Thermal Power Plants

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As is the case for any thermal engine, thermal power plants are devices that convert heat to rotational shaft work, which can be coupled with an electric generator to produce electricity.( a ) Unlike internal combustion engines, where the heat of combustion comes from fuel burned inside the engine, power plants use external heat – most often from fossil or nuclear fuels.

Most thermal power plants follow a thermodynamic cycle which consists of a boiler, a high-pressure steam turbine, a condenser, and a pump. Water is commonly used as the working fluid, although there are instances where ammonia and other refrigerants serve the purpose best. The cycle operates using a feed water pump to introduce water into a boiler or steam generator, where it boils and turns into high-pressure superheated steam. The superheated steam subsequently enters a steam turbine, where it is expanded and cooled to a saturated mixture of water vapor and liquid water. To close the cycle, the mixture must be turned into liquid and compressed to the boiler’s operating pressure. This is accomplished by cooling the mixture in a condenser and circulating it back into the boiler using a feed water pump. To boil the water, energy is required. The source of this energy could be fossil fuel, nuclear fuel, solar heat, or a geothermal reservoir (see Figure 1).

Question: What is the advantage of recycling water (as is done in closed cycles) over discarding steam and pumping fresh water (as is done in open cycles)?

Answer: Recycling the same water not only conserves water, but also saves on the cost of filtering. This is essential to avoid corrosion and prevent the buildup of mineral deposits in the system.

To condense steam, it must be cooled. This is done by pumping cold water from a nearby ocean, lake, or river and diverting it to the condenser. A simple analysis shows that for operation of even a moderately sized power plant, a tremendous amount of cooling water is needed. This is the main reason that most power plants are built near a large body of water such as a river or a lake. In most instances, the water out of the condenser is hotter by 10°C or more, too hot to be returned back to its source. The thermal shock of hot water can prove especially harmful to aquatic organisms whose survival depends on a narrow range of temperature fluctuations. ( b ) To safeguard fish and other marine habitats, there are regulations that require the condenser water to be cooled in a spray pond or a cooling tower before being returned to the lake or river from which it came.

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Figure 1: Thermal power plants consist of a boiler (or steam generator), a steam turbine, a condenser and a recirculating pump. Water boils in a boiler and turns into superheated steam. Steam will then expand in a steam turbine and deliver shaft work. The steam is condensed back to liquid water before being pumped into the boiler, beginning the cycle again.

Spray ponds are large, shallow bodies of water. Water from the condenser is sprayed across the large surface of the pond, where it is cooled by evaporation. Spray ponds are normally used in areas of low humidity and where land is available.

Cooling towers can be either wet or dry. Wet cooling towers are similar to spray ponds except that water is passed through coils which look like a shower head. As water is dripped down, it is cooled by the ambient air and is collected in a basin. Heat is transferred from water to the ambient air, where it stays (Figure 2). The air is usually either sucked out by giant blower fans placed on top of the cooling tower (forced draft) or flows naturally upward to replace the less-dense warm air, which is rising out of the tower (natural draft). Wet towers are large and use two to three times as much water as cooling ponds. Furthermore, many people find the towers aesthetically displeasing. Dry cooling towers are cooled by conduction and convection. Warm water is passed through a heat exchanger where air is passed over the coils containing water, cooling it. This is very much like the radiators in automobiles. No matter what method of cooling is used, the heat always ends up in the atmosphere. As we shall see, this heat cannot be disposed of and therefore directly contributes to global warming.

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Figure 2: Cooling towers in Didcot Power Station in England.

Example: Estimate the thermal efficiency of modern power plants using fossil fuels. Solution: A typical, modern steam power plant operates between temperatures of 600°C at the exit of the boiler and 35oC inside the condenser. An ideal (Carnot) plant operating between these two temperatures would have an efficiency of:

{{\eta}_{ideal}}=1-\frac{35+273}{600+273}=65%

Depending on the type of plant, practical plants have efficiencies far lower than this at around 30-40%. This is because much of the energy is used to raise water to high-temperature steam, or lost to friction in turbines and generators. In the case of fossil plants, if losses due to extraction, processing, and transport are included, the overall efficiencies will be much lower in the order of 10-20%. Figure 3 shows various losses at different stages of power generation from fossil resources.

Contents

Cogeneration

Cogeneration or CHP (combined heat and power) is the simultaneous production of electricity and heat. In this system, a primary power plant produces electricity, but unlike the simple power plants where exhaust is cooled to atmospheric temperatures, cogeneration utilizes the thermal energy left in the exhaust to drive a second thermodynamic device, which either produces additional power or is used directly for heating applications. Furthermore, some of the steam can be extracted at different pressures for industrial applications or home use.

