.  Energy Conversion in a Thermal Power Plant.  Source: Radovic (1997)
In a thermal power station, the chemical energy in the fuel is first converted to thermal energy in the boiler; thermal energy is then converted to mechanical energy in the turbine; finally, mechanical energy is converted to electricity in the generator.  These three sub-systems are shown in Figure 9.
 
	
	FIGURE 9.  Energy Sub-systems in a Thermal Power Plant.  Source: Adapted from Radovic (1997)
 
The system efficiency of a thermal power plant (ηpower plant) is the product of the efficiencies of the individual devices (boiler, turbine, generator):
Typical efficiencies for the boiler (0.88), turbine (0.4), and generator (0.98) produce an overall efficiency of (0.88) x (0.4)x (0.98) = 0.35 (35 percent). Thus, only 35 percent of the energy in the fuel input is converted to electricity, with most of the the rest released to the environment as heat.
 
Efficiency of Electricity Conversion Technologies
A wide range of technologies are used to generate electricity, and each has a characteristic range of efficiency (Figure 10).
	
	FIGURE 10.  Efficiency of Electricity Conversion Technologies.  Source:  EURELECTRIC
In electric power generation, a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators (Figure 11). The principle is that the exhaust of one heat engine is used as the heat source for another, thus extracting more useful energy from the heat, increasing the system's overall efficiency. This works because heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). In an ordinary (non combined cycle) heat engine the remaining heat (e.g., hot exhaust fumes) from combustion is generally wasted.
	
	FIGURE 11.  Combined Cycle Power Generation
 
History of Energy Efficiency
One of the hallmarks of energy conversion is a continuous improvement in the efficiency of the major prime movers (Figure 12).
	
	Figure 12.  Efficiency of Prime Movers.  Source:  Data from Smil (1991).
Improvements in efficiency manifest themselves in two ways.  The first is the improvement in the conversion efficiency for individual devices, as evidenced by the approximate doubling of steam engine efficiency in the second half of the nineteenth century.  The second is the continuous substitution of more efficient prime movers for their less efficient counterparts.  More recent devices such as the gas turbine have greater efficiencies compared to the steam engine and the internal combustion engine.
Greater efficiency in conversion produces a number of economic and environmental benefits.  The cost of fuel, whether it is wood, coal, oil, or natural gas, to effect a specific energy conversion decreases with increases in efficiency.  Other factors held constant, this lowers the overall cost of production. Greater efficiency also reduces the quantity of harmful emissions per unit of fuel input, and hence per unit of economic output as well. The depletion of forests and fossil fuels is also reduced when the efficiency of energy conversion improves.
History of the Power Output of Energy Converters
A second hallmark of energy conversion is the increase in power output of prime movers (Figure 13).
	
	Figure 13.  Power Output of Prime Movers.  Source:  Data from Smil (1991).
As was the case for the efficiency of prime movers, improvements in power put manifest themselves in two ways. The first is the increase in power for individual devices, as evidenced by the dramatic (orders of magnitude) advances in power for every major device. The second is the substitution of more powerful devices for their less powerful counterparts.  The power of individual steam engines passed waterwheels by about 1730; steam turbines rapidly replaced the steam engine in the first decade of the twentieth century.  The largest prime movers of the late twentieth century are about 200,000 times more powerful than their counterparts in 1700 (Smil, 1991).
It would be difficult to overstate the economic and social impact of the enormous increase in the power of prime movers (Hall et al., 1986; Smil, 1991). More powerful prime movers increase the rate at which economic work is done, i.e., the rate at which goods and services are produced.  Many of the dominant features of the industrial world—the spatial concentration of production and consumption activities, the rise in personal mobility, the growth of global trade, and the expansion of agricultural productivity—were made possible by the exponential increase in the power of prime movers (Smil, 1991).
One of the connections between prime movers and economic performance is labor productivity.  Energy conversion devices serve to enhance the productivity of labor, whether it be in the field, the mine, the factory, or the office. The "energy subsidy of labor" is the term used to describe this effect. The productivity of labor—the rate at which people produce goods and services—depends on the rates at which people can do economic work (output per unit time). In a modern economy the rate at which a laborer can do economic work depends on the types and quantities of machines that enhance that laborer’s effort, as well as the types and quantities of material used by the machines. In general, worker productivity is increased by enhancing human muscle power with machines that increase the amount of work each laborer can do, and by supplying those machines with natural resources that are easier to transform into goods and services. Developed nations are affluent
because they supplement human muscle power with powerful machines and diverse, sophisticated materials. Developing nations are poor because they depend more on the muscle power of people and animals to manipulate a relatively small quantity of simple materials.
Machines that have greater power are more productive than those with less power. For much of history, human labor powered very simple machines.  Examples include the use of a bow and arrow to hunt food or the use of a hoe to prepare a field for planting crops. But the human body has a limited capacity to do physical work.  A healthy adult can generate only about 75 W for sustained periods of work.
	
