Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering Summary of Energy Topics Chapter 1: Thermodynamics / Energy Introduction Chapter 2: Systems & Processes Chapter 3: Work, Energy, Temperature & Heat Chapter 4: Work Processes of Closed Systems Chapter 5: Thermodynamic Properties Chapter 6: Steam Tables Chapter 7: Ideal Gases Chapter 8: Conservation of Mass & Energy Chapter 9: 1 st Law of Thermodynamics Chapter 10: Steady Flow Energy Equation Chapter 11: Heat Engines and Reversibility Chapter 12: 2 nd Law of Thermodynamics Chapter 13: Entropy Chapter 14: General Energy

Chapter 11: Heat Engines and Reversibility Thermodynamics applications involve engines either producing work, or engines operating in reverse for producing heating or cooling effects. HEAT ENGINEHEAT PUMP Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE System that performs the conversion of thermal energy to mechanical work. Brings a working substance via a heat "source" from a high temperature to lower temperature state, via the "Working body/System" of the engine while transferring heat to the colder "sink" until it reaches a low temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance.

Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE EXAMPLES Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: NUCLEAR POWER PLANT In this plant heat transfer (Q H,in ) occurs at the rate of 900 MW from the hot reactor core (A) in which nuclear fission occurs to the primary coolant (pressurised water) in a closed steady-flow circuit. Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering Heat in

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: NUCLEAR POWER PLANT The primary coolant (B), in turn, transfers heat in the steam generators (only one of which is shown out of a total of four) to liquid water, producing steam that drives one of the two turbo-alternators (only one of the two is shown). Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: NUCLEAR POWER PLANT The water substance that receives the heat transfer and drives the turbo-alternators is contained within a closed steady-flow system (C). The turbo alternators produce MW of electric power (W net,out, F), of which MW are used within the power plant in order to operate pumps and auxiliary equipment, and to supply on-site electricity needs. Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering Work done

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: NUCLEAR POWER PLANT The balance of the electric power is supplied to the external electrical network. No component of the plant other than the turbo-alternators produces a work output. The steam that leaves the turbo-alternators is condensed in the condenser, transferring heat to sea water (E). The condensate is recirculated, via the feed pumps, to the steam generators. Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: NUCLEAR POWER PLANT Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: COMBUSTION ENGINE A combustion engine is a system in which a fuel and air or oxygen are taken in, the fuel is burned, combustion products are rejected, heat is rejected to a heat sink, and there is a net work output while the system undergoes no net change in its state. An internal combustion engine is a combustion engine in which the air or oxygen for combustion, the fuel (possibly), and the combustion products are directly involved in the main work processes. Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: COMBUSTION ENGINE Internal combustion engines such as the spark ignition engine used in cars or the gas turbine engine used in aircraft do not normally satisfy the strict requirements for a system to be considered a heat engine. However, for each type of internal combustion engine it is possible to define either a non-flow cycle or a steady-flow cycle that is very similar to the actual cycle and that can be considered a heat engine. Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility HEAT ENGINE: COMBUSTION ENGINE The top-dead-centre position is the position when the piston is nearest the cylinder head (containing the spark plug). The bottom-dead-centre position is the position when the piston is furthest from the cylinder head. Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility 1.Intake stroke: the piston moves to down. The inlet valve opens and the vaporized fuel mixture enters the combustion chamber. The inlet valve closes at the end of this stroke. 2.Compression stroke: Both valves are closed and the piston moves up compressing the fuel mixture, increasing pressure and temperature. 3.A Power stroke: The spark plug ignites the fuel mixture. The temperature and the pressure rise and work is done by the system. 4.Exhaust stroke: The exhaust valve opens, exhausts are emitted (H 2 O, CO 2 and nitrogen) and fresh fuel/air mix are pumped in. Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering Otto cycle 2-stroke compression cycle 1 2 3

Chapter 11: Heat Engines and Reversibility Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering

Chapter 11: Heat Engines and Reversibility Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering EXAMPLE: Calculate also the rate of energy transfer to the surroundings from the engine and the thermal efficiency of an internal combustion engine, which burns fuel at the rate of 4.11 kg/hour while it produces kW of output power. The calorific value of the fuel is 42,500 kJ/kg.

Chapter 11: Heat Engines and Reversibility Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering HEAT ENGINE OPERATING IN REVERSE A heat engine operating in reverse is known as a heat pump if its purpose is to provide a heating effect i.e. to cause heat transfer Q H,out to the thermal reservoir at temperature T H. It is a refrigerator if its purpose is to provide a cooling effect; i.e., to cause heat transfer Q L,in from the thermal reservoir at temperature T L.

Chapter 11: Heat Engines and Reversibility Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering Coefficient of performance, (c.o.p.) The ratio of the useful effect (whether heating or cooling) to the required input is used as a convenient way to describe the performance of heat pumps or refrigerators. This cannot be called an efficiency because its value is often greater than one.

Chapter 11: Heat Engines and Reversibility Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering Example: A refrigerator consumes 98 W of electric power on average. The average rate of heat transfer to the refrigerant circuit inside the cabinet is 183 W. Calculate the coefficient of performance of the fridge and the rate of heat rejection.

Chapter 11: Heat Engines and Reversibility Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering Example: A refrigerator consumes 98 W of electric power on average. The average rate of heat transfer to the refrigerant circuit inside the cabinet is 183 W. Calculate the coefficient of performance of the fridge and the rate of heat rejection.

Chapter 11: Heat Engines and Reversibility Dr. Joseph Stokes School of Mechanical & Manufacturing Engineering Example: A refrigerator consumes 98 W of electric power on average. The average rate of heat transfer to the refrigerant circuit inside the cabinet is 183 W. Calculate the coefficient of performance of the fridge and the rate of heat rejection.