How does the pinch point temperature difference in a heat recovery steam generator (HRSG) of a combined cycle power plant directly impact the overall cycle efficiency?

The pinch point temperature difference in a Heat Recovery Steam Generator (HRSG) directly influences the overall combined cycle efficiency by affecting the amount of steam that can be generated and its temperature. The pinch point is the minimum temperature difference between the hot exhaust gas from the gas turbine and the water/steam in the HRSG's evaporator section. A smaller pinch point allows for more heat recovery from the exhaust gas. If the pinch point is reduced, the water can be heated closer to the exhaust gas temperature, thus producing more steam or steam at a higher temperature. More steam generation translates directly to higher power output from the steam turbine. Similarly, higher steam temperature also contributes to increased steam turbine efficiency. A larger pinch point, conversely, implies that less heat is extracted from the exhaust gas, resulting in less steam generation or steam at lower temperatures. This leads to reduced power output from the steam turbine, consequently lowering the combined cycle efficiency. However, excessively reducing the pinch point increases the heat exchanger surface area needed for heat transfer, thus increasing the capital cost of the HRSG. Therefore, optimizing the pinch point involves a trade-off between cycle efficiency and equipment cost. In summary, reducing the pinch point generally increases the cycle efficiency up to a point where the incremental gains are offset by increased capital expenditure.