	FIGURE 14.  Draft Animals  Animals such as the ox in Tanzania improve the productivity of labor. Source: Maendeleo Agricultural Technology Fund
To increase labor productivity, humans developed technologies powered by domesticated animals. Humans breed and raise draft animals to help themselves do work (Figure 14). These are animals such as horses, oxen, and water buffalo that are used to pull heavy loads. Draft animals can convert greater quantities of food energy to useful work faster than humans, so they are more powerful than humans. Productivity rose when people learned how to make draft animals do tasks that are too strenuous or time-consuming for humans, such as plowing a field or pulling a cart. 
A recent demonstration project in Tanzania demonstrates how animal power is still being used to improve life in many developing nations (Figure 14).  Ox were introduced in several regions where planting, weeding and other work had been done entirely by hand. Ox-weeding alone reduced labor input per field by 25 to 35 hours, enabling farmers to expand crop production an average of seven to eight acres. Farmers who adopted the ox were also able to divert some land to sunflower and vegetable crops that generated extra income.  Maize yields increased from 324 to 1,188 kg per acre due to improved seed varieties and better field management made possible with implements powered by the ox.
Another boost in productivity occurred when humans developed machines such as the paddle wheel and the propeller to harness water and wind energy. Harnessing inanimate energy sources made it possible to build more powerful machines, such as large grain mills and sailing ships.
Though these changes were important at one time, they pale in comparison to the increases in work that were made possible when humans learned to harness fossil fuels and their associated prime movers:  the steam engine, the internal combustion engine, the steam turbine, and the gas turbine.
The effect of a larger energy subsidy for labor is illustrated by the changes that took place in agriculture (Figure 10.15). Using only a hoe a single farmer may need 400 hours to till one hectare (2.5 acres). By hitching oxen to a plow a single farmer can prepare the same hectare in about 65 hours. This change represents a sixfold increase in the farmer’s productivity. When attached to a tractor, a 50 horsepower internal combustion engine allows a farmer to prepare the same hectare in just 4 hours, a hundredfold increase in productivity. Similar improvements in productivity have occurred in every other sector of the economy when technologies capable of using fossil fuels have become widely available.
	
	FIGURE 15.  Energy Subsidy of Labor  The effect on labor productivity of increasing energy use per laborer in agriculture. Data from D. Pimentel and M. Pimentel, Food Energy and Society, Colorado University Press; Figure from Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).
 
Summary of Key Concepts
· An energy converter is a device that enables energy to be changed from one form to another, enabling work to be done in the process.
· The general definition of the efficiency of energy conversion is the ratio of the useful energy or work output to the energy input.
· What qualifies as "useful energy" in the context of energy efficiency is determined by the intended use or purpose of the device in question.
· The efficiency of a machine that transmits mechanical power is equal to the fraction of the power supplied to the transmission device that is delivered to another machine attached to its output.
· The efficiency of a heat engine is determined by the amount of heat energy required to produced a given amount of work output.
· In an energy  transfer device such as a refrigerator, the useful energy output is the quantity of heat extracted from the lower temperature body.
· The efficiency of a heat engine system is equal to the product of the efficiencies of the individual devices (sub-systems). 
· A combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators.
· Improvements in efficiency manifest themselves in two ways: improvement in the conversion efficiency for individual devices, and the continuous substitution of more efficient prime movers for their less efficient counterparts.  
· Improvements in the power of prime movers manifest themselves in two ways:  the increase in power for individual devices, and the substitution of more powerful devices for their less powerful counterparts.  
· The "energy subsidy of labor" is the term used to describe how energy conversion devices serve to enhance the productivity of labor.
Glossary 
· energy converter: a device that enables energy to be changed from one form to another, enabling work to be done in the process.
Publishing Information 
Published: January 23, 2017, 11:51 am
Author: Cutler J. Cleveland 
	Topics: 
	Energy Fundamentals 
Citation: Cleveland, C. (2017). Energy Conversion. Retrieved from http://www.trunity.net/energyandsustainability/view/article/51cbf4f87896bb431f6b0e28 
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