APPROACH
To estimate the amount of heat that may be available for conversion to electric power, the U.S. Environmental Protection Agency National Emissions Inventory Criteria Air Pollutants database (EPA CAP) was analyzed to determine both the number and size of waste heat sources as a function of temperature. The EPA CAP database was chosen for this analysis because it contains exhaust gas temperature, flow rate, industry classifications, and process classifications. This makes it a useful tool for estimating the power that could be generated by recovering the waste heat. The 2002 inventory was used in the study as it was the most recent complete inventory available at the time the analysis was performed. (U.S. EPA 2007)
In addition to being several years old, this source of information has other limitations. First, the database is limited to sources that are reported to the U.S. EPA by individual states. The states in turn gather their data from various local government agencies. As such, the data are subject to variations in quality as well as omissions and redundancies. Another limitation is that duty cycle is not disclosed in the database. Even with these limitations, the authors believe this to be the best available information on industrial waste heat sources.
Using the EPA CAP database as an input, recoverable power (the amount of electricity that can be generated from waste heat) was calculated for each individual source using available waste heat and modifications to the Carnot efficiency under assumptions described below. These estimates were then combined to produce an overall estimate of recoverable power as a function of exhaust temperature.
To reduce material and size related costs of an exhaust gas to working fluid (or thermal oil) heat exchanger, exhaust exit gas temperature was limited to 225°F (107°C) for purposes of the analysis. This constraint both reduces the required size of the heat exchanger and prevents condensation of water on the heat exchanger. Available heat for each source was then calculated based on the temperature difference between the EPA reported exhaust gas temperature and a heat capacity assumed to be 0.018 Btu/ft ·°F (1207 J/m ·°C).
WORKING FLUID CONSIDERATIONS
Two important factors determining the economic viability for ORC based waste heat to power applications are the installation and maintenance cost of the equipment and the amount of heat that can be successfully recovered in the form of electrical power. The choice of working fluid has significant implications for both of these factors. The Rankine cycle uses the phase change of a fluid from liquid to gas to replicate the Carnot cycle as closely as possible. The ultimate efficiency achieved is therefore limited by the high and low temperatures of the working fluid as well as by non-isentropic compression, heating, and expansion and by parasitic losses from pumps, 本文来自辣~文\论|文/网，毕业论文 www.751com.cn 加7位QQ324'9114找源文 . The T-s diagram of several working fluids (Figure 1) illustrates that none of them are ideal. Also, if the working fluid cannot be used at the maximum temperatures available from the heat source due to decomposition of the fluid, efficiency will be reduced. In the case of fluids such as water and ammonia with negative slope of the saturated vapor line, efficiency will be reduced by the requirement for superheat to avoid condensation in the turbine. In cases where the working fluid is operated at temperatures above its critical point, efficiency will be reduced, compared to a fluid which could operate at that temperature without supercritical operation as it more closely replicates the trilateral cycle than the Carnot cycle and reduces overall cycle efficiency (Crook 1994). Choice of working fluid therefore will determine how closely the cycle matches the Carnot ideal as well as the maximum allowable temperature of the cycle. One of the advantages of organic working fluids is that many of these compounds have saturated vapor lines with near vertical or slightly positive slope. This allows expansion through a turbine without the need for superheat. This property is tied to their larger number of atoms/molecule and the larger number of possible configurations that this allows. Note that water, ammonia, and propane, (3, 4, and 11 atoms per molecule respectively) have negative saturation slopes, while all of the other organic molecules (atoms per molecule > 13) shown in Figure 1 have positive slopes. It is also generally true that the enthalpy loss will be lower for a higher molecular weight fluid than a lower molecular weight fluid at a given boiler temperature (Marciniak et al. 1981). This allows for smaller and fewer turbine stages with a higher molecular weight fluid. Adding turbine stages for lower power applications tends to decrease turbine efficiency (Abbin and Leuenberger 1974).
上一页 [1] [2] [3] [4] [5] [6] [7] 下一页