Question

CCGT Parameters

The topping cycle is a simple gas turbine with a compression ratio of 2.

The efficiency of the compressor is 80%.

The max temperature across the gas cycle is 1400K.

The efficiency of the gas turbine is 85%.

Exhaust from the gas turbine is directed to the heat recovery steam generator (HRSG) where it is used to heat the water to the superheated steam state.

The mass flow rate of water is 80 kg/s and the pinch point is 12K.

The calorific value of the fuel is 50 MJ/kg.

The bottoming cycle is a simple Rankine cycle with a condenser pressure of 0.1 bar.

Steam enters the high-pressure turbine at 78 bar and 400°C.

The efficiency of the steam turbine is 88%.

The efficiency of the water pump is 92%.

The system is expected to deliver 100 MW of power.

r – gas cycle compression ratio = 2

k = 1.4

cp_air = cp_gas = 1 kJ/kgK

Using a Recuperator, show how you can increase the efficiency of the overall cycle.

Mathematical solution is must.

Answer 1

To enhance the efficiency of a CCGT system, a recuperator can be used to recover waste heat from exhaust gases and preheat incoming compressed air. The recuperator's efficiency is calculated based on the temperature increase of the air. Implementing a recuperator minimizes energy loss, decreases fuel consumption, and increases overall cycle efficiency.

Step-by-step explanation:

To increase the efficiency of a combined cycle gas turbine (CCGT) system, we can incorporate a recuperator. A recuperator is a heat exchanger that recovers waste heat from the gas turbine exhaust and preheats the compressed air entering the combustion chamber.

The efficiency of a recuperator can be defined as the ratio of the heat it transfers to the compressed air to the heat available in the exhaust. If Tc is the temperature of the compressed air before recuperation, Te is the temperature of the exhaust gases, and Tc' is the temperature after recuperation, then the recuperator's efficiency (Eta) can be calculated using the following equation:

Efficiency (Eta) = (Tc' - Tc) / (Te - Tc)

We can increase the overall cycle efficiency by designing the recuperator to minimize exergy losses and maximize the temperature of the compressed air before entering the combustion chamber. By doing so, we reduce the additional heat input required for reaching the high temperatures necessary for the combustion process, thus saving on fuel consumption.

Example Calculation

Using the available data:

• Exhaust gas temperature (Te): 1400K (after expansion)
• Pinch point: 12K
• Mass flow rate of water: 80 kg/s

Assuming that the compressed air leaves the compressor at a temperature Tc and the temperature after recuperation is Tc', with a recuperator efficiency of 100%, the temperature Tc' would be close to Te, respecting the pinch point limitation.

The enhanced efficiency can then be estimated by recalculating the thermal efficiency of the gas turbine cycle with the new, higher Tc' temperature. This would involve using the Brayton cycle efficiency formula:

Eta = 1 - (T1/T2)^(1 - 1/r)

Where:

• T1 is the absolute temperature of the air before compression (assumed to be at ambient temperature).
• T2 is the absolute temperature after heating (increased due to recuperation).
• r is the pressure ratio.

By increasing T2 through recuperation, the overall cycle efficiency increases as a result.