Thermal Efficiency Improvement – Rankine Cycle
There are several methods, how can be the thermal efficiency of the Rankine cycle improved. Assuming that the maximum temperature is limited by the pressure inside the reactor pressure vessel, these methods are:
Significant increases in the thermal efficiency of steam turbine power plants can be achieved by reducing the amount of fuel added to the boiler. This can be done by transferring heat (partially expanded steam) from certain steam turbine sections, which is normally well above the ambient temperature, to the feedwater. This process is known as heat regeneration, and a variety of heat regenerators can be used for this purpose. Sometimes engineers use the term economizer that is a heat exchanger intended to reduce energy consumption, especially in preheating a fluid.
As can be seen in the article “Steam Generator”, the feedwater (secondary circuit) at the inlet of the steam generator may have about ~230°C (446°F) and then is heated to the boiling point of that fluid (280°C; 536°F; 6,5MPa) and evaporated. But the condensate at the condenser outlet may have about 40°C, so the heat regeneration in typical PWR is significant:
- Heat regeneration increases the thermal efficiency since more of the heat flow into the cycle occurs at a higher temperature.
- Heat regeneration causes a decrease in the mass flow rate through the low-pressure stage of the steam turbine, thus increasing LP Isentropic Turbine Efficiency. Note that at the last stage of expansion, the steam has a very high specific volume.
- Heat regeneration causes an increase in working steam quality since the drains are situated at the periphery of the turbine casing, where is a higher concentration of water droplets.
Regeneration vs. Recuperation of Heat
In general, the heat exchangers used in regeneration may be classified as either regenerators or recuperators.
- A regenerator is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. It has a single flow path in which the hot and cold fluids alternately pass through.
- A recuperator is a heat exchanger with separate flow paths for each fluid along its passages, and heat is transferred through the separating walls. Recuperators (e.g.,, economizers) are often used in power engineering to increase the overall efficiency of thermodynamic cycles, for example, in a gas turbine engine. The recuperator transfers some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the combustion chamber. Many recuperators are designed as counterflow heat exchangers.