Prelim. Conclusion

The Rankine and Brayton cycles were analyzed and nodes identified as candidates to insert a Stirling engine to capture waste heat and utilize it in a secondary process. In all cases, it was determined that the available thermal energy was of such a low grade that it was not feasible to apply Stirling technology to the system without some form of supplemental firing. Electrical output of a Stirling generator improves as the temperature difference between the hot and cold sides of the engine increases. 

Comparing additional thermal energy required and added to operate the Stirling system at its highest efficiency, the best candidate for an application of this technology is supplemental firing, post exhaust combustor, adapted to the Brayton cycle. By varying the mass flow of ignited fuel applied to direct heating of the pressurized exhaust stream between 0.05 and 0.13 kg/s, enough thermal energy is created to produce the temperature differences required for the modeled 25kW Stirling generator system used in our analysis to operate. 

Although Node 10, post-Intermediate Pressure Turbine, appears to be a potential candidate, the Rankine cycle is balanced such that any supplemental heating of the working fluid at this location may create additional thermal waste at the condenser downstream and negatively interferes with the Low Pressure Turbine as a generator. This creates issues on a larger scale with respect to the rejected heat. The Rankine cycle is not a good candidate for utilization of Stirling technology in a secondary application.

Almost twice the thermal energy is required to direct fire a Stirling generator as is required with supplemental heating in the Brayton cycle. This is significant when considering the commercial costs for natural gas would be double that of the Brayton cycle with supplemental firing as compared to direct firing. 

The Stirling generator analyzed in this application is producing 25kW of electrical energy. The thermal energy available as an input to a Stirling engine in aggregate would be enough to power one single 25kW system. This is an extremely small amount of generated electricity. To increase the amount of available electrical energy out, therefore increasing engine size, both air flow and mass flow of fuel in would need to be increased. Considering installed cost of the engine, and additional fuel for supplemental firing, the electrical energy generated is less than 1% of a 300MW Brayton turbine plant. Upon closer inspection, this is not feasible. 

In conclusion “waste” heat recovery from commercial power generating operations is not a viable application of Stirling technology. However, the Stirling system does deserve consideration for smaller scale, scalable or personal use applications. 

Initial cost, energy wasted, and the mitigation of the rejected thermal energy from the engine are three of the larger deterrents (local ordinances and code requirements not considered). 

The Stirling engine is unique in that it may use virtually any heat source, including unscrubbed biogas and solar. Stirling engines using natural gas as the external combustion thermal energy fuel source have emissions that are less than 20% that of an equivalent coal fired generating system. 

The tangible costs related to the Stirling system are in maintenance and initial systems capital costs, including costs related to procurement, fabrication and installation of the engine and all related engine ancillaries. It is generally agreed that Stirling engines will run for more hours with less maintenance required than conventional internal combustion engines. The cost installed for solar applications is $6.40/W for photo voltaic compared to $4.67/W for dish/Stirling (October, 2009). Currently, dish/Stirling is not suited for residential installation where photo voltaic are easily adapted to existing residential settings. Dish/Stirling systems are scalable but massive areas of undeveloped land are necessary to generate meaningful amounts of electrical power. The dish/Stirling system has only limited applications and viability.

The technology is not viable at this time for large scale supplemental commercial power generation projects. The technology could find uses at the individual or corporate user level.

more to come…

~ by frazerthompson on March 18, 2010.

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