Continuing to Evaluate…


After modeling a typical, simple but accurate Rankine cycle steam plant producing 500MW of electricity, I chose four locations to consider as candidates to add a Stirling engine bank within the cycle itself. Keep in mind, the Rankine steam cycle is a delicately balanced system where most all of the energy is accounted for at every stage of the cycle. Not to be discouraged, I chose the following locations:


         the exit of the high pressure turbine (HPT)

         pre-intermediate pressure turbine (IPT) post reheat

         the exit of the low pressure turbine (LPT) pre-condenser

         a secondary leg, post reheat pre-IPT

         a secondary leg, adding reheat post LPT, pre-condenser


I modeled the nodes considering two scenarios:


  1. If no thermal energy is added back into the system, how are the overall output and cost parameters affected?
  2. If thermal energy is added back into the system, how are the same parameters affected?


It became immediately obvious that of the nodes I had chosen, the only two that may have any viability were the exit of the HPT and the exit of the LPT, pre-condenser.


The objective was to provide 1MW of power from the Stirling engines. The temperature of the fluid at the exit of the HPT was 322 oC. The temperature of the fluid at the outlet of the IPT was 45oC. I had built my Stirling model based on the geometry of the United Stirling 4-95 MkII, with a possible 25kW output operating at 20MPa with a working fluid of Helium.


At the HPT with no fuel added; the total system rating dropped to ~ 499,350kW, the plant efficiency decrease about 0.5%, the operating cost in fuel remained unchanged but there was a net revenue increase of about 1.2%. (This is the most interesting result.)


At the HPT with fuel added; the total system rating increased by 1MW, the plant efficiency decreased, the operating costs increased by ~ 0.5%, there was a net revenue decrease of ~ 0.75%  for an overall output increase of 0.2%. (Not viable.)


The LPT output is an interesting case. The temperature difference between the hot and cold sides of the Stirling engine would only be about 20 degrees, not enough of a delta T for the Stirling engine to operate. However, the working fluid at a very low pressure is still in a vapor state and must flow through the condenser prior to entering the low pressure pump. Lots of energy. What if we could replace the condenser, or reduce the amount of work the condenser does, by adding a large bank of Stirling engines to extract the energy from the working fluid? This would require further development of Stirling technology.


At such a low temperature, the number of engines required to produce 1MW would be excessive. This raises another question, what if we were to add a slight amount of reheat to the fluid prior to it entering the condenser? The results are devastating. If we increase the hot side temperature of the working fluid to 250oC, the plant efficiency decreases by >10%, the operating costs increase by >10% and there is was a net revenue decrease of >50% for an overall output increase of 0.2%. The condenser cooling water output temperature also increases 5 degrees. (Not viable.)


Initial conclusions: Adding engines without fuel and still seeing a net revenue increase seems to indicate the efficiencies of the Stirling engine bare further consideration. The increases are so small compared to overall plant losses, it does not seem viable. Adding engines and fuel did not produce a favorable result however, adding engines that operate on a very small delta T between hot and cold sides, at the condenser may warrant further investigation.


Modeling the Brayton Cycle. Thus far, adding a Stirling engine bank to the exhaust side of the cycle pre-stack has not produced favorable results. The gas temperature coming out of the turbine at 220oC is filled with carcinogens that contribute to acid rain if not disposed of in a combustion process. To do this, gas-fuel must be added to an afterburner. If just enough fuel is added to produce an adiabatic flame temperature of approximately 1450 oC (burning 100% of available oxygen), disposing of our ‘burnable gases’, and considering we must maintain at least 200 oC in the stack to fly the exhaust plume, there are only about 70K kJ/kg of energy left over at approximately 200 oC that may be fed into a secondary process.


What does this mean? It’s barely enough energy to produce 10kW of electrical output from a single Stirling engine using the same model as I have discussed above. At this point, the option is to add more gas-fuel. This further implies it may be equally efficient to consider the Stirling engine as a stand-alone generator.


Thus far, I am concluding that there is a very slight potential in the Rankine cycle pre-condenser, pending available engine technology, and there is little to no potential incorporating the engine in a Brayton cycle.


Let us further consider stand-alone and solar applications in the weeks to come.





~ by frazerthompson on March 14, 2009.

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