The Second Law of Thermodynamics Cengel & Boles, Chapter 5 ME 152

The Second Law of Thermodynamics So far we have studied: conservation of energy (i.e., First Law of Thermodynamics) conservation of mass tabulated thermodynamic properties and equations of state (e.g., ideal gas law) There is a need for another law – one that tells us what sort of processes are possible while satisfying conservation principles ME 152

Second Law Statements Like the 1st Law, the 2nd Law of Thermodynamics is based upon a long history of scientific experimentation There is no single verbal or math statement for this Law - instead, there is a collection of statements, deductions, and corollaries regarding thermodynamic processes that together form the 2nd Law Two popular statements: Clausius statement Kelvin-Planck statement ME 152

Kelvin-Planck Statement “It is impossible for any device that operates as a cycle to receive heat from a single thermal reservoir and produce an equivalent amount of work” ME 152

Clausius Statement “It is impossible to construct a device that operates as a cycle whose sole effect is the transfer of heat from a lower temper-ature reservoir to a higher temperature reservoir” ME 152

Thermodynamic Cycles Cycle energy balance Types of cycles heat engines, (aka power cycles) refrigeration and heat pump cycles ME 152

Heat Engines Net (cycle) work output: Thermal efficiency ME 152

Refrigeration & Heat Pump Cycles Net work input: Coefficient of performance (COP) ME 152

Reversible Processes Reversible Process: a process that can be reversed, allowing system and surroundings to be restored to their initial states no heat transfer no net work e.g., adiabatic compression/expansion of a gas in a frictionless piston device: ME 152

Reversible Processes, cont. Reversible processes are considered ideal processes – no energy is “wasted”, i.e., all energy can be recovered or restored they can produce the maximum amount of work (e.g., in a turbine) they can consume the least amount of work (e.g., in a compressor or pump) they can produce the maximum KE increase (e.g., in a nozzle) when configured as a cycle, they produce the maximum performance (i.e., the highest th or COP) ME 152

Irreversible Processes Irreversible Process - process that does not allow system and surroun-dings to be restored to initial state such a process contains “irreversibilities” all real processes have irreversibilities examples: heat transfer through a temperature difference unrestrained expansion of a fluid spontaneous chemical reaction spontaneous mixing of different fluids sliding friction or viscous fluid flow electric current through a resistance magnetization with hysteresis inelastic deformation ME 152

Internally Reversible Processes A process is called internally reversible if no irreversibilities occur within the boundary of the system the system can be restored to its initial state but not the surroundings comparable to concept of a point mass, frictionless pulley, rigid beam, etc. allows one to determine best theoretical performance of a system, then apply efficiencies or correction factors to obtain actual performance ME 152

Externally Reversible Processes A process is called externally reversible if no irreversibilities occur outside the boundary of the system heat transfer between a reservoir and a system is an externally reversible process if the outer surface of the system is at the reservoir temperature ME 152

The Carnot Principles Several corollaries (the Carnot principles) can be deduced from the Kelvin-Planck statement: the thermal efficiency of any heat engine must be less than 100% th of an irreversible heat engine is always less than that of a reversible heat engine all reversible heat engines operating between the same two thermal reservoirs must have the same th ME 152

The Kelvin Temperature Scale Consider a reversible heat engine operating between TH and TL : Kelvin proposed a simple relation: ME 152

The Kelvin Temperature Scale, cont. Kelvin’s choice equates the ratio of heat transfers in a reversible heat engine to a the ratio of absolute temperatures Need a reference to define the magnitude of a kelvin (1 K) - the triple point of water is assigned 273.16 K: ME 152

Maximum Performance of Cycles Carnot Heat Engine: Carnot Refrigerator: Carnot Heat Pump: ME 152

The Carnot Cycle The Carnot cycle is the best-known reversible cycle, consisting of four reversible processes: adiabatic compression from temperature TL to TH isothermal expansion with heat input QH from reservoir at TH adiabatic expansion from temperature TH to TL isothermal compression with heat rejection QL to reservoir at TL ME 152

The Carnot Cycle, cont. Note: the heat transfers (QH , QL) can only be reversible if no temperature difference exists between the reservoir and system (working fluid) the processes described constitute a power cycle; it produces net work and operates clockwise on a P-v diagram The Carnot heat engine can be reversed (operating counter-clockwise on a P-v diagram) to become a Carnot refrigerator or heat pump the thermal efficiency and coefficients of performance of Carnot cycles correspond to maximum performance ME 152