๐
Module 4 ยท Gas Turbine Power Plant
In gas turbine plants, combustion gases expand directly in the prime mover โ no steam generation needed. Based on the Brayton cycle, they offer fast startup, compact design, but high back-work ratio (40โ80% of turbine output drives compressor).
Gas Turbine Operation
$$\text{Working gas expands} \rightarrow \text{Heat energy} \rightarrow \text{Kinetic energy} \rightarrow \text{Shaft work (reaction)}$$
$$\boxed{\eta_{GT,simple} = 32\text{โ}38\%}$$
Brayton Cycle (Ideal Gas Turbine)
$$\text{1-2: Isentropic compression} \quad \text{2-3: Const. pressure heat addition}$$
$$\text{3-4: Isentropic expansion} \quad \text{4-1: Const. pressure heat rejection}$$
$$\boxed{\eta_{Brayton} = 1 - \frac{1}{r_p^{(\gamma-1)/\gamma}}}$$
$$r_p = \frac{P_2}{P_1}\;\text{(pressure ratio, typical 10โ30)} \quad \gamma = 1.4\;\text{(air)}$$
Work & Back Work Ratio
$$W_T = \dot{m}c_p(T_3 - T_4) \quad W_C = \dot{m}c_p(T_2 - T_1) \quad W_{net} = W_T - W_C$$
$$\text{Back work ratio} = \frac{W_C}{W_T} \approx 40\text{โ}80\% \quad \text{(much higher than steam turbine!)}$$
Fuels & Site Selection
$$CV_{natural\,gas} \approx 55{,}000\;\text{kJ/m}^3 \quad \text{Natural gas (methane) โ most used}$$
$$\text{Site: Near load center} \cdot \text{Gas pipeline access} \cdot \text{Land availability} \cdot \text{Transport}$$
Merits / Demerits
$$\text{โ Simple design} \cdot \text{High reliability} \cdot \text{Compact} \cdot \text{Low initial cost} \cdot \text{No standby losses}$$
$$\text{โ Low net output} \cdot \text{Low efficiency} \cdot \text{Noisy operation}$$
๐ Gas Turbine & Brayton Cycle
BRAYTON CYCLE EFFICIENCY
BACK WORK RATIO
โก
Module 4 ยท Power Plant Comparison Table
A comprehensive comparison of all major power plant types โ Thermal, Hydro, Nuclear, Diesel, and Gas Turbine โ across key parameters including efficiency, cost, startup time, pollution, and best use cases.
Complete Comparison
| Parameter | Thermal | Hydro | Nuclear | Diesel/GT |
|---|---|---|---|---|
| Fuel | Coal/gas | Water | U-235/Pu | Diesel/gas |
| Efficiency | 35โ42% | 85โ92% | 30โ42% | 35โ45% |
| Load Type | Base+Peak | Base/Peak | Base only | Peak/Standby |
| Capital Cost | Moderate | High | Very high | Low |
| Fuel Cost | High | Free | Very low | Very high |
| Startup Time | Hours | Minutes | Days | Seconds! |
| Pollution | High COโ | Nil | Radiation | Moderate |
| Location | Near coal | Remote | Remote | Anywhere |
| India % | 61.1% | 12.3% | 1.8% | Small % |
Best Use Cases
$$\text{Thermal: Base load, 24ร7, large cities} \quad \text{Hydro: Peaking, pumped storage, clean}$$
$$\text{Nuclear: Constant base load, low fuel cost} \quad \text{Diesel: Emergency, remote, mobile}$$
$$\text{Gas Turbine: Peak load, fast start, flexible}$$
โก Startup Race & Comparison
STARTUP TIME RACE
EFFICIENCY COMPARISON