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SECOND LAW OF THERMODYNAMICS SECOND LAW OF THERMODYNAMICS SECOND LAW OF THERMODYNAMICS SECOND LAW OF THERMODYNAMICS It is an observed fact that certain processes can only proceed spontaneously in one direction (hot coffee gets colder) TH TC TH TC Much Later TE TE SECOND LAW OF THERMODYNAMICS The following does not occur TE TH TE TC SECOND LAW OF THERMODYNAMICS Connecting high pressure tank with a low pressure tank: SECOND LAW OF THERMODYNAMICS Energy spontaneously tends to flow only from being concentrated in one place to becoming diffused or dispersed and spread out. SECOND LAW OF THERMODYNAMICS THERMAL ENERGY RESERVOIRS SECOND LAW OF THERMODYNAMICS THERMAL ENERGY RESERVOIRS A reservoir that supplies energy is a HEAT SOURCE A reservoir that absorbs energy is a HEAT SINK SECOND LAW OF THERMODYNAMICS Kelvin Planck Statement It is impossible for a system that operates in a cycle to generate work while transferring heat with a single reservoir Not Possible SECOND LAW OF THERMODYNAMICS Clausius Statement It is impossible for a system to operate in such a way that the sole result is the transfer of heat from a cold to a hot body Not Possible SECOND LAW OF THERMODYNAMICS Heat Engine Device that use heat to do work * Basic Heat Engine: SECOND LAW OF THERMODYNAMICS Heat Engine 1) Receive heat, QH, from a high temperature source 2) Convert part of this heat to work, W 3) Reject the remaining waste heat, QC, to a low temperature sink 4) Operate on a cycle SECOND LAW OF THERMODYNAMICS Heat Engine Work, SECOND LAW OF THERMODYNAMICS Heat Engine Efficiency work done W heat maximumwork WMAX engine QH QC QC heat 1 QH QH engine SECOND LAW OF THERMODYNAMICS Heat Engine Example: Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the net power output and the thermal efficiency for this heat engine SECOND LAW OF THERMODYNAMICS Heat Engine SECOND LAW OF THERMODYNAMICS Heat Engine Try: A car engine with a power output of 65hp has a thermal efficiency of 24 percent. Determine the fuel consumption rate of this car if the fuel has a heating value of 19000 Btu/lbm (that is, 19000 Btu of energy is released for each lbm of fuel burned). 36.3 lbm/h SECOND LAW OF THERMODYNAMICS Heat Engine Can we have a 100% thermally efficient engine? SECOND LAW OF THERMODYNAMICS Heat Engine SECOND LAW OF THERMODYNAMICS Heat Engine Every heat engine must waste some energy by transferring it to a lowtemperature reservoir in order to complete the cycle, Therefore, no engine can be 100% efficient SECOND LAW OF THERMODYNAMICS Heat Engine SECOND LAW OF THERMODYNAMICS Heat Pump SECOND LAW OF THERMODYNAMICS Heat pump A heat pump is a device which applies external work to extract an amount of heat QC from a cold reservoir and delivers heat QH to a hot reservoir. SECOND LAW OF THERMODYNAMICS Heat Pump SECOND LAW OF THERMODYNAMICS Heat Pump SECOND LAW OF THERMODYNAMICS Heat Pump The coefficient of performance (COP) for a heat pump is the ratio of the energy transferred for heating to the input electric energy used in the process. The coefficient is defined by Desired output QH COPHP Required Input Win QC 1 COPHP Q H QC 1 QC / QH SECOND LAW OF THERMODYNAMICS REFRIGERATOR SECOND LAW OF THERMODYNAMICS REFRIGERATOR Evaporator Condenser Expansion Valve SECOND LAW OF THERMODYNAMICS REFRIGERATOR Desired out put QC COPR Required Input Win QC 1 COPR Q H Q C QH / QC 1 SECOND LAW OF THERMODYNAMICS REFRIGERATOR 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 the COP of the refrigerator and the rate of heat rejection to the room that houses the refrigerator. REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBILITY • In a reversible process the state of working fluid and system's surroundings can be restored to the original ones. REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBILITY HEAT REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBILITY • No internal or mechanical friction is allowed. • The temperature and pressure difference between the working fluid and its surroundings should be infinitely small. REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBLE PROCESSES • Idealization • Do not occur in nature • Can be approximated, never achieve REVERSIBLE & IRREVERSIBLE PROCESSES IRREVERSIBILITY • Friction • Heat transfer • Non quasi equilibrium REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBLE PROCESSES Internally reversible -no irreversibilities within boundary Externally reversible -no irreversibilities outside system boundary REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBLE PROCESSES • No irreversibilities within the system or its surroundings Heat transfer Friction Non-quasi-equilibrium changes REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBLE PROCESSES • Require least amount of work • Delivers most heat • Maximum efficiency REVERSIBLE & IRREVERSIBLE PROCESSES REVERSIBLE PROCESSES Therefore, a reversible process is the most efficient process CARNOT CYCLE •Sadi Carnot •French engineer •Founder of Carnot Cycle in 1824 CARNOT CYCLE Reversible Isothermal Expansion Stage 1: In the first stage, the piston moves downward while the engine absorbs heat from a source and gas begins to expand. The portion of the graphic from point A to point B represents this behavior. Because the temperature of the gas does not change, this kind of expansion is called isothermic. CARNOT CYCLE Reversible Adiabatic Expansion Stage 2: In the second stage, the heat source is removed; the piston continues to move downward and the gas is still expanding while cooling (lowering in temperature). It is presented by the graphic from point B to point C. This stage is called a adiabatic expansion (Energy stays) CARNOT CYCLE Reversible Isothermal Compression Stage 3: The piston begins to move upward and the cool gas is recompressed in the third stage. The heat goes to sink. Point C point D represents the decrease in volume and increase in pressure. The engine gives energy to the environment. This stage is called isothermal compression. CARNOT CYCLE Reversible Adiabatic Compression Stage 4: In the final stage, the piston to move upward and the cool gas is secluded and compressed. Its temperature rises to its original state. Point C to point D illustrate this behavior; a continuing increase in pressure and decrease in volume to their initial position. Energy stays, so it's an adiabatic compression. CARNOT CYCLE CARNOT CYCLE CARNOT CYCLE CARNOT PRINCIPLES 1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs 2. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same. CARNOT CYCLE Thermodynamic Temperature Scale CARNOT CYCLE CARNOT HEAT ENGINE CARNOT CYCLE CARNOT HEAT ENGINE A power station contains a heat engine operating between two heat reservoirs, one consisting of steam at 100 C and the other consisting of water at 20 C. What is the maximum amount of electrical energy which can be produced for every Joule of heat extracted from the steam? CARNOT CYCLE CARNOT HEAT ENGINE CARNOT CYCLE CARNOT HEAT PUMP & REFRIGERATOR CARNOT CYCLE CARNOT HEAT PUMP & REFRIGERATOR When a fridge stands in a room at 20 C, the motor has to extract 500W of heat from the cabinet, at 4 C, to compensate for less than perfect insulation. How much power must be supplied to the motor if its efficiency is 80% of the maximum achievable? CARNOT CYCLE CARNOT HEAT PUMP & REFRIGERATOR