Powering the Power Sector

In this article, steam turbine and cooling tower technologies for power sector have been discussed in detail.

Arguably, electricity is one of the greatest inventions ever. It has given a quantum leap to the mankind. It has brought light to darkest regions of the world, enabled invention of many path breaking technologies and changed the way we live.

Electricity can be produced from various fuel sources. These sources are broadly divided under four categories:

• Thermal Power – Coal, natural gas and oil
• Hydro power
• Nuclear power
• Renewable power – Solar, wind, biomass, waste etc.

In a thermal power plant, heat energy produced from burning of Coal, Natural Gas or Oil is converted to electricity. A turbine or an engine is used which converts the heat energy into mechanical energy and an alternator is used to convert the mechanical energy into electricity. Thermal turbines can be split into steam turbine and gas turbine. Other than thermal power plants, steam turbine is also used in nuclear power plant, biomass or waste based power plant and solar thermal power plant. About 80 per cent of all electricity generation in the world is by use of steam turbines. In steam turbine based power plants, various fuel sources are burnt to generate steam which is then used to run the steam turbine. On the other hand, in a gas turbine based power plant, a mixture of gas and air is burnt to generate hot flue gas which is then used to run a gas turbine. Image 1 depicts diagram of a coal based power plant and its various components.

In a coal based power plant, coal is burnt to generate high pressure steam in a boiler which is then used to run a steam turbine. An alternator, also known as generator is connected to the turbine which generates electricity due to rotating motion of the turbine. Steam after hitting the turbine blades loses its pressure and enters into a condenser. Condenser condenses the steam into hot water to allow it to be pumped back to the boiler through a boiler feed water pump. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycle increases. To cool the condenser, a cooling tower is used in a thermal power plant. In the subsequent sections of this article, steam turbine and cooling tower technologies have been discussed in detail.

Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator or rotary equipment like pumps, compressors, and blowers etc. Its modern manifestation was invented by Sir Charles Parsons in 1884. The invention of Parson’s steam turbine made cheap and plentiful electricity possible and revolutionized marine transport and naval warfare. Parsons had the satisfaction of seeing his invention adopted for all major world power stations, and the size of generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity.

Steam turbines are made in a variety of sizes ranging from small <1 hp (<0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines. These types include condensing, non-condensing, reheat, extraction and induction. Non-condensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, sugar plants and desalination facilities where large amounts of low pressure process steam are available. Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90 per cent, at a pressure well below atmospheric to a condenser. Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power. Conventional steam power plants operate at a steam pressures in the range of 170 bar. These are Subcritical power plants. The new generation of power plants operate at pressures higher than the critical pressure. These are Supercritical power plants. The operating pressures are in the range of 230 to 265 bar. The concept of Supercritical technology has been explained below. Water is converted to steam in three stages:
As the heating continues, the temperature of water increases till it reaches 100 C. This is the sensible heat addition.
Further heating does not increase the temperature; instead small bubbles of steam start to form. The temperature remains constant at 100 C till all the water becomes steam. The water absorbs the heat without temperature change for conversion to steam. At atmospheric pressure, the Latent Heat of vaporization is 2,256 kJ/kg.
Further heating called superheating increases the temperature of the steam. How high the temperature can go depends on the withstanding capacity of the vessel.

As the pressure increases, the boiling temperature increases and the latent heat of vaporization decreases. When the pressure say at 100 bar, the boiling takes place at 311 C and the latent heat of vaporization is 1318 kJ/kg. If the water pressure is 200 bar then the boiling takes place at 366 C and the latent heat of vaporization is 584 kJ/kg. A further increase in pressure and temperature leads us to a point at which the latent heat of vaporization is zero, or there is no boiling. Water directly becomes steam. This is the Critical Pressure and the Critical Temperature. For steam, this occurs at 374 C and 220.6 bar. Capacity of a Supercritical steam turbine is 660 or above MW.

Cooling tower
Cooling tower is a heat transfer or heat rejection equipment which is widely being used across Power plants and Industries for process and space cooling. Cooling tower is a special type of heat exchanger that allows water and air to come in contact with each other to lower the temperature of the hot water. During the cooling tower working process, small volumes of water evaporate, lowering the temperature of the water that’s being circulated throughout the cooling tower. Cooling towers are considered as a better means of removing low grade heat in the system. Cooling towers are classified as:
By type of equipment: It is classified as natural draft and mechanical/ induced draft.
Based on construction: It is classified into RCC (reinforced cement concrete), FRP (fibre reinforced plastic) and Timber. FRP cooling towers are factory assembled and RCC and timber cooling towers are field erected units.
Based on design and mode: It is classified as packaged, field erected and industrial.
By application: Power plant is the major end-user, followed by industrial process cooling and other HVAC&R applications.

Thermal power plants predominantly use cooling towers to cool the circulating water used for condenser cooling. As water supply is limited, power plants have adopted the closed cooling system. With fresh water resources depleting and increase in population with more requirement of power, efficiency of water usage in cooling towers has become a major focus. Cooling towers help by reusing the cooling water, making power plants economical and more environmentally friendly.

Large utility scale power plants use natural draft cooling towers whereas mechanical or induced draft cooling towers are used in small capacity captive power plants and for industrial and HVAC applications.

Natural draft-cooling tower is a large hyperbolic tower, which pulls in air due to the stack effect, which is highly preferred in large Power plants. This type of cooling tower utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential compared to the dry, cooler outside air. This moist air buoyancy produces an upwards current of air through the tower. Even though the capital costs of these cooling towers are high, operating costs are less, as there is no fan to create the air flow.

Mechanical or forced draft-cooling tower is most commonly used across industries. A fan forces or sucks air through the cooling tower where the water falls through a packed heat transfer media. Operating costs are high for operating this, but they are simple and quick for construction.

Technologies are very much necessary in every sector. The new technologies that have been introduced in cooling tower includes minimum water evaporation and drift losses, pultruded FRP structure usage and energy efficiency in equipment like fans, blowers, etc. across cooling towers. All major cooling tower manufacturers are focusing on product development through R&D and have a tie-up/ collaboration with global players.

Authored by__

Rajkumar E.A.
Consultant – Energy Vertical
Feedback Business Consulting Services Pvt Ltd