Question

For the above gas turbine (question 1), it has been decided that for efficiency improvement some changes are required. You have been given task of helping the organization, as a consultant. Can you recommend practical changes that can lead to efficiency improvement?

One of the researchers at UA’s Mechanical Engineering Department, came-up with an idea of integrating above gas turbine with a steam powered cycle, so that end combination would be combined power plant cycle. It was also recommended to use a simple ideal Rankine cycle operating between the pressure limits of 6MPa and 6kPa, for ease of calculations. Steam is heated in a heat exchanger by the exhaust gases to a temperature of 550 degrees C. The exhaust gases leave the heat exchanger at 400 K. Use appropriate assumed values, where required. Gas/Steam tables available with you could be used in these questions. Being a consultant, you have been asked to evaluate this approach. Can you give details of efficiency improvements by taking above approach? Also give details of numerical method used to calculate improved efficiency?

Answer 1

I'd suggest to use the combined power plant (steam +gas cycle) because researches have shown that a single cycle (Rankine cycle) has a maximum efficiency of about 34%. Where as if we use a combination of both Brayton and Rankine cycle, we can increase the efficiency upto 64% which is a lot as compared to the single cycle. So my suggestion would be to used a combined cycle. Although the initial cost of this cycle will be large but thhe running cost will be much less as compared to the other cycles and that's what matters the most. If we talk numerically, by using combined cycle, we'll be able to achieve high input temperatures and low output temperatures. So this will be a cycle which would have a large difference between the source and sink temp. So the efficiency would increase.