ENERGY CONVERSION ES 832a Eric Savory Lecture 12 – Large-scale plants Department of Mechanical and Material Engineering University of Western Ontario

Motivation: Recognizing that a phase change is the most thermally effective way of transforming heat, many large - scale ( >500 MW ) plants use steam cycles (Rankine Cycles). The drawback is the need for large (bulky) equipment and additional safety concerns. Economically, these additional costs can be justified only for very large–scale operations. Objective: (1) Review of basic Rankine Cycle. (2) Factors affecting efficiency

Components & sub-systems of a simple vapour power plant We shall be examining sub-system A

(1) Simplified Rankine Cycle: 1 - 2Isentropic compression in pump : Water enters state 1 as saturated liquid and is compressed to the operating pressure in the boiler. A slight increase of temperature occurs due to the isentropic compression. ( ) 2 - 3Constant pressure heat addition in boiler: Water enters as compressed liquid at state 2 and exits as super heated vapour at state 3. The boiler (steam generator) is a heat exchanger transferring heat (from combustion of coal as propane, or heat of a nuclear reaction) to the water. ( )

3 - 4Isentropic expansion through a turbine: The superheated vapour at state 3 loses energy through the turbine and exits at state 4. At this state, the steam is usually a saturated liquid-vapour mixture with high quality. ( ) Constant pressure rejection of heat in condenser: Steam is condensed and heat is rejected to the environment. (Note: this heat can be reintroduced into the cycle to increase overall efficiency. This recuperation of heat is also called regeneration). The water leaves the condenser as saturated liquid to enter the pump at state 1. ( )

Boiler TP Condenser G P = Pump T = Turbine G = Generator Actual cycle includes pressure losses: P 2 – P 3 ’ = Pressure loss in boiler P 4 – P 1 = Pressure loss in condenser and entropy increases (dependant on the isenthalpic efficiencies (1  2, 3  4)) P2P2

(2) Increasing the efficiency of the Rankine cycle Given the large energy production rate, even a small increase in efficiency will result in economic benefits. Basic efficiencies of concern are the isentropic efficiencies: PumpTurbine BoilerOverall thermal Q add = heat addition to the boiler B

(a) Lowering the condenser pressure, P 1 - The steam enters the condenser as a saturated mixture corresponding to the pressure inside the condenser. Thus, lowering P 1 automatically lowers the temperature at which heat is rejected. Typically, P 1 is selected close to P atm. (b) Increasing the temperature of the superheated steam, T 3 at constant P 3 - Increases temperature drop through turbine. - Increases quality of vapour at exit.

(c) Increasing boiler pressure, P 3 This increases the temperature at which boiling takes place (i.e. increases heat transformed to steam). An undesirable effect is that the vapour quality can drop at outlet of turbine. This is usually remedied by reheating the steam. Reminder: Steam quality = proportion of saturated steam in a saturated water/steam mixture. A steam quality of 0 means 100% water while a steam quality of 1 means 100% steam. Steam quality is useful in determining enthalpy of saturated water/steam mixtures since the enthalpy of steam (gaseous state) is many orders of magnitude higher than the enthalpy of water (liquid state).

SUMMARY: For large-plants, phase changes can effectively be used to increase overall thermal performance. Typically, for steam cycles,  th ~ 40% - 50% whereas for gas cycles  th ~ 30% - 40%. The extra cost of large installations can only be justified by the large power outputs. Much of the cost arises from: (1) Higher operating pressures. (2) Fuel preparation. (3) Closed cycle operation. Thermal efficiency is increased through increasing the boiler pressure and temperature, while reducing the condenser pressure. The following example demonstrates some of these efficiency gains:

Example of Rankine cycle Consider a steam power plant operating on the ideal Rankine cycle. The steam enters the turbine at 3 MPa and 350°C and is condensed in the condenser at 10 kPa (all pressures are gauge). Determine: (a) the overall thermal efficiency of the plant (b) the overall thermal efficiency if the steam is superheated to 600°C instead of 350°C. (c) the overall thermal efficiency if the boiler pressure is raised to 15 MPa and the turbine inlet is maintained at 600°C. We’ll see how the the thermal efficiency increases from 33.5% to 43.0%:

Case (a)Case (b)Case (c) Steam is superheated to 600°C instead of 350°C Steam is superheated to 350°C Steam is superheated to 600°C + boiler pressure is raised to 15 MPa

Part (a): Superheated steam tables From tables

Part (a) - continued: Thus, the overall thermal efficiency is 33.5%

Part (b): Thus, the overall thermal efficiency is increased to 37.3%

Part (c): Thus, the overall thermal efficiency is increased to 43.0%