Rankine cycle

The Rankine cycle is a thermodynamic cycle. Like other thermodynamic cycles, the maximum efficiency of the Rankine cycle is given by calculating the maximum efficiency of the Carnot cycle. It is named after William John Macquorn Rankine, a Scottish polymath.

This article will deal with the Rankine cycle from an engineering point of view.

Processes of the Rankine cycle

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram above.

• Process 4-1: First, the working fluid is pumped (ideally isentropically) from low to high pressure by a compressor. Pumping requires a power output (for example mechanical or centrigugal).
• Process 1-2: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a saturated vapor. Common heat sources for power plant systems are coal (or other chemical energy), natural gas, or nuclear power.
• Process 2-3: The superheated vapor expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapor.
• Process 3-4: The vapor then enters a condenser where it is cooled to become a saturated liquid. This liquid then re-enters the pump and the cycle repeats.

The exposed Rankine cycle can also prevent vapor overheating^[1] , which reduces the amount of liquid condensed after the expansion in the turbine.

Description

Rankine cycles describe the operation of steam heat engines commonly found in power generation plants. In such vapour power plants, power is generated by alternately vaporizing and condensing a working fluid (in many cases water, although refrigerants such as ammonia may also be used).

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Water vapour seen billowing from power plants is evaporating cooling water, not working fluid. (Note that steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds seen leaving cooling towers).

Variables

${\dot {Q}}_{in}$	heat input rate (energy per unit time)
${\dot {m}}$	mass flow rate (mass per unit time)
${\dot {W}}$	mechanical power used by or provided to the system (energy per unit time)
$\eta$	thermodynamic efficiency of the process (power used for turbine per heat input, unitless)
$h_{1},h_{2},h_{3},h_{4}$	these are the "specific enthalpies" at indicated points on the T-S diagram

Equations

Each of the first four equations^[1] are easily derived from the energy and mass balance for a control volume. The fifth equation defines the thermodynamic efficiency of the cycle as the ratio of net power output to heat input.

${\frac {{\dot {Q}}_{\mathit {in}}}{\dot {m}}}=h_{2}-h_{1}$	${\frac {{\dot {W}}_{\mathit {turbine}}}{\dot {m}}}=h_{2}-h_{3}$
${\frac {{\dot {Q}}_{\mathit {out}}}{\dot {m}}}=h_{4}-h_{3}$	${\frac {{\dot {W}}_{\mathit {pump}}}{\dot {m}}}=h_{4}-h_{1}$
$\eta ={\frac {{\dot {W}}_{\mathit {turbine}}-{\dot {W}}_{\mathit {pump}}}{{\dot {Q}}_{\mathit {in}}}}$

Real Rankine cycle (non-ideal)

In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes (indicated in the figure as ΔS). This somewhat increases the power required by the pump and decreases the power generated by the turbine. It also makes calculations more involved and difficult.

Variations of the basic Rankine cycle

Two main variations of the basic Rankine cycle are used in modern practice.

Rankine cycle with reheat

In this variation, two turbines work in series. The first accepts vapor from the boiler at high pressure. After the vapour has passed through the first turbine, it re-enters the boiler and is reheated before passing through a second, lower pressure turbine. Among other advantages, this prevents the vapour from condensing during its expansion which can seriously damage the turbine blades, and improves the efficiency of the cycle.

Another variation is where 'bleed steam' from between turbine stages is sent to feedwater heaters to preheat the water on its way from the condenser to the boiler.

Regenerative Rankine cycle

The regenerative Rankine cycle is so named because after emerging from the condenser (possibly as a subcooled liquid) the working fluid is heated by steam tapped from the hot portion of the cycle. This increases the average temperature of heat addition which in turn increases the thermodynamic efficiency of the cycle.

Organic Rankine cycle

The Organic Rankine Cycle (ORC) uses organic fluids (such as toluene) in place of water and steam. For example, this allows use of lower temperature heat sources such as solar ponds, which typically operated at around 70-90 °C. The efficiency of the cycle is much lower as a result of the lower temperature range, but this can be worthwhile, because of the lower cost involved in gathering heat at this lower temperature.

Reverse Rankine cycle

A Rankine cycle that is driven in reverse, via net work input, is the vapor-compression refrigeration cycle. Its purpose is to move heat, rather than produce work.