RESULTS
A histogram of all sources over 300°F (149°C) is shown in Figure 6. Half of all potential sources are at 450°F (232°C) or below. This result may be somewhat misleading, however, as it considers only the number of sources and not their power production capability. Looking specifically at the temperature range important for ORC technology, between 300 and 1000°F (149 and 538°C), the EPA CAP database was analyzed first under the assumptions above assuming that the working fluid is capable of being used near the exhaust temperature without decomposition problems.
Under these assumptions total recoverable power potential is calculated to be 63 GW with 44 GW coming from sources under 1000°F (538°C). Results of this analysis are presented in Figure 7 as a function of waste heat source temperature. The total recoverable power available in each temperature regime remains roughly constant over the range of 300 to 1000°F (149 to 538°C) as a result of a much larger number of low temperature sources but poorer efficiency in recovering it. In the range of interest for ORC technology, 80% of recoverable power generation is in the temperature range of 500 to 1000°F (260 to 538°C) and 50% is in the range of 750 to 1000°F (399 to 538°C). Also, the average recoverable power per source does not get over 1 MW until exhaust gas temperatures rise above 600°F (316°C). Therefore, while most of the sources of industrial waste heat are at relatively low temperatures, most of the opportunity for waste heat to power applications is actually at intermediate temperature, 500 to 1000°F (260 to 538°C). These temperatures are high enough that working fluid decomposition must be considered when designing a system.
This analysis was repeated limiting maximum working 本文来自辣~文\论|文/网，
毕业论文 www.751com.cn 加7位QQ324'9114找源文 caused by this limitation reduces the total recoverable electric power from sources under 1000°F to 32 GW from 44 GW an increases capital costs, in the form of an additional heat exchanger for all sources over 450°F. The results of this analysis as a function of temperature are shown in Figure 8.
Including sources over 1000°F (538°C) assuming steam as a working fluid, with maximum temperatures equal to the source temperature, the total recoverable power opportunity is then calculated to be 51 GW. Even considering the high temperature limitations of the organic fluids, the overall opportunity for waste heat to power is still greater for ORC, 32 GW, than for steam, 19 GW. Figure 9 shows the same analysis as Figure 8 but includes sources 1000°F and hotter.
In the case of source temperatures near 1000°F (538°C), limiting working fluid temperature to 450°F brings the conversion efficiency to only 30% of the Carnot ideal. In this temperature range it may become advantageous to use a transcritical CO cycle instead of an ORC cycle, eliminating the issue of thermal decomposition completely. Another possibility for improvement is use of smaller chain alkanes such as propane or butane. Even though their critical temperatures are relatively low, the ability to use them at higher maximum temperature may provide better overall efficiency and may also provide better economics if the use of a thermal transfer fluid and additional heat exchanger can be avoided.
It is important to note that the estimates of recoverable power presented here almost certainly substantially overestimate the opportunity, as sources with hostile exhaust compositions have not been eliminated from the analysis and duty cycle is not contained in the EPA source database. However, this study illustrates the impact that working fluid decomposition has on the total opportunity for waste heat to power applications and the relative opportunity for ORC based recovery compared to steam Rankine cycle recovery.
CONCLUSIONS
The overall waste heat to power opportunity from industtrial sources in the U.S. based on analysis of the EPA National Emissions Inventory CAP database is estimated to be 51 GW. This result does not consider limitations that would certainly exist of exhaust gas suitability for heat exchange, difficulty in connecting some remote processes to the power grid, or low duty cycle of some sources included in the CAP database that would tend to decrease the opportunity. It also does not include opportunities that may exist from sources not included in the CAP database either accidentally or intentionally, such as non-stationary sources, that would tend to increase the opportunity.
Consideration of working fluid decomposition in the opportunity analysis reduces the potential for ORC energy recovery by 12 GW, 27%. However, 63% of the estimated recovery opportunity is still from sources below 1000°F (538°C) for which ORC is the most appropriate recovery technology.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the National Rural Electric Cooperative Association Cooperative Research Network for supporting this work.
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