The efficiency of a steam turbine is defined as the ratio of the turbine’s useful output energy to the energy delivered. The steam turbine is not completely efficient due to energy loss in the form of heat or friction.
A steam turbine’s efficiency is determined by a number of factors, including thermodynamic (temperature and steam pressure) and hydraulic (inlet fluid velocity) parameters, guide vanes angle, rotor blades angle, and rotor diameter.
Multistage (moderate to high-pressure ratio) steam turbine thermodynamic efficiency ranges from 65 percent for very small (under 1,000 kW) units to more than 90 percent for large industrial and utility-sized units. Small, single-stage steam turbines can have an efficiency as low as 40%.
The steam turbine is a type of turbine that converts the potential energy of heated and compressed steam generated in a steam generator or steam from natural sources such as geothermal sources into kinetic energy via steam expansion in the turbine blades and, as a result, mechanical work on the rotating shaft. The power transfer between the steam and the rows of rotary blades mounted on the turbine rotor alters the total steam enthalpy.
The steam turbine is a component of stationary and portable steam turbine power plants. In addition to turbine units, power plants have boilers that generate steam, steam condensers, and other equipment.
Why Is Steam Turbine Efficiency So Low?
The total Steam Turbine Efficiency is fairly low (around 29%) due to two factors. For starters, a significant amount of heat is lost in the condenser, and other heat losses occur throughout the plant. Heat loss in the condenser cannot be avoided.
Which Is More Efficient Gas or Steam Turbine?
Because of its simple design, the steam turbine is easier to maintain. In the case of industrial steam turbines, a steam turbine can achieve up to 90% efficiency, and sometimes even more. As a result, in most applications, steam turbines are the most efficient.
Principle of Operation
An ideal steam turbine should go through an isentropic process, or one with constant entropy. As a result, the entering steam entropy equals the left steam entropy during this process. However, there is no isentropic steam turbine, as predicted. Steam turbine isentropic efficiencies typically range from 20% to 90%, depending on the turbine’s application.
A steam turbine’s interior contains several sets of blades. A set of stationary blades is attached to the casing, while another set of rotary blades is attached to the shaft. These sets are designed and the clearances are determined so that the turbine can make the best use of the steam expansion.
Practical steam turbine thermal efficiency varies with turbine capacity, load, and physical losses. It can reach values of up to 50% in a 1200 MW turbine; as the turbine size decreases, the efficiency decreases.
The steam turbine structure is divided into stages to maximize turbine efficiency. The energy is extracted at each stage, and they are known as impulse or reaction turbines. The majority of steam turbines use a combination of impulse and reaction designs. Although each stage operates as either one or the other, the entire turbine makes use of both. Lower pressure parts are typically reaction turbines, while higher pressure stages are impulse turbines.
After leaving the boiler at high temperature and pressure, superheated steam enters the turbine. The kinetic energy of steam increases when it passes through a fixed nozzle in an impulse turbine or fixed vanes in a reaction turbine. The high-velocity steam exits the nozzle and travels to the turbine rotor blades.
The vapor pressure exerts a force on the blades, causing the rotor to spin. On the shaft, a generator or other similar device can be installed to store and use the energy contained in the steam. Following the turbine, steam enters the condenser to be cooled as saturated vapor or a mix of liquid and vapor at a lower temperature and pressure than that which entered the turbine.
We can obtain an equation for calculating the work produced per unit mass by applying the first law of thermodynamics. With the assumption that there is no heat transfer to the surrounding environment and that changes in kinetic and potential energy are ignored in comparison to changes in specific enthalpy.
Steam Turbine Efficiency
This section will discuss the efficiency of steam turbines in two types: impulse and reaction turbines.
An impulse turbine’s fixed nozzles direct the steam flow into high-speed jets with significant kinetic energy. The kinetic energy of the steam jet is converted into shaft rotation as it changes direction by striking the bucket-shaped rotor blades. Only over the stationary blades does pressure drop occur, as does an increase in net steam velocity across the stage. The pressure decreases as it passes through the nozzle. The outgoing steam through the nozzle has a very high velocity due to the high expansion ratio of steam.
According to the law of moment of momentum, the sum of the moments of applied external forces on a fluid equals the net time variation of angular momentum flux.
The ratio of work done on the blades to kinetic energy provided to the fluid is defined as blade efficiency. A stage in an impulse turbine is made up of a nozzle set and a moving wheel. The ratio of work done in the stage to the enthalpy drop in the nozzle is defined as the stage efficiency.
The rotor blades of a reaction turbine are designed to form convergent nozzles. The reaction force is combined with the acceleration of steam through the nozzles in a reaction turbine. The stationary guide vanes direct steam to the rotor. It exits the stationary part as a jet that fills the rotor’s entire periphery. The steam then reverses direction and its speed relative to the blades increases.
When the steam accelerates in the stator and decelerates in the rotor, it causes a pressure drop in both. There is no net change in steam velocity through the stage, but there is a decrease in both pressure and temperature, indicating that work is done in the rotor motion.
The isentropic steam turbine efficiency can be used to estimate how well a turbine is operating. This parameter compares the turbine’s performance in real-world conditions to the turbine’s performance under ideal conditions, which are assumed to be isentropic.
The heat lost to the surroundings is considered zero when calculating this isentropic steam turbine efficiency. Both the actual and isentropic turbines have the same turbine entrance pressure and temperature.
However, at the turbine exit, the real turbine has a higher steam-specific enthalpy than the ideal turbine. This is due to the irreversibility of the current conditions. To provide a fair comparison between the two cases, the steam-specific enthalpy of the actual and ideal turbines is calculated at the same pressure.