Unit 1

Introduction

Introduction

❖ Energy and their sources❖ Thermodynamic cycles❖ Types of power plants❖ Main components of power plants❖ Concepts of power plants : advantages & disadvantages❖ Fuels used in power plants

Energy

❖ Capacity for doing work, generating heat and emitting light.❖ Standard of living for any country can be directly related to energy

consumption/generation❖ An essential input for economic development❖ It exists in various forms : mechanical, thermal, electrical etc.❖ Electric Energy: an important gradient for industrial development

Electrical Energy

❖ can be generated centrally in bulk❖ can be easily and economically transported over long distances❖ losses in transportation are minimum❖ can be easily subdivided❖ can be adapted easily and efficiently to domestic and mechanical work

➢ Conventionally obtained by conversion from fossil fuels, nuclear and hydro sources

➢ Heat energy - Mechanical energy - Electrical energy

Energy

❖ With increasing population and their energy consumption, conventional energy sources will replenish in near future

❖ A coordinated world wide action plan is required to ensure that energy is available for longer period of time and at low cost.

❖ Following factors needs to be considered:➢ energy consumption curtailment➢ develop alternate energy sources➢ recycling nuclear wastes➢ development & application of antipollution technologies

Power

❖ Power is the rate of doing work, which equals energy per time

❖ Or power is defined as rate of flow of energy

❖ Mostly associated with mechanical and electrical forms of energy

❖ Power Plant : a unit built for production and delivery of a flow of mechanical and electrical energy

Review of Thermodynamic Cycles

❖ Laws of Thermodynamics

❖ Steam Engines : Rankine Cycle

❖ I.C Engines : Otto, Diesel and Dual Cycle

❖ Gas Turbine : Brayton Cycle

❖ Nuclear Power Plants : Fission and Fusion

Classification of power plant cycle❖ Vapour Power Cycles

➢ Carnot cycle

➢ Rankine Cycle

➢ Regenerative cycle

➢ Reheat Cycle

❖ Gas Power Cycles

➢ Otto Cycle

➢ Diesel Cycle

➢ Dual Cycle

➢ Gas Turbine Cycle

Carnot Cycle

Carnot Cycle

● Most efficient cycle. But to construct a device working on carnot cycle is practically impossible.● It used as a benchmark to compare the efficiency of different devices

Problem 1

A car engine with the power output of 65 hp has a thermal efficiency of 24%. Determine the fuel consumption rate of this car if the fuel has a heating value of 44,000 kJ/kg.

Problem 2

The food compartment of a refrigerator, is maintained at 4°C by removing heat from it at a rate of 360 kJ/min. If the required power input to the refrigerator is 2 kW, determine :(a) the coefficient of performance of the refrigerator (b) the rate of heat rejection to the room that houses the refrigerator.

Problem 3

A heat pump is used to meet the heating requirements of a house and maintain it at 20°C. On a day when the outdoor air temperature drops to -2°C, the house is estimatedto lose heat at a rate of 80,000 kJ/h. If the heat pump underthese conditions has a COP of 2.5, determine (a) the power consumed by the heat pump and (b) the rate at which heat is absorbed from the cold outdoor air.

Rankine Cycle

❖ Used to predict the performance of steam turbine systems

❖ The heat is supplied externally to a closed loop, which usually uses water as the working fluid

Re-Heat Cycle

❖ increases dryness fraction at exhaust so that turbine blade erosion reduces

❖ it increases thermal efficiency

❖ it increase the work done per kg of steam and this results in reduced size of boiler

❖ cost increases due to

reheater & connections❖ increases condenser

capacity due to increased x

Regeneration Cycle

❖ process of extracting steam from the turbine at certain points during its expansion and using this steam for heating for feed water

Binary Vapour Cycle

Reheat - Regeneration Cycle

Problem 4

❖ A simple rankine cycle works between pressure of 30 bar and 0.04 bar, the initial condition of steam being dry saturated, calculate the cycle efficiency, work ratio and specific steam consumption.

Problem 5

❖ A steam power plant works between 40 bar and 0.05 bar. If the steam supplied is dry saturated and the cycle of operation is Rankine, find (a) Rankine efficiency (b) specific steam consumption (c) work ratio (d) Turbine Power (e) condenser heat flow and (f) dryness fraction at the end of expansion. Assume flow rate of 10 kg/s

❖ Pump Work : 4 kJ/kg

❖ Efficiency : 35.5 %

❖ SSC : 3.8 kg/kW-hr

❖ WR : 0.9957

Problem 6

❖ A steam engine operates on ideal Carnot cycle using dry saturated steam at 17.5 bar. The exhaust takes place at 0.07 bar into a condenser. Assuming that the expansion and compression are isentropic and liquid enters the boiler as saturated liquid, find (a) power developed by the engine if the steam consumption is 20 kg/min and (b) the efficiency of the operating cycle.

Problem 7

❖ Dry saturated steam at 15 bar is supplied to a steam turbine. The exhaust takes at 1.1 bar. Determine the following: (a) Rankine Efficiency (b) Steam consumption per kWh if the efficiency ratio is 0.65 (c) carnot efficiency for the given pressure limit using steam as working fluid and (d) if the exhaust pressure is reduced to 0.2 bar, find the percentage increase in Rankine efficiency and percentage decrease in specific steam consumption.

❖ Neglect the pump work.

OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES

Actual and ideal cycles in spark-ignition engines and their P-v diagrams.

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The thermal efficiency of the Otto cycle increases with the specific heat ratio k of the working fluid.

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DIESEL CYCLE: THE IDEAL CYCLEFOR COMPRESSION-IGNITION ENGINES

In diesel engines, the spark plug is replaced by a fuel injector, and only air is compressed during the compression process.

In diesel engines, only air is compressed during the compression stroke, eliminating the possibility of auto ignition (engine knock). Therefore, diesel engines can be designed to operate at much higher compression ratios than SI engines, typically between 12 and 24.

• 1-2 isentropic compression

• 2-3 constant-pressure heat addition

• 3-4 isentropic expansion

• 4-1 constant-pressure heat rejection.

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Cutoff ratio

for the same compression ratio

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An ideal diesel cycle with air as the working fluid has a compression ratio of 18 and cutoff ratio of 2. At the beginning of the compression process, the working fluid is at 100kPa, 27°C, and 1917 cm3. Utilizing the cold air standard assumptions, determine (a) the temperature and pressure of air at the end of each process (b) the net work output and the thermal efficiency and (c) the mean effective pressure.

30P-v diagram of an ideal dual cycle.

Dual cycle: A more realistic ideal cycle model for modern, high-speed compression ignition engine.

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Problem : An air-standard Dual cycle operates with a compression ratio of 14. The conditions at the beginning of compression are 100 kPa and 300 K. The maximum temperature in the cycle is 2200 K and the heat added at constant volume is twice the heat added at constant pressure. Determined, (a) The pressure, temperature, and specific volume at each corner of the cycle, (b) The thermal efficiency of the cycle, and (c) The mean effective pressure.