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Chapter 7—Asynchronous Loads 7–35 gas in the line as their fuel. Typical line pressures are 4 to 5 MPa. To convert this to the English unit of pounds(force) per square inch (psi), we note that 1 psi = 6894.76 Pa, and find equivalent line pressures of 600 to 750 psi. A typical 0.91-m (36-in.) pipeline has a capacity of about 11,000 MW on an equivalent energy basis. That is, a pipeline of this size will transport about 10 times as much energy per hour as a three-phase 500-kV overhead transmission line. It requires less land area than the overhead lines and is more accepted by people because it is basically invisible. These factors combine to make the unit costs of energy transportation by pipeline much lower than for overhead transmission lines. Hydrogen has about one-third of the heating value per unit volume as natural gas. This means that a hydrogen volume of about three times the volume of natural gas must be moved in order to deliver the same energy. The density and viscosity of hydrogen are so much lower, however, that a given pipe can handle a hydrogen flow rate of three times the flow rate of natural gas. Thus where existing pipelines are properly located, they could be converted to hydrogen with the same capacity to move energy. Different compressors are required, however, to pump the lower density hydrogen. We have examined several asynchronous loads in this chapter, all of which involve some form of energy storage. Water is pumped and stored until needed. Heat is stored in the form of hot water. Wind generated electricity is stored in chemical form in batteries. Wind generated electricity can also be passed through electrolysis cells to produce hydrogen. Hydrogen can be stored for long periods of time and also transmitted over great distances. Technical and economic constraints indicate that only large facilities will be practical for hydrogen production, which is distinctly different from the cases of water pumping, space heating, and even battery charging. We shall examine some of the features of electrolysis cells as asynchronous loads for wind generators in the next section. First, however, we shall consider some of the properties of other fuels, to aid us in making the economic decisions which must be made. Table 7.2 shows the energy content of several different fuels, some of which are not extensively used for generating electrical power but are included for general interest. Both English and SI units are given since the British Thermal Unit (Btu), pound, and gallon are so deeply entrenched in the energy area. Anyone who would read the literature must be conversant with these English units so we shall present a portion of our discussion using these units. The energy content or heating values given are all the higher heating values. To explain this term, we recall that water vapor is one of the products of combustion for all fuels which contain hydrogen. The actual heat content of a fuel depends on whether this water vapor is allowed to remain in the vapor state or is condensed to liquid. The higher heating value is the heat content of the fuel with the heat of vaporization included. The lower heating value would then be the heat content when all products of combustion remain in the gaseous state. In the United States the practice is to use the higher heating value in utility reports and boiler combustion calculations. In Europe, the lower heating value is used. The lower heating value is smaller than the higher heating value by about 1040 Btu for each pound of water formed Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 7—Asynchronous Loads 7–36 Table 7.2 Heating Values of Various Fuels a Btu/gal MJ/L Btu/lb MJ/kg (liquid) (liquid) Hydrogen 63,375 147.3 37,442 10.42 Methane 23,875 55.49 83,945 23.37 Propane 21,666 50.35 104,870 29.20 Gasoline 20,460 47.55 120,000 33.4 Kerosene 19,750 45.90 136,000 37.9 Diesel Oil (1-D) 19,240 44.71 140,400 39.1 Diesel Oil (2-D) 19,110 44.41 146,600 40.8 Diesel Oil (4-D) 18,830 43.76 150,800 42.0 Ethyl Alcohol 12,780 29.70 83,730 23.31 Methyl Alcohol 9,612 22.34 63,090 17.56 Anthracite (Pa.) 12,880 29.9 Low-volatile 14,400 33.5 Bituminous (W. Va.) High-volatile A 14,040 32.6 Bituminous (W. Va.) High-volatile C 10,810 25.1 Bituminous (Ill.) Subbituminous A 10,650 24.8 (Wyo.) Subbituminous C 8,560 19.9 (Colo.) Lignite (N. Dak.) 7,000 16.3 a Source: Data compiled from CRC Handbook of Tables for Applied Engineering Science, first edition, 1970. Reprinted with permission. Copyright CRC Press, Inc., Boca Raton, FL. per pound of fuel. One pound of hydrogen produces about nine pounds of water, so the lower heating value of hydrogen, for example, would be about 63,375- 9(1040) = 54,000 Btu/lb. We see from the table that hydrogen has a very high heating value on a Btu/lb or a MJ/kg basis, but because of the low density of liquid hydrogen, the heating value per liter is lower than the other liquid fuels. Hydrogen becomes a liquid only at temperatures close to absolute zero and the energy required to liquefy it may be on the order of one third of the energy content of the resulting liquid hydrogen. The capital equipment and energy required for liquefaction will probably prevent liquid hydrogen from being used as a fuel except in special applications. These include space flights and large aircraft where the high energy content per kg may help produce significant cost savings. We also see that petroleum fuels have lower energy content per kg as their complexity Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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