Rankine Cycle Analysis

1.3Thermodynamic analysis of Rankine cycle

The heat source enters the system through the evaporator where the working fluid is heated to saturation or to its super heating point. After high pressure vaporized steam expands in the turbine. The stream of the expanded fluid is passed directly through condenser where it condenses to ambient temperature and condenser pressure. After leaving condenser the working fluid enters the pump the pressure is raised and the fluid is returned to the evaporator.

Below given figure1.3a shows Schematic diagram of Rankine cycle.

Pump stage (1-2):

In the pump stage condensing liquid from the condenser at pressure P1 and temperature T1 is pumped to higher pressure P2 by giving work input to the pump. There by

The stream of high pressure vapor expands in the turbine case; transfer its energy to rotating shaft of the turbine .Following an assumed adiabatic expansion process, the vapor returning to low pressure P2 and low temperature T4.By considering the turbine efficiency into account; turbine output is calculated by, Wt = ṁ*ηt*(h3-h4).

Where, ηt is turbine efficiency h3 is enthalpy at turbine inlet in (KJ/KgK). h4 is enthalpy at turbine outlet in (KJ/KgK).

Condenser Phase (Stage 4-1):

Vapor from the turbine at low pressure undergoes isobaric phase change into saturated liquid form by transferring heat to environment or to the coolant. The pressure of the working fluid in the condenser is considered the same as the lower pressure of the P1. The condenser load output, which defines the rate of latent heat transfer by working fluid, can be given by the equation:

Qout = ṁ*(h4-h1).

Where, h4 is enthalpy at condenser inlet in (KJ/KgK). h1 is enthalpy at condenser outlet in (KJ/KgK).

Condensed water again feed back to the boiler drum by pump so that cycle repeats.

Cycle

It can only increase overall efficiency of the working plant by 1-2% and emission regulations are not under control because there is no recovery of exhaust gases, so organic Rankine cycle is useful for recovering heat from the waste exhaust sources and can use in ORC this leads to further increase in plant thermal efficiency by 3-4% and harmful emission like CO, CO2, NOx, can be controlled.

1.4 Introduction to organic Rankine cycle

Ideal organic Rankine cycle was developed to generate power by using steam with high temperature heat source. The Rankine cycle that uses different organic fluid as working fluid called as organic Rankine cycle. ORC system uses the same component as in the normal Rankine cycle with steam as working fluid, but it operates at lower temperature because of lower heat source and characteristics of the working fluid.

ORC system recovers the waste heat temperature around 160- 180ºC temperature hence its beneficial in many aspects like reducing the electrical loads, economical utilization of heat and waste resources, reduces the emissions like CO,CO2,NOx. ORC cycle gives the lower thermal efficiency around 5-20%. The ORC cycle uses the organic working fluid as different thermodynamic properties as lower boiling temperature, higher molecular weight, higher mass flow rate then