Recent advances in gas turbine technology allow high-temperature combustion of natural gas, which can be used as the primary method of power generation. The exhausts of these turbines are sufficiently hot to be subsequently used in producing additional electricity or heat using conventional steam power plants, resulting in greater conversion efficiencies and lower pollution than the traditional generation methods. The addition of the second turbine boosts combined efficiency to about 60%, which is superior to conventional coal and nuclear power plants with efficiencies of around 33%. When the waste heat is used directly for industrial processes, efficiencies as high as 85% have been found to be possible. ( 1 ) This technology, generally referred to as the combined cycle combustion turbine (CCCT), has the additional advantage of releasing less heat into the atmosphere, limiting global warming and other environmental damage. The gas turbine operates by using gas (derived from the gasification of coal or natural gas) as its primary fuel. Natural gas turbines are particularly advantageous over conventional oil and coal plants because there is no emission of sulfur and negligible emission of particulates. Also, NOx emission is cut by 90% and carbon dioxide emission by 60%. Another advantage of this technology is that power can be distributed. This means that small-scale power generation facilities can be constructed using hybrid systems consisting of solar panels, micro-turbines, and wind turbines that can produce enough electricity for small communities such as shopping malls, large office buildings, etc. When power is not needed, the excess electricity could be sold to utility companies, reducing their peak loads. We will discuss this issue in greater detail in Electricity.( c )

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Figure 3: Fossil plant energy losses at various points from extraction to power generation.

Question: Explain the limitations inherent in the operation of steam and gas turbine cycles. What is the advantage of a combined cycle over steam and gas turbine cycles operating alone?

Answer: Because of material limitations, mainly the hydrogen embrittlement used in piping, the peak temperature of the steam cycles is limited to below 1100°C. The gas turbines, on the other hand, have exhaust at temperatures as high as 450°C, and much of the energy is lost to the atmosphere. The combined cycle takes advantage of the high TL of the gas turbine and the low TH of the steam cycle to achieve a greater efficiency than can be achieved by either when used alone. In this technique, heat is added at a high combustion temperature (topping cycle) and rejected at the relatively low temperature of the condenser or smoke stack (bottoming cycle).

Example: A cogeneration plant uses combined gas turbines and steam power plants. Assuming a total of 200 kg of natural gas is burned every second, what is the plant’s overall efficiency? The gas turbine has a thermal efficiency of 40%, whereas the efficiency of the steam power plant is only 33%. Assume methane has a heating value of 50,000 kJ/kg.


Solution: The total heat input into gas turbines is equal to the mass flow rate of the fuel multiplied by its heating value 200x50,000 = 10 MWt (megawatts thermal). The power output from the primary cycle (gas turbine) is equal to heat input times the efficiency, 10x0.40 = 4 MWe (megawatts electric), and heat rejected in the exhaust is 10-4 = 6 MWt.

In simple cycles, this heat is normally rejected to the atmosphere. In cogeneration cycles, however, we can generate additional electricity by using this heat to drive a second steam turbine. Since the thermal efficiency of steam turbines is only 33%, we have an additional 6x0.33 = 2 MWe (megawatts electric) from this turbine, for a total of 4+2 = 6 MWe from both the primary and secondary systems, and a production of 50% more power by cogeneration. As a result, the overall efficiency of the cogeneration plant has increased to 6 MWe/10 MWt = 60%.

References

(1) (DoE) Manufacturing Energy Consumption Survey, Energy Information Administration, DoE, 1996 (http://www.eia.doe.gov/emeu/mecs).

(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Additional Information

(a) The major difference between thermal, hydroelectric and wind power plants are that in hydroelectric and wind plants, water or air flows directly through water or wind turbines, whereas with thermal power plants a working fluid is heated to its boiling temperature before it is passed through a steam turbine.

(b) The optimal temperature for plankton, the major source of food for many aquatic ecosystems, varies by only a few degrees. For example, the optimum temperature for growth of green algae is 30°C, whereas blue-green algae thrive at about 30-35°C. Thermal discharges therefore favor production of blue-green algae over green algae. Blue-green algae are a poorer source of food and can be toxic to fish.

(c) A challenge problem for the more mathematically inclined: A cogeneration plant combines a gas turbine with efficiency ηGT with a steam turbine with efficiency hST. Show the cogeneration plant has a combined efficiency of: ηCC = ηGT + ηST(1 − ηGT). What is the overall efficiency of a combined plant if gas turbine and steam turbine efficiencies are 40% and 33% respectively? Compare your answer with the example given in this section.

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