Project Description:
Combined-cycle power plants that use both a gas and a steam turbine together are used to produce more energy output from the same fuel input than traditional single cycle plants.
This assignment involves designing and optimisation of a combined cycle power plant. A schematic of the basic system is shown in Figure 1.
Figure 1 Combined gas-vapour power cycle.
In essence a combined cycle power plant works by capturing the waste heat from the gas turbine (Brayton cycle) and using it as the heat input for the steam turbine (Rankine cycle). As the temperature of the exhaust gases from a gas turbine are very high considerable energy can be transferred to the steam in a Heat Recovery Steam Generator (HRSG). As the steam cycle produces additional energy form the waste heat the energy efficiency capable in the combined cycle is considerably higher than either of the individual cycles operating in isolation.
This assignment will be undertaken in groups of 2 and completed in 2 Parts.
Part 1:
You are to complete an analysis of the basic combined cycle shown in Figure 1 with the intention of producing a net power output of 2 MW.
The following are constraints on the system:
Brayton Cycle
- Working fluid is Air
- Air enters compressor at 300 K, 1 bar pressure
- Max turbine inlet temp = 1100°C
- Compressor and turbine are adiabatic both with isentropic efficiencies of ?=0.82
- Pressure ratio of the gas cycle can range from 8 to 14.
Rankine Cycle
- Condenser pressure is 0.5 bar
- Both high and low pressure turbines have isentropic efficiencies of ?=0.85
- Conditions at the exit of the open FWH are sat. liquid at the Open FWH pressure.
- The HRSG/Boiler has an effectiveness of 90% such that the heat transferred between the gas and the steam is defined by:
Q·HRSG = εm.a(ha@Ta,in - ha@Tw,in )
Where m.a is the mass flow rate of air, and are the temperatures of the air and water entering the HRSG respectively, and is the enthalpy evaluated for the air.
- A characteristic of the HRSG, provided by the manufacturer, is that the temperature of the steam exiting is always 30°C lower than the temperature of the gas entering:
- Maximum allowable steam temperature in either the low or high pressure turbine is 600°C
- Maximum steam pressure in the HRSG is 10 MPa.
- The minimum quality at the turbine exits is 0.95
Objective:
The primary goal of the design is to maximize the overall thermal efficiency of the system, whilst producing the desired power output. This is objective is mainly related to lowering the operating costs of the system, as a higher efficiency produces more electrical power for the same quantity of fuel consumed in the combustor.
A secondary goal is to minimize the total flow rate of air and water. This goal is more closely related to the capital cost of the system as higher mass flow rates will require larger system components which are more costly to purchase.
It is important to note that these two objectives not complementary. Higher efficiencies may result from higher flow rates.
Required Design Specifications:
For your system you will be required to present the following specifications:
1. The mass flow rates of the air and the water.
2. Your chosen gas turbine pressure ratio.
3. The steam cycle HRSG pressure, turbine inlet temperatures, Open FWH pressure, and Open FWH bleed fraction (the fraction of steam that is bled from the exit of the High Pressure Turbine and directed to the Open FWH).
Part 2:
You will now explore modifications to the basic cycle which can further increase the efficiency of the system. These modifications include adding a regenerator to the gas cycle and a reheater to the steam cycle, as shown in
Figure 2 Modified combined gas-vapour power cycle.
Report format
Part 1
The report must include a brief introduction describing the problem and the objectives of the project.
You should present all relevant formulas and methods and a detailed presentation of the analysis for the basic system shown in Figure 1 (it does not have to be your final design). You should present your final state points, heat inputs, power inputs and outputs in a table for clarity.
Part 2
Your final report should be an extension of Part 1 and include a complete analysis and optimisation of the total system including the regenerator and reheat elements. A detailed presentation of the final design is required. A table and/or plot of thermal efficiencies and mass flow rates for a range of preliminary designs should be given, and the rationale you use to select the final design should be clearly described.